Red Hat SummitDieser Inhalt ist in der von Ihnen ausgewählten Sprache nicht verfügbar.
Deployment Guide
Red Hat Enterprise Linux 6
Deployment, Configuration and Administration of Red Hat Enterprise Linux 6
Abstract
The Deployment Guide documents relevant information regarding the deployment, configuration and administration of Red Hat Enterprise Linux 6. It is oriented towards system administrators with a basic understanding of the system.
Part I. Basic System Configuration
Link kopierenLink in die Zwischenablage kopiert!
This part covers basic system administration tasks such as keyboard configuration, date and time configuration, managing users and groups, and gaining privileges.
Chapter 1. Keyboard Configuration
Link kopierenLink in die Zwischenablage kopiert!
This chapter describes how to change the keyboard layout, as well as how to add the Keyboard Indicator applet to the panel. It also covers the option to enforce a typing break, and explains both advantages and disadvantages of doing so.
1.1. Changing the Keyboard Layout
Link kopierenLink in die Zwischenablage kopiert!
The installation program has allowed you to configure a keyboard layout for your system. However, the default settings may not always suit your current needs. To configure a different keyboard layout after the installation, use the Keyboard Preferences tool.
To open Keyboard Layout Preferences, select → → from the panel, and click the Layouts tab.
Figure 1.1. Keyboard Layout Preferences
You will be presented with a list of available layouts. To add a new one, click the button below the list, and you will be prompted to choose which layout you want to add.
Figure 1.2. Choosing a layout
Currently, there are two ways how to choose the keyboard layout: you can either find it by the country it is associated with (the By country tab), or you can select it by language (the By language tab). In either case, first select the desired country or language from the Country or Language pulldown menu, then specify the variant from the Variants menu. The preview of the layout changes immediately. To confirm the selection, click .
Figure 1.3. Selecting the default layout
The layout should appear in the list. To make it the default, select the radio button next to its name. The changes take effect immediately. Note that there is a text-entry field at the bottom of the window where you can safely test your settings. Once you are satisfied, click to close the window.
Figure 1.4. Testing the layout
Note
By default, changing the keyboard layout affects the active window only. This means that if you change the layout and switch to another window, this window will use the old one, which might be confusing. To turn this behavior off, clear the Separate layout for each window check box.
Doing this has its drawbacks though, as you will no longer be able to choose the default layout by selecting the radio button as shown in Figure 1.3, “Selecting the default layout”. To make the layout the default, drag it to the beginning of the list.
1.2. Adding the Keyboard Layout Indicator
Link kopierenLink in die Zwischenablage kopiert!
If you want to see what keyboard layout you are currently using, or you would like to switch between different layouts with a single mouse click, add the Keyboard Indicator applet to the panel. To do so, right-click the empty space on the main panel, and select the option from the pulldown menu.
Figure 1.5. Adding a new applet
You will be presented with a list of available applets. Scroll through the list (or start typing “keyboard” into the search field at the top of the window), select , and click the button.
Figure 1.6. Selecting the Keyboard Indicator
The applet appears immediately, displaying the shortened name of the country the current layout is associated with. To display the actual variant, hover the pointer over the applet icon.
Figure 1.7. The Keyboard Indicator applet
1.3. Setting Up a Typing Break
Link kopierenLink in die Zwischenablage kopiert!
Typing for a long period of time can be not only tiring, but it can also increase the risk of serious health problems, such as carpal tunnel syndrome. One way of preventing this is to configure the system to enforce typing breaks. To do so, select → → from the panel, click the Typing Break tab, and select the Lock screen to enforce typing break check box.
Figure 1.8. Typing Break Properties
To increase or decrease the allowed typing time before the break is enforced, click the up or down button next to the Work interval lasts label respectively. You can do the same with the Break interval lasts setting to alter the length of the break itself. Finally, select the Allow postponing of breaks check box if you want to be able to delay the break in case you need to finish the work. The changes take effect immediately.
Figure 1.9. Taking a break
Next time you reach the time limit, you will be presented with a screen advising you to take a break, and a clock displaying the remaining time. If you have enabled it, the button will be located at the bottom right corner of the screen.
Chapter 2. Date and Time Configuration
Link kopierenLink in die Zwischenablage kopiert!
This chapter covers setting the system date and time in Red Hat Enterprise Linux, both manually and using the Network Time Protocol (NTP), as well as setting the adequate time zone. Two methods are covered: setting the date and time using the Date/Time Properties tool, and doing so on the command line.
2.1. Date/Time Properties Tool
Link kopierenLink in die Zwischenablage kopiert!
The Date/Time Properties tool allows the user to change the system date and time, to configure the time zone used by the system, and to set up the Network Time Protocol daemon to synchronize the system clock with a time server. Note that to use this application, you must be running the X Window System (see Appendix C, The X Window System for more information on this topic).
To start the tool, select → → from the panel, or type the
system-config-date command at a shell prompt (e.g., xterm or GNOME Terminal). Unless you are already authenticated, you will be prompted to enter the superuser password.
Figure 2.1. Authentication Query
2.1.1. Date and Time Properties
Link kopierenLink in die Zwischenablage kopiert!
As shown in Figure 2.2, “Date and Time Properties”, the Date/Time Properties tool is divided into two separate tabs. The tab containing the configuration of the current date and time is shown by default.
Figure 2.2. Date and Time Properties
To set up your system manually, follow these steps:
- Change the current date. Use the arrows to the left and right of the month and year to change the month and year respectively. Then click inside the calendar to select the day of the month.
- Change the current time. Use the up and down arrow buttons beside the Hour, Minute, and Second, or replace the values directly.
Click the button to apply the changes and exit the application.
2.1.2. Network Time Protocol Properties
Link kopierenLink in die Zwischenablage kopiert!
If you prefer an automatic setup, select the check box labeled Synchronize date and time over the network instead. This will display the list of available NTP servers as shown in Figure 2.3, “Network Time Protocol Properties”.
Figure 2.3. Network Time Protocol Properties
Here you can choose one of the predefined servers, edit a predefined server by clicking the button, or add a new server name by clicking . In the Advanced Options, you can also select whether you want to speed up the initial synchronization of the system clock, or if you want to use a local time source.
Note
Your system does not start synchronizing with the NTP server until you click the button at the bottom of the window to confirm your changes.
Click the button to apply any changes made to the date and time settings and exit the application.
2.1.3. Time Zone Properties
Link kopierenLink in die Zwischenablage kopiert!
To configure the system time zone, click the Time Zone tab as shown in Figure 2.4, “Time Zone Properties”.
Figure 2.4. Time Zone Properties
There are two common approaches to the time zone selection:
- Using the interactive map. Click “zoom in” and “zoom out” buttons next to the map, or click on the map itself to zoom into the selected region. Then choose the city specific to your time zone. A red X appears and the time zone selection changes in the list below the map.
- Use the list below the map. To make the selection easier, cities and countries are grouped within their specific continents. Note that non-geographic time zones have also been added to address needs in the scientific community.
If your system clock is set to use UTC, select the option. UTC stands for the Universal Time, Coordinated, also known as Greenwich Mean Time (GMT). Other time zones are determined by adding or subtracting from the UTC time.
Click to apply the changes and exit the program.
2.2. Command Line Configuration
Link kopierenLink in die Zwischenablage kopiert!
In case your system does not have the Date/Time Properties tool installed, or the X Window Server is not running, you will have to change the system date and time on the command line. Note that in order to perform actions described in this section, you have to be logged in as a superuser:
su -
~]$ su -
Password:2.2.1. Date and Time Setup
Link kopierenLink in die Zwischenablage kopiert!
The
date command allows the superuser to set the system date and time manually:
- Change the current date. Type the command in the following form at a shell prompt, replacing the YYYY with a four-digit year, MM with a two-digit month, and DD with a two-digit day of the month:
date +%D -s YYYY-MM-DD
~]# date +%D -s YYYY-MM-DDCopy to Clipboard Copied! Toggle word wrap Toggle overflow For example, to set the date to 2 June 2010, type:date +%D -s 2010-06-02
~]# date +%D -s 2010-06-02Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Change the current time. Use the following command, where HH stands for an hour, MM is a minute, and SS is a second, all typed in a two-digit form:
date +%T -s HH:MM:SS
~]# date +%T -s HH:MM:SSCopy to Clipboard Copied! Toggle word wrap Toggle overflow If your system clock is set to use UTC (Coordinated Universal Time), add the following option:date +%T -s HH:MM:SS -u
~]# date +%T -s HH:MM:SS -uCopy to Clipboard Copied! Toggle word wrap Toggle overflow For instance, to set the system clock to 11:26 PM using the UTC, type:date +%T -s 23:26:00 -u
~]# date +%T -s 23:26:00 -uCopy to Clipboard Copied! Toggle word wrap Toggle overflow
You can check your current settings by typing
date without any additional argument:
Example 2.1. Displaying the current date and time
date
~]$ date
Wed Jun 2 11:58:48 CEST 20102.2.2. Network Time Protocol Setup
Link kopierenLink in die Zwischenablage kopiert!
As opposed to the manual setup described above, you can also synchronize the system clock with a remote server over the Network Time Protocol (NTP). For the one-time synchronization only, use the ntpdate command:
- Firstly, check whether the selected NTP server is accessible:
ntpdate -q server_address
~]# ntpdate -q server_addressCopy to Clipboard Copied! Toggle word wrap Toggle overflow For example:ntpdate -q 0.rhel.pool.ntp.org
~]# ntpdate -q 0.rhel.pool.ntp.orgCopy to Clipboard Copied! Toggle word wrap Toggle overflow - When you find a satisfactory server, run the ntpdate command followed by one or more server addresses:
ntpdate server_address...
~]# ntpdate server_address...Copy to Clipboard Copied! Toggle word wrap Toggle overflow For instance:ntpdate 0.rhel.pool.ntp.org 1.rhel.pool.ntp.org
~]# ntpdate 0.rhel.pool.ntp.org 1.rhel.pool.ntp.orgCopy to Clipboard Copied! Toggle word wrap Toggle overflow Unless an error message is displayed, the system time should now be set. You can check the current by setting typingdatewithout any additional arguments as shown in Section 2.2.1, “Date and Time Setup”. - In most cases, these steps are sufficient. Only if you really need one or more system services to always use the correct time, enable running the ntpdate at boot time:
chkconfig ntpdate on
~]# chkconfig ntpdate onCopy to Clipboard Copied! Toggle word wrap Toggle overflow For more information about system services and their setup, see Chapter 12, Services and Daemons.Note
If the synchronization with the time server at boot time keeps failing, i.e., you find a relevant error message in the/var/log/boot.logsystem log, try to add the following line to/etc/sysconfig/network:NETWORKWAIT=1
NETWORKWAIT=1Copy to Clipboard Copied! Toggle word wrap Toggle overflow
However, the more convenient way is to set the ntpd daemon to synchronize the time at boot time automatically:
- Open the NTP configuration file
/etc/ntp.confin a text editor such as vi or nano, or create a new one if it does not already exist:nano /etc/ntp.conf
~]# nano /etc/ntp.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Now add or edit the list of public NTP servers. If you are using Red Hat Enterprise Linux 6, the file should already contain the following lines, but feel free to change or expand these according to your needs:
server 0.rhel.pool.ntp.org iburst server 1.rhel.pool.ntp.org iburst server 2.rhel.pool.ntp.org iburst server 3.rhel.pool.ntp.org iburst
server 0.rhel.pool.ntp.org iburst server 1.rhel.pool.ntp.org iburst server 2.rhel.pool.ntp.org iburst server 3.rhel.pool.ntp.org iburstCopy to Clipboard Copied! Toggle word wrap Toggle overflow Theiburstdirective at the end of each line is to speed up the initial synchronization. As of Red Hat Enterprise Linux 6.5 it is added by default. If upgrading from a previous minor release, and your/etc/ntp.conffile has been modified, then the upgrade to Red Hat Enterprise Linux 6.5 will create a new file/etc/ntp.conf.rpmnewand will not alter the existing/etc/ntp.conffile. - Once you have the list of servers complete, in the same file, set the proper permissions, giving the unrestricted access to localhost only:
restrict default kod nomodify notrap nopeer noquery restrict -6 default kod nomodify notrap nopeer noquery restrict 127.0.0.1 restrict -6 ::1
restrict default kod nomodify notrap nopeer noquery restrict -6 default kod nomodify notrap nopeer noquery restrict 127.0.0.1 restrict -6 ::1Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Save all changes, exit the editor, and restart the NTP daemon:
service ntpd restart
~]# service ntpd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make sure that
ntpdis started at boot time:chkconfig ntpd on
~]# chkconfig ntpd onCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Chapter 3. Managing Users and Groups
Link kopierenLink in die Zwischenablage kopiert!
3.1. What Users and Groups Are
Link kopierenLink in die Zwischenablage kopiert!
The control of users and groups is a core element of Red Hat Enterprise Linux system administration. The user of the system is either a human being or an account used by specific applications identified by a unique numerical identification number called user ID (UID). Users within a group can have read permissions, write permissions, execute permissions or any combination of read/write/execute permissions for files owned by that group.
Red Hat Enterprise Linux supports access control lists (ACLs) for files and directories which allow permissions for specific users outside of the owner to be set. For more information about this feature, see the Access Control Lists chapter of the Red Hat Enterprise Linux 6 Storage Administration Guide.
A group is an organization unit tying users together for a common purpose, which can be reading permissions, writing permission, or executing permission for files owned by that group. Similar to UID, each group is associated with a group ID (GID).
Note
Red Hat Enterprise Linux reserves user and group IDs below 500 for system users and groups. By default, the User Manager does not display the system users. Reserved user and group IDs are documented in the setup package. To view the documentation, use this command:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
The recommended practice is to assign non-reserved IDs starting at 5,000, as the reserved range can increase in the future. To make the IDs assigned to new users by default start at 5,000, change the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
cat /usr/share/doc/setup-2.8.14/uidgid
cat /usr/share/doc/setup-2.8.14/uidgidUID_MIN and GID_MIN directives in the /etc/login.defs file:
[file contents truncated] UID_MIN 5000 [file contents truncated] GID_MIN 5000 [file contents truncated]
[file contents truncated]
UID_MIN 5000
[file contents truncated]
GID_MIN 5000
[file contents truncated]
Even with new user and group IDs beginning with 5,000, it is recommended not to raise IDs reserved by system above 500 to avoid conflict with systems that retain the 500 limit.
Each user is a member of exactly one primary group and zero or more supplementary groups. By default, when a file is created, the file's owner is its creator and the file's group is the creator's primary group. A user can temporarily change what group is their primary group with the
newgrp command, after which all newly created files are owned by the new group. A supplementary group serves to grant a certain set of users, its members, access to a certain set of files, those owned by this group.
The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by
root, and access permissions can be changed by both the root user and file owner.
By default, a file or directory can be modified only by its creator. The setting that determines what permissions are applied to a newly created file or directory is called a
umask and can be configured in the /etc/bashrc file for all users, or in ~/.bashrc for each user individually . The configuration in ~/.bashrc overrides the configuration in /etc/bashrc. Additionally, the umask command overrides the default permissions for the duration of the shell session.
To authenticate, a user enters their password. A hash sum is generated from the entered string and compared to the hash sum of the user's password. If the hash sums match, the user authenticates successfully.
Hash sums of user passwords are stored in the
/etc/shadow file, which is only readable by the root user. The file also stores information about password aging and policies for specific accounts. The default values for a newly created account are stored in the /etc/login.defs and /etc/default/useradd files. The Red Hat Enterprise Linux 6 Security Guide provides more security-related information about users and groups.
3.2. Managing Users via the User Manager Application
Link kopierenLink in die Zwischenablage kopiert!
The User Manager application allows you to view, modify, add, and delete local users and groups in the graphical user interface.
To start the User Manager application:
- From the toolbar, select → → .
- Or, type
system-config-usersat the shell prompt.
Note
Unless you have superuser privileges, the application will prompt you to authenticate as
root.
3.2.1. Viewing Users
Link kopierenLink in die Zwischenablage kopiert!
In order to display the main window of the User Manager to view users, from the toolbar of User Manager select → . If you want to view all the users, that is including system users, clear the Hide system users and groups check box.
The Users tab provides a list of local users along with additional information about their user ID, primary group, home directory, login shell, and full name.
Figure 3.1. Viewing Users
To find a specific user, type the first few letters of the name in the Search filter field and either press Enter, or click the button. You can also sort the items according to any of the available columns by clicking the column header.
3.2.2. Adding a New User
Link kopierenLink in die Zwischenablage kopiert!
If there is a new user you need to add to the system, follow this procedure:
- Click the button.
- Enter the user name and full name in the appropriate fields
- Type the user's password in the Password and Confirm Password fields. The password must be at least six characters long.
Note
For safety reasons, choose a long password not based on a dictionary term; use a combination of letters, numbers, and special characters. - Select a login shell for the user from the Login Shell drop-down list or accept the default value of .
- Clear the Create home directory check box if you choose not to create the home directory for a new user in
/home/username/.You can also change this home directory by editing the content of the Home Directory text box. Note that when the home directory is created, default configuration files are copied into it from the/etc/skel/directory. - Clear the Create a private group for the user check box if you do not want a unique group with the same name as the user to be created. User private group (UPG) is a group assigned to a user account to which that user exclusively belongs, which is used for managing file permissions for individual users.
- Specify a user ID for the user by selecting Specify user ID manually. If the option is not selected, the next available user ID above 500 is assigned to the new user.
- Click the button to complete the process.
Look at the sample Add New User dialog box configuration:
To configure more advanced user properties, such as password expiration, modify the user's properties after adding the user.
3.2.3. Modifying User Properties
Link kopierenLink in die Zwischenablage kopiert!
- Select the user from the user list by clicking once on the user name.
- Click from the toolbar or choose → from the drop-down menu.
Figure 3.2. User Properties
- There are four tabs you can update to your preferences. When you have finished, click the button to save your changes.
3.3. Managing Groups via the User Manager Application
Link kopierenLink in die Zwischenablage kopiert!
3.3.1. Viewing Groups
Link kopierenLink in die Zwischenablage kopiert!
In order to display the main window of User Manager to view groups, from the toolbar select → . If you want to view all the groups, clear the Hide system users and groups check box.
The Groups tab provides a list of local groups with information about their group ID and group members as you can see in the picture below.
Figure 3.3. Viewing Groups
To find a specific group, type the first few letters of the name in the Search filter field and either press Enter, or click the button. You can also sort the items according to any of the available columns by clicking the column header.
3.3.2. Adding a New Group
Link kopierenLink in die Zwischenablage kopiert!
If there is a new group you need to add to the system, follow this procedure:
- Select from the User Manager toolbar:
Figure 3.4. New Group
- Type the name of the new group.
- Specify the group ID (GID) for the new group by checking the Specify group ID manually check box.
- Select the GID. Note that Red Hat Enterprise Linux also reserves group IDs lower than 500 for system groups.
- Click to create the group. The new group appears in the group list.
3.3.3. Modifying Group Properties
Link kopierenLink in die Zwischenablage kopiert!
- Select the group from the group list by clicking on its name.
- Click from the toolbar or choose → from the drop-down menu.
Figure 3.5. Group Properties
- The Group Users tab displays the list of group members. Use this tab to add or remove users from the group. Click to save your changes.
3.4. Managing Users via Command-Line Tools
Link kopierenLink in die Zwischenablage kopiert!
When managing users via command line, the following commands are used:
useradd, usermod, userdel, or passwd. The files affected include /etc/passwd which stores user accounts information and /etc/shadow, which stores secure user account information.
3.4.1. Creating Users
Link kopierenLink in die Zwischenablage kopiert!
The
useradd utility creates new users and adds them to the system. Following the short procedure below, you will create a default user account with its UID, automatically create a home directory where default user settings will be stored, /home/username/, and set the default shell to /bin/bash.
- Run the following command at a shell prompt as
rootsubstituting username with the name of your choice:useradd username
useradd usernameCopy to Clipboard Copied! Toggle word wrap Toggle overflow - By setting a password unlock the account to make it accessible. Type the password twice when the program prompts you to.
passwd
passwdCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Example 3.1. Creating a User with Default Settings
Running the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
useradd robert command creates an account named robert. If you run cat /etc/passwd to view the content of the /etc/passwd file, you can learn more about the new user from the line displayed to you:
robert:x:502:502::/home/robert:/bin/bash
robert:x:502:502::/home/robert:/bin/bashrobert has been assigned a UID of 502, which reflects the rule that the default UID values from 0 to 499 are typically reserved for system accounts. GID, group ID of User Private Group, equals to UID. The home directory is set to /home/robert and login shell to /bin/bash. The letter x signals that shadow passwords are used and that the hashed password is stored in /etc/shadow.
If you want to change the basic default setup for the user while creating the account, you can choose from a list of command-line options modifying the behavior of
useradd (see the useradd(8) man page for the whole list of options). As you can see from the basic syntax of the command, you can add one or more options:
useradd [option(s)] username
useradd [option(s)] username
As a system administrator, you can use the
-c option to specify, for example, the full name of the user when creating them. Use -c followed by a string, which adds a comment to the user:
useradd -c "string" username
useradd -c "string" usernameExample 3.2. Specifying a User's Full Name when Creating a User
useradd -c "Robert Smith" robert cat /etc/passwd
~]# useradd -c "Robert Smith" robert
~]# cat /etc/passwd
robert:x:502:502:Robert Smith:/home/robert:/bin/bash
A user account has been created with user name
robert, sometimes called the login name, and full name Robert Smith.
If you do not want to create the default
/home/username/ directory for the user account, set a different one instead of it. Execute the command below:
useradd -d home_directory
useradd -d home_directoryExample 3.3. Adding a User with non-default Home Directory
useradd -d /home/dir_1 robert
~]# useradd -d /home/dir_1 robert
robert's home directory is now not the default /home/robert but /home/dir_1/.
If you do not want to create the home directory for the user at all, you can do so by running
useradd with the -M option. However, when such a user logs into a system that has just booted and their home directory does not exist, their login directory will be the root directory. If such a user logs into a system using the su command, their login directory will be the current directory of the previous user.
useradd -M username
useradd -M username
If you need to copy a directory content to the
/home directory while creating a new user, make use of the -m and -k options together followed by the path.
Example 3.4. Creating a User while Copying Contents to the Home Directory
The following command copies the contents of a directory named
/dir_1 to /home/jane, which is the default home directory of a new user jane:
useradd -m -k /dir_1 jane
~]# useradd -m -k /dir_1 jane
As a system administrator, you may need to create a temporary account. Using the
useradd command, this means creating an account for a certain amount of time only and disabling it at a certain date. This is a particularly useful setting as there is no security risk resulting from forgetting to delete a certain account. For this, the -e option is used with the specified expire_date in the YYYY-MM-DD format.
Note
Do not confuse account expiration and password expiration. Account expiration is a particular date, after which it is impossible to log in to the account in any way, as the account no longer exists. Password expiration, the maximum password age and date of password creation or last password change, is the date, when it is not possible to log in using the password (but other ways exist, such as logging in using an SSH key).
useradd -e YYYY-MM-DD username
useradd -e YYYY-MM-DD usernameExample 3.5. Setting the Account Expiration Date
useradd -e 2015-11-05 emily
~]# useradd -e 2015-11-05 emily
The account
emily will be created now and automatically disabled on 5 November, 2015.
User's login shell defaults to
/bin/bash, but can be changed by the -s option to any other shell different from bash, ksh, csh, tsh, for example.
useradd -s login_shell username
useradd -s login_shell usernameExample 3.6. Adding a User with Non-default Shell
useradd -s /bin/ksh robert
~]# useradd -s /bin/ksh robert
This command creates the user
robert which has the /bin/ksh shell.
The
-r option creates a system account, which is an account for administrative use that has some, but not all, root privileges. Such accounts have a UID lower than the value of UID_MIN defined in /etc/login.defs, typically 500 and above for ordinary users.
useradd -r username
useradd -r username
3.4.2. Attaching New Users to Groups
Link kopierenLink in die Zwischenablage kopiert!
The
useradd command creates a User Private Group (UPG, a group assigned to a user account to which that user exclusively belongs) whenever a new user is added to the system and names the group after the user. For example, when the account robert is created, an UPG named robert is created at the same time, the only member of which is the user robert.
If you do not want to create a
User Private Group for a user for whatever reason, execute the useradd command with the following option:
useradd -N username
useradd -N username
Instead of automatically creating UPG or not creating it at all, you can specify the user's group membership with
-g and -G options. While the -g option specifies the primary group membership, -G refers to supplementary groups into which the user is also included. The group names you specify must already exist on the system.
Example 3.7. Adding a User to a Group
useradd -g "friends" -G "family,schoolmates" emily
~]# useradd -g "friends" -G "family,schoolmates" emily
The
useradd -g "friends" -G "family,schoolmates" emily command creates the user emily but emily's primary group is set to friends as specified by the -g option. emily is also a group member of the supplementary groups family and schoolmates.
Provided the user already exists and you want to add them to certain supplementary group(s), use the
usermod command with the -G option and a list of groups divided by commas, no spaces:
usermod -G group_1,group_2,group_3
usermod -G group_1,group_2,group_33.4.3. Updating Users' Authentication
Link kopierenLink in die Zwischenablage kopiert!
When running the basic
useradd username command, the password is automatically set to never expire (see the /etc/shadow file).
If you want to change this, use
passwd, the standard utility for administering the /etc/passwd file. The syntax of the passwd command look as follows:
passwd option(s) username
passwd option(s) username
You can, for example, lock the specified account. The locking is performed by rendering the encrypted password into an invalid string by prefixing the encrypted string with an the exclamation mark (
!). If you later find a reason to unlock the account, passwd has a reverse operation for locking. Only root can carry out these two operations.
passwd -l username passwd -u username
passwd -l username
passwd -u usernameExample 3.8. Unlocking a User Password
At first, the
-l option locks robert's account password successfully. However, running the passwd -u command does not unlock the password because by default passwd refuses to create a passwordless account.
If you want a password for an account to expire, run
passwd with the -e option. The user will be forced to change the password during the next login attempt:
passwd -e username
passwd -e username
As far as the password lifetime is concerned, setting the minimum time between password changes is useful for forcing the user to really change the password. The system administrator can set the minimum (the
-n option) and the maximum (the -x option) lifetimes. To inform the user about their password expiration, use the -w option. All these options must be accompanied with the number of days and can be run as root only.
Example 3.9. Adjusting Aging Data for User Passwords
passwd -n 10 -x 60 -w 3 jane
~]# passwd -n 10 -x 60 -w 3 jane
The above command has set the minimum password lifetime to 10 days, the maximum password lifetime to 60, and the number of days
jane will begin receiving warnings in advance that her password will expire to 3 day.
Later, when you cannot remember the password setting, make use of the
-S option which outputs a short information for you to know the status of the password for a given account:
passwd -S jane
~]# passwd -S jane
jane LK 2014-07-22 10 60 3 -1 (Password locked.)
You can also set the number of days after a password expires with the
useradd command, which disables the account permanently. A value of 0 disables the account as soon as the password has expired, and a value of -1 disables the feature, that is, the user will have to change his password when the password expires. The -f option is used to specify the number of days after a password expires until the account is disabled (but may be unblocked by system administrator):
useradd -f number-of-days username
useradd -f number-of-days username
For more information on the
passwd command see the passwd(1) man page.
3.4.4. Modifying User Settings
Link kopierenLink in die Zwischenablage kopiert!
When a user already exists and you need to specify any of the options now, use the
usermod command. The logic of using usermod is identical to useradd as well as its syntax:
usermod option(s) username
usermod option(s) username
If you need to change the user's user name, use the
-l option with the new user name (or login).
Example 3.10. Changing User's Login
usermod -l "emily-smith" emily
~]# usermod -l "emily-smith" emily
The
-l option changes the name of the user from the login emily to the new login, emily-smith. Nothing else is changed. In particular, emily's home directory name (/home/emily) remains the same unless it is changed manually to reflect the new user name.
In a similar way you can change the user's UID or user's home directory. See the example below:
Note
Find all files owned by the specified UID in system and change their owner. Do the same for Access Control List (ACL) referring to the UID. It is recommended to check there are no running processes as they keep running with the old UID.
Example 3.11. Changing User's UID and Home Directory
usermod -a -u 699 -d /home/dir_2 robert
~]# usermod -a -u 699 -d /home/dir_2 robert
The command with
-a -u and -d options changes the settings of user robert. Now, his ID is 699 instead of 501, and his home directory is no longer /home/robert but /home/dir_2.
With the
usermod command you can also move the content of the user's home directory to a new location, or lock the account by locking its password.
Example 3.12. Changing User's
usermod -m -d /home/jane -L jane
~]# usermod -m -d /home/jane -L jane
In this sample command, the
-m and -d options used together move the content of jane's home directory to the /home/dir_3 directory. The -L option locks the access to jane's account by locking its password.
For the whole list of options to be used with the
usermod command, see the usermod(8) man page or run usermod --help on the command line.
3.4.5. Deleting Users
Link kopierenLink in die Zwischenablage kopiert!
If you want to remove a user account from the system, use the
userdel command on the command line as root.
userdel username
userdel username
Combining
userdel with the -r option removes files in the user's home directory along with the home directory itself and the user's mail spool. Files located in other file systems have to be searched for and deleted manually.
userdel -r username
userdel -r usernameNote
The
-r option is relatively safer, and thus recommended, compared to -f which forces the removal of the user account even if the user is still logged in.
3.4.6. Displaying Comprehensive User Information
Link kopierenLink in die Zwischenablage kopiert!
When administering users and groups on your system, you need a good tool to monitor their configuration and activity on the system. Red Hat Enterprise Linux 6 provides you with the lslogins command-line utility, which gives you a comprehensive overview of users and groups, not only regarding user or group account configuration but also their activity on the system.
The general syntax of lslogins is the following:
lslogins [OPTIONS]
lslogins [OPTIONS]
where OPTIONS can be one or more available options and their related parameters. See the
lslogins(1) manual page or the output of the lslogins --help command for the complete list of available options and their usage.
The lslogins utility gives versatile information in a variety of formats based on the chosen options. The following examples introduce the most basic as well as some of the most useful combinations.
Running the
lslogins command without any options shows default information about all system and user accounts on the system. Specifically, their UID, user name, and GECOS information, as well as information about the user's last login to the system, and whether their password is locked or login by password disabled.
Example 3.13. Displaying basic information about all accounts on the system
To display detailed information about a single user, run the
lslogins LOGIN command, where LOGIN is either a UID or a user name. The following example displays detailed information about John Doe's account and his activity on the system:
Example 3.14. Displaying detailed information about a single account
If you use the
--logins=LOGIN option, you can display information about a group of accounts that are specified as a list of UIDs or user names. Specifying the --output=COLUMNS option, where COLUMNS is a list of available output parameters, you can customize the output of the lslogins command. For example, the following command shows login activity of the users root, jsmith, jdoe, and esmith:
Example 3.15. Displaying specific information about a group of users
The lslogins utility also distinguishes between system and user accounts. To address system accounts in your query, use the
--system-accs option. To address user accounts, use the --user-accs. For example, the following command displays information about supplementary groups and password expirations for all user accounts:
Example 3.16. Displaying information about supplementary groups and password expiration for all user accounts
The ability to format the output of
lslogins commands according to the user's needs makes lslogins an ideal tool to use in scripts and for automatic processing. For example, the following command returns a single string that represents the time and date of the last login. This string can be passed as input to another utility for further processing.
Example 3.17. Displaying a single piece of information without the heading
lslogins --logins=jsmith --output=LAST-LOGIN --time-format=iso | tail -1
~]# lslogins --logins=jsmith --output=LAST-LOGIN --time-format=iso | tail -1
2014-08-06T10:56:12+02003.5. Managing Groups via Command-Line Tools
Link kopierenLink in die Zwischenablage kopiert!
Groups are a useful tool for permitting co-operation between different users. There is a set of commands for operating with groups such as
groupadd, groupmod, groupdel, or gpasswd. The files affected include /etc/group which stores group account information and /etc/gshadow, which stores secure group account information.
3.5.1. Creating Groups
Link kopierenLink in die Zwischenablage kopiert!
To add a new group to the system with default settings, the
groupadd command is run at the shell prompt as root.
groupadd group_name
groupadd group_nameExample 3.18. Creating a Group with Default Settings
groupadd friends
~]# groupadd friends
The
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
groupadd command creates a new group called friends. You can read more information about the group from the newly-created line in the /etc/group file:
classmates:x:30005:
classmates:x:30005:
Automatically, the group
friends is attached with a unique GID (group ID) of 30005 and is not attached with any users. Optionally, you can set a password for a group by running gpasswd groupname.
Alternatively, you can add command options with specific settings.
groupadd option(s) groupname
groupadd option(s) groupname
If you, for example, want to specify the numerical value of the group's ID (GID) when creating the group, run the
groupadd command with the -g option. Remember that this value must be unique (unless the -o option is used) and the value must be non-negative.
groupadd -g GID
groupadd -g GIDExample 3.19. Creating a Group with Specified GID
The command below creates a group named
schoolmates and sets GID of 60002 for it:
groupadd -g 60002 schoolmates
~]# groupadd -g 60002 schoolmates
When used with
-g and GID already exists, groupadd refuses to create another group with existing GID. As a workaround, use the -f option, with which groupadd creates a group, but with a different GID.
groupadd -f GID
groupadd -f GID
You may also create a system group by attaching the
-r option to the groupadd command. System groups are used for system purposes, which practically means that GID is allocated from 1 to 499 within the reserved range of 999.
groupadd -r group_name
groupadd -r group_name
For more information on
groupadd, see the groupadd(8) man pages.
3.5.2. Attaching Users to Groups
Link kopierenLink in die Zwischenablage kopiert!
If you want to add an existing user to the named group, you can make use of the
gpasswd command.
gpasswd -a username which_group_to_edit
gpasswd -a username which_group_to_edit
To remove a user from the named group, run:
gpasswd -d username which_group_to_edit
gpasswd -d username which_group_to_edit
To set the list of group members, write the user names after the
--members option dividing them with commas and no spaces:
gpasswd --members username_1,username_2 which_group_to_edit
gpasswd --members username_1,username_2 which_group_to_edit3.5.3. Updating Group Authentication
Link kopierenLink in die Zwischenablage kopiert!
The
gpasswd command administers /etc/group and /etc/gshadow files. Note that this command works only if run by a group administrator.
Who is a group administrator? A group administrator can add and delete users as well as set, change, or remove the group password. A group can have more than one group administrator. The
root user can add group administrators with the gpasswd -A users groupname where users is a comma-separated list of existing users you want to be group administrators (without any spaces between commas).
For changing a group's password, run the
gpasswd command with the relevant group name. You will be prompted to type the new password of the group.
gpasswd groupname
gpasswd groupnameExample 3.20. Changing a Group Password
gpasswd crowd
~]# gpasswd crowd
Changing password for group crowd
New password:
Re-enter new password:
The password for the group
crowd has been changed.
You can also remove the password from the named group by using the
-r option.
gpasswd -r schoolmates
gpasswd -r schoolmates3.5.4. Modifying Group Settings
Link kopierenLink in die Zwischenablage kopiert!
When a group already exists and you need to specify any of the options now, use the
groupmod command. The logic of using groupmod is identical to groupadd as well as its syntax:
groupmod option(s) groupname
groupmod option(s) groupname
To change the group ID of a given group, use the
groupmod command in the following way:
groupmod -g GID_NEW which_group_to_edit
groupmod -g GID_NEW which_group_to_editNote
Find all files owned by the specified GID in system and change their owner. Do the same for Access Control List (ACL) referring to the GID. It is recommended to check there are no running processes as they keep running with the old GID.
To change the name of the group, run the following on the command line. The name of the group will be changed from GROUP_NAME to NEW_GROUP_NAME name.
groupmod -n new_groupname groupname
groupmod -n new_groupname groupnameExample 3.21. Changing a Group's Name
The following command changes the name of the group
schoolmates to crowd:
groupmod -n crowd schoolmates
~]# groupmod -n crowd schoolmates
3.5.5. Deleting Groups
Link kopierenLink in die Zwischenablage kopiert!
The
groupdel command modifies the system account files, deleting all entries that see the group. The named group must exist when you execute this command.
groupdel groupname
groupdel groupname3.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
See the following resources for more information about managing users and groups.
3.6.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
For information about various utilities for managing users and groups, see the following manual pages:
- chage(1) — A command to modify password aging policies and account expiration.
- gpasswd(1) — A command to administer the
/etc/groupfile. - groupadd(8) — A command to add groups.
- grpck(8) — A command to verify the
/etc/groupfile. - groupdel(8) — A command to remove groups.
- groupmod(8) — A command to modify group membership.
- pwck(8) — A command to verify the
/etc/passwdand/etc/shadowfiles. - pwconv(8) — A tool to convert standard passwords to shadow passwords.
- pwunconv(8) — A tool to convert shadow passwords to standard passwords.
- useradd(8) — A command to add users.
- userdel(8) — A command to remove users.
- usermod(8) — A command to modify users.
For information about related configuration files, see:
- group(5) — The file containing group information for the system.
- passwd(5) — The file containing user information for the system.
- shadow(5) — The file containing passwords and account expiration information for the system.
- login.defs(5) - The file containing shadow password suite configuration.
- useradd(8) - For
/etc/default/useradd, section “Changing the default values” in manual page.
Chapter 4. Gaining Privileges
Link kopierenLink in die Zwischenablage kopiert!
System administrators (and in some cases users) will need to perform certain tasks with administrative access. Accessing the system as
root is potentially dangerous and can lead to widespread damage to the system and data. This chapter covers ways to gain administrative privileges using the su and sudo programs. These programs allow specific users to perform tasks which would normally be available only to the root user while maintaining a higher level of control and system security.
See the Red Hat Enterprise Linux 6 Security Guide for more information on administrative controls, potential dangers and ways to prevent data loss resulting from improper use of privileged access.
4.1. The su Command
Link kopierenLink in die Zwischenablage kopiert!
When a user executes the
su command, they are prompted for the root password and, after authentication, are given a root shell prompt.
Once logged in via the
su command, the user is the root user and has absolute administrative access to the system[1]. In addition, once a user has become root, it is possible for them to use the su command to change to any other user on the system without being prompted for a password.
Because this program is so powerful, administrators within an organization may want to limit who has access to the command.
One of the simplest ways to do this is to add users to the special administrative group called wheel. To do this, type the following command as root:
usermod -a -G wheel username
~]# usermod -a -G wheel username
In the previous command, replace username with the user name you want to add to the
wheel group.
You can also use the User Manager to modify group memberships, as follows. Note: you need Administrator privileges to perform this procedure.
- Click the menu on the Panel, point to and then click to display the User Manager. Alternatively, type the command
system-config-usersat a shell prompt. - Click the Users tab, and select the required user in the list of users.
- Click on the toolbar to display the User Properties dialog box (or choose on the menu).
- Click the Groups tab, select the check box for the wheel group, and then click .
See Section 3.2, “Managing Users via the User Manager Application” for more information about the User Manager.
After you add the desired users to the
wheel group, it is advisable to only allow these specific users to use the su command. To do this, you will need to edit the PAM configuration file for su: /etc/pam.d/su. Open this file in a text editor and remove the comment (#) from the following line:
#auth required pam_wheel.so use_uid
#auth required pam_wheel.so use_uid
This change means that only members of the administrative group
wheel can switch to another user using the su command.
Note
The
root user is part of the wheel group by default.
4.2. The sudo Command
Link kopierenLink in die Zwischenablage kopiert!
The
sudo command offers another approach to giving users administrative access. When trusted users precede an administrative command with sudo, they are prompted for their own password. Then, when they have been authenticated and assuming that the command is permitted, the administrative command is executed as if they were the root user.
The basic format of the
sudo command is as follows:
sudo <command>
sudo <command>
In the above example, <command> would be replaced by a command normally reserved for the root user, such as
mount.
The
sudo command allows for a high degree of flexibility. For instance, only users listed in the /etc/sudoers configuration file are allowed to use the sudo command and the command is executed in the user's shell, not a root shell. This means the root shell can be completely disabled as shown in the Red Hat Enterprise Linux 6 Security Guide.
Each successful authentication using the
sudo is logged to the file /var/log/messages and the command issued along with the issuer's user name is logged to the file /var/log/secure. Should you require additional logging, use the pam_tty_audit module to enable TTY auditing for specified users by adding the following line to your /etc/pam.d/system-auth file:
session required pam_tty_audit.so disable=<pattern> enable=<pattern>
session required pam_tty_audit.so disable=<pattern> enable=<pattern>
where pattern represents a comma-separated listing of users with an optional use of globs. For example, the following configuration will enable TTY auditing for the root user and disable it for all other users:
session required pam_tty_audit.so disable=* enable=root
session required pam_tty_audit.so disable=* enable=root
Another advantage of the
sudo command is that an administrator can allow different users access to specific commands based on their needs.
Administrators wanting to edit the
sudo configuration file, /etc/sudoers, should use the visudo command.
To give someone full administrative privileges, type
visudo and add a line similar to the following in the user privilege specification section:
juan ALL=(ALL) ALL
juan ALL=(ALL) ALL
This example states that the user,
juan, can use sudo from any host and execute any command.
The example below illustrates the granularity possible when configuring
sudo:
%users localhost=/sbin/shutdown -h now
%users localhost=/sbin/shutdown -h now
This example states that any user can issue the command
/sbin/shutdown -h now as long as it is issued from the console.
The man page for
sudoers has a detailed listing of options for this file.
Important
There are several potential risks to keep in mind when using the
sudo command. You can avoid them by editing the /etc/sudoers configuration file using visudo as described above. Leaving the /etc/sudoers file in its default state gives every user in the wheel group unlimited root access.
- By default,
sudostores the sudoer's password for a five minute timeout period. Any subsequent uses of the command during this period will not prompt the user for a password. This could be exploited by an attacker if the user leaves his workstation unattended and unlocked while still being logged in. This behavior can be changed by adding the following line to the/etc/sudoersfile:Defaults timestamp_timeout=<value>
Defaults timestamp_timeout=<value>Copy to Clipboard Copied! Toggle word wrap Toggle overflow where <value> is the desired timeout length in minutes. Setting the <value> to 0 causessudoto require a password every time. - If a sudoer's account is compromised, an attacker can use
sudoto open a new shell with administrative privileges:sudo /bin/bash
sudo /bin/bashCopy to Clipboard Copied! Toggle word wrap Toggle overflow Opening a new shell as root in this or similar fashion gives the attacker administrative access for a theoretically unlimited amount of time, bypassing the timeout period specified in the/etc/sudoersfile and never requiring the attacker to input a password forsudoagain until the newly opened session is closed.
4.3. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
While programs allowing users to gain administrative privileges are a potential security risk, security itself is beyond the scope of this particular book. You should therefore see sources listed below for more information regarding security and privileged access.
Installed Documentation
- su(1) - the manual page for
suprovides information regarding the options available with this command. - sudo(8) - the manual page for
sudoincludes a detailed description of this command as well as a list of options available for customizingsudo's behavior. - pam(8) - the manual page describing the use of Pluggable Authentication Modules for Linux.
Online Documentation
- Red Hat Enterprise Linux 6 Security Guide - The Security Guide describes in detail security risks and mitigating techniques related to programs for gaining privileges.
- Red Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards - This guide provides, among other things, a detailed description of Pluggable Authentication Modules (PAM), their configuration and usage.
Chapter 5. Console Access
Link kopierenLink in die Zwischenablage kopiert!
When normal (non-root) users log into a computer locally, they are given two types of special permissions:
- They can run certain programs that they otherwise cannot run.
- They can access certain files that they otherwise cannot access. These files normally include special device files used to access diskettes, CD-ROMs, and so on.
Since there are multiple consoles on a single computer and multiple users can be logged into the computer locally at the same time, one of the users has to essentially win the race to access the files. The first user to log in at the console owns those files. Once the first user logs out, the next user who logs in owns the files.
In contrast, every user who logs in at the console is allowed to run programs that accomplish tasks normally restricted to the root user. If X is running, these actions can be included as menu items in a graphical user interface. As shipped, these console-accessible programs include
halt, poweroff, and reboot.
5.1. Disabling Console Program Access for Non-root Users
Link kopierenLink in die Zwischenablage kopiert!
Non-root users can be denied console access to any program in the
/etc/security/console.apps/ directory. To list these programs, run the following command:
For each of these programs, console access denial can be configured using the program's Pluggable Authentication Module (PAM) configuration file. For information about PAMs and their usage, see chapter Pluggable Authentication Modules of the Red Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards guide.
PAM configuration file for each program in
/etc/security/console.apps/ resides in the /etc/pam.d/ directory and is named the same as the program. Using this file, you can configure PAM to deny access to the program if the user is not root. To do that, insert line auth requisite pam_deny.so directly after the first uncommented line auth sufficient pam_rootok.so.
Example 5.1. Disabling Access to the Reboot Program
To disable non-root console access to
/etc/security/console.apps/reboot, insert line auth requisite pam_deny.so into the /etc/pam.d/reboot PAM configuration file:
With this setting, all non-root access to the
reboot utility is disabled.
Additionally, several programs in
/etc/security/console.apps/ partially derive their PAM configuration from the /etc/pam.d/config-util configuration file. This allows to change configuration for all these programs at once by editing /etc/pam.d/config-util. To find all these programs, search for PAM configuration files that refer to the config-util file:
Disabling console program access as described above may be useful in environments where the console is otherwise secured. Security measures may include password protection for BIOS and boot loader, disabling rebooting on pressing Ctrl+Alt+Delete, disabling the power and reset switches, and other. In these cases, you may want to restrict normal user's access to
halt, poweroff, reboot, and other programs, which by default are accessible from the console.
5.2. Disabling Rebooting Using Ctrl+Alt+Del
Link kopierenLink in die Zwischenablage kopiert!
The action that happens in response to pressing Ctrl+Alt+Del at console is specified in the
/etc/init/control-alt-delete.conf file. By default, the shutdown utility with the -r option is used to shutdown and reboot the system.
To disable this action, create an overriding configuration file that specifies the
exec true command, which does nothing. To do that, run the following command as root:
echo "exec true" >> /etc/init/control-alt-delete.override
~]# echo "exec true" >> /etc/init/control-alt-delete.overridePart II. Subscription and Support
Link kopierenLink in die Zwischenablage kopiert!
To receive updates to the software on a Red Hat Enterprise Linux system it must be subscribed to the Red Hat Content Delivery Network (CDN) and the appropriate repositories enabled. This part describes how to subscribe a system to the Red Hat Content Delivery Network.
Red Hat provides support via the Customer Portal, and you can access this support directly from the command line using the Red Hat Support Tool. This part describes the use of this command-line tool.
Chapter 6. Registering the System and Managing Subscriptions
Link kopierenLink in die Zwischenablage kopiert!
The subscription service provides a mechanism to handle Red Hat software inventory and allows you to install additional software or update already installed programs to newer versions using the yum or PackageKit package managers. In Red Hat Enterprise Linux 6 the recommended way to register your system and attach subscriptions is to use Red Hat Subscription Management.
Note
It is also possible to register the system and attach subscriptions after installation during the firstboot process. For detailed information about firstboot see the Firstboot chapter in the Installation Guide for Red Hat Enterprise Linux 6. Note that firstboot is only available on systems after a graphical installation or after a kickstart installation where a desktop and the X window system were installed and graphical login was enabled.
6.1. Registering the System and Attaching Subscriptions
Link kopierenLink in die Zwischenablage kopiert!
Complete the following steps to register your system and attach one or more subscriptions using Red Hat Subscription Management. Note that all
subscription-manager commands are supposed to be run as root.
- Run the following command to register your system. You will be prompted to enter your user name and password. Note that the user name and password are the same as your login credentials for Red Hat Customer Portal.
subscription-manager register
subscription-manager registerCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Determine the pool ID of a subscription that you require. To do so, type the following at a shell prompt to display a list of all subscriptions that are available for your system:
subscription-manager list --available
subscription-manager list --availableCopy to Clipboard Copied! Toggle word wrap Toggle overflow For each available subscription, this command displays its name, unique identifier, expiration date, and other details related to your subscription. To list subscriptions for all architectures, add the--alloption. The pool ID is listed on a line beginning withPool ID. - Attach the appropriate subscription to your system by entering a command as follows:
subscription-manager attach --pool=pool_id
subscription-manager attach --pool=pool_idCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace pool_id with the pool ID you determined in the previous step.To verify the list of subscriptions your system has currently attached, at any time, run:subscription-manager list --consumed
subscription-manager list --consumedCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Note
If you use a firewall or a proxy, you may need additional configuration to allow
yum and subscription-manager to work correctly. Refer to the "Setting Firewall Access for Content Delivery" section of the Red Hat Enterprise Linux 6 Subscription Management guide if you use a firewall and to the "Using an HTTP Proxy" section if you use a proxy.
For more details on how to register your system using Red Hat Subscription Management and associate it with subscriptions, see the designated solution article. For comprehensive information about subscriptions, see the Red Hat Subscription Management collection of guides.
6.2. Managing Software Repositories
Link kopierenLink in die Zwischenablage kopiert!
When a system is subscribed to the Red Hat Content Delivery Network, a repository file is created in the
/etc/yum.repos.d/ directory. To verify that, use yum to list all enabled repositories:
yum repolist
yum repolist
Red Hat Subscription Management also allows you to manually enable or disable software repositories provided by Red Hat. To list all available repositories, use the following command:
subscription-manager repos --list
subscription-manager repos --list
The repository names depend on the specific version of Red Hat Enterprise Linux you are using and are in the following format:
rhel-variant-rhscl-version-rpms rhel-variant-rhscl-version-debug-rpms rhel-variant-rhscl-version-source-rpms
rhel-variant-rhscl-version-rpms
rhel-variant-rhscl-version-debug-rpms
rhel-variant-rhscl-version-source-rpms
Where variant is the Red Hat Enterprise Linux system variant (
server or workstation), and version is the Red Hat Enterprise Linux system version (6 or 7), for example:
rhel-server-rhscl-6-eus-rpms rhel-server-rhscl-6-eus-source-rpms rhel-server-rhscl-6-eus-debug-rpms
rhel-server-rhscl-6-eus-rpms
rhel-server-rhscl-6-eus-source-rpms
rhel-server-rhscl-6-eus-debug-rpms
To enable a repository, enter a command as follows:
subscription-manager repos --enable repository
subscription-manager repos --enable repository
Replace repository with a name of the repository to enable.
Similarly, to disable a repository, use the following command:
subscription-manager repos --disable repository
subscription-manager repos --disable repository
Section 8.4, “Configuring Yum and Yum Repositories” provides detailed information about managing software repositories using yum.
6.3. Removing Subscriptions
Link kopierenLink in die Zwischenablage kopiert!
To remove a particular subscription, complete the following steps.
- Determine the serial number of the subscription you want to remove by listing information about already attached subscriptions:
subscription-manager list --consumed
subscription-manager list --consumedCopy to Clipboard Copied! Toggle word wrap Toggle overflow The serial number is the number listed asserial. For instance,744993814251016831in the example below:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter a command as follows to remove the selected subscription:
subscription-manager remove --serial=serial_number
subscription-manager remove --serial=serial_numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace serial_number with the serial number you determined in the previous step.
To remove all subscriptions attached to the system, run the following command:
subscription-manager remove --all
subscription-manager remove --all6.4. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For more information on how to register your system using Red Hat Subscription Management and associate it with subscriptions, see the resources listed below.
Installed Documentation
subscription-manager(8) — the manual page for Red Hat Subscription Management provides a complete list of supported options and commands.
Related Books
- Red Hat Subscription Management collection of guides — These guides contain detailed information how to use Red Hat Subscription Management.
- Installation Guide — see the Firstboot chapter for detailed information on how to register during the firstboot process.
Online Resources
- Red Hat Access Labs — The Red Hat Access Labs includes a “Registration Assistant”.
See Also
- Chapter 4, Gaining Privileges documents how to gain administrative privileges by using the
suandsudocommands. - Chapter 8, Yum provides information about using the yum packages manager to install and update software.
- Chapter 9, PackageKit provides information about using the PackageKit package manager to install and update software.
Chapter 7. Accessing Support Using the Red Hat Support Tool
Link kopierenLink in die Zwischenablage kopiert!
The Red Hat Support Tool, in the redhat-support-tool package, can function as both an interactive shell and as a single-execution program. It can be run over
SSH or from any terminal. It enables, for example, searching the Red Hat Knowledgebase from the command line, copying solutions directly on the command line, opening and updating support cases, and sending files to Red Hat for analysis.
7.1. Installing the Red Hat Support Tool
Link kopierenLink in die Zwischenablage kopiert!
The Red Hat Support Tool is installed by default on Red Hat Enterprise Linux. If required, to ensure that it is, enter the following command as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
root:
yum install redhat-support-tool
~]# yum install redhat-support-tool7.2. Registering the Red Hat Support Tool Using the Command Line
Link kopierenLink in die Zwischenablage kopiert!
To register the Red Hat Support Tool to the customer portal using the command line, proceed as follows:
- Where username is the user name of the Red Hat Customer Portal account.
redhat-support-tool config user username
~]# redhat-support-tool config user usernameCopy to Clipboard Copied! Toggle word wrap Toggle overflow redhat-support-tool config password
~]# redhat-support-tool config password Please enter the password for username:Copy to Clipboard Copied! Toggle word wrap Toggle overflow
7.3. Using the Red Hat Support Tool in Interactive Shell Mode
Link kopierenLink in die Zwischenablage kopiert!
To start the tool in interactive mode, enter the following command:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
The tool can be run as an unprivileged user, with a consequently reduced set of commands, or as
redhat-support-tool
~]$ redhat-support-tool
Welcome to the Red Hat Support Tool.
Command (? for help):root.
The commands can be listed by entering the
? character. The program or menu selection can be exited by entering the q or e character. You will be prompted for your Red Hat Customer Portal user name and password when you first search the Knowledgebase or support cases. Alternately, set the user name and password for your Red Hat Customer Portal account using interactive mode, and optionally save it to the configuration file.
7.4. Configuring the Red Hat Support Tool
Link kopierenLink in die Zwischenablage kopiert!
When in interactive mode, the configuration options can be listed by entering the command
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
config --help:
Procedure 7.1. Registering the Red Hat Support Tool Using Interactive Mode
To register the Red Hat Support Tool to the customer portal using interactive mode, proceed as follows:
- Start the tool by entering the following command:
redhat-support-tool
~]# redhat-support-toolCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter your Red Hat Customer Portal user name:To save your user name to the global configuration file, add the
Command (? for help): config user username
Command (? for help): config user usernameCopy to Clipboard Copied! Toggle word wrap Toggle overflow -goption. - Enter your Red Hat Customer Portal password:
Command (? for help): config password Please enter the password for username:
Command (? for help): config password Please enter the password for username:Copy to Clipboard Copied! Toggle word wrap Toggle overflow
7.4.1. Saving Settings to the Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
The Red Hat Support Tool, unless otherwise directed, stores values and options locally in the home directory of the current user, using the
~/.redhat-support-tool/redhat-support-tool.conf configuration file. If required, it is recommended to save passwords to this file because it is only readable by that particular user. When the tool starts, it will read values from the global configuration file /etc/redhat-support-tool.conf and from the local configuration file. Locally stored values and options take precedence over globally stored settings.
Warning
It is recommended not to save passwords in the global
/etc/redhat-support-tool.conf configuration file because the password is just base64 encoded and can easily be decoded. In addition, the file is world readable.
To save a value or option to the global configuration file, add the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
-g, --global option as follows:
Command (? for help): config setting -g value
Command (? for help): config setting -g valueNote
In order to be able to save settings globally, using the
-g, --global option, the Red Hat Support Tool must be run as root because normal users do not have the permissions required to write to /etc/redhat-support-tool.conf.
To remove a value or option from the local configuration file, add the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
This will clear, unset, the parameter from the tool and fall back to the equivalent setting in the global configuration file, if available.
-u, --unset option as follows:
Command (? for help): config setting -u value
Command (? for help): config setting -u valueNote
When running as an unprivileged user, values stored in the global configuration file cannot be removed using the
-u, --unset option, but they can be cleared, unset, from the current running instance of the tool by using the -g, --global option simultaneously with the -u, --unset option. If running as root, values and options can be removed from the global configuration file using -g, --global simultaneously with the -u, --unset option.
7.5. Opening and Updating Support Cases Using Interactive Mode
Link kopierenLink in die Zwischenablage kopiert!
Procedure 7.2. Opening a New Support Case Using Interactive Mode
To open a new support case using interactive mode, proceed as follows:
- Start the tool by entering the following command:
redhat-support-tool
~]# redhat-support-toolCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter the
opencasecommand:Command (? for help): opencase
Command (? for help): opencaseCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Follow the on screen prompts to select a product and then a version.
- Enter a summary of the case.
- Enter a description of the case and press Ctrl+D on an empty line when complete.
- Select a severity of the case.
- Optionally chose to see if there is a solution to this problem before opening a support case.
- Confirm you would still like to open the support case.
Support case 0123456789 has successfully been opened
Support case 0123456789 has successfully been openedCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Optionally chose to attach an SOS report.
- Optionally chose to attach a file.
Procedure 7.3. Viewing and Updating an Existing Support Case Using Interactive Mode
To view and update an existing support case using interactive mode, proceed as follows:
- Start the tool by entering the following command:
redhat-support-tool
~]# redhat-support-toolCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter the
getcasecommand:Where case-number is the number of the case you want to view and update.Command (? for help): getcase case-number
Command (? for help): getcase case-numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Follow the on screen prompts to view the case, modify or add comments, and get or add attachments.
Procedure 7.4. Modifying an Existing Support Case Using Interactive Mode
To modify the attributes of an existing support case using interactive mode, proceed as follows:
- Start the tool by entering the following command:
redhat-support-tool
~]# redhat-support-toolCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter the
modifycasecommand:Where case-number is the number of the case you want to view and update.Command (? for help): modifycase case-number
Command (? for help): modifycase case-numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow - The modify selection list appears:Follow the on screen prompts to modify one or more of the options.
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - For example, to modify the status, enter
3:Selection: 3 1 Waiting on Customer 2 Waiting on Red Hat 3 Closed Please select a status (or 'q' to exit):
Selection: 3 1 Waiting on Customer 2 Waiting on Red Hat 3 Closed Please select a status (or 'q' to exit):Copy to Clipboard Copied! Toggle word wrap Toggle overflow
7.6. Viewing Support Cases on the Command Line
Link kopierenLink in die Zwischenablage kopiert!
Viewing the contents of a case on the command line provides a quick and easy way to apply solutions from the command line.
To view an existing support case on the command line, enter a command as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where case-number is the number of the case you want to download.
redhat-support-tool getcase case-number
~]# redhat-support-tool getcase case-number7.7. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
The Red Hat Knowledgebase article Red Hat Support Tool has additional information, examples, and video tutorials.
Part III. Installing and Managing Software
Link kopierenLink in die Zwischenablage kopiert!
All software on a Red Hat Enterprise Linux system is divided into RPM packages, which can be installed, upgraded, or removed. This part focuses on product subscriptions and entitlements, and describes how to manage packages on Red Hat Enterprise Linux using both Yum and the PackageKit suite of graphical package management tools.
Chapter 8. Yum
Link kopierenLink in die Zwischenablage kopiert!
Yum is the Red Hat package manager that is able to query for information about available packages, fetch packages from repositories, install and uninstall them, and update an entire system to the latest available version. Yum performs automatic dependency resolution on packages you are updating, installing, or removing, and thus is able to automatically determine, fetch, and install all available dependent packages.
Yum can be configured with new, additional repositories, or package sources, and also provides many plug-ins which enhance and extend its capabilities. Yum is able to perform many of the same tasks that RPM can; additionally, many of the command-line options are similar. Yum enables easy and simple package management on a single machine or on groups of them.
The following sections assume your system was registered with Red Hat Subscription Management during installation as described in the Red Hat Enterprise Linux 6 Installation Guide. If your system is not subscribed, see Chapter 6, Registering the System and Managing Subscriptions.
Important
Yum provides secure package management by enabling GPG (Gnu Privacy Guard; also known as GnuPG) signature verification on GPG-signed packages to be turned on for all package repositories (i.e. package sources), or for individual repositories. When signature verification is enabled, Yum will refuse to install any packages not GPG-signed with the correct key for that repository. This means that you can trust that the RPM packages you download and install on your system are from a trusted source, such as Red Hat, and were not modified during transfer. See Section 8.4, “Configuring Yum and Yum Repositories” for details on enabling signature-checking with Yum, or Section B.3, “Checking a Package's Signature” for information on working with and verifying GPG-signed RPM packages in general.
Yum also enables you to easily set up your own repositories of RPM packages for download and installation on other machines.
Learning Yum is a worthwhile investment because it is often the fastest way to perform system administration tasks, and it provides capabilities beyond those provided by the PackageKit graphical package management tools. See Chapter 9, PackageKit for details on using PackageKit.
Note
You must have superuser privileges in order to use
yum to install, update or remove packages on your system. All examples in this chapter assume that you have already obtained superuser privileges by using either the su or sudo command.
8.1. Checking For and Updating Packages
Link kopierenLink in die Zwischenablage kopiert!
8.1.1. Checking For Updates
Link kopierenLink in die Zwischenablage kopiert!
To see which installed packages on your system have updates available, use the following command:
yum check-update
yum check-update
For example:
The packages in the above output are listed as having updates available. The first package in the list is PackageKit, the graphical package manager. The line in the example output tells us:
PackageKit— the name of the packagex86_64— the CPU architecture the package was built for0.5.8— the version of the updated package to be installedrhel— the repository in which the updated package is located
The output also shows us that we can update the kernel (the kernel package), Yum and RPM themselves (the yum and rpm packages), as well as their dependencies (such as the kernel-firmware, rpm-libs, and rpm-python packages), all using
yum.
8.1.2. Updating Packages
Link kopierenLink in die Zwischenablage kopiert!
You can choose to update a single package, multiple packages, or all packages at once. If any dependencies of the package (or packages) you update have updates available themselves, then they are updated too.
Updating a Single Package
To update a single package, run the following command as
root:
yum update package_name
yum update package_name
For example, to update the udev package, type:
This output contains several items of interest:
Loaded plugins: product-id, refresh-packagekit, subscription-manager—yumalways informs you which Yum plug-ins are installed and enabled. See Section 8.5, “Yum Plug-ins” for general information on Yum plug-ins, or to Section 8.5.3, “Plug-in Descriptions” for descriptions of specific plug-ins.udev.x86_64— you can download and install new udev package.yumpresents the update information and then prompts you as to whether you want it to perform the update;yumruns interactively by default. If you already know which transactions theyumcommand plans to perform, you can use the-yoption to automatically answeryesto any questions thatyumasks (in which case it runs non-interactively). However, you should always examine which changesyumplans to make to the system so that you can easily troubleshoot any problems that might arise.If a transaction does go awry, you can view Yum's transaction history by using theyum historycommand as described in Section 8.3, “Working with Transaction History”.
Important
yum always installs a new kernel in the same sense that RPM installs a new kernel when you use the command rpm -i kernel. Therefore, you do not need to worry about the distinction between installing and upgrading a kernel package when you use yum: it will do the right thing, regardless of whether you are using the yum update or yum install command.
When using RPM, on the other hand, it is important to use the
rpm -i kernel command (which installs a new kernel) instead of rpm -u kernel (which replaces the current kernel). See Section B.2.2, “Installing and Upgrading” for more information on installing/upgrading kernels with RPM.
Updating All Packages and Their Dependencies
To update all packages and their dependencies, enter
yum update (without any arguments):
yum update
yum updateUpdating Security-Related Packages
Discovering which packages have security updates available and then updating those packages quickly and easily is important. Yum provides the plug-in for this purpose. The security plug-in extends the
yum command with a set of highly-useful security-centric commands, subcommands and options. See Section 8.5.3, “Plug-in Descriptions” for specific information.
Updating Packages Automatically
It is also possible to set up periodical automatic updates for your packages. For this purpose, Red Hat Enterprise Linux 6 uses the yum-cron package. It provides a Yum interface for the
cron daemon and downloads metadata from your package repositories. With the yum-cron service enabled, the user can schedule an automated daily Yum update as a cron job.
Note
The yum-cron package is provided by the Optional subscription channel. See Section 8.4.8, “Adding the Optional and Supplementary Repositories” for more information on Red Hat additional channels.
To install yum-cron issue the following command:
yum install yum-cron
~]# yum install yum-cron
By default, the yum-cron service is disabled and needs to be activated and started manually:
chkconfig yum-cron on
~]# chkconfig yum-cron onservice yum-cron start
~]# service yum-cron start
To verify the status of the service, run the following command:
service yum-cron status
~]# service yum-cron status
The script included in the yum-cron package can be configured to change the extent and frequency of the updates, as well as to send notifications to e-mail. To customize yum-cron, edit the
/etc/sysconfig/yum-cron file.
Additional details and instructions for
yum-cron can be found in the comments within /etc/sysconfig/yum-cron and at the yum-cron(8) manual page.
8.1.3. Preserving Configuration File Changes
Link kopierenLink in die Zwischenablage kopiert!
You will inevitably make changes to the configuration files installed by packages as you use your Red Hat Enterprise Linux system. RPM, which Yum uses to perform changes to the system, provides a mechanism for ensuring their integrity. See Section B.2.2, “Installing and Upgrading” for details on how to manage changes to configuration files across package upgrades.
8.1.4. Upgrading the System Off-line with ISO and Yum
Link kopierenLink in die Zwischenablage kopiert!
For systems that are disconnected from the Internet or Red Hat Network, using the
yum update command with the Red Hat Enterprise Linux installation ISO image is an easy and quick way to upgrade systems to the latest minor version. The following steps illustrate the upgrading process:
- Create a target directory to mount your ISO image. This directory is not automatically created when mounting, so create it before proceeding to the next step. As
root, type:mkdir mount_dir
mkdir mount_dirCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace mount_dir with a path to the mount directory. Typically, users create it as a subdirectory in the/mediadirectory. - Mount the Red Hat Enterprise Linux 6 installation ISO image to the previously created target directory. As
root, type:mount -o loop iso_name mount_dir
mount -o loop iso_name mount_dirCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace iso_name with a path to your ISO image and mount_dir with a path to the target directory. Here, the-oloopoption is required to mount the file as a block device. - Copy the
media.repofile from the mount directory to the/etc/yum.repos.d/directory. Note that configuration files in this directory must have the .repo extension to function properly.cp mount_dir/media.repo /etc/yum.repos.d/new.repo
cp mount_dir/media.repo /etc/yum.repos.d/new.repoCopy to Clipboard Copied! Toggle word wrap Toggle overflow This creates a configuration file for the yum repository. Replace new.repo with the filename, for example rhel6.repo. - Edit the new configuration file so that it points to the Red Hat Enterprise Linux installation ISO. Add the following line into the
/etc/yum.repos.d/new.repofile:baseurl=file:///mount_dir
baseurl=file:///mount_dirCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace mount_dir with a path to the mount point. - Update all yum repositories including
/etc/yum.repos.d/new.repocreated in previous steps. Asroot, type:yum update
yum updateCopy to Clipboard Copied! Toggle word wrap Toggle overflow This upgrades your system to the version provided by the mounted ISO image. - After successful upgrade, you can unmount the ISO image. As
root, type:umount mount_dir
umount mount_dirCopy to Clipboard Copied! Toggle word wrap Toggle overflow where mount_dir is a path to your mount directory. Also, you can remove the mount directory created in the first step. Asroot, type:rmdir mount_dir
rmdir mount_dirCopy to Clipboard Copied! Toggle word wrap Toggle overflow - If you will not use the previously created configuration file for another installation or update, you can remove it. As
root, type:rm /etc/yum.repos.d/new.repo
rm /etc/yum.repos.d/new.repoCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Example 8.1. Upgrading from Red Hat Enterprise Linux 6.3 to 6.4
Imagine you need to upgrade your system without access to the Internet. To do so, you want to use an ISO image with the newer version of the system, called for instance
RHEL6.4-Server-20130130.0-x86_64-DVD1.iso. A target directory created for mounting is /media/rhel6/. As root, change into the directory with your ISO image and type:
mount -o loop RHEL6.4-Server-20130130.0-x86_64-DVD1.iso /media/rhel6/
~]# mount -o loop RHEL6.4-Server-20130130.0-x86_64-DVD1.iso /media/rhel6/
Then set up a yum repository for your image by copying the
media.repo file from the mount directory:
cp /media/rhel6/media.repo /etc/yum.repos.d/rhel6.repo
~]# cp /media/rhel6/media.repo /etc/yum.repos.d/rhel6.repo
To make yum recognize the mount point as a repository, add the following line into the
/etc/yum.repos.d/rhel6.repo copied in the previous step:
baseurl=file:///media/rhel6/
baseurl=file:///media/rhel6/
Now, updating the yum repository will upgrade your system to a version provided by
RHEL6.4-Server-20130130.0-x86_64-DVD1.iso. As root, execute:
yum update
~]# yum update
When your system is successfully upgraded, you can unmount the image, remove the target directory and the configuration file:
umount /media/rhel6/
~]# umount /media/rhel6/rmdir /media/rhel6/
~]# rmdir /media/rhel6/rm /etc/yum.repos.d/rhel6.repo
~]# rm /etc/yum.repos.d/rhel6.repo8.2. Packages and Package Groups
Link kopierenLink in die Zwischenablage kopiert!
8.2.1. Searching Packages
Link kopierenLink in die Zwischenablage kopiert!
You can search all RPM package names, descriptions and summaries by using the following command:
yum search term…
yum search term…
Replace term with a package name you want to search.
Example 8.2. Searching for packages matching a specific string
To list all packages that match “vim”, “gvim”, or “emacs”, type:
The
yum search command is useful for searching for packages you do not know the name of, but for which you know a related term. Note that by default, yum search returns matches in package name and summary, which makes the search faster. Use the yum search all command for a more exhaustive but slower search.
8.2.2. Listing Packages
Link kopierenLink in die Zwischenablage kopiert!
yum list and related commands provide information about packages, package groups, and repositories.
All of Yum's list commands allow you to filter the results by appending one or more glob expressions as arguments. Glob expressions are normal strings of characters which contain one or more of the wildcard characters
* (which expands to match any character multiple times) and ? (which expands to match any one character).
Note
Be careful to escape the glob expressions when passing them as arguments to a
yum command, otherwise the Bash shell will interpret these expressions as pathname expansions, and potentially pass all files in the current directory that match the globs to yum. To make sure the glob expressions are passed to yum as intended, either:
- escape the wildcard characters by preceding them with a backslash character
- double-quote or single-quote the entire glob expression.
See Example 8.3, “Listing all ABRT add-ons and plug-ins using glob expressions” and Example 8.5, “Listing available packages using a single glob expression with escaped wildcard characters” for an example usage of both these methods.
-
yum list glob_expression - Lists information on installed and available packages matching all glob expressions.
Example 8.3. Listing all ABRT add-ons and plug-ins using glob expressions
Packages with various ABRT add-ons and plug-ins either begin with “abrt-addon-”, or “abrt-plugin-”. To list these packages, type the following at a shell prompt:Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
yum list all - Lists all installed and available packages.
-
yum list installed - Lists all packages installed on your system. The rightmost column in the output lists the repository from which the package was retrieved.
Example 8.4. Listing installed packages using a double-quoted glob expression
To list all installed packages that begin with “krb” followed by exactly one character and a hyphen, type:Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
yum list available - Lists all available packages in all enabled repositories.
Example 8.5. Listing available packages using a single glob expression with escaped wildcard characters
To list all available packages with names that contain “gstreamer” and then “plugin”, run the following command:Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
yum grouplist - Lists all package groups.
-
yum repolist - Lists the repository ID, name, and number of packages it provides for each enabled repository.
8.2.3. Displaying Package Information
Link kopierenLink in die Zwischenablage kopiert!
To display information about one or more packages (glob expressions are valid here as well), use the following command:
yum info package_name
yum info package_name
For example, to display information about the abrt package, type:
The
yum info package_name command is similar to the rpm -q --info package_name command, but provides as additional information the ID of the Yum repository the RPM package is found in (look for the From repo: line in the output).
You can also query the Yum database for alternative and useful information about a package by using the following command:
yumdb info package_name
yumdb info package_name
This command provides additional information about a package, including the check sum of the package (and algorithm used to produce it, such as SHA-256), the command given on the command line that was invoked to install the package (if any), and the reason that the package is installed on the system (where
user indicates it was installed by the user, and dep means it was brought in as a dependency). For example, to display additional information about the yum package, type:
For more information on the
yumdb command, see the yumdb(8) manual page.
Listing Files Contained in a Package
repoquery is a program for querying information from yum repositories similarly to rpm queries. You can query both package groups and individual packages. To list all files contained in a specific package, type:
repoquery --list package_name
repoquery --list package_name
Replace package_name with a name of the package you want to inspect. For more information on the
repoquery command, see the repoquery manual page.
To find out which package provides a specific file, you can use the
yum provides command, described in Finding which package owns a file
8.2.4. Installing Packages
Link kopierenLink in die Zwischenablage kopiert!
Yum allows you to install both a single package and multiple packages, as well as a package group of your choice.
Installing Individual Packages
To install a single package and all of its non-installed dependencies, enter a command in the following form:
yum install package_name
yum install package_name
You can also install multiple packages simultaneously by appending their names as arguments:
yum install package_name package_name
yum install package_name package_name
If you are installing packages on a multilib system, such as an AMD64 or Intel 64 machine, you can specify the architecture of the package (as long as it is available in an enabled repository) by appending .arch to the package name. For example, to install the sqlite package for
i686, type:
yum install sqlite.i686
~]# yum install sqlite.i686
You can use glob expressions to quickly install multiple similarly-named packages:
yum install perl-Crypt-\*
~]# yum install perl-Crypt-\*
In addition to package names and glob expressions, you can also provide file names to
yum install. If you know the name of the binary you want to install, but not its package name, you can give yum install the path name:
yum install /usr/sbin/named
~]# yum install /usr/sbin/namedyum then searches through its package lists, finds the package which provides /usr/sbin/named, if any, and prompts you as to whether you want to install it.
Note
If you know you want to install the package that contains the
named binary, but you do not know in which bin or sbin directory is the file installed, use the yum provides command with a glob expression:
yum provides "*/file_name" is a common and useful trick to find the package(s) that contain file_name.
Installing a Package Group
A package group is similar to a package: it is not useful by itself, but installing one pulls a group of dependent packages that serve a common purpose. A package group has a name and a groupid. The
yum grouplist -v command lists the names of all package groups, and, next to each of them, their groupid in parentheses. The groupid is always the term in the last pair of parentheses, such as kde-desktop in the following example:
You can install a package group by passing its full group name (without the groupid part) to
groupinstall:
yum groupinstall group_name
yum groupinstall group_name
You can also install by groupid:
yum groupinstall groupid
yum groupinstall groupid
You can even pass the groupid (or quoted name) to the
install command if you prepend it with an @-symbol (which tells yum that you want to perform a groupinstall):
yum install @group
yum install @group
For example, the following are alternative but equivalent ways of installing the
KDE Desktop group:
yum groupinstall "KDE Desktop" yum groupinstall kde-desktop yum install @kde-desktop
~]# yum groupinstall "KDE Desktop"
~]# yum groupinstall kde-desktop
~]# yum install @kde-desktop8.2.5. Removing Packages
Link kopierenLink in die Zwischenablage kopiert!
Similarly to package installation, Yum allows you to uninstall (remove in RPM and Yum terminology) both individual packages and a package group.
Removing Individual Packages
To uninstall a particular package, as well as any packages that depend on it, run the following command as
root:
yum remove package_name
yum remove package_name
As when you install multiple packages, you can remove several at once by adding more package names to the command. For example, to remove totem, rhythmbox, and sound-juicer, type the following at a shell prompt:
yum remove totem rhythmbox sound-juicer
~]# yum remove totem rhythmbox sound-juicer
Similar to
install, remove can take these arguments:
- package names
- glob expressions
- file lists
- package provides
Warning
Yum is not able to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to misbehave and/or crash. For further information, see Section B.2.4, “Uninstalling” in the RPM chapter.
Removing a Package Group
You can remove a package group using syntax congruent with the
install syntax:
yum groupremove group
yum groupremove groupyum remove @group
yum remove @group
The following are alternative but equivalent ways of removing the
KDE Desktop group:
yum groupremove "KDE Desktop" yum groupremove kde-desktop yum remove @kde-desktop
~]# yum groupremove "KDE Desktop"
~]# yum groupremove kde-desktop
~]# yum remove @kde-desktopImportant
When you tell yum to remove a package group, it will remove every package in that group, even if those packages are members of other package groups or dependencies of other installed packages. However, you can instruct
yum to remove only those packages which are not required by any other packages or groups by adding the groupremove_leaf_only=1 directive to the [main] section of the /etc/yum.conf configuration file. For more information on this directive, see Section 8.4.1, “Setting [main] Options”.
8.3. Working with Transaction History
Link kopierenLink in die Zwischenablage kopiert!
The
yum history command allows users to review information about a timeline of Yum transactions, the dates and times they occurred, the number of packages affected, whether transactions succeeded or were aborted, and if the RPM database was changed between transactions. Additionally, this command can be used to undo or redo certain transactions.
8.3.1. Listing Transactions
Link kopierenLink in die Zwischenablage kopiert!
To display a list of twenty most recent transactions, as
root, either run yum history with no additional arguments, or type the following at a shell prompt:
yum history list
yum history list
To display all transactions, add the
all keyword:
yum history list all
yum history list all
To display only transactions in a given range, use the command in the following form:
yum history list start_id..end_id
yum history list start_id..end_id
You can also list only transactions regarding a particular package or packages. To do so, use the command with a package name or a glob expression:
yum history list glob_expression
yum history list glob_expression
For example, the list of the first five transactions looks as follows:
All forms of the
yum history list command produce tabular output with each row consisting of the following columns:
ID— an integer value that identifies a particular transaction.Login user— the name of the user whose login session was used to initiate a transaction. This information is typically presented in theFull Name <username>form. For transactions that were not issued by a user (such as an automatic system update),System <unset>is used instead.Date and time— the date and time when a transaction was issued.Action(s)— a list of actions that were performed during a transaction as described in Table 8.1, “Possible values of the Action(s) field”.Altered— the number of packages that were affected by a transaction, possibly followed by additional information as described in Table 8.2, “Possible values of the Altered field”.
| Action | Abbreviation | Description |
|---|---|---|
Downgrade | D | At least one package has been downgraded to an older version. |
Erase | E | At least one package has been removed. |
Install | I | At least one new package has been installed. |
Obsoleting | O | At least one package has been marked as obsolete. |
Reinstall | R | At least one package has been reinstalled. |
Update | U | At least one package has been updated to a newer version. |
| Symbol | Description |
|---|---|
< | Before the transaction finished, the rpmdb database was changed outside Yum. |
> | After the transaction finished, the rpmdb database was changed outside Yum. |
* | The transaction failed to finish. |
# | The transaction finished successfully, but yum returned a non-zero exit code. |
E | The transaction finished successfully, but an error or a warning was displayed. |
P | The transaction finished successfully, but problems already existed in the rpmdb database. |
s | The transaction finished successfully, but the --skip-broken command-line option was used and certain packages were skipped. |
Yum also allows you to display a summary of all past transactions. To do so, run the command in the following form as
root:
yum history summary
yum history summary
To display only transactions in a given range, type:
yum history summary start_id..end_id
yum history summary start_id..end_id
Similarly to the
yum history list command, you can also display a summary of transactions regarding a certain package or packages by supplying a package name or a glob expression:
yum history summary glob_expression
yum history summary glob_expression
For instance, a summary of the transaction history displayed above would look like the following:
All forms of the
yum history summary command produce simplified tabular output similar to the output of yum history list.
As shown above, both
yum history list and yum history summary are oriented towards transactions, and although they allow you to display only transactions related to a given package or packages, they lack important details, such as package versions. To list transactions from the perspective of a package, run the following command as root:
yum history package-list glob_expression
yum history package-list glob_expression
For example, to trace the history of subscription-manager and related packages, type the following at a shell prompt:
In this example, three packages were installed during the initial system installation: subscription-manager, subscription-manager-firstboot, and subscription-manager-gnome. In the third transaction, all these packages were updated from version 0.95.11 to version 0.95.17.
8.3.2. Examining Transactions
Link kopierenLink in die Zwischenablage kopiert!
To display the summary of a single transaction, as
root, use the yum history summary command in the following form:
yum history summary id
yum history summary id
To examine a particular transaction or transactions in more detail, run the following command as
root:
yum history info id
yum history info id
The id argument is optional and when you omit it,
yum automatically uses the last transaction. Note that when specifying more than one transaction, you can also use a range:
yum history info start_id..end_id
yum history info start_id..end_id
The following is sample output for two transactions, each installing one new package:
You can also view additional information, such as what configuration options were used at the time of the transaction, or from what repository and why were certain packages installed. To determine what additional information is available for a certain transaction, type the following at a shell prompt as
root:
yum history addon-info id
yum history addon-info id
Similarly to
yum history info, when no id is provided, yum automatically uses the latest transaction. Another way to see the latest transaction is to use the last keyword:
yum history addon-info last
yum history addon-info last
For instance, for the first transaction in the previous example, the
yum history addon-info command would provide the following output:
In this example, three types of information are available:
config-main— global Yum options that were in use during the transaction. See Section 8.4.1, “Setting [main] Options” for information on how to change global options.config-repos— options for individual Yum repositories. See Section 8.4.2, “Setting [repository] Options” for information on how to change options for individual repositories.saved_tx— the data that can be used by theyum load-transactioncommand in order to repeat the transaction on another machine (see below).
To display selected type of additional information, run the following command as
root:
yum history addon-info id information
yum history addon-info id information8.3.3. Reverting and Repeating Transactions
Link kopierenLink in die Zwischenablage kopiert!
Apart from reviewing the transaction history, the
yum history command provides means to revert or repeat a selected transaction. To revert a transaction, type the following at a shell prompt as root:
yum history undo id
yum history undo id
To repeat a particular transaction, as
root, run the following command:
yum history redo id
yum history redo id
Both commands also accept the
last keyword to undo or repeat the latest transaction.
Note that both
yum history undo and yum history redo commands only revert or repeat the steps that were performed during a transaction. If the transaction installed a new package, the yum history undo command will uninstall it, and if the transaction uninstalled a package the command will again install it. This command also attempts to downgrade all updated packages to their previous version, if these older packages are still available.
When managing several identical systems, Yum also allows you to perform a transaction on one of them, store the transaction details in a file, and after a period of testing, repeat the same transaction on the remaining systems as well. To store the transaction details to a file, type the following at a shell prompt as
root:
yum -q history addon-info id saved_tx > file_name
yum -q history addon-info id saved_tx > file_name
Once you copy this file to the target system, you can repeat the transaction by using the following command as
root:
yum load-transaction file_name
yum load-transaction file_name
Note, however that the
rpmdb version stored in the file must be identical to the version on the target system. You can verify the rpmdb version by using the yum version nogroups command.
8.3.4. Completing Transactions
Link kopierenLink in die Zwischenablage kopiert!
An unexpected situation, such as power loss or system crash, can prevent you from completing your yum transaction. When such event occurs in the middle of your transaction, you can try to resume it later with the following command as
root:
yum-complete-transaction
yum-complete-transaction
The yum-complete-transaction tool searches for incomplete or aborted yum transactions on a system and attempts to complete them. By default, these transactions are listed in the
/var/lib/yum/transaction-all and /var/lib/yum/transaction-done files. If there are more unfinished transactions, yum-complete-transaction attempts to complete the most recent one first.
To clean transaction journal files without attempting to resume the aborted transactions, use the
--cleanup-only option:
yum-complete-transaction --cleanup-only
yum-complete-transaction --cleanup-only8.3.5. Starting New Transaction History
Link kopierenLink in die Zwischenablage kopiert!
Yum stores the transaction history in a single SQLite database file. To start new transaction history, run the following command as
root:
yum history new
yum history new
This will create a new, empty database file in the
/var/lib/yum/history/ directory. The old transaction history will be kept, but will not be accessible as long as a newer database file is present in the directory.
8.4. Configuring Yum and Yum Repositories
Link kopierenLink in die Zwischenablage kopiert!
The configuration file for
yum and related utilities is located at /etc/yum.conf. This file contains one mandatory [main] section, which allows you to set Yum options that have global effect, and can also contain one or more [repository] sections, which allow you to set repository-specific options. However, it is recommended to define individual repositories in new or existing .repo files in the /etc/yum.repos.d/ directory. The values you define in individual [repository] sections of the /etc/yum.conf file override values set in the [main] section.
This section shows you how to:
- set global Yum options by editing the
[main]section of the/etc/yum.confconfiguration file; - set options for individual repositories by editing the
[repository]sections in/etc/yum.confand.repofiles in the/etc/yum.repos.d/directory; - use Yum variables in
/etc/yum.confand files in the/etc/yum.repos.d/directory so that dynamic version and architecture values are handled correctly; - add, enable, and disable Yum repositories on the command line; and,
- set up your own custom Yum repository.
8.4.1. Setting [main] Options
Link kopierenLink in die Zwischenablage kopiert!
The
/etc/yum.conf configuration file contains exactly one [main] section, and while some of the key-value pairs in this section affect how yum operates, others affect how Yum treats repositories. You can add many additional options under the [main] section heading in /etc/yum.conf.
A sample
/etc/yum.conf configuration file can look like this:
The following are the most commonly-used options in the
[main] section:
assumeyes=value- where value is one of:
0—yumshould prompt for confirmation of critical actions it performs. This is the default.1— Do not prompt for confirmation of criticalyumactions. Ifassumeyes=1is set,yumbehaves in the same way that the command-line option-ydoes. cachedir=directory- where directory is an absolute path to the directory where Yum should store its cache and database files. By default, Yum's cache directory is
/var/cache/yum/$basearch/$releasever.See Section 8.4.3, “Using Yum Variables” for descriptions of the$basearchand$releaseverYum variables. debuglevel=value- where value is an integer between
1and10. Setting a higherdebuglevelvalue causesyumto display more detailed debugging output.debuglevel=0disables debugging output, whiledebuglevel=2is the default. exactarch=value- where value is one of:
0— Do not take into account the exact architecture when updating packages.1— Consider the exact architecture when updating packages. With this setting,yumwill not install an i686 package to update an i386 package already installed on the system. This is the default. exclude=package_name [more_package_names]- This option allows you to exclude packages by keyword during installation/updates. Listing multiple packages for exclusion can be accomplished by quoting a space-delimited list of packages. Shell globs using wildcards (for example,
*and?) are allowed. gpgcheck=value- where value is one of:
0— Disable GPG signature-checking on packages in all repositories, including local package installation.1— Enable GPG signature-checking on all packages in all repositories, including local package installation.gpgcheck=1is the default, and thus all packages' signatures are checked.If this option is set in the[main]section of the/etc/yum.conffile, it sets the GPG-checking rule for all repositories. However, you can also setgpgcheck=valuefor individual repositories instead; that is, you can enable GPG-checking on one repository while disabling it on another. Settinggpgcheck=valuefor an individual repository in its corresponding.repofile overrides the default if it is present in/etc/yum.conf.For more information on GPG signature-checking, see Section B.3, “Checking a Package's Signature”. groupremove_leaf_only=value- where value is one of:
0—yumshould not check the dependencies of each package when removing a package group. With this setting,yumremoves all packages in a package group, regardless of whether those packages are required by other packages or groups.groupremove_leaf_only=0is the default.1—yumshould check the dependencies of each package when removing a package group, and remove only those packages which are not required by any other package or group.For more information on removing packages, see Intelligent package group removal. installonlypkgs=space separated list of packages- Here you can provide a space-separated list of packages which
yumcan install, but will never update. See the yum.conf(5) manual page for the list of packages which are install-only by default.If you add theinstallonlypkgsdirective to/etc/yum.conf, you should ensure that you list all of the packages that should be install-only, including any of those listed under theinstallonlypkgssection of yum.conf(5). In particular, kernel packages should always be listed ininstallonlypkgs(as they are by default), andinstallonly_limitshould always be set to a value greater than2so that a backup kernel is always available in case the default one fails to boot. installonly_limit=value- where value is an integer representing the maximum number of versions that can be installed simultaneously for any single package listed in the
installonlypkgsdirective.The defaults for theinstallonlypkgsdirective include several different kernel packages, so be aware that changing the value ofinstallonly_limitwill also affect the maximum number of installed versions of any single kernel package. The default value listed in/etc/yum.confisinstallonly_limit=3, and it is not recommended to decrease this value, particularly below2. keepcache=value- where value is one of:
0— Do not retain the cache of headers and packages after a successful installation. This is the default.1— Retain the cache after a successful installation. logfile=file_name- where file_name is an absolute path to the file in which
yumshould write its logging output. By default,yumlogs to/var/log/yum.log. multilib_policy=value- where value is one of:
best— install the best-choice architecture for this system. For example, settingmultilib_policy=beston an AMD64 system causesyumto install 64-bit versions of all packages.all— always install every possible architecture for every package. For example, withmultilib_policyset toallon an AMD64 system,yumwould install both the i686 and AMD64 versions of a package, if both were available. obsoletes=value- where value is one of:
0— Disableyum's obsoletes processing logic when performing updates.1— Enableyum's obsoletes processing logic when performing updates. When one package declares in its spec file that it obsoletes another package, the latter package will be replaced by the former package when the former package is installed. Obsoletes are declared, for example, when a package is renamed.obsoletes=1the default. plugins=value- where value is one of:
0— Disable all Yum plug-ins globally.Important
Disabling all plug-ins is not advised because certain plug-ins provide importantYumservices. In particular, rhnplugin provides support forRHN Classic, and product-id and subscription-manager plug-ins provide support for the certificate-basedContent Delivery Network(CDN). Disabling plug-ins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem withYum.1— Enable all Yum plug-ins globally. Withplugins=1, you can still disable a specific Yum plug-in by settingenabled=0in that plug-in's configuration file.For more information about various Yum plug-ins, see Section 8.5, “Yum Plug-ins”. For further information on controlling plug-ins, see Section 8.5.1, “Enabling, Configuring, and Disabling Yum Plug-ins”. reposdir=directory- where directory is an absolute path to the directory where
.repofiles are located. All.repofiles contain repository information (similar to the[repository]sections of/etc/yum.conf).yumcollects all repository information from.repofiles and the[repository]section of the/etc/yum.conffile to create a master list of repositories to use for transactions. Ifreposdiris not set,yumuses the default directory/etc/yum.repos.d/. retries=value- where value is an integer
0or greater. This value sets the number of timesyumshould attempt to retrieve a file before returning an error. Setting this to0makesyumretry forever. The default value is10.
For a complete list of available
[main] options, see the [main] OPTIONS section of the yum.conf(5) manual page.
8.4.2. Setting [repository] Options
Link kopierenLink in die Zwischenablage kopiert!
The
[repository] sections, where repository is a unique repository ID such as my_personal_repo (spaces are not permitted), allow you to define individual Yum repositories. To avoid conflicts, custom repositories should not use names used by Red Hat repositories.
The following is a bare-minimum example of the form a
[repository] section takes:
[repository] name=repository_name baseurl=repository_url
[repository]
name=repository_name
baseurl=repository_url
Every
[repository] section must contain the following directives:
name=repository_name- where repository_name is a human-readable string describing the repository.
baseurl=repository_url- where repository_url is a URL to the directory where the
repodatadirectory of a repository is located:- If the repository is available over HTTP, use:
http://path/to/repo - If the repository is available over FTP, use:
ftp://path/to/repo - If the repository is local to the machine, use:
file:///path/to/local/repo - If a specific online repository requires basic HTTP authentication, you can specify your user name and password by prepending it to the URL as
username:password@link. For example, if a repository on http://www.example.com/repo/ requires a user name of “user” and a password of “password”, then thebaseurllink could be specified ashttp://.user:password@www.example.com/repo/
Usually this URL is an HTTP link, such as:baseurl=http://path/to/repo/releases/$releasever/server/$basearch/os/
baseurl=http://path/to/repo/releases/$releasever/server/$basearch/os/Copy to Clipboard Copied! Toggle word wrap Toggle overflow Note that Yum always expands the$releasever,$arch, and$basearchvariables in URLs. For more information about Yum variables, see Section 8.4.3, “Using Yum Variables”.
Another useful
[repository] directive is the following:
enabled=value- where value is one of:
0— Do not include this repository as a package source when performing updates and installs. This is an easy way of quickly turning repositories on and off, which is useful when you desire a single package from a repository that you do not want to enable for updates or installs.1— Include this repository as a package source.Turning repositories on and off can also be performed by passing either the--enablerepo=repo_nameor--disablerepo=repo_nameoption toyum, or through the Add/Remove Software window of the PackageKit utility.
Many more
[repository] options exist. For a complete list, see the [repository] OPTIONS section of the yum.conf(5) manual page.
Example 8.6. A sample /etc/yum.repos.d/redhat.repo file
The following is a sample
/etc/yum.repos.d/redhat.repo file:
8.4.3. Using Yum Variables
Link kopierenLink in die Zwischenablage kopiert!
You can use and reference the following built-in variables in
yum commands and in all Yum configuration files (that is, /etc/yum.conf and all .repo files in the /etc/yum.repos.d/ directory):
$releasever- You can use this variable to reference the release version of Red Hat Enterprise Linux. Yum obtains the value of
$releaseverfrom thedistroverpkg=valueline in the/etc/yum.confconfiguration file. If there is no such line in/etc/yum.conf, thenyuminfers the correct value by deriving the version number from the redhat-release-server package. The value of$releasevertypically consists of the major release number and the variant of Red Hat Enterprise Linux, for example6Client, or6Server. $arch- You can use this variable to refer to the system's CPU architecture as returned when calling Python's
os.uname()function. Valid values for$archincludei686andx86_64. $basearch- You can use
$basearchto reference the base architecture of the system. For example, i686 machines have a base architecture ofi386, and AMD64 and Intel 64 machines have a base architecture ofx86_64. $YUM0-9- These ten variables are each replaced with the value of any shell environment variables with the same name. If one of these variables is referenced (in
/etc/yum.conffor example) and a shell environment variable with the same name does not exist, then the configuration file variable is not replaced.
To define a custom variable or to override the value of an existing one, create a file with the same name as the variable (without the “
$” sign) in the /etc/yum/vars/ directory, and add the desired value on its first line.
For example, repository descriptions often include the operating system name. To define a new variable called
$osname, create a new file with “Red Hat Enterprise Linux” on the first line and save it as /etc/yum/vars/osname:
echo "Red Hat Enterprise Linux" > /etc/yum/vars/osname
~]# echo "Red Hat Enterprise Linux" > /etc/yum/vars/osname
Instead of “Red Hat Enterprise Linux 6”, you can now use the following in the
.repo files:
name=$osname $releasever
name=$osname $releasever8.4.4. Viewing the Current Configuration
Link kopierenLink in die Zwischenablage kopiert!
To display the current values of global Yum options (that is, the options specified in the
[main] section of the /etc/yum.conf file), run the yum-config-manager with no command-line options:
yum-config-manager
yum-config-manager
To list the content of a different configuration section or sections, use the command in the following form:
yum-config-manager section
yum-config-manager section
You can also use a glob expression to display the configuration of all matching sections:
yum-config-manager glob_expression
yum-config-manager glob_expression
For example, to list all configuration options and their corresponding values, type the following at a shell prompt:
8.4.5. Adding, Enabling, and Disabling a Yum Repository
Link kopierenLink in die Zwischenablage kopiert!
Section 8.4.2, “Setting [repository] Options” described various options you can use to define a Yum repository. This section explains how to add, enable, and disable a repository by using the
yum-config-manager command.
Important
When the system is registered with the certificate-based
Red Hat Network, the Red Hat Subscription Manager tools are used to manage repositories in the /etc/yum.repos.d/redhat.repo file. See Chapter 6, Registering the System and Managing Subscriptions for more information how to register a system with Red Hat Network and use the Red Hat Subscription Manager tools to manage subscriptions.
Adding a Yum Repository
To define a new repository, you can either add a
[repository] section to the /etc/yum.conf file, or to a .repo file in the /etc/yum.repos.d/ directory. All files with the .repo file extension in this directory are read by yum, and it is recommended to define your repositories here instead of in /etc/yum.conf.
Warning
Obtaining and installing software packages from unverified or untrusted software sources other than Red Hat Network constitutes a potential security risk, and could lead to security, stability, compatibility, and maintainability issues.
Yum repositories commonly provide their own
.repo file. To add such a repository to your system and enable it, run the following command as root:
yum-config-manager --add-repo repository_url
yum-config-manager --add-repo repository_url
where repository_url is a link to the
.repo file. For example, to add a repository located at http://www.example.com/example.repo, type the following at a shell prompt:
Enabling a Yum Repository
To enable a particular repository or repositories, type the following at a shell prompt as
root:
yum-config-manager --enable repository
yum-config-manager --enable repository
where repository is the unique repository ID (use
yum repolist all to list available repository IDs). Alternatively, you can use a glob expression to enable all matching repositories:
yum-config-manager --enable glob_expression
yum-config-manager --enable glob_expression
For example, to enable repositories defined in the
[example], [example-debuginfo], and [example-source]sections, type:
When successful, the
yum-config-manager --enable command displays the current repository configuration.
Disabling a Yum Repository
To disable a Yum repository, run the following command as
root:
yum-config-manager --disable repository
yum-config-manager --disable repository
where repository is the unique repository ID (use
yum repolist all to list available repository IDs). Similarly to yum-config-manager --enable, you can use a glob expression to disable all matching repositories at the same time:
yum-config-manager --disable glob_expression
yum-config-manager --disable glob_expression
When successful, the
yum-config-manager --disable command displays the current configuration.
8.4.6. Creating a Yum Repository
Link kopierenLink in die Zwischenablage kopiert!
To set up a Yum repository, follow these steps:
- Install the createrepo package. To do so, type the following at a shell prompt as
root:yum install createrepo
yum install createrepoCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy all packages that you want to have in your repository into one directory, such as
/mnt/local_repo/. - Change to this directory and run the following command:
createrepo --database /mnt/local_repo
createrepo --database /mnt/local_repoCopy to Clipboard Copied! Toggle word wrap Toggle overflow This creates the necessary metadata for your Yum repository, as well as the sqlite database for speeding upyumoperations.Important
Compared to Red Hat Enterprise Linux 5, RPM packages for Red Hat Enterprise Linux 6 are compressed with the XZ lossless data compression format and can be signed with newer hash algorithms like SHA-256. Consequently, it is not recommended to use thecreaterepocommand on Red Hat Enterprise Linux 5 to create the package metadata for Red Hat Enterprise Linux 6.
8.4.7. Working with Yum Cache
Link kopierenLink in die Zwischenablage kopiert!
By default, yum deletes downloaded data files when they are no longer needed after a successful operation. This minimizes the amount of storage space that yum uses. However, you can enable caching, so that the package files downloaded by yum stay in cache directories. By using cached data, you can carry out certain operations without a network connection, you can also copy packages stored in the caches and reuse them elsewhere.
Yum stores temporary files in the
/var/cache/yum/$basearch/$releasever/ directory, where $basearch and $releasever are Yum variables referring to base architecture of the system and the release version of Red Hat Enterprise Linux. Each configured repository has one subdirectory. For example, the directory /var/cache/yum/$basearch/$releasever/development/packages/ holds packages downloaded from the development repository. You can find the values for the $basearch and $releasever variables in the output of the yum version command.
To change the default cache location, modify the
cachedir option in the [main] section of the /etc/yum.conf configuration file. See Section 8.4, “Configuring Yum and Yum Repositories” for more information on configuring yum.
Enabling the Caches
To retain the cache of packages after a successful installation, add the following text to the
[main] section of /etc/yum.conf.
keepcache = 1
keepcache = 1
Once you enabled caching, every yum operation may download package data from the configured repositories.
To download and make usable all the metadata for the currently enabled yum repositories, type:
yum makecache
yum makecache
This is useful if you want to make sure that the cache is fully up to date with all metadata. To set the time after which the metadata will expire, use the
metadata-expire setting in /etc/yum.conf.
Using yum in Cache-only Mode
To carry out a yum command without a network connection, add the
-C or --cacheonly command-line option. With this option, yum proceeds without checking any network repositories, and uses only cached files. In this mode, yum may only install packages that have been downloaded and cached by a previous operation.
For instance, to list packages that use the currently cached data with names that contain “gstreamer”, enter the following command:
yum -C list gstreamer*
yum -C list gstreamer*Clearing the yum Caches
It is often useful to remove entries accumulated in the
/var/cache/yum/ directory. If you remove a package from the cache, you do not affect the copy of the software installed on your system. To remove all entries for currently enabled repositories from the cache, type the following as a root:
yum clean all
yum clean all
There are various ways to invoke
yum in clean mode depending on the type of cached data you want to remove. See Table 8.3, “Available yum clean options” for a complete list of available configuration options.
| Option | Description |
|---|---|
| expire-cache | eliminates time records of the metadata and mirrorlists download for each repository. This forces yum to revalidate the cache for each repository the next time it is used. |
| packages | eliminates any cached packages from the system |
| headers | eliminates all header files that previous versions of yum used for dependency resolution |
| metadata | eliminates all files that yum uses to determine the remote availability of packages. These metadata are downloaded again the next time yum is run. |
| dbcache | eliminates the sqlite cache used for faster access to metadata. Using this option will force yum to download the sqlite metadata the next time it is run. This does not apply for repositories that contain only .xml data, in that case, sqlite data are deleted but without subsequent download |
| rpmdb | eliminates any cached data from the local rpmdb |
| plugins | enabled plugins are forced to eliminate their cached data |
| all | removes all of the above |
The
expire-cache option is most preferred from the above list. In many cases, it is a sufficient and much faster replacement for clean all.
8.4.8. Adding the Optional and Supplementary Repositories
Link kopierenLink in die Zwischenablage kopiert!
Optional and Supplementary subscription channels provide additional software packages for Red Hat Enterprise Linux that cover open source licensed software (in the Optional channel) and proprietary licensed software (in the Supplementary channel).
Before subscribing to the Optional and Supplementary channels see the Scope of Coverage Details. If you decide to install packages from these channels, follow the steps documented in the article called How to access Optional and Supplementary channels, and -devel packages using Red Hat Subscription Manager (RHSM)? on the Red Hat Customer Portal.
8.5. Yum Plug-ins
Link kopierenLink in die Zwischenablage kopiert!
Yum provides plug-ins that extend and enhance its operations. Certain plug-ins are installed by default. Yum always informs you which plug-ins, if any, are loaded and active whenever you call any
yum command. For example:
yum info yum
~]# yum info yum
Loaded plugins: product-id, refresh-packagekit, subscription-manager
[output truncated]
Note that the plug-in names which follow
Loaded plugins are the names you can provide to the --disableplugins=plugin_name option.
8.5.1. Enabling, Configuring, and Disabling Yum Plug-ins
Link kopierenLink in die Zwischenablage kopiert!
To enable Yum plug-ins, ensure that a line beginning with
plugins= is present in the [main] section of /etc/yum.conf, and that its value is 1:
plugins=1
plugins=1
You can disable all plug-ins by changing this line to
plugins=0.
Important
Disabling all plug-ins is not advised because certain plug-ins provide important
Yum services. In particular, rhnplugin provides support for RHN Classic, and product-id and subscription-manager plug-ins provide support for the certificate-based Content Delivery Network (CDN). Disabling plug-ins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with Yum.
Every installed plug-in has its own configuration file in the
/etc/yum/pluginconf.d/ directory. You can set plug-in specific options in these files. For example, here is the refresh-packagekit plug-in's refresh-packagekit.conf configuration file:
[main] enabled=1
[main]
enabled=1
Plug-in configuration files always contain a
[main] section (similar to Yum's /etc/yum.conf file) in which there is (or you can place if it is missing) an enabled= option that controls whether the plug-in is enabled when you run yum commands.
If you disable all plug-ins by setting
enabled=0 in /etc/yum.conf, then all plug-ins are disabled regardless of whether they are enabled in their individual configuration files.
If you merely want to disable all Yum plug-ins for a single
yum command, use the --noplugins option.
If you want to disable one or more Yum plug-ins for a single
yum command, add the --disableplugin=plugin_name option to the command. For example, to disable the presto plug-in while updating a system, type:
yum update --disableplugin=presto
~]# yum update --disableplugin=presto
The plug-in names you provide to the
--disableplugin= option are the same names listed after the Loaded plugins line in the output of any yum command. You can disable multiple plug-ins by separating their names with commas. In addition, you can match multiple plug-in names or shorten long ones by using glob expressions:
yum update --disableplugin=presto,refresh-pack*
~]# yum update --disableplugin=presto,refresh-pack*8.5.2. Installing Additional Yum Plug-ins
Link kopierenLink in die Zwischenablage kopiert!
Yum plug-ins usually adhere to the
yum-plugin-plugin_name package-naming convention, but not always: the package which provides the presto plug-in is named yum-presto, for example. You can install a Yum plug-in in the same way you install other packages. For instance, to install the security plug-in, type the following at a shell prompt:
yum install yum-plugin-security
~]# yum install yum-plugin-security8.5.3. Plug-in Descriptions
Link kopierenLink in die Zwischenablage kopiert!
The following list provides descriptions and usage instructions for several useful yum plug-ins. Plug-ins are listed by names, brackets contain the name of the package.
- search-disabled-repos (subscription-manager)
- The search-disabled-repos plug-in allows you to temporarily or permanently enable disabled repositories to help resolve dependencies. With this plug-in enabled, when Yum fails to install a package due to failed dependency resolution, it offers to temporarily enable disabled repositories and try again. If the installation succeeds, Yum also offers to enable the used repositories permanently. Note that the plug-in works only with the repositories that are managed by subscription-manager and not with custom repositories.
Important
Ifyumis executed with the--assumeyesor-yoption, or if theassumeyesdirective is enabled in/etc/yum.conf, the plug-in enables disabled repositories, both temporarily and permanently, without prompting for confirmation. This may lead to problems, for example, enabling repositories that you do not want enabled.To configure the search-disabled-repos plug-in, edit the configuration file located in/etc/yum/pluginconf.d/search-disabled-repos.conf. For the list of directives you can use in the[main]section, see the table below.Expand Table 8.4. Supported search-disabled-repos.conf directives Directive Description enabled=valueAllows you to enable or disable the plug-in. The value must be either 1(enabled), or0(disabled). The plug-in is enabled by default.notify_only=valueAllows you to restrict the behavior of the plug-in to notifications only. The value must be either 1(notify only without modifying the behavior of Yum), or0(modify the behavior of Yum). By default the plug-in only notifies the user.ignored_repos=repositoriesAllows you to specify the repositories that will not be enabled by the plug-in. - kabi (kabi-yum-plugins)
- The kabi plug-in checks whether a driver update package conforms with official Red Hat kernel Application Binary Interface (kABI). With this plug-in enabled, when a user attempts to install a package that uses kernel symbols which are not on a whitelist, a warning message is written to the system log. Additionally, configuring the plug-in to run in enforcing mode prevents such packages from being installed at all.To configure the kabi plug-in, edit the configuration file located in
/etc/yum/pluginconf.d/kabi.conf. See Table 8.5, “Supportedkabi.confdirectives” for a list of directives that can be used in the[main]section.Expand Table 8.5. Supported kabi.conf directives Directive Description enabled=valueAllows you to enable or disable the plug-in. The value must be either 1(enabled), or0(disabled). When installed, the plug-in is enabled by default.whitelists=directoryAllows you to specify the directory in which the files with supported kernel symbols are located. By default, the kabi plug-in uses files provided by the kernel-abi-whitelists package (that is, the /lib/modules/kabi/directory).enforce=valueAllows you to enable or disable enforcing mode. The value must be either 1(enabled), or0(disabled). By default, this option is commented out and the kabi plug-in only displays a warning message. - presto (yum-presto)
- The presto plug-in adds support to Yum for downloading delta RPM packages, during updates, from repositories which have presto metadata enabled. Delta RPMs contain only the differences between the version of the package installed on the client requesting the RPM package and the updated version in the repository.Downloading a delta RPM is much quicker than downloading the entire updated package, and can speed up updates considerably. Once the delta RPMs are downloaded, they must be rebuilt to apply the difference to the currently-installed package and thus create the full, updated package. This process takes CPU time on the installing machine. Using delta RPMs is therefore a compromise between time-to-download, which depends on the network connection, and time-to-rebuild, which is CPU-bound. Using the presto plug-in is recommended for fast machines and systems with slower network connections, while slower machines on very fast connections benefit more from downloading normal RPM packages, that is, by disabling presto.
- product-id (subscription-manager)
- The product-id plug-in manages product identity certificates for products installed from the Content Delivery Network. The product-id plug-in is installed by default.
- refresh-packagekit (PackageKit-yum-plugin)
- The refresh-packagekit plug-in updates metadata for PackageKit whenever
yumis run. The refresh-packagekit plug-in is installed by default. - rhnplugin (yum-rhn-plugin)
- The rhnplugin provides support for connecting to
RHN Classic. This allows systems registered withRHN Classicto update and install packages from this system. Note thatRHN Classicis only provided for older Red Hat Enterprise Linux systems (that is, Red Hat Enterprise Linux 4.x, Red Hat Enterprise Linux 5.x, and Satellite 5.x) in order to migrate them over to Red Hat Enterprise Linux 6. The rhnplugin is installed by default.See the rhnplugin(8) manual page for more information about the plug-in. - security (yum-plugin-security)
- Discovering information about and applying security updates easily and often is important to all system administrators. For this reason Yum provides the security plug-in, which extends
yumwith a set of highly-useful security-related commands, subcommands and options.You can check for security-related updates as follows:Copy to Clipboard Copied! Toggle word wrap Toggle overflow You can then use eitheryum update --securityoryum update-minimal --securityto update those packages which are affected by security advisories. Both of these commands update all packages on the system for which a security advisory has been issued.yum update-minimal --securityupdates them to the latest packages which were released as part of a security advisory, whileyum update --securitywill update all packages affected by a security advisory to the latest version of that package available.In other words, if:- the kernel-2.6.30.8-16 package is installed on your system;
- the kernel-2.6.30.8-32 package was released as a security update;
- then kernel-2.6.30.8-64 was released as a bug fix update,
...thenyum update-minimal --securitywill update you to kernel-2.6.30.8-32, andyum update --securitywill update you to kernel-2.6.30.8-64. Conservative system administrators probably want to useupdate-minimalto reduce the risk incurred by updating packages as much as possible.See the yum-security(8) manual page for usage details and further explanation of the enhancements the security plug-in adds toyum. - subscription-manager (subscription-manager)
- The subscription-manager plug-in provides support for connecting to
Red Hat Network. This allows systems registered withRed Hat Networkto update and install packages from the certificate-based Content Delivery Network. The subscription-manager plug-in is installed by default.See Chapter 6, Registering the System and Managing Subscriptions for more information how to manage product subscriptions and entitlements. - yum-downloadonly (yum-plugin-downloadonly)
- The yum-downloadonly plug-in provides the
--downloadonlycommand-line option which can be used to download packages from Red Hat Network or a configured Yum repository without installing the packages.To install the package, follow the instructions in Section 8.5.2, “Installing Additional Yum Plug-ins”. After the installation, see the contents of the/etc/yum/pluginconf.d/downloadonly.conffile to ensure that the plug-in is enabled:~]$ cat /etc/yum/pluginconf.d/downloadonly.conf [main] enabled=1
~]$ cat /etc/yum/pluginconf.d/downloadonly.conf [main] enabled=1Copy to Clipboard Copied! Toggle word wrap Toggle overflow In the following example, theyum install --downloadonlycommand is run to download the latest version of the httpd package, without installing it:Copy to Clipboard Copied! Toggle word wrap Toggle overflow By default, packages downloaded using the--downloadonlyoption are saved in one of the subdirectories of the/var/cache/yumdirectory, depending on the Red Hat Enterprise Linux variant and architecture.If you want to specify an alternate directory to save the packages, pass the--downloaddiroption along with--downloadonly:yum install --downloadonly --downloaddir=/path/to/directory httpd
~]# yum install --downloadonly --downloaddir=/path/to/directory httpdCopy to Clipboard Copied! Toggle word wrap Toggle overflow Note
As an alternative to the yum-downloadonly plugin — to download packages without installing them — you can use the yumdownloader utility that is provided by the yum-utils package.
8.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For more information on how to manage software packages on Red Hat Enterprise Linux, see the resources listed below.
Installed Documentation
yum(8) — The manual page for theyumcommand-line utility provides a complete list of supported options and commands.yumdb(8) — The manual page for theyumdbcommand-line utility documents how to use this tool to query and, if necessary, alter the yum database.yum.conf(5) — The manual page namedyum.confdocuments available yum configuration options.yum-utils(1) — The manual page namedyum-utilslists and briefly describes additional utilities for managing yum configuration, manipulating repositories, and working with yum database.
Online Resources
- Yum Guides — The Yum Guides page on the project home page provides links to further documentation.
- Red Hat Access Labs — The Red Hat Access Labs includes a “Yum Repository Configuration Helper”.
See Also
- Chapter 4, Gaining Privileges documents how to gain administrative privileges by using the
suandsudocommands. - Appendix B, RPM describes the RPM Package Manager (RPM), the packaging system used by Red Hat Enterprise Linux.
Chapter 9. PackageKit
Link kopierenLink in die Zwischenablage kopiert!
Red Hat provides PackageKit for viewing, managing, updating, installing and uninstalling packages compatible with your system. PackageKit consists of several graphical interfaces that can be opened from the GNOME panel menu, or from the Notification Area when PackageKit alerts you that updates are available. For more information on PackageKit's architecture and available front ends, see Section 9.3, “PackageKit Architecture”.
9.1. Updating Packages with Software Update
Link kopierenLink in die Zwischenablage kopiert!
PackageKit displays a starburst icon in the Notification Area whenever updates are available to be installed on your system.
Figure 9.1. PackageKit's icon in the Notification Area
Clicking on the notification icon opens the Software Update window. Alternatively, you can open Software Updates by clicking → → from the GNOME panel, or running the
gpk-update-viewer command at the shell prompt. In the Software Updates window, all available updates are listed along with the names of the packages being updated (minus the .rpm suffix, but including the CPU architecture), a short summary of the package, and, usually, short descriptions of the changes the update provides. Any updates you do not want to install can be de-selected here by unchecking the check box corresponding to the update.
Figure 9.2. Installing updates with Software Update
The updates presented in the Software Updates window only represent the currently-installed packages on your system for which updates are available; dependencies of those packages, whether they are existing packages on your system or new ones, are not shown until you click .
PackageKit utilizes the fine-grained user authentication capabilities provided by the PolicyKit toolkit whenever you request it to make changes to the system. Whenever you instruct PackageKit to update, install or remove packages, you will be prompted to enter the superuser password before changes are made to the system.
If you instruct PackageKit to update the kernel package, then it will prompt you after installation, asking you whether you want to reboot the system and thereby boot into the newly-installed kernel.
Setting the Update-Checking Interval
Right-clicking on PackageKit's Notification Area icon and clicking opens the Software Update Preferences window, where you can define the interval at which PackageKit checks for package updates, as well as whether or not to automatically install all updates or only security updates. Leaving the Check for updates when using mobile broadband box unchecked is handy for avoiding extraneous bandwidth usage when using a wireless connection on which you are charged for the amount of data you download.
Figure 9.3. Setting PackageKit's update-checking interval
9.2. Using Add/Remove Software
Link kopierenLink in die Zwischenablage kopiert!
To find and install a new package, on the GNOME panel click on → → , or run the
gpk-application command at the shell prompt.
Figure 9.4. PackageKit's Add/Remove Software window
9.2.1. Refreshing Software Sources (Yum Repositories)
Link kopierenLink in die Zwischenablage kopiert!
PackageKit refers to Yum repositories as software sources. It obtains all packages from enabled software sources. You can view the list of all configured and unfiltered (see below) Yum repositories by opening Add/Remove Software and clicking → . The Software Sources dialog shows the repository name, as written on the
name=<My Repository Name> field of all [repository] sections in the /etc/yum.conf configuration file, and in all repository.repo files in the /etc/yum.repos.d/ directory.
Entries which are checked in the Enabled column indicate that the corresponding repository will be used to locate packages to satisfy all update and installation requests (including dependency resolution). You can enable or disable any of the listed Yum repositories by selecting or clearing the check box. Note that doing so causes PolicyKit to prompt you for superuser authentication.
The Enabled column corresponds to the
enabled=<1 or 0> field in [repository] sections. When you click the check box, PackageKit inserts the enabled=<1 or 0> line into the correct [repository] section if it does not exist, or changes the value if it does. This means that enabling or disabling a repository through the Software Sources window causes that change to persist after closing the window or rebooting the system.
Note that it is not possible to add or remove Yum repositories through PackageKit.
Note
Checking the box at the bottom of the Software Sources window causes PackageKit to display source RPM, testing and debuginfo repositories as well. This box is unchecked by default.
After making a change to the available Yum repositories, click on → to make sure your package list is up-to-date.
9.2.2. Finding Packages with Filters
Link kopierenLink in die Zwischenablage kopiert!
Once the software sources have been updated, it is often beneficial to apply some filters so that PackageKit retrieves the results of our Find queries faster. This is especially helpful when performing many package searches. Four of the filters in the drop-down menu are used to split results by matching or not matching a single criterion. By default when PackageKit starts, these filters are all unapplied (), but once you do filter by one of them, that filter remains set until you either change it or close PackageKit.
Because you are usually searching for available packages that are not installed on the system, click → and select the radio button.
Figure 9.5. Filtering out already-installed packages
Also, unless you require development files such as C header files, click → and select the radio button. This filters out all of the
<package_name>-devel packages we are not interested in.
Figure 9.6. Filtering out development packages from the list of Find results
The two remaining filters with submenus are:
- Narrows the search to either applications which provide a GUI interface () or those that do not. This filter is useful when browsing for GUI applications that perform a specific function.
- Search for packages which are considered to be free software. See the Fedora Licensing List for details on approved licenses.
The remaining filters can be enabled by selecting the check boxes next to them:
- Checking the check box filters out generally-uninteresting packages that are typically only dependencies of other packages that we want. For example, checking and searching for
<package>would cause the following related packages to be filtered out of the results (if it exists):<package>-devel<package>-libs<package>-libs-devel<package>-debuginfo
- Checking filters out all older versions of the same package from the list of results, which is generally what we want. Note that this filter is often combined with the filter to search for the latest available versions of new (not installed) packages.
- Only native packages
- Checking the box on a multilib system causes PackageKit to omit listing results for packages compiled for the architecture that runs in compatibility mode. For example, enabling this filter on a 64-bit system with an AMD64 CPU would cause all packages built for the 32-bit x86 CPU architecture not to be shown in the list of results, even though those packages are able to run on an AMD64 machine. Packages which are architecture-agnostic (i.e. noarch packages such as
crontabs-1.10-32.1.el6.noarch.rpm) are never filtered out by checking . This filter has no affect on non-multilib systems, such as x86 machines.
9.2.3. Installing and Removing Packages (and Dependencies)
Link kopierenLink in die Zwischenablage kopiert!
With the two filters selected, and , search for the screen window manager for the command line and highlight the package. You now have access to some very useful information about it, including: a clickable link to the project homepage; the Yum package group it is found in, if any; the license of the package; a pointer to the GNOME menu location from where the application can be opened, if applicable; and the size of the package, which is relevant when we download and install it.
Figure 9.7. Viewing and installing a package with PackageKit's Add/Remove Software window
When the check box next to a package or group is checked, then that item is already installed on the system. Checking an unchecked box causes it to be marked for installation, which only occurs when the button is clicked. In this way, you can search for and select multiple packages or package groups before performing the actual installation transactions. Additionally, you can remove installed packages by unchecking the checked box, and the removal will occur along with any pending installations when is pressed. Dependency resolution, which may add additional packages to be installed or removed, is performed after pressing . PackageKit will then display a window listing those additional packages to install or remove, and ask for confirmation to proceed.
Select screen and click the button. You will then be prompted for the superuser password; enter it, and PackageKit will install screen. After finishing the installation, PackageKit sometimes presents you with a list of your newly-installed applications and offers you the choice of running them immediately. Alternatively, you will remember that finding a package and selecting it in the Add/Remove Software window shows you the Location of where in the GNOME menus its application shortcut is located, which is helpful when you want to run it.
Once it is installed, you can run
screen, a screen manager that allows you to have multiple logins on one terminal, by typing screen at a shell prompt.
screen is a very useful utility, but we decide that we do not need it and we want to uninstall it. Remembering that we need to change the filter we recently used to install it to in → , we search for screen again and uncheck it. The program did not install any dependencies of its own; if it had, those would be automatically removed as well, as long as they were not also dependencies of any other packages still installed on our system.
Warning
Although PackageKit automatically resolves dependencies during package installation and removal, it is unable to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to behave erratically and/or crash.
Figure 9.8. Removing a package with PackageKit's Add/Remove Software window
9.2.4. Installing and Removing Package Groups
Link kopierenLink in die Zwischenablage kopiert!
PackageKit also has the ability to install Yum package groups, which it calls Package collections. Clicking on Package collections in the top-left list of categories in the Software Updates window allows us to scroll through and find the package group we want to install. In this case, we want to install Czech language support (the Czech Support group). Checking the box and clicking informs us how many additional packages must be installed in order to fulfill the dependencies of the package group.
Figure 9.9. Installing the Czech Support package group
Similarly, installed package groups can be uninstalled by selecting Package collections, unchecking the appropriate check box, and applying.
9.2.5. Viewing the Transaction Log
Link kopierenLink in die Zwischenablage kopiert!
PackageKit maintains a log of the transactions that it performs. To view the log, from the Add/Remove Software window, click → , or run the
gpk-log command at the shell prompt.
The Software Log Viewer shows the following information:
- Date — the date on which the transaction was performed.
- Action — the action that was performed during the transaction, for example Updated packages or Installed packages.
- Details — the transaction type such as Updated, Installed, or Removed, followed by a list of affected packages.
- Username — the name of the user who performed the action.
- Application — the front end application that was used to perform the action, for example Update System.
Typing the name of a package in the top text entry field filters the list of transactions to those which affected that package.
Figure 9.10. Viewing the log of package management transactions with the Software Log Viewer
9.3. PackageKit Architecture
Link kopierenLink in die Zwischenablage kopiert!
Red Hat provides the PackageKit suite of applications for viewing, updating, installing and uninstalling packages and package groups compatible with your system. Architecturally, PackageKit consists of several graphical front ends that communicate with the
packagekitd daemon back end, which communicates with a package manager-specific back end that utilizes Yum to perform the actual transactions, such as installing and removing packages, etc.
Table 9.1, “PackageKit GUI windows, menu locations, and shell prompt commands” shows the name of the GUI window, how to start the window from the GNOME desktop or from the Add/Remove Software window, and the name of the command-line application that opens that window.
| Window Title | Function | How to Open | Shell Command |
|---|---|---|---|
| Add/Remove Software | Install, remove or view package info |
From the GNOME panel: → →
| gpk-application |
| Software Update | Perform package updates |
From the GNOME panel: → →
| gpk-update-viewer |
| Software Sources | Enable and disable Yum repositories |
From Add/Remove Software: →
| gpk-repo |
| Software Log Viewer | View the transaction log |
From Add/Remove Software: →
| gpk-log |
| Software Update Preferences | Set PackageKit preferences | gpk-prefs | |
| (Notification Area Alert) | Alerts you when updates are available |
From the GNOME panel: → → , the Startup Programs tab
| gpk-update-icon |
The
packagekitd daemon runs outside the user session and communicates with the various graphical front ends. The packagekitd daemon[2] communicates via the DBus system message bus with another back end, which utilizes Yum's Python API to perform queries and make changes to the system. On Linux systems other than Red Hat Enterprise Linux and Fedora, packagekitd can communicate with other back ends that are able to utilize the native package manager for that system. This modular architecture provides the abstraction necessary for the graphical interfaces to work with many different package managers to perform essentially the same types of package management tasks. Learning how to use the PackageKit front ends means that you can use the same familiar graphical interface across many different Linux distributions, even when they utilize a native package manager other than Yum.
In addition, PackageKit's separation of concerns provides reliability in that a crash of one of the GUI windows—or even the user's X Window session—will not affect any package management tasks being supervised by the
packagekitd daemon, which runs outside of the user session.
All of the front end graphical applications discussed in this chapter are provided by the gnome-packagekit package instead of by PackageKit and its dependencies.
Finally, PackageKit also comes with a console-based front end called
pkcon.
9.4. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For more information about PackageKit, see the resources listed below.
Installed Documentation
gpk-application(1)— The manual page containing information about thegpk-applicationcommand.gpk-backend-status(1)— The manual page containing information about thegpk-backend-statuscommand.gpk-install-local-file(1)— The manual page containing information about thegpk-install-local-filecommand.gpk-install-mime-type(1)— The manual page containing information about thegpk-install-mime-typecommand.gpk-install-package-name(1)— The manual page containing information about theqpk-install-package-namecommand.gpk-install-package-name(1)— The manual page containing information about thegpk-install-package-namecommand.gpk-prefs(1)— The manual page containing information about thegpk-prefscommand.gpk-repo(1)— The manual page containing information about thegpk-repocommand.gpk-update-icon(1)— The manual page containing information about thegpk-update-iconcommand.gpk-update-viewer(1)— The manual page containing information about thegpk-update-viewercommand.pkcon(1)andpkmon(1)— The manual pages containing information about the PackageKit console client.pkgenpack(1)— The manual page containing information about the PackageKit Pack Generator.
Online Documentation
- PackageKit home page — The PackageKit home page listing detailed information about the PackageKit software suite.
- PackageKit FAQ — An informative list of Frequently Asked Questions for the PackageKit software suite.
See Also
- Chapter 8, Yum documents Yum, the Red Hat package manager.
[2]
System daemons are typically long-running processes that provide services to the user or to other programs, and which are started, often at boot time, by special initialization scripts (often shortened to init scripts). Daemons respond to the
service command and can be turned on or off permanently by using the chkconfig on or chkconfig off commands. They can typically be recognized by a “d” appended to their name, such as the packagekitd daemon. See Chapter 12, Services and Daemons for information about system services.
Part IV. Networking
Link kopierenLink in die Zwischenablage kopiert!
This part describes how to configure the network on Red Hat Enterprise Linux.
Chapter 10. NetworkManager
Link kopierenLink in die Zwischenablage kopiert!
NetworkManager is a dynamic network control and configuration system that attempts to keep network devices and connections up and active when they are available. NetworkManager consists of a core daemon, a GNOME Notification Area applet that provides network status information, and graphical configuration tools that can create, edit and remove connections and interfaces. NetworkManager can be used to configure the following types of connections: Ethernet, wireless, mobile broadband (such as cellular 3G), and
DSL and PPPoE (Point-to-Point over Ethernet). In addition, NetworkManager allows for the configuration of network aliases, static routes, DNS information and VPN connections, as well as many connection-specific parameters. Finally, NetworkManager provides a rich API via D-Bus which allows applications to query and control network configuration and state.
Previous versions of Red Hat Enterprise Linux included the Network Administration Tool, which was commonly known as
system-config-network after its command-line invocation. In Red Hat Enterprise Linux 6, NetworkManager replaces the former Network Administration Tool while providing enhanced functionality, such as user-specific and mobile broadband configuration. It is also possible to configure the network in Red Hat Enterprise Linux 6 by editing interface configuration files; see Chapter 11, Network Interfaces for more information.
NetworkManager may be installed by default on your version of Red Hat Enterprise Linux. To ensure that it is, run the following command as
root:
yum install NetworkManager
~]# yum install NetworkManager10.1. The NetworkManager Daemon
Link kopierenLink in die Zwischenablage kopiert!
The NetworkManager daemon runs with
root privileges and is usually configured to start up at boot time. You can determine whether the NetworkManager daemon is running by entering this command as root:
service NetworkManager status
~]# service NetworkManager status
NetworkManager (pid 1527) is running...
The
service command will report NetworkManager is stopped if the NetworkManager service is not running. To start it for the current session:
service NetworkManager start
~]# service NetworkManager start
Run the
chkconfig command to ensure that NetworkManager starts up every time the system boots:
chkconfig NetworkManager on
~]# chkconfig NetworkManager on
For more information on starting, stopping and managing services and runlevels, see Chapter 12, Services and Daemons.
10.2. Interacting with NetworkManager
Link kopierenLink in die Zwischenablage kopiert!
Users do not interact with the NetworkManager system service directly. Instead, you can perform network configuration tasks via NetworkManager's Notification Area applet. The applet has multiple states that serve as visual indicators for the type of connection you are currently using. Hover the pointer over the applet icon for tooltip information on the current connection state.
Figure 10.1. NetworkManager applet states
If you do not see the NetworkManager applet in the GNOME panel, and assuming that the NetworkManager package is installed on your system, you can start the applet by running the following command as a normal user (not
root):
nm-applet &
~]$ nm-applet &
After running this command, the applet appears in your Notification Area. You can ensure that the applet runs each time you log in by clicking → → to open the Startup Applications Preferences window. Then, select the Startup Programs tab and check the box next to NetworkManager.
10.2.1. Connecting to a Network
Link kopierenLink in die Zwischenablage kopiert!
When you left-click on the applet icon, you are presented with:
- a list of categorized networks you are currently connected to (such as Wired and Wireless);
- a list of all Available Networks that NetworkManager has detected;
- options for connecting to any configured Virtual Private Networks (VPNs); and,
- options for connecting to hidden or new wireless networks.
If you are connected to a network, its name is presented in bold typeface under its network type, such as Wired or Wireless. When many networks are available, such as wireless access points, the More networks expandable menu entry appears.
Figure 10.2. The NetworkManager applet's left-click menu, showing all available and connected-to networks
10.2.2. Configuring New and Editing Existing Connections
Link kopierenLink in die Zwischenablage kopiert!
Next, right-click on the NetworkManager applet to open its context menu, which is the main point of entry for interacting with NetworkManager to configure connections.
Figure 10.3. The NetworkManager applet's context menu
Ensure that the Enable Networking box is checked. If the system has detected a wireless card, then you will also see an Enable Wireless menu option. Check the Enable Wireless check box as well. NetworkManager notifies you of network connection status changes if you check the Enable Notifications box. Clicking the Connection Information entry presents an informative Connection Information window that lists the connection type and interface, your IP address and routing details, and so on.
Finally, clicking on Edit Connections opens the Network Connections window, from where you can perform most of your network configuration tasks. Note that this window can also be opened by running, as a normal user:
nm-connection-editor &
~]$ nm-connection-editor &
Figure 10.4. Configure networks using the Network Connections window
There is an arrow head symbol to the left which can be clicked to hide and reveal entries as needed. To create a new connection, click the Add button to view the selection list, select the connection type and click the button. Alternatively, to edit an existing connection select the interface name from the list and click the button.
Then, to configure:
- wired Ethernet connections, proceed to Section 10.3.1, “Establishing a Wired (Ethernet) Connection”;
- wireless connections, proceed to Section 10.3.2, “Establishing a Wireless Connection”; or,
- mobile broadband connections, proceed to Section 10.3.3, “Establishing a Mobile Broadband Connection”; or,
- VPN connections, proceed to Section 10.3.4, “Establishing a VPN Connection”.
10.2.3. Connecting to a Network Automatically
Link kopierenLink in die Zwischenablage kopiert!
For any connection type you add or configure, you can choose whether you want NetworkManager to try to connect to that network automatically when it is available.
Procedure 10.1. Configuring NetworkManager to Connect to a Network Automatically When Detected
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the arrow head if necessary to reveal the list of connections.
- Select the specific connection that you want to configure and click .
- Check Connect automatically to cause NetworkManager to auto-connect to the connection whenever NetworkManager detects that it is available. Uncheck the check box if you do not want NetworkManager to connect automatically. If the box is unchecked, you will have to select that connection manually in the NetworkManager applet's left-click menu to cause it to connect.
10.2.4. User and System Connections
Link kopierenLink in die Zwischenablage kopiert!
NetworkManager connections are always either user connections or system connections. Depending on the system-specific policy that the administrator has configured, users may need
root privileges to create and modify system connections. NetworkManager's default policy enables users to create and modify user connections, but requires them to have root privileges to add, modify or delete system connections.
User connections are so-called because they are specific to the user who creates them. In contrast to system connections, whose configurations are stored under the
/etc/sysconfig/network-scripts/ directory (mainly in ifcfg-<network_type> interface configuration files), user connection settings are stored in the GConf configuration database and the GNOME keyring, and are only available during login sessions for the user who created them. Thus, logging out of the desktop session causes user-specific connections to become unavailable.
Note
Because NetworkManager uses the GConf and GNOME keyring applications to store user connection settings, and because these settings are specific to your desktop session, it is highly recommended to configure your personal VPN connections as user connections. If you do so, other Non-
root users on the system cannot view or access these connections in any way.
System connections, on the other hand, become available at boot time and can be used by other users on the system without first logging in to a desktop session.
NetworkManager can quickly and conveniently convert user to system connections and vice versa. Converting a user connection to a system connection causes NetworkManager to create the relevant interface configuration files under the
/etc/sysconfig/network-scripts/ directory, and to delete the GConf settings from the user's session. Conversely, converting a system to a user-specific connection causes NetworkManager to remove the system-wide configuration files and create the corresponding GConf/GNOME keyring settings.
Figure 10.5. The Available to all users check box controls whether connections are user-specific or system-wide
Procedure 10.2. Changing a Connection to be User-Specific instead of System-Wide, or Vice-Versa
Note
Depending on the system's policy, you may need
root privileges on the system in order to change whether a connection is user-specific or system-wide.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- If needed, select the arrow head (on the left hand side) to hide and reveal the types of available network connections.
- Select the specific connection that you want to configure and click .
- Check the Available to all users check box to ask NetworkManager to make the connection a system-wide connection. Depending on system policy, you may then be prompted for the
rootpassword by the PolicyKit application. If so, enter therootpassword to finalize the change.Conversely, uncheck the Available to all users check box to make the connection user-specific.
10.3. Establishing Connections
Link kopierenLink in die Zwischenablage kopiert!
10.3.1. Establishing a Wired (Ethernet) Connection
Link kopierenLink in die Zwischenablage kopiert!
To establish a wired network connection, Right-click on the NetworkManager applet to open its context menu, ensure that the Enable Networking box is checked, then click on Edit Connections. This opens the Network Connections window. Note that this window can also be opened by running, as a normal user:
nm-connection-editor &
~]$ nm-connection-editor &
You can click on the arrow head to reveal and hide the list of connections as needed.
Figure 10.6. The Network Connections window showing the newly created System eth0 connection
The system startup scripts create and configure a single wired connection called System eth0 by default on all systems. Although you can edit System eth0, creating a new wired connection for your custom settings is recommended. You can create a new wired connection by clicking the button, selecting the Wired entry from the list that appears and then clicking the button.
Figure 10.7. Selecting a new connection type from the "Choose a Connection Type" list
Note
When you add a new connection by clicking the button, a list of connection types appears. Once you have made a selection and clicked on the button, NetworkManager creates a new configuration file for that connection and then opens the same dialog that is used for editing an existing connection. There is no difference between these dialogs. In effect, you are always editing a connection; the difference only lies in whether that connection previously existed or was just created by NetworkManager when you clicked .
Figure 10.8. Editing the newly created Wired connection System eth0
Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings
Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Wired section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
Configuring the Wired Tab
The final three configurable settings are located within the Wired tab itself: the first is a text-entry field where you can specify a MAC (Media Access Control) address, and the second allows you to specify a cloned MAC address, and third allows you to specify the MTU (Maximum Transmission Unit) value. Normally, you can leave the MAC address field blank and the MTU set to
automatic. These defaults will suffice unless you are associating a wired connection with a second or specific NIC, or performing advanced networking. In such cases, see the following descriptions:
- MAC Address
- Network hardware such as a Network Interface Card (NIC) has a unique MAC address (Media Access Control; also known as a hardware address) that identifies it to the system. Running the
ip addrcommand will show the MAC address associated with each interface. For example, in the followingip addroutput, the MAC address for the eth0 interface (which is52:54:00:26:9e:f1) immediately follows thelink/etherkeyword:Copy to Clipboard Copied! Toggle word wrap Toggle overflow A single system can have one or more NICs installed on it. The MAC address field therefore allows you to associate a specific NIC with a specific connection (or connections). As mentioned, you can determine the MAC address using theip addrcommand, and then copy and paste that value into the MAC address text-entry field.The cloned MAC address field is mostly for use in such situations were a network service has been restricted to a specific MAC address and you need to emulate that MAC address. - MTU
- The MTU (Maximum Transmission Unit) value represents the size in bytes of the largest packet that the connection will use to transmit. This value defaults to
1500when using IPv4, or a variable number1280or higher for IPv6, and does not generally need to be specified or changed.
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your wired connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- port-based Network Access Control (PNAC), click the 802.1X Security tab and proceed to Section 10.3.9.1, “Configuring 802.1X Security”;
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”; or,
- IPv6 settings for the connection, click the IPv6 Settings tab and proceed to Section 10.3.9.5, “Configuring IPv6 Settings”.
10.3.2. Establishing a Wireless Connection
Link kopierenLink in die Zwischenablage kopiert!
This section explains how to use NetworkManager to configure a wireless (also known as Wi-Fi or 802.11a/b/g/n) connection to an Access Point.
To configure a mobile broadband (such as 3G) connection, see Section 10.3.3, “Establishing a Mobile Broadband Connection”.
Quickly Connecting to an Available Access Point
The easiest way to connect to an available access point is to left-click on the NetworkManager applet, locate the Service Set Identifier (SSID) of the access point in the list of Available networks, and click on it. If the access point is secured, a dialog prompts you for authentication.
Figure 10.9. Authenticating to a wireless access point
NetworkManager tries to auto-detect the type of security used by the access point. If there are multiple possibilities, NetworkManager guesses the security type and presents it in the Wireless security dropdown menu. To see if there are multiple choices, click the Wireless security dropdown menu and select the type of security the access point is using. If you are unsure, try connecting to each type in turn. Finally, enter the key or passphrase in the Password field. Certain password types, such as a 40-bit WEP or 128-bit WPA key, are invalid unless they are of a requisite length. The Connect button will remain inactive until you enter a key of the length required for the selected security type. To learn more about wireless security, see Section 10.3.9.2, “Configuring Wireless Security”.
Note
In the case of WPA and WPA2 (Personal and Enterprise), an option to select between Auto, WPA and WPA2 has been added. This option is intended for use with an access point that is offering both WPA and WPA2. Select one of the protocols if you would like to prevent roaming between the two protocols. Roaming between WPA and WPA2 on the same access point can cause loss of service.
If NetworkManager connects to the access point successfully, its applet icon will change into a graphical indicator of the wireless connection's signal strength.
Figure 10.10. Applet icon indicating a wireless connection signal strength of 75%
You can also edit the settings for one of these auto-created access point connections just as if you had added it yourself. The Wireless tab of the Network Connections window lists all of the connections you have ever tried to connect to: NetworkManager names each of them
Auto <SSID>, where SSID is the Service Set identifier of the access point.
Figure 10.11. An example of access points that have previously been connected to
Connecting to a Hidden Wireless Network
All access points have a Service Set Identifier (SSID) to identify them. However, an access point may be configured not to broadcast its SSID, in which case it is hidden, and will not show up in NetworkManager's list of Available networks. You can still connect to a wireless access point that is hiding its SSID as long as you know its SSID, authentication method, and secrets.
To connect to a hidden wireless network, left-click NetworkManager's applet icon and select Connect to Hidden Wireless Network to cause a dialog to appear. If you have connected to the hidden network before, use the Connection dropdown to select it, and click . If you have not, leave the Connection dropdown as , enter the SSID of the hidden network, select its Wireless security method, enter the correct authentication secrets, and click .
For more information on wireless security settings, see Section 10.3.9.2, “Configuring Wireless Security”.
Editing a Connection, or Creating a Completely New One
You can edit an existing connection that you have tried or succeeded in connecting to in the past by opening the Wireless tab of the Network Connections, selecting the connection by name (words which follow
Auto refer to the SSID of an access point), and clicking Edit.
You can create a new connection by opening the Network Connections window, clicking the Add button, selecting Wireless, and clicking the Create button.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button.
- Select the Wireless entry from the list.
- Click the button.
Figure 10.12. Editing the newly created Wireless connection 1
Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings
Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Wireless section of the Network Connections window. By default, wireless connections are named the same as the SSID of the wireless access point. You can rename the wireless connection without affecting its ability to connect, but it is recommended to retain the SSID name.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
Configuring the Wireless Tab
- SSID
- All access points have a Service Set identifier to identify them. However, an access point may be configured not to broadcast its SSID, in which case it is hidden, and will not show up in NetworkManager's list of Available networks. You can still connect to a wireless access point that is hiding its SSID as long as you know its SSID (and authentication secrets).For information on connecting to a hidden wireless network, see the section called “Connecting to a Hidden Wireless Network”.
- Mode
- — Set Mode to if you are connecting to a dedicated wireless access point or one built into a network device such as a router or a switch.— Set Mode to if you are creating a peer-to-peer network for two or more mobile devices to communicate directly with each other. If you use mode, referred to as Independent Basic Service Set (IBSS) in the 802.11 standard, you must ensure that the same SSID is set for all participating wireless devices, and that they are all communicating over the same channel.
- BSSID
- The Basic Service Set Identifier (BSSID) is the MAC address of the specific wireless access point you are connecting to when in Infrastructure mode. This field is blank by default, and you are able to connect to a wireless access point by SSID without having to specify its BSSID. If the BSSID is specified, it will force the system to associate to a specific access point only.For ad-hoc networks, the BSSID is generated randomly by the
mac80211subsystem when the ad-hoc network is created. It is not displayed by NetworkManager - MAC address
- Like an Ethernet Network Interface Card (NIC), a wireless adapter has a unique MAC address (Media Access Control; also known as a hardware address) that identifies it to the system. Running the
ip addrcommand will show the MAC address associated with each interface. For example, in the followingip addroutput, the MAC address for thewlan0interface (which is00:1c:bf:02:f8:70) immediately follows thelink/etherkeyword:Copy to Clipboard Copied! Toggle word wrap Toggle overflow A single system could have one or more wireless network adapters connected to it. The MAC address field therefore allows you to associate a specific wireless adapter with a specific connection (or connections). As mentioned, you can determine the MAC address using theip addrcommand, and then copy and paste that value into the MAC address text-entry field. - MTU
- The MTU (Maximum Transmission Unit) value represents the size in bytes of the largest packet that the connection will use to transmit. If set to a non-zero number, only packets of the specified size or smaller will be transmitted. Larger packets are broken up into multiple Ethernet frames. It is recommended to leave this setting on .
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing the wireless connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can successfully connect to your the modified connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for details on selecting and connecting to a network.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- security authentication for the wireless connection, click the Wireless Security tab and proceed to Section 10.3.9.2, “Configuring Wireless Security”;
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”; or,
- IPv6 settings for the connection, click the IPv6 Settings tab and proceed to Section 10.3.9.5, “Configuring IPv6 Settings”.
10.3.3. Establishing a Mobile Broadband Connection
Link kopierenLink in die Zwischenablage kopiert!
You can use NetworkManager's mobile broadband connection abilities to connect to the following 2G and 3G services:
- 2G — GPRS (General Packet Radio Service) or EDGE (Enhanced Data Rates for GSM Evolution)
- 3G — UMTS (Universal Mobile Telecommunications System) or HSPA (High Speed Packet Access)
Your computer must have a mobile broadband device (modem), which the system has discovered and recognized, in order to create the connection. Such a device may be built into your computer (as is the case on many notebooks and netbooks), or may be provided separately as internal or external hardware. Examples include PC card, USB Modem or Dongle, mobile or cellular telephone capable of acting as a modem.
Procedure 10.3. Adding a New Mobile Broadband Connection
You can configure a mobile broadband connection by opening the Network Connections window, clicking , and selecting Mobile Broadband from the list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button to open the selection list. Select Mobile Broadband and then click . The Set up a Mobile Broadband Connection assistant appears.
- Under Create a connection for this mobile broadband device, choose the 2G- or 3G-capable device you want to use with the connection. If the dropdown menu is inactive, this indicates that the system was unable to detect a device capable of mobile broadband. In this case, click Cancel, ensure that you do have a mobile broadband-capable device attached and recognized by the computer and then retry this procedure. Click the Forward button.
- Select the country where your service provider is located from the list and click the Forward button.
- Select your provider from the list or enter it manually. Click the Forward button.
- Select your payment plan from the dropdown menu and confirm the Access Point Name (APN) is correct. Click the Forward button.
- Review and confirm the settings and then click the Apply button.
- Edit the mobile broadband-specific settings by referring to the Configuring the Mobile Broadband Tab description below .
Procedure 10.4. Editing an Existing Mobile Broadband Connection
Follow these steps to edit an existing mobile broadband connection.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
- Select the Mobile Broadband tab.
- Configure the connection name, auto-connect behavior, and availability settings.Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Mobile Broadband section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
- Edit the mobile broadband-specific settings by referring to the Configuring the Mobile Broadband Tab description below .
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your mobile broadband connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- Point-to-point settings for the connection, click the PPP Settings tab and proceed to Section 10.3.9.3, “Configuring PPP (Point-to-Point) Settings”;
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”; or,
- IPv6 settings for the connection, click the IPv6 Settings tab and proceed to Section 10.3.9.5, “Configuring IPv6 Settings”.
Configuring the Mobile Broadband Tab
If you have already added a new mobile broadband connection using the assistant (see Procedure 10.3, “Adding a New Mobile Broadband Connection” for instructions), you can edit the Mobile Broadband tab to disable roaming if home network is not available, assign a network ID, or instruct NetworkManager to prefer a certain technology (such as 3G or 2G) when using the connection.
- Number
- The number that is dialed to establish a PPP connection with the GSM-based mobile broadband network. This field may be automatically populated during the initial installation of the broadband device. You can usually leave this field blank and enter the APN instead.
- Username
- Enter the user name used to authenticate with the network. Some providers do not provide a user name, or accept any user name when connecting to the network.
- Password
- Enter the password used to authenticate with the network. Some providers do not provide a password, or accept any password.
- APN
- Enter the Access Point Name (APN) used to establish a connection with the GSM-based network. Entering the correct APN for a connection is important because it often determines:
- how the user is billed for their network usage; and/or
- whether the user has access to the Internet, an intranet, or a subnetwork.
- Network ID
- Entering a Network ID causes NetworkManager to force the device to register only to a specific network. This can be used to ensure the connection does not roam when it is not possible to control roaming directly.
- Type
- Any — The default value of Any leaves the modem to select the fastest network.3G (UMTS/HSPA) — Force the connection to use only 3G network technologies.2G (GPRS/EDGE) — Force the connection to use only 2G network technologies.Prefer 3G (UMTS/HSPA) — First attempt to connect using a 3G technology such as HSPA or UMTS, and fall back to GPRS or EDGE only upon failure.Prefer 2G (GPRS/EDGE) — First attempt to connect using a 2G technology such as GPRS or EDGE, and fall back to HSPA or UMTS only upon failure.
- Allow roaming if home network is not available
- Uncheck this box if you want NetworkManager to terminate the connection rather than transition from the home network to a roaming one, thereby avoiding possible roaming charges. If the box is checked, NetworkManager will attempt to maintain a good connection by transitioning from the home network to a roaming one, and vice versa.
- PIN
- If your device's SIM (Subscriber Identity Module) is locked with a PIN (Personal Identification Number), enter the PIN so that NetworkManager can unlock the device. NetworkManager must unlock the SIM if a PIN is required in order to use the device for any purpose.
10.3.4. Establishing a VPN Connection
Link kopierenLink in die Zwischenablage kopiert!
Establishing an encrypted Virtual Private Network (VPN) enables you to communicate securely between your Local Area Network (LAN), and another, remote LAN. After successfully establishing a VPN connection, a VPN router or gateway performs the following actions upon the packets you transmit:
- it adds an Authentication Header for routing and authentication purposes;
- it encrypts the packet data; and,
- it encloses the data with an Encapsulating Security Payload (ESP), which constitutes the decryption and handling instructions.
The receiving VPN router strips the header information, decrypts the data, and routes it to its intended destination (either a workstation or other node on a network). Using a network-to-network connection, the receiving node on the local network receives the packets already decrypted and ready for processing. The encryption/decryption process in a network-to-network VPN connection is therefore transparent to clients.
Because they employ several layers of authentication and encryption, VPNs are a secure and effective means of connecting multiple remote nodes to act as a unified intranet.
Procedure 10.5. Adding a New VPN Connection
- You can configure a new VPN connection by opening the Network Connections window, clicking the button and selecting a type of VPN from the VPN section of the new connection list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button.
- The Choose a Connection Type list appears.
Note
The appropriate NetworkManager VPN plug-in for the VPN type you want to configure must be installed (see Section 8.2.4, “Installing Packages” for more information on how to install new packages in Red Hat Enterprise Linux 6).The VPN section in the Choose a Connection Type list will not appear if you do not have a suitable plug-in installed.- Select the VPN protocol for the gateway you are connecting to from the Choose a Connection Type list. The VPN protocols available for selection in the list correspond to the NetworkManager VPN plug-ins installed. For example, if NetworkManager-openswan, the NetworkManager VPN plug-in for libreswan, is installed, then the IPsec based VPN will be selectable from the Choose a Connection Type list.
Note
In Red Hat Enterprise Linux 6.8, openswan has been obsoleted by libreswan. NetworkManager-openswan has been modified to support both openswan and libreswan.After selecting the correct one, press the button. - The Editing VPN Connection 1 window then appears. This window presents settings customized for the type of VPN connection you selected in Step 6.
Procedure 10.6. Editing an Existing VPN Connection
You can configure an existing VPN connection by opening the Network Connections window and selecting the name of the connection from the list. Then click the button.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
Figure 10.13. Editing the newly created IPsec VPN connection 1
Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings
Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the VPN section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
Configuring the VPN Tab
- Gateway
- The name or IP address of the remote VPN gateway.
- Group name
- The name of a VPN group configured on the remote gateway.
- User password
- If required, enter the password used to authenticate with the VPN.
- Group password
- If required, enter the password used to authenticate with the VPN.
- User name
- If required, enter the user name used to authenticate with the VPN.
- Phase1 Algorithms
- If required, enter the algorithms to be used to authenticate and set up an encrypted channel.
- Phase2 Algorithms
- If required, enter the algorithms to be used for the IPsec negotiations.
- Domain
- If required, enter the Domain Name.
- NAT traversal
- — IPsec over UDP.— ESP encapsulation and IKE extensions are used to handle NAT Traversal.— No special NAT measures required.Disable Dead Peer Detection — Disable the sending of probes to the remote gateway or endpoint.
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your new VPN connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”.
10.3.5. Establishing a DSL Connection
Link kopierenLink in die Zwischenablage kopiert!
This section is intended for those installations which have a DSL card fitted within a host rather than the external combined DSL modem router combinations typical of private consumer or SOHO installations.
Procedure 10.7. Adding a New DSL Connection
You can configure a new DSL connection by opening the Network Connections window, clicking the button and selecting DSL from the Hardware section of the new connection list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button.
- The Choose a Connection Type list appears.
- Select DSL and press the button.
- The Editing DSL Connection 1 window appears.
Procedure 10.8. Editing an Existing DSL Connection
You can configure an existing DSL connection by opening the Network Connections window and selecting the name of the connection from the list. Then click the button.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings
Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the DSL section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
Configuring the DSL Tab
- Username
- Enter the user name used to authenticate with the service provider.
- Service
- Leave blank unless otherwise directed.
- Password
- Enter the password supplied by the service provider.
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your DSL connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- The MAC address and MTU settings, click the Wired tab and proceed to the section called “Configuring the Wired Tab”;
- Point-to-point settings for the connection, click the PPP Settings tab and proceed to Section 10.3.9.3, “Configuring PPP (Point-to-Point) Settings”;
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”.
10.3.6. Establishing a Bond Connection
Link kopierenLink in die Zwischenablage kopiert!
You can use NetworkManager to create a Bond from two or more Wired or Infiniband connections. It is not necessary to create the connections to be bonded first. They can be configured as part of the process to configure the bond. You must have the MAC addresses of the interfaces available in order to complete the configuration process.
Note
NetworkManager support for bonding must be enabled by means of the
NM_BOND_VLAN_ENABLED directive and then NetworkManager must be restarted. See Section 11.2.1, “Ethernet Interfaces” for an explanation of NM_CONTROLLED and the NM_BOND_VLAN_ENABLED directive. See Section 12.3.4, “Restarting a Service” for an explanation of restarting a service such as NetworkManager from the command line. Alternatively, for a graphical tool see Section 12.2.1, “Using the Service Configuration Utility”.
Procedure 10.9. Adding a New Bond Connection
You can configure a Bond connection by opening the Network Connections window, clicking , and selecting Bond from the list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button to open the selection list. Select Bond and then click . The Editing Bond connection 1 window appears.
- On the Bond tab, click and select the type of interface you want to use with the bond connection. Click the button. Note that the dialog to select the slave type only comes up when you create the first slave; after that, it will automatically use that same type for all further slaves.
- The Editing bond0 slave 1 window appears. Fill in the MAC address of the first interface to be bonded. The first slave's MAC address will be used as the MAC address for the bond interface. If required, enter a clone MAC address to be used as the bond's MAC address. Click the button.
- The Authenticate window appears. Enter the
rootpassword to continue. Click the button. - The name of the bonded slave appears in the Bonded Connections window. Click the button to add further slave connections.
- Review and confirm the settings and then click the Apply button.
- Edit the bond-specific settings by referring to the section called “Configuring the Bond Tab” below.
Figure 10.14. Editing the newly created Bond connection 1
Procedure 10.10. Editing an Existing Bond Connection
Follow these steps to edit an existing bond connection.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
- Select the Bond tab.
- Configure the connection name, auto-connect behavior, and availability settings.Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Bond section of the Network Connections window.
- Connect automatically — Select this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Select this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
- Edit the bond-specific settings by referring to the section called “Configuring the Bond Tab” below.
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your bond connection, click the button to save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”; or,
- IPv6 settings for the connection, click the IPv6 Settings tab and proceed to Section 10.3.9.5, “Configuring IPv6 Settings”.
Configuring the Bond Tab
If you have already added a new bond connection (see Procedure 10.9, “Adding a New Bond Connection” for instructions), you can edit the Bond tab to set the load sharing mode and the type of link monitoring to use to detect failures of a slave connection.
- Mode
- The mode that is used to share traffic over the slave connections which make up the bond. The default is Round-robin. Other load sharing modes, such as
802.3ad, can be selected by means of the drop-down list. - Link Monitoring
- The method of monitoring the slaves ability to carry network traffic.
The following modes of load sharing are selectable from the Mode drop-down list:
- Round-robin
- Sets a round-robin policy for fault tolerance and load balancing. Transmissions are received and sent out sequentially on each bonded slave interface beginning with the first one available. This mode might not work behind a bridge with virtual machines without additional switch configuration.
- Active backup
- Sets an active-backup policy for fault tolerance. Transmissions are received and sent out via the first available bonded slave interface. Another bonded slave interface is only used if the active bonded slave interface fails. Note that this is the only mode available for bonds of InfiniBand devices.
- XOR
- Sets an XOR (exclusive-or) policy. Transmissions are based on the selected hash policy. The default is to derive a hash by XOR of the source and destination MAC addresses multiplied by the modulo of the number of slave interfaces. In this mode traffic destined for specific peers will always be sent over the same interface. As the destination is determined by the MAC addresses this method works best for traffic to peers on the same link or local network. If traffic has to pass through a single router then this mode of traffic balancing will be suboptimal.
- Broadcast
- Sets a broadcast policy for fault tolerance. All transmissions are sent on all slave interfaces. This mode might not work behind a bridge with virtual machines without additional switch configuration.
- 802.3ad
- Sets an IEEE
802.3addynamic link aggregation policy. Creates aggregation groups that share the same speed and duplex settings. Transmits and receives on all slaves in the active aggregator. Requires a network switch that is802.3adcompliant. - Adaptive transmit load balancing
- Sets an adaptive Transmit Load Balancing (TLB) policy for fault tolerance and load balancing. The outgoing traffic is distributed according to the current load on each slave interface. Incoming traffic is received by the current slave. If the receiving slave fails, another slave takes over the MAC address of the failed slave. This mode is only suitable for local addresses known to the kernel bonding module and therefore cannot be used behind a bridge with virtual machines.
- Adaptive load balancing
- Sets an Adaptive Load Balancing (ALB) policy for fault tolerance and load balancing. Includes transmit and receive load balancing for
IPv4traffic. Receive load balancing is achieved throughARPnegotiation. This mode is only suitable for local addresses known to the kernel bonding module and therefore cannot be used behind a bridge with virtual machines.
The following types of link monitoring can be selected from the Link Monitoring drop-down list. It is a good idea to test which channel bonding module parameters work best for your bonded interfaces.
- MII (Media Independent Interface)
- The state of the carrier wave of the interface is monitored. This can be done by querying the driver, by querying MII registers directly, or by using ethtool to query the device. Three options are available:
- Monitoring Frequency
- The time interval, in milliseconds, between querying the driver or MII registers.
- Link up delay
- The time in milliseconds to wait before attempting to use a link that has been reported as up. This delay can be used if some gratuitous
ARPrequests are lost in the period immediately following the link being reported as “up”. This can happen during switch initialization for example. - Link down delay
- The time in milliseconds to wait before changing to another link when a previously active link has been reported as “down”. This delay can be used if an attached switch takes a relatively long time to change to backup mode.
- ARP
- The address resolution protocol (
ARP) is used to probe one or more peers to determine how well the link-layer connections are working. It is dependent on the device driver providing the transmit start time and the last receive time.Two options are available:- Monitoring Frequency
- The time interval, in milliseconds, between sending
ARPrequests. - ARP targets
- A comma separated list of
IPaddresses to sendARPrequests to.
10.3.7. Establishing a VLAN Connection
Link kopierenLink in die Zwischenablage kopiert!
You can use NetworkManager to create a VLAN using an existing interface. Currently, at time of writing, you can only make VLANs on Ethernet devices.
Procedure 10.11. Adding a New VLAN Connection
You can configure a VLAN connection by opening the Network Connections window, clicking , and selecting VLAN from the list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button to open the selection list. Select VLAN and then click . The Editing VLAN Connection 1 window appears.
- On the VLAN tab, select the parent interface from the drop-down list you want to use for the VLAN connection.
- Enter the VLAN ID
- Enter a VLAN interface name. This is the name of the VLAN interface that will be created. For example, "eth0.1" or "vlan2". (Normally this is either the parent interface name plus "." and the VLAN ID, or "vlan" plus the VLAN ID.)
- Review and confirm the settings and then click the Apply button.
- Edit the VLAN-specific settings by referring to the Configuring the VLAN Tab description below .
Procedure 10.12. Editing an Existing VLAN Connection
Follow these steps to edit an existing VLAN connection.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
- Select the VLAN tab.
- Configure the connection name, auto-connect behavior, and availability settings.Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the VLAN section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
- Edit the VLAN-specific settings by referring to the Configuring the VLAN Tab description below .
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your VLAN connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”.
Configuring the VLAN Tab
If you have already added a new VLAN connection (see Procedure 10.11, “Adding a New VLAN Connection” for instructions), you can edit the VLAN tab to set the parent interface and the VLAN ID.
- Parent Interface
- A previously configured interface can be selected in the drop-down list.
- VLAN ID
- The identification number to be used to tag the VLAN network traffic.
- VLAN interface name
- The name of the VLAN interface that will be created. For example, "eth0.1" or "vlan2".
- Cloned MAC address
- Optionally sets an alternate MAC address to use for identifying the VLAN interface. This can be used to change the source MAC address for packets sent on this VLAN.
- MTU
- Optionally sets a Maximum Transmission Unit (MTU) size to be used for packets to be sent over the VLAN connection.
10.3.8. Establishing an IP-over-InfiniBand (IPoIB) Connection
Link kopierenLink in die Zwischenablage kopiert!
You can use NetworkManager to create an InfiniBand connection.
Procedure 10.13. Adding a New InfiniBand Connection
You can configure an InfiniBand connection by opening the Network Connections window, clicking , and selecting InfiniBand from the list.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Click the button to open the selection list. Select InfiniBand and then click . The Editing InfiniBand Connection 1 window appears.
- On the InfiniBand tab, select the transport mode from the drop-down list you want to use for the InfiniBand connection.
- Enter the InfiniBand MAC address.
- Review and confirm the settings and then click the Apply button.
- Edit the InfiniBand-specific settings by referring to the Configuring the InfiniBand Tab description below .
Figure 10.15. Editing the newly created InfiniBand connection 1
Procedure 10.14. Editing an Existing InfiniBand Connection
Follow these steps to edit an existing InfiniBand connection.
- Right-click on the NetworkManager applet icon in the Notification Area and click Edit Connections. The Network Connections window appears.
- Select the connection you want to edit and click the Edit button.
- Select the InfiniBand tab.
- Configure the connection name, auto-connect behavior, and availability settings.Three settings in the Editing dialog are common to all connection types:
- Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the InfiniBand section of the Network Connections window.
- Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. See Section 10.2.3, “Connecting to a Network Automatically” for more information.
- Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require
rootprivileges. See Section 10.2.4, “User and System Connections” for details.
- Edit the InfiniBand-specific settings by referring to the Configuring the InfiniBand Tab description below .
Saving Your New (or Modified) Connection and Making Further Configurations
Once you have finished editing your InfiniBand connection, click the button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 10.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:
- IPv4 settings for the connection, click the IPv4 Settings tab and proceed to Section 10.3.9.4, “Configuring IPv4 Settings”; or,
- IPv6 settings for the connection, click the IPv6 Settings tab and proceed to Section 10.3.9.5, “Configuring IPv6 Settings”.
Configuring the InfiniBand Tab
If you have already added a new InfiniBand connection (see Procedure 10.13, “Adding a New InfiniBand Connection” for instructions), you can edit the InfiniBand tab to set the parent interface and the InfiniBand ID.
- Transport mode
- Datagram or Connected mode can be selected from the drop-down list. Select the same mode the rest of your IPoIB network is using.
- Device MAC address
- The MAC address of the InfiniBand capable device to be used for the InfiniBand network traffic.This hardware address field will be pre-filled if you have InfiniBand hardware installed.
- MTU
- Optionally sets a Maximum Transmission Unit (MTU) size to be used for packets to be sent over the InfiniBand connection.
10.3.9. Configuring Connection Settings
Link kopierenLink in die Zwischenablage kopiert!
10.3.9.1. Configuring 802.1X Security
Link kopierenLink in die Zwischenablage kopiert!
802.1X security is the name of the IEEE standard for port-based Network Access Control (PNAC). Simply put, 802.1X security is a way of defining a logical network out of a physical one. All clients who want to join the logical network must authenticate with the server (a router, for example) using the correct 802.1X authentication method.
802.1X security is most often associated with securing wireless networks (WLANs), but can also be used to prevent intruders with physical access to the network (LAN) from gaining entry. In the past, DHCP servers were configured not to lease IP addresses to unauthorized users, but for various reasons this practice is both impractical and insecure, and thus is no longer recommended. Instead, 802.1X security is used to ensure a logically-secure network through port-based authentication.
802.1X provides a framework for WLAN and LAN access control and serves as an envelope for carrying one of the Extensible Authentication Protocol (EAP) types. An EAP type is a protocol that defines how WLAN security is achieved on the network.
You can configure 802.1X security for a wired or wireless connection type by opening the Network Connections window (see Section 10.2.2, “Configuring New and Editing Existing Connections”) and following the applicable procedure:
Procedure 10.15. For a wired connection...
- Either click , select a new network connection for which you want to configure 802.1X security and then click , or select an existing connection and click .
- Then select the 802.1X Security tab and check the Use 802.1X security for this connection check box to enable settings configuration.
Procedure 10.16. For a wireless connection...
- Either click on , select a new network connection for which you want to configure 802.1X security and then click , or select an existing connection and click .
- Select the Wireless Security tab.
- Then click the Security dropdown and choose one of the following security methods: , , or .
- See Section 10.3.9.1.1, “Configuring TLS (Transport Layer Security) Settings” for descriptions of which EAP types correspond to your selection in the Security dropdown.
10.3.9.1.1. Configuring TLS (Transport Layer Security) Settings
Link kopierenLink in die Zwischenablage kopiert!
With Transport Layer Security, the client and server mutually authenticate using the TLS protocol. The server demonstrates that it holds a digital certificate, the client proves its own identity using its client-side certificate, and key information is exchanged. Once authentication is complete, the TLS tunnel is no longer used. Instead, the client and server use the exchanged keys to encrypt data using AES, TKIP or WEP.
The fact that certificates must be distributed to all clients who want to authenticate means that the EAP-TLS authentication method is very strong, but also more complicated to set up. Using TLS security requires the overhead of a public key infrastructure (PKI) to manage certificates. The benefit of using TLS security is that a compromised password does not allow access to the (W)LAN: an intruder must also have access to the authenticating client's private key.
NetworkManager does not determine the version of TLS supported. NetworkManager gathers the parameters entered by the user and passes them to the daemon, wpa_supplicant, that handles the procedure. It in turn uses OpenSSL to establish the TLS tunnel. OpenSSL itself negotiates the SSL/TLS protocol version. It uses the highest version both ends support.
- Identity
- Identity string for EAP authentication methods, such as a user name or login name.
- User certificate
- Click to browse for, and select, a user's certificate.
- CA certificate
- Click to browse for, and select, a Certificate Authority's certificate.
- Private key
- Click to browse for, and select, a user's private key file. Note that the key must be password protected.
- Private key password
- Enter the user password corresponding to the user's private key.
10.3.9.1.2. Configuring Tunneled TLS Settings
Link kopierenLink in die Zwischenablage kopiert!
- Anonymous identity
- This value is used as the unencrypted identity.
- CA certificate
- Click to browse for, and select, a Certificate Authority's certificate.
- Inner authentication
- — Password Authentication Protocol.— Challenge Handshake Authentication Protocol.— Microsoft Challenge Handshake Authentication Protocol version 2.— Challenge Handshake Authentication Protocol.
- Username
- Enter the user name to be used in the authentication process.
- Password
- Enter the password to be used in the authentication process.
10.3.9.1.3. Configuring Protected EAP (PEAP) Settings
Link kopierenLink in die Zwischenablage kopiert!
- Anonymous Identity
- This value is used as the unencrypted identity.
- CA certificate
- Click to browse for, and select, a Certificate Authority's certificate.
- PEAP version
- The version of Protected EAP to use. Automatic, 0 or 1.
- Inner authentication
- — Microsoft Challenge Handshake Authentication Protocol version 2.— Message Digest 5, a cryptographic hash function.— Generic Token Card.
- Username
- Enter the user name to be used in the authentication process.
- Password
- Enter the password to be used in the authentication process.
10.3.9.2. Configuring Wireless Security
Link kopierenLink in die Zwischenablage kopiert!
- Security
- — Do not encrypt the Wi-Fi connection.— Wired Equivalent Privacy (WEP), from the IEEE 802.11 standard. Uses a single pre-shared key (PSK).— An MD5 hash of the passphrase will be used to derive a WEP key.— Lightweight Extensible Authentication Protocol, from Cisco Systems.— WEP keys are changed dynamically.— Wi-Fi Protected Access (WPA), from the draft IEEE 802.11i standard. A replacement for WEP. Wi-Fi Protected Access II (WPA2), from the 802.11i-2004 standard. Personal mode uses a pre-shared key (WPA-PSK).— WPA for use with a RADIUS authentication server to provide IEEE 802.1X network access control.
- Password
- Enter the password to be used in the authentication process.
Note
In the case of WPA and WPA2 (Personal and Enterprise), an option to select between Auto, WPA and WPA2 has been added. This option is intended for use with an access point that is offering both WPA and WPA2. Select one of the protocols if you would like to prevent roaming between the two protocols. Roaming between WPA and WPA2 on the same access point can cause loss of service.
Figure 10.16. Editing the Wireless Security tab and selecting the WPA protocol
10.3.9.3. Configuring PPP (Point-to-Point) Settings
Link kopierenLink in die Zwischenablage kopiert!
- Configure Methods
- Use point-to-point encryption (MPPE)
- Microsoft Point-To-Point Encryption protocol (RFC 3078).
- Allow BSD data compression
- PPP BSD Compression Protocol (RFC 1977).
- Allow Deflate data compression
- PPP Deflate Protocol (RFC 1979).
- Use TCP header compression
- Compressing TCP/IP Headers for Low-Speed Serial Links (RFC 1144).
- Send PPP echo packets
- LCP Echo-Request and Echo-Reply Codes for loopback tests (RFC 1661).
10.3.9.4. Configuring IPv4 Settings
Link kopierenLink in die Zwischenablage kopiert!
Figure 10.17. Editing the IPv4 Settings Tab
The IPv4 Settings tab allows you to configure the method by which you connect to the Internet and enter IP address, route, and DNS information as required. The IPv4 Settings tab is available when you create and modify one of the following connection types: wired, wireless, mobile broadband, VPN or DSL.
If you are using DHCP to obtain a dynamic IP address from a DHCP server, you can set Method to .
Setting the Method
Available IPv4 Methods by Connection Type
When you click the Method dropdown menu, depending on the type of connection you are configuring, you are able to select one of the following IPv4 connection methods. All of the methods are listed here according to which connection type or types they are associated with.
- Method
- — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses. You do not need to fill in the DHCP client ID field.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.— Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be selected as per RFC 3927.— Choose this option if the interface you are configuring is for sharing an Internet or WAN connection.
- Wired, Wireless and DSL Connection Methods
- — Choose this option if the network you are connecting to does not have a DHCP server and you want to assign IP addresses manually.
- Mobile Broadband Connection Methods
- — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
- VPN Connection Methods
- — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
- DSL Connection Methods
- — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
PPPoE Specific Configuration Steps
If more than one NIC is installed, and PPPoE will only be run over one NIC but not the other, then for correct PPPoE operation it is also necessary to lock the connection to the specific Ethernet device PPPoE is supposed to be run over. To lock the connection to one specific NIC, do one of the following:
- Enter the MAC address in nm-connection-editor for that connection. Optionally select Connect automatically and Available to all users to make the connection come up without requiring user login after system start.
- Set the hardware-address in the [802-3-ethernet] section in the appropriate file for that connection in
/etc/NetworkManager/system-connections/as follows:Mere presence of the file in[802-3-ethernet] mac-address=00:11:22:33:44:55
[802-3-ethernet] mac-address=00:11:22:33:44:55Copy to Clipboard Copied! Toggle word wrap Toggle overflow /etc/NetworkManager/system-connections/means that it is “available to all users”. Ensure thatautoconnect=trueappears in the [connection] section for the connection to be brought up without requiring user login after system start.
For information on configuring static routes for the network connection, go to Section 10.3.9.6, “Configuring Routes”.
10.3.9.5. Configuring IPv6 Settings
Link kopierenLink in die Zwischenablage kopiert!
- Method
- — Choose this option if you want to disable IPv6 settings.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.— Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.— Choose this option if the network you are connecting to does not have a DHCP server and you want to assign IP addresses manually.— Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be selected as per RFC 4862.— Choose this option if the interface you are configuring is for sharing an Internet or WAN connection.
- Addresses
- — Enter a comma separated list of DNS servers.— Enter a comma separated list of domain controllers.
For information on configuring static routes for the network connection, go to Section 10.3.9.6, “Configuring Routes”.
10.3.9.6. Configuring Routes
Link kopierenLink in die Zwischenablage kopiert!
A host's routing table will be automatically populated with routes to directly connected networks. The routes are learned by observing the network interfaces when they are “up”. This section is for entering static routes to networks or hosts which can be reached by traversing an intermediate network or connection, such as a VPN or leased line.
Figure 10.18. Configuring static network routes
- Addresses
- — The IP address of a network, sub-net or host.— The netmask or prefix length of the IP address just entered.— The IP address of the gateway leading to the network, sub-net or host.— A network cost, that is to say a preference value to give to this route. Lower values will be preferred over higher values.
- Ignore automatically obtained routes
- Select this check box to only use manually entered routes for this connection.
- Use this connection only for resources on its network
- Select this check box to prevent the connection from becoming the default route. Typical examples are where a connection is a VPN or a leased line to a head office and you do not want any Internet bound traffic to pass over the connection. Selecting this option means that only traffic specifically destined for routes learned automatically over the connection or entered here manually will be routed over the connection.
Chapter 11. Network Interfaces
Link kopierenLink in die Zwischenablage kopiert!
Under Red Hat Enterprise Linux, all network communications occur between configured software interfaces and physical networking devices connected to the system.
The configuration files for network interfaces are located in the
/etc/sysconfig/network-scripts/ directory. The scripts used to activate and deactivate these network interfaces are also located here. Although the number and type of interface files can differ from system to system, there are three categories of files that exist in this directory:
- Interface configuration files
- Interface control scripts
- Network function files
The files in each of these categories work together to enable various network devices.
This chapter explores the relationship between these files and how they are used.
11.1. Network Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
Before delving into the interface configuration files, let us first itemize the primary configuration files used in network configuration. Understanding the role these files play in setting up the network stack can be helpful when customizing a Red Hat Enterprise Linux system.
The primary network configuration files are as follows:
-
/etc/hosts - The main purpose of this file is to resolve host names that cannot be resolved any other way. It can also be used to resolve host names on small networks with no
DNSserver. Regardless of the type of network the computer is on, this file should contain a line specifying theIPaddress of the loopback device (127.0.0.1) aslocalhost.localdomain. For more information, see thehosts(5)manual page. -
/etc/resolv.conf - This file specifies the
IPaddresses ofDNSservers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file. For more information about this file, see theresolv.conf(5)manual page. -
/etc/sysconfig/network - This file specifies routing and host information for all network interfaces. It is used to contain directives which are to have global effect and not to be interface specific. For more information about this file and the directives it accepts, see Section D.1.14, “/etc/sysconfig/network”.
-
/etc/sysconfig/network-scripts/ifcfg-interface-name - For each network interface, there is a corresponding interface configuration script. Each of these files provide information specific to a particular network interface. See Section 11.2, “Interface Configuration Files” for more information on this type of file and the directives it accepts.
Important
Network interface names may be different on different hardware types. See Appendix A, Consistent Network Device Naming for more information.
Warning
The
/etc/sysconfig/networking/ directory is used by the now deprecated Network Administration Tool (system-config-network). Its contents should not be edited manually. Using only one method for network configuration is strongly encouraged, due to the risk of configuration deletion. For more information about configuring network interfaces using graphical configuration tools, see Chapter 10, NetworkManager.
11.1.1. Setting the Host Name
Link kopierenLink in die Zwischenablage kopiert!
To permanently change the static host name, change the HOSTNAME directive in the
/etc/sysconfig/network file. For example:
HOSTNAME=penguin.example.com
HOSTNAME=penguin.example.com
Red Hat recommends the static host name matches the fully qualified domain name (FQDN) used for the machine in DNS, such as host.example.com. It is also recommended that the static host name consists only of 7 bit ASCII lower-case characters, no spaces or dots, and limits itself to the format allowed for DNS domain name labels, even though this is not a strict requirement. Older specifications do not permit the underscore, and so their use is not recommended. Changes will only take effect when the networking service, or the system, is restarted.
Note that the FQDN of the host can be supplied by a DNS resolver, by settings in
/etc/sysconfig/network, or by the /etc/hosts file. The default setting of hosts: files dns in /etc/nsswitch.conf causes the configuration files to be checked before a resolver. The default setting of multi on in the /etc/host.conf file means that all valid values in the /etc/hosts file are returned, not just the first.
Sometimes you may need to use the host table in the
/etc/hosts file instead of the HOSTNAME directive in /etc/sysconfig/network, for example, when DNS is not running during system bootup.
To change the host name using the
/etc/hosts file, add lines to it in the following format:
192.168.1.2 penguin.example.com penguin
192.168.1.2 penguin.example.com penguin11.2. Interface Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
Interface configuration files control the software interfaces for individual network devices. As the system boots, it uses these files to determine what interfaces to bring up and how to configure them. These files are usually named
ifcfg-name, where name refers to the name of the device that the configuration file controls.
11.2.1. Ethernet Interfaces
Link kopierenLink in die Zwischenablage kopiert!
One of the most common interface files is
/etc/sysconfig/network-scripts/ifcfg-eth0, which controls the first Ethernet network interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-ethX files (where X is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.
The following is a sample
ifcfg-eth0 file for a system using a fixed IP address:
The values required in an interface configuration file can change based on other values. For example, the
ifcfg-eth0 file for an interface using DHCP looks different because IP information is provided by the DHCP server:
DEVICE=eth0 BOOTPROTO=dhcp ONBOOT=yes
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
NetworkManager is graphical configuration tool which provides an easy way to make changes to the various network interface configuration files (see Chapter 10, NetworkManager for detailed instructions on using this tool).
However, it is also possible to manually edit the configuration files for a given network interface.
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
-
BONDING_OPTS=parameters - sets the configuration parameters for the bonding device, and is used in
/etc/sysconfig/network-scripts/ifcfg-bondN(see Section 11.2.4, “Channel Bonding Interfaces”). These parameters are identical to those used for bonding devices in/sys/class/net/bonding_device/bonding, and the module parameters for the bonding driver as described inbondingModule Directives.This configuration method is used so that multiple bonding devices can have different configurations. In Red Hat Enterprise Linux 6, place all interface-specific bonding options after theBONDING_OPTSdirective inifcfg-namefiles. See Where to specify bonding module parameters for more information. -
BOOTPROTO=protocol - where protocol is one of the following:
none— No boot-time protocol should be used.bootp— TheBOOTPprotocol should be used.dhcp— TheDHCPprotocol should be used.
BROADCAST=address- where address is the broadcast address. This directive is deprecated, as the value is calculated automatically with
ipcalc. -
DEVICE=name - where name is the name of the physical device (except for dynamically-allocated
PPPdevices where it is the logical name). DHCP_HOSTNAME=name- where name is a short host name to be sent to the
DHCPserver. Use this option only if theDHCPserver requires the client to specify a host name before receiving anIPaddress. DHCPV6C=answer- where answer is one of the following:
yes— UseDHCPto obtain anIPv6address for this interface.no— Do not useDHCPto obtain anIPv6address for this interface. This is the default value.
AnIPv6link-local address will still be assigned by default. The link-local address is based on the MAC address of the interface as per RFC 4862. DHCPV6C_OPTIONS=answer- where answer is one of the following:
-P— EnableIPv6prefix delegation.-S— UseDHCPto obtain stateless configuration only, not addresses, for this interface.-N— Restore normal operation after using the-Tor-Poptions.-T— UseDHCPto obtain a temporaryIPv6address for this interface.-D— Override the default when selecting the type of DHCP Unique Identifier (DUID) to use.By default, theDHCPclient (dhclient) creates a DHCP Unique Identifier (DUID) based on the link-layer address (DUID-LL) if it is running in stateless mode (with the-Soption, to not request an address), or it creates an identifier based on the link-layer address plus a timestamp (DUID-LLT) if it is running in stateful mode (without-S, requesting an address). The-Doption overrides this default, with a value of eitherLLorLLT.
DNS{1,2}=address- where address is a name server address to be placed in
/etc/resolv.confprovided that thePEERDNSdirective is not set tono. -
ETHTOOL_OPTS=options - where options are any device-specific options supported by
ethtool. For example, if you wanted to force 100Mb, full duplex:ETHTOOL_OPTS="autoneg off speed 100 duplex full"
ETHTOOL_OPTS="autoneg off speed 100 duplex full"Copy to Clipboard Copied! Toggle word wrap Toggle overflow Instead of a custom initscript, useETHTOOL_OPTSto set the interface speed and duplex settings. Custom initscripts run outside of the network init script lead to unpredictable results during a post-boot network service restart.Important
Changing speed or duplex settings almost always requires disabling auto-negotiation with theautoneg offoption. This option needs to be stated first, as the option entries are order-dependent.See Section 11.8, “Ethtool” for more ethtool options. HOTPLUG=answer- where answer is one of the following:
yes— This device should be activated when it is hot-plugged (this is the default option).no— This device should not be activated when it is hot-plugged.
TheHOTPLUG=nooption can be used to prevent a channel bonding interface from being activated when a bonding kernel module is loaded.See Section 11.2.4, “Channel Bonding Interfaces” for more information about channel bonding interfaces. -
HWADDR=MAC-address - where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive must be used in machines containing more than one NIC to ensure that the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with
MACADDR.Note
- Persistent device names are now handled by
/etc/udev/rules.d/70-persistent-net.rules. HWADDRmust not be used with System z network devices.- See Section 25.3.3, "Mapping subchannels and network device names", in the Red Hat Enterprise Linux 6 Installation Guide.
-
IPADDRn=address - where address is the
IPv4address and the n is expected to be consecutive positive integers starting from 0 (for example, IPADDR0). It is used for configurations with multiple IP addresses on an interface. It can be omitted if there is only one address being configured. -
IPV6ADDR=address - where address is the first static, or primary,
IPv6address on an interface.The format is Address/Prefix-length. If no prefix length is specified,/64is assumed. Note that this setting depends onIPV6INITbeing enabled. -
IPV6ADDR_SECONDARIES=address - where address is one or more, space separated, additional
IPv6addresses.The format is Address/Prefix-length. If no prefix length is specified,/64is assumed. Note that this setting depends onIPV6INITbeing enabled. -
IPV6INIT=answer - where answer is one of the following:
yes— Initialize this interface forIPv6addressing.no— Do not initialize this interface forIPv6addressing. This is the default value.This setting is required forIPv6static andDHCPassignment ofIPv6addresses. It does not affect IPv6 Stateless Address Autoconfiguration (SLAAC) as per RFC 4862.See Section D.1.14, “/etc/sysconfig/network” for information on disablingIPv6.
-
IPV6_AUTOCONF=answer - where answer is one of the following:
yes— EnableIPv6autoconf configuration for this interface.no— DisableIPv6autoconf configuration for this interface.
If enabled, anIPv6address will be requested using Neighbor Discovery (ND) from a router running theradvddaemon.Note that the default value ofIPV6_AUTOCONFdepends onIPV6FORWARDINGas follows:- If
IPV6FORWARDING=yes, thenIPV6_AUTOCONFwill default tono. - If
IPV6FORWARDING=no, thenIPV6_AUTOCONFwill default toyesandIPV6_ROUTERhas no effect.
-
IPV6_MTU=value - where value is an optional dedicated MTU for this interface.
-
IPV6_PRIVACY=rfc3041 - where rfc3041 optionally sets this interface to support RFC 3041 Privacy Extensions for Stateless Address Autoconfiguration in IPv6. Note that this setting depends on
IPV6INIToption being enabled.The default is for RFC 3041 support to be disabled. Stateless Autoconfiguration will derive addresses based on the MAC address, when available, using the modifiedEUI-64method. The address is appended to a prefix but as the address is normally derived from the MAC address it is globally unique even when the prefix changes. In the case of a link-local address the prefix isfe80::/64as per RFC 2462 IPv6 Stateless Address Autoconfiguration. -
LINKDELAY=time - where time is the number of seconds to wait for link negotiation before configuring the device. The default is 5 secs. Delays in link negotiation, caused by
STPfor example, can be overcome by increasing this value. -
MACADDR=MAC-address - where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF.This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction with the
HWADDRdirective. -
MASTER=bond-interface - where bond-interface is the channel bonding interface to which the Ethernet interface is linked.This directive is used in conjunction with the
SLAVEdirective.See Section 11.2.4, “Channel Bonding Interfaces” for more information about channel bonding interfaces. -
NETMASKn=mask - where mask is the netmask value and the n is expected to be consecutive positive integers starting from 0 (for example, NETMASK0). It is used for configurations with multiple IP addresses on an interface. It can be omitted if there is only one address being configured.
-
NETWORK=address - where address is the network address. This directive is deprecated, as the value is calculated automatically with
ipcalc. -
NM_CONTROLLED=answer - where answer is one of the following:
yes— NetworkManager is permitted to configure this device. This is the default behavior and can be omitted.no— NetworkManager is not permitted to configure this device.
Note
TheNM_CONTROLLEDdirective is now, as of Red Hat Enterprise Linux 6.3, dependent on theNM_BOND_VLAN_ENABLEDdirective in/etc/sysconfig/network. If and only if that directive is present and is one ofyes,y, ortrue, will NetworkManager detect and manage bonding and VLAN interfaces. ONBOOT=answer- where answer is one of the following:
yes— This device should be activated at boot-time.no— This device should not be activated at boot-time.
PEERDNS=answer- where answer is one of the following:
yes— Modify/etc/resolv.confif theDNSdirective is set, if usingDHCP, or if using Microsoft's RFC 1877IPCPextensions withPPP. In all casesyesis the default.no— Do not modify/etc/resolv.conf.
SLAVE=answer- where answer is one of the following:
yes— This device is controlled by the channel bonding interface specified in theMASTERdirective.no— This device is not controlled by the channel bonding interface specified in theMASTERdirective.
This directive is used in conjunction with theMASTERdirective.See Section 11.2.4, “Channel Bonding Interfaces” for more about channel bonding interfaces. -
SRCADDR=address - where address is the specified source
IPaddress for outgoing packets. -
USERCTL=answer - where answer is one of the following:
yes— Non-rootusers are allowed to control this device.no— Non-rootusers are not allowed to control this device.
11.2.2. Specific ifcfg Options for Linux on System z
Link kopierenLink in die Zwischenablage kopiert!
-
SUBCHANNELS=<read_device_bus_id>, <write_device_bus_id>, <data_device_bus_id> - where <read_device_bus_id>, <write_device_bus_id>, and <data_device_bus_id> are the three device bus IDs representing a network device.
-
PORTNAME=myname; - where myname is the Open Systems Adapter (OSA) portname or LAN Channel Station (LCS) portnumber.
CTCPROT=answer- where answer is one of the following:
0— Compatibility mode, TCP/IP for Virtual Machines (used with non-Linux peers other than IBM S/390 and IBM System z operating systems). This is the default mode.1— Extended mode, used for Linux-to-Linux Peers.3— Compatibility mode for S/390 and IBM System z operating systems.
This directive is used in conjunction with the NETTYPE directive. It specifies the CTC protocol for NETTYPE='ctc'. The default is 0. OPTION='answer'- where 'answer' is a quoted string of any valid sysfs attributes and their value. The Red Hat Enterprise Linux installer currently uses this to configure the layer mode, (layer2), and the relative port number, (portno), of QETH devices. For example:
OPTIONS='layer2=1 portno=0'
OPTIONS='layer2=1 portno=0'Copy to Clipboard Copied! Toggle word wrap Toggle overflow
11.2.3. Required ifcfg Options for Linux on System z
Link kopierenLink in die Zwischenablage kopiert!
NETTYPE=answer- where answer is one of the following:
ctc— Channel-to-Channel communication. For point-to-point TCP/IP or TTY.lcs— LAN Channel Station (LCS).qeth— QETH (QDIO Ethernet). This is the default network interface. It is the preferred installation method for supporting real or virtual OSA cards and HiperSockets devices.
11.2.4. Channel Bonding Interfaces
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux allows administrators to bind multiple network interfaces together into a single channel using the
bonding kernel module and a special network interface called a channel bonding interface. Channel bonding enables two or more network interfaces to act as one, simultaneously increasing the bandwidth and providing redundancy.
Warning
The use of direct cable connections without network switches is not supported for bonding. The failover mechanisms described here will not work as expected without the presence of network switches. See the Red Hat Knowledgebase article Why is bonding in not supported with direct connection using crossover cables? for more information.
Note
The active-backup, balance-tlb and balance-alb modes do not require any specific configuration of the switch. Other bonding modes require configuring the switch to aggregate the links. For example, a Cisco switch requires EtherChannel for Modes 0, 2, and 3, but for Mode 4 LACP and EtherChannel are required. See the documentation supplied with your switch and the
bonding.txt file in the kernel-doc package (see Section 31.9, “Additional Resources”).
11.2.4.1. Check if Bonding Kernel Module is Installed
Link kopierenLink in die Zwischenablage kopiert!
In Red Hat Enterprise Linux 6, the bonding module is not loaded by default. You can load the module by issuing the following command as
root:
modprobe --first-time bonding
~]# modprobe --first-time bondingBONDING_OPTS directive, the bonding module will be loaded as required and therefore does not need to be loaded separately.
To display information about the module, issue the following command:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
See the
modinfo bonding
~]$ modinfo bondingmodprobe(8) man page for more command options and see Chapter 31, Working with Kernel Modules for information on loading and unloading modules.
11.2.4.2. Create a Channel Bonding Interface
Link kopierenLink in die Zwischenablage kopiert!
To create a channel bonding interface, create a file in the
/etc/sysconfig/network-scripts/ directory called ifcfg-bondN, replacing N with the number for the interface, such as 0.
The contents of the file can be identical to whatever type of interface is getting bonded, such as an Ethernet interface. The only difference is that the
DEVICE directive is bondN, replacing N with the number for the interface. The NM_CONTROLLED directive can be added to prevent NetworkManager from configuring this device.
Example 11.1. Example ifcfg-bond0 interface configuration file
The following is an example of a channel bonding interface configuration file:
The MAC address of the bond will be taken from the first interface to be enslaved. It can also be specified using the HWADDR directive if required. If you want NetworkManager to control this interface, remove the
NM_CONTROLLED=no directive, or set it to yes, and add TYPE=Bond and BONDING_MASTER=yes.
After the channel bonding interface is created, the network interfaces to be bound together must be configured by adding the
MASTER and SLAVE directives to their configuration files. The configuration files for each of the channel-bonded interfaces can be nearly identical.
Example 11.2. Example ifcfg-ethX bonded interface configuration file
If two Ethernet interfaces are being channel bonded, both
eth0 and eth1 can be as follows:
In this example, replace X with the numerical value for the interface.
Once the interfaces have been configured, restart the network service to bring the bond up. As
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
root, issue the following command:
service network restart
~]# service network restart
To view the status of a bond, view the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
For example:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
/proc/ file by issuing a command in the following format: cat /proc/net/bonding/bondN
cat /proc/net/bonding/bondN
For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, see Section 31.8.1, “Using Channel Bonding”.
Support for bonding was added to NetworkManager in Red Hat Enterprise Linux 6.3. See Section 11.2.1, “Ethernet Interfaces” for an explanation of
NM_CONTROLLED and the NM_BOND_VLAN_ENABLED directive.
Important
In Red Hat Enterprise Linux 6, interface-specific parameters for the bonding kernel module must be specified as a space-separated list in the
BONDING_OPTS="bonding parameters" directive in the ifcfg-bondN interface file. Do not specify options specific to a bond in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf file.
The
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
However, the
max_bonds parameter is not interface specific and therefore, if required, should be specified in /etc/modprobe.d/bonding.conf as follows:
options bonding max_bonds=1
options bonding max_bonds=1max_bonds parameter should not be set when using ifcfg-bondN files with the BONDING_OPTS directive as this directive will cause the network scripts to create the bond interfaces as required.
Note that any changes to
/etc/modprobe.d/bonding.conf will not take effect until the module is next loaded. A running module must first be unloaded. See Chapter 31, Working with Kernel Modules for more information on loading and unloading modules.
11.2.4.2.1. Creating Multiple Bonds
Link kopierenLink in die Zwischenablage kopiert!
In Red Hat Enterprise Linux 6, for each bond a channel bonding interface is created including the
BONDING_OPTS directive. This configuration method is used so that multiple bonding devices can have different configurations. To create multiple channel bonding interfaces, proceed as follows:
- Create multiple
ifcfg-bondNfiles with theBONDING_OPTSdirective; this directive will cause the network scripts to create the bond interfaces as required. - Create, or edit existing, interface configuration files to be bonded and include the
SLAVEdirective. - Assign the interfaces to be bonded, the slave interfaces, to the channel bonding interfaces by means of the
MASTERdirective.
Example 11.3. Example multiple ifcfg-bondN interface configuration files
The following is an example of a channel bonding interface configuration file:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
In this example, replace N with the number for the bond interface. For example, to create two bonds create two configuration files,
ifcfg-bond0 and ifcfg-bond1.
Create the interfaces to be bonded as per Example 11.2, “Example ifcfg-ethX bonded interface configuration file” and assign them to the bond interfaces as required using the
MASTER=bondN directive. For example, continuing on from the example above, if two interfaces per bond are required, then for two bonds create four interface configuration files and assign the first two using MASTER=bond0 and the next two using MASTER=bond1.
11.2.5. Configuring a VLAN over a Bond
Link kopierenLink in die Zwischenablage kopiert!
This section will show configuring a VLAN over a bond consisting of two Ethernet links between a server and an Ethernet switch. The switch has a second bond to another server. Only the configuration for the first server will be shown as the other is essentially the same apart from the
IP addresses.
Warning
The use of direct cable connections without network switches is not supported for bonding. The failover mechanisms described here will not work as expected without the presence of network switches. See the Red Hat Knowledgebase article Why is bonding in not supported with direct connection using crossover cables? for more information.
Note
The active-backup, balance-tlb and balance-alb modes do not require any specific configuration of the switch. Other bonding modes require configuring the switch to aggregate the links. For example, a Cisco switch requires EtherChannel for Modes 0, 2, and 3, but for Mode 4 LACP and EtherChannel are required. See the documentation supplied with your switch and the
bonding.txt file in the kernel-doc package (see Section 31.9, “Additional Resources”).
Check the available interfaces on the server:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Procedure 11.1. Configuring the Interfaces on the Server
- Configure a slave interface using
eth0:The use of the NAME directive is optional. It is for display by a GUI interface, such as nm-connection-editor and nm-applet.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure a slave interface using
eth1:The use of the NAME directive is optional. It is for display by a GUI interface, such as nm-connection-editor and nm-applet.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure a channel bonding interface
ifcfg-bond0:The use of the NAME directive is optional. It is for display by a GUI interface, such as nm-connection-editor and nm-applet. In this example MII is used for link monitoring, see the Section 31.8.1.1, “Bonding Module Directives” section for more information on link monitoring.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Check the status of the interfaces on the server:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 11.2. Resolving Conflicts with Interfaces
The interfaces configured as slaves should not have
IP addresses assigned to them apart from the IPv6 link-local addresses (starting fe80). If you have an unexpected IP address, then there may be another configuration file with ONBOOT set to yes.
- If this occurs, issue the following command to list all
ifcfgfiles that may be causing a conflict:The above shows the expected result on a new installation. Any file having both the ONBOOT directive as well as the IPADDR or SLAVE directive will be displayed. For example, if thegrep -r "ONBOOT=yes" /etc/sysconfig/network-scripts/ | cut -f1 -d":" | xargs grep -E "IPADDR|SLAVE"
~]$ grep -r "ONBOOT=yes" /etc/sysconfig/network-scripts/ | cut -f1 -d":" | xargs grep -E "IPADDR|SLAVE" /etc/sysconfig/network-scripts/ifcfg-lo:IPADDR=127.0.0.1Copy to Clipboard Copied! Toggle word wrap Toggle overflow ifcfg-eth1file was incorrectly configured, the display might look similar to the following:grep -r "ONBOOT=yes" /etc/sysconfig/network-scripts/ | cut -f1 -d":" | xargs grep -E "IPADDR|SLAVE"
~]# grep -r "ONBOOT=yes" /etc/sysconfig/network-scripts/ | cut -f1 -d":" | xargs grep -E "IPADDR|SLAVE" /etc/sysconfig/network-scripts/ifcfg-lo:IPADDR=127.0.0.1 /etc/sysconfig/network-scripts/ifcfg-eth1:SLAVE=yes /etc/sysconfig/network-scripts/ifcfg-eth1:IPADDR=192.168.55.55Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Any other configuration files found should be moved to a different directory for backup, or assigned to a different interface by means of the HWADDR directive. After resolving any conflict set the interfaces “down” and “up” again or restart the network service as
root:If you are using NetworkManager, you might need to restart it at this point to make it forget the unwantedCopy to Clipboard Copied! Toggle word wrap Toggle overflow IPaddress. Asroot:service NetworkManager restart
~]# service NetworkManager restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 11.3. Checking the bond on the Server
- Bring up the bond on the server as
root:ifup /etc/sysconfig/network-scripts/ifcfg-bond0
~]# ifup /etc/sysconfig/network-scripts/ifcfg-bond0 Determining if ip address 192.168.100.100 is already in use for device bond0...Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Check the status of the interfaces on the server:Notice that
Copy to Clipboard Copied! Toggle word wrap Toggle overflow eth0andeth1havemaster bond0 state UPandbond0has status ofMASTER,UP. - View the bond configuration details:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Check the routes on the server:
ip route
~]$ ip route 192.168.100.0/24 dev bond0 proto kernel scope link src 192.168.100.100 169.254.0.0/16 dev bond0 scope link metric 1004Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 11.4. Configuring the VLAN on the Server
Important
At the time of writing, it is important that the bond has slaves and that they are “up” before bringing up the VLAN interface. At the time of writing, adding a VLAN interface to a bond without slaves does not work. In Red Hat Enterprise Linux 6, setting the ONPARENT directive to
yes is important to ensure that the VLAN interface does not attempt to come up before the bond is up. This is because a VLAN virtual device takes the MAC address of its parent, and when a NIC is enslaved, the bond changes its MAC address to that NIC's MAC address.
Note
A VLAN slave cannot be configured on a bond with the
fail_over_mac=follow option, because the VLAN virtual device cannot change its MAC address to match the parent's new MAC address. In such a case, traffic would still be sent with the now incorrect source MAC address.
Some older network interface cards, loopback interfaces, Wimax cards, and some Infiniband devices, are said to be VLAN challenged, meaning they cannot support VLANs. This is usually because the devices cannot cope with VLAN headers and the larger MTU size associated with VLANs.
- Create a VLAN interface file
bond0.192:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Bring up the VLAN interface as
root:ifup /etc/sysconfig/network-scripts/ifcfg-bond0.192
~]# ifup /etc/sysconfig/network-scripts/ifcfg-bond0.192 Determining if ip address 192.168.10.1 is already in use for device bond0.192...Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Enabling VLAN tagging on the network switch. Consult the documentation for the switch to see what configuration is required.
- Check the status of the interfaces on the server:Notice there is now
Copy to Clipboard Copied! Toggle word wrap Toggle overflow bond0.192@bond0in the list of interfaces and the status isMASTER,UP. - Check the route on the server:Notice there is now a route for the
ip route
~]$ ip route 192.168.100.0/24 dev bond0 proto kernel scope link src 192.168.100.100 192.168.10.0/24 dev bond0.192 proto kernel scope link src 192.168.10.1 169.254.0.0/16 dev bond0 scope link metric 1004 169.254.0.0/16 dev bond0.192 scope link metric 1005Copy to Clipboard Copied! Toggle word wrap Toggle overflow 192.168.10.0/24network pointing to the VLAN interfacebond0.192.
Configuring the Second Server
Repeat the configuration steps for the second server, using different
IP addresses but from the same subnets respectively.
Test the bond is up and the network switch is working as expected:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Testing the VLAN
To test that the network switch is configured for the VLAN, try to ping the first servers' VLAN interface:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
No packet loss suggests everything is configured correctly and that the VLAN and underlying interfaces are “up”.
Optional Steps
- If required, perform further tests by removing and replacing network cables one at a time to verify that failover works as expected. Make use of the ethtool utility to verify which interface is connected to which cable. For example:Where integer is the number of times to flash the LED on the network interface.
ethtool --identify ifname integer
ethtool --identify ifname integerCopy to Clipboard Copied! Toggle word wrap Toggle overflow - The bonding module does not support
STP, therefore consider disabling the sending of BPDU packets from the network switch. - If the system is not linked to the network except over the connection just configured, consider enabling the switch port to transition directly to sending and receiving. For example on a Cisco switch, by means of the
portfastcommand.
11.2.6. Network Bridge
Link kopierenLink in die Zwischenablage kopiert!
A network bridge is a Link Layer device which forwards traffic between networks based on MAC addresses and is therefore also referred to as a Layer 2 device. It makes forwarding decisions based on tables of MAC addresses which it builds by learning what hosts are connected to each network. A software bridge can be used within a Linux host in order to emulate a hardware bridge, for example in virtualization applications for sharing a NIC with one or more virtual NICs. This case will be illustrated here as an example.
To create a network bridge, create a file in the
/etc/sysconfig/network-scripts/ directory called ifcfg-brN, replacing N with the number for the interface, such as 0.
The contents of the file is similar to whatever type of interface is getting bridged to, such as an Ethernet interface. The differences in this example are as follows:
- The
DEVICEdirective is given an interface name as its argument in the formatbrN, where N is replaced with the number of the interface. - The
TYPEdirective is given an argumentBridge. This directive determines the device type and the argument is case sensitive. - The bridge interface configuration file now has the
IPaddress and the physical interface has only a MAC address. - An extra directive,
DELAY=0, is added to prevent the bridge from waiting while it monitors traffic, learns where hosts are located, and builds a table of MAC addresses on which to base its filtering decisions. The default delay of 15 seconds is not needed if no routing loops are possible. - The
NM_CONTROLLED=noshould be added to the Ethernet interface to prevent NetworkManager from altering the file. It can also be added to the bridge configuration file in case future versions of NetworkManager support bridge configuration.
The following is a sample bridge interface configuration file using a static
IP address:
Example 11.4. Sample ifcfg-br0 interface configuration file
To complete the bridge another interface is created, or an existing interface is modified, and pointed to the bridge interface. The following is a sample Ethernet interface configuration file pointing to a bridge interface. Configure your physical interface in
/etc/sysconfig/network-scripts/ifcfg-ethX, where X is a unique number corresponding to a specific interface, as follows:
Example 11.5. Sample ifcfg-ethX interface configuration file
Note
For the
DEVICE directive, almost any interface name could be used as it does not determine the device type. Other commonly used names include tap, dummy and bond for example. TYPE=Ethernet is not strictly required. If the TYPE directive is not set, the device is treated as an Ethernet device (unless its name explicitly matches a different interface configuration file.)
You can see Section 11.2, “Interface Configuration Files” for a review of the directives and options used in network interface config files.
Warning
If you are configuring bridging on a remote host, and you are connected to that host over the physical NIC you are configuring, please consider the implications of losing connectivity before proceeding. You will lose connectivity when restarting the service and may not be able to regain connectivity if any errors have been made. Console, or out-of-band access is advised.
Restart the networking service, in order for the changes to take effect, as follows:
service network restart
service network restart 11.2.6.1. Network Bridge with Bond
Link kopierenLink in die Zwischenablage kopiert!
An example of a network bridge formed from two or more bonded Ethernet interfaces will now be given as this is another common application in a virtualization environment. If you are not very familiar with the configuration files for bonded interfaces then please see Section 11.2.4, “Channel Bonding Interfaces”
Create or edit two or more Ethernet interface configuration files, which are to be bonded, as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Note
Using
ethX as the interface name is common practice but almost any name could be used. Names such as tap, dummy and bond are commonly used.
Create or edit one interface configuration file,
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, see Section 31.8.1, “Using Channel Bonding”.
/etc/sysconfig/network-scripts/ifcfg-bond0, as follows:
DEVICE=bond0 ONBOOT=yes BONDING_OPTS='mode=1 miimon=100' BRIDGE=br0 NM_CONTROLLED=no
DEVICE=bond0
ONBOOT=yes
BONDING_OPTS='mode=1 miimon=100'
BRIDGE=br0
NM_CONTROLLED=no
Create or edit one interface configuration file,
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
/etc/sysconfig/network-scripts/ifcfg-br0, as follows:
Figure 11.1. A network bridge consisting of two bonded Ethernet interfaces.
We now have two or more interface configuration files with the
MASTER=bond0 directive. These point to the configuration file named /etc/sysconfig/network-scripts/ifcfg-bond0, which contains the DEVICE=bond0 directive. This ifcfg-bond0 in turn points to the /etc/sysconfig/network-scripts/ifcfg-br0 configuration file, which contains the IP address, and acts as an interface to the virtual networks inside the host.
To bring up the new or recently configured interfaces, issue a command as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
root in the following format: ifup device
ifup device
Alternatively, restart the networking service, in order for the changes to take effect, as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
service network restart
~]# service network restart11.2.6.2. Network Bridge with Bonded VLAN
Link kopierenLink in die Zwischenablage kopiert!
Virtualization servers that intend to have distinct subnets for its guests while still ensuring availability in the event of a NIC failure will often combine bonds, VLANs, and bridges. An example of this configuration will now be given. By creating a bridge on the VLAN instead of the underlying device we allow VLAN tagging to be handled entirely through the host with no need to configure the guests' interfaces.
- Ensure the bond and VLAN have been configured as outlined in Section 11.2.5, “Configuring a VLAN over a Bond”.
- Create the bridge's configuration file,
ifcfg-br0:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Adjust the VLAN's configuration file,
ifcfg-bond0.192from the earlier example, to use the newly createdbr0as its master:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - To bring up the new or recently configured interfaces, issue a command as
rootin the following format:ifup device
ifup deviceCopy to Clipboard Copied! Toggle word wrap Toggle overflow Alternatively, restart the networking service, in order for the changes to take effect, as follows:service network restart
~]# service network restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
11.2.7. Setting Up 802.1Q VLAN Tagging
Link kopierenLink in die Zwischenablage kopiert!
- If required, start the VLAN 8021q module by issuing the following command as
root:No visual output indicates the module was not running and has now been loaded. Note that given a correct configuration file, the VLAN 8021q module will be loaded as required and therefore does not need to be loaded separately.modprobe --first-time 8021q
~]# modprobe --first-time 8021qCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure your physical interface in
/etc/sysconfig/network-scripts/ifcfg-ethX, where X is a unique number corresponding to a specific interface, as follows:DEVICE=ethX TYPE=Ethernet BOOTPROTO=none ONBOOT=yes
DEVICE=ethX TYPE=Ethernet BOOTPROTO=none ONBOOT=yesCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure the VLAN interface configuration in
/etc/sysconfig/network-scripts. The configuration filename should be the physical interface plus a.character plus the VLAN ID number. For example, if the VLAN ID is 192, and the physical interface iseth0, then the configuration filename should beifcfg-eth0.192:Copy to Clipboard Copied! Toggle word wrap Toggle overflow If there is a need to configure a second VLAN, with for example, VLAN ID 193, on the same interface,eth0, add a new file with the nameeth0.193with the VLAN configuration details. - Restart the networking service, in order for the changes to take effect. Issue the following command as
root:service network restart
~]# service network restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
11.2.8. Alias and Clone Files
Link kopierenLink in die Zwischenablage kopiert!
Two lesser-used types of interface configuration files are alias and clone files. As the ip utility now supports assigning multiple addresses to the same interface it is no longer necessary to use this method of binding multiple addresses to the same interface. The ip command to assign an address can be repeated multiple times in order to assign multiple address. For example:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
The commands for the ip utility, sometimes referred to as iproute2 after the upstream package name, are documented in the
man ip(8) page. The package name in Red Hat Enterprise Linux 6 is iproute.
Note
In Red Hat Enterprise Linux 6, NetworkManager now reads
ifcfg alias files and assigns the addresses in them to their master interface, using the alias name as the address label. For example, if ifcfg-eth0 and ifcfg-eth0:1 files are present, NetworkManager reads the alias file's DEVICE line and stores this as an address label. The use of secondary addresses rather than alias is still preferred.
For new installations, users should select the Manual method on the
IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, see Chapter 10, NetworkManager.
Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the
ifcfg-if-name:alias-value naming scheme.
For example, an
ifcfg-eth0:0 file could be configured to specify DEVICE=eth0:0 and a static IP address of 10.0.0.2, serving as an alias of an Ethernet interface already configured to receive its IP information via DHCP in ifcfg-eth0. Under this configuration, eth0 is bound to a dynamic IP address, but the same physical network card can receive requests via the fixed, 10.0.0.2 IP address.
Warning
Alias interfaces do not support
DHCP.
A clone interface configuration file should use the following naming convention:
ifcfg-if-name-clone-name. While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard DHCP Ethernet interface called eth0, may look similar to this:
DEVICE=eth0 ONBOOT=yes BOOTPROTO=dhcp
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
Since the default value for the
USERCTL directive is no if it is not specified, users cannot bring this interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user:
USERCTL=yes
USERCTL=yes
This way a user can bring up the
eth0 interface using the /sbin/ifup eth0-user command because the configuration options from ifcfg-eth0 and ifcfg-eth0-user are combined. While this is a very basic example, this method can be used with a variety of options and interfaces.
It is no longer possible to create alias and clone interface configuration files using a graphical tool. However, as explained at the beginning of this section, it is no longer necessary to use this method as it is now possible to directly assign multiple
IP address to the same interface. For new installations, users should select the Manual method on the IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, see Chapter 10, NetworkManager.
11.2.9. Dialup Interfaces
Link kopierenLink in die Zwischenablage kopiert!
If you are connecting to the Internet via a dialup connection, a configuration file is necessary for the interface.
PPP interface files are named using the following format:
ifcfg-pppX- where X is a unique number corresponding to a specific interface.
The
PPP interface configuration file is created automatically when wvdial, or Kppp is used to create a dialup account. It is also possible to create and edit this file manually.
The following is a typical
/etc/sysconfig/network-scripts/ifcfg-ppp0 file:
Serial Line Internet Protocol (SLIP) is another dialup interface, although it is used less frequently.
SLIP files have interface configuration file names such as ifcfg-sl0.
Other options that may be used in these files include:
-
DEFROUTE=answer - where answer is one of the following:
yes— Set this interface as the default route.no— Do not set this interface as the default route.
DEMAND=answer- where answer is one of the following:
yes— This interface allowspppdto initiate a connection when someone attempts to use it.no— A connection must be manually established for this interface.
IDLETIMEOUT=value- where value is the number of seconds of idle activity before the interface disconnects itself.
INITSTRING=string- where string is the initialization string passed to the modem device. This option is primarily used in conjunction with
SLIPinterfaces. LINESPEED=value- where value is the baud rate of the device. Possible standard values include
57600,38400,19200, and9600. MODEMPORT=device- where device is the name of the serial device that is used to establish the connection for the interface.
MTU=value- where value is the Maximum Transfer Unit (MTU) setting for the interface. The MTU refers to the largest number of bytes of data a frame can carry, not counting its header information. In some dialup situations, setting this to a value of
576results in fewer packets dropped and a slight improvement to the throughput for a connection. NAME=name- where name is the reference to the title given to a collection of dialup connection configurations.
PAPNAME=name- where name is the user name given during the Password Authentication Protocol (PAP) exchange that occurs to allow connections to a remote system.
PERSIST=answer- where answer is one of the following:
yes— This interface should be kept active at all times, even if deactivated after a modem hang up.no— This interface should not be kept active at all times.
REMIP=address- where address is the
IPaddress of the remote system. This is usually left unspecified. -
WVDIALSECT=name - where name associates this interface with a dialer configuration in
/etc/wvdial.conf. This file contains the phone number to be dialed and other important information for the interface.
11.2.10. Other Interfaces
Link kopierenLink in die Zwischenablage kopiert!
Other common interface configuration files include the following:
ifcfg-lo- A local loopback interface is often used in testing, as well as being used in a variety of applications that require an
IPaddress pointing back to the same system. Any data sent to the loopback device is immediately returned to the host's network layer.Warning
The loopback interface script,/etc/sysconfig/network-scripts/ifcfg-lo, should never be edited manually. Doing so can prevent the system from operating correctly. ifcfg-irlan0- An infrared interface allows information between devices, such as a laptop and a printer, to flow over an infrared link. This works in a similar way to an Ethernet device except that it commonly occurs over a peer-to-peer connection.
ifcfg-plip0- A Parallel Line Interface Protocol (PLIP) connection works much the same way as an Ethernet device, except that it utilizes a parallel port.
Interface configuration files for Linux on System z include the following:
ifcfg-hsiN- A HiperSockets interface is an interface for high-speed TCP/IP communication within and across z/VM guest virtual machines and logical partitions (LPARs) on an IBM System z mainframe.
11.3. Interface Control Scripts
Link kopierenLink in die Zwischenablage kopiert!
The interface control scripts activate and deactivate system interfaces. There are two primary interface control scripts that call on control scripts located in the
/etc/sysconfig/network-scripts/ directory: /sbin/ifdown and /sbin/ifup.
The
ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When either of these scripts are called, they require the value of the interface to be specified, such as:
ifup eth0
ifup eth0Warning
The
ifup and ifdown interface scripts are the only scripts that the user should use to bring up and take down network interfaces.
The following scripts are described for reference purposes only.
Two files used to perform a variety of network initialization tasks during the process of bringing up a network interface are
/etc/rc.d/init.d/functions and /etc/sysconfig/network-scripts/network-functions. See Section 11.7, “Network Function Files” for more information.
After verifying that an interface has been specified and that the user executing the request is allowed to control the interface, the correct script brings the interface up or down. The following are common interface control scripts found within the
/etc/sysconfig/network-scripts/ directory:
ifup-aliases- Configures
IPaliases from interface configuration files when more than oneIPaddress is associated with an interface. ifup-ipppandifdown-ippp- Brings ISDN interfaces up and down.
ifup-ipv6andifdown-ipv6- Brings
IPv6interfaces up and down. ifup-plip- Brings up a
PLIPinterface. ifup-plusb- Brings up a USB interface for network connections.
ifup-postandifdown-post- Contains commands to be executed after an interface is brought up or down.
ifup-pppandifdown-ppp- Brings a
PPPinterface up or down. ifup-routes- Adds static routes for a device as its interface is brought up.
ifdown-sitandifup-sit- Contains function calls related to bringing up and down an
IPv6tunnel within anIPv4connection. ifup-wireless- Brings up a wireless interface.
Warning
Removing or modifying any scripts in the
/etc/sysconfig/network-scripts/ directory can cause interface connections to act irregularly or fail. Only advanced users should modify scripts related to a network interface.
The easiest way to manipulate all network scripts simultaneously is to use the
/sbin/service command on the network service (/etc/rc.d/init.d/network), as illustrated by the following command:
/sbin/service network action
/sbin/service network action
Here, action can be either
start, stop, or restart.
To view a list of configured devices and currently active network interfaces, use the following command:
/sbin/service network status
/sbin/service network status11.4. Static Routes and the Default Gateway
Link kopierenLink in die Zwischenablage kopiert!
Static routes are for traffic that must not, or should not, go through the default gateway. Routing is often handled by devices on the network dedicated to routing (although any device can be configured to perform routing). Therefore, it is often not necessary to configure static routes on Red Hat Enterprise Linux servers or clients. Exceptions include traffic that must pass through an encrypted VPN tunnel or traffic that should take a specific route for reasons of cost or security. The default gateway is for any and all traffic which is not destined for the local network and for which no preferred route is specified in the routing table. The default gateway is traditionally a dedicated network router.
Configuring Static Routes Using the Command Line
If static routes are required, they can be added to the routing table by means of the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
See the
ip route add command and removed using the ip route del command. The more frequently used ip route commands take the following form: ip route [ add | del | change | append | replace ] destination-address
ip route [ add | del | change | append | replace ] destination-addressip-route(8) man page for more details on the options and formats.
Use the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
ip route command without options to display the IP routing table. For example:
ip route
~]$ ip route
default via 192.168.122.1 dev eth0 proto static metric 1024
192.168.122.0/24 dev ens9 proto kernel scope link src 192.168.122.107
192.168.122.0/24 dev eth0 proto kernel scope link src 192.168.122.126
To add a static route to a host address, in other words to a single
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where 192.0.2.1 is the
IP address, issue a command as root:
ip route add 192.0.2.1 via 10.0.0.1 [dev ifname]
~]# ip route add 192.0.2.1 via 10.0.0.1 [dev ifname]IP address of the host in dotted decimal notation, 10.0.0.1 is the next hop address and ifname is the exit interface leading to the next hop.
To add a static route to a network, in other words to an
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
where 192.0.2.0 is the
IP address representing a range of IP addresses, issue the following command as root:
ip route add 192.0.2.0/24 via 10.0.0.1 [dev ifname]
~]# ip route add 192.0.2.0/24 via 10.0.0.1 [dev ifname]IP address of the destination network in dotted decimal notation and /24 is the network prefix. The network prefix is the number of enabled bits in the subnet mask. This format of network address slash network prefix length is sometimes referred to as classless inter-domain routing (CIDR) notation.
Static route configuration can be stored per-interface in a
/etc/sysconfig/network-scripts/route-interface file. For example, static routes for the eth0 interface would be stored in the /etc/sysconfig/network-scripts/route-eth0 file. The route-interface file has two formats: ip command arguments and network/netmask directives. These are described below.
See the
ip-route(8) man page for more information on the ip route command.
Configuring The Default Gateway
The default gateway is determined by the network scripts which parse the
/etc/sysconfig/network file first and then the network interface ifcfg files for interfaces that are “up”. The ifcfg files are parsed in numerically ascending order, and the last GATEWAY directive to be read is used to compose a default route in the routing table.
The default route can thus be indicated by means of the GATEWAY directive and can be specified either globally or in interface-specific configuration files. Specifying the gateway globally has certain advantages in static networking environments, especially if more than one network interface is present. It can make fault finding simpler if applied consistently. There is also the GATEWAYDEV directive, which is a global option. If multiple devices specify GATEWAY, and one interface uses the GATEWAYDEV directive, that directive will take precedence. This option is not recommend as it can have unexpected consequences if an interface goes down and it can complicate fault finding.
In dynamic network environments, where mobile hosts are managed by NetworkManager, gateway information is likely to be interface specific and is best left to be assigned by
DHCP. In special cases where it is necessary to influence NetworkManager's selection of the exit interface to be used to reach a gateway, make use of the DEFROUTE=no command in the ifcfg files for those interfaces which do not lead to the default gateway.
Global default gateway configuration is stored in the
/etc/sysconfig/network file. This file specifies gateway and host information for all network interfaces. For more information about this file and the directives it accepts, see Section D.1.14, “/etc/sysconfig/network”.
11.5. Configuring Static Routes in ifcfg files
Link kopierenLink in die Zwischenablage kopiert!
Static routes set using ip commands at the command prompt will be lost if the system is shutdown or restarted. To configure static routes to be persistent after a system restart, they must be placed in per-interface configuration files in the
/etc/sysconfig/network-scripts/ directory. The file name should be of the format route-ifname. There are two types of commands to use in the configuration files; ip commands as explained in Section 11.5.1, “Static Routes Using the IP Command Arguments Format” and the Network/Netmask format as explained in Section 11.5.2, “Network/Netmask Directives Format”.
11.5.1. Static Routes Using the IP Command Arguments Format
Link kopierenLink in die Zwischenablage kopiert!
If required in a per-interface configuration file, for example
/etc/sysconfig/network-scripts/route-eth0, define a route to a default gateway on the first line. This is only required if the gateway is not set via DHCP and is not set globally in the /etc/sysconfig/network file:
default via 192.168.1.1 dev interface
default via 192.168.1.1 dev interface
where 192.168.1.1 is the
IP address of the default gateway. The interface is the interface that is connected to, or can reach, the default gateway. The dev option can be omitted, it is optional. Note that this setting takes precedence over a setting in the /etc/sysconfig/network file.
If a route to a remote network is required, a static route can be specified as follows. Each line is parsed as an individual route:
10.10.10.0/24 via 192.168.1.1 [dev interface]
10.10.10.0/24 via 192.168.1.1 [dev interface]
where 10.10.10.0/24 is the network address and prefix length of the remote or destination network. The address 192.168.1.1 is the
IP address leading to the remote network. It is preferably the next hop address but the address of the exit interface will work. The “next hop” means the remote end of a link, for example a gateway or router. The dev option can be used to specify the exit interface interface but it is not required. Add as many static routes as required.
The following is an example of a
route-interface file using the ip command arguments format. The default gateway is 192.168.0.1, interface eth0 and a leased line or WAN connection is available at 192.168.0.10. The two static routes are for reaching the 10.10.10.0/24 network and the 172.16.1.10/32 host:
default via 192.168.0.1 dev eth0 10.10.10.0/24 via 192.168.0.10 dev eth0 172.16.1.10/32 via 192.168.0.10 dev eth0
default via 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.10 dev eth0
172.16.1.10/32 via 192.168.0.10 dev eth0
In the above example, packets going to the local
192.168.0.0/24 network will be directed out the interface attached to that network. Packets going to the 10.10.10.0/24 network and 172.16.1.10/32 host will be directed to 192.168.0.10. Packets to unknown, remote, networks will use the default gateway therefore static routes should only be configured for remote networks or hosts if the default route is not suitable. Remote in this context means any networks or hosts that are not directly attached to the system.
Specifying an exit interface is optional. It can be useful if you want to force traffic out of a specific interface. For example, in the case of a VPN, you can force traffic to a remote network to pass through a tun0 interface even when the interface is in a different subnet to the destination network.
Important
If the default gateway is already assigned from
DHCP, the IP command arguments format can cause one of two errors during start-up, or when bringing up an interface from the down state using the ifup command: "RTNETLINK answers: File exists" or 'Error: either "to" is a duplicate, or "X.X.X.X" is a garbage.', where X.X.X.X is the gateway, or a different IP address. These errors can also occur if you have another route to another network using the default gateway. Both of these errors are safe to ignore.
11.5.2. Network/Netmask Directives Format
Link kopierenLink in die Zwischenablage kopiert!
You can also use the network/netmask directives format for
route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:
ADDRESS0=10.10.10.0 NETMASK0=255.255.255.0 GATEWAY0=192.168.1.1
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.1.1ADDRESS0=10.10.10.0is the network address of the remote network or host to be reached.NETMASK0=255.255.255.0is the netmask for the network address defined withADDRESS0=10.10.10.0.GATEWAY0=192.168.1.1is the default gateway, or anIPaddress that can be used to reachADDRESS0=10.10.10.0
The following is an example of a
route-interface file using the network/netmask directives format. The default gateway is 192.168.0.1 but a leased line or WAN connection is available at 192.168.0.10. The two static routes are for reaching the 10.10.10.0/24 and 172.16.1.0/24 networks:
Subsequent static routes must be numbered sequentially, and must not skip any values. For example,
ADDRESS0, ADDRESS1, ADDRESS2, and so on.
11.6. Configuring IPv6 Tokenized Interface Identifiers
Link kopierenLink in die Zwischenablage kopiert!
In a network, servers are generally given static addresses and these are usually configured manually to avoid relying on a
DHCP server which may fail or run out of addresses. The IPv6 protocol introduced Stateless Address Autoconfiguration (SLAAC) which enables clients to assign themselves an address without relying on a DHCPv6 server. SLAAC derives the IPv6 address based on the interface hardware, therefore it should not be used for servers in case the hardware is changed and the associated SLAAC generated address changes with it. In an IPv6 environment, if the network prefix is changed, or the system is moved to a new location, any manually configured static addresses would have to be edited due to the changed prefix.
To address these problems, the IETF draft Tokenised IPv6 Identifiers has been implemented in the kernel together with corresponding additions to the
ip utility. This enables the lower 64 bit interface identifier part of the IPv6 address to be based on a token, supplied by the administrator, leaving the network prefix, the higher 64 bits, to be obtained from router advertisements (RA). This means that if the network interface hardware is changed, the lower 64 bits of the address will not change, and if the system is moved to another network, the network prefix will be obtained from router advertisements automatically, thus no manual editing is required.
To configure an interface to use a tokenized
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where
IPv6 identifier, issue a command in the following format as root user:
ip token set ::1a:2b:3c:4d/64 dev eth4
~]# ip token set ::1a:2b:3c:4d/64 dev eth4::1a:2b:3c:4d/64 is the token to be used. This setting is not persistent. To make it persistent, add the command to an init script. See Section 11.3, “Interface Control Scripts”.
Using a memorable token is possible, but is limited to the range of valid hexadecimal digits. For example, for a
DNS server, which traditionally uses port 53, a token of ::53/64 could be used.
To view all the configured
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
IPv6 tokens, issue the following command:
To view the configured
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
IPv6 token for a specific interface, issue the following command:
ip token get dev eth4
~]$ ip token get dev eth4
token ::1a:2b:3c:4d dev eth4
Note that adding a token to an interface will replace a previously allocated token, and in turn invalidate the address derived from it. Supplying a new token causes a new address to be generated and applied, but this process will leave any other addresses unchanged. In other words, a new tokenized identifier only replaces a previously existing tokenized identifier, not any other
IP address.
Note
Take care not to add the same token to more than one system or interface as the duplicate address detection (DAD) mechanism will not be able to resolve the problem. Once a token is set, it cannot be cleared or reset, except by rebooting the machine.
11.7. Network Function Files
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux makes use of several files that contain important common functions used to bring interfaces up and down. Rather than forcing each interface control file to contain these functions, they are grouped together in a few files that are called upon when necessary.
The
/etc/sysconfig/network-scripts/network-functions file contains the most commonly used IPv4 functions, which are useful to many interface control scripts. These functions include contacting running programs that have requested information about changes in the status of an interface, setting host names, finding a gateway device, verifying whether or not a particular device is down, and adding a default route.
As the functions required for
IPv6 interfaces are different from IPv4 interfaces, a /etc/sysconfig/network-scripts/network-functions-ipv6 file exists specifically to hold this information. The functions in this file configure and delete static IPv6 routes, create and remove tunnels, add and remove IPv6 addresses to an interface, and test for the existence of an IPv6 address on an interface.
11.8. Ethtool
Link kopierenLink in die Zwischenablage kopiert!
Ethtool is a utility for configuration of Network Interface Cards (NICs). This utility allows querying and changing settings such as speed, port, auto-negotiation, PCI locations and checksum offload on many network devices, especially Ethernet devices.
We present here a short selection of often used ethtool commands together with some useful commands that are not well known. For a full list of commands type
ethtool -h or see the man page, ethtool(8), for a more comprehensive list and explanation. The first two examples are information queries and show the use of the different formats of the command.
But first, the command structure:
ethtool [option...] devname
ethtool [option...] devname
where option is none or more options, and devname is your Network Interface Card (NIC). For example eth0 or em1.
Some values can only be obtained when the command is run as
root. Here is an example of the output when the command is run as root:
Issue the following command, using the short or long form of the argument, to query the specified network device for associated driver information:
ethtool -i, --driver devname
ethtool -i, --driver devname
where devname is your Network Interface Card (NIC). For example eth0 or em1.
Here is an example of the output:
Here follows a list of command options to query, identify or reset the device. They are in the usual
-short and --long form:
-
--statistics - The
--statisticsor-Squeries the specified network device for NIC and driver statistics. It takes the following form:-S, --statistics devname
-S, --statistics devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where devname is your NIC. -
--identify - The
--identifyor-poption initiates adapter-specific action intended to enable an operator to easily identify the adapter by sight. Typically this involves blinking one or more LEDs on the specified network port. It takes the following form:-p, --identify devname integer
-p, --identify devname integerCopy to Clipboard Copied! Toggle word wrap Toggle overflow where integer is length of time in seconds to perform the action,and devname is your NIC. -
--show-time-stamping - The
--show-time-stampingor-Toption queries the specified network device for time stamping parameters. It takes the following form:-T, --show-time-stamping devname
-T, --show-time-stamping devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where devname is your NIC. -
--show-offload - The
--show-features, or--show-offload, or-koption queries the specified network device for the state of protocol offload and other features. It takes the following form:-k, --show-features, --show-offload devname
-k, --show-features, --show-offload devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where devname is your NIC. -
--test - The
--testor-toption is used to perform tests on a Network Interface Card. It takes the following form:-t, --test devname word
-t, --test devname wordCopy to Clipboard Copied! Toggle word wrap Toggle overflow where word is one of the following:offline— Perform a comprehensive set of tests. Service will be interrupted.online— Perform a reduced set of tests. Service should not be interrupted.external_lb— Perform full set of tests including loopback tests while fitted with a loopback cable.
and devname is your NIC.
Changing some or all settings of the specified network device requires the
-s or --change option. All the following options are only applied if the -s or --change option is also specified. For the sake of clarity we will omit it here.
To make these settings permanent you can make use of the
ETHTOOL_OPTS directive. It can be used in interface configuration files to set the desired options when the network interface is brought up. See Section 11.2.1, “Ethernet Interfaces” for more details on how to use this directive.
-
--offload - The
--features, or--offload, or-Koption changes the offload parameters and other features of the specified network device. It takes the following form:-K, --features, --offload devname feature boolean
-K, --features, --offload devname feature booleanCopy to Clipboard Copied! Toggle word wrap Toggle overflow where feature is a built-in or kernel supplied feature,boolean is one ofONorOFF,and devname is your NIC.Theethtool(8)man page lists most features. As the feature set is dependent on the NIC driver, you should consult the driver documentation for features not listed in the man page. -
--speed - The
--speedoption is used to set the speed in megabits per second (Mb/s). Omitting the speed value will show the supported device speeds. It takes the following form:--speed number devname
--speed number devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where number is the speed in megabits per second (Mb/s),and devname is your NIC. -
--duplex - The
--duplexoption is used to set the transmit and receive mode of operation. It takes the following form:--duplex word devname
--duplex word devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where word is one of the following:half— Sets half-duplex mode. Usually used when connected to a hub.full— Sets full-duplex mode. Usually used when connected to a switch or another host.
and devname is your NIC. -
--port - The
--portoption is used to select the device port . It takes the following form:--port value devname
--port value devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where value is one of the following:tp— An Ethernet interface using Twisted-Pair cable as the medium.aui— Attachment Unit Interface (AUI). Normally used with hubs.bnc— An Ethernet interface using BNC connectors and co-axial cable.mii— An Ethernet interface using a Media Independent Interface (MII).fibre— An Ethernet interface using Optical Fibre as the medium.
and devname is your NIC. -
--autoneg - The
--autonegoption is used to control auto-negotiation of network speed and mode of operation (full-duplex or half-duplex mode). If auto-negotiation is enabled you can initiate re-negotiation of network speeds and mode of operation by using the-r, --negotiateoption. You can display the auto-negotiation state using the--a, --show-pauseoption.It takes the following form:--autoneg value devname
--autoneg value devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where value is one of the following:yes— Allow auto-negotiating of network speed and mode of operation.no— Do not allow auto-negotiating of network speed and mode of operation.
and devname is your NIC. -
--advertise - The
--advertiseoption is used to set what speeds and modes of operation (duplex mode) are advertised for auto-negotiation. The argument is one or more hexadecimal values from Table 11.1, “Ethtool advertise options: speed and mode of operation”.It takes the following form:--advertise option devname
--advertise option devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where option is one or more of the hexadecimal values from the table below and devname is your NIC.Expand Table 11.1. Ethtool advertise options: speed and mode of operation Hex Value Speed Duplex Mode IEEE standard? 0x001 10 HalfYes 0x002 10 FullYes 0x004 100 HalfYes 0x008 100 FullYes 0x010 1000 Half No 0x020 1000 FullYes 0x8000 2500 Full Yes 0x1000 10000 Full Yes 0x20000 20000MLD2 Full No 0x20000 20000MLD2 Full No 0x40000 20000KR2 Full No -
--phyad - The
--phyadoption is used to change the physical address. Often referred to as the MAC or hardware address but in this context referred to as the physical address.It takes the following form:--phyad physical_address devname
--phyad physical_address devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where physical_address is the physical address in hexadecimal format and devname is your NIC. -
--xcvr - The
--xcvroption is used to select the transceiver type. Currently only “internal” and “external” can be specified. In the future other types might be added.It takes the following form:--xcvr word devname
--xcvr word devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where word is one of the following:internal— Use internal transceiver.external— Use external transceiver.
and devname is your NIC. -
--wol - The
--woloption is used to set “Wake-on-LAN” options. Not all devices support this. The argument to this option is a string of characters specifying which options to enable.It takes the following form:--wol value devname
--wol value devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where value is one or more of the following:p— Wake on PHY activity.u— Wake on unicast messages.m— Wake on multicast messages.b— Wake on broadcast messages.g— Wake-on-Lan; wake on receipt of a "magic packet".s— Enable security function using password for Wake-on-Lan.d— Disable Wake-on-Lan and clear all settings.
and devname is your NIC. -
--sopass - The
--sopassoption is used to set the “SecureOn” password. The argument to this option must be 6 bytes in Ethernet MAC hexadecimal format (xx:yy:zz:aa:bb:cc).It takes the following form:--sopass xx:yy:zz:aa:bb:cc devname
--sopass xx:yy:zz:aa:bb:cc devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where xx:yy:zz:aa:bb:cc is the password in the same format as a MAC address and devname is your NIC. -
--msglvl - The
--msglvloption is used to set the driver message-type flags by name or number. The precise meanings of these type flags differ between drivers.It takes the following form:--msglvl message_type devname
--msglvl message_type devnameCopy to Clipboard Copied! Toggle word wrap Toggle overflow where message_type is one of:- message type name in plain text.
- hexadecimal number indicating the message type.
and devname is your NIC.The defined message type names and numbers are shown in the table below:Expand Table 11.2. Driver message type Message Type Hex Value Description drv 0x0001 General driver statusprobe 0x0002 Hardware probinglink 0x0004 Link statetimer 0x0008 Periodic status checkifdown 0x0010 Interface being brought down ifup 0x0020 Interface being brought uprx_err 0x0040 Receive error tx_err 0x0080 Transmit error intr 0x0200 Interrupt handling tx_done 0x0400 Transmit completion rx_status 0x0800 Receive completion pktdata 0x1000 Packet contents hw 0x2000 Hardware status wol 0x4000 Wake-on-LAN status
11.9. Configuring netconsole
Link kopierenLink in die Zwischenablage kopiert!
The
netconsole kernel module enables logging of kernel messages over the network to another computer. It allows kernel debugging when disk logging fails or when using the serial console is not possible.
Configuring a Listening Machine
To enable receiving
netconsole logging messages, install the rsyslog package:
yum install rsyslog
]# yum install rsyslog
To configure
rsyslogd to listen on the 514/UDP port and receive messages from the network, uncomment the following lines in the MODULES section of /etc/rsyslog.conf:
$ModLoad imudp $UDPServerRun 514
$ModLoad imudp
$UDPServerRun 514
Restart the
rsyslogd service for the changes to take effect:
service rsyslog restart
]# service rsyslog restart
To verify that
rsyslogd is listening on the 514/udp port, use the following command:
netstat -l | grep syslog
]# netstat -l | grep syslog
udp 0 0 *:syslog *:*
udp 0 0 *:syslog *:*
The
0 *:syslog value in the netstat -l output mean that rsyslogd is listening on default netconsole port, which is defined in the /etc/services file:
]$ cat /etc/services | grep syslog syslog 514/udp syslog-conn 601/tcp # Reliable Syslog Service syslog-conn 601/udp # Reliable Syslog Service syslog-tls 6514/tcp # Syslog over TLS
]$ cat /etc/services | grep syslog
syslog 514/udp
syslog-conn 601/tcp # Reliable Syslog Service
syslog-conn 601/udp # Reliable Syslog Service
syslog-tls 6514/tcp # Syslog over TLSConfiguring a Sending Machine
In Red Hat Enterprise Linux 6,
netconsole is configured using the file /etc/sysconfig/netconsole, which is part of the initscripts package. This package is installed by default and it also provides the netconsole service.
To configure a sending machine, set the value of the
SYSLOGADDR variable in the /etc/sysconfig/netconsole file to match the IP address of the syslogd server, for example:
SYSLOGADDR=192.168.0.1
SYSLOGADDR=192.168.0.1
Restart the
netconsole service so the changes take effect. Then, use the chkconfig command to ensure netconsole service starts automatically after next reboot:
service netconsole restart chkconfig netconsole on
]# service netconsole restart
Initializing netconsole [ OK ]
]# chkconfig netconsole on
By default, the
rsyslogd server writes the netconsole messages from the client in /var/log/messages or in the file specified in rsyslog.conf.
Note
To set
rsyslogd and netconsole to use a different port, change the following line in /etc/rsyslog.conf to the desired port number:
$UDPServerRun <PORT>
$UDPServerRun <PORT>
On the sending machine, uncomment and edit the following line in the
/etc/sysconfig/netconsole file:
SYSLOGPORT=514
SYSLOGPORT=514
For more information about
netconsole configuration and troubleshooting tips, see Netconsole Kernel Documentation.
11.10. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
The following are resources which explain more about network interfaces.
Installed Documentation
/usr/share/doc/initscripts-version/sysconfig.txt— A guide to available options for network configuration files, includingIPv6options not covered in this chapter.
Online Resources
- http://linux-ip.net/gl/ip-cref/ — This document contains a wealth of information about the
ipcommand, which can be used to manipulate routing tables, among other things. - Red Hat Access Labs — The Red Hat Access Labs includes a “Network Bonding Helper”.
See Also
- Appendix E, The proc File System — Describes the sysctl utility and the virtual files within the
/proc/directory, which contain networking parameters and statistics among other things.
Part V. Infrastructure Services
Link kopierenLink in die Zwischenablage kopiert!
This part provides information how to configure services and daemons, configure authentication, and enable remote logins.
Chapter 12. Services and Daemons
Link kopierenLink in die Zwischenablage kopiert!
Maintaining security on your system is extremely important, and one approach for this task is to manage access to system services carefully. Your system may need to provide open access to particular services (for example,
httpd if you are running a web server). However, if you do not need to provide a service, you should turn it off to minimize your exposure to possible bug exploits.
This chapter explains the concept of runlevels, and describes how to set the default one. It also covers the setup of the services to be run in each of these runlevels, and provides information on how to start, stop, and restart the services on the command line using the service command.
Important
When you allow access for new services, always remember that both the firewall and SELinux need to be configured as well. One of the most common mistakes committed when configuring a new service is neglecting to implement the necessary firewall configuration and SELinux policies to allow access for it. For more information, see the Red Hat Enterprise Linux 6 Security Guide.
12.1. Configuring the Default Runlevel
Link kopierenLink in die Zwischenablage kopiert!
A runlevel is a state, or mode, defined by services that are meant to be run when this runlevel is selected. Seven numbered runlevels exist (indexed from 0):
| Runlevel | Description |
|---|---|
0 | Used to halt the system. This runlevel is reserved and cannot be changed. |
1 | Used to run in a single-user mode. This runlevel is reserved and cannot be changed. |
2 | Not used by default. You are free to define it yourself. |
3 | Used to run in a full multi-user mode with a command-line user interface. |
4 | Not used by default. You are free to define it yourself. |
5 | Used to run in a full multi-user mode with a graphical user interface. |
6 | Used to reboot the system. This runlevel is reserved and cannot be changed. |
To check in which runlevel you are operating, type the following:
runlevel
~]$ runlevel
N 5
The
runlevel command displays previous and current runlevel. In this case it is number 5, which means the system is running in a full multi-user mode with a graphical user interface.
The default runlevel can be changed by modifying the
/etc/inittab file, which contains a line near the end of the file similar to the following:
id:5:initdefault:
id:5:initdefault:
To do so, edit this file as
root and change the number on this line to the desired value. The change will take effect the next time you reboot the system.
12.2. Configuring the Services
Link kopierenLink in die Zwischenablage kopiert!
To allow you to configure which services are started at boot time, Red Hat Enterprise Linux is shipped with the following utilities: the Service Configuration graphical application, the ntsysv text user interface, and the chkconfig command-line tool.
Important
To ensure optimal performance on POWER architecture, it is recommended that the
irqbalance service is enabled. In most cases, this service is installed and configured to run during the Red Hat Enterprise Linux 6 installation. To verify that irqbalance is running, as root, type the following at a shell prompt:
service irqbalance status
~]# service irqbalance status
irqbalance (pid 1234) is running...
For information on how to enable and run a service using a graphical user interface, see Section 12.2.1, “Using the Service Configuration Utility”. For instructions on how to perform these task on the command line, see Section 12.2.3, “Using the chkconfig Utility” and Section 12.3, “Running Services” respectively.
12.2.1. Using the Service Configuration Utility
Link kopierenLink in die Zwischenablage kopiert!
The Service Configuration utility is a graphical application developed by Red Hat to configure which services are started in a particular runlevel, as well as to start, stop, and restart them from the menu. To start the utility, select → → from the panel, or type the command
system-config-services at a shell prompt.
Note
The
system-config-services utility is provided by the system-config-services package, which may not be installed by default on your version of Red Hat Enterprise Linux. To ensure that, first run the following command:
rpm -q system-config-services
~]$ rpm -q system-config-services
If the package is not installed by default, install it manually by running the following command as root:
yum install system-config-services
~]# yum install system-config-services
Figure 12.1. The Service Configuration utility
The utility displays the list of all available services (services from the
/etc/rc.d/init.d/ directory, as well as services controlled by xinetd) along with their description and the current status. For a complete list of used icons and an explanation of their meaning, see Table 12.2, “Possible service states”.
Note that unless you are already authenticated, you will be prompted to enter the superuser password the first time you make a change.
| Icon | Description |
|---|---|
| The service is enabled. |
| The service is disabled. |
| The service is enabled for selected runlevels only. |
| The service is running. |
| The service is stopped. |
| There is something wrong with the service. |
| The status of the service is unknown. |
12.2.1.1. Enabling and Disabling a Service
Link kopierenLink in die Zwischenablage kopiert!
To enable a service, select it from the list and either click the Enable button on the toolbar, or choose → from the main menu.
To disable a service, select it from the list and either click the Disable button on the toolbar, or choose → from the main menu.
12.2.1.2. Starting, Restarting, and Stopping a Service
Link kopierenLink in die Zwischenablage kopiert!
To start a service, select it from the list and either click the Start button on the toolbar, or choose → from the main menu. Note that this option is not available for services controlled by xinetd, as they are started by it on demand.
To restart a running service, select it from the list and either click the Restart button on the toolbar, or choose → from the main menu. Note that this option is not available for services controlled by xinetd, as they are started and stopped by it automatically.
To stop a service, select it from the list and either click the Stop button on the toolbar, or choose → from the main menu. Note that this option is not available for services controlled by xinetd, as they are stopped by it when their job is finished.
12.2.1.3. Selecting Runlevels
Link kopierenLink in die Zwischenablage kopiert!
To enable the service for certain runlevels only, select it from the list and either click the Customize button on the toolbar, or choose → from the main menu. Then select the check box beside each runlevel in which you want the service to run. Note that this option is not available for services controlled by xinetd.
12.2.2. Using the ntsysv Utility
Link kopierenLink in die Zwischenablage kopiert!
The ntsysv utility is a command-line application with a simple text user interface to configure which services are to be started in selected runlevels. To start the utility, type
ntsysv at a shell prompt as root.
Figure 12.2. The ntsysv utility
The utility displays the list of available services (the services from the
/etc/rc.d/init.d/ directory) along with their current status and a description obtainable by pressing F1. For a list of used symbols and an explanation of their meaning, see Table 12.3, “Possible service states”.
| Symbol | Description |
|---|---|
| [*] | The service is enabled. |
| [ ] | The service is disabled. |
12.2.2.1. Enabling and Disabling a Service
Link kopierenLink in die Zwischenablage kopiert!
To enable a service, navigate through the list using the Up and Down arrows keys, and select it with the Spacebar. An asterisk (*) appears in the brackets.
To disable a service, navigate through the list using the Up and Down arrows keys, and toggle its status with the Spacebar. An asterisk (*) in the brackets disappears.
Once you are done, use the Tab key to navigate to the button, and confirm the changes by pressing Enter. Keep in mind that ntsysv does not actually start or stop the service. If you need to start or stop the service immediately, use the
service command as described in Section 12.3.2, “Starting a Service”.
12.2.2.2. Selecting Runlevels
Link kopierenLink in die Zwischenablage kopiert!
By default, the ntsysv utility only affects the current runlevel. To enable or disable services for other runlevels, as
root, run the command with the additional --level option followed by numbers from 0 to 6 representing each runlevel you want to configure:
ntsysv --level runlevels
ntsysv --level runlevels
For example, to configure runlevels 3 and 5, type:
ntsysv --level 35
~]# ntsysv --level 3512.2.3. Using the chkconfig Utility
Link kopierenLink in die Zwischenablage kopiert!
The chkconfig utility is a command-line tool that allows you to specify in which runlevel to start a selected service, as well as to list all available services along with their current setting. Note that with the exception of listing, you must have superuser privileges to use this command.
12.2.3.1. Listing the Services
Link kopierenLink in die Zwischenablage kopiert!
To display a list of system services (services from the
/etc/rc.d/init.d/ directory, as well as the services controlled by xinetd), either type chkconfig --list, or use chkconfig with no additional arguments. You will be presented with an output similar to the following:
Each line consists of the name of the service followed by its status (on or off) for each of the seven numbered runlevels. For example, in the listing above, NetworkManager is enabled in runlevel 2, 3, 4, and 5, while abrtd runs in runlevel 3 and 5. The xinetd based services are listed at the end, being either on, or off.
To display the current settings for a selected service only, use
chkconfig --list followed by the name of the service:
chkconfig --list service_name
chkconfig --list service_name
For example, to display the current settings for the
sshd service, type:
chkconfig --list sshd
~]# chkconfig --list sshd
sshd 0:off 1:off 2:on 3:on 4:on 5:on 6:off
You can also use this command to display the status of a service that is managed by xinetd. In that case, the output will only contain the information whether the service is enabled or disabled:
chkconfig --list rsync
~]# chkconfig --list rsync
rsync off12.2.3.2. Enabling a Service
Link kopierenLink in die Zwischenablage kopiert!
To enable a service in runlevels 2, 3, 4, and 5, type the following at a shell prompt as
root:
chkconfig service_name on
chkconfig service_name on
For example, to enable the
httpd service in these four runlevels, type:
chkconfig httpd on
~]# chkconfig httpd on
To enable a service in certain runlevels only, add the
--level option followed by numbers from 0 to 6 representing each runlevel in which you want the service to run:
chkconfig service_name on --level runlevels
chkconfig service_name on --level runlevels
For instance, to enable the abrtd service in runlevels 3 and 5, type:
chkconfig abrtd on --level 35
~]# chkconfig abrtd on --level 35
The service will be started the next time you enter one of these runlevels. If you need to start the service immediately, use the
service command as described in Section 12.3.2, “Starting a Service”.
Do not use the
--level option when working with a service that is managed by xinetd, as it is not supported. For example, to enable the rsync service, type:
chkconfig rsync on
~]# chkconfig rsync on
If the xinetd daemon is running, the service is immediately enabled without having to manually restart the daemon.
12.2.3.3. Disabling a Service
Link kopierenLink in die Zwischenablage kopiert!
To disable a service in runlevels 2, 3, 4, and 5, type the following at a shell prompt as
root:
chkconfig service_name off
chkconfig service_name off
For instance, to disable the
httpd service in these four runlevels, type:
chkconfig httpd off
~]# chkconfig httpd off
To disable a service in certain runlevels only, add the
--level option followed by numbers from 0 to 6 representing each runlevel in which you do not want the service to run:
chkconfig service_name off --level runlevels
chkconfig service_name off --level runlevels
For instance, to disable the abrtd in runlevels 2 and 4, type:
chkconfig abrtd off --level 24
~]# chkconfig abrtd off --level 24
The service will be stopped the next time you enter one of these runlevels. If you need to stop the service immediately, use the
service command as described in Section 12.3.3, “Stopping a Service”.
Do not use the
--level option when working with a service that is managed by xinetd, as it is not supported. For example, to disable the rsync service, type:
chkconfig rsync off
~]# chkconfig rsync off
If the xinetd daemon is running, the service is immediately disabled without having to manually restart the daemon.
12.3. Running Services
Link kopierenLink in die Zwischenablage kopiert!
The service utility allows you to start, stop, or restart the services from the
/etc/init.d/ directory.
12.3.1. Determining the Service Status
Link kopierenLink in die Zwischenablage kopiert!
To determine the current status of a service, type the following at a shell prompt:
service service_name status
service service_name status
For example, to determine the status of the
httpd service, type:
service httpd status
~]# service httpd status
httpd (pid 7474) is running...
To display the status of all available services at once, run the
service command with the --status-all option:
Note that you can also use the Service Configuration utility as described in Section 12.2.1, “Using the Service Configuration Utility”.
12.3.2. Starting a Service
Link kopierenLink in die Zwischenablage kopiert!
To start a service, type the following at a shell prompt as
root:
service service_name start
service service_name start
For example, to start the
httpd service, type:
service httpd start
~]# service httpd start
Starting httpd: [ OK ]12.3.3. Stopping a Service
Link kopierenLink in die Zwischenablage kopiert!
To stop a running service, type the following at a shell prompt as
root:
service service_name stop
service service_name stop
For example, to stop the
httpd service, type:
service httpd stop
~]# service httpd stop
Stopping httpd: [ OK ]12.3.4. Restarting a Service
Link kopierenLink in die Zwischenablage kopiert!
To restart the service, type the following at a shell prompt as
root:
service service_name restart
service service_name restart
For example, to restart the
httpd service, type:
service httpd restart
~]# service httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]12.4. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
12.4.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
- chkconfig(8) — a manual page for the chkconfig utility.
- ntsysv(8) — a manual page for the ntsysv utility.
- service(8) — a manual page for the service utility.
- system-config-services(8) — a manual page for the system-config-services utility.
12.4.2. Related Books
Link kopierenLink in die Zwischenablage kopiert!
- Red Hat Enterprise Linux 6 Security Guide
- A guide to securing Red Hat Enterprise Linux 6. It contains valuable information on how to set up the firewall, as well as the configuration of SELinux.
Chapter 13. Configuring Authentication
Link kopierenLink in die Zwischenablage kopiert!
Authentication is the way that a user is identified and verified to a system. The authentication process requires presenting some sort of identity and credentials, like a user name and password. The credentials are then compared to information stored in some data store on the system. In Red Hat Enterprise Linux, the Authentication Configuration Tool helps configure what kind of data store to use for user credentials, such as LDAP.
For convenience and potentially part of single sign-on, Red Hat Enterprise Linux can use a central daemon to store user credentials for a number of different data stores. The System Security Services Daemon (SSSD) can interact with LDAP, Kerberos, and external applications to verify user credentials. The Authentication Configuration Tool can configure SSSD along with NIS, Winbind, and LDAP, so that authentication processing and caching can be combined.
13.1. Configuring System Authentication
Link kopierenLink in die Zwischenablage kopiert!
When a user logs into a Red Hat Enterprise Linux system, that user presents some sort of credential to establish the user identity. The system then checks those credentials against the configured authentication service. If the credentials match and the user account is active, then the user is authenticated. (Once a user is authenticated, then the information is passed to the access control service to determine what the user is permitted to do. Those are the resources the user is authorized to access.)
The information to verify the user can be located on the local system or the local system can reference a user database on a remote system, such as LDAP or Kerberos.
The system must have a configured list of valid account databases for it to check for user authentication. On Red Hat Enterprise Linux, the Authentication Configuration Tool has both GUI and command-line options to configure any user data stores.
A local system can use a variety of different data stores for user information, including Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind. Additionally, both LDAP and NIS data stores can use Kerberos to authenticate users.
Important
If a medium or high security level is set during installation or with the Security Level Configuration Tool, then the firewall prevents NIS authentication. For more information about firewalls, see the "Firewalls" section of the Security Guide.
13.1.1. Launching the Authentication Configuration Tool UI
Link kopierenLink in die Zwischenablage kopiert!
- Log into the system as root.
- Open the .
- Select the menu.
- Select the item.
Alternatively, run the
system-config-authentication command.
Important
Any changes take effect immediately when the Authentication Configuration Tool UI is closed.
There are two configuration tabs in the Authentication dialog box:
- Identity & Authentication, which configures the resource used as the identity store (the data repository where the user IDs and corresponding credentials are stored).
- Advanced Options, which allows authentication methods other than passwords or certificates, like smart cards and fingerprint.
13.1.2. Selecting the Identity Store for Authentication
Link kopierenLink in die Zwischenablage kopiert!
The Identity & Authentication tab sets how users should be authenticated. The default is to use local system authentication, meaning the users and their passwords are checked against local system accounts. A Red Hat Enterprise Linux machine can also use external resources which contain the users and credentials, including LDAP, NIS, and Winbind.
Figure 13.1. Local Authentication
13.1.2.1. Configuring LDAP Authentication
Link kopierenLink in die Zwischenablage kopiert!
Either the openldap-clients package or the sssd package is used to configure an LDAP server for the user database. Both packages are installed by default.
- Open the Authentication Configuration Tool, as in Section 13.1.1, “Launching the Authentication Configuration Tool UI”.
- Select in the User Account Database drop-down menu.
- Set the information that is required to connect to the LDAP server.
- LDAP Search Base DN gives the root suffix or distinguished name (DN) for the user directory. All of the user entries used for identity/authentication will exist below this parent entry. For example, ou=people,dc=example,dc=com.This field is optional. If it is not specified, then the System Security Services Daemon (SSSD) attempts to detect the search base using the
namingContextsanddefaultNamingContextattributes in the LDAP server's configuration entry. - LDAP Server gives the URL of the LDAP server. This usually requires both the host name and port number of the LDAP server, such as ldap://ldap.example.com:389.Entering the secure protocol in the URL,
ldaps://, enables the button. - Use TLS to encrypt connections sets whether to use Start TLS to encrypt the connections to the LDAP server. This enables a secure connection over a standard port.Selecting TLS enables the button, which retrieves the issuing CA certificate for the LDAP server from whatever certificate authority issued it. The CA certificate must be in the privacy enhanced mail (PEM) format.
Important
Do not select Use TLS to encrypt connections if the server URL uses a secure protocol (ldaps). This option uses Start TLS, which initiates a secure connection over a standard port; if a secure port is specified, then a protocol like SSL must be used instead of Start TLS.
- Select the authentication method. LDAP allows simple password authentication or Kerberos authentication.Using Kerberos is described in Section 13.1.2.4, “Using Kerberos with LDAP or NIS Authentication”.The LDAP password option uses PAM applications to use LDAP authentication. This option requires either a secure (
ldaps://) URL or the TLS option to connect to the LDAP server.
13.1.2.2. Configuring NIS Authentication
Link kopierenLink in die Zwischenablage kopiert!
- Install the ypbind package. This is required for NIS services, but is not installed by default.
yum install ypbind
~]# yum install ypbindCopy to Clipboard Copied! Toggle word wrap Toggle overflow When theypbindservice is installed, theportmapandypbindservices are started and enabled to start at boot time. - Open the Authentication Configuration Tool, as in Section 13.1.1, “Launching the Authentication Configuration Tool UI”.
- Select in the User Account Database drop-down menu.
- Set the information to connect to the NIS server, meaning the NIS domain name and the server host name. If the NIS server is not specified, the
authconfigdaemon scans for the NIS server. - Select the authentication method. NIS allows simple password authentication or Kerberos authentication.Using Kerberos is described in Section 13.1.2.4, “Using Kerberos with LDAP or NIS Authentication”.
For more information about NIS, see the "Securing NIS" section of the Security Guide.
13.1.2.3. Configuring Winbind Authentication
Link kopierenLink in die Zwischenablage kopiert!
- Install the samba-winbind package. This is required for Windows integration features in Samba services, but is not installed by default.
yum install samba-winbind
~]# yum install samba-winbindCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Open the Authentication Configuration Tool, as in Section 13.1.1, “Launching the Authentication Configuration Tool UI”.
- Select in the User Account Database drop-down menu.
- Set the information that is required to connect to the Microsoft Active Directory domain controller.
- Winbind Domain gives the Windows domain to connect to.This should be in the Windows 2000 format, such as
DOMAIN. - Security Model sets the security model to use for Samba clients.
authconfigsupports four types of security models:- ads configures Samba to act as a domain member in an Active Directory Server realm. To operate in this mode, the krb5-server package must be installed and Kerberos must be configured properly. Also, when joining to the Active Directory Server using the command line, the following command must be used:
net ads join
net ads joinCopy to Clipboard Copied! Toggle word wrap Toggle overflow - domain has Samba validate the user name/password by authenticating it through a Windows primary or backup domain controller, much like a Windows server.
- server has a local Samba server validate the user name/password by authenticating it through another server, such as a Windows server. If the server authentication attempt fails, the system then attempts to authenticate using
usermode. - user requires a client to log in with a valid user name and password. This mode does support encrypted passwords.The user name format must be domain\user, such as
EXAMPLE\jsmith.Note
When verifying that a given user exists in the Windows domain, always use Windows 2000-style formats and escape the backslash (\) character. For example:getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bashCopy to Clipboard Copied! Toggle word wrap Toggle overflow This is the default option.
- Winbind ADS Realm gives the Active Directory realm that the Samba server will join. This is only used with the ads security model.
- Winbind Domain Controllers gives the domain controller to use. For more information about domain controllers, see Section 21.1.6.3, “Domain Controller”.
- Template Shell sets which login shell to use for Windows user account settings.
- Allow offline login allows authentication information to be stored in a local cache. The cache is referenced when a user attempts to authenticate to system resources while the system is offline.
For more information about the
Winbind service, see Section 21.1.2, “Samba Daemons and Related Services”.
For additional information about configuring
Winbind and troubleshooting tips, see the Knowledgebase on the Red Hat Customer Portal.
Also, the Red Hat Access Labs page includes the
Winbind Mapper utility that generates a part of the smb.conf file to help you connect a Red Hat Enterprise Linux to an Active Directory.
13.1.2.4. Using Kerberos with LDAP or NIS Authentication
Link kopierenLink in die Zwischenablage kopiert!
Both LDAP and NIS authentication stores support Kerberos authentication methods. Using Kerberos has a couple of benefits:
- It uses a security layer for communication while still allowing connections over standard ports.
- It automatically uses credentials caching with SSSD, which allows offline logins.
Using Kerberos authentication requires the krb5-libs and krb5-workstation packages.
The option from the Authentication Method drop-down menu automatically opens the fields required to connect to the Kerberos realm.
Figure 13.2. Kerberos Fields
- Realm gives the name for the realm for the Kerberos server. The realm is the network that uses Kerberos, composed of one or more key distribution centers (KDC) and a potentially large number of clients.
- KDCs gives a comma-separated list of servers that issue Kerberos tickets.
- Admin Servers gives a list of administration servers running the
kadmindprocess in the realm. - Optionally, use DNS to resolve server host name and to find additional KDCs within the realm.
For more information about Kerberos, see section "Using Kerberos" of the Red Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards guide.
13.1.3. Configuring Alternative Authentication Features
Link kopierenLink in die Zwischenablage kopiert!
The Authentication Configuration Tool also configures settings related to authentication behavior, apart from the identity store. This includes entirely different authentication methods (fingerprint scans and smart cards) or local authentication rules. These alternative authentication options are configured in the Advanced Options tab.
Figure 13.3. Advanced Options
13.1.3.1. Using Fingerprint Authentication
Link kopierenLink in die Zwischenablage kopiert!
When there is appropriate hardware available, the Enable fingerprint reader support option allows fingerprint scans to be used to authenticate local users in addition to other credentials.
13.1.3.2. Setting Local Authentication Parameters
Link kopierenLink in die Zwischenablage kopiert!
There are two options in the Local Authentication Options area which define authentication behavior on the local system:
- Enable local access control instructs the
/etc/security/access.conffile to check for local user authorization rules. - Password Hashing Algorithm sets the hashing algorithm to use to encrypt locally-stored passwords.
13.1.3.3. Enabling Smart Card Authentication
Link kopierenLink in die Zwischenablage kopiert!
When there are appropriate smart card readers available, a system can accept smart cards (or tokens) instead of other user credentials to authenticate.
Once the Enable smart card support option is selected, then the behaviors of smart card authentication can be defined:
- Card Removal Action tells the system how to respond when the card is removed from the card reader during an active session. A system can either ignore the removal and allow the user to access resources as normal, or a system can immediately lock until the smart card is supplied.
- Require smart card login sets whether a smart card is required for logins or allowed for logins. When this option is selected, all other methods of authentication are immediately blocked.
Warning
Do not select this option until you have successfully authenticated to the system using a smart card.
Using smart cards requires the pam_pkcs11 package.
13.1.3.4. Creating User Home Directories
Link kopierenLink in die Zwischenablage kopiert!
There is an option (Create home directories on the first login) to create a home directory automatically the first time that a user logs in.
This option is beneficial with accounts that are managed centrally, such as with LDAP. However, this option should not be selected if a system like automount is used to manage user home directories.
13.1.4. Configuring Authentication from the Command Line
Link kopierenLink in die Zwischenablage kopiert!
The
authconfig command-line tool updates all of the configuration files and services required for system authentication, according to the settings passed to the script. Along with allowing all of the identity and authentication configuration options that can be set through the UI, the authconfig tool can also be used to create backup and kickstart files.
For a complete list of
authconfig options, check the help output and the man page.
13.1.4.1. Tips for Using authconfig
Link kopierenLink in die Zwischenablage kopiert!
There are some things to remember when running
authconfig:
- With every command, use either the
--updateor--testoption. One of those options is required for the command to run successfully. Using--updatewrites the configuration changes.--testprints the changes to stdout but does not apply the changes to the configuration. - Each enable option has a corresponding disable option.
13.1.4.2. Configuring LDAP User Stores
Link kopierenLink in die Zwischenablage kopiert!
To use an LDAP identity store, use the
--enableldap. To use LDAP as the authentication source, use --enableldapauth and then the requisite connection information, like the LDAP server name, base DN for the user suffix, and (optionally) whether to use TLS. The authconfig command also has options to enable or disable RFC 2307bis schema for user entries, which is not possible through the Authentication Configuration UI.
Be sure to use the full LDAP URL, including the protocol (
ldap or ldaps) and the port number. Do not use a secure LDAP URL (ldaps) with the --enableldaptls option.
authconfig --enableldap --enableldapauth --ldapserver=ldap://ldap.example.com:389,ldap://ldap2.example.com:389 --ldapbasedn="ou=people,dc=example,dc=com" --enableldaptls --ldaploadcacert=https://ca.server.example.com/caCert.crt --update
authconfig --enableldap --enableldapauth --ldapserver=ldap://ldap.example.com:389,ldap://ldap2.example.com:389 --ldapbasedn="ou=people,dc=example,dc=com" --enableldaptls --ldaploadcacert=https://ca.server.example.com/caCert.crt --update
Instead of using
--ldapauth for LDAP password authentication, it is possible to use Kerberos with the LDAP user store. These options are described in Section 13.1.4.5, “Configuring Kerberos Authentication”.
13.1.4.3. Configuring NIS User Stores
Link kopierenLink in die Zwischenablage kopiert!
To use a NIS identity store, use the
--enablenis. This automatically uses NIS authentication, unless the Kerberos parameters are explicitly set, so it uses Kerberos authentication (Section 13.1.4.5, “Configuring Kerberos Authentication”). The only parameters are to identify the NIS server and NIS domain; if these are not used, then the authconfig service scans the network for NIS servers.
authconfig --enablenis --nisdomain=EXAMPLE --nisserver=nis.example.com --update
authconfig --enablenis --nisdomain=EXAMPLE --nisserver=nis.example.com --update13.1.4.4. Configuring Winbind User Stores
Link kopierenLink in die Zwischenablage kopiert!
Windows domains have several different security models, and the security model used in the domain determines the authentication configuration for the local system.
For user and server security models, the Winbind configuration requires only the domain (or workgroup) name and the domain controller host names.
authconfig --enablewinbind --enablewinbindauth --smbsecurity=user|server --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --update
authconfig --enablewinbind --enablewinbindauth --smbsecurity=user|server --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --updateNote
The user name format must be domain\user, such as
EXAMPLE\jsmith.
When verifying that a given user exists in the Windows domain, always use Windows 2000-style formats and escape the backslash (\) character. For example:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
For ads and domain security models, the Winbind configuration allows additional configuration for the template shell and realm (ads only). For example:
authconfig --enablewinbind --enablewinbindauth --smbsecurity ads --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --smbrealm EXAMPLE.COM --winbindtemplateshell=/bin/sh --update
authconfig --enablewinbind --enablewinbindauth --smbsecurity ads --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --smbrealm EXAMPLE.COM --winbindtemplateshell=/bin/sh --update
There are a lot of other options for configuring Windows-based authentication and the information for Windows user accounts, such as name formats, whether to require the domain name with the user name, and UID ranges. These options are listed in the
authconfig help.
13.1.4.5. Configuring Kerberos Authentication
Link kopierenLink in die Zwischenablage kopiert!
Both LDAP and NIS allow Kerberos authentication to be used in place of their native authentication mechanisms. At a minimum, using Kerberos authentication requires specifying the realm, the KDC, and the administrative server. There are also options to use DNS to resolve client names and to find additional admin servers.
authconfig NIS or LDAP options --enablekrb5 --krb5realm EXAMPLE --krb5kdc kdc.example.com:88,server.example.com:88 --krb5adminserver server.example.com:749 --enablekrb5kdcdns --enablekrb5realmdns --update
authconfig NIS or LDAP options --enablekrb5 --krb5realm EXAMPLE --krb5kdc kdc.example.com:88,server.example.com:88 --krb5adminserver server.example.com:749 --enablekrb5kdcdns --enablekrb5realmdns --update13.1.4.6. Configuring Local Authentication Settings
Link kopierenLink in die Zwischenablage kopiert!
The Authentication Configuration Tool can also control some user settings that relate to security, such as creating home directories, setting password hash algorithms, and authorization. These settings are done independently of identity/user store settings.
For example, to create user home directories:
authconfig --enablemkhomedir --update
authconfig --enablemkhomedir --update
To set or change the hash algorithm used to encrypt user passwords:
authconfig --passalgo=sha512 --update
authconfig --passalgo=sha512 --update13.1.4.7. Configuring Fingerprint Authentication
Link kopierenLink in die Zwischenablage kopiert!
There is one option to enable support for fingerprint readers. This option can be used alone or in conjunction with other
authconfig settings, like LDAP user stores.
authconfig --enablefingerprint --update
~]# authconfig --enablefingerprint --update13.1.4.8. Configuring Smart Card Authentication
Link kopierenLink in die Zwischenablage kopiert!
All that is required to use smart cards with a system is to set the
--enablesmartcard option:
authconfig --enablesmartcard --update
~]# authconfig --enablesmartcard --update
There are other configuration options for smart cards, such as changing the default smart card module, setting the behavior of the system when the smart card is removed, and requiring smart cards for login.
For example, this command instructs the system to lock out a user immediately if the smart card is removed (a setting of 1 ignores it if the smart card is removed):
authconfig --enablesmartcard --smartcardaction=0 --update
~]# authconfig --enablesmartcard --smartcardaction=0 --update
Once smart card authentication has been successfully configured and tested, then the system can be configured to require smart card authentication for users rather than simple password-based authentication.
authconfig --enablerequiresmartcard --update
~]# authconfig --enablerequiresmartcard --updateWarning
Do not use the
--enablerequiresmartcard option until you have successfully authenticated to the system using a smart card. Otherwise, users may be unable to log into the system.
13.1.4.9. Managing Kickstart and Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
The
--update option updates all of the configuration files with the configuration changes. There are a couple of alternative options with slightly different behavior:
--kickstartwrites the updated configuration to a kickstart file.--testprints the full configuration, with changes, to stdout but does not edit any configuration files.
Additionally,
authconfig can be used to back up and restore previous configurations. All archives are saved to a unique subdirectory in the /var/lib/authconfig/ directory. For example, the --savebackup option gives the backup directory as 2011-07-01:
authconfig --savebackup=2011-07-01
~]# authconfig --savebackup=2011-07-01
This backs up all of the authentication configuration files beneath the
/var/lib/authconfig/backup-2011-07-01 directory.
Any of the saved backups can be used to restore the configuration using the
--restorebackup option, giving the name of the manually-saved configuration:
authconfig --restorebackup=2011-07-01
~]# authconfig --restorebackup=2011-07-01
Additionally,
authconfig automatically makes a backup of the configuration before it applies any changes (with the --update option). The configuration can be restored from the most recent automatic backup, without having to specify the exact backup, using the --restorelastbackup option.
13.1.5. Using Custom Home Directories
Link kopierenLink in die Zwischenablage kopiert!
If LDAP users have home directories that are not in
/home and the system is configured to create home directories the first time users log in, then these directories are created with the wrong permissions.
- Apply the correct SELinux context and permissions from the
/homedirectory to the home directory that is created on the local system. For example:semanage fcontext -a -e /home /home/locale
~]# semanage fcontext -a -e /home /home/localeCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Install the oddjob-mkhomedir package on the system.This package provides the
pam_oddjob_mkhomedir.solibrary, which the Authentication Configuration Tool uses to create home directories. Thepam_oddjob_mkhomedir.solibrary, unlike the defaultpam_mkhomedir.solibrary, can create SELinux labels.The Authentication Configuration Tool automatically uses thepam_oddjob_mkhomedir.solibrary if it is available. Otherwise, it will default to usingpam_mkhomedir.so. - Make sure the
oddjobdservice is running. - Re-run the Authentication Configuration Tool and enable home directories, as in Section 13.1.3, “Configuring Alternative Authentication Features”.
If home directories were created before the home directory configuration was changed, then correct the permissions and SELinux contexts. For example:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
semanage fcontext -a -e /home /home/locale
~]# semanage fcontext -a -e /home /home/locale
# restorecon -R -v /home/locale13.2. Using and Caching Credentials with SSSD
Link kopierenLink in die Zwischenablage kopiert!
The System Security Services Daemon (SSSD) provides access to different identity and authentication providers.
13.2.1. About SSSD
Link kopierenLink in die Zwischenablage kopiert!
Most system authentication is configured locally, which means that services must check with a local user store to determine users and credentials. What SSSD does is allow a local service to check with a local cache in SSSD, but that cache may be taken from any variety of remote identity providers — an LDAP directory, an Identity Management domain, Active Directory, possibly even a Kerberos realm.
SSSD also caches those users and credentials, so if the local system or the identity provider go offline, the user credentials are still available to services to verify.
SSSD is an intermediary between local clients and any configured data store. This relationship brings a number of benefits for administrators:
- Reducing the load on identification/authentication servers. Rather than having every client service attempt to contact the identification server directly, all of the local clients can contact SSSD which can connect to the identification server or check its cache.
- Permitting offline authentication. SSSD can optionally keep a cache of user identities and credentials that it retrieves from remote services. This allows users to authenticate to resources successfully, even if the remote identification server is offline or the local machine is offline.
- Using a single user account. Remote users frequently have two (or even more) user accounts, such as one for their local system and one for the organizational system. This is necessary to connect to a virtual private network (VPN). Because SSSD supports caching and offline authentication, remote users can connect to network resources by authenticating to their local machine and then SSSD maintains their network credentials.
Additional Resources
While this chapter covers the basics of configuring services and domains in SSSD, this is not a comprehensive resource. Many other configuration options are available for each functional area in SSSD; check out the man page for the specific functional area to get a complete list of options.
Some of the common man pages are listed in Table 13.1, “A Sampling of SSSD Man Pages”. There is also a complete list of SSSD man pages in the "See Also" section of the
sssd(8) man page.
| Functional Area | Man Page | ||
|---|---|---|---|
| General Configuration | sssd.conf(8) | ||
| sudo Services | sssd-sudo | ||
| LDAP Domains | sssd-ldap | ||
| Active Directory Domains |
| ||
| Identity Management (IdM or IPA) Domains |
| ||
| Kerberos Authentication for Domains | sssd-krb5 | ||
| OpenSSH Keys |
| ||
| Cache Maintenance |
|
13.2.2. Setting up the sssd.conf File
Link kopierenLink in die Zwischenablage kopiert!
SSSD services and domains are configured in a
.conf file. By default, this is /etc/sssd/sssd.conf — although that file must be created and configured manually, since SSSD is not configured after installation.
13.2.2.1. Creating the sssd.conf File
Link kopierenLink in die Zwischenablage kopiert!
There are three parts of the SSSD configuration file:
[sssd], for general SSSD process and operational configuration; this basically lists the configured services, domains, and configuration parameters for each- [service_name], for configuration options for each supported system service, as described in Section 13.2.4, “SSSD and System Services”
- [domain_type/DOMAIN_NAME], for configuration options for each configured identity provider
Important
While services are optional, at least one identity provider domain must be configured before the SSSD service can be started.
Example 13.1. Simple sssd.conf File
The
[sssd] section has three important parameters:
domainslists all of the domains, configured in thesssd.conf, which SSSD uses as identity providers. If a domain is not listed in thedomainskey, it is not used by SSSD, even if it has a configuration section.serviceslists all of the system services, configured in thesssd.conf, which use SSSD; when SSSD starts, the corresponding SSSD service is started for each configured system service. If a service is not listed in theserviceskey, it is not used by SSSD, even if it has a configuration section.config_file_versionsets the version of the configuration file to set file format expectations. This is version 2, for all recent SSSD versions.
Note
Even if a service or domain is configured in the
sssd.conf file, SSSD does not interact with that service or domain unless it is listed in the services or domains parameters, respectively, in the [sssd] section.
Other configuration parameters are listed in the
sssd.conf man page.
Each service and domain parameter is described in its respective configuration section in this chapter and in their man pages.
13.2.2.2. Using a Custom Configuration File
Link kopierenLink in die Zwischenablage kopiert!
By default, the
sssd process assumes that the configuration file is /etc/sssd/sssd.conf.
An alternative file can be passed to SSSD by using the
-c option with the sssd command:
sssd -c /etc/sssd/customfile.conf --daemon
~]# sssd -c /etc/sssd/customfile.conf --daemon13.2.3. Starting and Stopping SSSD
Link kopierenLink in die Zwischenablage kopiert!
Important
Configure at least one domain before starting SSSD for the first time. See Section 13.2.10, “SSSD and Identity Providers (Domains)”.
Either the
service command or the /etc/init.d/sssd script can start SSSD. For example:
service sssd start
~]# service sssd start
By default, SSSD is not configured to start automatically. There are two ways to change this behavior:
- Enabling SSSD through the
authconfigcommand:authconfig --enablesssd --enablesssdauth --update
~]# authconfig --enablesssd --enablesssdauth --updateCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Adding the SSSD process to the start list using the
chkconfigcommand:chkconfig sssd on
~]# chkconfig sssd onCopy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.4. SSSD and System Services
Link kopierenLink in die Zwischenablage kopiert!
SSSD and its associated services are configured in the
sssd.conf file. The [sssd] section also lists the services that are active and should be started when sssd starts within the services directive.
SSSD can provide credentials caches for several system services:
- A Name Service Switch (NSS) provider service that answers name service requests from the
sssd_nssmodule. This is configured in the[nss]section of the SSSD configuration.This is described in Section 13.2.5, “Configuring Services: NSS”. - A PAM provider service that manages a PAM conversation through the
sssd_pammodule. This is configured in the[pam]section of the configuration.This is described in Section 13.2.6, “Configuring Services: PAM”. - An SSH provider service that defines how SSSD manages the
known_hostsfile and other key-related configuration. Using SSSD with OpenSSH is described in Section 13.2.9, “Configuring Services: OpenSSH and Cached Keys”. - An
autofsprovider service that connects to an LDAP server to retrieve configured mount locations. This is configured as part of an LDAP identity provider in a[domain/NAME]section in the configuration file.This is described in Section 13.2.7, “Configuring Services: autofs”. - A
sudoprovider service that connects to an LDAP server to retrieve configuredsudopolicies. This is configured as part of an LDAP identity provider in a[domain/NAME]section in the configuration file.This is described in Section 13.2.8, “Configuring Services: sudo”. - A PAC responder service that defines how SSSD works with Kerberos to manage Active Directory users and groups. This is specifically part of managing Active Directory identity providers with domains, as described in Section 13.2.13, “Creating Domains: Active Directory”.
13.2.5. Configuring Services: NSS
Link kopierenLink in die Zwischenablage kopiert!
SSSD provides an NSS module,
sssd_nss, which instructs the system to use SSSD to retrieve user information. The NSS configuration must include a reference to the SSSD module, and then the SSSD configuration sets how SSSD interacts with NSS.
About NSS Service Maps and SSSD
The Name Service Switch (NSS) provides a central configuration for services to look up a number of configuration and name resolution services. NSS provides one method of mapping system identities and services with configuration sources.
SSSD works with NSS as a provider services for several types of NSS maps:
- Passwords (
passwd) - User groups (
shadow) - Groups (
groups) - Netgroups (
netgroups) - Services (
services)
Procedure 13.1. Configuring NSS Services to Use SSSD
NSS can use multiple identity and configuration providers for any and all of its service maps. The default is to use system files for services; for SSSD to be included, the
nss_sss module has to be included for the desired service type.
- Use the Authentication Configuration tool to enable SSSD. This automatically configured the
nsswitch.conffile to use SSSD as a provider.authconfig --enablesssd --update
~]# authconfig --enablesssd --updateCopy to Clipboard Copied! Toggle word wrap Toggle overflow This automatically configures the password, shadow, group, and netgroups services maps to use the SSSD module:passwd: files sss shadow: files sss group: files sss netgroup: files sss
passwd: files sss shadow: files sss group: files sss netgroup: files sssCopy to Clipboard Copied! Toggle word wrap Toggle overflow - The services map is not enabled by default when SSSD is enabled with
authconfig. To include that map, open thensswitch.conffile and add thesssmodule to theservicesmap:vim /etc/nsswitch.conf
~]# vim /etc/nsswitch.conf ... services: file sss ...Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 13.2. Configuring SSSD to Work with NSS
The options and configuration that SSSD uses to service NSS requests are configured in the SSSD configuration file, in the
[nss] services section.
- Open the
sssd.conffile.vim /etc/sssd/sssd.conf
~]# vim /etc/sssd/sssd.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make sure that NSS is listed as one of the services that works with SSSD.
[sssd] config_file_version = 2 reconnection_retries = 3 sbus_timeout = 30 services = nss, pam
[sssd] config_file_version = 2 reconnection_retries = 3 sbus_timeout = 30 services = nss, pamCopy to Clipboard Copied! Toggle word wrap Toggle overflow - In the
[nss]section, change any of the NSS parameters. These are listed in Table 13.2, “SSSD [nss] Configuration Parameters”.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD.
service sssd restart
~]# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
| Parameter | Value Format | Description |
|---|---|---|
| entry_cache_nowait_percentage | integer | Specifies how long sssd_nss should return cached entries before refreshing the cache. Setting this to zero (0) disables the entry cache refresh.
This configures the entry cache to update entries in the background automatically if they are requested if the time before the next update is a certain percentage of the next interval. For example, if the interval is 300 seconds and the cache percentage is 75, then the entry cache will begin refreshing when a request comes in at 225 seconds — 75% of the interval.
The allowed values for this option are 0 to 99, which sets the percentage based on the
entry_cache_timeout value. The default value is 50%.
|
| entry_negative_timeout | integer | Specifies how long, in seconds, sssd_nss should cache negative cache hits. A negative cache hit is a query for an invalid database entries, including non-existent entries. |
| filter_users, filter_groups | string | Tells SSSD to exclude certain users from being fetched from the NSS database. This is particularly useful for system accounts such as root. |
| filter_users_in_groups | Boolean | Sets whether users listed in the filter_users list appear in group memberships when performing group lookups. If set to FALSE, group lookups return all users that are members of that group. If not specified, this value defaults to true, which filters the group member lists. |
| debug_level | integer, 0 - 9 | Sets a debug logging level. |
NSS Compatibility Mode
NSS compatibility (compat) mode provides the support for additional entries in the
/etc/passwd file to ensure that users or members of netgroups have access to the system.
To enable NSS compatibility mode to work with SSSD, add the following entries to the
/etc/nsswitch.conf file:
passwd: compat passwd_compat: sss
passwd: compat
passwd_compat: sss
Once NSS compatibility mode is enabled, the following
passwd entries are supported:
+user-userInclude (+) or exclude (-) a specified user from the Network Information System (NIS) map.+@netgroup-@netgroupInclude (+) or exclude (-) all users in the given netgroup from the NIS map.+Exclude all users, except previously excluded ones from the NIS map.
For more information about NSS compatibility mode, see the
nsswitch.conf(5) manual page.
13.2.6. Configuring Services: PAM
Link kopierenLink in die Zwischenablage kopiert!
Warning
A mistake in the PAM configuration file can lock users out of the system completely. Always back up the configuration files before performing any changes, and keep a session open so that any changes can be reverted.
SSSD provides a PAM module,
sssd_pam, which instructs the system to use SSSD to retrieve user information. The PAM configuration must include a reference to the SSSD module, and then the SSSD configuration sets how SSSD interacts with PAM.
Procedure 13.3. Configuring PAM
- Use
authconfigto enable SSSD for system authentication.This automatically updates the PAM configuration to reference all of the SSSD modules:authconfig --update --enablesssd --enablesssdauth
# authconfig --update --enablesssd --enablesssdauthCopy to Clipboard Copied! Toggle word wrap Toggle overflow Copy to Clipboard Copied! Toggle word wrap Toggle overflow These modules can be set toincludestatements, as necessary. - Open the
sssd.conffile.vim /etc/sssd/sssd.conf
# vim /etc/sssd/sssd.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make sure that PAM is listed as one of the services that works with SSSD.
[sssd] config_file_version = 2 reconnection_retries = 3 sbus_timeout = 30 services = nss, pam
[sssd] config_file_version = 2 reconnection_retries = 3 sbus_timeout = 30 services = nss, pamCopy to Clipboard Copied! Toggle word wrap Toggle overflow - In the
[pam]section, change any of the PAM parameters. These are listed in Table 13.3, “SSSD [pam] Configuration Parameters”.[pam] reconnection_retries = 3 offline_credentials_expiration = 2 offline_failed_login_attempts = 3 offline_failed_login_delay = 5
[pam] reconnection_retries = 3 offline_credentials_expiration = 2 offline_failed_login_attempts = 3 offline_failed_login_delay = 5Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD.
service sssd restart
~]# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
| Parameter | Value Format | Description |
|---|---|---|
| offline_credentials_expiration | integer | Sets how long, in days, to allow cached logins if the authentication provider is offline. This value is measured from the last successful online login. If not specified, this defaults to zero (0), which is unlimited. |
| offline_failed_login_attempts | integer | Sets how many failed login attempts are allowed if the authentication provider is offline. If not specified, this defaults to zero (0), which is unlimited. |
| offline_failed_login_delay | integer | Sets how long to prevent login attempts if a user hits the failed login attempt limit. If set to zero (0), the user cannot authenticate while the provider is offline once he hits the failed attempt limit. Only a successful online authentication can re-enable offline authentication. If not specified, this defaults to five (5). |
13.2.7. Configuring Services: autofs
Link kopierenLink in die Zwischenablage kopiert!
About Automount, LDAP, and SSSD
Automount maps are commonly flat files, which define a relationship between a map, a mount directory, and a fileserver. (Automount is described in the Storage Administration Guide.)
For example, let's say that there is a fileserver called
nfs.example.com which hosts the directory pub, and automount is configured to mount directories in the /shares/ directory. So, the mount location is /shares/pub. All of the mounts are listed in the auto.master file, which identifies the different mount directories and the files which configure them. The auto.shares file then identifies each file server and mount directory which goes into the /shares/ directory. The relationships could be viewed like this:
Every mount point, then, is defined in two different files (at a minimum): the
auto.master and auto.whatever file, and those files have to be available to each local automount process.
One way for administrators to manage that for large environments is to store the automount configuration in a central LDAP directory, and just configure each local system to point to that LDAP directory. That means that updates only need to be made in a single location, and any new maps are automatically recognized by local systems.
For automount-LDAP configuration, the automount files are stored as LDAP entries, which are then translated into the requisite automount files. Each element is then translated into an LDAP attribute.
The LDAP entries look like this:
The schema elements, then, match up to the structure like this (with the RFC 2307 schema):
autofs uses those schema elements to derive the automount configuration. The /etc/sysconfig/autofs file identifies the LDAP server, directory location, and schema elements used for automount entities:
Rather than pointing the automount configuration to the LDAP directory, it can be configured to point to SSSD. SSSD, then, stores all of the information that automount needs, and as a user attempts to mount a directory, that information is cached into SSSD. This offers several advantages for configuration — such as failover, service discovery, and timeouts — as well as performance improvements by reducing the number of connections to the LDAP server. Most important, using SSSD allows all mount information to be cached, so that clients can still successfully mount directories even if the LDAP server goes offline.
Procedure 13.4. Configuring autofs Services in SSSD
- Make sure that the autofs and sssd-common packages are installed.
- Open the
sssd.conffile.vim /etc/sssd/sssd.conf
~]# vim /etc/sssd/sssd.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Add the
autofsservice to the list of services that SSSD manages.[sssd] services = nss,pam,autofs ....
[sssd] services = nss,pam,autofs ....Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Create a new
[autofs]service configuration section. This section can be left blank; there is only one configurable option, for timeouts for negative cache hits.This section is required, however, for SSSD to recognize theautofsservice and supply the default configuration.[autofs]
[autofs]Copy to Clipboard Copied! Toggle word wrap Toggle overflow - The automount information is read from a configured LDAP domain in the SSSD configuration, so an LDAP domain must be available. If no additional settings are made, then the configuration defaults to the RFC 2307 schema and the LDAP search base (
ldap_search_base) for the automount information. This can be customized:- The directory type,
autofs_provider; this defaults to theid_providervalue; a value of none explicitly disables autofs for the domain. - The search base,
ldap_autofs_search_base. - The object class to use to recognize map entries,
ldap_autofs_map_object_class - The attribute to use to recognize map names,
ldap_autofs_map_name - The object class to use to recognize mount point entries,
ldap_autofs_entry_object_class - The attribute to use to recognize mount point names,
ldap_autofs_entry_key - The attribute to use for additional configuration information for the mount point,
ldap_autofs_entry_value
For example:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Save and close the
sssd.conffile. - Configure
autofsto look for the automount map information in SSSD by editing thensswitch.conffile and changing the location fromldaptosss:vim /etc/nsswitch.conf
# vim /etc/nsswitch.conf automount: files sssCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD.
service sssd restart
# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.8. Configuring Services: sudo
Link kopierenLink in die Zwischenablage kopiert!
About sudo, LDAP, and SSSD
sudo rules are defined in the sudoers file, which must be distributed separately to every machine to maintain consistency.
One way for administrators to manage that for large environments is to store the
sudo configuration in a central LDAP directory, and just configure each local system to point to that LDAP directory. That means that updates only need to be made in a single location, and any new rules are automatically recognized by local systems.
For
sudo-LDAP configuration, each sudo rule is stored as an LDAP entry, with each component of the sudo rule defined in an LDAP attribute.
The
sudoers rule looks like this:
Defaults env_keep+=SSH_AUTH_SOCK ... %wheel ALL=(ALL) ALL
Defaults env_keep+=SSH_AUTH_SOCK
...
%wheel ALL=(ALL) ALL
The LDAP entry looks like this:
Note
SSSD only caches
sudo rules which apply to the local system, depending on the value of the sudoHost attribute. This can mean that the sudoHost value is set to ALL, uses a regular expression that matches the host name, matches the systems netgroup, or matches the systems host name, fully qualified domain name, or IP address.
The
sudo service can be configured to point to an LDAP server and to pull its rule configuration from those LDAP entries. Rather than pointing the sudo configuration to the LDAP directory, it can be configured to point to SSSD. SSSD, then, stores all of the information that sudo needs, and every time a user attempts a sudo-related operation, the latest sudo configuration can be pulled from the LDAP directory (through SSSD). SSSD, however, also caches all of the sudo riles, so that users can perform tasks, using that centralized LDAP configuration, even if the LDAP server goes offline.
Procedure 13.5. Configuring sudo with SSSD
All of the SSSD
sudo configuration options are listed in the sssd-ldap(5) man page.
- Make sure that the sssd-common package is installed.
rpm -q sssd-common
~]$ rpm -q sssd-commonCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Open the
sssd.conffile.vim /etc/sssd/sssd.conf
~]# vim /etc/sssd/sssd.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Add the
sudoservice to the list of services that SSSD manages.[sssd] services = nss,pam,sudo ....
[sssd] services = nss,pam,sudo ....Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Create a new
[sudo]service configuration section. This section can be left blank; there is only one configurable option, for evaluating the sudo not before/after period.This section is required, however, for SSSD to recognize thesudoservice and supply the default configuration.[sudo]
[sudo]Copy to Clipboard Copied! Toggle word wrap Toggle overflow - The
sudoinformation is read from a configured LDAP domain in the SSSD configuration, so an LDAP domain must be available. For an LDAP provider, these parameters are required:- The directory type,
sudo_provider; this is alwaysldap. - The search base,
ldap_sudo_search_base. - The URI for the LDAP server,
ldap_uri.
For example:Copy to Clipboard Copied! Toggle word wrap Toggle overflow For an Identity Management (IdM or IPA) provider, there are additional parameters required to perform Kerberos authentication when connecting to the server.Copy to Clipboard Copied! Toggle word wrap Toggle overflow Note
Thesudo_providertype for an Identity Management provider is stillldap. - Set the intervals to use to refresh the
sudorule cache.The cache for a specific system user is always checked and updated whenever that user performs a task. However, SSSD caches all rules which relate to the local system. That complete cache is updated in two ways:- Incrementally, meaning only changes to rules since the last full update (
ldap_sudo_smart_refresh_interval, the time in seconds); the default is 15 minutes, - Fully, which dumps the entire caches and pulls in all of the current rules on the LDAP server(
ldap_sudo_full_refresh_interval, the time in seconds); the default is six hours.
These two refresh intervals are set separately. For example:[domain/LDAP] ... ldap_sudo_full_refresh_interval=86400 ldap_sudo_smart_refresh_interval=3600
[domain/LDAP] ... ldap_sudo_full_refresh_interval=86400 ldap_sudo_smart_refresh_interval=3600Copy to Clipboard Copied! Toggle word wrap Toggle overflow Note
SSSD only cachessudorules which apply to the local system. This can mean that thesudoHostvalue is set to ALL, uses a regular expression that matches the host name, matches the systems netgroup, or matches the systems host name, fully qualified domain name, or IP address. - Optionally, set any values to change the schema used for
sudorules.Schema elements are set in theldap_sudorule_*attributes. By default, all of the schema elements use the schema defined in sudoers.ldap; these defaults will be used in almost all deployments. - Save and close the
sssd.conffile. - Configure
sudoto look for rules configuration in SSSD by editing thensswitch.conffile and adding thessslocation:vim /etc/nsswitch.conf
~]# vim /etc/nsswitch.conf sudoers: files sssCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD.
service sssd restart
~]# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.9. Configuring Services: OpenSSH and Cached Keys
Link kopierenLink in die Zwischenablage kopiert!
OpenSSH creates secure, encrypted connections between two systems. One machine authenticates to another machine to allow access; the authentication can be of the machine itself for server connections or of a user on that machine. OpenSSH is described in more detail in Chapter 14, OpenSSH.
This authentication is performed through public-private key pairs that identify the authenticating user or machine. The remote machine or user attempting to access the machine presents a key pair. The local machine then elects whether to trust that remote entity; if it is trusted, the public key for that remote machine is stored in the
known_hosts file or for the remote user in authorized_keys. Whenever that remote machine or user attempts to authenticate again, the local system checks the known_hosts or authorized_keys file first to see if that remote entity is recognized and trusted. If it is, then access is granted.
The first problem comes in verifying those identities reliably.
The
known_hosts file is a triplet of the machine name, its IP address, and its public key:
server.example.com,255.255.255.255 ssh-rsa AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776=
server.example.com,255.255.255.255 ssh-rsa AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776=
The
known_hosts file can quickly become outdated for a number of different reasons: systems using DHCP cycle through IP addresses, new keys can be re-issued periodically, or virtual machines or services can be brought online and removed. This changes the host name, IP address, and key triplet.
Administrators have to clean and maintain a current
known_hosts file to maintain security. (Or system users get in the habit of accepting any machine and key presented, which negates the security benefits of key-based security.)
Additionally, a problem for both machines and users is distributing keys in a scalable way. Machines can send their keys as part of establishing an encrypted session, but users have to supply their keys in advance. Simply propagating and then updating keys consistently is a difficult administrative task.
Lastly, SSH key and machine information are only maintained locally. There may be machines or users on the network which are recognized and trusted by some systems and not by others because the
known_hosts file has not been updated uniformly.
The goal of SSSD is to server as a credentials cache. This includes working as a credentials cache for SSH public keys for machines and users. OpenSSH is configured to reference SSSD to check for cached keys; SSSD uses Red Hat Linux's Identity Management (IPA) domain as an identity, and Identity Management actually stores the public keys and host information.
Note
Only Linux machines enrolled, or joined, in the Identity Management domain can use SSSD as a key cache for OpenSSH. Other Unix machines and Windows machines must use the regular authentication mechanisms with the
known_hosts file.
Configuring OpenSSH to Use SSSD for Host Keys
OpenSSH is configured in either a user-specific configuration file (
~/.ssh/config) or a system-wide configuration file (/etc/ssh/ssh_config). The user file has precedence over the system settings and the first obtained value for a parameter is used. The formatting and conventions for this file are covered in Chapter 14, OpenSSH.
In order to manage host keys, SSSD has a tool,
sss_ssh_knownhostsproxy, which performs two operations:
- Asks SSSD to retrieve the public host key from the Identity Management server and store it in the
/var/lib/sss/pubconf/known_hostsfile. - Establishes a connection with the host machine, using either a socket (the default) or a proxy command.
This tool has the format:
sss_ssh_knownhostsproxy [-d sssd_domain] [-p ssh_port] HOST [PROXY_COMMAND]
sss_ssh_knownhostsproxy [-d sssd_domain] [-p ssh_port] HOST [PROXY_COMMAND]| Short Argument | Long Argument | Description |
|---|---|---|
| HOSTNAME | Gives the host name of the host to check and connect to. In the OpenSSH configuration file, this can be a token, %h. | |
| PROXY_COMMAND | Passes a proxy command to use to connect to the SSH client. This is similar to running ssh -o ProxyCommand=value. This option is used when running sss_ssh_knownhostsproxy from the command line or through another script, but is not necessary in the OpenSSH configuration file. | |
| -d sssd_domain | --domain sssd_domain | Only searches for public keys in entries in the specified domain. If not given, SSSD searches for keys in all configured domains. |
| -p port | --port port | Uses this port to connect to the SSH client. By default, this is port 22. |
To use this SSSD tool, add or edit two parameters to the
ssh_config or ~/.ssh/config file:
- Specify the command to use to connect to the SSH client (
ProxyCommand). This is thesss_ssh_knownhostsproxy, with the desired arguments and host name. - Specify the location of the SSSD hosts file (
GlobalKnownHostsFile).
For example, this looks for public keys in all configured SSSD domains and connects over whatever port and host are supplied:
ProxyCommand /usr/bin/sss_ssh_knownhostsproxy -p %p %h GlobalKnownHostsFile /var/lib/sss/pubconf/known_hosts
ProxyCommand /usr/bin/sss_ssh_knownhostsproxy -p %p %h
GlobalKnownHostsFile /var/lib/sss/pubconf/known_hostsConfiguring OpenSSH to Use SSSD for User Keys
SSSD can provide user public keys to OpenSSH. The keys are read by the SSH daemon,
sshd, directly from the output of the sss_ssh_authorizedkeys tool and are not stored in a file.
To configure
sshd to read a user's public keys from an external program, in this case the sss_ssh_authorizedkeys tool, use the AuthorizedKeysCommand directive in the /etc/ssh/sshd_config file.
The
sss_ssh_authorizedkeys tool can be used to acquire SSH public keys from the user entries in the Identity Management (IPA) domain and output them in OpenSSH authorized_keys format. The command has the following format:
sss_ssh_authorizedkeys [-d sssd_domain] USER
sss_ssh_authorizedkeys [-d sssd_domain] USER| Short Argument | Long Argument | Description |
|---|---|---|
| USER | The user name or account name for which to obtain the public key. In the OpenSSH configuration file, this can be represented by a token, %u. | |
| -d sssd_domain | --domain sssd_domain | Only search for public keys in entries in the specified domain. If not given, SSSD searches for keys in all configured domains. |
This feature is configured in
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
These and further options are documented in the
/etc/ssh/sshd_config as follows:
AuthorizedKeysCommand /usr/bin/sss_ssh_authorizedkeys AuthorizedKeysCommandRunAs nobody
AuthorizedKeysCommand /usr/bin/sss_ssh_authorizedkeys
AuthorizedKeysCommandRunAs nobodysshd_config(5) man page. Note that the sshd service must be restarted for any changes to take effect.
13.2.10. SSSD and Identity Providers (Domains)
Link kopierenLink in die Zwischenablage kopiert!
SSSD recognizes domains, which are entries within the SSSD configuration file associated with different, external data sources. Domains are a combination of an identity provider (for user information) and, optionally, other providers such as authentication (for authentication requests) and for other operations, such as password changes. (The identity provider can also be used for all operations, if all operations are performed within a single domain or server.)
SSSD works with different LDAP identity providers (including OpenLDAP, Red Hat Directory Server, and Microsoft Active Directory) and can use native LDAP authentication, Kerberos authentication, or provider-specific authentication protocols (such as Active Directory).
A domain configuration defines the identity provider, the authentication provider, and any specific configuration to access the information in those providers. There are several types of identity and authentication providers:
- LDAP, for general LDAP servers
- Active Directory (an extension of the LDAP provider type)
- Identity Management (an extension of the LDAP provider type)
- Local, for the local SSSD database
- Proxy
- Kerberos (authentication provider only)
The identity and authentication providers can be configured in different combinations in the domain entry. The possible combinations are listed in Table 13.6, “Identity Store and Authentication Type Combinations”.
| Identification Provider | Authentication Provider |
|---|---|
| Identity Management (LDAP) | Identity Management (LDAP) |
| Active Directory (LDAP) | Active Directory (LDAP) |
| Active Directory (LDAP) | Kerberos |
| LDAP | LDAP |
| LDAP | Kerberos |
| proxy | LDAP |
| proxy | Kerberos |
| proxy | proxy |
Along with the domain entry itself, the domain name must be added to the list of domains that SSSD will query. For example:
global attributes are available to any type of domain, such as cache and time out settings. Each identity and authentication provider has its own set of required and optional configuration parameters.
| Parameter | Value Format | Description |
|---|---|---|
| id_provider | string | Specifies the data back end to use for this domain. The supported identity back ends are:
|
| auth_provider | string | Sets the authentication provider used for the domain. The default value for this option is the value of id_provider. The supported authentication providers are ldap, ipa, ad, krb5 (Kerberos), proxy, and none. |
| min_id,max_id | integer | Optional. Specifies the UID and GID range for the domain. If a domain contains entries that are outside that range, they are ignored. The default value for min_id is 1; the default value for max_id is 0, which is unlimited.
Important
The default min_id value is the same for all types of identity provider. If LDAP directories are using UID numbers that start at one, it could cause conflicts with users in the local /etc/passwd file. To avoid these conflicts, set min_id to 1000 or higher as possible.
|
| cache_credentials | Boolean | Optional. Specifies whether to store user credentials in the local SSSD domain database cache. The default value for this parameter is false. Set this value to true for domains other than the LOCAL domain to enable offline authentication. |
| entry_cache_timeout | integer | Optional. Specifies how long, in seconds, SSSD should cache positive cache hits. A positive cache hit is a successful query. |
| use_fully_qualified_names | Boolean | Optional. Specifies whether requests to this domain require fully qualified domain names. If set to true, all requests to this domain must use fully qualified domain names. It also means that the output from the request displays the fully-qualified name. Restricting requests to fully qualified user names allows SSSD to differentiate between domains with users with conflicting user names.
If
use_fully_qualified_names is set to false, it is possible to use the fully-qualified name in the requests, but only the simplified version is displayed in the output.
SSSD can only parse names based on the domain name, not the realm name. The same name can be used for both domains and realms, however.
|
13.2.11. Creating Domains: LDAP
Link kopierenLink in die Zwischenablage kopiert!
An LDAP domain means that SSSD uses an LDAP directory as the identity provider (and, optionally, also as an authentication provider). SSSD supports several major directory services:
- Red Hat Directory Server
- OpenLDAP
- Identity Management (IdM or IPA)
- Microsoft Active Directory 2008 R2
Note
All of the parameters available to a general LDAP identity provider are also available to Identity Management and Active Directory identity providers, which are subsets of the LDAP provider.
Parameters for Configuring an LDAP Domain
An LDAP directory can function as both an identity provider and an authentication provider. The configuration requires enough information to identify and connect to the user directory in the LDAP server, but the way that those connection parameters are defined is flexible.
Other options are available to provide more fine-grained control, like specifying a user account to use to connect to the LDAP server or using different LDAP servers for password operations. The most common options are listed in Table 13.8, “LDAP Domain Configuration Parameters”.
Note
Server-side password policies always take precedence over the policy enabled from the client side. For example, when setting the
ldap_pwd_policy=shadow option, the policies defined with the shadow LPAD attributes for a user have no effect on whether the password policy is enabled on the OpenLDAP server.
Note
Many other options are listed in the man page for LDAP domain configuration,
sssd-ldap(5).
| Parameter | Description |
|---|---|
| ldap_uri | Gives a comma-separated list of the URIs of the LDAP servers to which SSSD will connect. The list is given in order of preference, so the first server in the list is tried first. Listing additional servers provides failover protection. This can be detected from the DNS SRV records if it is not given. |
| ldap_search_base |
Gives the base DN to use for performing LDAP user operations.
Important
If used incorrectly, ldap_search_base might cause SSSD lookups to fail.
With an AD provider, setting
ldap_search_base is not required. The AD provider automatically discovers all the necessary information. Red Hat recommends not to set the parameter in this situation and instead rely on what the AD provider discovers.
|
| ldap_tls_reqcert | Specifies how to check for SSL server certificates in a TLS session. There are four options:
|
| ldap_tls_cacert | Gives the full path and file name to the file that contains the CA certificates for all of the CAs that SSSD recognizes. SSSD will accept any certificate issued by these CAs.
This uses the OpenLDAP system defaults if it is not given explicitly.
|
| ldap_referrals | Sets whether SSSD will use LDAP referrals, meaning forwarding queries from one LDAP database to another. SSSD supports database-level and subtree referrals. For referrals within the same LDAP server, SSSD will adjust the DN of the entry being queried. For referrals that go to different LDAP servers, SSSD does an exact match on the DN. Setting this value to true enables referrals; this is the default.
Referrals can negatively impact overall performance because of the time spent attempting to trace referrals. Disabling referral checking can significantly improve performance.
|
| ldap_schema | Sets what version of schema to use when searching for user entries. This can be rfc2307, rfc2307bis, ad, or ipa. The default is rfc2307.
In RFC 2307, group objects use a multi-valued attribute,
memberuid, which lists the names of the users that belong to that group. In RFC 2307bis, group objects use the member attribute, which contains the full distinguished name (DN) of a user or group entry. RFC 2307bis allows nested groups using the member attribute. Because these different schema use different definitions for group membership, using the wrong LDAP schema with SSSD can affect both viewing and managing network resources, even if the appropriate permissions are in place.
For example, with RFC 2307bis, all groups are returned when using nested groups or primary/secondary groups.
id
If SSSD is using RFC 2307 schema, only the primary group is returned.
This setting only affects how SSSD determines the group members. It does not change the actual user data.
|
| ldap_search_timeout | Sets the time, in seconds, that LDAP searches are allowed to run before they are canceled and cached results are returned.
When an LDAP search times out, SSSD automatically switches to offline mode.
|
| ldap_network_timeout | Sets the time, in seconds, SSSD attempts to poll an LDAP server after a connection attempt fails. The default is six seconds. |
| ldap_opt_timeout | Sets the time, in seconds, to wait before aborting synchronous LDAP operations if no response is received from the server. This option also controls the timeout when communicating with the KDC in case of a SASL bind. The default is five seconds. |
LDAP Domain Example
The LDAP configuration is very flexible, depending on your specific environment and the SSSD behavior. These are some common examples of an LDAP domain, but the SSSD configuration is not limited to these examples.
Note
Along with creating the domain entry, add the new domain to the list of domains for SSSD to query in the
sssd.conf file. For example:
domains = LOCAL,LDAP1,AD,PROXYNIS
domains = LOCAL,LDAP1,AD,PROXYNISExample 13.2. A Basic LDAP Domain Configuration
An LDAP domain requires three things:
- An LDAP server
- The search base
- A way to establish a secure connection
The last item depends on the LDAP environment. SSSD requires a secure connection since it handles sensitive information. This connection can be a dedicated TLS/SSL connection or it can use Start TLS.
Using a dedicated TLS/SSL connection uses an LDAPS connection to connect to the server and is therefore set as part of the
ldap_uri option:
Using Start TLS requires a way to input the certificate information to establish a secure connection dynamically over an insecure port. This is done using the
ldap_id_use_start_tls option to use Start TLS and then ldap_tls_cacert to identify the CA certificate which issued the SSL server certificates.
13.2.12. Creating Domains: Identity Management (IdM)
Link kopierenLink in die Zwischenablage kopiert!
The Identity Management (IdM or IPA) identity provider is an extension of a generic LDAP provider. All of the configuration options for an LDAP provider are available to the IdM provider, as well as some additional parameters which allow SSSD to work as a client of the IdM domain and extend IdM functionality.
Identity Management can work as a provider for identities, authentication, access control rules, and passwords, all of the
*_provider parameters for a domain. Additionally, Identity Management has configuration options within its own domain to manage SELinux policies, automount information, and host-based access control. All of those features in IdM domains can be tied to SSSD configuraiton, allowing those security-related policies to be applied and cached for system users.
Example 13.3. Basic IdM Provider
An IdM provider, like an LDAP provider, can be set to serve several different services, including identity, authentication, and access control
For IdM servers, there are two additional settings which are very useful (although not required):
- Use the specific IdM schema rather than the default RFC 2307 schema.
- Set SSSD to update the Identity Management domain's DNS server with the IP address of this client when the client first connects to the IdM domain.
Identity Management defines and maintains security policies and identities for users across a Linux domain. This includes access control policies, SELinux policies, and other rules. Some of these elements in the IdM domain interact directly with SSSD, using SSSD as an IdM client — and those features can be managed in the IdM domain entry in
sssd.conf.
Most of the configuration parameters relate to setting schema elements (which is not relevant in most deployments because IdM uses a fixed schema) and never need to be changed. In fact, none of the features in IdM require client-side settings. But there may be circumstances where tweaking the behavior is helpful.
Example 13.4. IdM Provider with SELinux
IdM can define SELinux user policies for system users, so it can work as an SELinux provider for SSSD. This is set in the
selinux_provider parameter. The provider defaults to the id_provider value, so this is not necessary to set explicitly to support SELinux rules. However, it can be useful to explicitly disable SELinux support for the IdM provider in SSSD.
selinux_provider = ipa
selinux_provider = ipaExample 13.5. IdM Provider with Host-Based Access Control
IdM can define host-based access controls, restricting access to services or entire systems based on what host a user is using to connect or attempting to connect to. This rules can be evaluated and enforced by SSSD as part of the access provider behavior.
For host-based access controls to be in effect, the Identity Management server must be the access provider, at a minimum.
There are two options which can be set for how SSSD evaluates host-based access control rules:
- SSSD can evaluate what machine (source host) the user is using to connect to the IdM resource; this is disabled by default, so that only the target host part of the rule is evaluated.
- SSSD can refresh the host-based access control rules in its cache at a specified interval.
For example:
access_provider = ipa ipa_hbac_refresh = 120 # check for source machine rules; disabled by default ipa_hbac_support_srchost = true
access_provider = ipa
ipa_hbac_refresh = 120
# check for source machine rules; disabled by default
ipa_hbac_support_srchost = trueExample 13.6. Identity Management with Cross-Realm Kerberos Trusts
Identity Management (IdM or IPA) can be configured with trusted relationships between Active Directory DNS domains and Kerberos realms. This allows Active Directory users to access services and hosts on Linux systems.
There are two configuration settings in SSSD that are used with cross-realm trusts:
- A service that adds required data to Kerberos tickets
- A setting to support subdomains
Kerberos Ticket Data
Microsoft uses a special authorization structure called privileged access certificates or MS-PAC. A PAC is embedded in a Kerberos ticket as a way of identifying the entity to other Windows clients and servers in the Windows domain.
SSSD has a special PAC service which generates the additional data for Kerberos tickets. When using an Active Directory domain, it may be necessary to include the PAC data for Windows users. In that case, enable the
pac service in SSSD:
[sssd] services = nss, pam, pac ...
[sssd]
services = nss, pam, pac
...
Windows Subdomains
Normally, a domain entry in SSSD corresponds directly to a single identity provider. However, with IdM cross-realm trusts, the IdM domain can trust another domain, so that the domains are transparent to each other. SSSD can follow that trusted relationship, so that if an IdM domain is configured, any Windows domain is also automatically searched and supported by SSSD — without having to be configured in a domain section in SSSD.
This is configured by adding the
subdomains_provider parameter to the IdM domain section. This is actually an optional parameter; if a subdomain is discovered, then SSSD defaults to using the ipa provider type. However, this parameter can also be used to disable subdomain fetches by setting a value of none.
[domain/IDM] ... subdomains_provider = ipa get_domains_timeout = 300
[domain/IDM]
...
subdomains_provider = ipa
get_domains_timeout = 30013.2.13. Creating Domains: Active Directory
Link kopierenLink in die Zwischenablage kopiert!
The Active Directory identity provider is an extension of a generic LDAP provider. All of the configuration options for an LDAP provider are available to the Active Directory provider, as well as some additional parameters related to user accounts and identity mapping between Active Directory and system users.
There are some fundamental differences between standard LDAP servers and an Active Directory server. When configuring an Active Directory provider, there are some configuration areas, then, which require specific configuration:
- Identities using a Windows security ID must be mapped to the corresponding Linux system user ID.
- Searches must account for the range retrieval extension.
- There may be performance issues with LDAP referrals.
Mapping Active Directory Securiy IDs and Linux User IDs
There are inherent structural differences between how Windows and Linux handle system users and in the user schemas used in Active Directory and standard LDAPv3 directory services. When using an Active Directory identity provider with SSSD to manage system users, it is necessary to reconcile the Active Directory-style user to the new SSSD user. There are two ways to do this:
- Using Services for Unix to insert POSIX attributes on Windows user and group entries, and then having those attributes pulled into PAM/NSS
- Using ID mapping on SSSD to create a map between Active Directory security IDs (SIDs) and the generated UIDs on Linux
ID mapping is the simplest option for most environments because it requires no additional packages or configuration on Active Directory.
The Mechanism of ID Mapping
Linux/Unix systems use a local user ID number and group ID number to identify users on the system. These UID:GID numbers are a simple integer, such as 501:501. These numbers are simple because they are always created and administered locally, even for systems which are part of a larger Linux/Unix domain.
Microsoft Windows and Active Directory use a different user ID structure to identify users, groups, and machines. Each ID is constructed of different segments that identify the security version, the issuing authority type, the machine, and the identity itself. For example:
S-1-5-21-3623811015-3361044348-30300820-1013
S-1-5-21-3623811015-3361044348-30300820-1013
The third through sixth blocks are the machine identifier:
S-1-5-21-3623811015-3361044348-30300820-1013
S-1-5-21-3623811015-3361044348-30300820-1013
The last block is the relative identifier (RID) which identifies the specific entity:
S-1-5-21-3623811015-3361044348-30300820-1013
S-1-5-21-3623811015-3361044348-30300820-1013
A range of possible ID numbers are always assigned to SSSD. (This is a local range, so it is the same for every machine.)
|_____________________________| | | minimum ID max ID
|_____________________________|
| |
minimum ID max ID
This range is divided into 10,000 sections (by default), with each section allocated 200,000 IDs.
| slice 1 | slice 2 | ... | |_________|_________|_________| | | | | minimum ID max ID
| slice 1 | slice 2 | ... |
|_________|_________|_________|
| | | |
minimum ID max ID
When a new Active Directory domain is detected, the SID is hashed. Then, SSSD takes the modulus of the hash and the number of available sections to determine which ID section to assign to the Active Directory domain. This is a reliably consistent means of assigning ID sections, so the same ID range is assigned to the same Active Directory domain on most client machines.
Note
While the method of assigning ID sections is consistent, ID mapping is based on the order that an Active Directory domain is encountered on a client machine — so it may not result in consistent ID range assignments on all Linux client machines. If consistency is required, then consider disabling ID mapping and using explicit POSIX attributes.
ID Mapping Parameters
ID mapping is enabled in two parameters, one to enable the mapping and one to load the appropriate Active Directory user schema:
ldap_id_mapping = True ldap_schema = ad
ldap_id_mapping = True
ldap_schema = adNote
When ID mapping is enabled, the
uidNumber and gidNumber attributes are ignored. This prevents any manually-assigned values. If any values must be manually assigned, then all values must be manually assigned, and ID mapping should be disabled.
Mapping Users
When an Active Directory user attempts to log into a local system service for the first time, an entry for that user is created in the SSSD cache. The remote user is set up much like a system user:
- A system UID is created for the user based on his SID and the ID range for that domain.
- A GID is created for the user, which is identical to the UID.
- A private group is created for the user.
- A home directory is created, based on the home directory format in the
sssd.conffile. - A shell is created, according to the system defaults or the setting in the
sssd.conffile. - If the user belongs to any groups in the Active Directory domain, then, using the SID, SSSD adds the user to those groups on the Linux system.
Active Directory Users and Range Retrieval Searches
Microsoft Active Directory has an attribute,
MaxValRange, which sets a limit on how many values for a multi-valued attribute will be returned. This is the range retrieval search extension. Essentially, this runs multiuple mini-searches, each returning a subset of the results within a given range, until all matches are returned.
For example, when doing a search for the
member attribute, each entry could have multiple values, and there can be multiple entries with that attribute. If there are 2000 matching results (or more), then MaxValRange limits how many are displayed at once; this is the value range. The given attribute then has an additional flag set, showing which range in the set the result is in:
attribute:range=low-high:value
attribute:range=low-high:value
For example, results 100 to 500 in a search:
member;range=99-499: cn=John Smith...
member;range=99-499: cn=John Smith...
This is described in the Microsoft documentation at http://msdn.microsoft.com/en-us/library/cc223242.aspx.
SSSD supports range retrievals with Active Directory providers as part of user and group management, without any additional configuration.
However, some LDAP provider attributes which are available to configure searches — such as
ldap_user_search_base — are not performant with range retrievals. Be cautious when configuring search bases in the Active Directory provider domain and consider what searches may trigger a range retrieval.
Performance and LDAP Referrals
Referrals can negatively impact overall performance because of the time spent attempting to trace referrals. There is particularly bad performance degradation when referral chasing is used with an Active Directory identity provider. Disabling referral checking can significantly improve performance.
LDAP referrals are enabled by default, so they must be explicitly disabled in the LDAP domain configuration. For example:
ldap_referrals = false
ldap_referrals = falseActive Directory as Other Provider Types
Active Directory can be used as an identity provider and as an access, password, and authentication provider.
There are a number of options in the generic LDAP provider configuration which can be used to configure an Active Directory provider. Using the
ad value is a short-cut which automatically pulls in the parameters and values to configure a given provider for Active Directory. For example, using access_provider = ad to configure an Active Directory access provider expands to this configuration using the explicit LDAP provider parameters:
access_provider = ldap ldap_access_order = expire ldap_account_expire_policy = ad
access_provider = ldap
ldap_access_order = expire
ldap_account_expire_policy = adProcedure 13.6. Configuring an Active Directory Identity Provider
Active Directory can work as a provider for identities, authentication, access control rules, and passwords, all of the
*_provider parameters for a domain. Additionally, it is possible to load the native Active Directory schema for user and group entries, rather than using the default RFC 2307.
- Make sure that both the Active Directory and Linux systems have a properly configured environment.
- Name resolution must be properly configured, particularly if service discovery is used with SSSD.
- The clocks on both systems must be in sync for Kerberos to work properly.
- Set up the Linux system as an Active Directory client and enroll it within the Active Directory domain. This is done by configuring the Kerberos and Samba services on the Linux system.
- Set up Kerberos to use the Active Directory Kerberos realm.
- Open the Kerberos client configuration file.
vim /etc/krb5.conf
~]# vim /etc/krb5.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure the
[logging]and[libdefaults]sections so that they connect to the Active Directory realm.Copy to Clipboard Copied! Toggle word wrap Toggle overflow If autodiscovery is not used with SSSD, then also configure the[realms]and[domain_realm]sections to explicitly define the Active Directory server.
- Configure the Samba server to connect to the Active directory server.
- Open the Samba configuration file.
vim /etc/samba/smb.conf
~]# vim /etc/samba/smb.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Set the Active Directory domain information in the
[global]section.Copy to Clipboard Copied! Toggle word wrap Toggle overflow
- Add the Linux machine to the Active Directory domain.
- Obtain Kerberos credentials for a Windows administrative user.
kinit Administrator
~]# kinit AdministratorCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Add the machine to the domain using the
netcommand.net ads join -k
~]# net ads join -k Joined 'server' to dns domain 'example.com'Copy to Clipboard Copied! Toggle word wrap Toggle overflow This creates a new keytab file,/etc/krb5.keytab.List the keys for the system and check that the host principal is there.klist -k
~]# klist -kCopy to Clipboard Copied! Toggle word wrap Toggle overflow
- Use
authconfigto enable SSSD for system authentication.authconfig --update --enablesssd --enablesssdauth
# authconfig --update --enablesssd --enablesssdauthCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Set the Active Directory domain as an identity provider in the SSSD configuration, as shown in Example 13.7, “An Active Directory 2008 R2 Domain” and Example 13.8, “An Active Directory 2008 R2 Domain with ID Mapping”.
- Restart the SSH service to load the new PAM configuration.
service sshd restart
~]# service sshd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD after changing the configuration file.
service sssd restart
~]# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Example 13.7. An Active Directory 2008 R2 Domain
There are two parameters that are critical for ID mapping: the Active Directory schema must be loaded (
ldap_schema) and ID mapping must be explicitly enabled (ldap_id_mapping).
Example 13.8. An Active Directory 2008 R2 Domain with ID Mapping
All of the potential configuration attributes for an Active Directory domain are listed in the
sssd-ldap(5) and sssd-ad(5) man pages.
13.2.14. Configuring Domains: Active Directory as an LDAP Provider (Alternative)
Link kopierenLink in die Zwischenablage kopiert!
While Active Directory can be configured as a type-specific identity provider, it can also be configured as a pure LDAP provider with a Kerberos authentication provider.
Procedure 13.7. Configuring Active Directory as an LDAP Provider
- It is recommended that SSSD connect to the Active Directory server using SASL, which means that the local host must have a service keytab for the Windows domain on the Linux host.This keytab can be created using Samba.
- Configure the
/etc/krb5.conffile to use the Active Directory realm.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Set the Samba configuration file,
/etc/samba/smb.conf, to point to the Windows Kerberos realm.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - To initialize Kerberos, type the following command as
root:kinit Administrator@EXAMPLE.COM
~]# kinit Administrator@EXAMPLE.COMCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Then, run the
net adscommand to log in as an administrator principal. This administrator account must have sufficient rights to add a machine to the Windows domain, but it does not require domain administrator privileges.net ads join -U Administrator
~]# net ads join -U AdministratorCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Run
net adsagain to add the host machine to the domain. This can be done with the host principal (host/FQDN) or, optionally, with the NFS service (nfs/FQDN).net ads join createupn="host/rhel-server.example.com@AD.EXAMPLE.COM" -U Administrator
~]# net ads join createupn="host/rhel-server.example.com@AD.EXAMPLE.COM" -U AdministratorCopy to Clipboard Copied! Toggle word wrap Toggle overflow
- Make sure that the Services for Unix package is installed on the Windows server.
- Set up the Windows domain which will be used with SSSD.
- On the Windows machine, open Server Manager.
- Create the Active Directory Domain Services role.
- Create a new domain, such as
ad.example.com. - Add the Identity Management for UNIX service to the Active Directory Domain Services role. Use the Unix NIS domain as the domain name in the configuration.
- On the Active Directory server, create a group for the Linux users.
- Open Administrative Tools and select Active Directory Users and Computers.
- Select the Active Directory domain,
ad.example.com. - In the Users tab, right-click and select .
- Name the new group
unixusers, and save. - Double-click the
unixusersgroup entry, and open the Users tab. - Open the Unix Attributes tab.
- Set the NIS domain to the NIS domain that was configured for
ad.example.comand, optionally, set a group ID (GID) number.
- Configure a user to be part of the Unix group.
- Open Administrative Tools and select Active Directory Users and Computers.
- Select the Active Directory domain,
ad.example.com. - In the Users tab, right-click and select .
- Name the new user
aduser, and make sure that the User must change password at next logon and Lock account check boxes are not selected.Then save the user. - Double-click the
aduseruser entry, and open the Unix Attributes tab. Make sure that the Unix configuration matches that of the Active Directory domain and theunixgroupgroup:- The NIS domain, as created for the Active Directory domain
- The UID
- The login shell, to
/bin/bash - The home directory, to
/home/aduser - The primary group name, to
unixusers
Note
Password lookups on large directories can take several seconds per request. The initial user lookup is a call to the LDAP server. Unindexed searches are much more resource-intensive, and therefore take longer, than indexed searches because the server checks every entry in the directory for a match. To speed up user lookups, index the attributes that are searched for by SSSD:- uid
- uidNumber
- gidNumber
- gecos
- On the Linux system, configure the SSSD domain.
vim /etc/sssd/sssd.conf
~]# vim /etc/sssd/sssd.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow For a complete list of LDAP provider parameters, see thesssd-ldap(5)man pages.Example 13.9. An Active Directory 2008 R2 Domain with Services for Unix
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart SSSD.
service sssd restart
~]# service sssd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.15. Domain Options: Setting Username Formats
Link kopierenLink in die Zwischenablage kopiert!
One of the primary actions that SSSD performs is mapping a local system user to an identity in the remote identity provider. SSSD uses a combination of the user name and the domain back end name to create the login identity.
As long as they belong to different domains, SSSD can recognize different users with the same user name. For example, SSSD can successfully authenticate both
jsmith in the ldap.example.com domain and jsmith in the ldap.otherexample.com domain.
The name format used to construct full user name is (optionally) defined universally in the
[sssd] section of the configuration and can then be defined individually in each domain section.
Usernames for different services — LDAP, Samba, Active Directory, Identity Management, even the local system — all have different formats. The expression that SSSD uses to identify user name/domain name sets must be able to interpret names in different formats. This expression is set in the
re_expression parameter.
In the global default, this filter constructs a name in the form name@domain:
(?P<name>[^@]+)@?(?P<domain>[^@]*$)
(?P<name>[^@]+)@?(?P<domain>[^@]*$)Note
The regular expression format is Python syntax.
The domain part may be supplied automatically, based on the domain name of the identity provider. Therefore, a user can log in as
jsmith and if the user belongs to the LOCAL domain (for example), then his user name is interpreted by SSSD as jsmith@LOCAL.
However, other identity providers may have other formats. Samba, for example, has a very strict format so that user name must match the form DOMAIN\username. For Samba, then, the regular expression must be:
(?P<domain>[^\\]*?)\\?(?P<name>[^\\]+$)
(?P<domain>[^\\]*?)\\?(?P<name>[^\\]+$)
Some providers, such as Active Directory, support multiple different name formats. Active Directory and Identity Management, for example, support three different formats by default:
- username
- username@domain.name
- DOMAIN\username
The default value for Active Directory and Identity Management providers, then, is a more complex filter that allows all three name formats:
(((?P<domain>[^\\]+)\\(?P<name>.+$))|((?P<name>[^@]+)@(?P<domain>.+$))|(^(?P<name>[^@\\]+)$))
(((?P<domain>[^\\]+)\\(?P<name>.+$))|((?P<name>[^@]+)@(?P<domain>.+$))|(^(?P<name>[^@\\]+)$)) Note
Requesting information with the fully-qualified name, such as
jsmith@ldap.example.com, always returns the proper user account. If there are multiple users with the same user name in different domains, specifying only the user name returns the user for whichever domain comes first in the lookup order.
While
re_expression is the most important method for setting user name formats, there are two other options which are useful for other applications.
Default Domain Name Value
The first sets a default domain name to be used with all users, default_domain_suffix. (This is a global setting, available in the [sssd] section only.) There may be a case where multiple domains are configured but only one stores user data and the others are used for host or service identities. Setting a default domain name allows users to log in with only their user name, not specifying the domain name (which would be required for users outside the primary domain).
[sssd] ... default_domain_suffix = USERS.EXAMPLE.COM
[sssd]
...
default_domain_suffix = USERS.EXAMPLE.COMFull Name Format for Output
The other parameter is related to re_expression, only instead of defining how to interpret a user name, it defines how to print an identified name. The full_name_format parameter sets how the user name and domain name (once determined) are displayed.
full_name_format = %1$s@%2$s
full_name_format = %1$s@%2$s13.2.16. Domain Options: Enabling Offline Authentication
Link kopierenLink in die Zwischenablage kopiert!
User identities are always cached, as well as information about the domain services. However, user credentials are not cached by default. This means that SSSD always checks with the back end identity provider for authentication requests. If the identity provider is offline or unavailable, there is no way to process those authentication requests, so user authentication could fail.
It is possible to enable offline credentials caching, which stores credentials (after successful login) as part of the user account in the SSSD cache. Therefore, even if an identity provider is unavailable, users can still authenticate, using their stored credentials. Offline credentials caching is primarily configured in each individual domain entry, but there are some optional settings that can be set in the PAM service section, because credentials caching interacts with the local PAM service as well as the remote domain.
[domain/EXAMPLE] cache_credentials = true
[domain/EXAMPLE]
cache_credentials = true
There are optional parameters that set when those credentials expire. Expiration is useful because it can prevent a user with a potentially outdated account or credentials from accessing local services indefinitely.
The credentials expiration itself is set in the PAM service, which processes authentication requests for the system.
offline_credentials_expiration sets the number of days after a successful login that a single credentials entry for a user is preserved in cache. Setting this to zero (0) means that entries are kept forever.
While not related to the credentials cache specifically, each domain has configuration options on when individual user and service caches expire:
account_cache_expirationsets the number of days after a successful login that the entire user account entry is removed from the SSSD cache. This must be equal to or longer than the individual offline credentials cache expiration period.entry_cache_timeoutsets a validity period, in seconds, for all entries stored in the cache before SSSD requests updated information from the identity provider. There are also individual cache timeout parameters for group, service, netgroup, sudo, and autofs entries; these are listed in thesssd.confman page. The default time is 5400 seconds (90 minutes).
For example:
13.2.17. Domain Options: Setting Password Expirations
Link kopierenLink in die Zwischenablage kopiert!
Password policies generally set an expiration time, after which passwords expire and must be replaced. Password expiration policies are evaluated on the server side through the identity provider, then a warning can be processed and displayed in SSSD through its PAM service.
There are two ways to display password expiration warnings:
- The
pam_pwd_expiration_warningparameter defines the global default setting for all domains on how far in advance of the password expiration to display a warning. This is set for the PAM service. - The
pwd_expiration_warningparameter defines the per-domain setting on how far in advance of the password expiration to display a warning.When using a domain-level password expiration warning, an authentication provider (auth_provider) must also be configured for the domain.
For example:
The password expiration warning must be sent from the server to SSSD for the warning to be displayed. If no password warning is sent from the server, no message is displayed through SSSD, even if the password expiration time is within the period set in SSSD.
If the password expiration warning is not set in SSSD or is set to
0, then the SSSD password warning filter is not applied and the server-side password warning is automatically displayed.
Note
As long as the password warning is sent from the server, the PAM or domain password expirations in effect override the password warning settings on the back end identity provider. For example, consider a back end identity provider that has the warning period set at 28 days, but the PAM service in SSSD has it set to 7 days. The provider sends the warning to SSSD starting at 28 days, but the warning is not displayed locally until 7 days, according to the password expiration set in the SSSD configuration.
Password Expiration Warnings for Non-Password Authentication
By default, password expiration is verified only if the user enters the password during authentication. However, you can configure SSSD to perform the expiration check and display the warning even when a non-password authentication method is used, for example, during SSH login.
To enable password expiration warnings with non-password authentication methods:
- Make sure the
access_providerparameter is set toldapin thesssd.conffile. - Make sure the
ldap_pwd_policyparameter is set insssd.conf. In most situations, the appropriate value isshadow. - Add one of the following
pwd_expire_*values to theldap_access_orderparameter insssd.conf. If the password is about to expire, each one of these values only displays the expiration warning. In addition:pwd_expire_policy_rejectprevents the user from logging in if the password is already expired.pwd_expire_policy_warnallows the user to log in even if the password is already expired.pwd_expire_policy_renewprompts the user to immediately change the password if the user attempts to log in with an expired password.
For example:[domain/EXAMPLE] access_provider = ldap ldap_pwd_policy = shadow ldap_access_order = pwd_expire_policy_warn
[domain/EXAMPLE] access_provider = ldap ldap_pwd_policy = shadow ldap_access_order = pwd_expire_policy_warnCopy to Clipboard Copied! Toggle word wrap Toggle overflow
For more details on using
ldap_access_order and its values, see the sssd-ldap(5) man page.
13.2.18. Domain Options: Using DNS Service Discovery
Link kopierenLink in die Zwischenablage kopiert!
DNS service discovery, defined in RFC 2782, allows applications to check the SRV records in a given domain for certain services of a certain type; it then returns any servers discovered of that type.
With SSSD, the identity and authentication providers can either be explicitly defined (by IP address or host name) or they can be discovered dynamically, using service discovery. If no provider server is listed — for example, if
id_provider = ldap is set without a corresponding ldap_uri parameter — then discovery is automatically used.
The DNS discovery query has this format:
_service._protocol.domain
_service._protocol.domain
For example, a scan for an LDAP server using TCP in the
example.com domain looks like this:
_ldap._tcp.example.com
_ldap._tcp.example.comNote
For every service with which to use service discovery, add a special DNS record to the DNS server:
_service._protocol._domain TTL priority weight port hostname
_service._protocol._domain TTL priority weight port hostname
For SSSD, the service type is LDAP by default, and almost all services use TCP (except for Kerberos, which starts with UDP). For service discovery to be enabled, the only thing that is required is the domain name. The default is to use the domain portion of the machine host name, but another domain can be specified (using the
dns_discovery_domain parameter).
So, by default, no additional configuration needs to be made for service discovery — with one exception. The password change provider has server discovery disabled by default, and it must be explicitly enabled by setting a service type.
[domain/EXAMPLE] ... chpass_provider = ldap ldap_chpass_dns_service_name = ldap
[domain/EXAMPLE]
...
chpass_provider = ldap
ldap_chpass_dns_service_name = ldap
While no configuration is necessary, it is possible for server discovery to be customized by using a different DNS domain (
dns_discovery_domain) or by setting a different service type to scan for. For example:
Lastly, service discovery is never used with backup servers; it is only used for the primary server for a provider. What this means is that discovery can be used initially to locate a server, and then SSSD can fall back to using a backup server. To use discovery for the primary server, use
_srv_ as the primary server value, and then list the backup servers. For example:
Note
Service discovery cannot be used with backup servers, only primary servers.
If a DNS lookup fails to return an IPv4 address for a host name, SSSD attempts to look up an IPv6 address before returning a failure. This only ensures that the asynchronous resolver identifies the correct address.
The host name resolution behavior is configured in the
lookup family order option in the sssd.conf configuration file.
13.2.19. Domain Options: Using IP Addresses in Certificate Subject Names (LDAP Only)
Link kopierenLink in die Zwischenablage kopiert!
Using an IP address in the
ldap_uri option instead of the server name may cause the TLS/SSL connection to fail. TLS/SSL certificates contain the server name, not the IP address. However, the subject alternative name field in the certificate can be used to include the IP address of the server, which allows a successful secure connection using an IP address.
Procedure 13.8. Using IP Addresses in Certificate Subject Names
- Convert an existing certificate into a certificate request. The signing key (
-signkey) is the key of the issuer of whatever CA originally issued the certificate. If this is done by an external CA, it requires a separate PEM file; if the certificate is self-signed, then this is the certificate itself. For example:openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey key.pem
openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey key.pemCopy to Clipboard Copied! Toggle word wrap Toggle overflow With a self-signed certificate:openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey old_cert.pem
openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey old_cert.pemCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
/etc/pki/tls/openssl.cnfconfiguration file to include the server's IP address under the[ v3_ca ]section:subjectAltName = IP:10.0.0.10
subjectAltName = IP:10.0.0.10Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Use the generated certificate request to generate a new self-signed certificate with the specified IP address:
openssl x509 -req -in req.pem -out new_cert.pem -extfile ./openssl.cnf -extensions v3_ca -signkey old_cert.pem
openssl x509 -req -in req.pem -out new_cert.pem -extfile ./openssl.cnf -extensions v3_ca -signkey old_cert.pemCopy to Clipboard Copied! Toggle word wrap Toggle overflow The-extensionsoption sets which extensions to use with the certificate. For this, it should be v3_ca to load the appropriate section. - Copy the private key block from the
old_cert.pemfile into thenew_cert.pemfile to keep all relevant information in one file.
When creating a certificate through the certutil utility provided by the nss-tools package, note that certutil supports DNS subject alternative names for certificate creation only.
13.2.20. Creating Domains: Proxy
Link kopierenLink in die Zwischenablage kopiert!
A proxy with SSSD is just a relay, an intermediary configuration. SSSD connects to its proxy service, and then that proxy loads the specified libraries. This allows SSSD to use some resources that it otherwise would not be able to use. For example, SSSD only supports LDAP and Kerberos as authentication providers, but using a proxy allows SSSD to use alternative authentication methods like a fingerprint scanner or smart card.
| Parameter | Description |
|---|---|
| proxy_pam_target | Specifies the target to which PAM must proxy as an authentication provider. The PAM target is a file containing PAM stack information in the default PAM directory, /etc/pam.d/.
This is used to proxy an authentication provider.
Important
Ensure that the proxy PAM stack does not recursively include pam_sss.so.
|
| proxy_lib_name | Specifies which existing NSS library to proxy identity requests through.
This is used to proxy an identity provider.
|
Example 13.10. Proxy Identity and Kerberos Authentication
The proxy library is loaded using the
proxy_lib_name parameter. This library can be anything as long as it is compatible with the given authentication service. For a Kerberos authentication provider, it must be a Kerberos-compatible library, like NIS.
Example 13.11. LDAP Identity and Proxy Authentication
The proxy library is loaded using the
proxy_pam_target parameter. This library must be a PAM module that is compatible with the given identity provider. For example, this uses a PAM fingerprint module with LDAP:
After the SSSD domain is configured, make sure that the specified PAM files are configured. In this example, the target is
sssdpamproxy, so create a /etc/pam.d/sssdpamproxy file and load the PAM/LDAP modules:
auth required pam_frprint.so account required pam_frprint.so password required pam_frprint.so session required pam_frprint.so
auth required pam_frprint.so
account required pam_frprint.so
password required pam_frprint.so
session required pam_frprint.soExample 13.12. Proxy Identity and Authentication
SSSD can have a domain with both identity and authentication proxies. The only configuration given then are the proxy settings,
proxy_pam_target for the authentication PAM module and proxy_lib_name for the service, like NIS or LDAP.
This example illustrates a possible configuration, but this is not a realistic configuration. If LDAP is used for identity and authentication, then both the identity and authentication providers should be set to the LDAP configuration, not a proxy.
Once the SSSD domain is added, then update the system settings to configure the proxy service:
- Create a
/etc/pam.d/sssdproxyldapfile which requires thepam_ldap.somodule:auth required pam_ldap.so account required pam_ldap.so password required pam_ldap.so session required pam_ldap.so
auth required pam_ldap.so account required pam_ldap.so password required pam_ldap.so session required pam_ldap.soCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make sure the nss-pam-ldapd package is installed.
yum install nss-pam-ldapd
~]# yum install nss-pam-ldapdCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
/etc/nslcd.conffile, the configuration file for the LDAP name service daemon, to contain the information for the LDAP directory:Copy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.21. Creating Domains: Kerberos Authentication
Link kopierenLink in die Zwischenablage kopiert!
Both LDAP and proxy identity providers can use a separate Kerberos domain to supply authentication. Configuring a Kerberos authentication provider requires the key distribution center (KDC) and the Kerberos domain. All of the principal names must be available in the specified identity provider; if they are not, SSSD constructs the principals using the format username@REALM.
Note
Kerberos can only provide authentication; it cannot provide an identity database.
SSSD assumes that the Kerberos KDC is also a Kerberos kadmin server. However, production environments commonly have multiple, read-only replicas of the KDC and only a single kadmin server. Use the
krb5_kpasswd option to specify where the password changing service is running or if it is running on a non-default port. If the krb5_kpasswd option is not defined, SSSD tries to use the Kerberos KDC to change the password.
The basic Kerberos configuration options are listed in Table 13.10, “Kerberos Authentication Configuration Parameters”. The
sssd-krb5(5) man page has more information about Kerberos configuration options.
Example 13.13. Basic Kerberos Authentication
Example 13.14. Setting Kerberos Ticket Renewal Options
The Kerberos authentication provider, among other tasks, requests ticket granting tickets (TGT) for users and services. These tickets are used to generate other tickets dynamically for specific services, as accessed by the ticket principal (the user).
The TGT initially granted to the user principal is valid only for the lifetime of the ticket (by default, whatever is configured in the configured KDC). After that, the ticket cannot be renewed or extended. However, not renewing tickets can cause problems with some services when they try to access a service in the middle of operations and their ticket has expired.
Kerberos tickets are not renewable by default, but ticket renewal can be enabled using the
krb5_renewable_lifetime and krb5_renew_interval parameters.
The lifetime for a ticket is set in SSSD with the
krb5_lifetime parameter. This specifies how long a single ticket is valid, and overrides any values in the KDC.
Ticket renewal itself is enabled in the
krb5_renewable_lifetime parameter, which sets the maximum lifetime of the ticket, counting all renewals.
For example, the ticket lifetime is set at one hour and the renewable lifetime is set at 24 hours:
krb5_lifetime = 1h krb5_renewable_lifetime = 1d
krb5_lifetime = 1h
krb5_renewable_lifetime = 1d
This means that the ticket expires every hour and can be renewed continually up to one day.
The lifetime and renewable lifetime values can be in seconds (s), minutes (m), hours (h), or days (d).
The other option — which must also be set for ticket renewal — is the
krb5_renew_interval parameter, which sets how frequently SSSD checks to see if the ticket needs to be renewed. At half of the ticket lifetime (whatever that setting is), the ticket is renewed automatically. (This value is always in seconds.)
krb5_lifetime = 1h krb5_renewable_lifetime = 1d krb5_renew_interval = 60s
krb5_lifetime = 1h
krb5_renewable_lifetime = 1d
krb5_renew_interval = 60sNote
If the
krb5_renewable_lifetime value is not set or the krb5_renew_interval parameter is not set or is set to zero (0), then ticket renewal is disabled. Both krb5_renewable_lifetime and krb5_renew_interval are required for ticket renewal to be enabled.
| Parameter | Description |
|---|---|
| chpass_provider | Specifies which service to use for password change operations. This is assumed to be the same as the authentication provider. To use Kerberos, set this to krb5. |
| krb5_server | Gives the primary Kerberos server, by IP address or host names, to which SSSD will connect. |
| krb5_backup_server | Gives a comma-separated list of IP addresses or host names of Kerberos servers to which SSSD will connect if the primary server is not available. The list is given in order of preference, so the first server in the list is tried first.
After an hour, SSSD will attempt to reconnect to the primary service specified in the
krb5_server parameter.
When using service discovery for KDC or kpasswd servers, SSSD first searches for DNS entries that specify UDP as the connection protocol, and then falls back to TCP.
|
| krb5_realm | Identifies the Kerberos realm served by the KDC. |
| krb5_lifetime | Requests a Kerberos ticket with the specified lifetime in seconds (s), minutes (m), hours (h) or days (d). |
| krb5_renewable_lifetime | Requests a renewable Kerberos ticket with a total lifetime that is specified in seconds (s), minutes (m), hours (h) or days (d). |
| krb5_renew_interval | Sets the time, in seconds, for SSSD to check if tickets should be renewed. Tickets are renewed automatically once they exceed half their lifetime. If this option is missing or set to zero, then automatic ticket renewal is disabled. |
| krb5_store_password_if_offline | Sets whether to store user passwords if the Kerberos authentication provider is offline, and then to use that cache to request tickets when the provider is back online. The default is false, which does not store passwords. |
| krb5_kpasswd | Lists alternate Kerberos kadmin servers to use if the change password service is not running on the KDC. |
| krb5_ccname_template | Gives the directory to use to store the user's credential cache. This can be templatized, and the following tokens are supported:
krb5_ccname_template = FILE:%d/krb5cc_%U_XXXXXX |
| krb5_ccachedir | Specifies the directory to store credential caches. This can be templatized, using the same tokens as krb5_ccname_template, except for %d and %P. If %u, %U, %p, or %h are used, then SSSD creates a private directory for each user; otherwise, it creates a public directory. |
| krb5_auth_timeout | Gives the time, in seconds, before an online authentication or change password request is aborted. If possible, the authentication request is continued offline. The default is 15 seconds. |
13.2.22. Creating Domains: Access Control
Link kopierenLink in die Zwischenablage kopiert!
SSSD provides a rudimentary access control for domain configuration, allowing either simple user allow/deny lists or using the LDAP back end itself.
Using the Simple Access Provider
The Simple Access Provider allows or denies access based on a list of user names or groups.
The Simple Access Provider is a way to restrict access to certain, specific machines. For example, if a company uses laptops, the Simple Access Provider can be used to restrict access to only a specific user or a specific group, even if a different user authenticated successfully against the same authentication provider.
The most common options are
simple_allow_users and simple_allow_groups, which grant access explicitly to specific users (either the given users or group members) and deny access to everyone else. It is also possible to create deny lists (which deny access only to explicit people and implicitly allow everyone else access).
The Simple Access Provider adheres to the following four rules to determine which users should or should not be granted access:
- If both the allow and deny lists are empty, access is granted.
- If any list is provided, allow rules are evaluated first, and then deny rules. Practically, this means that deny rules supersede allow rules.
- If an allowed list is provided, then all users are denied access unless they are in the list.
- If only deny lists are provided, then all users are allowed access unless they are in the list.
This example grants access to two users and anyone who belongs to the IT group; implicitly, all other users are denied:
[domain/example.com] access_provider = simple simple_allow_users = jsmith,bjensen simple_allow_groups = itgroup
[domain/example.com]
access_provider = simple
simple_allow_users = jsmith,bjensen
simple_allow_groups = itgroupNote
The LOCAL domain in SSSD does not support
simple as an access provider.
Other options are listed in the
sssd-simple man page, but these are rarely used.
Using the Access Filters
An LDAP, Active Directory, or Identity Management server can provide access control rules for a domain. The associated options (
ldap_access_filter for LDAP and IdM and ad_access_filter for AD) specify which users are granted access to the specified host. The user filter must be used or all users are denied access. See the examples below:
[domain/example.com] access_provider = ldap ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com
[domain/example.com]
access_provider = ldap
ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com[domain/example.com] access_provider = ad ad_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com
[domain/example.com]
access_provider = ad
ad_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=comNote
Offline caching for LDAP access providers is limited to determining whether the user's last online login attempt was successful. Users that were granted access during their last login will continue to be granted access while offline.
SSSD can also check results by the
authorizedService or host attribute in an entry. In fact, all options — LDAP filter, authorizedService, and host — can be evaluated, depending on the user entry and the configuration. The ldap_access_order parameter lists all access control methods to use, in order of how they should be evaluated.
[domain/example.com] access_provider = ldap ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com ldap_access_order = filter, host, authorized_service
[domain/example.com]
access_provider = ldap
ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com
ldap_access_order = filter, host, authorized_service
The attributes in the user entry to use to evaluate authorized services or allowed hosts can be customized. Additional access control parameters are listed in the
sssd-ldap(5) man page.
13.2.23. Creating Domains: Primary Server and Backup Servers
Link kopierenLink in die Zwischenablage kopiert!
Identity and authentication providers for a domain can be configured for automatic failover. SSSD attempts to connect to the specified, primary server first. If that server cannot be reached, then SSSD then goes through the listed backup servers, in order.
Note
SSSD tries to connect to the primary server every 30 seconds, until the connection can be re-established, and then switches from the backup to the primary.
All of the major service areas have optional settings for primary and backup servers[3].
| Service Area | Primary Server Attribute | Backup Server Attribute |
|---|---|---|
| LDAP identity provider | ldap_uri | ldap_backup_uri |
| Active Directory identity provider | ad_server | ad_backup_server |
| Identity Management (IdM or IPA) identity provider | ipa_server | ipa_backup_server |
| Kerberos authentication provider | krb5_server | krb5_backup_server |
| Kerberos authentication provider | krb5_server | krb5_backup_server |
| Password change provider | ldap_chpass_uri | ldap_chpass_backup_uri |
One and only one server can be set as the primary server. (And, optionally, the primary server can be set to service discovery, using
_srv_ rather than a host name.) Multiple backup servers can be set, in a comma-separated list. The backup server list is in order of preference, so the first server listed is tried first.
[domain/EXAMPLE] id_provider = ad ad_server = ad.example.com ad_backup_server = ad1.example.com, ad-backup.example.com
[domain/EXAMPLE]
id_provider = ad
ad_server = ad.example.com
ad_backup_server = ad1.example.com, ad-backup.example.com13.2.24. Installing SSSD Utilities
Link kopierenLink in die Zwischenablage kopiert!
Additional tools to handle the SSSD cache, user entries, and group entries are contained in the sssd-tools package. This package is not required, but it is useful to install to help administer user accounts.
yum install sssd-tools
~]# yum install sssd-toolsNote
The sssd-tools package is provided by the Optional subscription channel. See Section 8.4.8, “Adding the Optional and Supplementary Repositories” for more information on Red Hat additional channels.
13.2.25. SSSD and UID and GID Numbers
Link kopierenLink in die Zwischenablage kopiert!
When a user is created — using system tools such as
useradd or through an application such as Red Hat Identity Management or other client tools — the user is automatically assigned a user ID number and a group ID number.
When the user logs into a system or service, SSSD caches that user name with the associated UID/GID numbers. The UID number is then used as the identifying key for the user. If a user with the same name but a different UID attempts to log into the system, then SSSD treats it as two different users with a name collision.
What this means is that SSSD does not recognize UID number changes. It interprets it as a different and new user, not an existing user with a different UID number. If an existing user changes the UID number, that user is prevented from logging into SSSD and associated services and domains. This also has an impact on any client applications which use SSSD for identity information; the user with the conflict will not be found or accessible to those applications.
Important
UID/GID changes are not supported in SSSD.
If a user for some reason has a changed UID/GID number, then the SSSD cache must be cleared for that user before that user can log in again. For example:
sss_cache -u jsmith
~]# sss_cache -u jsmith
Cleaning the SSSD cache is covered in the section called “Purging the SSSD Cache”.
13.2.26. Creating Local System Users
Link kopierenLink in die Zwischenablage kopiert!
There can be times when it is useful to seed users into the SSSD database rather than waiting for users to login and be added.
Note
Adding user accounts manually requires the sssd-tools package to be installed.
When creating new system users, it is possible to create a user within the SSSD local identity provider domain. This can be useful for creating new system users, for troubleshooting SSSD configuration, or for creating specialized or nested groups.
New users can be added using the
sss_useradd command.
At its most basic, the
sss_useradd command only requires the new user name.
sss_useradd jsmith
~]# sss_useradd jsmith
There are other options (listed in the
sss_useradd(8) man page) which can be used to set attributes on the account, like the UID and GID, the home directory, or groups which the user belongs to.
sss_useradd --UID 501 --home /home/jsmith --groups admin,dev-group jsmith
~]# sss_useradd --UID 501 --home /home/jsmith --groups admin,dev-group jsmith13.2.27. Seeding Users into the SSSD Cache During Kickstart
Link kopierenLink in die Zwischenablage kopiert!
Note
Adding user accounts manually requires the sssd-tools package to be installed.
With SSSD, users in a remote domain are not available in a local system until that identity is retrieved from the identity provider. However, some network interfaces are not available until a user has logged in — which is not possible if the user identity is somewhere over the network. In that case, it is possible to seed the SSSD cache with that user identity, associated with the appropriate domain, so that the user can log in locally and active the appropriate interfaces.
This is done using the
sss_seed utility:
sss_seed --domain EXAMPLE.COM --username testuser --password-file /tmp/sssd-pwd.txt
sss_seed --domain EXAMPLE.COM --username testuser --password-file /tmp/sssd-pwd.txt
This utility requires options that identify, at a minimum, the user name, domain name, and password.
--domaingives the domain name from the SSSD configuration. This domain must already exist in the SSSD configuration.--usernamefor the short name of the user account.--password-filefor the path and name of a file containing a temporary password for the seed entry. If the user account already exists in the SSSD cache, then the temporary password in this file overwrites the stored password in the SSSD cache.
Additional account configuration options are listed in the
sss_seed(8) man page.
This would almost always be run as part of a kickstart or automated setup, so it would be part of a larger set of scripts, which would also enable SSSD, set up an SSSD domain, and create the password file. For example:
13.2.28. Managing the SSSD Cache
Link kopierenLink in die Zwischenablage kopiert!
SSSD can define multiple domains of the same type and different types of domain. SSSD maintains a separate database file for each domain, meaning each domain has its own cache. These cache files are stored in the
/var/lib/sss/db/ directory.
Purging the SSSD Cache
As LDAP updates are made to the identity provider for the domains, it can be necessary to clear the cache to reload the new information quickly.
The cache purge utility,
sss_cache, invalidates records in the SSSD cache for a user, a domain, or a group. Invalidating the current records forces the cache to retrieve the updated records from the identity provider, so changes can be realized quickly.
Note
This utility is included with SSSD in the sssd package.
Most commonly, this is used to clear the cache and update all records:
sss_cache -E
~]# sss_cache -E
The
sss_cache command can also clear all cached entries for a particular domain:
sss_cache -Ed LDAP1
~]# sss_cache -Ed LDAP1
If the administrator knows that a specific record (user, group, or netgroup) has been updated, then
sss_cache can purge the records for that specific account and leave the rest of the cache intact:
sss_cache -u jsmith
~]# sss_cache -u jsmith| Short Argument | Long Argument | Description |
|---|---|---|
| -E | --everything | Invalidates all cached entries with the exception of sudo rules. |
| -d name | --domain name | Invalidates cache entries for users, groups, and other entries only within the specified domain. |
| -G | --groups | Invalidates all group records. If -g is also used, -G takes precedence and -g is ignored. |
| -g name | --group name | Invalidates the cache entry for the specified group. |
| -N | --netgroups | Invalidates cache entries for all netgroup cache records. If -n is also used, -N takes precedence and -n is ignored. |
| -n name | --netgroup name | Invalidates the cache entry for the specified netgroup. |
| -U | --users | Invalidates cache entries for all user records. If the -u option is also used, -U takes precedence and -u is ignored. |
| -u name | --user name | Invalidates the cache entry for the specified user. |
Deleting Domain Cache Files
All cache files are named for the domain. For example, for a domain named
exampleldap, the cache file is named cache_exampleldap.ldb.
Be careful when you delete a cache file. This operation has significant effects:
- Deleting the cache file deletes all user data, both identification and cached credentials. Consequently, do not delete a cache file unless the system is online and can authenticate with a user name against the domain's servers. Without a credentials cache, offline authentication will fail.
- If the configuration is changed to reference a different identity provider, SSSD will recognize users from both providers until the cached entries from the original provider time out.It is possible to avoid this by purging the cache, but the better option is to use a different domain name for the new provider. When SSSD is restarted, it creates a new cache file with the new name and the old file is ignored.
13.2.29. Downgrading SSSD
Link kopierenLink in die Zwischenablage kopiert!
When downgrading — either downgrading the version of SSSD or downgrading the operating system itself — then the existing SSSD cache needs to be removed. If the cache is not removed, then SSSD process is dead but a PID file remains. The SSSD logs show that it cannot connect to any of its associated domains because the cache version is unrecognized.
(Wed Nov 28 21:25:50 2012) [sssd] [sysdb_domain_init_internal] (0x0010): Unknown DB version [0.14], expected [0.10] for domain AD!
(Wed Nov 28 21:25:50 2012) [sssd] [sysdb_domain_init_internal] (0x0010): Unknown DB version [0.14], expected [0.10] for domain AD!
Users are then no longer recognized and are unable to authenticate to domain services and hosts.
After downgrading the SSSD version:
- Delete the existing cache database files.
rm -rf /var/lib/sss/db/*
~]# rm -rf /var/lib/sss/db/*Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart the SSSD process.
service sssd restart
~]# service sssd restart Stopping sssd: [FAILED] Starting sssd: [ OK ]Copy to Clipboard Copied! Toggle word wrap Toggle overflow
13.2.30. Using NSCD with SSSD
Link kopierenLink in die Zwischenablage kopiert!
SSSD is not designed to be used with the NSCD daemon. Even though SSSD does not directly conflict with NSCD, using both services can result in unexpected behavior, especially with how long entries are cached.
The most common evidence of a problem is conflicts with NFS. When using Network Manager to manage network connections, it may take several minutes for the network interface to come up. During this time, various services attempt to start. If these services start before the network is up and the DNS servers are available, these services fail to identify the forward or reverse DNS entries they need. These services will read an incorrect or possibly empty
resolv.conf file. This file is typically only read once, and so any changes made to this file are not automatically applied. This can cause NFS locking to fail on the machine where the NSCD service is running, unless that service is manually restarted.
To avoid this problem, enable caching for hosts and services in the
/etc/nscd.conf file and rely on the SSSD cache for the passwd, group, and netgroup entries.
Change the
/etc/nscd.conf file:
enable-cache hosts yes enable-cache passwd no enable-cache group no enable-cache netgroup no
enable-cache hosts yes
enable-cache passwd no
enable-cache group no
enable-cache netgroup no
With NSCD answering hosts requests, these entries will be cached by NSCD and returned by NSCD during the boot process. All other entries are handled by SSSD.
13.2.31. Troubleshooting SSSD
Link kopierenLink in die Zwischenablage kopiert!
Setting Debug Logs for SSSD Domains
Each domain sets its own debug log level. Increasing the log level can provide more information about problems with SSSD or with the domain configuration.
To change the log level, set the
debug_level parameter for each section in the sssd.conf file for which to produce extra logs. For example:
[domain/LDAP] cache_credentials = true debug_level = 9
[domain/LDAP]
cache_credentials = true
debug_level = 9| Level | Description |
|---|---|
| 0 | Fatal failures. Anything that would prevent SSSD from starting up or causes it to cease running. |
| 1 | Critical failures. An error that doesn't kill the SSSD, but one that indicates that at least one major feature is not going to work properly. |
| 2 | Serious failures. An error announcing that a particular request or operation has failed. |
| 3 | Minor failures. These are the errors that would percolate down to cause the operation failure of 2. |
| 4 | Configuration settings. |
| 5 | Function data. |
| 6 | Trace messages for operation functions. |
| 7 | Trace messages for internal control functions. |
| 8 | Contents of function-internal variables that may be interesting. |
| 9 | Extremely low-level tracing information. |
Note
In versions of SSSD older than 1.8, debug log levels could be set globally in the
[sssd] section. Now, each domain and service must configure its own debug log level.
To copy the global SSSD debug log levels into each configuration area in the SSSD configuration file, use the
sssd_update_debug_levels.py script.
python -m SSSDConfig.sssd_update_debug_levels.py
python -m SSSDConfig.sssd_update_debug_levels.pyChecking SSSD Log Files
SSSD uses a number of log files to report information about its operation, located in the
/var/log/sssd/ directory. SSSD produces a log file for each domain, as well as an sssd_pam.log and an sssd_nss.log file.
Additionally, the
/var/log/secure file logs authentication failures and the reason for the failure.
Problems with SSSD Configuration
- Q: SSSD fails to start
- Q: I don't see any groups with 'id' or group members with 'getent group'.
- Q: Authentication fails against LDAP.
- Q: Connecting to LDAP servers on non-standard ports fail.
- Q: NSS fails to return user information
- Q: NSS returns incorrect user information
- Q: Setting the password for the local SSSD user prompts twice for the password
- Q: I am trying to use sudo rules with an Identity Management (IPA) provider, but no sudo rules are being found, even though sudo is properly configured.
- Q: Password lookups on large directories can take several seconds per request. How can this be improved?
- Q: An Active Directory identity provider is properly configured in my sssd.conf file, but SSSD fails to connect to it, with GSS-API errors.
- Q: I configured SSSD for central authentication, but now several of my applications (such as Firefox or Adobe) will not start.
- Q: SSSD is showing an automount location that I removed.
Q:
SSSD fails to start
A:
SSSD requires that the configuration file be properly set up, with all the required entries, before the daemon will start.
- SSSD requires at least one properly configured domain before the service will start. Without a domain, attempting to start SSSD returns an error that no domains are configured:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow Edit the/etc/sssd/sssd.conffile and create at least one domain. - SSSD also requires at least one available service provider before it will start. If the problem is with the service provider configuration, the error message indicates that there are no services configured:
[sssd] [get_monitor_config] (0): No services configured!
[sssd] [get_monitor_config] (0): No services configured!Copy to Clipboard Copied! Toggle word wrap Toggle overflow Edit the/etc/sssd/sssd.conffile and configure at least one service provider.Important
SSSD requires that service providers be configured as a comma-separated list in a singleservicesentry in the/etc/sssd/sssd.conffile. If services are listed in multiple entries, only the last entry is recognized by SSSD.
Q:
I don't see any groups with 'id' or group members with 'getent group'.
A:
This may be due to an incorrect
ldap_schema setting in the [domain/DOMAINNAME] section of sssd.conf.
SSSD supports RFC 2307 and RFC 2307bis schema types. By default, SSSD uses the more common RFC 2307 schema.
The difference between RFC 2307 and RFC 2307bis is the way which group membership is stored in the LDAP server. In an RFC 2307 server, group members are stored as the multi-valued
memberuid attribute, which contains the name of the users that are members. In an RFC2307bis server, group members are stored as the multi-valued member or uniqueMember attribute which contains the DN of the user or group that is a member of this group. RFC2307bis allows nested groups to be maintained as well.
If group lookups are not returning any information:
- Set
ldap_schematorfc2307bis. - Delete
/var/lib/sss/db/cache_DOMAINNAME.ldb. - Restarting SSSD.
If that doesn't work, add this line to
sssd.conf:
ldap_group_name = uniqueMember
ldap_group_name = uniqueMember
Then delete the cache and restart SSSD again.
Q:
Authentication fails against LDAP.
A:
To perform authentication, SSSD requires that the communication channel be encrypted. This means that if
sssd.conf is configured to connect over a standard protocol (ldap://), it attempts to encrypt the communication channel with Start TLS. If sssd.conf is configured to connect over a secure protocol (ldaps://), then SSSD uses SSL.
This means that the LDAP server must be configured to run in SSL or TLS. TLS must be enabled for the standard LDAP port (389) or SSL enabled on the secure LDAPS port (636). With either SSL or TLS, the LDAP server must also be configured with a valid certificate trust.
An invalid certificate trust is one of the most common issues with authenticating against LDAP. If the client does not have proper trust of the LDAP server certificate, it is unable to validate the connection, and SSSD refuses to send the password. The LDAP protocol requires that the password be sent in plaintext to the LDAP server. Sending the password in plaintext over an unencrypted connection is a security problem.
If the certificate is not trusted, a
syslog message is written, indicating that TLS encryption could not be started. The certificate configuration can be tested by checking if the LDAP server is accessible apart from SSSD. For example, this tests an anonymous bind over a TLS connection to test.example.com:
ldapsearch -x -ZZ -h test.example.com -b dc=example,dc=com
$ ldapsearch -x -ZZ -h test.example.com -b dc=example,dc=com
If the certificate trust is not properly configured, the test fails with this error:
ldap_start_tls: Connect error (-11) additional info: TLS error -8179:Unknown code ___f 13
ldap_start_tls: Connect error (-11) additional info: TLS error -8179:Unknown code ___f 13
To trust the certificate:
- Obtain a copy of the public CA certificate for the certificate authority used to sign the LDAP server certificate and save it to the local system.
- Add a line to the
sssd.conffile that points to the CA certificate on the filesystem.ldap_tls_cacert = /path/to/cacert
ldap_tls_cacert = /path/to/cacertCopy to Clipboard Copied! Toggle word wrap Toggle overflow - If the LDAP server uses a self-signed certificate, remove the
ldap_tls_reqcertline from thesssd.conffile.This parameter directs SSSD to trust any certificate issued by the CA certificate, which is a security risk with a self-signed CA certificate.
Q:
Connecting to LDAP servers on non-standard ports fail.
A:
When running SELinux in enforcing mode, the client's SELinux policy has to be modified to connect to the LDAP server over the non-standard port. For example:
semanage port -a -t ldap_port_t -p tcp 1389
# semanage port -a -t ldap_port_t -p tcp 1389Q:
NSS fails to return user information
A:
This usually means that SSSD cannot connect to the NSS service.
- Ensure that NSS is running:
service sssd status
# service sssd statusCopy to Clipboard Copied! Toggle word wrap Toggle overflow - If NSS is running, make sure that the provider is properly configured in the
[nss]section of the/etc/sssd/sssd.conffile. Especially check thefilter_usersandfilter_groupsattributes. - Make sure that NSS is included in the list of services that SSSD uses.
- Check the configuration in the
/etc/nsswitch.conffile.
Q:
NSS returns incorrect user information
A:
If searches are returning the incorrect user information, check that there are not conflicting user names in separate domains. When there are multiple domains, set the
use_fully_qualified_domains attribute to true in the /etc/sssd/sssd.conf file. This differentiates between different users in different domains with the same name.
Q:
Setting the password for the local SSSD user prompts twice for the password
A:
When attempting to change a local SSSD user's password, it may prompt for the password twice:
This is the result of an incorrect PAM configuration. Ensure that the
use_authtok option is correctly configured in your /etc/pam.d/system-auth file.
Q:
I am trying to use sudo rules with an Identity Management (IPA) provider, but no sudo rules are being found, even though sudo is properly configured.
A:
The SSSD client can successfully authenticate to the Identity Management server, and it is properly searching the LDAP directory for sudo rules. However, it is showing that no rules exist. For example, in the logs:
When using an Identity Management provider for SSSD, SSSD attempts to connect to the underlying LDAP directory using Kerberos/GSS-API. However, by default, SSSD uses an anonymous connection to an LDAP server to retrieve sudo rules. This means that SSSD cannot retrieve the sudo rules from the Identity Management server with its default configuration.
To support retrieving sudo rules with a Kerberos/GSS-API connection, enable GSS-API as the authentication mechanism in the identity provider configuration in
sssd.conf. For example:
Q:
Password lookups on large directories can take several seconds per request. How can this be improved?
A:
The initial user lookup is a call to the LDAP server. Unindexed searches are much more resource-intensive, and therefore take longer, than indexed searches because the server checks every entry in the directory for a match. To speed up user lookups, index the attributes that are searched for by SSSD:
- uid
- uidNumber
- gidNumber
- gecos
Q:
An Active Directory identity provider is properly configured in my
sssd.conf file, but SSSD fails to connect to it, with GSS-API errors.
A:
SSSD can only connect with an Active Directory provider using its host name. If the host name is not given, the SSSD client cannot resolve the IP address to the host, and authentication fails.
For example, with this configuration:
The SSSD client returns this GSS-API failure, and the authentication request fails:
(Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0020): ldap_sasl_bind failed (-2)[Local error] (Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0080): Extended failure message: [SASL(-1): generic failure: GSSAPI Error: Unspecified GSS failure. Minor code may provide more information (Cannot determine realm for numeric host address)]
(Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0020): ldap_sasl_bind failed (-2)[Local error]
(Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0080): Extended failure message: [SASL(-1): generic failure: GSSAPI Error: Unspecified GSS failure. Minor code may provide more information (Cannot determine realm for numeric host address)]
To avoid this error, set the
ad_server to the name of the Active Directory host.
Q:
I configured SSSD for central authentication, but now several of my applications (such as Firefox or Adobe) will not start.
A:
Even on 64-bit systems, 32-bit applications require a 32-bit version of SSSD to use to access the password and identity cache. If a 32-bit version of SSSD is not available, but the system is configured to use the SSSD cache, then 32-bit applications can fail to start.
For example, Firefox can fail with permission denied errors:
Failed to contact configuration server. See http://www.gnome.org/projects/gconf/ for information. (Details - 1: IOR file '/tmp/gconfd-somebody/lock/ior' not opened successfully, no gconfd located: Permission denied 2: IOR file '/tmp/gconfd-somebody/lock/ior' not opened successfully, no gconfd located: Permission denied)
Failed to contact configuration server. See http://www.gnome.org/projects/gconf/
for information. (Details - 1: IOR file '/tmp/gconfd-somebody/lock/ior'
not opened successfully, no gconfd located: Permission denied 2: IOR
file '/tmp/gconfd-somebody/lock/ior' not opened successfully, no gconfd
located: Permission denied)
For Adobe Reader, the error shows that the current system user is not recognized:
acroread
~]$ acroread
(acroread:12739): GLib-WARNING **: getpwuid_r(): failed due to unknown
user id (366)
Other applications may show similar user or permissions errors.
Q:
SSSD is showing an automount location that I removed.
A:
The SSSD cache for the automount location persists even if the location is subsequently changed or removed. To update the autofs information in SSSD:
- Remove the autofs cache, as described in the section called “Purging the SSSD Cache”.
- Restart SSSD, as in Section 13.2.3, “Starting and Stopping SSSD”.
[3]
Most services default to the identity provider server if a specific server for that service is not set.
Chapter 14. OpenSSH
Link kopierenLink in die Zwischenablage kopiert!
SSH (Secure Shell) is a protocol which facilitates secure communications between two systems using a client-server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP, Telnet, or rlogin, SSH encrypts the login session, rendering the connection difficult for intruders to collect unencrypted passwords.
The ssh program is designed to replace older, less secure terminal applications used to log into remote hosts, such as
telnet or rsh. A related program called scp replaces older programs designed to copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the remote host.
Red Hat Enterprise Linux includes the general OpenSSH package, openssh, as well as the OpenSSH server, openssh-server, and client, openssh-clients, packages.
14.1. The SSH Protocol
Link kopierenLink in die Zwischenablage kopiert!
14.1.1. Why Use SSH?
Link kopierenLink in die Zwischenablage kopiert!
Potential intruders have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:
- Interception of communication between two systems
- The attacker can be somewhere on the network between the communicating parties, copying any information passed between them. He may intercept and keep the information, or alter the information and send it on to the intended recipient.This attack is usually performed using a packet sniffer, a rather common network utility that captures each packet flowing through the network, and analyzes its content.
- Impersonation of a particular host
- Attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.This attack can be performed using a technique known as DNS poisoning, or via so-called IP spoofing. In the first case, the intruder uses a cracked DNS server to point client systems to a maliciously duplicated host. In the second case, the intruder sends falsified network packets that appear to be from a trusted host.
Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous. If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.
14.1.2. Main Features
Link kopierenLink in die Zwischenablage kopiert!
The SSH protocol provides the following safeguards:
- No one can pose as the intended server
- After an initial connection, the client can verify that it is connecting to the same server it had connected to previously.
- No one can capture the authentication information
- The client transmits its authentication information to the server using strong, 128-bit encryption.
- No one can intercept the communication
- All data sent and received during a session is transferred using 128-bit encryption, making intercepted transmissions extremely difficult to decrypt and read.
Additionally, it also offers the following options:
- It provides secure means to use graphical applications over a network
- Using a technique called X11 forwarding, the client can forward X11 (X Window System) applications from the server. Note that if you set the
ForwardX11Trustedoption toyesor you use SSH with the-Yoption, you bypass the X11 SECURITY extension controls, which can result in a security threat. - It provides a way to secure otherwise insecure protocols
- The SSH protocol encrypts everything it sends and receives. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.
- It can be used to create a secure channel
- The OpenSSH server and client can be configured to create a tunnel similar to a virtual private network for traffic between server and client machines.
- It supports the Kerberos authentication
- OpenSSH servers and clients can be configured to authenticate using the GSSAPI (Generic Security Services Application Program Interface) implementation of the Kerberos network authentication protocol.
14.1.3. Protocol Versions
Link kopierenLink in die Zwischenablage kopiert!
Two varieties of SSH currently exist: version 1 and version 2. The OpenSSH suite under Red Hat Enterprise Linux uses SSH version 2, which has an enhanced key exchange algorithm not vulnerable to the known exploit in version 1. However, for compatibility reasons, the OpenSSH suite does support version 1 connections as well, although version 1 is disabled by default and needs to be enabled in the configuration files.
Important
For maximum security, avoid using SSH version 1 and use SSH version 2-compatible servers and clients whenever possible.
14.1.4. Event Sequence of an SSH Connection
Link kopierenLink in die Zwischenablage kopiert!
The following series of events help protect the integrity of SSH communication between two hosts.
- A cryptographic handshake is made so that the client can verify that it is communicating with the correct server.
- The transport layer of the connection between the client and remote host is encrypted using a symmetric cipher.
- The client authenticates itself to the server.
- The client interacts with the remote host over the encrypted connection.
14.1.4.1. Transport Layer
Link kopierenLink in die Zwischenablage kopiert!
The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of authentication and during subsequent communication. The transport layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also provides compression, speeding the transfer of information.
Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange:
- Keys are exchanged
- The public key encryption algorithm is determined
- The symmetric encryption algorithm is determined
- The message authentication algorithm is determined
- The hash algorithm is determined
During the key exchange, the server identifies itself to the client with a unique host key. If the client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH notifies the user that the authenticity of the host cannot be established and prompts the user to accept or reject it. The user is expected to independently verify the new host key before accepting it. In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is indeed communicating with the intended server. If, in the future, the host key no longer matches, the user must remove the client's saved version before a connection can occur.
Warning
Always verify the integrity of a new SSH server. During the initial contact, an attacker can pretend to be the intended SSH server to the local system without being recognized. To verify the integrity of a new SSH server, contact the server administrator before the first connection or if a host key mismatch occurs.
SSH is designed to work with almost any kind of public key algorithm or encoding format. After an initial key exchange creates a hash value used for exchanges and a shared secret value, the two systems immediately begin calculating new keys and algorithms to protect authentication and future data sent over the connection.
After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine the hash and shared secret value, this information is only useful for a limited period of time.
14.1.4.2. Authentication
Link kopierenLink in die Zwischenablage kopiert!
Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate itself to the server using one of these supported methods.
SSH servers and clients can be configured to allow different types of authentication, which gives each side the optimal amount of control. The server can decide which encryption methods it supports based on its security model, and the client can choose the order of authentication methods to attempt from the available options.
14.1.4.3. Channels
Link kopierenLink in die Zwischenablage kopiert!
After a successful authentication over the SSH transport layer, multiple channels are opened via a technique called multiplexing[4]. Each of these channels handles communication for different terminal sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel is then assigned a different number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be closed without disrupting the primary SSH connection.
Channels also support flow-control, which allows them to send and receive data in an orderly fashion. In this way, data is not sent over the channel until the client receives a message that the channel is open.
The client and server negotiate the characteristics of each channel automatically, depending on the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change the basic infrastructure of the protocol.
14.2. Configuring OpenSSH
Link kopierenLink in die Zwischenablage kopiert!
14.2.1. Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
There are two different sets of configuration files: those for client programs (that is,
ssh, scp, and sftp), and those for the server (the sshd daemon).
System-wide SSH configuration information is stored in the
/etc/ssh/ directory as described in Table 14.1, “System-wide configuration files”. User-specific SSH configuration information is stored in ~/.ssh/ within the user's home directory as described in Table 14.2, “User-specific configuration files”.
| File | Description |
|---|---|
/etc/ssh/moduli | Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is critical for constructing a secure transport layer. When keys are exchanged at the beginning of an SSH session, a shared, secret value is created which cannot be determined by either party alone. This value is then used to provide host authentication. |
/etc/ssh/ssh_config | The default SSH client configuration file. Note that it is overridden by ~/.ssh/config if it exists. |
/etc/ssh/sshd_config | The configuration file for the sshd daemon. |
/etc/ssh/ssh_host_dsa_key | The DSA private key used by the sshd daemon. |
/etc/ssh/ssh_host_dsa_key.pub | The DSA public key used by the sshd daemon. |
/etc/ssh/ssh_host_key | The RSA private key used by the sshd daemon for version 1 of the SSH protocol. |
/etc/ssh/ssh_host_key.pub | The RSA public key used by the sshd daemon for version 1 of the SSH protocol. |
/etc/ssh/ssh_host_rsa_key | The RSA private key used by the sshd daemon for version 2 of the SSH protocol. |
/etc/ssh/ssh_host_rsa_key.pub | The RSA public key used by the sshd daemon for version 2 of the SSH protocol. |
/etc/pam.d/sshd | The PAM configuration file for the sshd daemon. |
/etc/sysconfig/sshd | Configuration file for the sshd service. |
| File | Description |
|---|---|
~/.ssh/authorized_keys | Holds a list of authorized public keys for servers. When the client connects to a server, the server authenticates the client by checking its signed public key stored within this file. |
~/.ssh/id_dsa | Contains the DSA private key of the user. |
~/.ssh/id_dsa.pub | The DSA public key of the user. |
~/.ssh/id_rsa | The RSA private key used by ssh for version 2 of the SSH protocol. |
~/.ssh/id_rsa.pub | The RSA public key used by ssh for version 2 of the SSH protocol. |
~/.ssh/identity | The RSA private key used by ssh for version 1 of the SSH protocol. |
~/.ssh/identity.pub | The RSA public key used by ssh for version 1 of the SSH protocol. |
~/.ssh/known_hosts | Contains DSA host keys of SSH servers accessed by the user. This file is very important for ensuring that the SSH client is connecting the correct SSH server. |
For information concerning various directives that can be used in the SSH configuration files, see the
ssh_config(5) and sshd_config(5) manual pages.
14.2.2. Starting an OpenSSH Server
Link kopierenLink in die Zwischenablage kopiert!
In order to run an OpenSSH server, you must have the openssh-server installed (see Section 8.2.4, “Installing Packages” for more information on how to install new packages in Red Hat Enterprise Linux 6).
To start the
sshd daemon, type the following at a shell prompt:
service sshd start
~]# service sshd start
To stop the running
sshd daemon, use the following command:
service sshd stop
~]# service sshd stop
If you want the daemon to start automatically at the boot time, type:
chkconfig sshd on
~]# chkconfig sshd on
This will enable the service for levels 2, 3, 4, and 5. For more configuration options, see Chapter 12, Services and Daemons for the detailed information on how to manage services.
Note that if you reinstall the system, a new set of identification keys will be created. As a result, clients who had connected to the system with any of the OpenSSH tools before the reinstall will see the following message:
To prevent this, you can backup the relevant files from the
/etc/ssh/ directory (see Table 14.1, “System-wide configuration files” for a complete list), and restore them whenever you reinstall the system.
14.2.3. Requiring SSH for Remote Connections
Link kopierenLink in die Zwischenablage kopiert!
For SSH to be truly effective, using insecure connection protocols should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet. Some services to disable include
telnet, rsh, rlogin, and vsftpd.
To disable these services, type the following commands at a shell prompt:
chkconfig telnet off chkconfig rsh off chkconfig rlogin off chkconfig vsftpd off
~]# chkconfig telnet off
~]# chkconfig rsh off
~]# chkconfig rlogin off
~]# chkconfig vsftpd off
For more information on runlevels and configuring services in general, see Chapter 12, Services and Daemons.
14.2.4. Using Key-Based Authentication
Link kopierenLink in die Zwischenablage kopiert!
To improve the system security even further, you can enforce key-based authentication by disabling the standard password authentication. To do so, open the
/etc/ssh/sshd_config configuration file in a text editor such as vi or nano, and change the PasswordAuthentication option as follows:
PasswordAuthentication no
PasswordAuthentication no
To be able to use
ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be generated for each user separately.
Red Hat Enterprise Linux 6 uses SSH Protocol 2 and RSA keys by default (see Section 14.1.3, “Protocol Versions” for more information).
Important
Do not generate key pairs as
root, as only root would be able to use those keys.
Note
Before reinstalling your system, back up the
~/.ssh/ directory to keep the generated key pair. Copy the backed-up data to the home directory in the new system for any user you require, including root.
14.2.4.1. Generating Key Pairs
Link kopierenLink in die Zwischenablage kopiert!
To generate an RSA key pair for version 2 of the SSH protocol, follow these steps:
- Generate an RSA key pair by typing the following at a shell prompt:
ssh-keygen -t rsa
~]$ ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/home/john/.ssh/id_rsa):Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Press Enter to confirm the default location (that is,
~/.ssh/id_rsa) for the newly created key. - Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.After this, you will be presented with a message similar to this:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Change the permissions of the
~/.ssh/directory:chmod 700 ~/.ssh
~]$ chmod 700 ~/.sshCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the content of
~/.ssh/id_rsa.pubinto the~/.ssh/authorized_keyson the machine to which you want to connect, appending it to its end if the file already exists. - Change the permissions of the
~/.ssh/authorized_keysfile using the following command:chmod 600 ~/.ssh/authorized_keys
~]$ chmod 600 ~/.ssh/authorized_keysCopy to Clipboard Copied! Toggle word wrap Toggle overflow
To generate a DSA key pair for version 2 of the SSH protocol, follow these steps:
- Generate a DSA key pair by typing the following at a shell prompt:
ssh-keygen -t dsa
~]$ ssh-keygen -t dsa Generating public/private dsa key pair. Enter file in which to save the key (/home/john/.ssh/id_dsa):Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Press Enter to confirm the default location (that is,
~/.ssh/id_dsa) for the newly created key. - Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.After this, you will be presented with a message similar to this:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Change the permissions of the
~/.ssh/directory:chmod 700 ~/.ssh
~]$ chmod 700 ~/.sshCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the content of
~/.ssh/id_dsa.pubinto the~/.ssh/authorized_keyson the machine to which you want to connect, appending it to its end if the file already exists. - Change the permissions of the
~/.ssh/authorized_keysfile using the following command:chmod 600 ~/.ssh/authorized_keys
~]$ chmod 600 ~/.ssh/authorized_keysCopy to Clipboard Copied! Toggle word wrap Toggle overflow
To generate an RSA key pair for version 1 of the SSH protocol, follow these steps:
- Generate an RSA key pair by typing the following at a shell prompt:
ssh-keygen -t rsa1
~]$ ssh-keygen -t rsa1 Generating public/private rsa1 key pair. Enter file in which to save the key (/home/john/.ssh/identity):Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Press Enter to confirm the default location (that is,
~/.ssh/identity) for the newly created key. - Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log into your account.After this, you will be presented with a message similar to this:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Change the permissions of the
~/.ssh/directory:chmod 700 ~/.ssh
~]$ chmod 700 ~/.sshCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the content of
~/.ssh/identity.pubinto the~/.ssh/authorized_keyson the machine to which you want to connect, appending it to its end if the file already exists. - Change the permissions of the
~/.ssh/authorized_keysfile using the following command:chmod 600 ~/.ssh/authorized_keys
~]$ chmod 600 ~/.ssh/authorized_keysCopy to Clipboard Copied! Toggle word wrap Toggle overflow
See Section 14.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember the passphrase.
Important
Never share your private key with anybody; it is for your personal use only.
14.2.4.2. Configuring ssh-agent
Link kopierenLink in die Zwischenablage kopiert!
To store your passphrase so that you do not have to enter it each time you initiate a connection with a remote machine, you can use the
ssh-agent authentication agent. If you are running GNOME, you can configure it to prompt you for your passphrase whenever you log in and remember it during the whole session. Otherwise you can store the passphrase for a certain shell prompt.
To save your passphrase during your GNOME session, follow these steps:
- Make sure you have the openssh-askpass package installed. If not, see Section 8.2.4, “Installing Packages” for more information on how to install new packages in Red Hat Enterprise Linux.
- Select → → from the panel. The Startup Applications Preferences will be started, and the tab containing a list of available startup programs will be shown by default.
Figure 14.1. Startup Applications Preferences
- Click the button on the right, and enter
/usr/bin/ssh-addin the Command field.Figure 14.2. Adding new application
- Click and make sure the check box next to the newly added item is selected.
Figure 14.3. Enabling the application
- Log out and then log back in. A dialog box will appear prompting you for your passphrase. From this point on, you should not be prompted for a password by
ssh,scp, orsftp.Figure 14.4. Entering a passphrase
To save your passphrase for a certain shell prompt, use the following command:
ssh-add
~]$ ssh-add
Enter passphrase for /home/john/.ssh/id_rsa:
Note that when you log out, your passphrase will be forgotten. You must execute the command each time you log in to a virtual console or a terminal window.
14.2.4.3. Multiple required methods of authentication for sshd
Link kopierenLink in die Zwischenablage kopiert!
For higher security, SSH can require multiple methods of authentication to log in successfully, for example both a passphrase and a public key. Set the
RequiredAuthentications2 option in the /etc/ssh/sshd_config file as desired, for example by running:
echo "RequiredAuthentications2 publickey,password" >> /etc/ssh/sshd_config
~]# echo "RequiredAuthentications2 publickey,password" >> /etc/ssh/sshd_config
For more information on the available options, see the
sshd_config(5) manual page.
14.3. Using OpenSSH Certificate Authentication
Link kopierenLink in die Zwischenablage kopiert!
14.3.1. Introduction to SSH Certificates
Link kopierenLink in die Zwischenablage kopiert!
Using public key cryptography for authentication requires copying the public key from every client to every server that the client intends to log into. This system does not scale well and can be an administrative burden. Using a public key from a certificate authority (CA) to authenticate client certificates removes the need to copy keys between multiple systems. While the X.509 Public Key Infrastructure Certificate system provides a solution to this issue, there is a submission and validation process, with associated fees, to go through in order to get a certificate signed. As an alternative, OpenSSH supports the creation of simple certificates and associated CA infrastructure.
OpenSSH certificates contain a public key, identity information, and validity constraints. They are signed with a standard SSH public key using the
ssh-keygen utility. The format of the certificate is described in /usr/share/doc/openssh-version/PROTOCOL.certkeys.
The
ssh-keygen utility supports two types of certificates: user and host. User certificates authenticate users to servers, whereas host certificates authenticate server hosts to users. For certificates to be used for user or host authentication, sshd must be configured to trust the CA public key.
14.3.2. Support for SSH Certificates
Link kopierenLink in die Zwischenablage kopiert!
Support for certificate authentication of users and hosts using the new OpenSSH certificate format was introduced in Red Hat Enterprise Linux 6.5, in the openssh-5.3p1-94.el6 package. If required, to ensure the latest OpenSSH package is installed, enter the following command as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
root:
yum install openssh
~]# yum install openssh
Package openssh-5.3p1-104.el6_6.1.i686 already installed and latest version
Nothing to do14.3.3. Creating SSH CA Certificate Signing Keys
Link kopierenLink in die Zwischenablage kopiert!
Two types of certificates are required, host certificates and user certificates. It is considered better to have two separate keys for signing the two certificates, for example
ca_user_key and ca_host_key, however it is possible to use just one CA key to sign both certificates. It is also easier to follow the procedures if separate keys are used, so the examples that follow will use separate keys.
The basic format of the command to sign user's public key to create a user certificate is as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where
ssh-keygen -s ca_user_key -I certificate_ID id_rsa.pub
ssh-keygen -s ca_user_key -I certificate_ID id_rsa.pub-s indicates the private key used to sign the certificate, -I indicates an identity string, the certificate_ID, which can be any alpha numeric value. It is stored as a zero terminated string in the certificate. The certificate_ID is logged whenever the certificate is used for identification and it is also used when revoking a certificate. Having a long value would make logs hard to read, therefore using the host name for host certificates and the user name for user certificates is a safe choice.
To sign a host's public key to create a host certificate, add the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
-h option: ssh-keygen -s ca_host_key -I certificate_ID -h ssh_host_rsa_key.pub
ssh-keygen -s ca_host_key -I certificate_ID -h ssh_host_rsa_key.pub
Host keys are generated on the system by default, to list the keys, enter a command as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Important
It is recommended to create and store CA keys in a safe place just as with any other private key. In these examples the
root user will be used. In a real production environment using an offline computer with an administrative user account is recommended. For guidance on key lengths see NIST Special Publication 800-131A.
Procedure 14.1. Generating SSH CA Certificate Signing Keys
- On the server designated to be the CA, generate two keys for use in signing certificates. These are the keys that all other hosts need to trust. Choose suitable names, for example
ca_user_keyandca_host_key. To generate the user certificate signing key, enter the following command asroot:Generate a host certificate signing key,Copy to Clipboard Copied! Toggle word wrap Toggle overflow ca_host_key, as follows:If required, confirm the permissions are correct:Copy to Clipboard Copied! Toggle word wrap Toggle overflow Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Create the CA server's own host certificate by signing the server's host public key together with an identification string such as the host name, the CA server's fully qualified domain name (FQDN) but without the trailing
., and a validity period. The command takes the following form:Thessh-keygen -s ~/.ssh/ca_host_key -I certificate_ID -h -Z host_name.example.com -V -start:+end /etc/ssh/ssh_host_rsa.pub
ssh-keygen -s ~/.ssh/ca_host_key -I certificate_ID -h -Z host_name.example.com -V -start:+end /etc/ssh/ssh_host_rsa.pubCopy to Clipboard Copied! Toggle word wrap Toggle overflow -Zoption restricts this certificate to a specific host within the domain. The-Voption is for adding a validity period; this is highly recommend. Where the validity period is intended to be one year, fifty two weeks, consider the need for time to change the certificates and any holiday periods around the time of certificate expiry.For example:ssh-keygen -s ~/.ssh/ca_host_key -I host_name -h -Z host_name.example.com -V -1w:+54w5d /etc/ssh/ssh_host_rsa.pub
~]# ssh-keygen -s ~/.ssh/ca_host_key -I host_name -h -Z host_name.example.com -V -1w:+54w5d /etc/ssh/ssh_host_rsa.pub Enter passphrase: Signed host key /root/.ssh/ssh_host_rsa-cert.pub: id "host_name" serial 0 for host_name.example.com valid from 2015-05-15T13:52:29 to 2016-06-08T13:52:29Copy to Clipboard Copied! Toggle word wrap Toggle overflow
14.3.4. Distributing and Trusting SSH CA Public Keys
Link kopierenLink in die Zwischenablage kopiert!
Hosts that are to allow certificate authenticated log in from users must be configured to trust the CA's public key that was used to sign the user certificates, in order to authenticate user's certificates. In this example that is the
ca_user_key.pub.
Publish the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where host_name is the host name of a server the is required to authenticate user's certificates presented during the login process. Ensure you copy the public key not the private key. For example, as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
ca_user_key.pub key and download it to all hosts that are required to allow remote users to log in. Alternately, copy the CA user public key to all the hosts. In a production environment, consider copying the public key to an administrator account first. The secure copy command can be used to copy the public key to remote hosts. The command has the following format: scp ~/.ssh/ca_user_key.pub root@host_name.example.com:/etc/ssh/
scp ~/.ssh/ca_user_key.pub root@host_name.example.com:/etc/ssh/root:
For remote user authentication, CA keys can be marked as trusted per-user in the
~/.ssh/authorized_keys file using the cert-authority directive or for global use by means of the TrustedUserCAKeys directive in the /etc/ssh/sshd_config file. For remote host authentication, CA keys can be marked as trusted globally in the /etc/ssh/known_hosts file or per-user in the ~/.ssh/ssh_known_hosts file.
Procedure 14.2. Trusting the User Signing Key
- For user certificates which have one or more principles listed, and where the setting is to have global effect, edit the
/etc/ssh/sshd_configfile as follows:RestartTrustedUserCAKeys /etc/ssh/ca_user_key.pub
TrustedUserCAKeys /etc/ssh/ca_user_key.pubCopy to Clipboard Copied! Toggle word wrap Toggle overflow sshdto make the changes take effect:service sshd restart
~]# service sshd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
To avoid being presented with the warning about an unknown host, a user's system must trust the CA's public key that was used to sign the host certificates. In this example that is
ca_host_key.pub.
Procedure 14.3. Trusting the Host Signing Key
- Extract the contents of the public key used to sign the host certificate. For example, on the CA:
cat ~/.ssh/ca_host_key.pub ssh-rsa AAAAB5Wm.== root@ca-server.example.com
cat ~/.ssh/ca_host_key.pub ssh-rsa AAAAB5Wm.== root@ca-server.example.comCopy to Clipboard Copied! Toggle word wrap Toggle overflow - To configure client systems to trust servers' signed host certificates, add the contents of the
ca_host_key.pubinto the globalknown_hostsfile. This will automatically check a server's host advertised certificate against the CA public key for all users every time a new machine is connected to in the domain*.example.com. Login asrootand configure the/etc/ssh/ssh_known_hostsfile, as follows:Wherevi /etc/ssh/ssh_known_hosts
~]# vi /etc/ssh/ssh_known_hosts # A CA key, accepted for any host in *.example.com @cert-authority *.example.com ssh-rsa AAAAB5Wm.Copy to Clipboard Copied! Toggle word wrap Toggle overflow ssh-rsa AAAAB5Wm.is the contents ofca_host_key.pub. The above configures the system to trust the CA servers host public key. This enables global authentication of the certificates presented by hosts to remote users.
14.3.5. Creating SSH Certificates
Link kopierenLink in die Zwischenablage kopiert!
A certifcate is a signed public key. The user's and host's public keys must be copied to the CA server for signing by the CA server's private key.
Important
Copying many keys to the CA to be signed can create confusion if they are not uniquely named. If the default name is always used then the latest key to be copied will overwrite the previously copied key, which may be an acceptable method for one administrator. In the example below the default name is used. In a production environment, consider using easily recognizable names. It is recommend to have a designated directory on the CA server owned by an administrative user for the keys to be copied into. Copying these keys to the
root user's /etc/ssh/ directory is not recommend. In the examples below an account named admin with a directory named keys/ will be used.
Create an administrator account, in this example
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Set the permissions to allow keys to be copied in:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
admin, and a directory to receive the user's keys. For example:
mkdir keys
~]$ mkdir keyschmod o+w keys
~]$ chmod o+w keys
ls -la keys
total 8
drwxrwxrwx. 2 admin admin 4096 May 22 16:17 .
drwx------. 3 admin admin 4096 May 22 16:17 ..14.3.5.1. Creating SSH Certificates to Authenticate Hosts
Link kopierenLink in die Zwischenablage kopiert!
The command to sign a host certificate has the following format:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
The host certificate will named
ssh-keygen -s ca_host_key -I host_name -h ssh_host_rsa_key.pub
ssh-keygen -s ca_host_key -I host_name -h ssh_host_rsa_key.pubssh_host_rsa_key-cert.pub.
Procedure 14.4. Generating a Host Certificate
To authenticate a host to a user, a public key must be generated on the host, passed to the CA server, signed by the CA, and then passed back to be stored on the host to present to a user attempting to log into the host.
- Host keys are generated automatically on the system. To list them enter the following command:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the chosen public key to the server designated as the CA. For example, from the host:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow Alternately, from the CA:scp root@host_name.example.com:/etc/ssh/ssh_host_rsa_key.pub ~/keys/ssh_host_rsa_key.pub
~]$ scp root@host_name.example.com:/etc/ssh/ssh_host_rsa_key.pub ~/keys/ssh_host_rsa_key.pubCopy to Clipboard Copied! Toggle word wrap Toggle overflow - On the CA server, sign the host's public key. For example, as
root:Where host_name is the host name of the system requiring the certificate.ssh-keygen -s ~/.ssh/ca_host_key -I host_name -h -Z host_name.example.com -V -1d:+54w /home/admin/keys/ssh_host_rsa_key.pub
~]# ssh-keygen -s ~/.ssh/ca_host_key -I host_name -h -Z host_name.example.com -V -1d:+54w /home/admin/keys/ssh_host_rsa_key.pub Enter passphrase: Signed host key /home/admin/keys/ssh_host_rsa_key-cert.pub: id "host_name" serial 0 for host_name.example.com valid from 2015-05-26T12:21:54 to 2016-06-08T12:21:54Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the certificate to the host. For example, from the CA:
scp /home/admin/keys/ssh_host_rsa_key-cert.pub root@host_name.example.com:/etc/ssh/
~]# scp /home/admin/keys/ssh_host_rsa_key-cert.pub root@host_name.example.com:/etc/ssh/ root@host_name.example.com's password: ssh_host_rsa_key-cert.pub 100% 1384 1.5KB/s 00:00Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure the host to present the certificate to a user's system when a user initiates the login process. As
root, edit the/etc/ssh/sshd_configfile as follows:HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub
HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pubCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart
sshdto make the changes take effect:service sshd restart
~]# service sshd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow - On user's systems. remove keys belonging to hosts from the
~/.ssh/known_hostsfile if the user has previously logged into the host configured above. When a user logs into the host they should no longer be presented with the warning about the hosts authenticity.
To test the host certificate, on a client system, ensure the client has set up the global
/etc/ssh/known_hosts file, as described in Procedure 14.3, “Trusting the Host Signing Key”, and that the server's public key is not in the ~/.ssh/known_hosts file. Then attempt to log into the server over SSH as a remote user. You should not see a warning about the authenticity of the host. If required, add the -v option to the SSH command to see logging information.
14.3.5.2. Creating SSH Certificates for Authenticating Users
Link kopierenLink in die Zwischenablage kopiert!
To sign a user's certificate, use a command in the following format:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
The resulting certificate will be named
ssh-keygen -s ca_user_key -I user_name -Z user_name -V -start:+end id_rsa.pub
ssh-keygen -s ca_user_key -I user_name -Z user_name -V -start:+end id_rsa.pubid_rsa-cert.pub.
The default behavior of OpenSSH is that a user is allowed to log in as a remote user if one of the principals specified in the certificate matches the remote user's name. This can be adjusted in the following ways:
- Add more user's names to the certificate during the signing process using the
-Zoption:-Z "name1[,name2,...]"
-Z "name1[,name2,...]"Copy to Clipboard Copied! Toggle word wrap Toggle overflow - On the user's system, add the public key of the CA in the
~/.ssh/authorized_keysfile using thecert-authoritydirective and list the principals names as follows:vi ~/.ssh/authorized_keys
~]# vi ~/.ssh/authorized_keys # A CA key, accepted for any host in *.example.com @cert-authority principals="name1,name2" *.example.com ssh-rsa AAAAB5Wm.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - On the server, create an
AuthorizedPrincipalsFilefile, either per user or globally, and add the principles' names to the file for those users allowed to log in. Then in the/etc/ssh/sshd_configfile, specify the file using theAuthorizedPrincipalsFiledirective.
Procedure 14.5. Generating a User Certificate
To authenticate a user to a remote host, a public key must be generated by the user, passed to the CA server, signed by the CA, and then passed back to be stored by the user for use when logging in to a host.
- On client systems, login as the user who requires the certificate. Check for available keys as follows:If no suitable public key exists, generate one and set the directory permissions if the directory is not the default directory. For example, enter the following command:
ls -l ~/.ssh/
~]$ ls -l ~/.ssh/Copy to Clipboard Copied! Toggle word wrap Toggle overflow By default the directory permissions for a user's keys areCopy to Clipboard Copied! Toggle word wrap Toggle overflow drwx------., or octal 0700. If required, confirm the permissions are correct:See Section 14.2.4, “Using Key-Based Authentication” for more examples of key generation and for instructions on setting the correct directory permissions.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - The chosen public key must be copied to the server designated as the CA, in order to be signed. The secure copy command can be used to do this, the command has the following format:Where protocol is the part of the file name indicating the protocol used to generate the key, for example
scp ~/.ssh/id_protocol.pub admin@ca_server.example.com:~/keys/
scp ~/.ssh/id_protocol.pub admin@ca_server.example.com:~/keys/Copy to Clipboard Copied! Toggle word wrap Toggle overflow rsa, admin is an account on the CA server, and /keys/ is a directory setup to receive the keys to be signed.Copy the chosen public key to the server designated as the CA. For example:If you have configured the client system to trust the host signing key as described in Procedure 14.3, “Trusting the Host Signing Key” then you should not see a warning about the authenticity of the remote host.scp ~/.ssh/id_rsa.pub admin@ca-server.example.com:~/keys/
~]$ scp ~/.ssh/id_rsa.pub admin@ca-server.example.com:~/keys/ admin@ca-server.example.com's password: id_rsa.pub 100% 421 0.4KB/s 00:00Copy to Clipboard Copied! Toggle word wrap Toggle overflow - On the CA server, sign the user's public key. For example, as
root:ssh-keygen -s ~/.ssh/ca_user_key -I user1 -Z user1 -V -1d:+54w /home/admin/keys/id_rsa.pub
~]# ssh-keygen -s ~/.ssh/ca_user_key -I user1 -Z user1 -V -1d:+54w /home/admin/keys/id_rsa.pub Enter passphrase: Signed user key /home/admin/keys/id_rsa-cert.pub: id "user1" serial 0 for host_name.example.com valid from 2015-05-21T16:43:17 to 2016-06-03T16:43:17Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Copy the resulting certificate to the user's
~/.ssh/directory on their system. For example:scp /home/admin/keys/id_rsa-cert.pub user1@host_name.example.com:~/.ssh/
~]# scp /home/admin/keys/id_rsa-cert.pub user1@host_name.example.com:~/.ssh/ user1@host_name.example.com's password: id_rsa-cert.pub 100% 1498 1.5KB/s 00:00Copy to Clipboard Copied! Toggle word wrap Toggle overflow - If using the standard file names and location then no further configuration is required as the SSH daemon will search for user certificates ending in
-cert.puband use them automatically if it finds them. Note that the default location and file names for for SSH version 2 keys are:~/.ssh/id_dsa,~/.ssh/id_ecdsaand~/.ssh/id_rsaas explained in thessh_config(5)manual page. If you use these locations and naming conventions then there is no need for editing the configuration files to enablesshdto present the certificate. They will be used automatically when logging in to a remote system. In this is the case then skip to step 6.If required to use a non-default directory or file naming convention, then asroot, add the following line to the/etc/ssh/ssh_configor~/.ssh/configfiles:Note that this must be the private key name, do not hadIdentityFile ~/path/key_file
IdentityFile ~/path/key_fileCopy to Clipboard Copied! Toggle word wrap Toggle overflow .pubor-cert.pub. Ensure the file permission are correct. For example:This will enable the user of this system to be authenticated by a user certificate when logging into a remote system configured to trust the CA user certificate signing key.ls -la ~/.ssh/config ls -la ~/.ssh/config
~]$ ls -la ~/.ssh/config -rw-rw-r--. 1 user1 user1 36 May 27 21:49 /home/user1/.ssh/config chmod 700 ~/.ssh/config ~]$ ls -la ~/.ssh/config -rwx------. 1 user1 user1 36 May 27 21:49 /home/user1/.ssh/configCopy to Clipboard Copied! Toggle word wrap Toggle overflow - To test the user certificate, attempt to log into a server over SSH from the user's account. You should do this as the user listed as a principle in the certificate, if any are specified. You should not be prompted for a password. If required, add the
-voption to the SSH command to see logging information.
14.3.6. Signing an SSH Certificate Using a PKCS#11 Token
Link kopierenLink in die Zwischenablage kopiert!
It is possible to sign a host key using a CA key stored in a PKCS#11 token by providing the token library using the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
In all cases, certificate_ID is a “key identifier” that is logged by the server when the certificate is used for authentication.
-D and identifying the CA key by providing its public half as an argument to the -s option: ssh-keygen -s ca_host_key.pub -D libpkcs11.so -I certificate_ID host_key.pub
ssh-keygen -s ca_host_key.pub -D libpkcs11.so -I certificate_ID host_key.pub
Certificates may be configured to be valid only for a set of users or host names, the principals. By default, generated certificates are valid for all users or hosts. To generate a certificate for a specified set of principals, use a comma separated list with the
-Z option as follows:
ssh-keygen -s ca_user_key.pub -D libpkcs11.so -I certificate_ID -Z user1,user2 id_rsa.pub
ssh-keygen -s ca_user_key.pub -D libpkcs11.so -I certificate_ID -Z user1,user2 id_rsa.pub
and for hosts:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
ssh-keygen -s ca_host_key.pub -D libpkcs11.so -I certificate_ID -h -Z host.domain ssh_host_rsa_key.pub
ssh-keygen -s ca_host_key.pub -D libpkcs11.so -I certificate_ID -h -Z host.domain ssh_host_rsa_key.pub
Additional limitations on the validity and use of user certificates may be specified through certificate options. A certificate option may disable features of the SSH session, may be valid only when presented from particular source addresses or may force the use of a specific command. For a list of valid certificate options, see the
ssh-keygen(1) manual page for the -O option.
Certificates may be defined to be valid for a specific lifetime. The
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
A certificate that is presented at a time outside this range will not be considered valid. By default, certificates are valid indefinitely starting from UNIX Epoch.
-V option allows specifying a certificates start and end times. For example: ssh-keygen -s ca_user_key -I certificate_ID id_rsa.pub -V "-1w:+54w5d"
ssh-keygen -s ca_user_key -I certificate_ID id_rsa.pub -V "-1w:+54w5d"14.3.7. Viewing an SSH CA Certificate
Link kopierenLink in die Zwischenablage kopiert!
To view a certificate, use the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
-L to list the contents. For example, for a user's certificate:
To vew a host certificate:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
14.3.8. Revoking an SSH CA Certificate
Link kopierenLink in die Zwischenablage kopiert!
If a certificate is stolen, it should be revoked. Although OpenSSH does not provide a mechanism to distribute the revocation list it is still easier to create the revocation list and distribute it by other means then to change the CA keys and all host and user certificates previously created and distributed.
Keys can be revoked by adding them to the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Note that if this file is not readable, then public key authentication will be refused for all users.
revoked_keys file and specifying the file name in the sshd_config file as follows: RevokedKeys /etc/ssh/revoked_keys
RevokedKeys /etc/ssh/revoked_keys
To test if a key has been revoked, query the revocation list for the presence of the key. Use a command as follows:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
ssh-keygen -Qf /etc/ssh/revoked_keys ~/.ssh/id_rsa.pub
ssh-keygen -Qf /etc/ssh/revoked_keys ~/.ssh/id_rsa.pub
A user can revoke a CA certificate by changing the
cert-authority directive to revoke in the known_hosts file.
14.4. OpenSSH Clients
Link kopierenLink in die Zwischenablage kopiert!
To connect to an OpenSSH server from a client machine, you must have the openssh-clients and openssh packages installed (see Section 8.2.4, “Installing Packages” for more information on how to install new packages in Red Hat Enterprise Linux).
14.4.1. Using the ssh Utility
Link kopierenLink in die Zwischenablage kopiert!
The
ssh utility allows you to log in to a remote machine and execute commands there. It is a secure replacement for the rlogin, rsh, and telnet programs.
Similarly to the
telnet command, log in to a remote machine by using the following command:
ssh hostname
ssh hostname
For example, to log in to a remote machine named
penguin.example.com, type the following at a shell prompt:
ssh penguin.example.com
~]$ ssh penguin.example.com
This will log you in with the same user name you are using on the local machine. If you want to specify a different user name, use a command in the following form:
ssh username@hostname
ssh username@hostname
For example, to log in to
penguin.example.com as john, type:
ssh john@penguin.example.com
~]$ ssh john@penguin.example.com
The first time you initiate a connection, you will be presented with a message similar to this:
The authenticity of host 'penguin.example.com' can't be established. RSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c. Are you sure you want to continue connecting (yes/no)?
The authenticity of host 'penguin.example.com' can't be established.
RSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c.
Are you sure you want to continue connecting (yes/no)?
Type
yes to confirm. You will see a notice that the server has been added to the list of known hosts, and a prompt asking for your password:
Warning: Permanently added 'penguin.example.com' (RSA) to the list of known hosts. john@penguin.example.com's password:
Warning: Permanently added 'penguin.example.com' (RSA) to the list of known hosts.
john@penguin.example.com's password:Important
Update the host key of an SSH server if the key changes. The client notifies the user that the connection cannot proceed until the server's host key is deleted from the
~/.ssh/known_hosts file. Contact the system administrator of the SSH server to verify the server is not compromised, then remove the line with the name of the remote machine at the beginning.
After entering the password, you will be provided with a shell prompt for the remote machine.
Alternatively, the
ssh program can be used to execute a command on the remote machine without logging in to a shell prompt:
ssh [username@]hostname command
ssh [username@]hostname command
For example, the
/etc/redhat-release file provides information about the Red Hat Enterprise Linux version. To view the contents of this file on penguin.example.com, type:
~]$ ssh john@penguin.example.com cat /etc/redhat-release john@penguin.example.com's password: Red Hat Enterprise Linux Server release 6.2 (Santiago)
~]$ ssh john@penguin.example.com cat /etc/redhat-release
john@penguin.example.com's password:
Red Hat Enterprise Linux Server release 6.2 (Santiago)
After you enter the correct password, the user name will be displayed, and you will return to your local shell prompt.
14.4.2. Using the scp Utility
Link kopierenLink in die Zwischenablage kopiert!
scp can be used to transfer files between machines over a secure, encrypted connection. In its design, it is very similar to rcp.
To transfer a local file to a remote system, use a command in the following form:
scp localfile username@hostname:remotefile
scp localfile username@hostname:remotefile
For example, if you want to transfer
taglist.vim to a remote machine named penguin.example.com, type the following at a shell prompt:
scp taglist.vim john@penguin.example.com:.vim/plugin/taglist.vim
~]$ scp taglist.vim john@penguin.example.com:.vim/plugin/taglist.vim
john@penguin.example.com's password:
taglist.vim 100% 144KB 144.5KB/s 00:00
Multiple files can be specified at once. To transfer the contents of
.vim/plugin/ to the same directory on the remote machine penguin.example.com, type the following command:
scp .vim/plugin/* john@penguin.example.com:.vim/plugin/
~]$ scp .vim/plugin/* john@penguin.example.com:.vim/plugin/
john@penguin.example.com's password:
closetag.vim 100% 13KB 12.6KB/s 00:00
snippetsEmu.vim 100% 33KB 33.1KB/s 00:00
taglist.vim 100% 144KB 144.5KB/s 00:00
To transfer a remote file to the local system, use the following syntax:
scp username@hostname:remotefile localfile
scp username@hostname:remotefile localfile
For instance, to download the
.vimrc configuration file from the remote machine, type:
scp john@penguin.example.com:.vimrc .vimrc
~]$ scp john@penguin.example.com:.vimrc .vimrc
john@penguin.example.com's password:
.vimrc 100% 2233 2.2KB/s 00:0014.4.3. Using the sftp Utility
Link kopierenLink in die Zwischenablage kopiert!
The
sftp utility can be used to open a secure, interactive FTP session. In its design, it is similar to ftp except that it uses a secure, encrypted connection.
To connect to a remote system, use a command in the following form:
sftp username@hostname
sftp username@hostname
For example, to log in to a remote machine named
penguin.example.com with john as a user name, type:
sftp john@penguin.example.com
~]$ sftp john@penguin.example.com
john@penguin.example.com's password:
Connected to penguin.example.com.
sftp>
After you enter the correct password, you will be presented with a prompt. The
sftp utility accepts a set of commands similar to those used by ftp (see Table 14.3, “A selection of available sftp commands”).
| Command | Description |
|---|---|
ls [directory] | List the content of a remote directory. If none is supplied, a current working directory is used by default. |
cd directory | Change the remote working directory to directory. |
mkdir directory | Create a remote directory. |
rmdir path | Remove a remote directory. |
put localfile [remotefile] | Transfer localfile to a remote machine. |
get remotefile [localfile] | Transfer remotefile from a remote machine. |
For a complete list of available commands, see the
sftp(1) manual page.
14.5. More Than a Secure Shell
Link kopierenLink in die Zwischenablage kopiert!
A secure command-line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP forwarding, previously insecure port connections between systems can be mapped to specific SSH channels.
14.5.1. X11 Forwarding
Link kopierenLink in die Zwischenablage kopiert!
To open an X11 session over an SSH connection, use a command in the following form:
ssh -Y username@hostname
ssh -Y username@hostname
For example, to log in to a remote machine named
penguin.example.com with john as a user name, type:
ssh -Y john@penguin.example.com
~]$ ssh -Y john@penguin.example.com
john@penguin.example.com's password:
When an X program is run from the secure shell prompt, the SSH client and server create a new secure channel, and the X program data is sent over that channel to the client machine transparently.
X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session of the Printer Configuration utility. To do this, connect to the server using ssh and type:
system-config-printer &
~]$ system-config-printer &
The Printer Configuration Tool will appear, allowing the remote user to safely configure printing on the remote system.
Please note that X11 Forwarding does not distinguish between trusted and untrusted forwarding.
14.5.2. Port Forwarding
Link kopierenLink in die Zwischenablage kopiert!
SSH can secure otherwise insecure
TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.
Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client. Port numbers do not need to match for this technique to work.
Note
If you want to use reserved port numbers, please note that setting up port forwarding to listen on ports below 1024 requires
root level access.
To create a TCP/IP port forwarding channel which listens for connections on the
localhost, use a command in the following form:
ssh -L local-port:remote-hostname:remote-port username@hostname
ssh -L local-port:remote-hostname:remote-port username@hostname
For example, to check email on a server called
mail.example.com using POP3 through an encrypted connection, use the following command:
ssh -L 1100:mail.example.com:110 mail.example.com
~]$ ssh -L 1100:mail.example.com:110 mail.example.com
Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port
1100 on the localhost to check for new email. Any requests sent to port 1100 on the client system will be directed securely to the mail.example.com server.
If
mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:
ssh -L 1100:mail.example.com:110 other.example.com
~]$ ssh -L 1100:mail.example.com:110 other.example.com
In this example, POP3 requests from port
1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new email. Note that when using this technique, only the connection between the client system and other.example.com SSH server is secure.
Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (that is, port 22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.
Important
The connection is only as secure as the client system because forwarding connections in this way allows any user on the client system to connect to that service. If the client system becomes compromised, an attacker can also access the forwarded services.
If preferred, disable this functionality on the server by specifying a
No parameter for the AllowTcpForwarding line in the /etc/ssh/sshd_config file and restarting the sshd service.
14.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For more information about OpenSSH and OpenSSL, see the resources listed below.
14.6.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
- sshd(8) — a manual page for the
sshddaemon. - ssh(1) — a manual page for the ssh client.
- scp(1) — a manual page for the scp utility.
- sftp(1) — a manual page for the sftp utility.
- ssh-keygen(1) — a manual page for the ssh-keygen utility.
- ssh_config(5) — a manual page with a full description of available SSH client configuration options.
- sshd_config(5) — a manual page with a full description of available SSH daemon configuration options.
/usr/share/doc/openssh-version/Contains detailed information on the protocols supported by OpenSSH.
14.6.2. Useful Websites
Link kopierenLink in die Zwischenablage kopiert!
- http://www.openssh.com/
- The OpenSSH home page containing further documentation, frequently asked questions, links to the mailing lists, bug reports, and other useful resources.
- http://www.openssl.org/
- The OpenSSL home page containing further documentation, frequently asked questions, links to the mailing lists, and other useful resources.
[4]
A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH, different channels are sent over a common secure connection.
Chapter 15. TigerVNC
Link kopierenLink in die Zwischenablage kopiert!
TigerVNC (Tiger Virtual Network Computing) is a system for graphical desktop sharing which allows you to remotely control other computers.
TigerVNC works on the client-server principle: a server shares its output (vncserver) and a client (vncviewer) connects to the server.
15.1. VNC Server
Link kopierenLink in die Zwischenablage kopiert!
vncserver is a utility which starts a VNC (Virtual Network Computing) desktop. It runs Xvnc with appropriate options and starts a window manager on the VNC desktop. vncserver allows users to run separate sessions in parallel on a machine which can then be accessed by any number of clients from anywhere.
15.1.1. Installing VNC Server
Link kopierenLink in die Zwischenablage kopiert!
To install the TigerVNC server, run the following command as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
root:
yum install tigervnc-server
~]# yum install tigervnc-server15.1.2. Configuring VNC Server
Link kopierenLink in die Zwischenablage kopiert!
The VNC server can be configured to start a display for one or more users, provided that accounts for the users exist on the system, with optional parameters such as for display settings, network address and port, and security settings.
Procedure 15.1. Configuring a VNC Display for a Single User
- Specify the user name and the display number by editing
/etc/sysconfig/vncserversand adding a line in the following format:The VNC user names must correspond to users of the system.VNCSERVERS="display_number:user"
VNCSERVERS="display_number:user"Copy to Clipboard Copied! Toggle word wrap Toggle overflow Example 15.1. Setting the Display Number for a User
For example, to configure display number3for userjoe, open the configuration file for editing:vi /etc/sysconfig/vncservers
~]# vi /etc/sysconfig/vncserversCopy to Clipboard Copied! Toggle word wrap Toggle overflow Add a line as follows:Save and close the file.VNCSERVERS="3:joe"
VNCSERVERS="3:joe"Copy to Clipboard Copied! Toggle word wrap Toggle overflow In the example above, display number 3 and the user joe are set. Do not use 0 as the display number since the main X display of a workstation is usually indicated as 0.
Procedure 15.2. Configuring a VNC Display for Multiple Users
- To set a VNC display for more than one user, specify the user names and display numbers by editing
/etc/sysconfig/vncserversand adding a line in the following format:The VNC user names must correspond to users of the system.VNCSERVERS="display_number:user display_number:user"
VNCSERVERS="display_number:user display_number:user"Copy to Clipboard Copied! Toggle word wrap Toggle overflow Example 15.2. Setting the Display Numbers for Two Users
For example, to configure two users, open the configuration file for editing:vi /etc/sysconfig/vncservers
~]# vi /etc/sysconfig/vncserversCopy to Clipboard Copied! Toggle word wrap Toggle overflow Add a line as follows:VNCSERVERS="3:joe 4:jill"
VNCSERVERS="3:joe 4:jill"Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 15.3. Configuring VNC Display Arguments
- Specify additional settings in the
/etc/sysconfig/vncserversfile by adding arguments using the VNCSERVERARGS directive as follows:VNCSERVERS="display_number:user display_number:user" VNCSERVERARGS[display_number]="arguments"
VNCSERVERS="display_number:user display_number:user" VNCSERVERARGS[display_number]="arguments"Copy to Clipboard Copied! Toggle word wrap Toggle overflow Expand Table 15.1. Frequently Used VNC Server Parameters VNCSERVERARGS Definition -geometry specifies the size of the VNC desktop to be created, default is 1024x768. -nolisten tcp prevents connections to your VNC server through TCP (Transmission Control Protocol) -localhost prevents remote VNC clients from connecting except when doing so through a secure tunnel See theXvnc(1)man page for further options.Example 15.3. Setting vncserver Arguments
Following on from the example above, to add arguments for two users, edit the/etc/sysconfig/vncserversfile as follows:VNCSERVERS="3:joe 4:jill" VNCSERVERARGS[1]="-geometry 800x600 -nolisten tcp -localhost" VNCSERVERARGS[2]="-geometry 1920×1080 -nolisten tcp -localhost"
VNCSERVERS="3:joe 4:jill" VNCSERVERARGS[1]="-geometry 800x600 -nolisten tcp -localhost" VNCSERVERARGS[2]="-geometry 1920×1080 -nolisten tcp -localhost"Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 15.4. Configuring VNC User Passwords
- To set the VNC password for all users defined in the
/etc/sysconfig/vncserversfile, enter the following command asroot:To set the VNC password individually for a user:vncpasswd
~]# vncpasswd Password: Verify:Copy to Clipboard Copied! Toggle word wrap Toggle overflow su - user vncpasswd
~]# su - user ~]$ vncpasswd Password: Verify:Copy to Clipboard Copied! Toggle word wrap Toggle overflow Important
The stored password is not encrypted; anyone who has access to the password file can find the plain-text password.
15.1.3. Starting VNC Server
Link kopierenLink in die Zwischenablage kopiert!
In order to start a VNC desktop, the
vncserver utility is used. It is a Perl script which simplifies the process of starting an Xvnc server. It runs Xvnc with appropriate options and starts a window manager on the VNC desktop. There are three ways to start vncserver:
- You can allow
vncserverto choose the first available display number, start Xvnc with that display number, and start the default window manager in the Xvnc session. All these steps are provided by one command:You will be prompted to enter a VNC password the first time the command is run if no VNC password has been set.vncserver
~]$ vncserverCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Alternately, you can specify a specific display number:
vncserver :display_number
vncserver :display_numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow vncserverattempts to start Xvnc with that display number and exits if the display number is not available.For example:vncserver :20
~]$ vncserver :20Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Alternately, to start VNC server with displays for the users configured in the
/etc/sysconfig/vncserversconfiguration file, asrootenter:service vncserver start
~]# service vncserver startCopy to Clipboard Copied! Toggle word wrap Toggle overflow You can enable thevncserverservice automatically at system start. Every time you log in,vncserveris automatically started. Asroot, runchkconfig vncserver on
~]# chkconfig vncserver onCopy to Clipboard Copied! Toggle word wrap Toggle overflow
15.1.4. Terminating a VNC Session
Link kopierenLink in die Zwischenablage kopiert!
Similarly to enabling the
vncserver service, you can disable the automatic start of the service at system start:
chkconfig vncserver off
~]# chkconfig vncserver off
Or, when your system is running, you can stop the service by issuing the following command as
root:
service vncserver stop
~]# service vncserver stop
To terminate a specific display, terminate
vncserver using the -kill option along with the display number.
Example 15.4. Terminating a Specific Display
For example, to terminate display number 2, run:
vncserver -kill :2
~]# vncserver -kill :2Example 15.5. Terminating an Xvnc process
If it is not possible to terminate the VNC service or display, terminate the Xvnc session using the process ID (PID). To view the processes, enter:
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
service vncserver status
~]$ service vncserver status
Xvnc (pid 4290 4189) is running...
To terminate process
4290, enter as root:
kill -s 15 4290
~]# kill -s 15 429015.2. Sharing an Existing Desktop
Link kopierenLink in die Zwischenablage kopiert!
By default a logged in user has a desktop provided by X Server on display
0. A user can share their desktop using the TigerVNC server x0vncserver.
Procedure 15.5. Sharing an X Desktop
To share the desktop of a logged in user, using the
x0vncserver, proceed as follows:
- Enter the following command as
rootyum install tigervnc-server
~]# yum install tigervnc-serverCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Set the VNC password for the user:
vncpasswd
~]$ vncpasswd Password: Verify:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Enter the following command as that user:
x0vncserver -PasswordFile=.vnc/passwd -AlwaysShared=1
~]$ x0vncserver -PasswordFile=.vnc/passwd -AlwaysShared=1Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Provided the firewall is configured to allow connections to port
5900, the remote viewer can now connect to display 0, and view the logged in users desktop. See Section 15.3.2.1, “Configuring the Firewall for VNC” for information on how to configure the firewall.
15.3. Using a VNC Viewer
Link kopierenLink in die Zwischenablage kopiert!
A VNC viewer is a program which shows the graphical user interface created by the VNC server and can control the VNC server remotely. The desktop that is shared is not by default the same as the desktop that is displayed to a user directly logged into the system. The VNC server creates a unique desktop for every display number. Any number of clients can connect to a VNC server.
15.3.1. Installing the VNC Viewer
Link kopierenLink in die Zwischenablage kopiert!
To install the TigerVNC client,
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
vncviewer, as root, run the following command: yum install tigervnc
~]# yum install tigervnc
The TigerVNC client has a graphical user interface (GUI) which can be started by entering the command
vncviewer. Alternatively, you can operate vncviewer through the command-line interface (CLI). To view a list of parameters for vncviewer enter vncviewer -h on the command line.
15.3.2. Connecting to a VNC Server
Link kopierenLink in die Zwischenablage kopiert!
Once the VNC server is configured, you can connect to it from any VNC viewer.
Procedure 15.6. Connecting to a VNC Server Using a GUI
- Enter the
vncviewercommand with no arguments, the VNC Viewer: Connection Details utility appears. It prompts for a VNC server to connect to. - If required, to prevent disconnecting any existing VNC connections to the same display, select the option to allow sharing of the desktop as follows:
- Select the button.
- Select the Misc. tab.
- Select the button.
- Press OK to return to the main menu.
- Enter an address and display number to connect to:
address:display_number
address:display_numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Press Connect to connect to the VNC server display.
- You will be prompted to enter the VNC password. This will be the VNC password for the user corresponding to the display number unless a global default VNC password was set.A window appears showing the VNC server desktop. Note that this is not the desktop the normal user sees, it is an Xvnc desktop.
Procedure 15.7. Connecting to a VNC Server Using the CLI
- Enter the
viewercommand with the address and display number as arguments:Where address is anvncviewer address:display_number
vncviewer address:display_numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow IPaddress or host name. - Authenticate yourself by entering the VNC password. This will be the VNC password for the user corresponding to the display number unless a global default VNC password was set.
- A window appears showing the VNC server desktop. Note that this is not the desktop the normal user sees, it is the Xvnc desktop.
15.3.2.1. Configuring the Firewall for VNC
Link kopierenLink in die Zwischenablage kopiert!
When using a non-encrypted connection, the firewall might block your connection. The VNC protocol is remote framebuffer (RFB), which is transported in
TCP packets. If required, open a port for the TCP protocol as described below. When using the -via option, traffic is redirected over SSH which is enabled by default.
Note
The default port of VNC server is 5900. To reach the port through which a remote desktop will be accessible, sum the default port and the user's assigned display number. For example, for the second display: 2 + 5900 = 5902.
Procedure 15.8. Opening a Port Using lokkit
The
lokkit command provides a way to quickly enable a port using the command line.
- To enable a specific port, for example port
5902forTCP, issue the following command asroot:Note that this will restart the firewall as long as it has not been disabled with thelokkit --port=5902:tcp --update
~]# lokkit --port=5902:tcp --updateCopy to Clipboard Copied! Toggle word wrap Toggle overflow --disabledoption. Active connections will be terminated and time out on the initiating machine. - Verify whether the chosen port is open. As
root, enter:iptables -L -n | grep 'tcp.*59'
~]# iptables -L -n | grep 'tcp.*59' ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:5902Copy to Clipboard Copied! Toggle word wrap Toggle overflow - If you are unsure of the port numbers in use for VNC, as
root, enter:Ports startingCopy to Clipboard Copied! Toggle word wrap Toggle overflow 59XXare for the VNCRFBprotocol. Ports starting60XXare for the X windows protocol.To list the ports and the Xvnc session's associated user, asroot, enter:Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 15.9. Configuring the Firewall Using an Editor
When preparing a configuration file for multiple installations using administration tools, it is useful to edit the firewall configuration file directly. Note that any mistakes in the configuration file could have unexpected consequences, cause an error, and prevent the firewall settings from being applied. Therefore, check the
/etc/sysconfig/system-config-firewall file thoroughly after editing.
- To check what the firewall is configured to allow, issue the following command as
rootto view the firewall configuration file:In this example taken from a default installation, the firewall is enabled but VNC ports have not been configured to pass through.less /etc/sysconfig/system-config-firewall
~]# less /etc/sysconfig/system-config-firewall # Configuration file for system-config-firewall --enabled --service=sshCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Open
/etc/sysconfig/system-config-firewallfor editing asrootand add lines in the following format to the firewall configuration file:For example, to add port--port=port_number:tcp
--port=port_number:tcpCopy to Clipboard Copied! Toggle word wrap Toggle overflow 5902:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Note that these changes will not take effect even if the firewall is reloaded or the system rebooted. To apply the settings in
/etc/sysconfig/system-config-firewall, issue the following command asroot:lokkit --update
~]# lokkit --updateCopy to Clipboard Copied! Toggle word wrap Toggle overflow
15.3.3. Connecting to VNC Server Using SSH
Link kopierenLink in die Zwischenablage kopiert!
VNC is a clear text network protocol with no security against possible attacks on the communication. To make the communication secure, you can encrypt your server-client connection by using the
-via option. This will create an SSH tunnel between the VNC server and the client.
The format of the command to encrypt a VNC server-client connection is as follows:
vncviewer -via user@host:display_number
vncviewer -via user@host:display_numberExample 15.6. Using the -via Option
- To connect to a VNC server using
SSH, enter a command as follows:vncviewer -via joe@192.168.2.101 127.0.0.1:3
$ vncviewer -via joe@192.168.2.101 127.0.0.1:3Copy to Clipboard Copied! Toggle word wrap Toggle overflow - When you are prompted to, type the password, and confirm by pressing Enter.
- A window with a remote desktop appears on your screen.
For more information on using
SSH, see Chapter 14, OpenSSH.
15.4. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For more information about TigerVNC, see the resources listed below.
Installed Documentation
vncserver(1)— The manual page for the VNC server utility.vncviewer(1)— The manual page for the VNC viewer.vncpasswd(1)— The manual page for the VNC password command.Xvnc(1)— The manual page for the Xvnc server configuration options.x0vncserver(1)— The manual page for theTigerVNCserver for sharing existing X servers.
Part VI. Servers
Link kopierenLink in die Zwischenablage kopiert!
This part discusses various topics related to servers such as how to set up a Web server or share files and directories over the network.
Chapter 16. DHCP Servers
Link kopierenLink in die Zwischenablage kopiert!
Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns the network configuration (including the IP address, gateway, and DNS servers) of that client.
16.1. Why Use DHCP?
Link kopierenLink in die Zwischenablage kopiert!
DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, you can choose DHCP instead of specifying an IP address, netmask, gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if you want to change the IP addresses of a large number of systems. Instead of reconfiguring all the systems, you can just edit one configuration file on the server for the new set of IP addresses. If the DNS servers for an organization changes, the changes happen on the DHCP server, not on the DHCP clients. When you restart the network or reboot the clients, the changes go into effect.
If an organization has a functional DHCP server correctly connected to a network, laptops and other mobile computer users can move these devices from office to office.
16.2. Configuring a DHCPv4 Server
Link kopierenLink in die Zwischenablage kopiert!
The dhcp package contains an Internet Systems Consortium (ISC) DHCP server. First, install the package as the superuser:
yum install dhcp
~]# yum install dhcp
Installing the dhcp package creates a file,
/etc/dhcp/dhcpd.conf, which is merely an empty configuration file:
~]# cat /etc/dhcp/dhcpd.conf # # DHCP Server Configuration file. see /usr/share/doc/dhcp*/dhcpd.conf.sample
~]# cat /etc/dhcp/dhcpd.conf
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.sample
The sample configuration file can be found at
/usr/share/doc/dhcp-<version>/dhcpd.conf.sample. You should use this file to help you configure /etc/dhcp/dhcpd.conf, which is explained in detail below.
DHCP also uses the file
/var/lib/dhcpd/dhcpd.leases to store the client lease database. See Section 16.2.2, “Lease Database” for more information.
16.2.1. Configuration File
Link kopierenLink in die Zwischenablage kopiert!
The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (#) are considered comments.
There are two types of statements in the configuration file:
- Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.
- Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.
The parameters that start with the keyword option are referred to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves.
Parameters (including options) declared before a section enclosed in curly brackets ({ }) are considered global parameters. Global parameters apply to all the sections below it.
Important
If the configuration file is changed, the changes do not take effect until the DHCP daemon is restarted with the command
service dhcpd restart.
Note
Instead of changing a DHCP configuration file and restarting the service each time, using the
omshell command provides an interactive way to connect to, query, and change the configuration of a DHCP server. By using omshell, all changes can be made while the server is running. For more information on omshell, see the omshell man page.
In Example 16.1, “Subnet Declaration”, the
routers, subnet-mask, domain-search, domain-name-servers, and time-offset options are used for any host statements declared below it.
For every
subnet which will be served, and for every subnet to which the DHCP server is connected, there must be one subnet declaration, which tells the DHCP daemon how to recognize that an address is on that subnet. A subnet declaration is required for each subnet even if no addresses will be dynamically allocated to that subnet.
In this example, there are global options for every DHCP client in the subnet and a
range declared. Clients are assigned an IP address within the range.
Example 16.1. Subnet Declaration
To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 16.2, “Range Parameter” with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the
range 192.168.1.10 and 192.168.1.100 to client systems.
Example 16.2. Range Parameter
To assign an IP address to a client based on the MAC address of the network interface card, use the
hardware ethernet parameter within a host declaration. As demonstrated in Example 16.3, “Static IP Address Using DHCP”, the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4.
Note that you can also use the optional parameter
host-name to assign a host name to the client.
Example 16.3. Static IP Address Using DHCP
All subnets that share the same physical network should be declared within a
shared-network declaration as shown in Example 16.4, “Shared-network Declaration”. Parameters within the shared-network, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.
As demonstrated in Example 16.5, “Group Declaration”, the
group declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.
Example 16.5. Group Declaration
Note
You can use the provided sample configuration file as a starting point and add custom configuration options to it. To copy this file to the proper location, use the following command as
root:
cp /usr/share/doc/dhcp-<version_number>/dhcpd.conf.sample /etc/dhcp/dhcpd.conf
~]# cp /usr/share/doc/dhcp-<version_number>/dhcpd.conf.sample /etc/dhcp/dhcpd.conf
... where <version_number> is the DHCP version number.
For a complete list of option statements and what they do, see the
dhcp-options man page.
16.2.2. Lease Database
Link kopierenLink in die Zwischenablage kopiert!
On the DHCP server, the file
/var/lib/dhcpd/dhcpd.leases stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.
All times in the lease database are in Coordinated Universal Time (UTC), not local time.
The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The
dhcpd.leases file is renamed dhcpd.leases~ and the temporary lease database is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the
dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.
16.2.3. Starting and Stopping the Server
Link kopierenLink in die Zwischenablage kopiert!
Important
When the DHCP server is started for the first time, it fails unless the
dhcpd.leases file exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file if it does not exist.
If the same server is also running BIND as a DNS server, this step is not necessary, as starting the
named service automatically checks for a dhcpd.leases file.
To start the DHCP service, use the command
/sbin/service dhcpd start. To stop the DHCP server, use the command /sbin/service dhcpd stop.
By default, the DHCP service does not start at boot time. For information on how to configure the daemon to start automatically at boot time, see Chapter 12, Services and Daemons.
If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interfaces, configure the DHCP server to start only on that device. In
/etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:
Command line options here
# Command line options here
DHCPDARGS=eth0
This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users cannot connect to the daemon through the Internet.
Other command-line options that can be specified in
/etc/sysconfig/dhcpd include:
-p <portnum>— Specifies the UDP port number on whichdhcpdshould listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responds to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified. See Section 16.2.4, “DHCP Relay Agent” for details.-f— Runs the daemon as a foreground process. This is mostly used for debugging.-d— Logs the DHCP server daemon to the standard error descriptor. This is mostly used for debugging. If this is not specified, the log is written to/var/log/messages.-cf <filename>— Specifies the location of the configuration file. The default location is/etc/dhcp/dhcpd.conf.-lf <filename>— Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is/var/lib/dhcpd/dhcpd.leases.-q— Do not print the entire copyright message when starting the daemon.
16.2.4. DHCP Relay Agent
Link kopierenLink in die Zwischenablage kopiert!
The DHCP Relay Agent (
dhcrelay) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.
The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in
/etc/sysconfig/dhcrelay with the INTERFACES directive.
To start the DHCP Relay Agent, use the command
service dhcrelay start.
16.3. Configuring a DHCPv4 Client
Link kopierenLink in die Zwischenablage kopiert!
To configure a DHCP client manually, modify the
/etc/sysconfig/network file to enable networking and the configuration file for each network device in the /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where eth0 is the network device name.
Make sure that the
/etc/sysconfig/network-scripts/ifcfg-eth0 file contains the following lines:
DEVICE=eth0 BOOTPROTO=dhcp ONBOOT=yes
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
To use DHCP, set a configuration file for each device.
Other options for the network script include:
DHCP_HOSTNAME— Only use this option if the DHCP server requires the client to specify a host name before receiving an IP address.PEERDNS=<answer>, where<answer>is one of the following:yes— Modify/etc/resolv.confwith information from the server. This is the default.no— Do not modify/etc/resolv.conf.
If you prefer using a graphical interface, see Chapter 10, NetworkManager for instructions on using NetworkManager to configure a network interface to use DHCP.
Note
For advanced configurations of client DHCP options such as protocol timing, lease requirements and requests, dynamic DNS support, aliases, as well as a wide variety of values to override, prepend, or append to client-side configurations, see the
dhclient and dhclient.conf man pages.
16.4. Configuring a Multihomed DHCP Server
Link kopierenLink in die Zwischenablage kopiert!
A multihomed DHCP server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a DHCP server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.
Before making any changes, back up the existing
/etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
The DHCP daemon listens on all network interfaces unless otherwise specified. Use the
/etc/sysconfig/dhcpd file to specify which network interfaces the DHCP daemon listens on. The following /etc/sysconfig/dhcpd example specifies that the DHCP daemon listens on the eth0 and eth1 interfaces:
DHCPDARGS="eth0 eth1";
DHCPDARGS="eth0 eth1";
If a system has three network interfaces cards —
eth0, eth1, and eth2 — and it is only desired that the DHCP daemon listens on the eth0 card, then only specify eth0 in /etc/sysconfig/dhcpd:
DHCPDARGS="eth0";
DHCPDARGS="eth0";
The following is a basic
/etc/dhcp/dhcpd.conf file, for a server that has two network interfaces, eth0 in a 10.0.0.0/24 network, and eth1 in a 172.16.0.0/24 network. Multiple subnet declarations allow you to define different settings for multiple networks:
-
subnet 10.0.0.0 netmask 255.255.255.0; - A
subnetdeclaration is required for every network your DHCP server is serving. Multiple subnets require multiplesubnetdeclarations. If the DHCP server does not have a network interface in a range of asubnetdeclaration, the DHCP server does not serve that network.If there is only onesubnetdeclaration, and no network interfaces are in the range of that subnet, the DHCP daemon fails to start, and an error such as the following is logged to/var/log/messages:Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
option subnet-mask 255.255.255.0; - The
option subnet-maskoption defines a subnet mask, and overrides thenetmaskvalue in thesubnetdeclaration. In simple cases, the subnet and netmask values are the same. -
option routers 10.0.0.1; - The
option routersoption defines the default gateway for the subnet. This is required for systems to reach internal networks on a different subnet, as well as external networks. -
range 10.0.0.5 10.0.0.15; - The
rangeoption specifies the pool of available IP addresses. Systems are assigned an address from the range of specified IP addresses.
For further information, see the
dhcpd.conf(5) man page.
16.4.1. Host Configuration
Link kopierenLink in die Zwischenablage kopiert!
Before making any changes, back up the existing
/etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
Configuring a Single System for Multiple Networks
The following /etc/dhcp/dhcpd.conf example creates two subnets, and configures an IP address for the same system, depending on which network it connects to:
-
host example0 - The
hostdeclaration defines specific parameters for a single system, such as an IP address. To configure specific parameters for multiple hosts, use multiplehostdeclarations.Most DHCP clients ignore the name inhostdeclarations, and as such, this name can be anything, as long as it is unique to otherhostdeclarations. To configure the same system for multiple networks, use a different name for eachhostdeclaration, otherwise the DHCP daemon fails to start. Systems are identified by thehardware ethernetoption, not the name in thehostdeclaration. -
hardware ethernet 00:1A:6B:6A:2E:0B; - The
hardware ethernetoption identifies the system. To find this address, run theip linkcommand. -
fixed-address 10.0.0.20; - The
fixed-addressoption assigns a valid IP address to the system specified by thehardware ethernetoption. This address must be outside the IP address pool specified with therangeoption.
If
option statements do not end with a semicolon, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
Configuring Systems with Multiple Network Interfaces
The following host declarations configure a single system, which has multiple network interfaces, so that each interface receives the same IP address. This configuration will not work if both network interfaces are connected to the same network at the same time:
For this example,
interface0 is the first network interface, and interface1 is the second interface. The different hardware ethernet options identify each interface.
If such a system connects to another network, add more
host declarations, remembering to:
- assign a valid
fixed-addressfor the network the host is connecting to. - make the name in the
hostdeclaration unique.
When a name given in a
host declaration is not unique, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
dhcpd: /etc/dhcp/dhcpd.conf line 31: host interface0: already exists dhcpd: } dhcpd: ^ dhcpd: Configuration file errors encountered -- exiting
dhcpd: /etc/dhcp/dhcpd.conf line 31: host interface0: already exists
dhcpd: }
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting
This error was caused by having multiple
host interface0 declarations defined in /etc/dhcp/dhcpd.conf.
16.5. DHCP for IPv6 (DHCPv6)
Link kopierenLink in die Zwischenablage kopiert!
The ISC DHCP includes support for IPv6 (DHCPv6) since the 4.x release with a DHCPv6 server, client and relay agent functionality. The server, client and relay agents support both IPv4 and IPv6. However, the client and the server can only manage one protocol at a time — for dual support they must be started separately for IPv4 and IPv6.
16.5.1. Configuring a DHCPv6 Server
Link kopierenLink in die Zwischenablage kopiert!
The DHCPv6 server configuration file is installed together with the dhcp package and it can be found at
/etc/dhcp/dhcpd6.conf.
The sample server configuration file can be found at
/usr/share/doc/dhcp-<version>/dhcpd6.conf.sample, in Red Hat Enterprise Linux 6 at /usr/share/doc/dhcp-4.1.1/dhcpd6.conf.sample.
A simple DHCPv6 server configuration file can look like this:
subnet6 2001:db8:0:1::/64 {
range6 2001:db8:0:1::129 2001:db8:0:1::254;
option dhcp6.name-servers fec0:0:0:1::1;
option dhcp6.domain-search "domain.example";
}
subnet6 2001:db8:0:1::/64 {
range6 2001:db8:0:1::129 2001:db8:0:1::254;
option dhcp6.name-servers fec0:0:0:1::1;
option dhcp6.domain-search "domain.example";
}
For more examples, see the
dhcpd.conf(5) man page.
To start the DHCPv6 service, enter the command
service dhcpd6 start as root. To stop the DHCPv6 server, use the command service dhcpdv6 stop.
To pass command-line options to
dhcpd daemon when the DHCPv6 service starts, use the /etc/sysconfig/dhcpd6 file. This file uses the same structure like the /etc/sysconfig/dhcpd:
cat /etc/sysconfig/dhcpd6 # Command line options here DHCPDARGS=
# cat /etc/sysconfig/dhcpd6
# Command line options here
DHCPDARGS=
The value added to the
DHCPDARGS option is passed to the DHCPv6 service, which passes it to the dhcpd daemon. For more information, see the STANDARD DHCPV6 OPTIONS section in the dhcpd-options(5) man page. For additional examples, see the Dynamic IPv6 configuration on the Fedora Project wiki.
16.5.2. Configuring a DHCPv6 Client
Link kopierenLink in die Zwischenablage kopiert!
The default configuration of the DHCPv6 client works fine in the most cases. However, to configure a DHCP client manually, create and modify the
/etc/dhcp/dhclient.conf file. See the /usr/share/doc/dhclient-4.1.1/dhclient6.conf.sample for a client configuration file example.
For advanced configurations of DHCPv6 client options such as protocol timing, lease requirements and requests, dynamic DNS support, aliases, as well as a wide variety of values to override, prepend, or append to client-side configurations, see the
dhclient.conf(5) man page and the STANDARD DHCPV6 OPTIONS section in the dhcpd-options(5) man page.
Important
In Red Hat Enterprise Linux 6, a DHCPv6 client is correctly handled only by NetworkManager and should not generally be run separately. That is because DHCPv6, unlike DHCPv4, is not a standalone network configuration protocol but is always supposed to be used together with router discovery.
16.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
For additional information, see The DHCP Handbook; Ralph Droms and Ted Lemon; 2003 or the following resources.
16.6.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
dhcpdman page — Describes how the DHCP daemon works.dhcpd.confman page — Explains how to configure the DHCP configuration file; includes some examples.dhcpd.leasesman page — Describes a persistent database of leases.dhcp-optionsman page — Explains the syntax for declaring DHCP options indhcpd.conf; includes some examples.dhcrelayman page — Explains the DHCP Relay Agent and its configuration options./usr/share/doc/dhcp-<version>/— Contains sample files, README files, and release notes for current versions of the DHCP service.
Chapter 17. DNS Servers
Link kopierenLink in die Zwischenablage kopiert!
DNS (Domain Name System), also known as a nameserver, is a network system that associates host names with their respective IP addresses. For users, this has the advantage that they can refer to machines on the network by names that are usually easier to remember than the numerical network addresses. For system administrators, using the nameserver allows them to change the IP address for a host without ever affecting the name-based queries, or to decide which machines handle these queries.
17.1. Introduction to DNS
Link kopierenLink in die Zwischenablage kopiert!
DNS is usually implemented using one or more centralized servers that are authoritative for certain domains. When a client host requests information from a nameserver, it usually connects to port 53. The nameserver then attempts to resolve the name requested. If it does not have an authoritative answer, or does not already have the answer cached from an earlier query, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the name in question, and then queries them to get the requested name.
17.1.1. Nameserver Zones
Link kopierenLink in die Zwischenablage kopiert!
In a DNS server such as BIND (Berkeley Internet Name Domain), all information is stored in basic data elements called resource records (RR). The resource record is usually a fully qualified domain name (FQDN) of a host, and is broken down into multiple sections organized into a tree-like hierarchy. This hierarchy consists of a main trunk, primary branches, secondary branches, and so on.
Example 17.1. A simple resource record
bob.sales.example.com
bob.sales.example.com
Each level of the hierarchy is divided by a period (that is,
.). In Example 17.1, “A simple resource record”, com defines the top-level domain, example its subdomain, and sales the subdomain of example. In this case, bob identifies a resource record that is part of the sales.example.com domain. With the exception of the part furthest to the left (that is, bob), each of these sections is called a zone and defines a specific namespace.
Zones are defined on authoritative nameservers through the use of zone files, which contain definitions of the resource records in each zone. Zone files are stored on primary nameservers (also called master nameservers), where changes are made to the files, and secondary nameservers (also called slave nameservers), which receive zone definitions from the primary nameservers. Both primary and secondary nameservers are authoritative for the zone and look the same to clients. Depending on the configuration, any nameserver can also serve as a primary or secondary server for multiple zones at the same time.
17.1.2. Nameserver Types
Link kopierenLink in die Zwischenablage kopiert!
There are two nameserver configuration types:
- authoritative
- Authoritative nameservers answer to resource records that are part of their zones only. This category includes both primary (master) and secondary (slave) nameservers.
- recursive
- Recursive nameservers offer resolution services, but they are not authoritative for any zone. Answers for all resolutions are cached in a memory for a fixed period of time, which is specified by the retrieved resource record.
Although a nameserver can be both authoritative and recursive at the same time, it is recommended not to combine the configuration types. To be able to perform their work, authoritative servers should be available to all clients all the time. On the other hand, since the recursive lookup takes far more time than authoritative responses, recursive servers should be available to a restricted number of clients only, otherwise they are prone to distributed denial of service (DDoS) attacks.
17.1.3. BIND as a Nameserver
Link kopierenLink in die Zwischenablage kopiert!
BIND consists of a set of DNS-related programs. It contains a nameserver called
named, an administration utility called rndc, and a debugging tool called dig. See Chapter 12, Services and Daemons for more information on how to run a service in Red Hat Enterprise Linux.
17.2. BIND
Link kopierenLink in die Zwischenablage kopiert!
This chapter covers
BIND (Berkeley Internet Name Domain), the DNS server included in Red Hat Enterprise Linux. It focuses on the structure of its configuration files, and describes how to administer it both locally and remotely.
17.2.1. Configuring the named Service
Link kopierenLink in die Zwischenablage kopiert!
When the
named service is started, it reads the configuration from the files as described in Table 17.1, “The named service configuration files”.
| Path | Description |
|---|---|
/etc/named.conf | The main configuration file. |
/etc/named/ | An auxiliary directory for configuration files that are included in the main configuration file. |
The configuration file consists of a collection of statements with nested options surrounded by opening and closing curly brackets. Note that when editing the file, you have to be careful not to make any syntax error, otherwise the
named service will not start. A typical /etc/named.conf file is organized as follows:
Note
If you have installed the bind-chroot package, the BIND service will run in the
/var/named/chroot environment. In that case, the initialization script will mount the above configuration files using the mount --bind command, so that you can manage the configuration outside this environment. There is no need to copy anything into the /var/named/chroot directory because it is mounted automatically. This simplifies maintenance since you do not need to take any special care of BIND configuration files if it is run in a chroot environment. You can organize everything as you would with BIND not running in a chroot environment.
The following directories are automatically mounted into
/var/named/chroot if they are empty in the /var/named/chroot directory. They must be kept empty if you want them to be mounted into /var/named/chroot:
/var/named/etc/pki/dnssec-keys/etc/named/usr/lib64/bindor/usr/lib/bind(architecture dependent).
The following files are also mounted if the target file does not exist in
/var/named/chroot.
/etc/named.conf/etc/rndc.conf/etc/rndc.key/etc/named.rfc1912.zones/etc/named.dnssec.keys/etc/named.iscdlv.key/etc/named.root.key
17.2.1.1. Common Statement Types
Link kopierenLink in die Zwischenablage kopiert!
The following types of statements are commonly used in
/etc/named.conf:
-
acl - The
acl(Access Control List) statement allows you to define groups of hosts, so that they can be permitted or denied access to the nameserver. It takes the following form:acl acl-name { match-element; ... };acl acl-name { match-element; ... };Copy to Clipboard Copied! Toggle word wrap Toggle overflow The acl-name statement name is the name of the access control list, and the match-element option is usually an individual IP address (such as10.0.1.1) or a CIDR (Classless Inter-Domain Routing) network notation (for example,10.0.1.0/24). For a list of already defined keywords, see Table 17.2, “Predefined access control lists”.Expand Table 17.2. Predefined access control lists Keyword Description anyMatches every IP address. localhostMatches any IP address that is in use by the local system. localnetsMatches any IP address on any network to which the local system is connected. noneDoes not match any IP address. Theaclstatement can be especially useful in conjunction with other statements such asoptions. Example 17.2, “Using acl in conjunction with options” defines two access control lists,black-hatsandred-hats, and addsblack-hatson the blacklist while grantingred-hatsa normal access.Example 17.2. Using acl in conjunction with options
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
include - The
includestatement allows you to include files in the/etc/named.conf, so that potentially sensitive data can be placed in a separate file with restricted permissions. It takes the following form:include "file-name"
include "file-name"Copy to Clipboard Copied! Toggle word wrap Toggle overflow The file-name statement name is an absolute path to a file.Example 17.3. Including a file to /etc/named.conf
include "/etc/named.rfc1912.zones";
include "/etc/named.rfc1912.zones";Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
options - The
optionsstatement allows you to define global server configuration options as well as to set defaults for other statements. It can be used to specify the location of thenamedworking directory, the types of queries allowed, and much more. It takes the following form:options { option; ... };options { option; ... };Copy to Clipboard Copied! Toggle word wrap Toggle overflow For a list of frequently used option directives, see Table 17.3, “Commonly used options” below.Expand Table 17.3. Commonly used options Option Description allow-querySpecifies which hosts are allowed to query the nameserver for authoritative resource records. It accepts an access control list, a collection of IP addresses, or networks in the CIDR notation. All hosts are allowed by default. allow-query-cacheSpecifies which hosts are allowed to query the nameserver for non-authoritative data such as recursive queries. Only localhostandlocalnetsare allowed by default.blackholeSpecifies which hosts are not allowed to query the nameserver. This option should be used when particular host or network floods the server with requests. The default option is none.directorySpecifies a working directory for the namedservice. The default option is/var/named/.dnssec-enableSpecifies whether to return DNSSEC related resource records. The default option is yes.dnssec-validationSpecifies whether to prove that resource records are authentic via DNSSEC. The default option is yes.forwardersSpecifies a list of valid IP addresses for nameservers to which the requests should be forwarded for resolution. forwardSpecifies the behavior of theforwardersdirective. It accepts the following options:first— The server will query the nameservers listed in theforwardersdirective before attempting to resolve the name on its own.only— When unable to query the nameservers listed in theforwardersdirective, the server will not attempt to resolve the name on its own.
listen-onSpecifies the IPv4 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv4 interfaces are used by default. listen-on-v6Specifies the IPv6 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv6 interfaces are used by default. max-cache-sizeSpecifies the maximum amount of memory to be used for server caches. When the limit is reached, the server causes records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default option is 32M.notifySpecifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:yes— The server will notify all secondary nameservers.no— The server will not notify any secondary nameserver.master-only— The server will notify primary server for the zone only.explicit— The server will notify only the secondary servers that are specified in thealso-notifylist within a zone statement.
pid-fileSpecifies the location of the process ID file created by the namedservice.recursionSpecifies whether to act as a recursive server. The default option is yes.statistics-fileSpecifies an alternate location for statistics files. The /var/named/named.statsfile is used by default.Important
To prevent distributed denial of service (DDoS) attacks, it is recommended that you use theallow-query-cacheoption to restrict recursive DNS services for a particular subset of clients only.See the BIND 9 Administrator Reference Manual referenced in Section 17.2.7.1, “Installed Documentation”, and thenamed.confmanual page for a complete list of available options.Example 17.4. Using the options statement
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
zone - The
zonestatement allows you to define the characteristics of a zone, such as the location of its configuration file and zone-specific options, and can be used to override the globaloptionsstatements. It takes the following form:zone zone-name [zone-class] { option; ... };zone zone-name [zone-class] { option; ... };Copy to Clipboard Copied! Toggle word wrap Toggle overflow The zone-name attribute is the name of the zone, zone-class is the optional class of the zone, and option is azonestatement option as described in Table 17.4, “Commonly used options”.The zone-name attribute is particularly important, as it is the default value assigned for the$ORIGINdirective used within the corresponding zone file located in the/var/named/directory. Thenameddaemon appends the name of the zone to any non-fully qualified domain name listed in the zone file. For example, if azonestatement defines the namespace forexample.com, useexample.comas the zone-name so that it is placed at the end of host names within theexample.comzone file.For more information about zone files, see Section 17.2.2, “Editing Zone Files”.Expand Table 17.4. Commonly used options Option Description allow-querySpecifies which clients are allowed to request information about this zone. This option overrides global allow-queryoption. All query requests are allowed by default.allow-transferSpecifies which secondary servers are allowed to request a transfer of the zone's information. All transfer requests are allowed by default. allow-updateSpecifies which hosts are allowed to dynamically update information in their zone. The default option is to deny all dynamic update requests.Note that you should be careful when allowing hosts to update information about their zone. Do not set IP addresses in this option unless the server is in the trusted network. Instead, use TSIG key as described in Section 17.2.5.3, “Transaction SIGnatures (TSIG)”.fileSpecifies the name of the file in the namedworking directory that contains the zone's configuration data.mastersSpecifies from which IP addresses to request authoritative zone information. This option is used only if the zone is defined as typeslave.notifySpecifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:yes— The server will notify all secondary nameservers.no— The server will not notify any secondary nameserver.master-only— The server will notify primary server for the zone only.explicit— The server will notify only the secondary servers that are specified in thealso-notifylist within a zone statement.
typeSpecifies the zone type. It accepts the following options:delegation-only— Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated asNXDOMAIN. This option is only applicable in TLDs (Top-Level Domain) or root zone files used in recursive or caching implementations.forward— Forwards all requests for information about this zone to other nameservers.hint— A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with ahintzone.master— Designates the nameserver as authoritative for this zone. A zone should be set as themasterif the zone's configuration files reside on the system.slave— Designates the nameserver as a slave server for this zone. Master server is specified inmastersdirective.
Most changes to the/etc/named.conffile of a primary or secondary nameserver involve adding, modifying, or deletingzonestatements, and only a small subset ofzonestatement options is usually needed for a nameserver to work efficiently.In Example 17.5, “A zone statement for a primary nameserver”, the zone is identified asexample.com, the type is set tomaster, and thenamedservice is instructed to read the/var/named/example.com.zonefile. It also allows only a secondary nameserver (192.168.0.2) to transfer the zone.Example 17.5. A zone statement for a primary nameserver
zone "example.com" IN { type master; file "example.com.zone"; allow-transfer { 192.168.0.2; }; };zone "example.com" IN { type master; file "example.com.zone"; allow-transfer { 192.168.0.2; }; };Copy to Clipboard Copied! Toggle word wrap Toggle overflow A secondary server'szonestatement is slightly different. The type is set toslave, and themastersdirective is tellingnamedthe IP address of the master server.In Example 17.6, “A zone statement for a secondary nameserver”, thenamedservice is configured to query the primary server at the192.168.0.1IP address for information about theexample.comzone. The received information is then saved to the/var/named/slaves/example.com.zonefile. Note that you have to put all slave zones to/var/named/slavesdirectory, otherwise the service will fail to transfer the zone.Example 17.6. A zone statement for a secondary nameserver
zone "example.com" { type slave; file "slaves/example.com.zone"; masters { 192.168.0.1; }; };zone "example.com" { type slave; file "slaves/example.com.zone"; masters { 192.168.0.1; }; };Copy to Clipboard Copied! Toggle word wrap Toggle overflow
17.2.1.2. Other Statement Types
Link kopierenLink in die Zwischenablage kopiert!
The following types of statements are less commonly used in
/etc/named.conf:
-
controls - The
controlsstatement allows you to configure various security requirements necessary to use therndccommand to administer thenamedservice.See Section 17.2.3, “Using the rndc Utility” for more information on therndcutility and its usage. -
key - The
keystatement allows you to define a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of therndccommand. Two options are used withkey:algorithm algorithm-name— The type of algorithm to be used (for example,hmac-md5).secret "key-value"— The encrypted key.
See Section 17.2.3, “Using the rndc Utility” for more information on therndcutility and its usage. -
logging - The
loggingstatement allows you to use multiple types of logs, so called channels. By using thechanneloption within the statement, you can construct a customized type of log with its own file name (file), size limit (size), versioning (version), and level of importance (severity). Once a customized channel is defined, acategoryoption is used to categorize the channel and begin logging when thenamedservice is restarted.By default,namedsends standard messages to thersyslogdaemon, which places them in/var/log/messages. Several standard channels are built into BIND with various severity levels, such asdefault_syslog(which handles informational logging messages) anddefault_debug(which specifically handles debugging messages). A default category, calleddefault, uses the built-in channels to do normal logging without any special configuration.Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, see the BIND 9 Administrator Reference Manual referenced in Section 17.2.7.1, “Installed Documentation”. -
server - The
serverstatement allows you to specify options that affect how thenamedservice should respond to remote nameservers, especially with regard to notifications and zone transfers.Thetransfer-formatoption controls the number of resource records that are sent with each message. It can be eitherone-answer(only one resource record), ormany-answers(multiple resource records). Note that while themany-answersoption is more efficient, it is not supported by older versions of BIND. -
trusted-keys - The
trusted-keysstatement allows you to specify assorted public keys used for secure DNS (DNSSEC). See Section 17.2.5.4, “DNS Security Extensions (DNSSEC)” for more information on this topic. -
view - The
viewstatement allows you to create special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones.Multiple views can be used as long as their names are unique. Thematch-clientsoption allows you to specify the IP addresses that apply to a particular view. If theoptionsstatement is used within a view, it overrides the already configured global options. Finally, mostviewstatements contain multiplezonestatements that apply to thematch-clientslist.Note that the order in which theviewstatements are listed is important, as the first statement that matches a particular client's IP address is used. For more information on this topic, see Section 17.2.5.1, “Multiple Views”.
17.2.1.3. Comment Tags
Link kopierenLink in die Zwischenablage kopiert!
Additionally to statements, the
/etc/named.conf file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to a user. The following are valid comment tags:
-
// - Any text after the
//characters to the end of the line is considered a comment. For example:notify yes; // notify all secondary nameservers
notify yes; // notify all secondary nameserversCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
# - Any text after the
#character to the end of the line is considered a comment. For example:notify yes; # notify all secondary nameservers
notify yes; # notify all secondary nameserversCopy to Clipboard Copied! Toggle word wrap Toggle overflow /*and*/- Any block of text enclosed in
/*and*/is considered a comment. For example:notify yes; /* notify all secondary nameservers */
notify yes; /* notify all secondary nameservers */Copy to Clipboard Copied! Toggle word wrap Toggle overflow
17.2.2. Editing Zone Files
Link kopierenLink in die Zwischenablage kopiert!
As outlined in Section 17.1.1, “Nameserver Zones”, zone files contain information about a namespace. They are stored in the
named working directory located in /var/named/ by default, and each zone file is named according to the file option in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.
| Path | Description |
|---|---|
/var/named/ | The working directory for the named service. The nameserver is not allowed to write to this directory. |
/var/named/slaves/ | The directory for secondary zones. This directory is writable by the named service. |
/var/named/dynamic/ | The directory for other files, such as dynamic DNS (DDNS) zones or managed DNSSEC keys. This directory is writable by the named service. |
/var/named/data/ | The directory for various statistics and debugging files. This directory is writable by the named service. |
A zone file consists of directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone, resource records define the parameters of the zone and assign identities to individual hosts. While the directives are optional, the resource records are required in order to provide name service to a zone.
All directives and resource records should be entered on individual lines.
17.2.2.1. Common Directives
Link kopierenLink in die Zwischenablage kopiert!
Directives begin with the dollar sign character followed by the name of the directive, and usually appear at the top of the file. The following directives are commonly used in zone files:
-
$INCLUDE - The
$INCLUDEdirective allows you to include another file at the place where it appears, so that other zone settings can be stored in a separate zone file.Example 17.7. Using the $INCLUDE directive
$INCLUDE /var/named/penguin.example.com
$INCLUDE /var/named/penguin.example.comCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
$ORIGIN - The
$ORIGINdirective allows you to append the domain name to unqualified records, such as those with the host name only. Note that the use of this directive is not necessary if the zone is specified in/etc/named.conf, since the zone name is used by default.In Example 17.8, “Using the $ORIGIN directive”, any names used in resource records that do not end in a trailing period are appended withexample.com.Example 17.8. Using the $ORIGIN directive
$ORIGIN example.com.
$ORIGIN example.com.Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
$TTL - The
$TTLdirective allows you to set the default Time to Live (TTL) value for the zone, that is, how long is a zone record valid. Each resource record can contain its own TTL value, which overrides this directive.Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to propagate resource record changes.Example 17.9. Using the $TTL directive
$TTL 1D
$TTL 1DCopy to Clipboard Copied! Toggle word wrap Toggle overflow
17.2.2.2. Common Resource Records
Link kopierenLink in die Zwischenablage kopiert!
The following resource records are commonly used in zone files:
-
A - The Address record specifies an IP address to be assigned to a name. It takes the following form:
hostname IN A IP-address
hostname IN A IP-addressCopy to Clipboard Copied! Toggle word wrap Toggle overflow If the hostname value is omitted, the record will point to the last specified hostname.In Example 17.10, “Using the A resource record”, the requests forserver1.example.comare pointed to10.0.1.3or10.0.1.5.Example 17.10. Using the A resource record
server1 IN A 10.0.1.3 IN A 10.0.1.5server1 IN A 10.0.1.3 IN A 10.0.1.5Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
CNAME - The Canonical Name record maps one name to another. Because of this, this type of record is sometimes referred to as an alias record. It takes the following form:
alias-name IN CNAME real-name
alias-name IN CNAME real-nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow CNAMErecords are most commonly used to point to services that use a common naming scheme, such aswwwfor Web servers. However, there are multiple restrictions for their usage:- CNAME records should not point to other CNAME records. This is mainly to avoid possible infinite loops.
- CNAME records should not contain other resource record types (such as A, NS, MX, etc.). The only exception are DNSSEC related records (that is, RRSIG, NSEC, etc.) when the zone is signed.
- Other resource record that point to the fully qualified domain name (FQDN) of a host (that is, NS, MX, PTR) should not point to a CNAME record.
In Example 17.11, “Using the CNAME resource record”, theArecord binds a host name to an IP address, while theCNAMErecord points the commonly usedwwwhost name to it.Example 17.11. Using the CNAME resource record
server1 IN A 10.0.1.5 www IN CNAME server1
server1 IN A 10.0.1.5 www IN CNAME server1Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MX - The Mail Exchange record specifies where the mail sent to a particular namespace controlled by this zone should go. It takes the following form:
IN MX preference-value email-server-name
IN MX preference-value email-server-nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The email-server-name is a fully qualified domain name (FQDN). The preference-value allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. TheMXresource record with the lowest preference-value is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them.In Example 17.12, “Using the MX resource record”, the firstmail.example.comemail server is preferred to themail2.example.comemail server when receiving email destined for theexample.comdomain.Example 17.12. Using the MX resource record
example.com. IN MX 10 mail.example.com. IN MX 20 mail2.example.com.example.com. IN MX 10 mail.example.com. IN MX 20 mail2.example.com.Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
NS - The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form:
IN NS nameserver-name
IN NS nameserver-nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The nameserver-name should be a fully qualified domain name (FQDN). Note that when two nameservers are listed as authoritative for the domain, it is not important whether these nameservers are secondary nameservers, or if one of them is a primary server. They are both still considered authoritative.Example 17.13. Using the NS resource record
IN NS dns1.example.com. IN NS dns2.example.com.
IN NS dns1.example.com. IN NS dns2.example.com.Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
PTR - The Pointer record points to another part of the namespace. It takes the following form:
last-IP-digit IN PTR FQDN-of-system
last-IP-digit IN PTR FQDN-of-systemCopy to Clipboard Copied! Toggle word wrap Toggle overflow The last-IP-digit directive is the last number in an IP address, and the FQDN-of-system is a fully qualified domain name (FQDN).PTRrecords are primarily used for reverse name resolution, as they point IP addresses back to a particular name. See Section 17.2.2.4.2, “A Reverse Name Resolution Zone File” for more examples ofPTRrecords in use. -
SOA - The Start of Authority record announces important authoritative information about a namespace to the nameserver. Located after the directives, it is the first resource record in a zone file. It takes the following form:
Copy to Clipboard Copied! Toggle word wrap Toggle overflow The directives are as follows:- The
@symbol places the$ORIGINdirective (or the zone's name if the$ORIGINdirective is not set) as the namespace being defined by thisSOAresource record. - The primary-name-server directive is the host name of the primary nameserver that is authoritative for this domain.
- The hostmaster-email directive is the email of the person to contact about the namespace.
- The serial-number directive is a numerical value incremented every time the zone file is altered to indicate it is time for the
namedservice to reload the zone. - The time-to-refresh directive is the numerical value secondary nameservers use to determine how long to wait before asking the primary nameserver if any changes have been made to the zone.
- The time-to-retry directive is a numerical value used by secondary nameservers to determine the length of time to wait before issuing a refresh request in the event that the primary nameserver is not answering. If the primary server has not replied to a refresh request before the amount of time specified in the time-to-expire directive elapses, the secondary servers stop responding as an authority for requests concerning that namespace.
- In BIND 4 and 8, the minimum-TTL directive is the amount of time other nameservers cache the zone's information. In BIND 9, it defines how long negative answers are cached for. Caching of negative answers can be set to a maximum of 3 hours (that is,
3H).
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). Table 17.6, “Seconds compared to other time units” shows an amount of time in seconds and the equivalent time in another format.Expand Table 17.6. Seconds compared to other time units Seconds Other Time Units 60 1M1800 30M3600 1H10800 3H21600 6H43200 12H86400 1D259200 3D604800 1W31536000 365DExample 17.14. Using the SOA resource record
Copy to Clipboard Copied! Toggle word wrap Toggle overflow
17.2.2.3. Comment Tags
Link kopierenLink in die Zwischenablage kopiert!
Additionally to resource records and directives, a zone file can also contain comments. Comments are ignored by the
named service, but can prove useful when providing additional information to the user. Any text after the semicolon character to the end of the line is considered a comment. For example:
604800 ; expire after 1 week
604800 ; expire after 1 week17.2.2.4. Example Usage
Link kopierenLink in die Zwischenablage kopiert!
The following examples show the basic usage of zone files.
17.2.2.4.1. A Simple Zone File
Link kopierenLink in die Zwischenablage kopiert!
Example 17.15, “A simple zone file” demonstrates the use of standard directives and
SOA values.
Example 17.15. A simple zone file
In this example, the authoritative nameservers are set as
dns1.example.com and dns2.example.com, and are tied to the 10.0.1.1 and 10.0.1.2 IP addresses respectively using the A record.
The email servers configured with the
MX records point to mail and mail2 via A records. Since these names do not end in a trailing period, the $ORIGIN domain is placed after them, expanding them to mail.example.com and mail2.example.com.
Services available at the standard names, such as
www.example.com (WWW), are pointed at the appropriate servers using the CNAME record.
This zone file would be called into service with a
zone statement in the /etc/named.conf similar to the following:
zone "example.com" IN {
type master;
file "example.com.zone";
allow-update { none; };
};
zone "example.com" IN {
type master;
file "example.com.zone";
allow-update { none; };
};17.2.2.4.2. A Reverse Name Resolution Zone File
Link kopierenLink in die Zwischenablage kopiert!
A reverse name resolution zone file is used to translate an IP address in a particular namespace into an fully qualified domain name (FQDN). It looks very similar to a standard zone file, except that the
PTR resource records are used to link the IP addresses to a fully qualified domain name as shown in Example 17.16, “A reverse name resolution zone file”.
Example 17.16. A reverse name resolution zone file
In this example, IP addresses
10.0.1.1 through 10.0.1.6 are pointed to the corresponding fully qualified domain name.
This zone file would be called into service with a
zone statement in the /etc/named.conf file similar to the following:
zone "1.0.10.in-addr.arpa" IN {
type master;
file "example.com.rr.zone";
allow-update { none; };
};
zone "1.0.10.in-addr.arpa" IN {
type master;
file "example.com.rr.zone";
allow-update { none; };
};
There is very little difference between this example and a standard
zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.
17.2.3. Using the rndc Utility
Link kopierenLink in die Zwischenablage kopiert!
The
rndc utility is a command-line tool that allows you to administer the named service, both locally and from a remote machine. Its usage is as follows:
rndc [option...] command [command-option]
rndc [option...] command [command-option]17.2.3.1. Configuring the Utility
Link kopierenLink in die Zwischenablage kopiert!
To prevent unauthorized access to the service,
named must be configured to listen on the selected port (that is, 953 by default), and an identical key must be used by both the service and the rndc utility.
The
rndc configuration is located in /etc/rndc.conf. If the file does not exist, the utility will use the key located in /etc/rndc.key, which was generated automatically during the installation process using the rndc-confgen -a command.
The
named service is configured using the controls statement in the /etc/named.conf configuration file as described in Section 17.2.1.2, “Other Statement Types”. Unless this statement is present, only the connections from the loopback address (that is, 127.0.0.1) will be allowed, and the key located in /etc/rndc.key will be used.
For more information on this topic, see manual pages and the BIND 9 Administrator Reference Manual listed in Section 17.2.7, “Additional Resources”.
Important
To prevent unprivileged users from sending control commands to the service, make sure only root is allowed to read the
/etc/rndc.key file:
chmod o-rwx /etc/rndc.key
~]# chmod o-rwx /etc/rndc.key17.2.3.2. Checking the Service Status
Link kopierenLink in die Zwischenablage kopiert!
To check the current status of the
named service, use the following command:
17.2.3.3. Reloading the Configuration and Zones
Link kopierenLink in die Zwischenablage kopiert!
To reload both the configuration file and zones, type the following at a shell prompt:
rndc reload
~]# rndc reload
server reload successful
This will reload the zones while keeping all previously cached responses, so that you can make changes to the zone files without losing all stored name resolutions.
To reload a single zone, specify its name after the
reload command, for example:
rndc reload localhost
~]# rndc reload localhost
zone reload up-to-date
Finally, to reload the configuration file and newly added zones only, type:
rndc reconfig
~]# rndc reconfigNote
If you intend to manually modify a zone that uses Dynamic DNS (DDNS), make sure you run the
freeze command first:
rndc freeze localhost
~]# rndc freeze localhost
Once you are finished, run the
thaw command to allow the DDNS again and reload the zone:
rndc thaw localhost
~]# rndc thaw localhost
The zone reload and thaw was successful.17.2.3.4. Updating Zone Keys
Link kopierenLink in die Zwischenablage kopiert!
To update the DNSSEC keys and sign the zone, use the
sign command. For example:
rndc sign localhost
~]# rndc sign localhost
Note that to sign a zone with the above command, the
auto-dnssec option has to be set to maintain in the zone statement. For instance:
17.2.3.5. Enabling the DNSSEC Validation
Link kopierenLink in die Zwischenablage kopiert!
To enable the DNSSEC validation, type the following at a shell prompt:
rndc validation on
~]# rndc validation on
Similarly, to disable this option, type:
rndc validation off
~]# rndc validation off
See the
options statement described in Section 17.2.1.1, “Common Statement Types” for information on how to configure this option in /etc/named.conf.
17.2.3.6. Enabling the Query Logging
Link kopierenLink in die Zwischenablage kopiert!
To enable (or disable in case it is currently enabled) the query logging, run the following command:
rndc querylog
~]# rndc querylog
To check the current setting, use the
status command as described in Section 17.2.3.2, “Checking the Service Status”.
17.2.4. Using the dig Utility
Link kopierenLink in die Zwischenablage kopiert!
The
dig utility is a command-line tool that allows you to perform DNS lookups and debug a nameserver configuration. Its typical usage is as follows:
dig [@server] [option...] name type
dig [@server] [option...] name type
See Section 17.2.2.2, “Common Resource Records” for a list of common types.
17.2.4.1. Looking Up a Nameserver
Link kopierenLink in die Zwischenablage kopiert!
To look up a nameserver for a particular domain, use the command in the following form:
dig name NS
dig name NS
In Example 17.17, “A sample nameserver lookup”, the
dig utility is used to display nameservers for example.com.
Example 17.17. A sample nameserver lookup
17.2.4.2. Looking Up an IP Address
Link kopierenLink in die Zwischenablage kopiert!
To look up an IP address assigned to a particular domain, use the command in the following form:
dig name A
dig name A
In Example 17.18, “A sample IP address lookup”, the
dig utility is used to display the IP address of example.com.
Example 17.18. A sample IP address lookup
17.2.4.3. Looking Up a Hostname
Link kopierenLink in die Zwischenablage kopiert!
To look up a host name for a particular IP address, use the command in the following form:
dig -x address
dig -x address
In Example 17.19, “A sample host name lookup”, the
dig utility is used to display the host name assigned to 192.0.32.10.
Example 17.19. A sample host name lookup
17.2.5. Advanced Features of BIND
Link kopierenLink in die Zwischenablage kopiert!
Most BIND implementations only use the
named service to provide name resolution services or to act as an authority for a particular domain. However, BIND version 9 has a number of advanced features that allow for a more secure and efficient DNS service.
Important
Before attempting to use advanced features like DNSSEC, TSIG, or IXFR (Incremental Zone Transfer), make sure that the particular feature is supported by all nameservers in the network environment, especially when you use older versions of BIND or non-BIND servers.
All of the features mentioned are discussed in greater detail in the BIND 9 Administrator Reference Manual referenced in Section 17.2.7.1, “Installed Documentation”.
17.2.5.1. Multiple Views
Link kopierenLink in die Zwischenablage kopiert!
Optionally, different information can be presented to a client depending on the network a request originates from. This is primarily used to deny sensitive DNS entries from clients outside of the local network, while allowing queries from clients inside the local network.
To configure multiple views, add the
view statement to the /etc/named.conf configuration file. Use the match-clients option to match IP addresses or entire networks and give them special options and zone data.
17.2.5.2. Incremental Zone Transfers (IXFR)
Link kopierenLink in die Zwischenablage kopiert!
Incremental Zone Transfers (IXFR) allow a secondary nameserver to only download the updated portions of a zone modified on a primary nameserver. Compared to the standard transfer process, this makes the notification and update process much more efficient.
Note that IXFR is only available when using dynamic updating to make changes to master zone records. If manually editing zone files to make changes, Automatic Zone Transfer (AXFR) is used.
17.2.5.3. Transaction SIGnatures (TSIG)
Link kopierenLink in die Zwischenablage kopiert!
Transaction SIGnatures (TSIG) ensure that a shared secret key exists on both primary and secondary nameserver before allowing a transfer. This strengthens the standard IP address-based method of transfer authorization, since attackers would not only need to have access to the IP address to transfer the zone, but they would also need to know the secret key.
Since version 9, BIND also supports TKEY, which is another shared secret key method of authorizing zone transfers.
Important
When communicating over an insecure network, do not rely on IP address-based authentication only.
17.2.5.4. DNS Security Extensions (DNSSEC)
Link kopierenLink in die Zwischenablage kopiert!
Domain Name System Security Extensions (DNSSEC) provide origin authentication of DNS data, authenticated denial of existence, and data integrity. When a particular domain is marked as secure, the
SERFVAIL response is returned for each resource record that fails the validation.
Note that to debug a DNSSEC-signed domain or a DNSSEC-aware resolver, you can use the
dig utility as described in Section 17.2.4, “Using the dig Utility”. Useful options are +dnssec (requests DNSSEC-related resource records by setting the DNSSEC OK bit), +cd (tells recursive nameserver not to validate the response), and +bufsize=512 (changes the packet size to 512B to get through some firewalls).
17.2.5.5. Internet Protocol version 6 (IPv6)
Link kopierenLink in die Zwischenablage kopiert!
Internet Protocol version 6 (IPv6) is supported through the use of
AAAA resource records, and the listen-on-v6 directive as described in Table 17.3, “Commonly used options”.
17.2.6. Common Mistakes to Avoid
Link kopierenLink in die Zwischenablage kopiert!
The following is a list of recommendations on how to avoid common mistakes users make when configuring a nameserver:
- Use semicolons and curly brackets correctly
- An omitted semicolon or unmatched curly bracket in the
/etc/named.conffile can prevent thenamedservice from starting. - Use period correctly
- In zone files, a period at the end of a domain name denotes a fully qualified domain name. If omitted, the
namedservice will append the name of the zone or the value of$ORIGINto complete it. - Increment the serial number when editing a zone file
- If the serial number is not incremented, the primary nameserver will have the correct, new information, but the secondary nameservers will never be notified of the change, and will not attempt to refresh their data of that zone.
- Configure the firewall
- If a firewall is blocking connections from the
namedservice to other nameservers, the recommended practice is to change the firewall settings.Warning
According to the recent research in DNS security, using a fixed UDP source port for DNS queries is a potential security vulnerability that could allow an attacker to conduct cache-poisoning attacks more easily. To prevent this, configure your firewall to allow queries from a random UDP source port.
17.2.7. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
The following sources of information provide additional resources regarding BIND.
17.2.7.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
BIND features a full range of installed documentation covering many different topics, each placed in its own subject directory. For each item below, replace version with the version of the bind package installed on the system:
-
/usr/share/doc/bind-version/ - The main directory containing the most recent documentation.
-
/usr/share/doc/bind-version/arm/ - The directory containing the BIND 9 Administrator Reference Manual in HTML and SGML formats, which details BIND resource requirements, how to configure different types of nameservers, how to perform load balancing, and other advanced topics. For most new users of BIND, this is the best place to start.
-
/usr/share/doc/bind-version/draft/ - The directory containing assorted technical documents that review issues related to the DNS service, and propose some methods to address them.
-
/usr/share/doc/bind-version/misc/ - The directory designed to address specific advanced issues. Users of BIND version 8 should consult the
migrationdocument for specific changes they must make when moving to BIND 9. Theoptionsfile lists all of the options implemented in BIND 9 that are used in/etc/named.conf. -
/usr/share/doc/bind-version/rfc/ - The directory providing every RFC document related to BIND.
There is also a number of man pages for the various applications and configuration files involved with BIND:
-
man rndc - The manual page for
rndccontaining the full documentation on its usage. -
man named - The manual page for
namedcontaining the documentation on assorted arguments that can be used to control the BIND nameserver daemon. -
man lwresd - The manual page for
lwresdcontaining the full documentation on the lightweight resolver daemon and its usage. -
man named.conf - The manual page with a comprehensive list of options available within the
namedconfiguration file. -
man rndc.conf - The manual page with a comprehensive list of options available within the
rndcconfiguration file.
17.2.7.2. Useful Websites
Link kopierenLink in die Zwischenablage kopiert!
- http://www.isc.org/software/bind
- The home page of the BIND project containing information about current releases as well as a PDF version of the BIND 9 Administrator Reference Manual.
Chapter 18. Web Servers
Link kopierenLink in die Zwischenablage kopiert!
HTTP (Hypertext Transfer Protocol) server, or a web server, is a network service that serves content to a client over the web. This typically means web pages, but any other documents can be served as well.
The web servers available in Red Hat Enterprise Linux 6 are:
- Apache HTTP Server
- nginx
Important
Note that the nginx web server is available only as a Software Collection for Red Hat Enterprise Linux 6. See the Red Hat Software Collections Release Notes for information regarding getting access to nginx, usage of Software Collections, and other.
18.1. The Apache HTTP Server
Link kopierenLink in die Zwischenablage kopiert!
This section focuses on the Apache HTTP Server 2.2, a robust, full-featured open source web server developed by the Apache Software Foundation, that is included in Red Hat Enterprise Linux 6. It describes the basic configuration of the
httpd service, and covers advanced topics such as adding server modules, setting up virtual hosts, or configuring the secure HTTP server.
There are important differences between the Apache HTTP Server 2.2 and version 2.0, and if you are upgrading from a previous release of Red Hat Enterprise Linux, you will need to update the
httpd service configuration accordingly. This section reviews some of the newly added features, outlines important changes, and guides you through the update of older configuration files.
18.1.1. New Features
Link kopierenLink in die Zwischenablage kopiert!
The Apache HTTP Server version 2.2 introduces the following enhancements:
- Support for large files on 32-bit architectures, allowing the web server to handle files greater than 2GB.
- A new structure for authentication and authorization support, replacing the authentication modules provided in previous versions.
18.1.2. Notable Changes
Link kopierenLink in die Zwischenablage kopiert!
Since version 2.0, few changes have been made to the default
httpd service configuration:
18.1.3. Updating the Configuration
Link kopierenLink in die Zwischenablage kopiert!
To update the configuration files from the Apache HTTP Server version 2.0, take the following steps:
- Make sure all module names are correct, since they may have changed. Adjust the
LoadModuledirective for each module that has been renamed. - Recompile all third party modules before attempting to load them. This typically means authentication and authorization modules.
- If you use the Apache HTTP Secure Server, see Section 18.1.9, “Enabling the mod_ssl Module” for important information on enabling the Secure Sockets Layer (SSL) protocol.
Note that you can check the configuration for possible errors by using the following command:
service httpd configtest
~]# service httpd configtest
Syntax OK
For more information on upgrading the Apache HTTP Server configuration from version 2.0 to 2.2, see http://httpd.apache.org/docs/2.2/upgrading.html.
18.1.4. Running the httpd Service
Link kopierenLink in die Zwischenablage kopiert!
This section describes how to start, stop, restart, and check the current status of the Apache HTTP Server. To be able to use the
httpd service, make sure you have the httpd installed. You can do so by using the following command:
yum install httpd
~]# yum install httpd
For more information on the concept of runlevels and how to manage system services in Red Hat Enterprise Linux in general, see Chapter 12, Services and Daemons.
18.1.4.1. Starting the Service
Link kopierenLink in die Zwischenablage kopiert!
To run the
httpd service, type the following at a shell prompt as root:
service httpd start
~]# service httpd start
Starting httpd: [ OK ]
If you want the service to start automatically at the boot time, use the following command:
chkconfig httpd on
~]# chkconfig httpd on
This will enable the service for runlevel 2, 3, 4, and 5. Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.
Note
If running the Apache HTTP Server as a secure server, a password may be required after the machine boots if using an encrypted private SSL key.
18.1.4.2. Stopping the Service
Link kopierenLink in die Zwischenablage kopiert!
To stop the running
httpd service, type the following at a shell prompt as root:
service httpd stop
~]# service httpd stop
Stopping httpd: [ OK ]
To prevent the service from starting automatically at the boot time, type:
chkconfig httpd off
~]# chkconfig httpd off
This will disable the service for all runlevels. Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.
18.1.4.3. Restarting the Service
Link kopierenLink in die Zwischenablage kopiert!
There are three different ways to restart a running
httpd service:
- To restart the service completely, enter the following command as
root:service httpd restart
~]# service httpd restart Stopping httpd: [ OK ] Starting httpd: [ OK ]Copy to Clipboard Copied! Toggle word wrap Toggle overflow This stops the runninghttpdservice and immediately starts it again. Use this command after installing or removing a dynamically loaded module such as PHP. - To only reload the configuration, as
root, type:service httpd reload
~]# service httpd reloadCopy to Clipboard Copied! Toggle word wrap Toggle overflow This causes the runninghttpdservice to reload its configuration file. Any requests being currently processed will be interrupted, which may cause a client browser to display an error message or render a partial page. - To reload the configuration without affecting active requests, enter the following command as
root:service httpd graceful
~]# service httpd gracefulCopy to Clipboard Copied! Toggle word wrap Toggle overflow This causes the runninghttpdservice to reload its configuration file. Any requests being currently processed will use the old configuration.
Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.2, “Starting, Restarting, and Stopping a Service”.
18.1.4.4. Verifying the Service Status
Link kopierenLink in die Zwischenablage kopiert!
To verify that the
httpd service is running, type the following at a shell prompt:
service httpd status
~]# service httpd status
httpd (pid 19014) is running...
Alternatively, you can use the Service Configuration utility as described in Section 12.2.1, “Using the Service Configuration Utility”.
18.1.5. Editing the Configuration Files
Link kopierenLink in die Zwischenablage kopiert!
When the
httpd service is started, by default, it reads the configuration from locations that are listed in Table 18.1, “The httpd service configuration files”.
Although the default configuration should be suitable for most situations, it is a good idea to become at least familiar with some of the more important configuration options. Note that for any changes to take effect, the web server has to be restarted first. See Section 18.1.4.3, “Restarting the Service” for more information on how to restart the
httpd service.
To check the configuration for possible errors, type the following at a shell prompt:
service httpd configtest
~]# service httpd configtest
Syntax OK
To make the recovery from mistakes easier, it is recommended that you make a copy of the original file before editing it.
18.1.5.1. Common httpd.conf Directives
Link kopierenLink in die Zwischenablage kopiert!
The following directives are commonly used in the
/etc/httpd/conf/httpd.conf configuration file:
-
<Directory> - The
<Directory>directive allows you to apply certain directives to a particular directory only. It takes the following form:<Directory directory> directive … </Directory>
<Directory directory> directive … </Directory>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The directory can be either a full path to an existing directory in the local file system, or a wildcard expression.This directive can be used to configure additionalcgi-bindirectories for server-side scripts located outside the directory that is specified byScriptAlias. In this case, theExecCGIandAddHandlerdirectives must be supplied, and the permissions on the target directory must be set correctly (that is,0755).Example 18.1. Using the <Directory> directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
<IfDefine> - The
IfDefinedirective allows you to use certain directives only when a particular parameter is supplied on the command line. It takes the following form:<IfDefine [!]parameter> directive … </IfDefine>
<IfDefine [!]parameter> directive … </IfDefine>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The parameter can be supplied at a shell prompt using the-Dparameter command-line option (for example,httpd -DEnableHome). If the optional exclamation mark (that is,!) is present, the enclosed directives are used only when the parameter is not specified.Example 18.2. Using the <IfDefine> directive
<IfDefine EnableHome> UserDir public_html </IfDefine>
<IfDefine EnableHome> UserDir public_html </IfDefine>Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
<IfModule> - The
<IfModule>directive allows you to use certain directive only when a particular module is loaded. It takes the following form:<IfModule [!]module> directive … </IfModule>
<IfModule [!]module> directive … </IfModule>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The module can be identified either by its name, or by the file name. If the optional exclamation mark (that is,!) is present, the enclosed directives are used only when the module is not loaded.Example 18.3. Using the <IfModule> directive
<IfModule mod_disk_cache.c> CacheEnable disk / CacheRoot /var/cache/mod_proxy </IfModule>
<IfModule mod_disk_cache.c> CacheEnable disk / CacheRoot /var/cache/mod_proxy </IfModule>Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
<Location> - The
<Location>directive allows you to apply certain directives to a particular URL only. It takes the following form:<Location url> directive … </Location>
<Location url> directive … </Location>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The url can be either a path relative to the directory specified by theDocumentRootdirective (for example,/server-info), or an external URL such ashttp://example.com/server-info.Example 18.4. Using the <Location> directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
<Proxy> - The
<Proxy>directive allows you to apply certain directives to the proxy server only. It takes the following form:<Proxy pattern> directive … </Proxy>
<Proxy pattern> directive … </Proxy>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The pattern can be an external URL, or a wildcard expression (for example,http://example.com/*).Example 18.5. Using the <Proxy> directive
<Proxy *> Order deny,allow Deny from all Allow from .example.com </Proxy>
<Proxy *> Order deny,allow Deny from all Allow from .example.com </Proxy>Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
<VirtualHost> - The
<VirtualHost>directive allows you apply certain directives to particular virtual hosts only. It takes the following form:<VirtualHost address[:port]…> directive … </VirtualHost>
<VirtualHost address[:port]…> directive … </VirtualHost>Copy to Clipboard Copied! Toggle word wrap Toggle overflow The address can be an IP address, a fully qualified domain name, or a special form as described in Table 18.2, “Available <VirtualHost> options”.Expand Table 18.2. Available <VirtualHost> options Option Description *Represents all IP addresses. _default_Represents unmatched IP addresses. Example 18.6. Using the <VirtualHost> directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
AccessFileName - The
AccessFileNamedirective allows you to specify the file to be used to customize access control information for each directory. It takes the following form:AccessFileName filename…
AccessFileName filename…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The filename is a name of the file to look for in the requested directory. By default, the server looks for.htaccess.For security reasons, the directive is typically followed by theFilestag to prevent the files beginning with.htfrom being accessed by web clients. This includes the.htaccessand.htpasswdfiles.Example 18.7. Using the AccessFileName directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
Action - The
Actiondirective allows you to specify a CGI script to be executed when a certain media type is requested. It takes the following form:Action content-type path
Action content-type pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The content-type has to be a valid MIME type such astext/html,image/png, orapplication/pdf. The path refers to an existing CGI script, and must be relative to the directory specified by theDocumentRootdirective (for example,/cgi-bin/process-image.cgi).Example 18.8. Using the Action directive
Action image/png /cgi-bin/process-image.cgi
Action image/png /cgi-bin/process-image.cgiCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddDescription - The
AddDescriptiondirective allows you to specify a short description to be displayed in server-generated directory listings for a given file. It takes the following form:AddDescription "description" filename…
AddDescription "description" filename…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The description should be a short text enclosed in double quotes (that is,"). The filename can be a full file name, a file extension, or a wildcard expression.Example 18.9. Using the AddDescription directive
AddDescription "GZIP compressed tar archive" .tgz
AddDescription "GZIP compressed tar archive" .tgzCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddEncoding - The
AddEncodingdirective allows you to specify an encoding type for a particular file extension. It takes the following form:AddEncoding encoding extension…
AddEncoding encoding extension…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The encoding has to be a valid MIME encoding such asx-compress,x-gzip, etc. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example,.gz).This directive is typically used to instruct web browsers to decompress certain file types as they are downloaded.Example 18.10. Using the AddEncoding directive
AddEncoding x-gzip .gz .tgz
AddEncoding x-gzip .gz .tgzCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddHandler - The
AddHandlerdirective allows you to map certain file extensions to a selected handler. It takes the following form:AddHandler handler extension…
AddHandler handler extension…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The handler has to be a name of a previously defined handler. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example,.cgi).This directive is typically used to treat files with the.cgiextension as CGI scripts regardless of the directory they are in. Additionally, it is also commonly used to process server-parsed HTML and image-map files.Example 18.11. Using the AddHandler option
AddHandler cgi-script .cgi
AddHandler cgi-script .cgiCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddIcon - The
AddIcondirective allows you to specify an icon to be displayed for a particular file in server-generated directory listings. It takes the following form:AddIcon path pattern…
AddIcon path pattern…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to an existing icon file, and must be relative to the directory specified by theDocumentRootdirective (for example,/icons/folder.png). The pattern can be a file name, a file extension, a wildcard expression, or a special form as described in the following table:Expand Table 18.3. Available AddIcon options Option Description ^^DIRECTORY^^Represents a directory. ^^BLANKICON^^Represents a blank line. Example 18.12. Using the AddIcon directive
AddIcon /icons/text.png .txt README
AddIcon /icons/text.png .txt READMECopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddIconByEncoding - The
AddIconByEncodingdirective allows you to specify an icon to be displayed for a particular encoding type in server-generated directory listings. It takes the following form:AddIconByEncoding path encoding…
AddIconByEncoding path encoding…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to an existing icon file, and must be relative to the directory specified by theDocumentRootdirective (for example,/icons/compressed.png). The encoding has to be a valid MIME encoding such asx-compress,x-gzip, etc.Example 18.13. Using the AddIconByEncoding directive
AddIconByEncoding /icons/compressed.png x-compress x-gzip
AddIconByEncoding /icons/compressed.png x-compress x-gzipCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddIconByType - The
AddIconByTypedirective allows you to specify an icon to be displayed for a particular media type in server-generated directory listings. It takes the following form:AddIconByType path content-type…
AddIconByType path content-type…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to an existing icon file, and must be relative to the directory specified by theDocumentRootdirective (for example,/icons/text.png). The content-type has to be either a valid MIME type (for example,text/htmlorimage/png), or a wildcard expression such astext/*,image/*, etc.Example 18.14. Using the AddIconByType directive
AddIconByType /icons/video.png video/*
AddIconByType /icons/video.png video/*Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddLanguage - The
AddLanguagedirective allows you to associate a file extension with a specific language. It takes the following form:AddLanguage language extension…
AddLanguage language extension…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The language has to be a valid MIME language such ascs,en, orfr. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example,.cs).This directive is especially useful for web servers that serve content in multiple languages based on the client's language settings.Example 18.15. Using the AddLanguage directive
AddLanguage cs .cs .cz
AddLanguage cs .cs .czCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
AddType - The
AddTypedirective allows you to define or override the media type for a particular file extension. It takes the following form:AddType content-type extension…
AddType content-type extension…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The content-type has to be a valid MIME type such astext/html,image/png, etc. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example,.cs).Example 18.16. Using the AddType directive
AddType application/x-gzip .gz .tgz
AddType application/x-gzip .gz .tgzCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Alias - The
Aliasdirective allows you to refer to files and directories outside the default directory specified by theDocumentRootdirective. It takes the following form:Alias url-path real-path
Alias url-path real-pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The url-path must be relative to the directory specified by theDocumentRootdirective (for example,/images/). The real-path is a full path to a file or directory in the local file system.This directive is typically followed by theDirectorytag with additional permissions to access the target directory. By default, the/icons/alias is created so that the icons from/var/www/icons/are displayed in server-generated directory listings.Example 18.17. Using the Alias directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
Allow - The
Allowdirective allows you to specify which clients have permission to access a given directory. It takes the following form:Allow from client…
Allow from client…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The client can be a domain name, an IP address (both full and partial), a network/netmask pair, orallfor all clients.Example 18.18. Using the Allow directive
Allow from 192.168.1.0/255.255.255.0
Allow from 192.168.1.0/255.255.255.0Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
AllowOverride - The
AllowOverridedirective allows you to specify which directives in a.htaccessfile can override the default configuration. It takes the following form:AllowOverride type…
AllowOverride type…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The type has to be one of the available grouping options as described in Table 18.4, “Available AllowOverride options”.Expand Table 18.4. Available AllowOverride options Option Description AllAll directives in .htaccessare allowed to override earlier configuration settings.NoneNo directive in .htaccessis allowed to override earlier configuration settings.AuthConfigAllows the use of authorization directives such as AuthName,AuthType, orRequire.FileInfoAllows the use of file type, metadata, and mod_rewritedirectives such asDefaultType,RequestHeader, orRewriteEngine, as well as theActiondirective.IndexesAllows the use of directory indexing directives such as AddDescription,AddIcon, orFancyIndexing.LimitAllows the use of host access directives, that is, Allow,Deny, andOrder.Options[=option,…]Allows the use of the Optionsdirective. Additionally, you can provide a comma-separated list of options to customize which options can be set using this directive.Example 18.19. Using the AllowOverride directive
AllowOverride FileInfo AuthConfig Limit
AllowOverride FileInfo AuthConfig LimitCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
BrowserMatch - The
BrowserMatchdirective allows you to modify the server behavior based on the client's web browser type. It takes the following form:BrowserMatch pattern variable…
BrowserMatch pattern variable…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The pattern is a regular expression to match the User-Agent HTTP header field. The variable is an environment variable that is set when the header field matches the pattern.By default, this directive is used to deny connections to specific browsers with known issues, and to disable keepalives and HTTP header flushes for browsers that are known to have problems with these actions.Example 18.20. Using the BrowserMatch directive
BrowserMatch "Mozilla/2" nokeepalive
BrowserMatch "Mozilla/2" nokeepaliveCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheDefaultExpire - The
CacheDefaultExpireoption allows you to set how long to cache a document that does not have any expiration date or the date of its last modification specified. It takes the following form:CacheDefaultExpire time
CacheDefaultExpire timeCopy to Clipboard Copied! Toggle word wrap Toggle overflow The time is specified in seconds. The default option is3600(that is, one hour).Example 18.21. Using the CacheDefaultExpire directive
CacheDefaultExpire 3600
CacheDefaultExpire 3600Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheDisable - The
CacheDisabledirective allows you to disable caching of certain URLs. It takes the following form:CacheDisable path
CacheDisable pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path must be relative to the directory specified by theDocumentRootdirective (for example,/files/).Example 18.22. Using the CacheDisable directive
CacheDisable /temporary
CacheDisable /temporaryCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheEnable - The
CacheEnabledirective allows you to specify a cache type to be used for certain URLs. It takes the following form:CacheEnable type url
CacheEnable type urlCopy to Clipboard Copied! Toggle word wrap Toggle overflow The type has to be a valid cache type as described in Table 18.5, “Available cache types”. The url can be a path relative to the directory specified by theDocumentRootdirective (for example,/images/), a protocol (for example,ftp://), or an external URL such ashttp://example.com/.Expand Table 18.5. Available cache types Type Description memThe memory-based storage manager. diskThe disk-based storage manager. fdThe file descriptor cache. Example 18.23. Using the CacheEnable directive
CacheEnable disk /
CacheEnable disk /Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheLastModifiedFactor - The
CacheLastModifiedFactordirective allows you to customize how long to cache a document that does not have any expiration date specified, but that provides information about the date of its last modification. It takes the following form:CacheLastModifiedFactor number
CacheLastModifiedFactor numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow The number is a coefficient to be used to multiply the time that passed since the last modification of the document. The default option is0.1(that is, one tenth).Example 18.24. Using the CacheLastModifiedFactor directive
CacheLastModifiedFactor 0.1
CacheLastModifiedFactor 0.1Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheMaxExpire - The
CacheMaxExpiredirective allows you to specify the maximum amount of time to cache a document. It takes the following form:CacheMaxExpire time
CacheMaxExpire timeCopy to Clipboard Copied! Toggle word wrap Toggle overflow The time is specified in seconds. The default option is86400(that is, one day).Example 18.25. Using the CacheMaxExpire directive
CacheMaxExpire 86400
CacheMaxExpire 86400Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheNegotiatedDocs - The
CacheNegotiatedDocsdirective allows you to enable caching of the documents that were negotiated on the basis of content. It takes the following form:CacheNegotiatedDocs option
CacheNegotiatedDocs optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.6, “Available CacheNegotiatedDocs options”. Since the content-negotiated documents may change over time or because of the input from the requester, the default option isOff.Expand Table 18.6. Available CacheNegotiatedDocs options Option Description OnEnables caching the content-negotiated documents. OffDisables caching the content-negotiated documents. Example 18.26. Using the CacheNegotiatedDocs directive
CacheNegotiatedDocs On
CacheNegotiatedDocs OnCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
CacheRoot - The
CacheRootdirective allows you to specify the directory to store cache files in. It takes the following form:CacheRoot directory
CacheRoot directoryCopy to Clipboard Copied! Toggle word wrap Toggle overflow The directory must be a full path to an existing directory in the local file system. The default option is/var/cache/mod_proxy/.Example 18.27. Using the CacheRoot directive
CacheRoot /var/cache/mod_proxy
CacheRoot /var/cache/mod_proxyCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
CustomLog - The
CustomLogdirective allows you to specify the log file name and the log file format. It takes the following form:CustomLog path format
CustomLog path formatCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to a log file, and must be relative to the directory that is specified by theServerRootdirective (that is,/etc/httpd/by default). The format has to be either an explicit format string, or a format name that was previously defined using theLogFormatdirective.Example 18.28. Using the CustomLog directive
CustomLog logs/access_log combined
CustomLog logs/access_log combinedCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
DefaultIcon - The
DefaultIcondirective allows you to specify an icon to be displayed for a file in server-generated directory listings when no other icon is associated with it. It takes the following form:DefaultIcon path
DefaultIcon pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to an existing icon file, and must be relative to the directory specified by theDocumentRootdirective (for example,/icons/unknown.png).Example 18.29. Using the DefaultIcon directive
DefaultIcon /icons/unknown.png
DefaultIcon /icons/unknown.pngCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
DefaultType - The
DefaultTypedirective allows you to specify a media type to be used in case the proper MIME type cannot be determined by the server. It takes the following form:DefaultType content-type
DefaultType content-typeCopy to Clipboard Copied! Toggle word wrap Toggle overflow The content-type has to be a valid MIME type such astext/html,image/png,application/pdf, etc.Example 18.30. Using the DefaultType directive
DefaultType text/plain
DefaultType text/plainCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Deny - The
Denydirective allows you to specify which clients are denied access to a given directory. It takes the following form:Deny from client…
Deny from client…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The client can be a domain name, an IP address (both full and partial), a network/netmask pair, orallfor all clients.Example 18.31. Using the Deny directive
Deny from 192.168.1.1
Deny from 192.168.1.1Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
DirectoryIndex - The
DirectoryIndexdirective allows you to specify a document to be served to a client when a directory is requested (that is, when the URL ends with the/character). It takes the following form:DirectoryIndex filename…
DirectoryIndex filename…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The filename is a name of the file to look for in the requested directory. By default, the server looks forindex.html, andindex.html.var.Example 18.32. Using the DirectoryIndex directive
DirectoryIndex index.html index.html.var
DirectoryIndex index.html index.html.varCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
DocumentRoot - The
DocumentRootdirective allows you to specify the main directory from which the content is served. It takes the following form:DocumentRoot directory
DocumentRoot directoryCopy to Clipboard Copied! Toggle word wrap Toggle overflow The directory must be a full path to an existing directory in the local file system. The default option is/var/www/html/.Example 18.33. Using the DocumentRoot directive
DocumentRoot /var/www/html
DocumentRoot /var/www/htmlCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ErrorDocument - The
ErrorDocumentdirective allows you to specify a document or a message to be displayed as a response to a particular error. It takes the following form:ErrorDocument error-code action
ErrorDocument error-code actionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The error-code has to be a valid code such as403(Forbidden),404(Not Found), or500(Internal Server Error). The action can be either a URL (both local and external), or a message string enclosed in double quotes (that is,").Example 18.34. Using the ErrorDocument directive
ErrorDocument 403 "Access Denied" ErrorDocument 404 /404-not_found.html
ErrorDocument 403 "Access Denied" ErrorDocument 404 /404-not_found.htmlCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ErrorLog - The
ErrorLogdirective allows you to specify a file to which the server errors are logged. It takes the following form:ErrorLog path
ErrorLog pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to a log file, and can be either absolute, or relative to the directory that is specified by theServerRootdirective (that is,/etc/httpd/by default). The default option islogs/error_logExample 18.35. Using the ErrorLog directive
ErrorLog logs/error_log
ErrorLog logs/error_logCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ExtendedStatus - The
ExtendedStatusdirective allows you to enable detailed server status information. It takes the following form:ExtendedStatus option
ExtendedStatus optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.7, “Available ExtendedStatus options”. The default option isOff.Expand Table 18.7. Available ExtendedStatus options Option Description OnEnables generating the detailed server status. OffDisables generating the detailed server status. Example 18.36. Using the ExtendedStatus directive
ExtendedStatus On
ExtendedStatus OnCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Group - The
Groupdirective allows you to specify the group under which thehttpdservice will run. It takes the following form:Group group
Group groupCopy to Clipboard Copied! Toggle word wrap Toggle overflow The group has to be an existing UNIX group. The default option isapache.Note thatGroupis no longer supported inside<VirtualHost>, and has been replaced by theSuexecUserGroupdirective.Example 18.37. Using the Group directive
Group apache
Group apacheCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
HeaderName - The
HeaderNamedirective allows you to specify a file to be prepended to the beginning of the server-generated directory listing. It takes the following form:HeaderName filename
HeaderName filenameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The filename is a name of the file to look for in the requested directory. By default, the server looks forHEADER.html.Example 18.38. Using the HeaderName directive
HeaderName HEADER.html
HeaderName HEADER.htmlCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
HostnameLookups - The
HostnameLookupsdirective allows you to enable automatic resolving of IP addresses. It takes the following form:HostnameLookups option
HostnameLookups optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.8, “Available HostnameLookups options”. To conserve resources on the server, the default option isOff.Expand Table 18.8. Available HostnameLookups options Option Description OnEnables resolving the IP address for each connection so that the host name can be logged. However, this also adds a significant processing overhead. DoubleEnables performing the double-reverse DNS lookup. In comparison to the above option, this adds even more processing overhead. OffDisables resolving the IP address for each connection. Note that when the presence of host names is required in server log files, it is often possible to use one of the many log analyzer tools that perform the DNS lookups more efficiently.Example 18.39. Using the HostnameLookups directive
HostnameLookups Off
HostnameLookups OffCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Include - The
Includedirective allows you to include other configuration files. It takes the following form:Include filename
Include filenameCopy to Clipboard Copied! Toggle word wrap Toggle overflow Thefilenamecan be an absolute path, a path relative to the directory specified by theServerRootdirective, or a wildcard expression. All configuration files from the/etc/httpd/conf.d/directory are loaded by default.Example 18.40. Using the Include directive
Include conf.d/*.conf
Include conf.d/*.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
IndexIgnore - The
IndexIgnoredirective allows you to specify a list of file names to be omitted from the server-generated directory listings. It takes the following form:IndexIgnore filename…
IndexIgnore filename…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The filename option can be either a full file name, or a wildcard expression.Example 18.41. Using the IndexIgnore directive
IndexIgnore .??* *~ *# HEADER* README* RCS CVS *,v *,t
IndexIgnore .??* *~ *# HEADER* README* RCS CVS *,v *,tCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
IndexOptions - The
IndexOptionsdirective allows you to customize the behavior of server-generated directory listings. It takes the following form:IndexOptions option…
IndexOptions option…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.9, “Available directory listing options”. The default options areCharset=UTF-8,FancyIndexing,HTMLTable,NameWidth=*, andVersionSort.Expand Table 18.9. Available directory listing options Option Description Charset=encodingSpecifies the character set of a generated web page. The encoding has to be a valid character set such as UTF-8orISO-8859-2.Type=content-typeSpecifies the media type of a generated web page. The content-type has to be a valid MIME type such as text/htmlortext/plain.DescriptionWidth=valueSpecifies the width of the description column. The value can be either a number of characters, or an asterisk (that is, *) to adjust the width automatically.FancyIndexingEnables advanced features such as different icons for certain files or possibility to re-sort a directory listing by clicking on a column header. FolderFirstEnables listing directories first, always placing them above files. HTMLTableEnables the use of HTML tables for directory listings. IconsAreLinksEnables using the icons as links. IconHeight=valueSpecifies an icon height. The value is a number of pixels. IconWidth=valueSpecifies an icon width. The value is a number of pixels. IgnoreCaseEnables sorting files and directories in a case-sensitive manner. IgnoreClientDisables accepting query variables from a client. NameWidth=valueSpecifies the width of the file name column. The value can be either a number of characters, or an asterisk (that is, *) to adjust the width automatically.ScanHTMLTitlesEnables parsing the file for a description (that is, the titleelement) in case it is not provided by theAddDescriptiondirective.ShowForbiddenEnables listing the files with otherwise restricted access. SuppressColumnSortingDisables re-sorting a directory listing by clicking on a column header. SuppressDescriptionDisables reserving a space for file descriptions. SuppressHTMLPreambleDisables the use of standard HTML preamble when a file specified by the HeaderNamedirective is present.SuppressIconDisables the use of icons in directory listings. SuppressLastModifiedDisables displaying the date of the last modification field in directory listings. SuppressRulesDisables the use of horizontal lines in directory listings. SuppressSizeDisables displaying the file size field in directory listings. TrackModifiedEnables returning the Last-ModifiedandETagvalues in the HTTP header.VersionSortEnables sorting files that contain a version number in the expected manner. XHTMLEnables the use of XHTML 1.0 instead of the default HTML 3.2. Example 18.42. Using the IndexOptions directive
IndexOptions FancyIndexing VersionSort NameWidth=* HTMLTable Charset=UTF-8
IndexOptions FancyIndexing VersionSort NameWidth=* HTMLTable Charset=UTF-8Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
KeepAlive - The
KeepAlivedirective allows you to enable persistent connections. It takes the following form:KeepAlive option
KeepAlive optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.10, “Available KeepAlive options”. The default option isOff.Expand Table 18.10. Available KeepAlive options Option Description OnEnables the persistent connections. In this case, the server will accept more than one request per connection. OffDisables the keep-alive connections. Note that when the persistent connections are enabled, on a busy server, the number of child processes can increase rapidly and eventually reach the maximum limit, slowing down the server significantly. To reduce the risk, it is recommended that you setKeepAliveTimeoutto a low number, and monitor the/var/log/httpd/logs/error_loglog file carefully.Example 18.43. Using the KeepAlive directive
KeepAlive Off
KeepAlive OffCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
KeepAliveTimeout - The
KeepAliveTimeoutdirective allows you to specify the amount of time to wait for another request before closing the connection. It takes the following form:KeepAliveTimeout time
KeepAliveTimeout timeCopy to Clipboard Copied! Toggle word wrap Toggle overflow The time is specified in seconds. The default option is15.Example 18.44. Using the KeepAliveTimeout directive
KeepAliveTimeout 15
KeepAliveTimeout 15Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
LanguagePriority - The
LanguagePrioritydirective allows you to customize the precedence of languages. It takes the following form:LanguagePriority language…
LanguagePriority language…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The language has to be a valid MIME language such ascs,en, orfr.This directive is especially useful for web servers that serve content in multiple languages based on the client's language settings.Example 18.45. Using the LanguagePriority directive
LanguagePriority sk cs en
LanguagePriority sk cs enCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Listen - The Listen directive allows you to specify IP addresses or ports to listen to. It takes the following form:
Listen [ip-address:]port [protocol]
Listen [ip-address:]port [protocol]Copy to Clipboard Copied! Toggle word wrap Toggle overflow The ip-address is optional and unless supplied, the server will accept incoming requests on a given port from all IP addresses. Since the protocol is determined automatically from the port number, it can be usually omitted. The default option is to listen to port80.Note that if the server is configured to listen to a port under 1024, only superuser will be able to start thehttpdservice.Example 18.46. Using the Listen directive
Listen 80
Listen 80Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
LoadModule - The
LoadModuledirective allows you to load a Dynamic Shared Object (DSO) module. It takes the following form:LoadModule name path
LoadModule name pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The name has to be a valid identifier of the required module. The path refers to an existing module file, and must be relative to the directory in which the libraries are placed (that is,/usr/lib/httpd/on 32-bit and/usr/lib64/httpd/on 64-bit systems by default).See Section 18.1.6, “Working with Modules” for more information on the Apache HTTP Server's DSO support.Example 18.47. Using the LoadModule directive
LoadModule php5_module modules/libphp5.so
LoadModule php5_module modules/libphp5.soCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
LogFormat - The LogFormat directive allows you to specify a log file format. It takes the following form:
LogFormat format name
LogFormat format nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The format is a string consisting of options as described in Table 18.11, “Common LogFormat options”. The name can be used instead of the format string in theCustomLogdirective.Expand Table 18.11. Common LogFormat options Option Description %bRepresents the size of the response in bytes. %hRepresents the IP address or host name of a remote client. %lRepresents the remote log name if supplied. If not, a hyphen (that is, -) is used instead.%rRepresents the first line of the request string as it came from the browser or client. %sRepresents the status code. %tRepresents the date and time of the request. %uIf the authentication is required, it represents the remote user. If not, a hyphen (that is, -) is used instead.%{field}Represents the content of the HTTP header field. The common options include %{Referer}(the URL of the web page that referred the client to the server) and%{User-Agent}(the type of the web browser making the request).Example 18.48. Using the LogFormat directive
LogFormat "%h %l %u %t \"%r\" %>s %b" common
LogFormat "%h %l %u %t \"%r\" %>s %b" commonCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
LogLevel - The
LogLeveldirective allows you to customize the verbosity level of the error log. It takes the following form:LogLevel option
LogLevel optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.12, “Available LogLevel options”. The default option iswarn.Expand Table 18.12. Available LogLevel options Option Description emergOnly the emergency situations when the server cannot perform its work are logged. alertAll situations when an immediate action is required are logged. critAll critical conditions are logged. errorAll error messages are logged. warnAll warning messages are logged. noticeEven normal, but still significant situations are logged. infoVarious informational messages are logged. debugVarious debugging messages are logged. Example 18.49. Using the LogLevel directive
LogLevel warn
LogLevel warnCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
MaxKeepAliveRequests - The
MaxKeepAliveRequestsdirective allows you to specify the maximum number of requests for a persistent connection. It takes the following form:MaxKeepAliveRequests number
MaxKeepAliveRequests numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow A high number can improve the performance of the server. Note that using0allows unlimited number of requests. The default option is100.Example 18.50. Using the MaxKeepAliveRequests option
MaxKeepAliveRequests 100
MaxKeepAliveRequests 100Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
NameVirtualHost - The
NameVirtualHostdirective allows you to specify the IP address and port number for a name-based virtual host. It takes the following form:NameVirtualHost ip-address[:port]
NameVirtualHost ip-address[:port]Copy to Clipboard Copied! Toggle word wrap Toggle overflow The ip-address can be either a full IP address, or an asterisk (that is,*) representing all interfaces. Note that IPv6 addresses have to be enclosed in square brackets (that is,[and]). The port is optional.Name-based virtual hosting allows one Apache HTTP Server to serve different domains without using multiple IP addresses.Important
Name-based virtual hosts only work with non-secure HTTP connections. If using virtual hosts with a secure server, use IP address-based virtual hosts instead.Example 18.51. Using the NameVirtualHost directive
NameVirtualHost *:80
NameVirtualHost *:80Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
Options - The
Optionsdirective allows you to specify which server features are available in a particular directory. It takes the following form:Options option…
Options option…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.13, “Available server features”.Expand Table 18.13. Available server features Option Description ExecCGIEnables the execution of CGI scripts. FollowSymLinksEnables following symbolic links in the directory. IncludesEnables server-side includes. IncludesNOEXECEnables server-side includes, but does not allow the execution of commands. IndexesEnables server-generated directory listings. MultiViewsEnables content-negotiated “MultiViews”. SymLinksIfOwnerMatchEnables following symbolic links in the directory when both the link and the target file have the same owner. AllEnables all of the features above with the exception of MultiViews.NoneDisables all of the features above. Important
TheSymLinksIfOwnerMatchoption is not a security feature as it can be bypassed by an attacker.Example 18.52. Using the Options directive
Options Indexes FollowSymLinks
Options Indexes FollowSymLinksCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Order - The
Orderdirective allows you to specify the order in which theAllowandDenydirectives are evaluated. It takes the following form:Order option
Order optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.14, “Available Order options”. The default option isallow,deny.Expand Table 18.14. Available Order options Option Description allow,denyAllowdirectives are evaluated first.deny,allowDenydirectives are evaluated first.Example 18.53. Using the Order directive
Order allow,deny
Order allow,denyCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
PidFile - The
PidFiledirective allows you to specify a file to which the process ID (PID) of the server is stored. It takes the following form:PidFile path
PidFile pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to a pid file, and can be either absolute, or relative to the directory that is specified by theServerRootdirective (that is,/etc/httpd/by default). The default option isrun/httpd.pid.Example 18.54. Using the PidFile directive
PidFile run/httpd.pid
PidFile run/httpd.pidCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ProxyRequests - The
ProxyRequestsdirective allows you to enable forward proxy requests. It takes the following form:ProxyRequests option
ProxyRequests optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.15, “Available ProxyRequests options”. The default option isOff.Expand Table 18.15. Available ProxyRequests options Option Description OnEnables forward proxy requests. OffDisables forward proxy requests. Example 18.55. Using the ProxyRequests directive
ProxyRequests On
ProxyRequests OnCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ReadmeName - The
ReadmeNamedirective allows you to specify a file to be appended to the end of the server-generated directory listing. It takes the following form:ReadmeName filename
ReadmeName filenameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The filename is a name of the file to look for in the requested directory. By default, the server looks forREADME.html.Example 18.56. Using the ReadmeName directive
ReadmeName README.html
ReadmeName README.htmlCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Redirect - The
Redirectdirective allows you to redirect a client to another URL. It takes the following form:Redirect [status] path url
Redirect [status] path urlCopy to Clipboard Copied! Toggle word wrap Toggle overflow The status is optional, and if provided, it has to be a valid keyword as described in Table 18.16, “Available status options”. The path refers to the old location, and must be relative to the directory specified by theDocumentRootdirective (for example,/docs). The url refers to the current location of the content (for example,http://docs.example.com).Expand Table 18.16. Available status options Status Description permanentIndicates that the requested resource has been moved permanently. The 301(Moved Permanently) status code is returned to a client.tempIndicates that the requested resource has been moved only temporarily. The 302(Found) status code is returned to a client.seeotherIndicates that the requested resource has been replaced. The 303(See Other) status code is returned to a client.goneIndicates that the requested resource has been removed permanently. The 410(Gone) status is returned to a client.Note that for more advanced redirection techniques, you can use themod_rewritemodule that is part of the Apache HTTP Server installation.Example 18.57. Using the Redirect directive
Redirect permanent /docs http://docs.example.com
Redirect permanent /docs http://docs.example.comCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ScriptAlias - The
ScriptAliasdirective allows you to specify the location of CGI scripts. It takes the following form:ScriptAlias url-path real-path
ScriptAlias url-path real-pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The url-path must be relative to the directory specified by theDocumentRootdirective (for example,/cgi-bin/). The real-path is a full path to a file or directory in the local file system.This directive is typically followed by theDirectorytag with additional permissions to access the target directory. By default, the/cgi-bin/alias is created so that the scripts located in the/var/www/cgi-bin/are accessible.TheScriptAliasdirective is used for security reasons to prevent CGI scripts from being viewed as ordinary text documents.Example 18.58. Using the ScriptAlias directive
Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
ServerAdmin - The
ServerAdmindirective allows you to specify the email address of the server administrator to be displayed in server-generated web pages. It takes the following form:ServerAdmin email
ServerAdmin emailCopy to Clipboard Copied! Toggle word wrap Toggle overflow The default option isroot@localhost.This directive is commonly set towebmaster@hostname, where hostname is the address of the server. Once set, aliaswebmasterto the person responsible for the web server in/etc/aliases, and as superuser, run thenewaliasescommand.Example 18.59. Using the ServerAdmin directive
ServerAdmin webmaster@penguin.example.com
ServerAdmin webmaster@penguin.example.comCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ServerName - The
ServerNamedirective allows you to specify the host name and the port number of a web server. It takes the following form:ServerName hostname[:port]
ServerName hostname[:port]Copy to Clipboard Copied! Toggle word wrap Toggle overflow The hostname has to be a fully qualified domain name (FQDN) of the server. The port is optional, but when supplied, it has to match the number specified by theListendirective.When using this directive, make sure that the IP address and server name pair are included in the/etc/hostsfile.Example 18.60. Using the ServerName directive
ServerName penguin.example.com:80
ServerName penguin.example.com:80Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
ServerRoot - The
ServerRootdirective allows you to specify the directory in which the server operates. It takes the following form:ServerRoot directory
ServerRoot directoryCopy to Clipboard Copied! Toggle word wrap Toggle overflow The directory must be a full path to an existing directory in the local file system. The default option is/etc/httpd/.Example 18.61. Using the ServerRoot directive
ServerRoot /etc/httpd
ServerRoot /etc/httpdCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ServerSignature - The
ServerSignaturedirective allows you to enable displaying information about the server on server-generated documents. It takes the following form:ServerSignature option
ServerSignature optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.17, “Available ServerSignature options”. The default option isOn.Expand Table 18.17. Available ServerSignature options Option Description OnEnables appending the server name and version to server-generated pages. OffDisables appending the server name and version to server-generated pages. EMailEnables appending the server name, version, and the email address of the system administrator as specified by the ServerAdmindirective to server-generated pages.Example 18.62. Using the ServerSignature directive
ServerSignature On
ServerSignature OnCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
ServerTokens - The
ServerTokensdirective allows you to customize what information is included in the Server response header. It takes the following form:ServerTokens option
ServerTokens optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.18, “Available ServerTokens options”. The default option isOS.Expand Table 18.18. Available ServerTokens options Option Description ProdIncludes the product name only (that is, Apache).MajorIncludes the product name and the major version of the server (for example, 2).MinorIncludes the product name and the minor version of the server (for example, 2.2).MinIncludes the product name and the minimal version of the server (for example, 2.2.15).OSIncludes the product name, the minimal version of the server, and the type of the operating system it is running on (for example, Red Hat).FullIncludes all the information above along with the list of loaded modules. Note that for security reasons, it is recommended to reveal as little information about the server as possible.Example 18.63. Using the ServerTokens directive
ServerTokens Prod
ServerTokens ProdCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
SuexecUserGroup - The
SuexecUserGroupdirective allows you to specify the user and group under which the CGI scripts will be run. It takes the following form:SuexecUserGroup user group
SuexecUserGroup user groupCopy to Clipboard Copied! Toggle word wrap Toggle overflow The user has to be an existing user, and the group must be a valid UNIX group.For security reasons, the CGI scripts should not be run with root privileges. Note that in<VirtualHost>,SuexecUserGroupreplaces theUserandGroupdirectives.Example 18.64. Using the SuexecUserGroup directive
SuexecUserGroup apache apache
SuexecUserGroup apache apacheCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
Timeout - The
Timeoutdirective allows you to specify the amount of time to wait for an event before closing a connection. It takes the following form:Timeout time
Timeout timeCopy to Clipboard Copied! Toggle word wrap Toggle overflow The time is specified in seconds. The default option is60.Example 18.65. Using the Timeout directive
Timeout 60
Timeout 60Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
TypesConfig - The
TypesConfigallows you to specify the location of the MIME types configuration file. It takes the following form:TypesConfig path
TypesConfig pathCopy to Clipboard Copied! Toggle word wrap Toggle overflow The path refers to an existing MIME types configuration file, and can be either absolute, or relative to the directory that is specified by theServerRootdirective (that is,/etc/httpd/by default). The default option is/etc/mime.types.Note that instead of editing/etc/mime.types, the recommended way to add MIME type mapping to the Apache HTTP Server is to use theAddTypedirective.Example 18.66. Using the TypesConfig directive
TypesConfig /etc/mime.types
TypesConfig /etc/mime.typesCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
UseCanonicalName - The
UseCanonicalNameallows you to specify the way the server refers to itself. It takes the following form:UseCanonicalName option
UseCanonicalName optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option has to be a valid keyword as described in Table 18.19, “Available UseCanonicalName options”. The default option isOff.Expand Table 18.19. Available UseCanonicalName options Option Description OnEnables the use of the name that is specified by the ServerNamedirective.OffDisables the use of the name that is specified by the ServerNamedirective. The host name and port number provided by the requesting client are used instead.DNSDisables the use of the name that is specified by the ServerNamedirective. The host name determined by a reverse DNS lookup is used instead.Example 18.67. Using the UseCanonicalName directive
UseCanonicalName Off
UseCanonicalName OffCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
User - The
Userdirective allows you to specify the user under which thehttpdservice will run. It takes the following form:User user
User userCopy to Clipboard Copied! Toggle word wrap Toggle overflow The user has to be an existing UNIX user. The default option isapache.For security reasons, thehttpdservice should not be run with root privileges. Note thatUseris no longer supported inside<VirtualHost>, and has been replaced by theSuexecUserGroupdirective.Example 18.68. Using the User directive
User apache
User apacheCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
UserDir - The
UserDirdirective allows you to enable serving content from users' home directories. It takes the following form:UserDir option
UserDir optionCopy to Clipboard Copied! Toggle word wrap Toggle overflow The option can be either a name of the directory to look for in user's home directory (typicallypublic_html), or a valid keyword as described in Table 18.20, “Available UserDir options”. The default option isdisabled.Expand Table 18.20. Available UserDir options Option Description enableduser…Enables serving content from home directories of given users. disabled[user…]Disables serving content from home directories, either for all users, or, if a space separated list of users is supplied, for given users only. Note
In order for the web server to access the content, the permissions on relevant directories and files must be set correctly. Make sure that all users are able to access the home directories, and that they can access and read the content of the directory specified by theUserDirdirective. For example:chmod a+x /home/username/ chmod a+rx /home/username/public_html/
~]# chmod a+x /home/username/ ~]# chmod a+rx /home/username/public_html/Copy to Clipboard Copied! Toggle word wrap Toggle overflow All files in this directory must be set accordingly.Example 18.69. Using the UserDir directive
UserDir public_html
UserDir public_htmlCopy to Clipboard Copied! Toggle word wrap Toggle overflow
18.1.5.2. Common ssl.conf Directives
Link kopierenLink in die Zwischenablage kopiert!
The Secure Sockets Layer (SSL) directives allow you to customize the behavior of the Apache HTTP Secure Server, and in most cases, they are configured appropriately during the installation. Be careful when changing these settings, as incorrect configuration can lead to security vulnerabilities.
The following directive is commonly used in
/etc/httpd/conf.d/ssl.conf:
-
SetEnvIf - The
SetEnvIfdirective allows you to set environment variables based on the headers of incoming connections. It takes the following form:SetEnvIf option pattern [!]variable[=value]…
SetEnvIf option pattern [!]variable[=value]…Copy to Clipboard Copied! Toggle word wrap Toggle overflow The option can be either a HTTP header field, a previously defined environment variable name, or a valid keyword as described in Table 18.21, “Available SetEnvIf options”. The pattern is a regular expression. The variable is an environment variable that is set when the option matches the pattern. If the optional exclamation mark (that is,!) is present, the variable is removed instead of being set.Expand Table 18.21. Available SetEnvIf options Option Description Remote_HostRefers to the client's host name. Remote_AddrRefers to the client's IP address. Server_AddrRefers to the server's IP address. Request_MethodRefers to the request method (for example, GET).Request_ProtocolRefers to the protocol name and version (for example, HTTP/1.1).Request_URIRefers to the requested resource. TheSetEnvIfdirective is used to disable HTTP keepalives, and to allow SSL to close the connection without a closing notification from the client browser. This is necessary for certain web browsers that do not reliably shut down the SSL connection.Example 18.70. Using the SetEnvIf directive
SetEnvIf User-Agent ".*MSIE.*" \ nokeepalive ssl-unclean-shutdown \ downgrade-1.0 force-response-1.0SetEnvIf User-Agent ".*MSIE.*" \ nokeepalive ssl-unclean-shutdown \ downgrade-1.0 force-response-1.0Copy to Clipboard Copied! Toggle word wrap Toggle overflow
Note that for the
/etc/httpd/conf.d/ssl.conf file to be present, the mod_ssl needs to be installed. See Section 18.1.8, “Setting Up an SSL Server” for more information on how to install and configure an SSL server.
18.1.5.3. Common Multi-Processing Module Directives
Link kopierenLink in die Zwischenablage kopiert!
The Multi-Processing Module (MPM) directives allow you to customize the behavior of a particular MPM specific server-pool. Since its characteristics differ depending on which MPM is used, the directives are embedded in
IfModule. By default, the server-pool is defined for both the prefork and worker MPMs.
The following MPM directives are commonly used in
/etc/httpd/conf/httpd.conf:
-
MaxClients - The
MaxClientsdirective allows you to specify the maximum number of simultaneously connected clients to process at one time. It takes the following form:MaxClients number
MaxClients numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow A high number can improve the performance of the server, although it is not recommended to exceed256when using thepreforkMPM.Example 18.71. Using the MaxClients directive
MaxClients 256
MaxClients 256Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MaxRequestsPerChild - The
MaxRequestsPerChilddirective allows you to specify the maximum number of request a child process can serve before it dies. It takes the following form:MaxRequestsPerChild number
MaxRequestsPerChild numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow Setting the number to0allows unlimited number of requests.TheMaxRequestsPerChilddirective is used to prevent long-lived processes from causing memory leaks.Example 18.72. Using the MaxRequestsPerChild directive
MaxRequestsPerChild 4000
MaxRequestsPerChild 4000Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MaxSpareServers - The
MaxSpareServersdirective allows you to specify the maximum number of spare child processes. It takes the following form:MaxSpareServers number
MaxSpareServers numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow This directive is used by thepreforkMPM only.Example 18.73. Using the MaxSpareServers directive
MaxSpareServers 20
MaxSpareServers 20Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MaxSpareThreads - The
MaxSpareThreadsdirective allows you to specify the maximum number of spare server threads. It takes the following form:MaxSpareThreads number
MaxSpareThreads numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow The number must be greater than or equal to the sum ofMinSpareThreadsandThreadsPerChild. This directive is used by theworkerMPM only.Example 18.74. Using the MaxSpareThreads directive
MaxSpareThreads 75
MaxSpareThreads 75Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MinSpareServers - The
MinSpareServersdirective allows you to specify the minimum number of spare child processes. It takes the following form:MinSpareServers number
MinSpareServers numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow Note that a high number can create a heavy processing load on the server. This directive is used by thepreforkMPM only.Example 18.75. Using the MinSpareServers directive
MinSpareServers 5
MinSpareServers 5Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
MinSpareThreads - The
MinSpareThreadsdirective allows you to specify the minimum number of spare server threads. It takes the following form:MinSpareThreads number
MinSpareThreads numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow This directive is used by theworkerMPM only.Example 18.76. Using the MinSpareThreads directive
MinSpareThreads 75
MinSpareThreads 75Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
StartServers - The
StartServersdirective allows you to specify the number of child processes to create when the service is started. It takes the following form:StartServers number
StartServers numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow Since the child processes are dynamically created and terminated according to the current traffic load, it is usually not necessary to change this value.Example 18.77. Using the StartServers directive
StartServers 8
StartServers 8Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
ThreadsPerChild - The
ThreadsPerChilddirective allows you to specify the number of threads a child process can create. It takes the following form:ThreadsPerChild number
ThreadsPerChild numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow This directive is used by theworkerMPM only.Example 18.78. Using the ThreadsPerChild directive
ThreadsPerChild 25
ThreadsPerChild 25Copy to Clipboard Copied! Toggle word wrap Toggle overflow
18.1.6. Working with Modules
Link kopierenLink in die Zwischenablage kopiert!
Being a modular application, the
httpd service is distributed along with a number of Dynamic Shared Objects (DSOs), which can be dynamically loaded or unloaded at runtime as necessary. By default, these modules are located in /usr/lib/httpd/modules/ on 32-bit and in /usr/lib64/httpd/modules/ on 64-bit systems.
18.1.6.1. Loading a Module
Link kopierenLink in die Zwischenablage kopiert!
To load a particular DSO module, use the
LoadModule directive as described in Section 18.1.5.1, “Common httpd.conf Directives”. Note that modules provided by a separate package often have their own configuration file in the /etc/httpd/conf.d/ directory.
Example 18.79. Loading the mod_ssl DSO
LoadModule ssl_module modules/mod_ssl.so
LoadModule ssl_module modules/mod_ssl.so
Once you are finished, restart the web server to reload the configuration. See Section 18.1.4.3, “Restarting the Service” for more information on how to restart the
httpd service.
18.1.6.2. Writing a Module
Link kopierenLink in die Zwischenablage kopiert!
If you intend to create a new DSO module, make sure you have the httpd-devel package installed. To do so, enter the following command as
root:
yum install httpd-devel
~]# yum install httpd-devel
This package contains the include files, the header files, and the APache eXtenSion (
apxs) utility required to compile a module.
Once written, you can build the module with the following command:
apxs -i -a -c module_name.c
~]# apxs -i -a -c module_name.c
If the build was successful, you should be able to load the module the same way as any other module that is distributed with the Apache HTTP Server.
18.1.7. Setting Up Virtual Hosts
Link kopierenLink in die Zwischenablage kopiert!
The Apache HTTP Server's built in virtual hosting allows the server to provide different information based on which IP address, host name, or port is being requested.
To create a name-based virtual host, find the virtual host container provided in
/etc/httpd/conf/httpd.conf as an example, remove the hash sign (that is, #) from the beginning of each line, and customize the options according to your requirements as shown in Example 18.80, “Example virtual host configuration”.
Example 18.80. Example virtual host configuration
Note that
ServerName must be a valid DNS name assigned to the machine. The <VirtualHost> container is highly customizable, and accepts most of the directives available within the main server configuration. Directives that are not supported within this container include User and Group, which were replaced by SuexecUserGroup.
Note
If you configure a virtual host to listen on a non-default port, make sure you update the
Listen directive in the global settings section of the /etc/httpd/conf/httpd.conf file accordingly.
To activate a newly created virtual host, the web server has to be restarted first. See Section 18.1.4.3, “Restarting the Service” for more information on how to restart the
httpd service.
18.1.8. Setting Up an SSL Server
Link kopierenLink in die Zwischenablage kopiert!
Secure Sockets Layer (SSL) is a cryptographic protocol that allows a server and a client to communicate securely. Along with its extended and improved version called Transport Layer Security (TLS), it ensures both privacy and data integrity. The Apache HTTP Server in combination with
mod_ssl, a module that uses the OpenSSL toolkit to provide the SSL/TLS support, is commonly referred to as the SSL server. Red Hat Enterprise Linux also supports the use of Mozilla NSS as the TLS implementation. Support for Mozilla NSS is provided by the mod_nss module.
Unlike an HTTP connection that can be read and possibly modified by anybody who is able to intercept it, the use of SSL/TLS over HTTP, referred to as HTTPS, prevents any inspection or modification of the transmitted content. This section provides basic information on how to enable this module in the Apache HTTP Server configuration, and guides you through the process of generating private keys and self-signed certificates.
18.1.8.1. An Overview of Certificates and Security
Link kopierenLink in die Zwischenablage kopiert!
Secure communication is based on the use of keys. In conventional or symmetric cryptography, both ends of the transaction have the same key they can use to decode each other's transmissions. On the other hand, in public or asymmetric cryptography, two keys co-exist: a private key that is kept a secret, and a public key that is usually shared with the public. While the data encoded with the public key can only be decoded with the private key, data encoded with the private key can in turn only be decoded with the public key.
To provide secure communications using SSL, an SSL server must use a digital certificate signed by a Certificate Authority (CA). The certificate lists various attributes of the server (that is, the server host name, the name of the company, its location, etc.), and the signature produced using the CA's private key. This signature ensures that a particular certificate authority has signed the certificate, and that the certificate has not been modified in any way.
When a web browser establishes a new SSL connection, it checks the certificate provided by the web server. If the certificate does not have a signature from a trusted CA, or if the host name listed in the certificate does not match the host name used to establish the connection, it refuses to communicate with the server and usually presents a user with an appropriate error message.
By default, most web browsers are configured to trust a set of widely used certificate authorities. Because of this, an appropriate CA should be chosen when setting up a secure server, so that target users can trust the connection, otherwise they will be presented with an error message, and will have to accept the certificate manually. Since encouraging users to override certificate errors can allow an attacker to intercept the connection, you should use a trusted CA whenever possible. For more information on this, see Table 18.22, “Information about CA lists used by common web browsers”.
| Web Browser | Link |
|---|---|
| Mozilla Firefox | Mozilla root CA list. |
| Opera | Information on root certificates used by Opera. |
| Internet Explorer | Information on root certificates used by Microsoft Windows. |
| Chromium | Information on root certificates used by the Chromium project. |
When setting up an SSL server, you need to generate a certificate request and a private key, and then send the certificate request, proof of the company's identity, and payment to a certificate authority. Once the CA verifies the certificate request and your identity, it will send you a signed certificate you can use with your server. Alternatively, you can create a self-signed certificate that does not contain a CA signature, and thus should be used for testing purposes only.
18.1.9. Enabling the mod_ssl Module
Link kopierenLink in die Zwischenablage kopiert!
If you intend to set up an SSL or HTTPS server using
mod_ssl, you cannot have another application or module, such as mod_nss configured to use the same port. Port 443 is the default port for HTTPS.
To set up an SSL server using the
mod_ssl module and the OpenSSL toolkit, install the mod_ssl and openssl packages. Enter the following command as root:
yum install mod_ssl openssl
~]# yum install mod_ssl openssl
This will create the
mod_ssl configuration file at /etc/httpd/conf.d/ssl.conf, which is included in the main Apache HTTP Server configuration file by default. For the module to be loaded, restart the httpd service as described in Section 18.1.4.3, “Restarting the Service”.
Important
Due to the vulnerability described in POODLE: SSLv3 vulnerability (CVE-2014-3566), Red Hat recommends disabling
SSL, if it is enabled, and using only TLSv1.1 or TLSv1.2. Backwards compatibility can be achieved using TLSv1.0. Many products Red Hat supports have the ability to use SSLv2 or SSLv3 protocols. However, the use of SSLv2 or SSLv3 is now strongly recommended against.
18.1.9.1. Enabling and Disabling SSL and TLS in mod_ssl
Link kopierenLink in die Zwischenablage kopiert!
To disable and enable specific versions of the SSL and TLS protocol, either do it globally by adding the
SSLProtocol directive in the “## SSL Global Context” section of the configuration file and removing it everywhere else, or edit the default entry under “# SSL Protocol support” in all “VirtualHost” sections. If you do not specify it in the per-domain VirtualHost section then it will inherit the settings from the global section. To make sure that a protocol version is being disabled the administrator should either only specify SSLProtocol in the “SSL Global Context” section, or specify it in all per-domain VirtualHost sections.
Note that in Red Hat Enterprise Linux 6.8 SSLv2 is disabled by default.
Procedure 18.1. Disable SSLv2 and SSLv3
To disable SSL version 2 and SSL version 3, which implies enabling everything except SSL version 2 and SSL version 3, in all VirtualHost sections, proceed as follows:
- As
root, open the/etc/httpd/conf.d/ssl.conffile and search for all instances of theSSLProtocoldirective. By default, the configuration file contains one section that looks as follows:This section is within the VirtualHost section.vi /etc/httpd/conf.d/ssl.conf
~]# vi /etc/httpd/conf.d/ssl.conf # SSL Protocol support: # List the enable protocol levels with which clients will be able to # connect. Disable SSLv2 access by default: SSLProtocol all -SSLv2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
SSLProtocolline as follows:Repeat this action for all VirtualHost sections. Save and close the file.SSL Protocol support: List the enable protocol levels with which clients will be able to connect. Disable SSLv2 access by default:
# SSL Protocol support: # List the enable protocol levels with which clients will be able to # connect. Disable SSLv2 access by default: SSLProtocol all -SSLv2 -SSLv3Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Verify that all occurrences of the
SSLProtocoldirective have been changed as follows:This step is particularly important if you have more than the one default VirtualHost section.grep SSLProtocol /etc/httpd/conf.d/ssl.conf
~]# grep SSLProtocol /etc/httpd/conf.d/ssl.conf SSLProtocol all -SSLv2 -SSLv3Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart the Apache daemon as follows:Note that any sessions will be interrupted.
service httpd restart
~]# service httpd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 18.2. Disable All SSL and TLS Protocols Except TLS 1 and Up
To disable all SSL and TLS protocol versions except TLS version 1 and higher, proceed as follows:
- As
root, open the/etc/httpd/conf.d/ssl.conffile and search for all instances ofSSLProtocoldirective. By default the file contains one section that looks as follows:vi /etc/httpd/conf.d/ssl.conf
~]# vi /etc/httpd/conf.d/ssl.conf # SSL Protocol support: # List the enable protocol levels with which clients will be able to # connect. Disable SSLv2 access by default: SSLProtocol all -SSLv2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
SSLProtocolline as follows:Save and close the file.SSL Protocol support: List the enable protocol levels with which clients will be able to connect. Disable SSLv2 access by default:
# SSL Protocol support: # List the enable protocol levels with which clients will be able to # connect. Disable SSLv2 access by default: SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Verify the change as follows:
grep SSLProtocol /etc/httpd/conf.d/ssl.conf
~]# grep SSLProtocol /etc/httpd/conf.d/ssl.conf SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart the Apache daemon as follows:Note that any sessions will be interrupted.
service httpd restart
~]# service httpd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 18.3. Testing the Status of SSL and TLS Protocols
To check which versions of SSL and TLS are enabled or disabled, make use of the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where port is the port to test and protocol is the protocol version to test for. To test the SSL server running locally, use
openssl s_client -connect command. The command has the following form: openssl s_client -connect hostname:port -protocol
openssl s_client -connect hostname:port -protocollocalhost as the host name. For example, to test the default port for secure HTTPS connections, port 443 to see if SSLv3 is enabled, issue a command as follows:
- The above output indicates that the handshake failed and therefore no cipher was negotiated.
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - The above output indicates that no failure of the handshake occurred and a set of ciphers was negotiated.
Copy to Clipboard Copied! Toggle word wrap Toggle overflow
The
openssl s_client command options are documented in the s_client(1) manual page.
For more information on the SSLv3 vulnerability and how to test for it, see the Red Hat Knowledgebase article POODLE: SSLv3 vulnerability (CVE-2014-3566).
18.1.10. Enabling the mod_nss Module
Link kopierenLink in die Zwischenablage kopiert!
If you intend to set up an HTTPS server using
mod_nss, the HTTPS server cannot simultaneously use mod_ssl with its default settings as mod_ssl will use port 443 by default, however this is the default HTTPS port. If is recommend to remove the package if it is not required.
To remove mod_ssl, enter the following command as
root:
yum remove mod_ssl
~]# yum remove mod_sslNote
If
mod_ssl is required for other purposes, modify the /etc/httpd/conf.d/ssl.conf file to use a port other than 443 to prevent mod_ssl conflicting with mod_nss when its port to listen on is changed to 443.
For a specific VirtualHost where HTTPS is required,
mod_nss and mod_ssl can only co-exist at the same time if they use unique ports. For this reason mod_nss by default uses 8443, but the default port for HTTPS is port 443. The port is specified by the Listen directive as well as in the VirtualHost name or address.
Everything in NSS is associated with a “token”. The software token exists in the NSS database but you can also have a physical token containing certificates. With OpenSSL, discrete certificates and private keys are held in PEM files. With NSS, these are stored in a database. Each certificate and key is associated with a token and each token can have a password protecting it. This password is optional, but if a password is used then the Apache HTTP server needs a copy of it in order to open the database without user intervention at system start.
Procedure 18.4. Configuring mod_nss
- Install mod_nss as
root:yum install mod_nss
~]# yum install mod_nssCopy to Clipboard Copied! Toggle word wrap Toggle overflow This will create themod_nssconfiguration file at/etc/httpd/conf.d/nss.conf. The/etc/httpd/conf.d/directory is included in the main Apache HTTP Server configuration file by default. For the module to be loaded, restart thehttpdservice as described in Section 18.1.4.3, “Restarting the Service”. - As
root, open the/etc/httpd/conf.d/nss.conffile and search for all instances of theListendirective.Edit theListen 8443line as follows:PortListen 443
Listen 443Copy to Clipboard Copied! Toggle word wrap Toggle overflow 443is the default port forHTTPS. - Edit the default
VirtualHost _default_:8443line as follows:Edit any other non-default virtual host sections if they exist. Save and close the file.VirtualHost _default_:443
VirtualHost _default_:443Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Mozilla NSS stores certificates in a server certificate database indicated by the
NSSCertificateDatabasedirective in the/etc/httpd/conf.d/nss.conffile. By default the path is set to/etc/httpd/alias, the NSS database created during installation.To view the default NSS database, issue a command as follows:In the above command output,Copy to Clipboard Copied! Toggle word wrap Toggle overflow Server-Certis the defaultNSSNickname. The-Loption lists all the certificates, or displays information about a named certificate, in a certificate database. The-doption specifies the database directory containing the certificate and key database files. See thecertutil(1)man page for more command line options. - To configure mod_nss to use another database, edit the
NSSCertificateDatabaseline in the/etc/httpd/conf.d/nss.conffile. The default file has the following lines within the VirtualHost section.In the above command output,Server Certificate Database: The NSS security database directory that holds the certificates and keys. The database consists of 3 files: cert8.db, key3.db and secmod.db. Provide the directory that these files exist.
# Server Certificate Database: # The NSS security database directory that holds the certificates and # keys. The database consists of 3 files: cert8.db, key3.db and secmod.db. # Provide the directory that these files exist. NSSCertificateDatabase /etc/httpd/aliasCopy to Clipboard Copied! Toggle word wrap Toggle overflow aliasis the default NSS database directory,/etc/httpd/alias/. - To apply a password to the default NSS certificate database, use the following command as
root:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Before deploying the HTTPS server, create a new certificate database using a certificate signed by a certificate authority (CA).
Example 18.81. Adding a Certificate to the Mozilla NSS database
Thecertutilcommand is used to add a CA certificate to the NSS database files:certutil -d /etc/httpd/nss-db-directory/ -A -n "CA_certificate" -t CT,, -a -i certificate.pem
certutil -d /etc/httpd/nss-db-directory/ -A -n "CA_certificate" -t CT,, -a -i certificate.pemCopy to Clipboard Copied! Toggle word wrap Toggle overflow The above command adds a CA certificate stored in a PEM-formatted file named certificate.pem. The-doption specifies the NSS database directory containing the certificate and key database files, the-noption sets a name for the certificate,-tCT,,means that the certificate is trusted to be used in TLS clients and servers. The-Aoption adds an existing certificate to a certificate database. If the database does not exist it will be created. The-aoption allows the use of ASCII format for input or output, and the-ioption passes thecertificate.peminput file to the command.See thecertutil(1)man page for more command line options. - The NSS database should be password protected to safeguard the private key.
Example 18.82. Setting_a_Password_for_a_Mozilla_NSS_database
Thecertutiltool can be used to set a password for an NSS database as follows:certutil -W -d /etc/httpd/nss-db-directory/
certutil -W -d /etc/httpd/nss-db-directory/Copy to Clipboard Copied! Toggle word wrap Toggle overflow For example, for the default database, issue a command asrootas follows:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Configure
mod_nssto use the NSS internal software token by changing the line with theNSSPassPhraseDialogdirective as follows:This is to avoid manual password entry on system start. The software token exists in the NSS database but you can also have a physical token containing your certificates.vi /etc/httpd/conf.d/nss.conf
~]# vi /etc/httpd/conf.d/nss.conf NSSPassPhraseDialog file:/etc/httpd/password.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - If the SSL Server Certificate contained in the NSS database is an RSA certificate, make certain that the
NSSNicknameparameter is uncommented and matches the nickname displayed in step 4 above:vi /etc/httpd/conf.d/nss.conf
~]# vi /etc/httpd/conf.d/nss.conf NSSNickname Server-CertCopy to Clipboard Copied! Toggle word wrap Toggle overflow If the SSL Server Certificate contained in the NSS database is an ECC certificate, make certain that theNSSECCNicknameparameter is uncommented and matches the nickname displayed in step 4 above:vi /etc/httpd/conf.d/nss.conf
~]# vi /etc/httpd/conf.d/nss.conf NSSECCNickname Server-CertCopy to Clipboard Copied! Toggle word wrap Toggle overflow Make certain that theNSSCertificateDatabaseparameter is uncommented and points to the NSS database directory displayed in step 4 or configured in step 5 above:Replacevi /etc/httpd/conf.d/nss.conf
~]# vi /etc/httpd/conf.d/nss.conf NSSCertificateDatabase /etc/httpd/aliasCopy to Clipboard Copied! Toggle word wrap Toggle overflow /etc/httpd/aliaswith the path to the certificate database to be used. - Create the
/etc/httpd/password.conffile asroot:Add a line with the following form:vi /etc/httpd/password.conf
~]# vi /etc/httpd/password.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replacing password with the password that was applied to the NSS security databases in step 6 above.internal:password
internal:passwordCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Apply the appropriate ownership and permissions to the
/etc/httpd/password.conffile:chgrp apache /etc/httpd/password.conf chmod 640 /etc/httpd/password.conf ls -l /etc/httpd/password.conf
~]# chgrp apache /etc/httpd/password.conf ~]# chmod 640 /etc/httpd/password.conf ~]# ls -l /etc/httpd/password.conf -rw-r-----. 1 root apache 10 Dec 4 17:13 /etc/httpd/password.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - To configure
mod_nssto use the NSS the software token in/etc/httpd/password.conf, edit/etc/httpd/conf.d/nss.confas follows:vi /etc/httpd/conf.d/nss.conf
~]# vi /etc/httpd/conf.d/nss.confCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart the Apache server for the changes to take effect as described in Section 18.1.4.3, “Restarting the Service”
Important
Due to the vulnerability described in POODLE: SSLv3 vulnerability (CVE-2014-3566), Red Hat recommends disabling
SSL, if it is enabled, and using only TLSv1.1 or TLSv1.2. Backwards compatibility can be achieved using TLSv1.0. Many products Red Hat supports have the ability to use SSLv2 or SSLv3 protocols. However, the use of SSLv2 or SSLv3 is now strongly recommended against.
18.1.10.1. Enabling and Disabling SSL and TLS in mod_nss
Link kopierenLink in die Zwischenablage kopiert!
To disable and enable specific versions of the SSL and TLS protocol, either do it globally by adding the
NSSProtocol directive in the “## SSL Global Context” section of the configuration file and removing it everywhere else, or edit the default entry under “# SSL Protocol” in all “VirtualHost” sections. If you do not specify it in the per-domain VirtualHost section then it will inherit the settings from the global section. To make sure that a protocol version is being disabled the administrator should either only specify NSSProtocol in the “SSL Global Context” section, or specify it in all per-domain VirtualHost sections.
Note that in Red Hat Enterprise Linux 6.8 SSLv2 is disabled by default.
Procedure 18.5. Disable All SSL and TLS Protocols Except TLS 1 and Up in mod_nss
To disable all SSL and TLS protocol versions except TLS version 1 and higher, proceed as follows:
- As
root, open the/etc/httpd/conf.d/nss.conffile and search for all instances of theNSSProtocoldirective. By default, the configuration file contains one section that looks as follows:This section is within the VirtualHost section.Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
NSSProtocolline as follows:Repeat this action for all VirtualHost sections.SSL Protocol:
# SSL Protocol: NSSProtocol TLSv1.0,TLSv1.1,TLSv1.2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the
Listen 8443line as follows:Listen 443
Listen 443Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Edit the default
VirtualHost _default_:8443line as follows:Edit any other non-default virtual host sections if they exist. Save and close the file.VirtualHost _default_:443
VirtualHost _default_:443Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Verify that all occurrences of the
NSSProtocoldirective have been changed as follows:This step is particularly important if you have more than one VirtualHost section.grep NSSProtocol /etc/httpd/conf.d/nss.conf
~]# grep NSSProtocol /etc/httpd/conf.d/nss.conf # middle of a range may be excluded, the entry "NSSProtocol SSLv3,TLSv1.1" # is identical to the entry "NSSProtocol SSLv3,TLSv1.0,TLSv1.1". NSSProtocol TLSv1.0,TLSv1.1,TLSv1.2Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Restart the Apache daemon as follows:Note that any sessions will be interrupted.
service httpd restart
~]# service httpd restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 18.6. Testing the Status of SSL and TLS Protocols in mod_nss
To check which versions of SSL and TLS are enabled or disabled in mod_nss, make use of the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
openssl s_client -connect command. Install the openssl package as root:
yum install openssl
~]# yum install openssl
The
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Where port is the port to test and protocol is the protocol version to test for. To test the SSL server running locally, use
openssl s_client -connect command has the following form: openssl s_client -connect hostname:port -protocol
openssl s_client -connect hostname:port -protocollocalhost as the host name. For example, to test the default port for secure HTTPS connections, port 443 to see if SSLv3 is enabled, issue a command as follows:
- The above output indicates that the handshake failed and therefore no cipher was negotiated.
Copy to Clipboard Copied! Toggle word wrap Toggle overflow - The above output indicates that no failure of the handshake occurred and a set of ciphers was negotiated.
Copy to Clipboard Copied! Toggle word wrap Toggle overflow
The
openssl s_client command options are documented in the s_client(1) manual page.
For more information on the SSLv3 vulnerability and how to test for it, see the Red Hat Knowledgebase article POODLE: SSLv3 vulnerability (CVE-2014-3566).
18.1.11. Using an Existing Key and Certificate
Link kopierenLink in die Zwischenablage kopiert!
If you have a previously created key and certificate, you can configure the SSL server to use these files instead of generating new ones. There are only two situations where this is not possible:
- You are changing the IP address or domain name.Certificates are issued for a particular IP address and domain name pair. If one of these values changes, the certificate becomes invalid.
- You have a certificate from VeriSign, and you are changing the server software.VeriSign, a widely used certificate authority, issues certificates for a particular software product, IP address, and domain name. Changing the software product renders the certificate invalid.
In either of the above cases, you will need to obtain a new certificate. For more information on this topic, see Section 18.1.12, “Generating a New Key and Certificate”.
If you want to use an existing key and certificate, move the relevant files to the
/etc/pki/tls/private/ and /etc/pki/tls/certs/ directories respectively. You can do so by issuing the following commands as root:
mv key_file.key /etc/pki/tls/private/hostname.key mv certificate.crt /etc/pki/tls/certs/hostname.crt
~]# mv key_file.key /etc/pki/tls/private/hostname.key
~]# mv certificate.crt /etc/pki/tls/certs/hostname.crt
Then add the following lines to the
/etc/httpd/conf.d/ssl.conf configuration file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
To load the updated configuration, restart the
httpd service as described in Section 18.1.4.3, “Restarting the Service”.
Example 18.83. Using a key and certificate from the Red Hat Secure Web Server
mv /etc/httpd/conf/httpsd.key /etc/pki/tls/private/penguin.example.com.key mv /etc/httpd/conf/httpsd.crt /etc/pki/tls/certs/penguin.example.com.crt
~]# mv /etc/httpd/conf/httpsd.key /etc/pki/tls/private/penguin.example.com.key
~]# mv /etc/httpd/conf/httpsd.crt /etc/pki/tls/certs/penguin.example.com.crt18.1.12. Generating a New Key and Certificate
Link kopierenLink in die Zwischenablage kopiert!
In order to generate a new key and certificate pair, the crypto-utils package must be installed on the system. In Red Hat Enterprise Linux 6, the mod_ssl package is required by the genkey utility. To install these, enter the following command as
root:
yum install crypto-utils mod_ssl
~]# yum install crypto-utils mod_ssl
This package provides a set of tools to generate and manage SSL certificates and private keys, and includes genkey, the Red Hat Keypair Generation utility that will guide you through the key generation process.
Important
If the server already has a valid certificate and you are replacing it with a new one, specify a different serial number. This ensures that client browsers are notified of this change, update to this new certificate as expected, and do not fail to access the page. To create a new certificate with a custom serial number, use the following command instead of genkey:
openssl req -x509 -new -set_serial number -key hostname.key -out hostname.crt
~]# openssl req -x509 -new -set_serial number -key hostname.key -out hostname.crtNote
If there already is a key file for a particular host name in your system, genkey will refuse to start. In this case, remove the existing file using the following command as
root:
rm /etc/pki/tls/private/hostname.key
~]# rm /etc/pki/tls/private/hostname.key
To run the utility enter the
genkey command as root, followed by the appropriate host name (for example, penguin.example.com):
genkey hostname
~]# genkey hostname
To complete the key and certificate creation, take the following steps:
- Review the target locations in which the key and certificate will be stored.
Figure 18.1. Running the genkey utility
Use the Tab key to select the button, and press Enter to proceed to the next screen. - Using the up and down arrow keys, select a suitable key size. Note that while a larger key increases the security, it also increases the response time of your server. The NIST recommends using
2048 bits. See NIST Special Publication 800-131A.Figure 18.2. Selecting the key size
Once finished, use the Tab key to select the button, and press Enter to initiate the random bits generation process. Depending on the selected key size, this may take some time. - Decide whether you want to send a certificate request to a certificate authority.
Figure 18.3. Generating a certificate request
Use the Tab key to select to compose a certificate request, or to generate a self-signed certificate. Then press Enter to confirm your choice. - Using the Spacebar key, enable (
[*]) or disable ([ ]) the encryption of the private key.Figure 18.4. Encrypting the private key
Use the Tab key to select the button, and press Enter to proceed to the next screen. - If you have enabled the private key encryption, enter an adequate passphrase. Note that for security reasons, it is not displayed as you type, and it must be at least five characters long.
Figure 18.5. Entering a passphrase
Use the Tab key to select the button, and press Enter to proceed to the next screen.Important
Entering the correct passphrase is required in order for the server to start. If you lose it, you will need to generate a new key and certificate. - Customize the certificate details.
Figure 18.6. Specifying certificate information
Use the Tab key to select the button, and press Enter to finish the key generation. - If you have previously enabled the certificate request generation, you will be prompted to send it to a certificate authority.
Figure 18.7. Instructions on how to send a certificate request
Press Enter to return to a shell prompt.
Once generated, add the key and certificate locations to the
/etc/httpd/conf.d/ssl.conf configuration file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
Finally, restart the
httpd service as described in Section 18.1.4.3, “Restarting the Service”, so that the updated configuration is loaded.
18.1.13. Configure the Firewall for HTTP and HTTPS Using the Command Line
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux does not allow
HTTP and HTTPS traffic by default. To enable the system to act as a web server, enable ports and protocols as required. The default port for HTTP is 80 and the default port for HTTPS is 443. In both cases the TCP should be allowed to pass through the firewall.
To enable port
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Note that this will restart the firewall as long as it has not been disabled with the
80 for HTTP using the command line, issue the following command as root:
lokkit --port=80:tcp --update
~]# lokkit --port=80:tcp --update--disabled option. Active connections will be terminated and time out on the initiating machine. Use the lokkit --help command to view the built in help.
To enable port
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Note that this will restart the firewall as long as it has not been disabled with the
443 for HTTPS using the command line, issue the following command as root:
lokkit --port=443:tcp --update
~]# lokkit --port=443:tcp --update--disabled option. Active connections will be terminated and time out on the initiating machine. See the /etc/services file for list of services and their associated ports.
When preparing a configuration file for multiple installations using administration tools, it is useful to edit the firewall configuration file directly. Note that any mistakes in the configuration file could have unexpected consequences, cause an error, and prevent the firewall settings from being applied. Therefore, check the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
/etc/sysconfig/system-config-firewall file thoroughly after editing. To apply the settings in /etc/sysconfig/system-config-firewall, issue the following command as root:
lokkit --update
~]# lokkit --update
For example, to enable
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
Note that these changes will not take effect even if the firewall is reloaded or the system rebooted. To apply the changes in
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
HTTPS to pass through the firewall, by editing the configuration file, become the root user and add the following line to /etc/sysconfig/system-config-firewall:
--port=443:tcp
--port=443:tcp/etc/sysconfig/system-config-firewall, issue the following command as root:
lokkit --update
~]# lokkit --update18.1.13.1. Checking Network Access for Incoming HTTPS and HTTPS Using the Command Line
Link kopierenLink in die Zwischenablage kopiert!
To check what the firewall is configured to allow, using the command line, issue the following command as
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
In this example taken from a default installation, the firewall is enabled but
root:
less /etc/sysconfig/system-config-firewall
~]# less /etc/sysconfig/system-config-firewall
# Configuration file for system-config-firewall
--enabled
--service=sshHTTP and HTTPS have not been allowed to pass through.
Once the default port for
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
HTTP is enabled, the following line appears as output in addition to the lines shown above:
--port=80:tcp
--port=80:tcp
To check if the firewall is currently allowing incoming
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
HTTP traffic for clients, issue the following command as root:
iptables -L -n | grep 'tcp.*80'
~]# iptables -L -n | grep 'tcp.*80'
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:80
Once the default port for
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
HTTPS is enabled, the following line appears as output in addition to the lines shown above:
--port=443:tcp
--port=443:tcp
To check if the firewall is currently allowing incoming
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
HTTPS traffic for clients, issue the following command as root:
iptables -L -n | grep 'tcp.*443'
~]# iptables -L -n | grep 'tcp.*443'
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:44318.1.14. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
To learn more about the Apache HTTP Server, see the following resources.
Installed Documentation
httpd(8)— The manual page for thehttpdservice containing the complete list of its command-line options.genkey(1)— The manual page forgenkeyutility, provided by the crypto-utils package.
Installable Documentation
- http://localhost/manual/ — The official documentation for the Apache HTTP Server with the full description of its directives and available modules. Note that in order to access this documentation, you must have the httpd-manual package installed, and the web server must be running.Before accessing the documentation, issue the following commands as
root:yum install httpd-manual service httpd graceful
~]# yum install httpd-manual ~]# service httpd gracefulCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Online Documentation
- http://httpd.apache.org/ — The official website for the Apache HTTP Server with documentation on all the directives and default modules.
- http://www.openssl.org/ — The OpenSSL home page containing further documentation, frequently asked questions, links to the mailing lists, and other useful resources.
Chapter 19. Mail Servers
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux offers many advanced applications to serve and access email. This chapter describes modern email protocols in use today, and some of the programs designed to send and receive email.
19.1. Email Protocols
Link kopierenLink in die Zwischenablage kopiert!
Today, email is delivered using a client/server architecture. An email message is created using a mail client program. This program then sends the message to a server. The server then forwards the message to the recipient's email server, where the message is then supplied to the recipient's email client.
To enable this process, a variety of standard network protocols allow different machines, often running different operating systems and using different email programs, to send and receive email.
The following protocols discussed are the most commonly used in the transfer of email.
19.1.1. Mail Transport Protocols
Link kopierenLink in die Zwischenablage kopiert!
Mail delivery from a client application to the server, and from an originating server to the destination server, is handled by the Simple Mail Transfer Protocol (SMTP).
19.1.1.1. SMTP
Link kopierenLink in die Zwischenablage kopiert!
The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email clients as well. To send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an SMTP server when configuring an email client.
Under Red Hat Enterprise Linux, a user can configure an SMTP server on the local machine to handle mail delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.
One important point to make about the SMTP protocol is that it does not require authentication. This allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this characteristic of SMTP that makes junk email or spam possible. Imposing relay restrictions limits random users on the Internet from sending email through your SMTP server, to other servers on the internet. Servers that do not impose such restrictions are called open relay servers.
Red Hat Enterprise Linux provides the Postfix and Sendmail SMTP programs.
19.1.2. Mail Access Protocols
Link kopierenLink in die Zwischenablage kopiert!
There are two primary protocols used by email client applications to retrieve email from mail servers: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).
19.1.2.1. POP
Link kopierenLink in die Zwischenablage kopiert!
The default POP server under Red Hat Enterprise Linux is Dovecot and is provided by the dovecot package.
Note
In order to use Dovecot, first ensure the dovecot package is installed on your system by running, as
root:
yum install dovecot
~]# yum install dovecot
For more information on installing packages with Yum, see Section 8.2.4, “Installing Packages”.
When using a
POP server, email messages are downloaded by email client applications. By default, most POP email clients are automatically configured to delete the message on the email server after it has been successfully transferred, however this setting usually can be changed.
POP is fully compatible with important Internet messaging standards, such as Multipurpose Internet Mail Extensions (MIME), which allow for email attachments.
POP works best for users who have one system on which to read email. It also works well for users who do not have a persistent connection to the Internet or the network containing the mail server. Unfortunately for those with slow network connections, POP requires client programs upon authentication to download the entire content of each message. This can take a long time if any messages have large attachments.
The most current version of the standard
POP protocol is POP3.
There are, however, a variety of lesser-used
POP protocol variants:
- APOP —
POP3withMD5authentication. An encoded hash of the user's password is sent from the email client to the server rather than sending an unencrypted password. - KPOP —
POP3with Kerberos authentication. - RPOP —
POP3withRPOPauthentication. This uses a per-user ID, similar to a password, to authenticate POP requests. However, this ID is not encrypted, soRPOPis no more secure than standardPOP.
For added security, it is possible to use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. This can be enabled by using the
pop3s service, or by using the stunnel application. For more information on securing email communication, see Section 19.5.1, “Securing Communication”.
19.1.2.2. IMAP
Link kopierenLink in die Zwischenablage kopiert!
The default
IMAP server under Red Hat Enterprise Linux is Dovecot and is provided by the dovecot package. See Section 19.1.2.1, “POP” for information on how to install Dovecot.
When using an
IMAP mail server, email messages remain on the server where users can read or delete them. IMAP also allows client applications to create, rename, or delete mail directories on the server to organize and store email.
IMAP is particularly useful for users who access their email using multiple machines. The protocol is also convenient for users connecting to the mail server via a slow connection, because only the email header information is downloaded for messages until opened, saving bandwidth. The user also has the ability to delete messages without viewing or downloading them.
For convenience,
IMAP client applications are capable of caching copies of messages locally, so the user can browse previously read messages when not directly connected to the IMAP server.
IMAP, like POP, is fully compatible with important Internet messaging standards, such as MIME, which allow for email attachments.
For added security, it is possible to use
SSL encryption for client authentication and data transfer sessions. This can be enabled by using the imaps service, or by using the stunnel program. For more information on securing email communication, see Section 19.5.1, “Securing Communication”.
Other free, as well as commercial, IMAP clients and servers are available, many of which extend the IMAP protocol and provide additional functionality.
19.1.2.3. Dovecot
Link kopierenLink in die Zwischenablage kopiert!
The
imap-login and pop3-login processes which implement the IMAP and POP3 protocols are spawned by the master dovecot daemon included in the dovecot package. The use of IMAP and POP is configured through the /etc/dovecot/dovecot.conf configuration file; by default dovecot runs IMAP and POP3 together with their secure versions using SSL. To configure dovecot to use POP, complete the following steps:
- Edit the
/etc/dovecot/dovecot.confconfiguration file to make sure theprotocolsvariable is uncommented (remove the hash sign (#) at the beginning of the line) and contains thepop3argument. For example:protocols = imap pop3 lmtp
protocols = imap pop3 lmtpCopy to Clipboard Copied! Toggle word wrap Toggle overflow When theprotocolsvariable is left commented out,dovecotwill use the default values as described above. - Make the change operational for the current session by running the following command:
service dovecot restart
~]# service dovecot restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make the change operational after the next reboot by running the command:
chkconfig dovecot on
~]# chkconfig dovecot onCopy to Clipboard Copied! Toggle word wrap Toggle overflow Note
Please note thatdovecotonly reports that it started theIMAPserver, but also starts thePOP3server.
Unlike
SMTP, both IMAP and POP3 require connecting clients to authenticate using a user name and password. By default, passwords for both protocols are passed over the network unencrypted.
To configure
SSL on dovecot:
- Edit the
/etc/dovecot/conf.d/10-ssl.confconfiguration to make sure thessl_cipher_listvariable is uncommented, and append:!SSLv3:ssl_cipher_list = ALL:!LOW:!SSLv2:!EXP:!aNULL:!SSLv3
ssl_cipher_list = ALL:!LOW:!SSLv2:!EXP:!aNULL:!SSLv3Copy to Clipboard Copied! Toggle word wrap Toggle overflow These values ensure thatdovecotavoids SSL versions 2 and also 3, which are both known to be insecure. This is due to the vulnerability described in POODLE: SSLv3 vulnerability (CVE-2014-3566). See Resolution for POODLE SSL 3.0 vulnerability (CVE-2014-3566) in Postfix and Dovecot for details. - Edit the
/etc/pki/dovecot/dovecot-openssl.cnfconfiguration file as you prefer. However, in a typical installation, this file does not require modification. - Rename, move or delete the files
/etc/pki/dovecot/certs/dovecot.pemand/etc/pki/dovecot/private/dovecot.pem. - Execute the
/usr/libexec/dovecot/mkcert.shscript which creates thedovecotself signed certificates. These certificates are copied in the/etc/pki/dovecot/certsand/etc/pki/dovecot/privatedirectories. To implement the changes, restartdovecot:service dovecot restart
~]# service dovecot restartCopy to Clipboard Copied! Toggle word wrap Toggle overflow
More details on
dovecot can be found online at http://www.dovecot.org.
19.2. Email Program Classifications
Link kopierenLink in die Zwischenablage kopiert!
In general, all email applications fall into at least one of three classifications. Each classification plays a specific role in the process of moving and managing email messages. While most users are only aware of the specific email program they use to receive and send messages, each one is important for ensuring that email arrives at the correct destination.
19.2.1. Mail Transport Agent
Link kopierenLink in die Zwischenablage kopiert!
A Mail Transport Agent (MTA) transports email messages between hosts using
SMTP. A message may involve several MTAs as it moves to its intended destination.
While the delivery of messages between machines may seem rather straightforward, the entire process of deciding if a particular MTA can or should accept a message for delivery is quite complicated. In addition, due to problems from spam, use of a particular MTA is usually restricted by the MTA's configuration or the access configuration for the network on which the MTA resides.
Many modern email client programs can act as an MTA when sending email. However, this action should not be confused with the role of a true MTA. The sole reason email client programs are capable of sending email like an MTA is because the host running the application does not have its own MTA. This is particularly true for email client programs on non-UNIX-based operating systems. However, these client programs only send outbound messages to an MTA they are authorized to use and do not directly deliver the message to the intended recipient's email server.
Since Red Hat Enterprise Linux offers two MTAs, Postfix and Sendmail, email client programs are often not required to act as an MTA. Red Hat Enterprise Linux also includes a special purpose MTA called Fetchmail.
For more information on Postfix, Sendmail, and Fetchmail, see Section 19.3, “Mail Transport Agents”.
19.2.2. Mail Delivery Agent
Link kopierenLink in die Zwischenablage kopiert!
A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user's mailbox. In many cases, the MDA is actually a Local Delivery Agent (LDA), such as
mail or Procmail.
Any program that actually handles a message for delivery to the point where it can be read by an email client application can be considered an MDA. For this reason, some MTAs (such as Sendmail and Postfix) can fill the role of an MDA when they append new email messages to a local user's mail spool file. In general, MDAs do not transport messages between systems nor do they provide a user interface; MDAs distribute and sort messages on the local machine for an email client application to access.
19.2.3. Mail User Agent
Link kopierenLink in die Zwischenablage kopiert!
A Mail User Agent (MUA) is synonymous with an email client application. An MUA is a program that, at a minimum, allows a user to read and compose email messages. Many MUAs are capable of retrieving messages via the
POP or IMAP protocols, setting up mailboxes to store messages, and sending outbound messages to an MTA.
MUAs may be graphical, such as Evolution, or have simple text-based interfaces, such as pine.
19.3. Mail Transport Agents
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux offers two primary MTAs: Postfix and Sendmail. Postfix is configured as the default MTA, although it is easy to switch the default MTA to Sendmail. To switch the default MTA to Sendmail, you can either uninstall Postfix or use the following command to switch to Sendmail:
alternatives --config mta
~]# alternatives --config mta
You can also use a command in the following format to enable or disable the desired service:
chkconfig service_name on | off
chkconfig service_name on | off19.3.1. Postfix
Link kopierenLink in die Zwischenablage kopiert!
Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.
To improve security, Postfix uses a modular design, where small processes with limited privileges are launched by a master daemon. The smaller, less privileged processes perform very specific tasks related to the various stages of mail delivery and run in a changed root environment to limit the effects of attacks.
Configuring Postfix to accept network connections from hosts other than the local computer takes only a few minor changes in its configuration file. Yet for those with more complex needs, Postfix provides a variety of configuration options, as well as third party add-ons that make it a very versatile and full-featured MTA.
The configuration files for Postfix are human readable and support upward of 250 directives. Unlike Sendmail, no macro processing is required for changes to take effect and the majority of the most commonly used options are described in the heavily commented files.
19.3.1.1. The Default Postfix Installation
Link kopierenLink in die Zwischenablage kopiert!
The Postfix executable is
/usr/sbin/postfix. This daemon launches all related processes needed to handle mail delivery.
Postfix stores its configuration files in the
/etc/postfix/ directory. The following is a list of the more commonly used files:
access— Used for access control, this file specifies which hosts are allowed to connect to Postfix.main.cf— The global Postfix configuration file. The majority of configuration options are specified in this file.master.cf— Specifies how Postfix interacts with various processes to accomplish mail delivery.transport— Maps email addresses to relay hosts.
The
aliases file can be found in the /etc/ directory. This file is shared between Postfix and Sendmail. It is a configurable list required by the mail protocol that describes user ID aliases.
Important
The default
/etc/postfix/main.cf file does not allow Postfix to accept network connections from a host other than the local computer. For instructions on configuring Postfix as a server for other clients, see Section 19.3.1.2, “Basic Postfix Configuration”.
Restart the
Copy to Clipboard
Copied!
Toggle word wrap
Toggle overflow
postfix service after changing any options in the configuration files under the /etc/postfix directory in order for those changes to take effect:
service postfix restart
~]# service postfix restart19.3.1.2. Basic Postfix Configuration
Link kopierenLink in die Zwischenablage kopiert!
By default, Postfix does not accept network connections from any host other than the local host. Perform the following steps as
root to enable mail delivery for other hosts on the network:
- Edit the
/etc/postfix/main.cffile with a text editor, such asvi. - Uncomment the
mydomainline by removing the hash sign (#), and replace domain.tld with the domain the mail server is servicing, such asexample.com. - Uncomment the
myorigin = $mydomainline. - Uncomment the
myhostnameline, and replace host.domain.tld with the host name for the machine. - Uncomment the
mydestination = $myhostname, localhost.$mydomainline. - Uncomment the
mynetworksline, and replace 168.100.189.0/28 with a valid network setting for hosts that can connect to the server. - Uncomment the
inet_interfaces = allline. - Comment the
inet_interfaces = localhostline. - Restart the
postfixservice.
Once these steps are complete, the host accepts outside emails for delivery.
Postfix has a large assortment of configuration options. One of the best ways to learn how to configure Postfix is to read the comments within the
/etc/postfix/main.cf configuration file. Additional resources including information about Postfix configuration, SpamAssassin integration, or detailed descriptions of the /etc/postfix/main.cf parameters are available online at http://www.postfix.org/.
19.3.1.2.1. Configuring Postfix to Use Transport Layer Security
Link kopierenLink in die Zwischenablage kopiert!
Configuring postfix to use transport layer security (TLS) is described in the Red Hat Knowledgebase solution How to configure postfix with TLS?
Important
Due to the vulnerability described in Resolution for POODLE SSL 3.0 vulnerability (CVE-2014-3566) in Postfix and Dovecot, Red Hat recommends disabling
SSL, if it is enabled, and using only TLSv1.1 or TLSv1.2. Backwards compatibility can be achieved using TLSv1.0. Many products Red Hat supports have the ability to use SSLv2 or SSLv3 protocols. However, the use of SSLv2 or SSLv3 is now strongly recommended against.
19.3.1.3. Using Postfix with LDAP
Link kopierenLink in die Zwischenablage kopiert!
Postfix can use an
LDAP directory as a source for various lookup tables (e.g.: aliases, virtual, canonical, etc.). This allows LDAP to store hierarchical user information and Postfix to only be given the result of LDAP queries when needed. By not storing this information locally, administrators can easily maintain it.
19.3.1.3.1. The /etc/aliases lookup example
Link kopierenLink in die Zwischenablage kopiert!
The following is a basic example for using
LDAP to look up the /etc/aliases file. Make sure your /etc/postfix/main.cf file contains the following:
alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf
alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf
Create a
/etc/postfix/ldap-aliases.cf file if you do not have one already and make sure it contains the following:
server_host = ldap.example.com search_base = dc=example, dc=com
server_host = ldap.example.com
search_base = dc=example, dc=com
where
ldap.example.com, example, and com are parameters that need to be replaced with specification of an existing available LDAP server.
Note
The
/etc/postfix/ldap-aliases.cf file can specify various parameters, including parameters that enable LDAP SSL and STARTTLS. For more information, see the ldap_table(5) man page.
For more information on
LDAP, see Section 20.1, “OpenLDAP”.
19.3.2. Sendmail
Link kopierenLink in die Zwischenablage kopiert!
Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using the
SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every aspect of how email is handled, including the protocol used. Many system administrators elect to use Sendmail as their MTA due to its power and scalability.
19.3.2.1. Purpose and Limitations
Link kopierenLink in die Zwischenablage kopiert!
It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only application needed to run an email server within an organization. Technically, this is true, as Sendmail can spool mail to each users' directory and deliver outbound mail for users. However, most users actually require much more than simple email delivery. Users usually want to interact with their email using an MUA, that uses
POP or IMAP, to download their messages to their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other applications can work in conjunction with Sendmail, but they actually exist for different reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With literally hundreds of different options and rule sets, entire volumes have been dedicated to helping explain everything that can be done and how to fix things that go wrong. See the Section 19.6, “Additional Resources” for a list of Sendmail resources.
This section reviews the files installed with Sendmail by default and reviews basic configuration changes, including how to stop unwanted email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).
19.3.2.2. The Default Sendmail Installation
Link kopierenLink in die Zwischenablage kopiert!
In order to use Sendmail, first ensure the sendmail package is installed on your system by running, as
root:
yum install sendmail
~]# yum install sendmail
In order to configure Sendmail, ensure the sendmail-cf package is installed on your system by running, as
root:
yum install sendmail-cf
~]# yum install sendmail-cf
For more information on installing packages with Yum, see Section 8.2.4, “Installing Packages”.
Before using Sendmail, the default MTA has to be switched from Postfix. For more information how to switch the default MTA see Section 19.3, “Mail Transport Agents”.
The Sendmail executable is
/usr/sbin/sendmail.
Sendmail's lengthy and detailed configuration file is
/etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly. To make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf file, and use the following alternatives to generate a new configuration file:
- Use the included makefile in
/etc/mail/to create a new/etc/mail/sendmail.cfconfiguration file:make all -C /etc/mail/
~]# make all -C /etc/mail/Copy to Clipboard Copied! Toggle word wrap Toggle overflow All other generated files in/etc/mail(db files) will be regenerated if needed. The old makemap commands are still usable. The make command is automatically used whenever you start or restart thesendmailservice. - Alternatively you may use the
m4macro processor to create a new/etc/mail/sendmail.cf. Them4macro processor is not installed by default. Before using it to create/etc/mail/sendmail.cf, install the m4 package as root:yum install m4
~]# yum install m4Copy to Clipboard Copied! Toggle word wrap Toggle overflow
More information on configuring Sendmail can be found in Section 19.3.2.3, “Common Sendmail Configuration Changes”.
Various Sendmail configuration files are installed in the
/etc/mail/ directory including:
access— Specifies which systems can use Sendmail for outbound email.domaintable— Specifies domain name mapping.local-host-names— Specifies aliases for the host.mailertable— Specifies instructions that override routing for particular domains.virtusertable— Specifies a domain-specific form of aliasing, allowing multiple virtual domains to be hosted on one machine.
Several of the configuration files in
/etc/mail/, such as access, domaintable, mailertable and virtusertable, must actually store their information in database files before Sendmail can use any configuration changes. To include any changes made to these configurations in their database files, run the following command, as root:
makemap hash /etc/mail/<name> < /etc/mail/<name>
~]# makemap hash /etc/mail/<name> < /etc/mail/<name>
where <name> represents the name of the configuration file to be updated. You may also restart the
sendmail service for the changes to take effect by running:
service sendmail restart
~]# service sendmail restart
For example, to have all emails addressed to the
example.com domain delivered to bob@other-example.com, add the following line to the virtusertable file:
@example.com bob@other-example.com
@example.com bob@other-example.com
To finalize the change, the
virtusertable.db file must be updated:
makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
~]# makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
Sendmail will create an updated
virtusertable.db file containing the new configuration.
19.3.2.3. Common Sendmail Configuration Changes
Link kopierenLink in die Zwischenablage kopiert!
When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an entirely new
/etc/mail/sendmail.cf file.
Warning
Before replacing or making any changes to the
sendmail.cf file, create a backup copy.
To add the desired functionality to Sendmail, edit the
/etc/mail/sendmail.mc file as root. Once you are finished, restart the sendmail service and, if the m4 package is installed, the m4 macro processor will automatically generate a new sendmail.cf configuration file:
service sendmail restart
~]# service sendmail restartImportant
The default
sendmail.cf file does not allow Sendmail to accept network connections from any host other than the local computer. To configure Sendmail as a server for other clients, edit the /etc/mail/sendmail.mc file, and either change the address specified in the Addr= option of the DAEMON_OPTIONS directive from 127.0.0.1 to the IP address of an active network device or comment out the DAEMON_OPTIONS directive all together by placing dnl at the beginning of the line. When finished, regenerate /etc/mail/sendmail.cf by restarting the service
service sendmail restart
~]# service sendmail restart
The default configuration in Red Hat Enterprise Linux works for most
SMTP-only sites. However, it does not work for UUCP (UNIX-to-UNIX Copy Protocol) sites. If using UUCP mail transfers, the /etc/mail/sendmail.mc file must be reconfigured and a new /etc/mail/sendmail.cf file must be generated.
Consult the
/usr/share/sendmail-cf/README file before editing any files in the directories under the /usr/share/sendmail-cf directory, as they can affect the future configuration of the /etc/mail/sendmail.cf file.
19.3.2.4. Masquerading
Link kopierenLink in die Zwischenablage kopiert!
One common Sendmail configuration is to have a single machine act as a mail gateway for all machines on the network. For example, a company may want to have a machine called
mail.example.com that handles all of their email and assigns a consistent return address to all outgoing mail.
In this situation, the Sendmail server must masquerade the machine names on the company network so that their return address is
user@example.com instead of user@host.example.com.
To do this, add the following lines to
/etc/mail/sendmail.mc:
After generating a new
sendmail.cf file using the m4 macro processor, this configuration makes all mail from inside the network appear as if it were sent from example.com.
19.3.2.5. Stopping Spam
Link kopierenLink in die Zwischenablage kopiert!
Email spam can be defined as unnecessary and unwanted email received by a user who never requested the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.
Sendmail makes it relatively easy to block new spamming techniques being employed to send junk email. It even blocks many of the more usual spamming methods by default. Main anti-spam features available in sendmail are header checks, relaying denial (default from version 8.9), access database and sender information checks.
For example, forwarding of
SMTP messages, also called relaying, has been disabled by default since Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host (x.edu) to accept messages from one party (y.com) and sent them to a different party (z.net). Now, however, Sendmail must be configured to permit any domain to relay mail through the server. To configure relay domains, edit the /etc/mail/relay-domains file and restart Sendmail
service sendmail restart
~]# service sendmail restart
However users can also be sent spam from from servers on the Internet. In these instances, Sendmail's access control features available through the
/etc/mail/access file can be used to prevent connections from unwanted hosts. The following example illustrates how this file can be used to both block and specifically allow access to the Sendmail server:
badspammer.com ERROR:550 "Go away and do not spam us anymore" tux.badspammer.com OK 10.0 RELAY
badspammer.com ERROR:550 "Go away and do not spam us anymore" tux.badspammer.com OK 10.0 RELAY
This example shows that any email sent from
badspammer.com is blocked with a 550 RFC-821 compliant error code, with a message sent back. Email sent from the tux.badspammer.com sub-domain, is accepted. The last line shows that any email sent from the 10.0.*.* network can be relayed through the mail server.
Because the
/etc/mail/access.db file is a database, use the makemap command to update any changes. Do this using the following command as root:
makemap hash /etc/mail/access < /etc/mail/access
~]# makemap hash /etc/mail/access < /etc/mail/access
Message header analysis allows you to reject mail based on header contents.
SMTP servers store information about an email's journey in the message header. As the message travels from one MTA to another, each puts in a Received header above all the other Received headers. It is important to note that this information may be altered by spammers.
The above examples only represent a small part of what Sendmail can do in terms of allowing or blocking access. See the
/usr/share/sendmail-cf/README file for more information and examples.
Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam filtering program, such as SpamAssassin, to identify and file spam for users. See Section 19.4.2.6, “Spam Filters” for more information about using SpamAssassin.
19.3.2.6. Using Sendmail with LDAP
Link kopierenLink in die Zwischenablage kopiert!
Using
LDAP is a very quick and powerful way to find specific information about a particular user from a much larger group. For example, an LDAP server can be used to look up a particular email address from a common corporate directory by the user's last name. In this kind of implementation, LDAP is largely separate from Sendmail, with LDAP storing the hierarchical user information and Sendmail only being given the result of LDAP queries in pre-addressed email messages.
However, Sendmail supports a much greater integration with
LDAP, where it uses LDAP to replace separately maintained files, such as /etc/aliases and /etc/mail/virtusertables, on different mail servers that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the mail routing level from Sendmail and its separate configuration files to a powerful LDAP cluster that can be leveraged by many different applications.
The current version of Sendmail contains support for
LDAP. To extend the Sendmail server using LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then edit the /etc/mail/sendmail.mc to include the following:
LDAPROUTE_DOMAIN('yourdomain.com')dnl
FEATURE('ldap_routing')dnl
LDAPROUTE_DOMAIN('yourdomain.com')dnl
FEATURE('ldap_routing')dnlNote
This is only for a very basic configuration of Sendmail with
LDAP. The configuration can differ greatly from this depending on the implementation of LDAP, especially when configuring several Sendmail machines to use a common LDAP server.
Consult
/usr/share/sendmail-cf/README for detailed LDAP routing configuration instructions and examples.
Next, recreate the
/etc/mail/sendmail.cf file by running the m4 macro processor and again restarting Sendmail. See Section 19.3.2.3, “Common Sendmail Configuration Changes” for instructions.
For more information on
LDAP, see Section 20.1, “OpenLDAP”.
19.3.3. Fetchmail
Link kopierenLink in die Zwischenablage kopiert!
Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA. Many users appreciate the ability to separate the process of downloading their messages located on a remote server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-up users in mind, Fetchmail connects and quickly downloads all of the email messages to the mail spool file using any number of protocols, including
POP3 and IMAP. It can even forward email messages to an SMTP server, if necessary.
Note
In order to use Fetchmail, first ensure the fetchmail package is installed on your system by running, as
root:
yum install fetchmail
~]# yum install fetchmail
For more information on installing packages with Yum, see Section 8.2.4, “Installing Packages”.
Fetchmail is configured for each user through the use of a
.fetchmailrc file in the user's home directory. If it does not already exist, create the .fetchmailrc file in your home directory
Using preferences in the
.fetchmailrc file, Fetchmail checks for email on a remote server and downloads it. It then delivers it to port 25 on the local machine, using the local MTA to place the email in the correct user's spool file. If Procmail is available, it is launched to filter the email and place it in a mailbox so that it can be read by an MUA.
19.3.3.1. Fetchmail Configuration Options
Link kopierenLink in die Zwischenablage kopiert!
Although it is possible to pass all necessary options on the command line to check for email on a remote server when executing Fetchmail, using a
.fetchmailrc file is much easier. Place any desired configuration options in the .fetchmailrc file for those options to be used each time the fetchmail command is issued. It is possible to override these at the time Fetchmail is run by specifying that option on the command line.
A user's
.fetchmailrc file contains three classes of configuration options:
- global options — Gives Fetchmail instructions that control the operation of the program or provide settings for every connection that checks for email.
- server options — Specifies necessary information about the server being polled, such as the host name, as well as preferences for specific email servers, such as the port to check or number of seconds to wait before timing out. These options affect every user using that server.
- user options — Contains information, such as user name and password, necessary to authenticate and check for email using a specified email server.
Global options appear at the top of the
.fetchmailrc file, followed by one or more server options, each of which designate a different email server that Fetchmail should check. User options follow server options for each user account checking that email server. Like server options, multiple user options may be specified for use with a particular server as well as to check multiple email accounts on the same server.
Server options are called into service in the
.fetchmailrc file by the use of a special option verb, poll or skip, that precedes any of the server information. The poll action tells Fetchmail to use this server option when it is run, which checks for email using the specified user options. Any server options after a skip action, however, are not checked unless this server's host name is specified when Fetchmail is invoked. The skip option is useful when testing configurations in the .fetchmailrc file because it only checks skipped servers when specifically invoked, and does not affect any currently working configurations.
The following is an example of a
.fetchmailrc file:
In this example, the global options specify that the user is sent email as a last resort (
postmaster option) and all email errors are sent to the postmaster instead of the sender (bouncemail option). The set action tells Fetchmail that this line contains a global option. Then, two email servers are specified, one set to check using POP3, the other for trying various protocols to find one that works. Two users are checked using the second server option, but all email found for any user is sent to user1's mail spool. This allows multiple mailboxes to be checked on multiple servers, while appearing in a single MUA inbox. Each user's specific information begins with the user action.
Note
Users are not required to place their password in the
.fetchmailrc file. Omitting the with password '<password>' section causes Fetchmail to ask for a password when it is launched.
Fetchmail has numerous global, server, and local options. Many of these options are rarely used or only apply to very specific situations. The
fetchmail man page explains each option in detail, but the most common ones are listed in the following three sections.
19.3.3.2. Global Options
Link kopierenLink in die Zwischenablage kopiert!
Each global option should be placed on a single line after a
set action.
daemon seconds— Specifies daemon-mode, where Fetchmail stays in the background. Replace seconds with the number of seconds Fetchmail is to wait before polling the server.postmaster— Specifies a local user to send mail to in case of delivery problems.syslog— Specifies the log file for errors and status messages. By default, this is/var/log/maillog.
19.3.3.3. Server Options
Link kopierenLink in die Zwischenablage kopiert!
Server options must be placed on their own line in
.fetchmailrc after a poll or skip action.
auth auth-type— Replace auth-type with the type of authentication to be used. By default,passwordauthentication is used, but some protocols support other types of authentication, includingkerberos_v5,kerberos_v4, andssh. If theanyauthentication type is used, Fetchmail first tries methods that do not require a password, then methods that mask the password, and finally attempts to send the password unencrypted to authenticate to the server.interval number— Polls the specified server everynumberof times that it checks for email on all configured servers. This option is generally used for email servers where the user rarely receives messages.port port-number— Replace port-number with the port number. This value overrides the default port number for the specified protocol.proto protocol— Replace protocol with the protocol, such aspop3orimap, to use when checking for messages on the server.timeout seconds— Replace seconds with the number of seconds of server inactivity after which Fetchmail gives up on a connection attempt. If this value is not set, a default of300seconds is used.
19.3.3.4. User Options
Link kopierenLink in die Zwischenablage kopiert!
User options may be placed on their own lines beneath a server option or on the same line as the server option. In either case, the defined options must follow the
user option (defined below).
fetchall— Orders Fetchmail to download all messages in the queue, including messages that have already been viewed. By default, Fetchmail only pulls down new messages.fetchlimit number— Replace number with the number of messages to be retrieved before stopping.flush— Deletes all previously viewed messages in the queue before retrieving new messages.limit max-number-bytes— Replace max-number-bytes with the maximum size in bytes that messages are allowed to be when retrieved by Fetchmail. This option is useful with slow network links, when a large message takes too long to download.password 'password'— Replace password with the user's password.preconnect "command"— Replace command with a command to be executed before retrieving messages for the user.postconnect "command"— Replace command with a command to be executed after retrieving messages for the user.ssl— Activates SSL encryption. At the time of writing, the default action is to use the best available fromSSL2,SSL3,SSL23,TLS1,TLS1.1andTLS1.2. Note thatSSL2is considered obsolete and due to the POODLE: SSLv3 vulnerability (CVE-2014-3566),SSLv3should not be used. However there is no way to force the use of TLS1 or newer, therefore ensure the mail server being connected to is configured not to useSSLv2andSSLv3. Usestunnelwhere the server cannot be configured not to useSSLv2andSSLv3.sslproto— Defines allowed SSL or TLS protocols. Possible values areSSL2,SSL3,SSL23, andTLS1. The default value, ifsslprotois omitted, unset, or set to an invalid value, isSSL23. The default action is to use the best fromSSLv3,TLSv1,TLS1.1andTLS1.2. Note that setting any other value for SSL or TLS will disable all the other protocols. Due to the POODLE: SSLv3 vulnerability (CVE-2014-3566), it is recommend to omit this option, or set it toSSLv23, and configure the corresponding mail server not to useSSLv2andSSLv3. Usestunnelwhere the server cannot be configured not to useSSLv2andSSLv3.user "username"— Replace username with the username used by Fetchmail to retrieve messages. This option must precede all other user options.
19.3.3.5. Fetchmail Command Options
Link kopierenLink in die Zwischenablage kopiert!
Most Fetchmail options used on the command line when executing the
fetchmail command mirror the .fetchmailrc configuration options. In this way, Fetchmail may be used with or without a configuration file. These options are not used on the command line by most users because it is easier to leave them in the .fetchmailrc file.
There may be times when it is desirable to run the
fetchmail command with other options for a particular purpose. It is possible to issue command options to temporarily override a .fetchmailrc setting that is causing an error, as any options specified at the command line override configuration file options.
19.3.3.6. Informational or Debugging Options
Link kopierenLink in die Zwischenablage kopiert!
Certain options used after the
fetchmail command can supply important information.
--configdump— Displays every possible option based on information from.fetchmailrcand Fetchmail defaults. No email is retrieved for any users when using this option.-s— Executes Fetchmail in silent mode, preventing any messages, other than errors, from appearing after thefetchmailcommand.-v— Executes Fetchmail in verbose mode, displaying every communication between Fetchmail and remote email servers.-V— Displays detailed version information, lists its global options, and shows settings to be used with each user, including the email protocol and authentication method. No email is retrieved for any users when using this option.
19.3.3.7. Special Options
Link kopierenLink in die Zwischenablage kopiert!
These options are occasionally useful for overriding defaults often found in the
.fetchmailrc file.
-a— Fetchmail downloads all messages from the remote email server, whether new or previously viewed. By default, Fetchmail only downloads new messages.-k— Fetchmail leaves the messages on the remote email server after downloading them. This option overrides the default behavior of deleting messages after downloading them.-l max-number-bytes— Fetchmail does not download any messages over a particular size and leaves them on the remote email server.--quit— Quits the Fetchmail daemon process.
More commands and
.fetchmailrc options can be found in the fetchmail man page.
19.3.4. Mail Transport Agent (MTA) Configuration
Link kopierenLink in die Zwischenablage kopiert!
A Mail Transport Agent (MTA) is essential for sending email. A Mail User Agent (MUA) such as Evolution, Thunderbird, or Mutt, is used to read and compose email. When a user sends an email from an MUA, the message is handed off to the MTA, which sends the message through a series of MTAs until it reaches its destination.
Even if a user does not plan to send email from the system, some automated tasks or system programs might use the
/bin/mail command to send email containing log messages to the root user of the local system.
Red Hat Enterprise Linux 6 provides two MTAs: Postfix and Sendmail. If both are installed, Postfix is the default MTA.
19.4. Mail Delivery Agents
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux includes two primary MDAs, Procmail and
mail. Both of the applications are considered LDAs and both move email from the MTA's spool file into the user's mailbox. However, Procmail provides a robust filtering system.
This section details only Procmail. For information on the
mail command, consult its man page (man mail).
Procmail delivers and filters email as it is placed in the mail spool file of the localhost. It is powerful, gentle on system resources, and widely used. Procmail can play a critical role in delivering email to be read by email client applications.
Procmail can be invoked in several different ways. Whenever an MTA places an email into the mail spool file, Procmail is launched. Procmail then filters and files the email for the MUA and quits. Alternatively, the MUA can be configured to execute Procmail any time a message is received so that messages are moved into their correct mailboxes. By default, the presence of
/etc/procmailrc or of a ~/.procmailrc file (also called an rc file) in the user's home directory invokes Procmail whenever an MTA receives a new message.
By default, no system-wide
rc files exist in the /etc/ directory and no .procmailrc files exist in any user's home directory. Therefore, to use Procmail, each user must construct a .procmailrc file with specific environment variables and rules.
Whether Procmail acts upon an email message depends upon whether the message matches a specified set of conditions or recipes in the
rc file. If a message matches a recipe, then the email is placed in a specified file, is deleted, or is otherwise processed.
When Procmail starts, it reads the email message and separates the body from the header information. Next, Procmail looks for a
/etc/procmailrc file and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Procmail then searches for a .procmailrc file in the user's home directory. Many users also create additional rc files for Procmail that are referred to within the .procmailrc file in their home directory.
19.4.1. Procmail Configuration
Link kopierenLink in die Zwischenablage kopiert!
The Procmail configuration file contains important environmental variables. These variables specify things such as which messages to sort and what to do with the messages that do not match any recipes.
These environmental variables usually appear at the beginning of the
~/.procmailrc file in the following format:
env-variable="value"
env-variable="value"
In this example,
env-variable is the name of the variable and value defines the variable.
There are many environment variables not used by most Procmail users and many of the more important environment variables are already defined by a default value. Most of the time, the following variables are used:
DEFAULT— Sets the default mailbox where messages that do not match any recipes are placed.The defaultDEFAULTvalue is the same as$ORGMAIL.INCLUDERC— Specifies additionalrcfiles containing more recipes for messages to be checked against. This breaks up the Procmail recipe lists into individual files that fulfill different roles, such as blocking spam and managing email lists, that can then be turned off or on by using comment characters in the user's~/.procmailrcfile.For example, lines in a user's~/.procmailrcfile may look like this:MAILDIR=$HOME/Msgs INCLUDERC=$MAILDIR/lists.rc INCLUDERC=$MAILDIR/spam.rc
MAILDIR=$HOME/Msgs INCLUDERC=$MAILDIR/lists.rc INCLUDERC=$MAILDIR/spam.rcCopy to Clipboard Copied! Toggle word wrap Toggle overflow To turn off Procmail filtering of email lists but leaving spam control in place, comment out the firstINCLUDERCline with a hash sign (#). Note that it uses paths relative to the current directory.LOCKSLEEP— Sets the amount of time, in seconds, between attempts by Procmail to use a particular lockfile. The default is8seconds.LOCKTIMEOUT— Sets the amount of time, in seconds, that must pass after a lockfile was last modified before Procmail assumes that the lockfile is old and can be deleted. The default is1024seconds.LOGFILE— The file to which any Procmail information or error messages are written.MAILDIR— Sets the current working directory for Procmail. If set, all other Procmail paths are relative to this directory.ORGMAIL— Specifies the original mailbox, or another place to put the messages if they cannot be placed in the default or recipe-required location.By default, a value of/var/spool/mail/$LOGNAMEis used.SUSPEND— Sets the amount of time, in seconds, that Procmail pauses if a necessary resource, such as swap space, is not available.SWITCHRC— Allows a user to specify an external file containing additional Procmail recipes, much like theINCLUDERCoption, except that recipe checking is actually stopped on the referring configuration file and only the recipes on theSWITCHRC-specified file are used.VERBOSE— Causes Procmail to log more information. This option is useful for debugging.
Other important environmental variables are pulled from the shell, such as
LOGNAME, the login name; HOME, the location of the home directory; and SHELL, the default shell.
A comprehensive explanation of all environments variables, and their default values, is available in the
procmailrc man page.
19.4.2. Procmail Recipes
Link kopierenLink in die Zwischenablage kopiert!
New users often find the construction of recipes the most difficult part of learning to use Procmail. This difficulty is often attributed to recipes matching messages by using regular expressions which are used to specify qualifications for string matching. However, regular expressions are not very difficult to construct and even less difficult to understand when read. Additionally, the consistency of the way Procmail recipes are written, regardless of regular expressions, makes it easy to learn by example. To see example Procmail recipes, see Section 19.4.2.5, “Recipe Examples”.
Procmail recipes take the following form:
The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed after the zero to control how Procmail processes the recipe. A colon after the
flags section specifies that a lockfile is created for this message. If a lockfile is created, the name can be specified by replacing lockfile-name.
A recipe can contain several conditions to match against the message. If it has no conditions, every message matches the recipe. Regular expressions are placed in some conditions to facilitate message matching. If multiple conditions are used, they must all match for the action to be performed. Conditions are checked based on the flags set in the recipe's first line. Optional special characters placed after the asterisk character (
*) can further control the condition.
The
action-to-perform argument specifies the action taken when the message matches one of the conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used here to direct matching messages into that file, effectively sorting the email. Special action characters may also be used before the action is specified. See Section 19.4.2.4, “Special Conditions and Actions” for more information.
19.4.2.1. Delivering vs. Non-Delivering Recipes
Link kopierenLink in die Zwischenablage kopiert!
The action used if the recipe matches a particular message determines whether it is considered a delivering or non-delivering recipe. A delivering recipe contains an action that writes the message to a file, sends the message to another program, or forwards the message to another email address. A non-delivering recipe covers any other actions, such as a nesting block. A nesting block is a set of actions, contained in braces
{ }, that are performed on messages which match the recipe's conditions. Nesting blocks can be nested inside one another, providing greater control for identifying and performing actions on messages.
When messages match a delivering recipe, Procmail performs the specified action and stops comparing the message against any other recipes. Messages that match non-delivering recipes continue to be compared against other recipes.
19.4.2.2. Flags
Link kopierenLink in die Zwischenablage kopiert!
Flags are essential to determine how or if a recipe's conditions are compared to a message. The egrep utility is used internally for matching of the conditions. The following flags are commonly used:
A— Specifies that this recipe is only used if the previous recipe without anAoraflag also matched this message.a— Specifies that this recipe is only used if the previous recipe with anAoraflag also matched this message and was successfully completed.B— Parses the body of the message and looks for matching conditions.b— Uses the body in any resulting action, such as writing the message to a file or forwarding it. This is the default behavior.c— Generates a carbon copy of the email. This is useful with delivering recipes, since the required action can be performed on the message and a copy of the message can continue being processed in thercfiles.D— Makes theegrepcomparison case-sensitive. By default, the comparison process is not case-sensitive.E— While similar to theAflag, the conditions in the recipe are only compared to the message if the immediately preceding recipe without anEflag did not match. This is comparable to an else action.e— The recipe is compared to the message only if the action specified in the immediately preceding recipe fails.f— Uses the pipe as a filter.H— Parses the header of the message and looks for matching conditions. This is the default behavior.h— Uses the header in a resulting action. This is the default behavior.w— Tells Procmail to wait for the specified filter or program to finish, and reports whether or not it was successful before considering the message filtered.W— Is identical towexcept that "Program failure" messages are suppressed.
For a detailed list of additional flags, see the
procmailrc man page.
19.4.2.3. Specifying a Local Lockfile
Link kopierenLink in die Zwischenablage kopiert!
Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a message simultaneously. Specify a local lockfile by placing a colon (
:) after any flags on a recipe's first line. This creates a local lockfile based on the destination file name plus whatever has been set in the LOCKEXT global environment variable.
Alternatively, specify the name of the local lockfile to be used with this recipe after the colon.
19.4.2.4. Special Conditions and Actions
Link kopierenLink in die Zwischenablage kopiert!
Special characters used before Procmail recipe conditions and actions change the way they are interpreted.
The following characters may be used after the asterisk character (
*) at the beginning of a recipe's condition line:
!— In the condition line, this character inverts the condition, causing a match to occur only if the condition does not match the message.<— Checks if the message is under a specified number of bytes.>— Checks if the message is over a specified number of bytes.
The following characters are used to perform special actions:
!— In the action line, this character tells Procmail to forward the message to the specified email addresses.$— Refers to a variable set earlier in thercfile. This is often used to set a common mailbox that is referred to by various recipes.|— Starts a specified program to process the message.{and}— Constructs a nesting block, used to contain additional recipes to apply to matching messages.
If no special character is used at the beginning of the action line, Procmail assumes that the action line is specifying the mailbox in which to write the message.
19.4.2.5. Recipe Examples
Link kopierenLink in die Zwischenablage kopiert!
Procmail is an extremely flexible program, but as a result of this flexibility, composing Procmail recipes from scratch can be difficult for new users.
The best way to develop the skills to build Procmail recipe conditions stems from a strong understanding of regular expressions combined with looking at many examples built by others. A thorough explanation of regular expressions is beyond the scope of this section. The structure of Procmail recipes and useful sample Procmail recipes can be found at various places on the Internet. The proper use and adaptation of regular expressions can be derived by viewing these recipe examples. In addition, introductory information about basic regular expression rules can be found in the
grep(1) man page.
The following simple examples demonstrate the basic structure of Procmail recipes and can provide the foundation for more intricate constructions.
A basic recipe may not even contain conditions, as is illustrated in the following example:
:0: new-mail.spool
:0:
new-mail.spool
The first line specifies that a local lockfile is to be created but does not specify a name, so Procmail uses the destination file name and appends the value specified in the
LOCKEXT environment variable. No condition is specified, so every message matches this recipe and is placed in the single spool file called new-mail.spool, located within the directory specified by the MAILDIR environment variable. An MUA can then view messages in this file.
A basic recipe, such as this, can be placed at the end of all
rc files to direct messages to a default location.
The following example matched messages from a specific email address and throws them away.
:0 * ^From: spammer@domain.com /dev/null
:0
* ^From: spammer@domain.com
/dev/null
With this example, any messages sent by
spammer@domain.com are sent to the /dev/null device, deleting them.
Warning
Be certain that rules are working as intended before sending messages to
/dev/null for permanent deletion. If a recipe inadvertently catches unintended messages, and those messages disappear, it becomes difficult to troubleshoot the rule.
A better solution is to point the recipe's action to a special mailbox, which can be checked from time to time to look for false positives. Once satisfied that no messages are accidentally being matched, delete the mailbox and direct the action to send the messages to
/dev/null.
The following recipe grabs email sent from a particular mailing list and places it in a specified folder.
:0: * ^(From|Cc|To).*tux-lug tuxlug
:0:
* ^(From|Cc|To).*tux-lug
tuxlug
Any messages sent from the
tux-lug@domain.com mailing list are placed in the tuxlug mailbox automatically for the MUA. Note that the condition in this example matches the message if it has the mailing list's email address on the From, Cc, or To lines.
Consult the many Procmail online resources available in Section 19.6, “Additional Resources” for more detailed and powerful recipes.
19.4.2.6. Spam Filters
Link kopierenLink in die Zwischenablage kopiert!
Because it is called by Sendmail, Postfix, and Fetchmail upon receiving new emails, Procmail can be used as a powerful tool for combating spam.
This is particularly true when Procmail is used in conjunction with SpamAssassin. When used together, these two applications can quickly identify spam emails, and sort or destroy them.
SpamAssassin uses header analysis, text analysis, blacklists, a spam-tracking database, and self-learning Bayesian spam analysis to quickly and accurately identify and tag spam.
Note
In order to use SpamAssassin, first ensure the spamassassin package is installed on your system by running, as
root:
yum install spamassassin
~]# yum install spamassassin
For more information on installing packages with Yum, see Section 8.2.4, “Installing Packages”.
The easiest way for a local user to use SpamAssassin is to place the following line near the top of the
~/.procmailrc file:
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc
The
/etc/mail/spamassassin/spamassassin-default.rc contains a simple Procmail rule that activates SpamAssassin for all incoming email. If an email is determined to be spam, it is tagged in the header as such and the title is prepended with the following pattern:
*****SPAM*****
*****SPAM*****
The message body of the email is also prepended with a running tally of what elements caused it to be diagnosed as spam.
To file email tagged as spam, a rule similar to the following can be used:
:0 Hw * ^X-Spam-Status: Yes spam
:0 Hw * ^X-Spam-Status: Yes spam
This rule files all email tagged in the header as spam into a mailbox called
spam.
Since SpamAssassin is a Perl script, it may be necessary on busy servers to use the binary SpamAssassin daemon (
spamd) and the client application (spamc). Configuring SpamAssassin this way, however, requires root access to the host.
To start the
spamd daemon, type the following command:
service spamassassin start
~]# service spamassassin start
To start the SpamAssassin daemon when the system is booted, use an initscript utility, such as the Services Configuration Tool (
system-config-services), to turn on the spamassassin service. See Chapter 12, Services and Daemons for more information about starting and stopping services.
To configure Procmail to use the SpamAssassin client application instead of the Perl script, place the following line near the top of the
~/.procmailrc file. For a system-wide configuration, place it in /etc/procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc19.5. Mail User Agents
Link kopierenLink in die Zwischenablage kopiert!
Red Hat Enterprise Linux offers a variety of email programs, both, graphical email client programs, such as Evolution, and text-based email programs such as
mutt.
The remainder of this section focuses on securing communication between a client and a server.
19.5.1. Securing Communication
Link kopierenLink in die Zwischenablage kopiert!
Popular MUAs included with Red Hat Enterprise Linux, such as Evolution and Mutt offer SSL-encrypted email sessions.
Like any other service that flows over a network unencrypted, important email information, such as user names, passwords, and entire messages, may be intercepted and viewed by users on the network. Additionally, since the standard
POP and IMAP protocols pass authentication information unencrypted, it is possible for an attacker to gain access to user accounts by collecting user names and passwords as they are passed over the network.
19.5.1.1. Secure Email Clients
Link kopierenLink in die Zwischenablage kopiert!
Most Linux MUAs designed to check email on remote servers support SSL encryption. To use SSL when retrieving email, it must be enabled on both the email client and the server.
SSL is easy to enable on the client-side, often done with the click of a button in the MUA's configuration window or via an option in the MUA's configuration file. Secure
IMAP and POP have known port numbers (993 and 995, respectively) that the MUA uses to authenticate and download messages.
19.5.1.2. Securing Email Client Communications
Link kopierenLink in die Zwischenablage kopiert!
Offering SSL encryption to
IMAP and POP users on the email server is a simple matter.
First, create an SSL certificate. This can be done in two ways: by applying to a Certificate Authority (CA) for an SSL certificate or by creating a self-signed certificate.
Warning
Self-signed certificates should be used for testing purposes only. Any server used in a production environment should use an SSL certificate granted by a CA.
To create a self-signed SSL certificate for
IMAP or POP, change to the /etc/pki/dovecot/ directory, edit the certificate parameters in the /etc/pki/dovecot/dovecot-openssl.cnf configuration file as you prefer, and type the following commands, as root:
dovecot]# rm -f certs/dovecot.pem private/dovecot.pem dovecot]# /usr/libexec/dovecot/mkcert.sh
dovecot]# rm -f certs/dovecot.pem private/dovecot.pem
dovecot]# /usr/libexec/dovecot/mkcert.sh
Once finished, make sure you have the following configurations in your
/etc/dovecot/conf.d/10-ssl.conf file:
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem ssl_key = </etc/pki/dovecot/private/dovecot.pem
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
Execute the
service dovecot restart command to restart the dovecot daemon.
Alternatively, the
stunnel command can be used as an encryption wrapper around the standard, non-secure connections to IMAP or POP services.
The
stunnel utility uses external OpenSSL libraries included with Red Hat Enterprise Linux to provide strong cryptography and to protect the network connections. It is recommended to apply to a CA to obtain an SSL certificate, but it is also possible to create a self-signed certificate.
See Using stunnel in the Red Hat Enterprise Linux 6 Security Guide for instructions on how to install
stunnel and create its basic configuration. To configure stunnel as a wrapper for IMAPS and POP3S, add the following lines to the /etc/stunnel/stunnel.conf configuration file:
The Security Guide also explains how to start and stop
stunnel. Once you start it, it is possible to use an IMAP or a POP email client and connect to the email server using SSL encryption.
19.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
The following is a list of additional documentation about email applications.
19.6.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
- Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.
/usr/share/sendmail-cf/README— Contains information on them4macro processor, file locations for Sendmail, supported mailers, how to access enhanced features, and more.
In addition, thesendmailandaliasesman pages contain helpful information covering various Sendmail options and the proper configuration of the Sendmail/etc/mail/aliasesfile. /usr/share/doc/postfix-version-number/— Contains a large amount of information on how to configure Postfix. Replace version-number with the version number of Postfix./usr/share/doc/fetchmail-version-number— Contains a full list of Fetchmail features in theFEATURESfile and an introductoryFAQdocument. Replace version-number with the version number of Fetchmail./usr/share/doc/procmail-version-number/— Contains aREADMEfile that provides an overview of Procmail, aFEATURESfile that explores every program feature, and anFAQfile with answers to many common configuration questions. Replace version-number with the version number of Procmail.When learning how Procmail works and creating new recipes, the following Procmail man pages are invaluable:procmail— Provides an overview of how Procmail works and the steps involved with filtering email.procmailrc— Explains thercfile format used to construct recipes.procmailex— Gives a number of useful, real-world examples of Procmail recipes.procmailsc— Explains the weighted scoring technique used by Procmail to match a particular recipe to a message./usr/share/doc/spamassassin-version-number/— Contains a large amount of information pertaining to SpamAssassin. Replace version-number with the version number of the spamassassin package.
19.6.2. Online Documentation
Link kopierenLink in die Zwischenablage kopiert!
- How to configure postfix with TLS? — A Red Hat Knowledgebase article that describes configuring postfix to use TLS.
- The Red Hat Knowledgebase article How to Configure a System to Manage Multiple Virtual Mailboxes Using Postfix and Dovecot describes managing multiple virtual users under one real-user account using Postfix as Mail Transporting Agent (MTA) and Dovecot as IMAP server.
- http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features, documentation and configuration examples.
- http://www.sendmail.com/ — Contains news, interviews and articles concerning Sendmail, including an expanded view of the many options available.
- http://www.postfix.org/ — The Postfix project home page contains a wealth of information about Postfix. The mailing list is a particularly good place to look for information.
- http://www.fetchmail.info/fetchmail-FAQ.html — A thorough FAQ about Fetchmail.
- http://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.
- http://www.spamassassin.org/ — The official site of the SpamAssassin project.
Chapter 20. Directory Servers
Link kopierenLink in die Zwischenablage kopiert!
20.1. OpenLDAP
Link kopierenLink in die Zwischenablage kopiert!
LDAP (Lightweight Directory Access Protocol) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to as “X.500 Lite”.
Like X.500, LDAP organizes information in a hierarchical manner using directories. These directories can store a variety of information such as names, addresses, or phone numbers, and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.
LDAP is commonly used for centrally managed users and groups, user authentication, or system configuration. It can also serve as a virtual phone directory, allowing users to easily access contact information for other users. Additionally, it can refer a user to other LDAP servers throughout the world, and thus provide an ad-hoc global repository of information. However, it is most frequently used within individual organizations such as universities, government departments, and private companies.
This section covers the installation and configuration of OpenLDAP 2.4, an open source implementation of the LDAPv2 and LDAPv3 protocols.
20.1.1. Introduction to LDAP
Link kopierenLink in die Zwischenablage kopiert!
Using a client-server architecture, LDAP provides a reliable means to create a central information directory accessible from the network. When a client attempts to modify information within this directory, the server verifies the user has permission to make the change, and then adds or updates the entry as requested. To ensure the communication is secure, the Transport Layer Security (TLS) cryptographic protocol can be used to prevent an attacker from intercepting the transmission.
Important
The OpenLDAP suite in Red Hat Enterprise Linux 6 no longer uses OpenSSL. Instead, it uses the Mozilla implementation of Network Security Services (NSS). OpenLDAP continues to work with existing certificates, keys, and other TLS configuration. For more information on how to configure it to use Mozilla certificate and key database, see How do I use TLS/SSL with Mozilla NSS.
Important
Due to the vulnerability described in Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings, Red Hat recommends that you do not rely on the
SSLv3 protocol for security. OpenLDAP is one of the system components that do not provide configuration parameters that allow SSLv3 to be effectively disabled. To mitigate the risk, it is recommended that you use the stunnel command to provide a secure tunnel, and disable stunnel from using SSLv3. For more information on using stunnel, see the Red Hat Enterprise Linux 6 Security Guide.
The LDAP server supports several database systems, which gives administrators the flexibility to choose the best suited solution for the type of information they are planning to serve. Because of a well-defined client Application Programming Interface (API), the number of applications able to communicate with an LDAP server is numerous, and increasing in both quantity and quality.
20.1.1.1. LDAP Terminology
Link kopierenLink in die Zwischenablage kopiert!
The following is a list of LDAP-specific terms that are used within this chapter:
- entry
- A single unit within an LDAP directory. Each entry is identified by its unique Distinguished Name (DN).
- attribute
- Information directly associated with an entry. For example, if an organization is represented as an LDAP entry, attributes associated with this organization might include an address, a fax number, etc. Similarly, people can be represented as entries with common attributes such as personal telephone number or email address.An attribute can either have a single value, or an unordered space-separated list of values. While certain attributes are optional, others are required. Required attributes are specified using the
objectClassdefinition, and can be found in schema files located in the/etc/openldap/slapd.d/cn=config/cn=schema/directory.The assertion of an attribute and its corresponding value is also referred to as a Relative Distinguished Name (RDN). Unlike distinguished names that are unique globally, a relative distinguished name is only unique per entry. - LDIF
- The LDAP Data Interchange Format (LDIF) is a plain text representation of an LDAP entry. It takes the following form:
[id] dn: distinguished_name attribute_type: attribute_value attribute_type: attribute_value ...
[id] dn: distinguished_name attribute_type: attribute_value attribute_type: attribute_value ...Copy to Clipboard Copied! Toggle word wrap Toggle overflow The optional id is a number determined by the application that is used to edit the entry. Each entry can contain as many attribute_type and attribute_value pairs as needed, as long as they are all defined in a corresponding schema file. A blank line indicates the end of an entry.
20.1.1.2. OpenLDAP Features
Link kopierenLink in die Zwischenablage kopiert!
OpenLDAP suite provides a number of important features:
- LDAPv3 Support — Many of the changes in the protocol since LDAP version 2 are designed to make LDAP more secure. Among other improvements, this includes the support for Simple Authentication and Security Layer (SASL), and Transport Layer Security (TLS) protocols.
- LDAP Over IPC — The use of inter-process communication (IPC) enhances security by eliminating the need to communicate over a network.
- IPv6 Support — OpenLDAP is compliant with Internet Protocol version 6 (IPv6), the next generation of the Internet Protocol.
- LDIFv1 Support — OpenLDAP is fully compliant with LDIF version 1.
- Updated C API — The current C API improves the way programmers can connect to and use LDAP directory servers.
- Enhanced Standalone LDAP Server — This includes an updated access control system, thread pooling, better tools, and much more.
20.1.1.3. OpenLDAP Server Setup
Link kopierenLink in die Zwischenablage kopiert!
The typical steps to set up an LDAP server on Red Hat Enterprise Linux are as follows:
- Install the OpenLDAP suite. See Section 20.1.2, “Installing the OpenLDAP Suite” for more information on required packages.
- Customize the configuration as described in Section 20.1.3, “Configuring an OpenLDAP Server”.
- Start the
slapdservice as described in Section 20.1.4, “Running an OpenLDAP Server”. - Use the
ldapaddutility to add entries to the LDAP directory. - Use the
ldapsearchutility to verify that theslapdservice is accessing the information correctly.
20.1.2. Installing the OpenLDAP Suite
Link kopierenLink in die Zwischenablage kopiert!
The suite of OpenLDAP libraries and tools is provided by the following packages:
| Package | Description |
|---|---|
| openldap | A package containing the libraries necessary to run the OpenLDAP server and client applications. |
| openldap-clients | A package containing the command-line utilities for viewing and modifying directories on an LDAP server. |
| openldap-servers | A package containing both the services and utilities to configure and run an LDAP server. This includes the Standalone LDAP Daemon, slapd. |
| compat-openldap | A package containing the OpenLDAP compatibility libraries. |
Additionally, the following packages are commonly used along with the LDAP server:
| Package | Description |
|---|---|
| sssd | A package containing the System Security Services Daemon (SSSD), a set of daemons to manage access to remote directories and authentication mechanisms. It provides the Name Service Switch (NSS) and the Pluggable Authentication Modules (PAM) interfaces toward the system and a pluggable back-end system to connect to multiple different account sources. |
| mod_authz_ldap |
A package containing
mod_authz_ldap, the LDAP authorization module for the Apache HTTP Server. This module uses the short form of the distinguished name for a subject and the issuer of the client SSL certificate to determine the distinguished name of the user within an LDAP directory. It is also capable of authorizing users based on attributes of that user's LDAP directory entry, determining access to assets based on the user and group privileges of the asset, and denying access for users with expired passwords. Note that the mod_ssl module is required when using the mod_authz_ldap module.
|
To install these packages, use the
yum command in the following form:
yum install package
yum install package
For example, to perform the basic LDAP server installation, type the following at a shell prompt:
yum install openldap openldap-clients openldap-servers
~]# yum install openldap openldap-clients openldap-servers
Note that you must have superuser privileges (that is, you must be logged in as
root) to run this command. For more information on how to install new packages in Red Hat Enterprise Linux, see Section 8.2.4, “Installing Packages”.
20.1.2.1. Overview of OpenLDAP Server Utilities
Link kopierenLink in die Zwischenablage kopiert!
To perform administrative tasks, the openldap-servers package installs the following utilities along with the
slapd service:
| Command | Description |
|---|---|
slapacl | Allows you to check the access to a list of attributes. |
slapadd | Allows you to add entries from an LDIF file to an LDAP directory. |
slapauth | Allows you to check a list of IDs for authentication and authorization permissions. |
slapcat | Allows you to pull entries from an LDAP directory in the default format and save them in an LDIF file. |
slapdn | Allows you to check a list of Distinguished Names (DNs) based on available schema syntax. |
slapindex | Allows you to re-index the slapd directory based on the current content. Run this utility whenever you change indexing options in the configuration file. |
slappasswd | Allows you to create an encrypted user password to be used with the ldapmodify utility, or in the slapd configuration file. |
slapschema | Allows you to check the compliance of a database with the corresponding schema. |
slaptest | Allows you to check the LDAP server configuration. |
For a detailed description of these utilities and their usage, see the corresponding manual pages as referred to in Section 20.1.6.1, “Installed Documentation”.
Important
Although only
root can run slapadd, the slapd service runs as the ldap user. Because of this, the directory server is unable to modify any files created by slapadd. To correct this issue, after running the slapd utility, type the following at a shell prompt:
chown -R ldap:ldap /var/lib/ldap
~]# chown -R ldap:ldap /var/lib/ldapWarning
To preserve the data integrity, stop the
slapd service before using slapadd, slapcat, or slapindex. You can do so by typing the following at a shell prompt:
service slapd stop
~]# service slapd stop
Stopping slapd: [ OK ]
For more information on how to start, stop, restart, and check the current status of the
slapd service, see Section 20.1.4, “Running an OpenLDAP Server”.
20.1.2.2. Overview of OpenLDAP Client Utilities
Link kopierenLink in die Zwischenablage kopiert!
The openldap-clients package installs the following utilities which can be used to add, modify, and delete entries in an LDAP directory:
| Command | Description |
|---|---|
ldapadd | Allows you to add entries to an LDAP directory, either from a file, or from standard input. It is a symbolic link to ldapmodify -a. |
ldapcompare | Allows you to compare given attribute with an LDAP directory entry. |
ldapdelete | Allows you to delete entries from an LDAP directory. |
ldapexop | Allows you to perform extended LDAP operations. |
ldapmodify | Allows you to modify entries in an LDAP directory, either from a file, or from standard input. |
ldapmodrdn | Allows you to modify the RDN value of an LDAP directory entry. |
ldappasswd | Allows you to set or change the password for an LDAP user. |
ldapsearch | Allows you to search LDAP directory entries. |
ldapurl | Allows you to compose or decompose LDAP URLs. |
ldapwhoami | Allows you to perform a whoami operation on an LDAP server. |
With the exception of
ldapsearch, each of these utilities is more easily used by referencing a file containing the changes to be made rather than typing a command for each entry to be changed within an LDAP directory. The format of such a file is outlined in the man page for each utility.
20.1.2.3. Overview of Common LDAP Client Applications
Link kopierenLink in die Zwischenablage kopiert!
Although there are various graphical LDAP clients capable of creating and modifying directories on the server, none of them is included in Red Hat Enterprise Linux. Popular applications that can access directories in a read-only mode include Mozilla Thunderbird, Evolution, or Ekiga.
20.1.3. Configuring an OpenLDAP Server
Link kopierenLink in die Zwischenablage kopiert!
By default, OpenLDAP stores its configuration in the
/etc/openldap/ directory. Table 20.5, “List of OpenLDAP configuration files and directories” highlights the most important files and directories within this directory.
In Red Hat Enterprise Linux 6, the
slapd service uses a configuration database located in the /etc/openldap/slapd.d/ directory and only reads the old /etc/openldap/slapd.conf configuration file if this directory does not exist. If you have an existing slapd.conf file from a previous installation, you can either wait for the openldap-servers package to convert it to the new format the next time you update this package, or type the following at a shell prompt as root to convert it immediately:
slaptest -f /etc/openldap/slapd.conf -F /etc/openldap/slapd.d/
~]# slaptest -f /etc/openldap/slapd.conf -F /etc/openldap/slapd.d/
The
slapd configuration consists of LDIF entries organized in a hierarchical directory structure, and the recommended way to edit these entries is to use the server utilities described in Section 20.1.2.1, “Overview of OpenLDAP Server Utilities”.
Important
An error in an LDIF file can render the
slapd service unable to start. Because of this, it is strongly advised that you avoid editing the LDIF files within the /etc/openldap/slapd.d/ directory directly.
20.1.3.1. Changing the Global Configuration
Link kopierenLink in die Zwischenablage kopiert!
Global configuration options for the LDAP server are stored in the
/etc/openldap/slapd.d/cn=config.ldif file. The following directives are commonly used:
-
olcAllows - The
olcAllowsdirective allows you to specify which features to enable. It takes the following form:olcAllows: feature
olcAllows: featureCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts a space-separated list of features as described in Table 20.6, “Available olcAllows options”. The default option isbind_v2.Expand Table 20.6. Available olcAllows options Option Description bind_v2Enables the acceptance of LDAP version 2 bind requests. bind_anon_credEnables an anonymous bind when the Distinguished Name (DN) is empty. bind_anon_dnEnables an anonymous bind when the Distinguished Name (DN) is not empty. update_anonEnables processing of anonymous update operations. proxy_authz_anonEnables processing of anonymous proxy authorization control. Example 20.1. Using the olcAllows directive
olcAllows: bind_v2 update_anon
olcAllows: bind_v2 update_anonCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcConnMaxPending - The
olcConnMaxPendingdirective allows you to specify the maximum number of pending requests for an anonymous session. It takes the following form:olcConnMaxPending: number
olcConnMaxPending: numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow The default option is100.Example 20.2. Using the olcConnMaxPending directive
olcConnMaxPending: 100
olcConnMaxPending: 100Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcConnMaxPendingAuth - The
olcConnMaxPendingAuthdirective allows you to specify the maximum number of pending requests for an authenticated session. It takes the following form:olcConnMaxPendingAuth: number
olcConnMaxPendingAuth: numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow The default option is1000.Example 20.3. Using the olcConnMaxPendingAuth directive
olcConnMaxPendingAuth: 1000
olcConnMaxPendingAuth: 1000Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcDisallows - The
olcDisallowsdirective allows you to specify which features to disable. It takes the following form:olcDisallows: feature
olcDisallows: featureCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts a space-separated list of features as described in Table 20.7, “Available olcDisallows options”. No features are disabled by default.Expand Table 20.7. Available olcDisallows options Option Description bind_anonDisables the acceptance of anonymous bind requests. bind_simpleDisables the simple bind authentication mechanism. tls_2_anonDisables the enforcing of an anonymous session when the STARTTLS command is received. tls_authcDisallows the STARTTLS command when authenticated. Example 20.4. Using the olcDisallows directive
olcDisallows: bind_anon
olcDisallows: bind_anonCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcIdleTimeout - The
olcIdleTimeoutdirective allows you to specify how many seconds to wait before closing an idle connection. It takes the following form:olcIdleTimeout: number
olcIdleTimeout: numberCopy to Clipboard Copied! Toggle word wrap Toggle overflow This option is disabled by default (that is, set to0).Example 20.5. Using the olcIdleTimeout directive
olcIdleTimeout: 180
olcIdleTimeout: 180Copy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcLogFile - The
olcLogFiledirective allows you to specify a file in which to write log messages. It takes the following form:olcLogFile: file_name
olcLogFile: file_nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow The log messages are written to standard error by default.Example 20.6. Using the olcLogFile directive
olcLogFile: /var/log/slapd.log
olcLogFile: /var/log/slapd.logCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcReferral - The
olcReferraloption allows you to specify a URL of a server to process the request in case the server is not able to handle it. It takes the following form:olcReferral: URL
olcReferral: URLCopy to Clipboard Copied! Toggle word wrap Toggle overflow This option is disabled by default.Example 20.7. Using the olcReferral directive
olcReferral: ldap://root.openldap.org
olcReferral: ldap://root.openldap.orgCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcWriteTimeout - The
olcWriteTimeoutoption allows you to specify how many seconds to wait before closing a connection with an outstanding write request. It takes the following form:olcWriteTimeout
olcWriteTimeoutCopy to Clipboard Copied! Toggle word wrap Toggle overflow This option is disabled by default (that is, set to0).Example 20.8. Using the olcWriteTimeout directive
olcWriteTimeout: 180
olcWriteTimeout: 180Copy to Clipboard Copied! Toggle word wrap Toggle overflow
20.1.3.2. Changing the Database-Specific Configuration
Link kopierenLink in die Zwischenablage kopiert!
By default, the OpenLDAP server uses Berkeley DB (BDB) as a database back end. The configuration for this database is stored in the
/etc/openldap/slapd.d/cn=config/olcDatabase={2}bdb.ldif file. The following directives are commonly used in a database-specific configuration:
-
olcReadOnly - The
olcReadOnlydirective allows you to use the database in a read-only mode. It takes the following form:olcReadOnly: boolean
olcReadOnly: booleanCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts eitherTRUE(enable the read-only mode), orFALSE(enable modifications of the database). The default option isFALSE.Example 20.9. Using the olcReadOnly directive
olcReadOnly: TRUE
olcReadOnly: TRUECopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcRootDN - The
olcRootDNdirective allows you to specify the user that is unrestricted by access controls or administrative limit parameters set for operations on the LDAP directory. It takes the following form:olcRootDN: distinguished_name
olcRootDN: distinguished_nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts a Distinguished Name (DN). The default option iscn=Manager,dc=my-domain,dc=com.Example 20.10. Using the olcRootDN directive
olcRootDN: cn=root,dc=example,dc=com
olcRootDN: cn=root,dc=example,dc=comCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcRootPW - The
olcRootPWdirective allows you to set a password for the user that is specified using theolcRootDNdirective. It takes the following form:olcRootPW: password
olcRootPW: passwordCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts either a plain text string, or a hash. To generate a hash, type the following at a shell prompt:slappaswd
~]$ slappaswd New password: Re-enter new password: {SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxDCopy to Clipboard Copied! Toggle word wrap Toggle overflow Example 20.11. Using the olcRootPW directive
olcRootPW: {SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxDolcRootPW: {SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxDCopy to Clipboard Copied! Toggle word wrap Toggle overflow -
olcSuffix - The
olcSuffixdirective allows you to specify the domain for which to provide information. It takes the following form:olcSuffix: domain_name
olcSuffix: domain_nameCopy to Clipboard Copied! Toggle word wrap Toggle overflow It accepts a fully qualified domain name (FQDN). The default option isdc=my-domain,dc=com.Example 20.12. Using the olcSuffix directive
olcSuffix: dc=example,dc=com
olcSuffix: dc=example,dc=comCopy to Clipboard Copied! Toggle word wrap Toggle overflow
20.1.3.3. Extending Schema
Link kopierenLink in die Zwischenablage kopiert!
Since OpenLDAP 2.3, the
/etc/openldap/slapd.d/cn=config/cn=schema/ directory also contains LDAP definitions that were previously located in /etc/openldap/schema/. It is possible to extend the schema used by OpenLDAP to support additional attribute types and object classes using the default schema files as a guide. However, this task is beyond the scope of this chapter. For more information on this topic, see http://www.openldap.org/doc/admin/schema.html.
20.1.4. Running an OpenLDAP Server
Link kopierenLink in die Zwischenablage kopiert!
This section describes how to start, stop, restart, and check the current status of the Standalone LDAP Daemon. For more information on how to manage system services in general, see Chapter 12, Services and Daemons.
20.1.4.1. Starting the Service
Link kopierenLink in die Zwischenablage kopiert!
To run the
slapd service, type the following at a shell prompt:
service slapd start
~]# service slapd start
Starting slapd: [ OK ]
If you want the service to start automatically at the boot time, use the following command:
chkconfig slapd on
~]# chkconfig slapd on
Note that you can also use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.
20.1.4.2. Stopping the Service
Link kopierenLink in die Zwischenablage kopiert!
To stop the running
slapd service, type the following at a shell prompt:
service slapd stop
~]# service slapd stop
Stopping slapd: [ OK ]
To prevent the service from starting automatically at the boot time, type:
chkconfig slapd off
~]# chkconfig slapd off
Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.
20.1.4.3. Restarting the Service
Link kopierenLink in die Zwischenablage kopiert!
To restart the running
slapd service, type the following at a shell prompt:
service slapd restart
~]# service slapd restart
Stopping slapd: [ OK ]
Starting slapd: [ OK ]
This stops the service, and then starts it again. Use this command to reload the configuration.
20.1.4.4. Checking the Service Status
Link kopierenLink in die Zwischenablage kopiert!
To check whether the service is running, type the following at a shell prompt:
service slapd status
~]# service slapd status
slapd (pid 3672) is running...20.1.5. Configuring a System to Authenticate Using OpenLDAP
Link kopierenLink in die Zwischenablage kopiert!
In order to configure a system to authenticate using OpenLDAP, make sure that the appropriate packages are installed on both LDAP server and client machines. For information on how to set up the server, follow the instructions in Section 20.1.2, “Installing the OpenLDAP Suite” and Section 20.1.3, “Configuring an OpenLDAP Server”. On a client, type the following at a shell prompt:
yum install openldap openldap-clients sssd
~]# yum install openldap openldap-clients sssd
Chapter 13, Configuring Authentication provides detailed instructions on how to configure applications to use LDAP for authentication.
20.1.5.1. Migrating Old Authentication Information to LDAP Format
Link kopierenLink in die Zwischenablage kopiert!
The migrationtools package provides a set of shell and Perl scripts to help you migrate authentication information into an LDAP format. To install this package, type the following at a shell prompt:
yum install migrationtools
~]# yum install migrationtools
This will install the scripts to the
/usr/share/migrationtools/ directory. Once installed, edit the /usr/share/migrationtools/migrate_common.ph file and change the following lines to reflect the correct domain, for example:
Default DNS domain Default base
# Default DNS domain
$DEFAULT_MAIL_DOMAIN = "example.com";
# Default base
$DEFAULT_BASE = "dc=example,dc=com";
Alternatively, you can specify the environment variables directly on the command line. For example, to run the
migrate_all_online.sh script with the default base set to dc=example,dc=com, type:
export DEFAULT_BASE="dc=example,dc=com" \ /usr/share/migrationtools/migrate_all_online.sh
~]# export DEFAULT_BASE="dc=example,dc=com" \
/usr/share/migrationtools/migrate_all_online.sh
To decide which script to run in order to migrate the user database, see Table 20.8, “Commonly used LDAP migration scripts”.
| Existing Name Service | Is LDAP Running? | Script to Use |
|---|---|---|
/etc flat files | yes | migrate_all_online.sh |
/etc flat files | no | migrate_all_offline.sh |
| NetInfo | yes | migrate_all_netinfo_online.sh |
| NetInfo | no | migrate_all_netinfo_offline.sh |
| NIS (YP) | yes | migrate_all_nis_online.sh |
| NIS (YP) | no | migrate_all_nis_offline.sh |
For more information on how to use these scripts, see the
README and the migration-tools.txt files in the /usr/share/doc/migrationtools-version/ directory.
20.1.6. Additional Resources
Link kopierenLink in die Zwischenablage kopiert!
The following resources offer additional information on the Lightweight Directory Access Protocol. Before configuring LDAP on your system, it is highly recommended that you review these resources, especially the OpenLDAP Software Administrator's Guide.
20.1.6.1. Installed Documentation
Link kopierenLink in die Zwischenablage kopiert!
The following documentation is installed with the openldap-servers package:
/usr/share/doc/openldap-servers-version/guide.html- A copy of the OpenLDAP Software Administrator's Guide.
/usr/share/doc/openldap-servers-version/README.schema- A README file containing the description of installed schema files.
Additionally, there is also a number of manual pages that are installed with the openldap, openldap-servers, and openldap-clients packages:
- Client Applications
man ldapadd— Describes how to add entries to an LDAP directory.man ldapdelete— Describes how to delete entries within an LDAP directory.man ldapmodify— Describes how to modify entries within an LDAP directory.man ldapsearch— Describes how to search for entries within an LDAP directory.man ldappasswd— Describes how to set or change the password of an LDAP user.man ldapcompare— Describes how to use theldapcomparetool.man ldapwhoami— Describes how to use theldapwhoamitool.man ldapmodrdn— Describes how to modify the RDNs of entries.
- Server Applications
man slapd— Describes command-line options for the LDAP server.
- Administrative Applications
man slapadd— Describes command-line options used to add entries to aslapddatabase.man slapcat— Describes command-line options used to generate an LDIF file from aslapddatabase.man slapindex— Describes command-line options used to regenerate an index based upon the contents of aslapddatabase.man slappasswd— Describes command-line options used to generate user passwords for LDAP directories.
- Configuration Files
man ldap.conf— Describes the format and options available within the configuration file for LDAP clients.man slapd-config— Describes the format and options available within the configuration directory.
20.1.6.2. Useful Websites
Link kopierenLink in die Zwischenablage kopiert!
- http://www.openldap.org/doc/admin24/
- The current version of the OpenLDAP Software Administrator's Guide.
Chapter 21. File and Print Servers
Link kopierenLink in die Zwischenablage kopiert!
21.1. Samba
Link kopierenLink in die Zwischenablage kopiert!
Samba is the standard open source Windows interoperability suite of programs for Linux. It implements the server message block (
SMB) protocol. Modern versions of this protocol are also known as the common Internet file system (CIFS) protocol. It allows the networking of Microsoft Windows®, Linux, UNIX, and other operating systems together, enabling access to Windows-based file and printer shares. Samba's use of SMB allows it to appear as a Windows server to Windows clients.
Note
In order to use Samba, first ensure the samba package is installed on your system by running the following command as
root:
yum install samba
~]# yum install samba
For more information on installing packages with Yum, see Section 8.2.4, “Installing Packages”.
21.1.1. Introduction to Samba
Link kopierenLink in die Zwischenablage kopiert!
Samba is an important component to seamlessly integrate Linux Servers and Desktops into Active Directory (AD) environments. It can function both as a domain controller (NT4-style) or as a regular domain member (AD or NT4-style).
What Samba can do:
- Serve directory trees and printers to Linux, UNIX, and Windows clients
- Assist in network browsing (with NetBIOS)
- Authenticate Windows domain logins
- Provide Windows Internet Name Service (
WINS) name server resolution - Act as a Windows NT®-style Primary Domain Controller (PDC)
- Act as a Backup Domain Controller (BDC) for a Samba-based PDC
- Act as an Active Directory domain member server
- Join a Windows NT/2000/2003/2008 PDC
What Samba cannot do:
- Act as a BDC for a Windows PDC (and vice versa)
- Act as an Active Directory domain controller
21.1.2. Samba Daemons and Related Services
Link kopierenLink in die Zwischenablage kopiert!
Samba is comprised of three daemons (
smbd, nmbd, and winbindd). Three services (smb, nmb, and winbind) control how the daemons are started, stopped, and other service-related features. These services act as different init scripts. Each daemon is listed in detail below, as well as which specific service has control over it.
smbd
The
smbd server daemon provides file sharing and printing services to Windows clients. In addition, it is responsible for user authentication, resource locking, and data sharing through the SMB protocol. The default ports on which the server listens for SMB traffic are TCP ports 139 and 445.
The
smbd daemon is controlled by the smb service.
nmbd
The
nmbd server daemon understands and replies to NetBIOS name service requests such as those produced by SMB/CIFS in Windows-based systems. These systems include Windows 95/98/ME, Windows NT, Windows 2000, Windows XP, and LanManager clients. It also participates in the browsing protocols that make up the Windows Network Neighborhood view. The default port that the server listens to for NMB traffic is UDP port 137.
The
nmbd daemon is controlled by the nmb service.
winbindd
The
winbind service resolves user and group information received from a server running Windows NT, 2000, 2003, Windows Server 2008, or Windows Server 2012. This makes Windows user and group information understandable by UNIX platforms. This is achieved by using Microsoft RPC calls, Pluggable Authentication Modules (PAM), and the Name Service Switch (NSS). This allows Windows NT domain and Active Directory users to appear and operate as UNIX users on a UNIX machine. Though bundled with the Samba distribution, the winbind service is controlled separately from the smb service.
The
winbind daemon is controlled by the winbind service and does not require the smb service to be started in order to operate. winbind is also used when Samba is an Active Directory member, and may also be used on a Samba domain controller (to implement nested groups and interdomain trust). Because winbind is a client-side service used to connect to Windows NT-based servers, further discussion of winbind is beyond the scope of this chapter.
For information on how to configure
winbind for authentication, see Section 13.1.2.3, “Configuring Winbind Authentication”.
Note
See Section 21.1.11, “Samba Distribution Programs” for a list of utilities included in the Samba distribution.
21.1.3. Connecting to a Samba Share
Link kopierenLink in die Zwischenablage kopiert!
You can use either Nautilus or command line to connect to available Samba shares.
Procedure 21.1. Connecting to a Samba Share Using Nautilus
- To view a list of Samba workgroups and domains on your network, select → from the GNOME panel, and then select the desired network. Alternatively, type
smb:in the → bar of Nautilus.As shown in Figure 21.1, “SMB Workgroups in Nautilus”, an icon appears for each availableSMBworkgroup or domain on the network.Figure 21.1. SMB Workgroups in Nautilus
- Double-click one of the workgroup or domain icon to view a list of computers within the workgroup or domain.
Figure 21.2. SMB Machines in Nautilus
- As displayed in Figure 21.2, “SMB Machines in Nautilus”, an icon exists for each machine within the workgroup. Double-click on an icon to view the Samba shares on the machine. If a user name and password combination is required, you are prompted for them.Alternately, you can also specify the Samba server and sharename in the Location: bar for Nautilus using the following syntax (replace servername and sharename with the appropriate values):
smb://servername/sharename
smb://servername/sharenameCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Procedure 21.2. Connecting to a Samba Share Using the Command Line
- To query the network for Samba servers, use the
findsmbcommand. For each server found, it displays itsIPaddress, NetBIOS name, workgroup name, operating system, andSMBserver version:findsmb
findsmbCopy to Clipboard Copied! Toggle word wrap Toggle overflow - To connect to a Samba share from a shell prompt, type the following command:
smbclient //hostname/sharename -U username
smbclient //hostname/sharename -U usernameCopy to Clipboard Copied! Toggle word wrap Toggle overflow Replace hostname with the host name orIPaddress of the Samba server you want to connect to, sharename with the name of the shared directory you want to browse, and username with the Samba user name for the system. Enter the correct password or press Enter if no password is required for the user.If you see thesmb:\>prompt, you have successfully logged in. Once you are logged in, typehelpfor a list of commands. If you want to browse the contents of your home directory, replace sharename with your user name. If the-Uswitch is not used, the user name of the current user is passed to the Samba server. - To exit
smbclient, typeexitat thesmb:\>prompt.
21.1.4. Configuring a Samba Server
Link kopierenLink in die Zwischenablage kopiert!
The default configuration file (
/etc/samba/smb.conf) allows users to view their home directories as a Samba share. It also shares all printers configured for the system as Samba shared printers. You can attach a printer to the system and print to it from the Windows machines on your network.
21.1.4.1. Graphical Configuration
Link kopierenLink in die Zwischenablage kopiert!
To configure Samba using a graphical interface, use one of the available Samba graphical user interfaces. A list of available GUIs can be found at http://www.samba.org/samba/GUI/.
21.1.4.2. Command-Line Configuration
Link kopierenLink in die Zwischenablage kopiert!
Samba uses
/etc/samba/smb.conf as its configuration file. If you change this configuration file, the changes do not take effect until you restart the Samba daemon with the following command as root:
service smb restart
~]# service smb restart
To specify the Windows workgroup and a brief description of the Samba server, edit the following lines in your
/etc/samba/smb.conf file:
workgroup = WORKGROUPNAME server string = BRIEF COMMENT ABOUT SERVER
workgroup = WORKGROUPNAME
server string = BRIEF COMMENT ABOUT SERVER
Replace WORKGROUPNAME with the name of the Windows workgroup to which this machine should belong. The BRIEF COMMENT ABOUT SERVER is optional and is used as the Windows comment about the Samba system.
To create a Samba share directory on your Linux system, add the following section to your
/etc/samba/smb.conf file (after modifying it to reflect your needs and your system):
Example 21.1. An Example Configuration of a Samba Server
The above example allows the users
tfox and carole to read and write to the directory /home/share/, on the Samba server, from a Samba client.
21.1.4.3. Encrypted Passwords
Link kopierenLink in die Zwischenablage kopiert!
Encrypted passwords are enabled by default because it is more secure to use them. To create a user with an encrypted password, use the
smbpasswd utility:
smbpasswd -a username
smbpasswd -a username21.1.5. Starting and Stopping Samba
Link kopierenLink in die Zwischenablage kopiert!
To start a Samba server, type the following command in a shell prompt, as
root:
service smb start
~]# service smb startImportant
To set up a domain member server, you must first join the domain or Active Directory using the
net join command before starting the smb service. Also it is recommended to run winbind before smbd.
To stop the server, type the following command in a shell prompt, as
root:
service smb stop
~]# service smb stop
The
restart option is a quick way of stopping and then starting Samba. This is the most reliable way to make configuration changes take effect after editing the configuration file for Samba. Note that the restart option starts the daemon even if it was not running originally.
To restart the server, type the following command in a shell prompt, as
root:
service smb restart
~]# service smb restart
The
condrestart (conditional restart) option only stops and starts smb on the condition that it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running.
Note
When the
/etc/samba/smb.conf file is changed, Samba automatically reloads it after a few minutes. Issuing a manual restart or reload is just as effective.
To conditionally restart the server, type the following command as
root:
service smb condrestart
~]# service smb condrestart
A manual reload of the
/etc/samba/smb.conf file can be useful in case of a failed automatic reload by the smb service. To ensure that the Samba server configuration file is reloaded without restarting the service, type the following command, as root:
service smb reload
~]# service smb reload
By default, the
smb service does not start automatically at boot time. To configure Samba to start at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program. See Chapter 12, Services and Daemons for more information regarding these tools.
21.1.6. Samba Server Types and the smb.conf File
Link kopierenLink in die Zwischenablage kopiert!
Samba configuration is straightforward. All modifications to Samba are done in the
/etc/samba/smb.conf configuration file. Although the default smb.conf file is well documented, it does not address complex topics such as LDAP, Active Directory, and the numerous domain controller implementations.
The following sections describe the different ways a Samba server can be configured. Keep in mind your needs and the changes required to the
/etc/samba/smb.conf file for a successful configuration.
21.1.6.1. Stand-alone Server
Link kopierenLink in die Zwischenablage kopiert!
A stand-alone server can be a workgroup server or a member of a workgroup environment. A stand-alone server is not a domain controller and does not participate in a domain in any way. The following examples include several user-level security configurations. For more information on security modes, see Section 21.1.7, “Samba Security Modes”.
Anonymous Read-Only
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement anonymous read-only file sharing. Two directives are used to configure anonymous access – map to guest = Bad user and guest account = nobody.
Example 21.2. An Example Configuration of a Anonymous Read-Only Samba Server
Anonymous Read/Write
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement anonymous read/write file sharing. To enable anonymous read/write file sharing, set the read only directive to no. The force user and force group directives are also added to enforce the ownership of any newly placed files specified in the share.
Note
Although having an anonymous read/write server is possible, it is not recommended. Any files placed in the share space, regardless of user, are assigned the user/group combination as specified by a generic user (
force user) and group (force group) in the /etc/samba/smb.conf file.
Example 21.3. An Example Configuration of a Anonymous Read/Write Samba Server
Anonymous Print Server
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement an anonymous print server. Setting browseable to no as shown does not list the printer in Windows Network Neighborhood. Although hidden from browsing, configuring the printer explicitly is possible. By connecting to DOCS_SRV using NetBIOS, the client can have access to the printer if the client is also part of the DOCS workgroup. It is also assumed that the client has the correct local printer driver installed, as the use client driver directive is set to yes. In this case, the Samba server has no responsibility for sharing printer drivers to the client.
Example 21.4. An Example Configuration of a Anonymous Print Samba Server
Secure Read/Write File and Print Server
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement a secure read/write file and print server. Setting the security directive to user forces Samba to authenticate client connections. Notice the [homes] share does not have a force user or force group directive as the [public] share does. The [homes] share uses the authenticated user details for any files created as opposed to the force user and force group in [public].
Example 21.5. An Example Configuration of a Secure Read/Write File and Print Samba Server
21.1.6.2. Domain Member Server
Link kopierenLink in die Zwischenablage kopiert!
A domain member, while similar to a stand-alone server, is logged into a domain controller (either Windows or Samba) and is subject to the domain's security rules. An example of a domain member server would be a departmental server running Samba that has a machine account on the Primary Domain Controller (PDC). All of the department's clients still authenticate with the PDC, and desktop profiles and all network policy files are included. The difference is that the departmental server has the ability to control printer and network shares.
Active Directory Domain Member Server
To implement an Active Directory domain member server, follow procedure below:
Procedure 21.3. Adding a Member Server to an Active Directory Domain
- Create the
/etc/samba/smb.confconfiguration file on a member server to be added to the Active Directory domain. Add the following lines to the configuration file:Copy to Clipboard Copied! Toggle word wrap Toggle overflow With the above configuration, Samba authenticates users for services being run locally but is also a client of the Active Directory. Ensure that your kerberosrealmparameter is shown in all caps (for examplerealm = EXAMPLE.COM). Since Windows 2000/2003/2008 requires Kerberos for Active Directory authentication, therealmdirective is required. If Active Directory and Kerberos are running on different servers, thepassword serverdirective is required to help the distinction. - Configure Kerberos on the member server. Create the
/etc/krb5.confconfiguration file with the following content:Copy to Clipboard Copied! Toggle word wrap Toggle overflow Uncomment the[realms]and[domain_realm]sections if DNS lookups are not working.For more information on Kerberos, and the/etc/krb5.conffile, see the Using Kerberos section of the Red Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards. - To join an Active Directory server, type the following command as
rooton the member server:net ads join -U administrator%password
~]# net ads join -U administrator%passwordCopy to Clipboard Copied! Toggle word wrap Toggle overflow Thenetcommand authenticates asAdministratorusing the NT LAN Manager (NTLM) protocol and creates the machine account. Thennetuses the machine account credentials to authenticate with Kerberos.Note
Sincesecurity = adsand notsecurity = useris used, a local password back end such assmbpasswdis not needed. Older clients that do not supportsecurity = adsare authenticated as ifsecurity = domainhad been set. This change does not affect functionality and allows local users not previously in the domain.
Windows NT4-based Domain Member Server
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement a Windows NT4-based domain member server. Becoming a member server of an NT4-based domain is similar to connecting to an Active Directory. The main difference is NT4-based domains do not use Kerberos in their authentication method, making the /etc/samba/smb.conf file simpler. In this instance, the Samba member server functions as a pass through to the NT4-based domain server.
Example 21.6. An Example Configuration of Samba Windows NT4-based Domain Member Server
Having Samba as a domain member server can be useful in many situations. There are times where the Samba server can have other uses besides file and printer sharing. It may be beneficial to make Samba a domain member server in instances where Linux-only applications are required for use in the domain environment. Administrators appreciate keeping track of all machines in the domain, even if not Windows-based. In the event the Windows-based server hardware is deprecated, it is quite easy to modify the
/etc/samba/smb.conf file to convert the server to a Samba-based PDC. If Windows NT-based servers are upgraded to Windows 2000/2003/2008 the /etc/samba/smb.conf file is easily modifiable to incorporate the infrastructure change to Active Directory if needed.
Important
After configuring the
/etc/samba/smb.conf file, join the domain before starting Samba by typing the following command as root:
net rpc join -U administrator%password
~]# net rpc join -U administrator%password
Note that the
-S option, which specifies the domain server host name, does not need to be stated in the net rpc join command. Samba uses the host name specified by the workgroup directive in the /etc/samba/smb.conf file instead of it being stated explicitly.
21.1.6.3. Domain Controller
Link kopierenLink in die Zwischenablage kopiert!
A domain controller in Windows NT is functionally similar to a Network Information Service (NIS) server in a Linux environment. Domain controllers and NIS servers both host user and group information databases as well as related services. Domain controllers are mainly used for security, including the authentication of users accessing domain resources. The service that maintains the user and group database integrity is called the Security Account Manager (SAM). The SAM database is stored differently between Windows and Linux Samba-based systems, therefore SAM replication cannot be achieved and platforms cannot be mixed in a PDC/BDC environment.
In a Samba environment, there can be only one PDC and zero or more BDCs.
Important
Samba cannot exist in a mixed Samba/Windows domain controller environment (Samba cannot be a BDC of a Windows PDC or vice versa). Alternatively, Samba PDCs and BDCs can coexist.
Primary Domain Controller (PDC) Using tdbsam
The simplest and most common implementation of a Samba PDC uses the new default
tdbsam password database back end. Replacing the aging smbpasswd back end, tdbsam has numerous improvements that are explained in more detail in Section 21.1.8, “Samba Account Information Databases”. The passdb backend directive controls which back end is to be used for the PDC.
The following
/etc/samba/smb.conf file shows a sample configuration needed to implement a tdbsam password database back end.
Example 21.7. An Example Configuration of Primary Domain Controller (PDC) Using tdbsam
To provide a functional PDC system which uses
tdbsam follow these steps:
- Adjust the
smb.confconfiguration file as shown in Example 21.7, “An Example Configuration of Primary Domain Controller (PDC) Usingtdbsam”. - Add the
rootuser to the Samba password database. You will be prompted to provide a new Samba password for therootuser:smbpasswd -a root
~]# smbpasswd -a root New SMB password:Copy to Clipboard Copied! Toggle word wrap Toggle overflow - Start the
smbservice:service smb start
~]# service smb startCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Make sure all profile, user, and netlogon directories are created.
- Add groups that users can be members of:
groupadd -f users groupadd -f nobody groupadd -f ntadmins
~]# groupadd -f users ~]# groupadd -f nobody ~]# groupadd -f ntadminsCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Associate the UNIX groups with their respective Windows groups.
net groupmap add ntgroup="Domain Users" unixgroup=users net groupmap add ntgroup="Domain Guests" unixgroup=nobody net groupmap add ntgroup="Domain Admins" unixgroup=ntadmins
~]# net groupmap add ntgroup="Domain Users" unixgroup=users ~]# net groupmap add ntgroup="Domain Guests" unixgroup=nobody ~]# net groupmap add ntgroup="Domain Admins" unixgroup=ntadminsCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Grant access rights to a user or a group. For example, to grant the right to add client machines to the domain on a Samba domain controller, to the members to the Domain Admins group, execute the following command:
net rpc rights grant 'DOCS\Domain Admins' SetMachineAccountPrivilege -S PDC -U root
~]# net rpc rights grant 'DOCS\Domain Admins' SetMachineAccountPrivilege -S PDC -U rootCopy to Clipboard Copied! Toggle word wrap Toggle overflow
Keep in mind that Windows systems prefer to have a primary group which is mapped to a domain group such as Domain Users.
Windows groups and users use the same namespace thus not allowing the existence of a group and a user with the same name like in UNIX.
Note
If you need more than one domain controller or have more than 250 users, do not use the
tdbsam authentication back end. LDAP is recommended in these cases.
Primary Domain Controller (PDC) with Active Directory
Although it is possible for Samba to be a member of an Active Directory, it is not possible for Samba to operate as an Active Directory domain controller.
21.1.7. Samba Security Modes
Link kopierenLink in die Zwischenablage kopiert!
There are only two types of security modes for Samba, share-level and user-level, which are collectively known as security levels. Share-level security is deprecated and Red Hat recommends to use user-level security instead. User-level security can be implemented in one of three different ways. The different ways of implementing a security level are called security modes.
21.1.7.1. User-Level Security
Link kopierenLink in die Zwischenablage kopiert!
User-level security is the default and recommended setting for Samba. Even if the
security = user directive is not listed in the /etc/samba/smb.conf file, it is used by Samba. If the server accepts the client's user name and password, the client can then mount multiple shares without specifying a password for each instance. Samba can also accept session-based user name and password requests. The client maintains multiple authentication contexts by using a unique UID for each logon.
In the
/etc/samba/smb.conf file, the security = user directive that sets user-level security is:
[GLOBAL] ... security = user ...
[GLOBAL]
...
security = user
...Samba Guest Shares
As mentioned above, share-level security mode is deprecated and highly recommended to not use. To configure a Samba guest share without using the
security = share parameter, follow the procedure below:
Procedure 21.4. Configuring Samba Guest Shares
- Create a username map file, in this example
/etc/samba/smbusers, and add the following line to it:nobody = guest
nobody = guestCopy to Clipboard Copied! Toggle word wrap Toggle overflow - Add the following directives to the main section in the
/etc/samba/smb.conffile. Also, do not use thevalid usersdirective:Copy to Clipboard Copied! Toggle word wrap Toggle overflow Theusername mapdirective provides a path to the username map file specified in the previous step. - Add the following directive to the share section in the
/ect/samba/smb.conffile. Do not use thevalid usersdirective.[SHARE] ... guest ok = yes ...
[SHARE] ... guest ok = yes ...Copy to Clipboard Copied! Toggle word wrap Toggle overflow
The following sections describe other implementations of user-level security.
Domain Security Mode (User-Level Security)
In domain security mode, the Samba server has a machine account (domain security trust account) and causes all authentication requests to be passed through to the domain controllers. The Samba server is made into a domain member server by using the following directives in the
/etc/samba/smb.conf file:
[GLOBAL] ... security = domain workgroup = MARKETING ...
[GLOBAL]
...
security = domain
workgroup = MARKETING
...Active Directory Security Mode (User-Level Security)
If you have an Active Directory environment, it is possible to join the domain as a native Active Directory member. Even if a security policy restricts the use of NT-compatible authentication protocols, the Samba server can join an ADS using Kerberos. Samba in Active Directory member mode can accept Kerberos tickets.
In the
/etc/samba/smb.conf file, the following directives make Samba an Active Directory member server:
21.1.8. Samba Account Information Databases
Link kopierenLink in die Zwischenablage kopiert!
The following is a list different back ends you can use with Samba. Other back ends not listed here may also be available.
- Plain Text
- Plain text back ends are nothing more than the
/etc/passwdtype back ends. With a plain text back end, all user names and passwords are sent unencrypted between the client and the Samba server. This method is very insecure and is not recommended for use by any means. It is possible that different Windows clients connecting to the Samba server with plain text passwords cannot support such an authentication method. smbpasswd- The
smbpasswdback end utilizes a plain ASCII text layout that includes the MS Windows LanMan and NT account, and encrypted password information. Thesmbpasswdback end lacks the storage of the Windows NT/2000/2003 SAM extended controls. Thesmbpasswdback end is not recommended because it does not scale well or hold any Windows information, such as RIDs for NT-based groups. Thetdbsamback end solves these issues for use in a smaller database (250 users), but is still not an enterprise-class solution. ldapsam_compat- The
ldapsam_compatback end allows continued OpenLDAP support for use with upgraded versions of Samba. tdbsam- The default
tdbsampassword back end provides a database back end for local servers, servers that do not need built-in database replication, and servers that do not require the scalability or complexity of LDAP. Thetdbsamback end includes all of thesmbpasswddatabase information as well as the previously-excluded SAM information. The inclusion of the extended SAM data allows Samba to implement the same account and system access controls as seen with Windows NT/2000/2003/2008-based systems.Thetdbsamback end is recommended for 250 users at most. Larger organizations should require Active Directory or LDAP integration due to scalability and possible network infrastructure concerns. ldapsam- The
ldapsamback end provides an optimal distributed account installation method for Samba. LDAP is optimal because of its ability to replicate its database to any number of servers such as the Red Hat Directory Server or an OpenLDAP Server. LDAP databases are light-weight and scalable, and as such are preferred by large enterprises. Installation and configuration of directory servers is beyond the scope of this chapter. For more information on the Red Hat Directory Server, see the Red Hat Directory Server 9.0 Deployment Guide. For more information on LDAP, see Section 20.1, “OpenLDAP”.If you are upgrading from a previous version of Samba to 3.0, note that the OpenLDAP schema file (/usr/share/doc/samba-version/LDAP/samba.schema) and the Red Hat Directory Server schema file (/usr/share/doc/samba-version/LDAP/samba-schema-FDS.ldif) have changed. These files contain the attribute syntax definitions and objectclass definitions that theldapsamback end needs in order to function properly.As such, if you are using theldapsamback end for your Samba server, you will need to configureslapdto include one of these schema file. See Section 20.1.3.3, “Extending Schema” for directions on how to do this.Note
You need to have the openldap-servers package installed if you want to use theldapsamback end. To ensure that the package is installed, execute the following command asroots:yum install openldap-servers
~]# yum install openldap-serversCopy to Clipboard Copied! Toggle word wrap Toggle overflow
21.1.9. Samba Network Browsing
Link kopierenLink in die Zwischenablage kopiert!
Network browsing enables Windows and Samba servers to appear in the Windows Network Neighborhood. Inside the Network Neighborhood, icons are represented as servers and if opened, the server's shares and printers that are available are displayed.
Network browsing capabilities require NetBIOS over
TCP/IP. NetBIOS-based networking uses broadcast (UDP) messaging to accomplish browse list management. Without NetBIOS and WINS as the primary method for TCP/IP host name resolution, other methods such as static files (/etc/hosts) or DNS, must be used.
A domain master browser collates the browse lists from local master browsers on all subnets so that browsing can occur between workgroups and subnets. Also, the domain master browser should preferably be the local master browser for its own subnet.
21.1.9.1. Domain Browsing
Link kopierenLink in die Zwischenablage kopiert!
By default, a Windows server PDC for a domain is also the domain master browser for that domain. A Samba server must not be set up as a domain master server in this type of situation.
For subnets that do not include the Windows server PDC, a Samba server can be implemented as a local master browser. Configuring the
/etc/samba/smb.conf file for a local master browser (or no browsing at all) in a domain controller environment is the same as workgroup configuration (see Section 21.1.4, “Configuring a Samba Server”).
21.1.9.2. WINS (Windows Internet Name Server)
Link kopierenLink in die Zwischenablage kopiert!
Either a Samba server or a Windows NT server can function as a WINS server. When a WINS server is used with NetBIOS enabled, UDP unicasts can be routed which allows name resolution across networks. Without a WINS server, the UDP broadcast is limited to the local subnet and therefore cannot be routed to other subnets, workgroups, or domains. If WINS replication is necessary, do not use Samba as your primary WINS server, as Samba does not currently support WINS replication.
In a mixed NT/2000/2003/2008 server and Samba environment, it is recommended that you use the Microsoft WINS capabilities. In a Samba-only environment, it is recommended that you use only one Samba server for WINS.
The following is an example of the
/etc/samba/smb.conf file in which the Samba server is serving as a WINS server:
Example 21.8. An Example Configuration of WINS Server
[global] wins support = yes
[global]
wins support = yesNote
All servers (including Samba) should connect to a WINS server to resolve NetBIOS names. Without WINS, browsing only occurs on the local subnet. Furthermore, even if a domain-wide list is somehow obtained, hosts cannot be resolved for the client without WINS.
21.1.10. Samba with CUPS Printing Support
Link kopierenLink in die Zwischenablage kopiert!
Samba allows client machines to share printers connected to the Samba server. In addition, Samba also allows client machines to send documents built in Linux to Windows printer shares. Although there are other printing systems that function with Red Hat Enterprise Linux, CUPS (Common UNIX Print System) is the recommended printing system due to its close integration with Samba.
21.1.10.1. Simple smb.conf Settings
Link kopierenLink in die Zwischenablage kopiert!
The following example shows a very basic
/etc/samba/smb.conf configuration for CUPS support:
Example 21.9. An Example Configuration of Samba with CUPS Support
Other printing configurations are also possible. To add additional security and privacy for printing confidential documents, users can have their own print spooler not located in a public path. If a job fails, other users would not have access to the file.
The
print$ directive contains printer drivers for clients to access if not available locally. The print$ directive is optional and may not be required depending on the organization.
Setting
browseable to yes enables the printer to be viewed in the Windows Network Neighborhood, provided the Samba server is set up correctly in the domain or workgroup.
21.1.11. Samba Distribution Programs
Link kopierenLink in die Zwischenablage kopiert!
findsmb
findsmb <subnet_broadcast_address>
findsmb <subnet_broadcast_address>