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Security Guide
Concepts and techniques to secure RHEL servers and workstations
Abstract
Chapter 1. Overview of Security Topics
Note
/lib
directory. When using 64-bit systems, some of the files mentioned may instead be located in /lib64
.
1.1. What is Computer Security?
1.1.1. Standardizing Security
- Confidentiality — Sensitive information must be available only to a set of pre-defined individuals. Unauthorized transmission and usage of information should be restricted. For example, confidentiality of information ensures that a customer's personal or financial information is not obtained by an unauthorized individual for malicious purposes such as identity theft or credit fraud.
- Integrity — Information should not be altered in ways that render it incomplete or incorrect. Unauthorized users should be restricted from the ability to modify or destroy sensitive information.
- Availability — Information should be accessible to authorized users any time that it is needed. Availability is a warranty that information can be obtained with an agreed-upon frequency and timeliness. This is often measured in terms of percentages and agreed to formally in Service Level Agreements (SLAs) used by network service providers and their enterprise clients.
1.1.2. Cryptographic Software and Certifications
1.2. Security Controls
- Physical
- Technical
- Administrative
1.2.1. Physical Controls
- Closed-circuit surveillance cameras
- Motion or thermal alarm systems
- Security guards
- Picture IDs
- Locked and dead-bolted steel doors
- Biometrics (includes fingerprint, voice, face, iris, handwriting, and other automated methods used to recognize individuals)
1.2.2. Technical Controls
- Encryption
- Smart cards
- Network authentication
- Access control lists (ACLs)
- File integrity auditing software
1.2.3. Administrative Controls
- Training and awareness
- Disaster preparedness and recovery plans
- Personnel recruitment and separation strategies
- Personnel registration and accounting
1.3. Vulnerability Assessment
- The expertise of the staff responsible for configuring, monitoring, and maintaining the technologies.
- The ability to patch and update services and kernels quickly and efficiently.
- The ability of those responsible to keep constant vigilance over the network.
1.3.1. Defining Assessment and Testing
Warning
- Creates proactive focus on information security.
- Finds potential exploits before crackers find them.
- Results in systems being kept up to date and patched.
- Promotes growth and aids in developing staff expertise.
- Abates financial loss and negative publicity.
1.3.2. Establishing a Methodology for Vulnerability Assessment
- https://www.owasp.org/ — The Open Web Application Security Project
1.3.3. Vulnerability Assessment Tools
README
file or man page for the tools. Additionally, look to the Internet for more information, such as articles, step-by-step guides, or even mailing lists specific to the tools.
1.3.3.1. Scanning Hosts with Nmap
yum install nmap
command as the root
user.
1.3.3.1.1. Using Nmap
nmap
command followed by the host name or IP
address of the machine to scan:
nmap
<hostname>
foo.example.com
, type the following at a shell prompt:
~]$ nmap foo.example.com
Interesting ports on foo.example.com: Not shown: 1710 filtered ports PORT STATE SERVICE 22/tcp open ssh 53/tcp open domain 80/tcp open http 113/tcp closed auth
1.3.3.2. Nessus
Note
1.3.3.3. OpenVAS
1.3.3.4. Nikto
1.4. Security Threats
1.4.1. Threats to Network Security
Insecure Architectures
Broadcast Networks
Centralized Servers
1.4.2. Threats to Server Security
Unused Services and Open Ports
Unpatched Services
Inattentive Administration
Inherently Insecure Services
1.4.3. Threats to Workstation and Home PC Security
Bad Passwords
Vulnerable Client Applications
1.5. Common Exploits and Attacks
Exploit | Description | Notes |
---|---|---|
Null or Default Passwords | Leaving administrative passwords blank or using a default password set by the product vendor. This is most common in hardware such as routers and firewalls, but some services that run on Linux can contain default administrator passwords as well (though Red Hat Enterprise Linux 7 does not ship with them). |
Commonly associated with networking hardware such as routers, firewalls, VPNs, and network attached storage (NAS) appliances.
Common in many legacy operating systems, especially those that bundle services (such as UNIX and Windows.)
Administrators sometimes create privileged user accounts in a rush and leave the password null, creating a perfect entry point for malicious users who discover the account.
|
Default Shared Keys | Secure services sometimes package default security keys for development or evaluation testing purposes. If these keys are left unchanged and are placed in a production environment on the Internet, all users with the same default keys have access to that shared-key resource, and any sensitive information that it contains. |
Most common in wireless access points and preconfigured secure server appliances.
|
IP Spoofing | A remote machine acts as a node on your local network, finds vulnerabilities with your servers, and installs a backdoor program or Trojan horse to gain control over your network resources. |
Spoofing is quite difficult as it involves the attacker predicting TCP/IP sequence numbers to coordinate a connection to target systems, but several tools are available to assist crackers in performing such a vulnerability.
Depends on target system running services (such as
rsh , telnet , FTP and others) that use source-based authentication techniques, which are not recommended when compared to PKI or other forms of encrypted authentication used in ssh or SSL/TLS.
|
Eavesdropping | Collecting data that passes between two active nodes on a network by eavesdropping on the connection between the two nodes. |
This type of attack works mostly with plain text transmission protocols such as Telnet, FTP, and HTTP transfers.
Remote attacker must have access to a compromised system on a LAN in order to perform such an attack; usually the cracker has used an active attack (such as IP spoofing or man-in-the-middle) to compromise a system on the LAN.
Preventative measures include services with cryptographic key exchange, one-time passwords, or encrypted authentication to prevent password snooping; strong encryption during transmission is also advised.
|
Service Vulnerabilities | An attacker finds a flaw or loophole in a service run over the Internet; through this vulnerability, the attacker compromises the entire system and any data that it may hold, and could possibly compromise other systems on the network. |
HTTP-based services such as CGI are vulnerable to remote command execution and even interactive shell access. Even if the HTTP service runs as a non-privileged user such as "nobody", information such as configuration files and network maps can be read, or the attacker can start a denial of service attack which drains system resources or renders it unavailable to other users.
Services sometimes can have vulnerabilities that go unnoticed during development and testing; these vulnerabilities (such as buffer overflows, where attackers crash a service using arbitrary values that fill the memory buffer of an application, giving the attacker an interactive command prompt from which they may execute arbitrary commands) can give complete administrative control to an attacker.
Administrators should make sure that services do not run as the root user, and should stay vigilant of patches and errata updates for applications from vendors or security organizations such as CERT and CVE.
|
Application Vulnerabilities | Attackers find faults in desktop and workstation applications (such as email clients) and execute arbitrary code, implant Trojan horses for future compromise, or crash systems. Further exploitation can occur if the compromised workstation has administrative privileges on the rest of the network. |
Workstations and desktops are more prone to exploitation as workers do not have the expertise or experience to prevent or detect a compromise; it is imperative to inform individuals of the risks they are taking when they install unauthorized software or open unsolicited email attachments.
Safeguards can be implemented such that email client software does not automatically open or execute attachments. Additionally, the automatic update of workstation software using Red Hat Network; or other system management services can alleviate the burdens of multi-seat security deployments.
|
Denial of Service (DoS) Attacks | Attacker or group of attackers coordinate against an organization's network or server resources by sending unauthorized packets to the target host (either server, router, or workstation). This forces the resource to become unavailable to legitimate users. |
The most reported DoS case in the US occurred in 2000. Several highly-trafficked commercial and government sites were rendered unavailable by a coordinated ping flood attack using several compromised systems with high bandwidth connections acting as zombies, or redirected broadcast nodes.
Source packets are usually forged (as well as rebroadcast), making investigation as to the true source of the attack difficult.
Advances in ingress filtering (IETF rfc2267) using
iptables and Network Intrusion Detection Systems such as snort assist administrators in tracking down and preventing distributed DoS attacks.
|
Chapter 2. Security Tips for Installation
2.1. Securing BIOS
2.1.1. BIOS Passwords
- Preventing Changes to BIOS Settings — If an intruder has access to the BIOS, they can set it to boot from a CD-ROM or a flash drive. This makes it possible for them to enter rescue mode or single user mode, which in turn allows them to start arbitrary processes on the system or copy sensitive data.
- Preventing System Booting — Some BIOSes allow password protection of the boot process. When activated, an attacker is forced to enter a password before the BIOS launches the boot loader.
2.1.1.1. Securing Non-BIOS-based Systems
2.2. Partitioning the Disk
/boot
, /
, /home
, /tmp
, and /var/tmp/
directories. The reasons for each are different, and we will address each partition.
/boot
- This partition is the first partition that is read by the system during boot up. The boot loader and kernel images that are used to boot your system into Red Hat Enterprise Linux 7 are stored in this partition. This partition should not be encrypted. If this partition is included in / and that partition is encrypted or otherwise becomes unavailable then your system will not be able to boot.
/home
- When user data (
/home
) is stored in/
instead of in a separate partition, the partition can fill up causing the operating system to become unstable. Also, when upgrading your system to the next version of Red Hat Enterprise Linux 7 it is a lot easier when you can keep your data in the/home
partition as it will not be overwritten during installation. If the root partition (/
) becomes corrupt your data could be lost forever. By using a separate partition there is slightly more protection against data loss. You can also target this partition for frequent backups. /tmp
and/var/tmp/
- Both the
/tmp
and/var/tmp/
directories are used to store data that does not need to be stored for a long period of time. However, if a lot of data floods one of these directories it can consume all of your storage space. If this happens and these directories are stored within/
then your system could become unstable and crash. For this reason, moving these directories into their own partitions is a good idea.
Note
2.3. Installing the Minimum Amount of Packages Required
Minimal install
environment, see the Software Selection chapter of the Red Hat Enterprise Linux 7 Installation Guide. A minimal installation can also be performed by a Kickstart file using the --nobase
option. For more information about Kickstart installations, see the Package Selection section from the Red Hat Enterprise Linux 7 Installation Guide.
2.4. Restricting Network Connectivity During the Installation Process
2.5. Post-installation Procedures
- Update your system. enter the following command as root:
~]#
yum update
- Even though the firewall service,
firewalld
, is automatically enabled with the installation of Red Hat Enterprise Linux, there are scenarios where it might be explicitly disabled, for example in the kickstart configuration. In such a case, it is recommended to consider re-enabling the firewall.To startfirewalld
enter the following commands as root:~]#
systemctl start firewalld
~]#systemctl enable firewalld
- To enhance security, disable services you do not need. For example, if there are no printers installed on your computer, disable the
cups
service using the following command:~]#
systemctl disable cups
To review active services, enter the following command:~]$
systemctl list-units | grep service
2.6. Additional Resources
Chapter 3. Keeping Your System Up-to-Date
3.1. Maintaining Installed Software
3.1.1. Planning and Configuring Security Updates
3.1.1.1. Using the Security Features of Yum
root
:
~]# yum check-update --security
Loaded plugins: langpacks, product-id, subscription-manager
rhel-7-workstation-rpms/x86_64 | 3.4 kB 00:00:00
No packages needed for security; 0 packages available
~]# yum update --security
updateinfo
subcommand to display or act upon information provided by repositories about available updates. The updateinfo
subcommand itself accepts a number of commands, some of which pertain to security-related uses. See Table 3.1, “Security-related commands usable with yum updateinfo” for an overview of these commands.
Command | Description | |
---|---|---|
advisory [advisories] | Displays information about one or more advisories. Replace advisories with an advisory number or numbers. | |
cves | Displays the subset of information that pertains to CVE (Common Vulnerabilities and Exposures). | |
security or sec | Displays all security-related information. | |
severity [severity_level] or sev [severity_level] | Displays information about security-relevant packages of the supplied severity_level. |
3.1.2. Updating and Installing Packages
3.1.2.1. Verifying Signed Packages
gpgcheck
configuration directive is set to 1
in the /etc/yum.conf
configuration file.
rpmkeys --checksig package_file.rpm
3.1.2.2. Installing Signed Packages
yum install
command as the root
user as follows:
yum install package_file.rpm
.rpm
packages in the current directory:
yum install *.rpm
Important
3.1.3. Applying Changes Introduced by Installed Updates
Note
- Applications
- User-space applications are any programs that can be initiated by the user. Typically, such applications are used only when the user, a script, or an automated task utility launch them.Once such a user-space application is updated, halt any instances of the application on the system, and launch the program again to use the updated version.
- Kernel
- The kernel is the core software component for the Red Hat Enterprise Linux 7 operating system. It manages access to memory, the processor, and peripherals, and it schedules all tasks.Because of its central role, the kernel cannot be restarted without also rebooting the computer. Therefore, an updated version of the kernel cannot be used until the system is rebooted.
- KVM
- When the qemu-kvm and libvirt packages are updated, it is necessary to stop all guest virtual machines, reload relevant virtualization modules (or reboot the host system), and restart the virtual machines.Use the
lsmod
command to determine which modules from the following are loaded:kvm
,kvm-intel
, orkvm-amd
. Then use themodprobe -r
command to remove and subsequently themodprobe -a
command to reload the affected modules. Fox example:~]#
lsmod | grep kvm
kvm_intel 143031 0 kvm 460181 1 kvm_intel ~]#modprobe -r kvm-intel
~]#modprobe -r kvm
~]#modprobe -a kvm kvm-intel
- Shared Libraries
- Shared libraries are units of code, such as
glibc
, that are used by a number of applications and services. Applications utilizing a shared library typically load the shared code when the application is initialized, so any applications using an updated library must be halted and relaunched.To determine which running applications link against a particular library, use thelsof
command:lsof library
For example, to determine which running applications link against thelibwrap.so.0
library, type:~]#
lsof /lib64/libwrap.so.0
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME pulseaudi 12363 test mem REG 253,0 42520 34121785 /usr/lib64/libwrap.so.0.7.6 gnome-set 12365 test mem REG 253,0 42520 34121785 /usr/lib64/libwrap.so.0.7.6 gnome-she 12454 test mem REG 253,0 42520 34121785 /usr/lib64/libwrap.so.0.7.6This command returns a list of all the running programs that useTCP
wrappers for host-access control. Therefore, any program listed must be halted and relaunched when the tcp_wrappers package is updated. - systemd Services
- systemd services are persistent server programs usually launched during the boot process. Examples of systemd services include
sshd
orvsftpd
.Because these programs usually persist in memory as long as a machine is running, each updated systemd service must be halted and relaunched after its package is upgraded. This can be done as theroot
user using thesystemctl
command:systemctl restart service_name
Replace service_name with the name of the service you want to restart, such assshd
. - Other Software
- Follow the instructions outlined by the resources linked below to correctly update the following applications.
- Red Hat Directory Server — See the Release Notes for the version of the Red Hat Directory Server in question at https://access.redhat.com/documentation/en-US/Red_Hat_Directory_Server/.
- Red Hat Enterprise Virtualization Manager — See the Installation Guide for the version of the Red Hat Enterprise Virtualization in question at https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Virtualization/.
3.2. Using the Red Hat Customer Portal
3.2.1. Viewing Security Advisories on the Customer Portal
3.2.3. Understanding Issue Severity Classification
3.3. Additional Resources
Installed Documentation
- yum(8) — The manual page for the Yum package manager provides information about the way Yum can be used to install, update, and remove packages on your systems.
- rpmkeys(8) — The manual page for the
rpmkeys
utility describes the way this program can be used to verify the authenticity of downloaded packages.
Online Documentation
- Red Hat Enterprise Linux 7 System Administrator's Guide — The System Administrator's Guide for Red Hat Enterprise Linux 7 documents the use of the
Yum
andrpm
commands that are used to install, update, and remove packages on Red Hat Enterprise Linux 7 systems. - Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide — The SELinux User's and Administrator's Guide for Red Hat Enterprise Linux 7 documents the configuration of the SELinux mandatory access control mechanism.
Red Hat Customer Portal
- Red Hat Customer Portal, Security — The Security section of the Customer Portal contains links to the most important resources, including the Red Hat CVE database, and contacts for Red Hat Product Security.
- Red Hat Security Blog — Articles about latest security-related issues from Red Hat security professionals.
See Also
- Chapter 2, Security Tips for Installation describes how to configure your system securely from the beginning to make it easier to implement additional security settings later.
- Section 4.9.2, “Creating GPG Keys” describes how to create a set of personal GPG keys to authenticate your communications.
Chapter 4. Hardening Your System with Tools and Services
4.1. Desktop Security
4.1.1. Password Security
/etc/passwd
file, which makes the system vulnerable to offline password cracking attacks. If an intruder can gain access to the machine as a regular user, he can copy the /etc/passwd
file to his own machine and run any number of password cracking programs against it. If there is an insecure password in the file, it is only a matter of time before the password cracker discovers it.
/etc/shadow
, which is readable only by the root user.
Note
4.1.1.1. Creating Strong Passwords
randomword1 randomword2 randomword3 randomword4
1!
". Note that such a modification does not increase the security of the passphrase significantly.
/dev/urandom
. The minimum number of bits you can specify is 56, which is enough for passwords on systems and services where brute force attacks are rare. 64 bits is adequate for applications where the attacker does not have direct access to the password hash file. For situations when the attacker might obtain the direct access to the password hash or the password is used as an encryption key, 80 to 128 bits should be used. If you specify an invalid number of entropy bits, pwmake will use the default of bits. To create a password of 128 bits, enter the following command:
pwmake 128
- Using a single dictionary word, a word in a foreign language, an inverted word, or only numbers.
- Using less than 10 characters for a password or passphrase.
- Using a sequence of keys from the keyboard layout.
- Writing down your passwords.
- Using personal information in a password, such as birth dates, anniversaries, family member names, or pet names.
- Using the same passphrase or password on multiple machines.
4.1.1.2. Forcing Strong Passwords
passwd
command-line utility, which is PAM-aware (Pluggable Authentication Modules) and checks to see if the password is too short or otherwise easy to crack. This checking is performed by the pam_pwquality.so
PAM module.
Note
pam_pwquality
PAM module replaced pam_cracklib
, which was used in Red Hat Enterprise Linux 6 as a default module for password quality checking. It uses the same back end as pam_cracklib
.
pam_pwquality
module is used to check a password's strength against a set of rules. Its procedure consists of two steps: first it checks if the provided password is found in a dictionary. If not, it continues with a number of additional checks. pam_pwquality
is stacked alongside other PAM modules in the password
component of the /etc/pam.d/passwd
file, and the custom set of rules is specified in the /etc/security/pwquality.conf
configuration file. For a complete list of these checks, see the pwquality.conf (8)
manual page.
Example 4.1. Configuring password strength-checking in pwquality.conf
pam_quality
, add the following line to the password
stack in the /etc/pam.d/passwd
file:
password required pam_pwquality.so retry=3
/etc/security/pwquality.conf
file:
minlen = 8 minclass = 4
/etc/security/pwquality.conf
:
maxsequence = 3 maxrepeat = 3
abcd
, and more than 3 identical consecutive characters, such as 1111
.
Note
4.1.1.3. Configuring Password Aging
chage
command.
Important
-M
option of the chage
command specifies the maximum number of days the password is valid. For example, to set a user's password to expire in 90 days, use the following command:
chage
-M 90
username
-1
after the -M
option.
chage
command, see the table below.
Option | Description |
---|---|
-d days | Specifies the number of days since January 1, 1970 the password was changed. |
-E date | Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used. |
-I days | Specifies the number of inactive days after the password expiration before locking the account. If the value is 0 , the account is not locked after the password expires. |
-l | Lists current account aging settings. |
-m days | Specify the minimum number of days after which the user must change passwords. If the value is 0 , the password does not expire. |
-M days | Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account. |
-W days | Specifies the number of days before the password expiration date to warn the user. |
chage
command in interactive mode to modify multiple password aging and account details. Use the following command to enter interactive mode:
chage
<username>
~]#chage juan
Changing the aging information for juan Enter the new value, or press ENTER for the default Minimum Password Age [0]:10
Maximum Password Age [99999]:90
Last Password Change (YYYY-MM-DD) [2006-08-18]: Password Expiration Warning [7]: Password Inactive [-1]: Account Expiration Date (YYYY-MM-DD) [1969-12-31]:
- Set up an initial password. To assign a default password, enter the following command at a shell prompt as
root
:passwd
usernameWarning
Thepasswd
utility has the option to set a null password. Using a null password, while convenient, is a highly insecure practice, as any third party can log in and access the system using the insecure user name. Avoid using null passwords wherever possible. If it is not possible, always make sure that the user is ready to log in before unlocking an account with a null password. - Force immediate password expiration by running the following command as
root
:chage
-d
0
usernameThis command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
4.1.2. Account Locking
pam_faillock
PAM module allows system administrators to lock out user accounts after a specified number of failed attempts. Limiting user login attempts serves mainly as a security measure that aims to prevent possible brute force attacks targeted to obtain a user's account password.
pam_faillock
module, failed login attempts are stored in a separate file for each user in the /var/run/faillock
directory.
Note
root
user account when the even_deny_root
option is used.
- To lock out any non-root user after three unsuccessful attempts and unlock that user after 10 minutes, add two lines to the
auth
section of the/etc/pam.d/system-auth
and/etc/pam.d/password-auth
files. After your edits, the entireauth
section in both files should look like this:1 auth required pam_env.so 2 auth required pam_faillock.so preauth silent audit deny=3 unlock_time=600 3 auth sufficient pam_unix.so nullok try_first_pass 4 auth [default=die] pam_faillock.so authfail audit deny=3 unlock_time=600 5 auth requisite pam_succeed_if.so uid >= 1000 quiet_success 6 auth required pam_deny.so
Lines number 2 and 4 have been added. - Add the following line to the
account
section of both files specified in the previous step:account required pam_faillock.so
- To apply account locking for the root user as well, add the
even_deny_root
option to thepam_faillock
entries in the/etc/pam.d/system-auth
and/etc/pam.d/password-auth
files:auth required pam_faillock.so preauth silent audit deny=3 even_deny_root unlock_time=600 auth sufficient pam_unix.so nullok try_first_pass auth [default=die] pam_faillock.so authfail audit deny=3 even_deny_root unlock_time=600 account required pam_faillock.so
john
attempts to log in for the fourth time after failing to log in three times previously, his account is locked upon the fourth attempt:
~]$ su - john
Account locked due to 3 failed logins
su: incorrect password
pam_faillock
is called for the first time in both /etc/pam.d/system-auth
and /etc/pam.d/password-auth
. Also replace user1
, user2
, and user3
with the actual user names.
auth [success=1 default=ignore] pam_succeed_if.so user in user1:user2:user3
root
, the following command:
~]$ faillock
john:
When Type Source Valid
2013-03-05 11:44:14 TTY pts/0 V
root
, the following command:
faillock
--user <username> --reset
Important
cron
jobs resets the failure counter of pam_faillock
of that user that is running the cron
job, and thus pam_faillock
should not be configured for cron
. See the Knowledge Centered Support (KCS) solution for more information.
Keeping Custom Settings with authconfig
system-auth
and password-auth
files are overwritten with the settings from the authconfig utility. This can be avoided by creating symbolic links in place of the configuration files, which authconfig recognizes and does not overwrite. In order to use custom settings in the configuration files and authconfig simultaneously, configure account locking using the following steps:
- Check whether the
system-auth
andpassword-auth
files are already symbolic links pointing tosystem-auth-ac
andpassword-auth-ac
(this is the system default):~]#
ls -l /etc/pam.d/{password,system}-auth
If the output is similar to the following, the symbolic links are in place, and you can skip to step number 3:lrwxrwxrwx. 1 root root 16 24. Feb 09.29 /etc/pam.d/password-auth -> password-auth-ac lrwxrwxrwx. 1 root root 28 24. Feb 09.29 /etc/pam.d/system-auth -> system-auth-ac
If thesystem-auth
andpassword-auth
files are not symbolic links, continue with the next step. - Rename the configuration files:
~]#
mv /etc/pam.d/system-auth /etc/pam.d/system-auth-ac
~]#mv /etc/pam.d/password-auth /etc/pam.d/password-auth-ac
- Create configuration files with your custom settings:
~]#
vi /etc/pam.d/system-auth-local
The/etc/pam.d/system-auth-local
file should contain the following lines:auth required pam_faillock.so preauth silent audit deny=3 unlock_time=600 auth include system-auth-ac auth [default=die] pam_faillock.so authfail silent audit deny=3 unlock_time=600 account required pam_faillock.so account include system-auth-ac password include system-auth-ac session include system-auth-ac
~]#
vi /etc/pam.d/password-auth-local
The/etc/pam.d/password-auth-local
file should contain the following lines:auth required pam_faillock.so preauth silent audit deny=3 unlock_time=600 auth include password-auth-ac auth [default=die] pam_faillock.so authfail silent audit deny=3 unlock_time=600 account required pam_faillock.so account include password-auth-ac password include password-auth-ac session include password-auth-ac
- Create the following symbolic links:
~]#
ln -sf /etc/pam.d/system-auth-local /etc/pam.d/system-auth
~]#ln -sf /etc/pam.d/password-auth-local /etc/pam.d/password-auth
pam_faillock
configuration options, see the pam_faillock(8) manual page.
Removing the nullok
option
nullok
option, which allows users to log in with a blank password if the password field in the /etc/shadow
file is empty, is enabled by default. To disable the nullok
option, remove the nullok
string from configuration files in the /etc/pam.d/
directory, such as /etc/pam.d/system-auth
or /etc/pam.d/password-auth
.
nullok
option allow users to login without entering a password? KCS solution for more information.
4.1.3. Session Locking
Note
4.1.3.1. Locking Virtual Consoles Using vlock
vlock
utility. Install it by entering the following command as root:
~]# yum install kbd
vlock
command without any additional parameters. This locks the currently active virtual console session while still allowing access to the others. To prevent access to all virtual consoles on the workstation, execute the following:
vlock
-a
vlock
locks the currently active console and the -a
option prevents switching to other virtual consoles.
vlock(1)
man page for additional information.
4.1.4. Enforcing Read-Only Mounting of Removable Media
udev
rule to detect removable media and configure them to be mounted read-only using the blockdev utility. This is sufficient for enforcing read-only mounting of physical media.
Using blockdev to Force Read-Only Mounting of Removable Media
udev
configuration file named, for example, 80-readonly-removables.rules
in the /etc/udev/rules.d/
directory with the following content:
SUBSYSTEM=="block",ATTRS{removable}=="1",RUN{program}="/sbin/blockdev --setro %N"
udev
rule ensures that any newly connected removable block (storage) device is automatically configured as read-only using the blockdev
utility.
Applying New udev Settings
udev
rules need to be applied. The udev
service automatically detects changes to its configuration files, but new settings are not applied to already existing devices. Only newly connected devices are affected by the new settings. Therefore, you need to unmount and unplug all connected removable media to ensure that the new settings are applied to them when they are next plugged in.
udev
to re-apply all rules to already existing devices, enter the following command as root
:
~#
udevadm trigger
udev
to re-apply all rules using the above command does not affect any storage devices that are already mounted.
udev
to reload all rules (in case the new rules are not automatically detected for some reason), use the following command:
~#
udevadm control --reload
4.2. Controlling Root Access
root
user or by acquiring effective root
privileges using a setuid program, such as sudo
or su
. A setuid program is one that operates with the user ID (UID) of the program's owner rather than the user operating the program. Such programs are denoted by an s
in the owner section of a long format listing, as in the following example:
~]$ ls -l /bin/su
-rwsr-xr-x. 1 root root 34904 Mar 10 2011 /bin/su
Note
s
may be upper case or lower case. If it appears as upper case, it means that the underlying permission bit has not been set.
pam_console.so
, some activities normally reserved only for the root user, such as rebooting and mounting removable media, are allowed for the first user that logs in at the physical console. However, other important system administration tasks, such as altering network settings, configuring a new mouse, or mounting network devices, are not possible without administrative privileges. As a result, system administrators must decide how much access the users on their network should receive.
4.2.1. Disallowing Root Access
root
for these or other reasons, the root password should be kept secret, and access to runlevel one or single user mode should be disallowed through boot loader password protection (see Section 4.2.5, “Securing the Boot Loader” for more information on this topic.)
root
logins are disallowed:
- Changing the root shell
- To prevent users from logging in directly as
root
, the system administrator can set theroot
account's shell to/sbin/nologin
in the/etc/passwd
file.Table 4.2. Disabling the Root Shell Effects Does Not Affect Prevents access to aroot
shell and logs any such attempts. The following programs are prevented from accessing theroot
account:login
gdm
kdm
xdm
su
ssh
scp
sftp
Programs that do not require a shell, such as FTP clients, mail clients, and many setuid programs. The following programs are not prevented from accessing theroot
account:sudo
- FTP clients
- Email clients
- Disabling root access using any console device (tty)
- To further limit access to the
root
account, administrators can disableroot
logins at the console by editing the/etc/securetty
file. This file lists all devices theroot
user is allowed to log into. If the file does not exist at all, theroot
user can log in through any communication device on the system, whether through the console or a raw network interface. This is dangerous, because a user can log in to their machine asroot
using Telnet, which transmits the password in plain text over the network.By default, Red Hat Enterprise Linux 7's/etc/securetty
file only allows theroot
user to log in at the console physically attached to the machine. To prevent theroot
user from logging in, remove the contents of this file by typing the following command at a shell prompt asroot
:echo > /etc/securetty
To enablesecuretty
support in the KDM, GDM, and XDM login managers, add the following line:auth [user_unknown=ignore success=ok ignore=ignore default=bad] pam_securetty.so
to the files listed below:/etc/pam.d/gdm
/etc/pam.d/gdm-autologin
/etc/pam.d/gdm-fingerprint
/etc/pam.d/gdm-password
/etc/pam.d/gdm-smartcard
/etc/pam.d/kdm
/etc/pam.d/kdm-np
/etc/pam.d/xdm
Warning
A blank/etc/securetty
file does not prevent theroot
user from logging in remotely using the OpenSSH suite of tools because the console is not opened until after authentication.Table 4.3. Disabling Root Logins Effects Does Not Affect Prevents access to theroot
account using the console or the network. The following programs are prevented from accessing theroot
account:login
gdm
kdm
xdm
- Other network services that open a tty
Programs that do not log in asroot
, but perform administrative tasks through setuid or other mechanisms. The following programs are not prevented from accessing theroot
account:su
sudo
ssh
scp
sftp
- Disabling root SSH logins
- To prevent
root
logins through the SSH protocol, edit the SSH daemon's configuration file,/etc/ssh/sshd_config
, and change the line that reads:#PermitRootLogin yes
to read as follows:PermitRootLogin no
Table 4.4. Disabling Root SSH Logins Effects Does Not Affect Preventsroot
access using the OpenSSH suite of tools. The following programs are prevented from accessing theroot
account:ssh
scp
sftp
Programs that are not part of the OpenSSH suite of tools. - Using PAM to limit root access to services
- PAM, through the
/lib/security/pam_listfile.so
module, allows great flexibility in denying specific accounts. The administrator can use this module to reference a list of users who are not allowed to log in. To limitroot
access to a system service, edit the file for the target service in the/etc/pam.d/
directory and make sure thepam_listfile.so
module is required for authentication.The following is an example of how the module is used for thevsftpd
FTP server in the/etc/pam.d/vsftpd
PAM configuration file (the\
character at the end of the first line is not necessary if the directive is on a single line):auth required /lib/security/pam_listfile.so item=user \ sense=deny file=/etc/vsftpd.ftpusers onerr=succeed
This instructs PAM to consult the/etc/vsftpd.ftpusers
file and deny access to the service for any listed user. The administrator can change the name of this file, and can keep separate lists for each service or use one central list to deny access to multiple services.If the administrator wants to deny access to multiple services, a similar line can be added to the PAM configuration files, such as/etc/pam.d/pop
and/etc/pam.d/imap
for mail clients, or/etc/pam.d/ssh
for SSH clients.For more information about PAM, see The Linux-PAM System Administrator's Guide, located in the/usr/share/doc/pam-<version>/html/
directory.Table 4.5. Disabling Root Using PAM Effects Does Not Affect Preventsroot
access to network services that are PAM aware. The following services are prevented from accessing theroot
account:login
gdm
kdm
xdm
ssh
scp
sftp
- FTP clients
- Email clients
- Any PAM aware services
Programs and services that are not PAM aware.
4.2.2. Allowing Root Access
root
access may not be an issue. Allowing root
access by users means that minor activities, like adding devices or configuring network interfaces, can be handled by the individual users, leaving system administrators free to deal with network security and other important issues.
root
access to individual users can lead to the following issues:
- Machine Misconfiguration — Users with
root
access can misconfigure their machines and require assistance to resolve issues. Even worse, they might open up security holes without knowing it. - Running Insecure Services — Users with
root
access might run insecure servers on their machine, such as FTP or Telnet, potentially putting usernames and passwords at risk. These services transmit this information over the network in plain text. - Running Email Attachments As Root — Although rare, email viruses that affect Linux do exist. A malicious program poses the greatest threat when run by the
root
user. - Keeping the audit trail intact — Because the
root
account is often shared by multiple users, so that multiple system administrators can maintain the system, it is impossible to figure out which of those users wasroot
at a given time. When using separate logins, the account a user logs in with, as well as a unique number for session tracking purposes, is put into the task structure, which is inherited by every process that the user starts. When using concurrent logins, the unique number can be used to trace actions to specific logins. When an action generates an audit event, it is recorded with the login account and the session associated with that unique number. Use theaulast
command to view these logins and sessions. The--proof
option of theaulast
command can be used suggest a specificausearch
query to isolate auditable events generated by a particular session. For more information about the Audit system, see Chapter 7, System Auditing.
4.2.3. Limiting Root Access
root
user, the administrator may want to allow access only through setuid programs, such as su
or sudo
. For more information on su
and sudo
, see the Gaining Privileges chapter in Red Hat Enterprise Linux 7 System Administrator's Guide, and the su(1)
and sudo(8)
man pages.
4.2.4. Enabling Automatic Logouts
root
, an unattended login session may pose a significant security risk. To reduce this risk, you can configure the system to automatically log out idle users after a fixed period of time.
- As
root
, add the following line at the beginning of the/etc/profile
file to make sure the processing of this file cannot be interrupted:trap "" 1 2 3 15
- As
root
, insert the following lines to the/etc/profile
file to automatically log out after 120 seconds:export TMOUT=120 readonly TMOUT
TheTMOUT
variable terminates the shell if there is no activity for the specified number of seconds (set to120
in the above example). You can change the limit according to the needs of the particular installation.
4.2.5. Securing the Boot Loader
- Preventing Access to Single User Mode — If attackers can boot the system into single user mode, they are logged in automatically as
root
without being prompted for theroot
password.Warning
Protecting access to single user mode with a password by editing theSINGLE
parameter in the/etc/sysconfig/init
file is not recommended. An attacker can bypass the password by specifying a custom initial command (using theinit=
parameter) on the kernel command line in GRUB 2. It is recommended to password-protect the GRUB 2 boot loader, as described in the Protecting GRUB 2 with a Password chapter in Red Hat Enterprise Linux 7 System Administrator's Guide. - Preventing Access to the GRUB 2 Console — If the machine uses GRUB 2 as its boot loader, an attacker can use the GRUB 2 editor interface to change its configuration or to gather information using the
cat
command. - Preventing Access to Insecure Operating Systems — If it is a dual-boot system, an attacker can select an operating system at boot time, for example DOS, which ignores access controls and file permissions.
4.2.5.1. Disabling Interactive Startup
root
, disable the PROMPT
parameter in the /etc/sysconfig/init
file:
PROMPT=no
4.2.6. Protecting Hard and Symbolic Links
- The user owns the file to which they link.
- The user already has read and write access to the file to which they link.
- The process following the symbolic link is the owner of the symbolic link.
- The owner of the directory is the same as the owner of the symbolic link.
/usr/lib/sysctl.d/50-default.conf
file:
fs.protected_hardlinks = 1 fs.protected_symlinks = 1
51-no-protect-links.conf
in the /etc/sysctl.d/
directory with the following content:
fs.protected_hardlinks = 0 fs.protected_symlinks = 0
Note
.conf
extension, and it needs to be read after the default system file (the files are read in lexicographic order, therefore settings contained in a file with a higher number at the beginning of the file name take precedence).
sysctl
mechanism.
4.3. Securing Services
4.3.1. Risks To Services
- Denial of Service Attacks (DoS) — By flooding a service with requests, a denial of service attack can render a system unusable as it tries to log and answer each request.
- Distributed Denial of Service Attack (DDoS) — A type of DoS attack which uses multiple compromised machines (often numbering in the thousands or more) to direct a coordinated attack on a service, flooding it with requests and making it unusable.
- Script Vulnerability Attacks — If a server is using scripts to execute server-side actions, as Web servers commonly do, an attacker can target improperly written scripts. These script vulnerability attacks can lead to a buffer overflow condition or allow the attacker to alter files on the system.
- Buffer Overflow Attacks — Services that want to listen on ports 1 through 1023 must start either with administrative privileges or the
CAP_NET_BIND_SERVICE
capability needs to be set for them. Once a process is bound to a port and is listening on it, the privileges or the capability are often dropped. If the privileges or the capability are not dropped, and the application has an exploitable buffer overflow, an attacker could gain access to the system as the user running the daemon. Because exploitable buffer overflows exist, crackers use automated tools to identify systems with vulnerabilities, and once they have gained access, they use automated rootkits to maintain their access to the system.
Note
Important
4.3.2. Identifying and Configuring Services
cups
— The default print server for Red Hat Enterprise Linux 7.cups-lpd
— An alternative print server.xinetd
— A super server that controls connections to a range of subordinate servers, such asgssftp
andtelnet
.sshd
— The OpenSSH server, which is a secure replacement for Telnet.
cups
running. The same is true for portreserve
. If you do not mount NFSv3 volumes or use NIS (the ypbind
service), then rpcbind
should be disabled. Checking which network services are available to start at boot time is not sufficient. It is recommended to also check which ports are open and listening. Refer to Section 4.4.2, “Verifying Which Ports Are Listening” for more information.
4.3.3. Insecure Services
- Transmit Usernames and Passwords Over a Network Unencrypted — Many older protocols, such as Telnet and FTP, do not encrypt the authentication session and should be avoided whenever possible.
- Transmit Sensitive Data Over a Network Unencrypted — Many protocols transmit data over the network unencrypted. These protocols include Telnet, FTP, HTTP, and SMTP. Many network file systems, such as NFS and SMB, also transmit information over the network unencrypted. It is the user's responsibility when using these protocols to limit what type of data is transmitted.
rlogin
, rsh
, telnet
, and vsftpd
.
rlogin
, rsh
, and telnet
) should be avoided in favor of SSH
. See Section 4.3.11, “Securing SSH” for more information about sshd
.
FTP
is not as inherently dangerous to the security of the system as remote shells, but FTP
servers must be carefully configured and monitored to avoid problems. See Section 4.3.9, “Securing FTP” for more information about securing FTP
servers.
auth
nfs-server
smb
andnbm
(Samba)yppasswdd
ypserv
ypxfrd
4.3.4. Securing rpcbind
rpcbind
service is a dynamic port assignment daemon for RPC services such as NIS and NFS. It has weak authentication mechanisms and has the ability to assign a wide range of ports for the services it controls. For these reasons, it is difficult to secure.
Note
rpcbind
only affects NFSv2 and NFSv3 implementations, since NFSv4 no longer requires it. If you plan to implement an NFSv2 or NFSv3 server, then rpcbind
is required, and the following section applies.
4.3.4.1. Protect rpcbind With TCP Wrappers
rpcbind
service since it has no built-in form of authentication.
4.3.4.2. Protect rpcbind With firewalld
rpcbind
service, it is a good idea to add firewalld
rules to the server and restrict access to specific networks.
firewalld
rich language commands. The first allows TCP connections to the port 111 (used by the rpcbind
service) from the 192.168.0.0/24 network. The second allows TCP connections to the same port from the localhost. All other packets are dropped.
~]#firewall-cmd --add-rich-rule='rule family="ipv4" port port="111" protocol="tcp" source address="192.168.0.0/24" invert="True" drop'
~]#firewall-cmd --add-rich-rule='rule family="ipv4" port port="111" protocol="tcp" source address="127.0.0.1" accept'
~]# firewall-cmd --add-rich-rule='rule family="ipv4" port port="111" protocol="udp" source address="192.168.0.0/24" invert="True" drop'
Note
--permanent
to the firewalld
rich language commands to make the settings permanent. See Chapter 5, Using Firewalls for more information about implementing firewalls.
4.3.5. Securing rpc.mountd
rpc.mountd
daemon implements the server side of the NFS MOUNT protocol, a protocol used by NFS version 2 (RFC 1904) and NFS version 3 (RFC 1813).
4.3.5.1. Protect rpc.mountd With TCP Wrappers
rpc.mountd
service since it has no built-in form of authentication.
IP
addresses when limiting access to the service. Avoid using host names, as they can be forged by DNS
poisoning and other methods.
4.3.5.2. Protect rpc.mountd With firewalld
rpc.mountd
service, add firewalld
rich language rules to the server and restrict access to specific networks.
firewalld
rich language commands. The first allows mountd
connections from the 192.168.0.0/24
network. The second allows mountd
connections from the local host. All other packets are dropped.
~]#firewall-cmd --add-rich-rule 'rule family="ipv4" source NOT address="192.168.0.0/24" service name="mountd" drop'
~]#firewall-cmd --add-rich-rule 'rule family="ipv4" source address="127.0.0.1" service name="mountd" accept'
Note
--permanent
to the firewalld
rich language commands to make the settings permanent. See Chapter 5, Using Firewalls for more information about implementing firewalls.
4.3.6. Securing NIS
ypserv
, which is used in conjunction with rpcbind
and other related services to distribute maps of user names, passwords, and other sensitive information to any computer claiming to be within its domain.
/usr/sbin/rpc.yppasswdd
— Also called theyppasswdd
service, this daemon allows users to change their NIS passwords./usr/sbin/rpc.ypxfrd
— Also called theypxfrd
service, this daemon is responsible for NIS map transfers over the network./usr/sbin/ypserv
— This is the NIS server daemon.
rpcbind
service as outlined in Section 4.3.4, “Securing rpcbind”, then address the following issues, such as network planning.
4.3.6.1. Carefully Plan the Network
4.3.6.2. Use a Password-like NIS Domain Name and Hostname
/etc/passwd
map:
ypcat
-d
<NIS_domain>-h
<DNS_hostname>passwd
/etc/shadow
file by typing the following command:
ypcat
-d
<NIS_domain>-h
<DNS_hostname>shadow
Note
/etc/shadow
file is not stored within a NIS map.
o7hfawtgmhwg.domain.com
. Similarly, create a different randomized NIS domain name. This makes it much more difficult for an attacker to access the NIS server.
4.3.6.3. Edit the /var/yp/securenets
File
/var/yp/securenets
file is blank or does not exist (as is the case after a default installation), NIS listens to all networks. One of the first things to do is to put netmask/network pairs in the file so that ypserv
only responds to requests from the appropriate network.
/var/yp/securenets
file:
255.255.255.0 192.168.0.0
Warning
/var/yp/securenets
file.
4.3.6.4. Assign Static Ports and Use Rich Language Rules
rpc.yppasswdd
— the daemon that allows users to change their login passwords. Assigning ports to the other two NIS server daemons, rpc.ypxfrd
and ypserv
, allows for the creation of firewall rules to further protect the NIS server daemons from intruders.
/etc/sysconfig/network
:
YPSERV_ARGS="-p 834" YPXFRD_ARGS="-p 835"
firewalld
rules can then be used to enforce which network the server listens to for these ports:
~]#firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.0.0/24" invert="True" port port="834-835" protocol="tcp" drop'
~]#firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.0.0/24" invert="True" port port="834-835" protocol="udp" drop'
192.168.0.0/24
network. The first rule is for TCP
and the second for UDP
.
Note
4.3.6.5. Use Kerberos Authentication
/etc/shadow
map is sent over the network. If an intruder gains access to a NIS domain and sniffs network traffic, they can collect user names and password hashes. With enough time, a password cracking program can guess weak passwords, and an attacker can gain access to a valid account on the network.
4.3.7. Securing NFS
Important
RPCSEC_GSS
kernel module. Information on rpcbind
is still included, since Red Hat Enterprise Linux 7 supports NFSv3 which utilizes rpcbind
.
4.3.7.1. Carefully Plan the Network
4.3.7.2. Securing NFS Mount Options
mount
command in the /etc/fstab
file is explained in the Using the mount Command chapter of the Red Hat Enterprise Linux 7 Storage Administration Guide. From a security administration point of view it is worthwhile to note that the NFS mount options can also be specified in /etc/nfsmount.conf
, which can be used to set custom default options.
4.3.7.2.1. Review the NFS Server
Warning
exports(5)
man page.
ro
option to export the file system as read-only whenever possible to reduce the number of users able to write to the mounted file system. Only use the rw
option when specifically required. See the man exports(5)
page for more information. Allowing write access increases the risk from symlink attacks for example. This includes temporary directories such as /tmp
and /usr/tmp
.
rw
option avoid making them world-writable whenever possible to reduce risk. Exporting home directories is also viewed as a risk as some applications store passwords in clear text or weakly encrypted. This risk is being reduced as application code is reviewed and improved. Some users do not set passwords on their SSH keys so this too means home directories present a risk. Enforcing the use of passwords or using Kerberos would mitigate that risk.
showmount -e
command on an NFS server to review what the server is exporting. Do not export anything that is not specifically required.
no_root_squash
option and review existing installations to make sure it is not used. See Section 4.3.7.4, “Do Not Use the no_root_squash Option” for more information.
secure
option is the server-side export option used to restrict exports to “reserved” ports. By default, the server allows client communication only from “reserved” ports (ports numbered less than 1024), because traditionally clients have only allowed “trusted” code (such as in-kernel NFS clients) to use those ports. However, on many networks it is not difficult for anyone to become root on some client, so it is rarely safe for the server to assume that communication from a reserved port is privileged. Therefore the restriction to reserved ports is of limited value; it is better to rely on Kerberos, firewalls, and restriction of exports to particular clients.
4.3.7.2.2. Review the NFS Client
nosuid
option to disallow the use of a setuid program. The nosuid
option disables the set-user-identifier
or set-group-identifier
bits. This prevents remote users from gaining higher privileges by running a setuid program. Use this option on the client and the server side.
noexec
option disables all executable files on the client. Use this to prevent users from inadvertently executing files placed in the file system being shared. The nosuid
and noexec
options are standard options for most, if not all, file systems.
nodev
option to prevent “device-files” from being processed as a hardware device by the client.
resvport
option is a client-side mount option and secure
is the corresponding server-side export option (see explanation above). It restricts communication to a "reserved port". The reserved or "well known" ports are reserved for privileged users and processes such as the root user. Setting this option causes the client to use a reserved source port to communicate with the server.
sec=krb5
.
krb5i
for integrity and krb5p
for privacy protection. These are used when mounting with sec=krb5
, but need to be configured on the NFS server. See the man page on exports (man 5 exports
) for more information.
man 5 nfs
) has a “SECURITY CONSIDERATIONS” section which explains the security enhancements in NFSv4 and contains all the NFS specific mount options.
Important
4.3.7.3. Beware of Syntax Errors
/etc/exports
file. Be careful not to add extraneous spaces when editing this file.
/etc/exports
file shares the directory /tmp/nfs/
to the host bob.example.com
with read/write permissions.
/tmp/nfs/ bob.example.com(rw)
/etc/exports
file, on the other hand, shares the same directory to the host bob.example.com
with read-only permissions and shares it to the world with read/write permissions due to a single space character after the host name.
/tmp/nfs/ bob.example.com (rw)
showmount
command to verify what is being shared:
showmount
-e
<hostname>
4.3.7.4. Do Not Use the no_root_squash Option
nfsnobody
user, an unprivileged user account. This changes the owner of all root-created files to nfsnobody
, which prevents uploading of programs with the setuid bit set.
no_root_squash
is used, remote root users are able to change any file on the shared file system and leave applications infected by Trojans for other users to inadvertently execute.
4.3.7.5. NFS Firewall Configuration
Configuring Ports for NFSv3
rpcbind
service, which might cause problems when creating firewall rules. To simplify this process, use the /etc/sysconfig/nfs
file to specify which ports are to be used:
MOUNTD_PORT
— TCP and UDP port for mountd (rpc.mountd)STATD_PORT
— TCP and UDP port for status (rpc.statd)
/etc/modprobe.d/lockd.conf
file:
nlm_tcpport
— TCP port for nlockmgr (rpc.lockd)nlm_udpport
— UDP port nlockmgr (rpc.lockd)
/etc/modprobe.d/lockd.conf
for descriptions of additional customizable NFS lock manager parameters.
rpcinfo -p
command on the NFS server to see which ports and RPC programs are being used.
4.3.7.6. Securing NFS with Red Hat Identity Management
4.3.8. Securing HTTP Servers
4.3.8.1. Securing the Apache HTTP Server
chown
root
<directory_name>
chmod
755
<directory_name>
/etc/httpd/conf/httpd.conf
):
FollowSymLinks
- This directive is enabled by default, so be sure to use caution when creating symbolic links to the document root of the Web server. For instance, it is a bad idea to provide a symbolic link to
/
. Indexes
- This directive is enabled by default, but may not be desirable. To prevent visitors from browsing files on the server, remove this directive.
UserDir
- The
UserDir
directive is disabled by default because it can confirm the presence of a user account on the system. To enable user directory browsing on the server, use the following directives:UserDir enabled UserDir disabled root
These directives activate user directory browsing for all user directories other than/root/
. To add users to the list of disabled accounts, add a space-delimited list of users on theUserDir disabled
line. ServerTokens
- The
ServerTokens
directive controls the server response header field which is sent back to clients. It includes various information which can be customized using the following parameters:ServerTokens Full
(default option) — provides all available information (OS type and used modules), for example:Apache/2.0.41 (Unix) PHP/4.2.2 MyMod/1.2
ServerTokens Prod
orServerTokens ProductOnly
— provides the following information:Apache
ServerTokens Major
— provides the following information:Apache/2
ServerTokens Minor
— provides the following information:Apache/2.0
ServerTokens Min
orServerTokens Minimal
— provides the following information:Apache/2.0.41
ServerTokens OS
— provides the following information:Apache/2.0.41 (Unix)
It is recommended to use theServerTokens Prod
option so that a possible attacker does not gain any valuable information about your system.
Important
IncludesNoExec
directive. By default, the Server-Side Includes (SSI) module cannot execute commands. It is recommended that you do not change this setting unless absolutely necessary, as it could, potentially, enable an attacker to execute commands on the system.
Removing httpd Modules
httpd
modules to limit the functionality of the HTTP Server. To do so, edit configuration files in the /etc/httpd/conf.modules.d
directory. For example, to remove the proxy module:
echo '# All proxy modules disabled' > /etc/httpd/conf.modules.d/00-proxy.conf
/etc/httpd/conf.d/
directory contains configuration files which are used to load modules as well.
httpd and SELinux
4.3.8.2. Securing NGINX
server
section of your NGINX configuration files.
Disabling Version Strings
server_tokens off;
nginx
in all requests served by NGINX:
$ curl -sI http://localhost | grep Server Server: nginx
Including Additional Security-related Headers
add_header X-Frame-Options SAMEORIGIN;
— this option denies any page outside of your domain to frame any content served by NGINX, effectively mitigating clickjacking attacks.add_header X-Content-Type-Options nosniff;
— this option prevents MIME-type sniffing in certain older browsers.add_header X-XSS-Protection "1; mode=block";
— this option enables the Cross-Site Scripting (XSS) filtering, which prevents a browser from rendering potentially malicious content included in a response by NGINX.
Disabling Potentially Harmful HTTP Methods
# Allow GET, PUT, POST; return "405 Method Not Allowed" for all others. if ( $request_method !~ ^(GET|PUT|POST)$ ) { return 405; }
Configuring SSL
4.3.9. Securing FTP
- Red Hat Content Accelerator (
tux
) — A kernel-space Web server with FTP capabilities. vsftpd
— A standalone, security oriented implementation of the FTP service.
vsftpd
FTP service.
4.3.9.1. FTP Greeting Banner
vsftpd
, add the following directive to the /etc/vsftpd/vsftpd.conf
file:
ftpd_banner=<insert_greeting_here>
/etc/banners/
. The banner file for FTP connections in this example is /etc/banners/ftp.msg
. Below is an example of what such a file may look like:
######### Hello, all activity on ftp.example.com is logged. #########
Note
220
as specified in Section 4.4.1, “Securing Services With TCP Wrappers and xinetd”.
vsftpd
, add the following directive to the /etc/vsftpd/vsftpd.conf
file:
banner_file=/etc/banners/ftp.msg
4.3.9.2. Anonymous Access
/var/ftp/
directory activates the anonymous account.
vsftpd
package. This package establishes a directory tree for anonymous users and configures the permissions on directories to read-only for anonymous users.
Warning
4.3.9.2.1. Anonymous Upload
/var/ftp/pub/
. To do this, enter the following command as root:
~]# mkdir /var/ftp/pub/upload
~]# chmod 730 /var/ftp/pub/upload
~]# ls -ld /var/ftp/pub/upload
drwx-wx---. 2 root ftp 4096 Nov 14 22:57 /var/ftp/pub/upload
vsftpd
, add the following line to the /etc/vsftpd/vsftpd.conf
file:
anon_upload_enable=YES
4.3.9.3. User Accounts
vsftpd
, add the following directive to /etc/vsftpd/vsftpd.conf
:
local_enable=NO
4.3.9.3.1. Restricting User Accounts
sudo
privileges, the easiest way is to use a PAM list file as described in Section 4.2.1, “Disallowing Root Access”. The PAM configuration file for vsftpd
is /etc/pam.d/vsftpd
.
vsftpd
, add the user name to /etc/vsftpd/ftpusers
4.3.9.4. Use TCP Wrappers To Control Access
4.3.10. Securing Postfix
4.3.10.1. Limiting a Denial of Service Attack
/etc/postfix/main.cf
file. You can change the value of the directives which are already there or you can add the directives you need with the value you want in the following format:
<directive> = <value>. The following is a list of directives that can be used for limiting a denial of service attack:
smtpd_client_connection_rate_limit
— The maximum number of connection attempts any client is allowed to make to this service per time unit (described below). The default value is 0, which means a client can make as many connections per time unit as Postfix can accept. By default, clients in trusted networks are excluded.anvil_rate_time_unit
— This time unit is used for rate limit calculations. The default value is 60 seconds.smtpd_client_event_limit_exceptions
— Clients that are excluded from the connection and rate limit commands. By default, clients in trusted networks are excluded.smtpd_client_message_rate_limit
— The maximum number of message deliveries a client is allowed to request per time unit (regardless of whether or not Postfix actually accepts those messages).default_process_limit
— The default maximum number of Postfix child processes that provide a given service. This limit can be overruled for specific services in themaster.cf
file. By default the value is 100.queue_minfree
— The minimum amount of free space in bytes in the queue file system that is needed to receive mail. This is currently used by the Postfix SMTP server to decide if it will accept any mail at all. By default, the Postfix SMTP server rejectsMAIL FROM
commands when the amount of free space is less than 1.5 times the message_size_limit. To specify a higher minimum free space limit, specify a queue_minfree value that is at least 1.5 times the message_size_limit. By default the queue_minfree value is 0.header_size_limit
— The maximum amount of memory in bytes for storing a message header. If a header is larger, the excess is discarded. By default the value is 102400.message_size_limit
— The maximum size in bytes of a message, including envelope information. By default the value is 10240000.
4.3.10.2. NFS and Postfix
/var/spool/postfix/
, on an NFS shared volume. Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail.
Note
SECRPC_GSS
kernel module does not utilize UID-based authentication. However, it is still considered good practice not to put the mail spool directory on NFS shared volumes.
4.3.10.3. Mail-only Users
/etc/passwd
file should be set to /sbin/nologin
(with the possible exception of the root user).
4.3.10.4. Disable Postfix Network Listening
/etc/postfix/main.cf
.
/etc/postfix/main.cf
to ensure that only the following inet_interfaces
line appears:
inet_interfaces = localhost
inet_interfaces = all
setting can be used.
4.3.10.5. Configuring Postfix to Use SASL
SASL
implementations for SMTP Authentication (or SMTP AUTH). SMTP Authentication is an extension of the Simple Mail Transfer Protocol
. When enabled, SMTP
clients are required to authenticate to the SMTP
server using an authentication method supported and accepted by both the server and the client. This section describes how to configure Postfix to make use of the Dovecot SASL
implementation.
POP
/IMAP
server, and thus make the Dovecot SASL
implementation available on your system, issue the following command as the root
user:
~]# yum install dovecot
SMTP
server can communicate with the Dovecot SASL
implementation using either a UNIX-domain socket or a TCP socket. The latter method is only needed in case the Postfix and Dovecot applications are running on separate machines. This guide gives preference to the UNIX-domain socket method, which affords better privacy.
SASL
implementation, a number of configuration changes need to be performed for both applications. Follow the procedures below to effect these changes.
Setting Up Dovecot
- Modify the main Dovecot configuration file,
/etc/dovecot/conf.d/10-master.conf
, to include the following lines (the default configuration file already includes most of the relevant section, and the lines just need to be uncommented):service auth { unix_listener /var/spool/postfix/private/auth { mode = 0660 user = postfix group = postfix } }
The above example assumes the use of UNIX-domain sockets for communication between Postfix and Dovecot. It also assumes default settings of the PostfixSMTP
server, which include the mail queue located in the/var/spool/postfix/
directory, and the application running under thepostfix
user and group. In this way, read and write permissions are limited to thepostfix
user and group.Alternatively, you can use the following configuration to set up Dovecot to listen for Postfix authentication requests throughTCP
:service auth { inet_listener { port = 12345 } }
In the above example, replace12345
with the number of the port you want to use. - Edit the
/etc/dovecot/conf.d/10-auth.conf
configuration file to instruct Dovecot to provide the PostfixSMTP
server with theplain
andlogin
authentication mechanisms:auth_mechanisms = plain login
Setting Up Postfix
/etc/postfix/main.cf
, needs to be modified. Add or edit the following configuration directives:
- Enable SMTP Authentication in the Postfix
SMTP
server:smtpd_sasl_auth_enable = yes
- Instruct Postfix to use the Dovecot
SASL
implementation for SMTP Authentication:smtpd_sasl_type = dovecot
- Provide the authentication path relative to the Postfix queue directory (note that the use of a relative path ensures that the configuration works regardless of whether the Postfix server runs in a chroot or not):
smtpd_sasl_path = private/auth
This step assumes that you want to use UNIX-domain sockets for communication between Postfix and Dovecot. To configure Postfix to look for Dovecot on a different machine in case you useTCP
sockets for communication, use configuration values similar to the following:smtpd_sasl_path = inet:127.0.0.1:12345
In the above example,127.0.0.1
needs to be substituted by theIP
address of the Dovecot machine and12345
by the port specified in Dovecot's/etc/dovecot/conf.d/10-master.conf
configuration file. - Specify
SASL
mechanisms that the PostfixSMTP
server makes available to clients. Note that different mechanisms can be specified for encrypted and unencrypted sessions.smtpd_sasl_security_options = noanonymous, noplaintext smtpd_sasl_tls_security_options = noanonymous
The above example specifies that during unencrypted sessions, no anonymous authentication is allowed and no mechanisms that transmit unencrypted user names or passwords are allowed. For encrypted sessions (usingTLS
), only non-anonymous authentication mechanisms are allowed.See http://www.postfix.org/SASL_README.html#smtpd_sasl_security_options for a list of all supported policies for limiting allowedSASL
mechanisms.
Additional Resources
SASL
.
- http://wiki2.dovecot.org/HowTo/PostfixAndDovecotSASL — Contains information on how to set up Postfix to use the Dovecot
SASL
implementation for SMTP Authentication. - http://www.postfix.org/SASL_README.html#server_sasl — Contains information on how to set up Postfix to use either the Dovecot or Cyrus
SASL
implementations for SMTP Authentication.
4.3.11. Securing SSH
SSH
are encrypted and protected from interception. See the OpenSSH chapter of the Red Hat Enterprise Linux 7 System Administrator's Guide for general information about the SSH
protocol and about using the SSH
service in Red Hat Enterprise Linux 7.
Important
SSH
setup. By no means should this list of suggested measures be considered exhaustive or definitive. See sshd_config(5)
for a description of all configuration directives available for modifying the behavior of the sshd
daemon and to ssh(1)
for an explanation of basic SSH
concepts.
4.3.11.1. Cryptographic Login
SSH
supports the use of cryptographic keys for logging in to computers. This is much more secure than using only a password. If you combine this method with other authentication methods, it can be considered a multi-factor authentication. See Section 4.3.11.2, “Multiple Authentication Methods” for more information about using multiple authentication methods.
PubkeyAuthentication
configuration directive in the /etc/ssh/sshd_config
file needs to be set to yes
. Note that this is the default setting. Set the PasswordAuthentication
directive to no
to disable the possibility of using passwords for logging in.
SSH
keys can be generated using the ssh-keygen
command. If invoked without additional arguments, it creates a 2048-bit RSA key set. The keys are stored, by default, in the ~/.ssh/
directory. You can utilize the -b
switch to modify the bit-strength of the key. Using 2048-bit keys is normally sufficient. The Configuring OpenSSH chapter in the Red Hat Enterprise Linux 7 System Administrator's Guide includes detailed information about generating key pairs.
~/.ssh/
directory. If you accepted the defaults when running the ssh-keygen
command, then the generated files are named id_rsa
and id_rsa.pub
and contain the private and public key respectively. You should always protect the private key from exposure by making it unreadable by anyone else but the file's owner. The public key, however, needs to be transferred to the system you are going to log in to. You can use the ssh-copy-id
command to transfer the key to the server:
~]$ ssh-copy-id -i [user@]server
~/.ssh/authorized_keys
file on the server. The sshd
daemon will check this file when you attempt to log in to the server.
SSH
keys regularly. When you do, make sure you remove any unused keys from the authorized_keys
file.
4.3.11.2. Multiple Authentication Methods
AuthenticationMethods
configuration directive in the /etc/ssh/sshd_config
file to specify which authentication methods are to be utilized. Note that it is possible to define more than one list of required authentication methods using this directive. If that is the case, the user must complete every method in at least one of the lists. The lists need to be separated by blank spaces, and the individual authentication-method names within the lists must be comma-separated. For example:
AuthenticationMethods publickey,gssapi-with-mic publickey,keyboard-interactive
sshd
daemon configured using the above AuthenticationMethods
directive only grants access if the user attempting to log in successfully completes either publickey
authentication followed by gssapi-with-mic
or by keyboard-interactive
authentication. Note that each of the requested authentication methods needs to be explicitly enabled using a corresponding configuration directive (such as PubkeyAuthentication
) in the /etc/ssh/sshd_config
file. See the AUTHENTICATION section of ssh(1)
for a general list of available authentication methods.
4.3.11.3. Other Ways of Securing SSH
Protocol Version
SSH
protocol supplied with Red Hat Enterprise Linux 7 still supports both the SSH-1 and SSH-2 versions of the protocol for SSH clients, only the latter should be used whenever possible. The SSH-2 version contains a number of improvements over the older SSH-1, and the majority of advanced configuration options is only available when using SSH-2.
SSH
protocol protects the authentication and communication for which it is used. The version or versions of the protocol supported by the sshd
daemon can be specified using the Protocol
configuration directive in the /etc/ssh/sshd_config
file. The default setting is 2
. Note that the SSH-2 version is the only version supported by the Red Hat Enterprise Linux 7 SSH server.
Key Types
ssh-keygen
command generates a pair of SSH-2 RSA keys by default, using the -t
option, it can be instructed to generate DSA or ECDSA keys as well. The ECDSA (Elliptic Curve Digital Signature Algorithm) offers better performance at the same equivalent symmetric key length. It also generates shorter keys.
Non-Default Port
sshd
daemon listens on TCP port 22
. Changing the port reduces the exposure of the system to attacks based on automated network scanning, thus increasing security through obscurity. The port can be specified using the Port
directive in the /etc/ssh/sshd_config
configuration file. Note also that the default SELinux policy must be changed to allow for the use of a non-default port. You can do this by modifying the ssh_port_t
SELinux type by typing the following command as root
:
~]# semanage -a -t ssh_port_t -p tcp port_number
Port
directive.
No Root Login
root
user, you should consider setting the PermitRootLogin
configuration directive to no
in the /etc/ssh/sshd_config
file. By disabling the possibility of logging in as the root
user, the administrator can audit which user runs what privileged command after they log in as regular users and then gain root
rights.
Using the X Security extension
ForwardX11Trusted
option in the /etc/ssh/ssh_config
file is set to yes
, and there is no difference between the ssh -X remote_machine
(untrusted host) and ssh -Y remote_machine
(trusted host) command.
Warning
4.3.12. Securing PostgreSQL
postgresql-server
package provides PostgreSQL. If it is not installed, enter the following command as the root user to install it:
~]# yum install postgresql-server
-D
option. For example:
~]$ initdb -D
/home/postgresql/db1
initdb
command will attempt to create the directory you specify if it does not already exist. We use the name /home/postgresql/db1 in this example. The /home/postgresql/db1 directory contains all the data stored in the database and also the client authentication configuration file:
~]$cat
pg_hba.conf
# PostgreSQL Client Authentication Configuration File # This file controls: which hosts are allowed to connect, how clients # are authenticated, which PostgreSQL user names they can use, which # databases they can access. Records take one of these forms: # # local DATABASE USER METHOD [OPTIONS] # host DATABASE USER ADDRESS METHOD [OPTIONS] # hostssl DATABASE USER ADDRESS METHOD [OPTIONS] # hostnossl DATABASE USER ADDRESS METHOD [OPTIONS]
pg_hba.conf
file allows any authenticated local users to access any databases with their user names:
local all all trust
pg_hba.conf
file.
4.3.13. Securing Docker
4.3.14. Securing memcached against DDoS Attacks
memcached Vulnerabilities
Hardening memcached
- Configure a firewall in your LAN. If your memcached server should be accessible only from within your local network, do not allow external traffic to ports used by memcached. For example, remove the port 11211, which is used by memcached by default, from the list of allowed ports:
~]#
See Section 5.8, “Using Zones to Manage Incoming Traffic Depending on Source” forfirewall-cmd --remove-port=11211/udp
~]#firewall-cmd --runtime-to-permanent
firewalld
commands that allow specific IP ranges to use the port 11211. - Disable UDP by adding the
-U 0 -p 11211
value to theOPTIONS
variable in the/etc/sysconfig/memcached
file unless your clients really need this protocol:OPTIONS="-U 0 -p 11211"
- If you use just a single memcached server on the same machine as your application, set up memcached to listen to localhost traffic only. Add the
-l 127.0.0.1,::1
value toOPTIONS
in/etc/sysconfig/memcached
:OPTIONS="-l 127.0.0.1,::1"
- If changing the authentication is possible, enable SASL (Simple Authentication and Security Layer) authentication:
- Modify or add in the
/etc/sasl2/memcached.conf
file:sasldb_path: /path.to/memcached.sasldb
- Add an account in the SASL database:
~]#
saslpasswd2 -a memcached -c cacheuser -f /path.to/memcached.sasldb
- Ensure the database is accessible for the memcached user and group.
~]#
chown memcached:memcached /path.to/memcached.sasldb
- Enable SASL support in memcached by adding the
-S
value toOPTIONS
to/etc/sysconfig/memcached
:OPTIONS="-S"
- Restart the memcached server to apply the changes.
- Add the user name and password created in the SASL database to the memcached client configuration of your application.
- Encrypt communication between memcached clients and servers with stunnel. Since memcached does not support TLS, a workaround is to use a proxy, such as stunnel, which provides TLS on top of the memcached protocol.You could either configure stunnel to use PSK (Pre Shared Keys) or even better to use user certificates. When using certificates, only authenticated users can reach your memcached servers and your traffic is encrypted.
Important
If you use a tunnel to access memcached, ensure that the service is either listening only on localhost or a firewall prevents access from the network to the memcached port.See Section 4.8, “Using stunnel” for more information.
4.4. Securing Network Access
4.4.1. Securing Services With TCP Wrappers and xinetd
4.4.1.1. TCP Wrappers and Connection Banners
banner
option.
vsftpd
. To begin, create a banner file. It can be anywhere on the system, but it must have same name as the daemon. For this example, the file is called /etc/banners/vsftpd
and contains the following lines:
220-Hello, %c 220-All activity on ftp.example.com is logged. 220-Inappropriate use will result in your access privileges being removed.
%c
token supplies a variety of client information, such as the user name and host name, or the user name and IP address to make the connection even more intimidating.
/etc/hosts.allow
file:
vsftpd : ALL : banners /etc/banners/
4.4.1.2. TCP Wrappers and Attack Warnings
spawn
directive.
/etc/hosts.deny
file to deny any connection attempts from that network, and to log the attempts to a special file:
ALL : 206.182.68.0 : spawn /bin/echo `date` %c %d >> /var/log/intruder_alert
%d
token supplies the name of the service that the attacker was trying to access.
spawn
directive in the /etc/hosts.allow
file.
Note
spawn
directive executes any shell command, it is a good idea to create a special script to notify the administrator or execute a chain of commands in the event that a particular client attempts to connect to the server.
4.4.1.3. TCP Wrappers and Enhanced Logging
severity
option.
emerg
flag in the log files instead of the default flag, info
, and deny the connection.
/etc/hosts.deny
:
in.telnetd : ALL : severity emerg
authpriv
logging facility, but elevates the priority from the default value of info
to emerg
, which posts log messages directly to the console.
4.4.2. Verifying Which Ports Are Listening
Using netstat for Open Ports Scan
root
to determine which ports are listening for connections from the network:
~]# netstat -pan -A inet,inet6 | grep -v ESTABLISHED
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN 1/systemd
tcp 0 0 192.168.124.1:53 0.0.0.0:* LISTEN 1829/dnsmasq
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 1176/sshd
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 1177/cupsd
tcp6 0 0 :::111 :::* LISTEN 1/systemd
tcp6 0 0 ::1:25 :::* LISTEN 1664/master
sctp 0.0.0.0:2500 LISTEN 20985/sctp_darn
udp 0 0 192.168.124.1:53 0.0.0.0:* 1829/dnsmasq
udp 0 0 0.0.0.0:67 0.0.0.0:* 977/dhclient
...
-l
option of the netstat
command to display only listening server sockets:
~]# netstat -tlnw
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN
tcp 0 0 192.168.124.1:53 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
tcp6 0 0 :::111 :::* LISTEN
tcp6 0 0 :::22 :::* LISTEN
tcp6 0 0 ::1:631 :::* LISTEN
tcp6 0 0 ::1:25 :::* LISTEN
raw6 0 0 :::58 :::* 7
Using ss for Open Ports Scan
~]# ss -tlw
etid State Recv-Q Send-Q Local Address:Port Peer Address:Port
udp UNCONN 0 0 :::ipv6-icmp :::*
tcp LISTEN 0 128 *:sunrpc *:*
tcp LISTEN 0 5 192.168.124.1:domain *:*
tcp LISTEN 0 128 *:ssh *:*
tcp LISTEN 0 128 127.0.0.1:ipp *:*
tcp LISTEN 0 100 127.0.0.1:smtp *:*
tcp LISTEN 0 128 :::sunrpc :::*
tcp LISTEN 0 128 :::ssh :::*
tcp LISTEN 0 128 ::1:ipp :::*
tcp LISTEN 0 100 ::1:smtp :::*
~]# ss -plno -A tcp,udp,sctp
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
udp UNCONN 0 0 192.168.124.1:53 *:* users:(("dnsmasq",pid=1829,fd=5))
udp UNCONN 0 0 *%virbr0:67 *:* users:(("dnsmasq",pid=1829,fd=3))
udp UNCONN 0 0 *:68 *:* users:(("dhclient",pid=977,fd=6))
...
tcp LISTEN 0 5 192.168.124.1:53 *:* users:(("dnsmasq",pid=1829,fd=6))
tcp LISTEN 0 128 *:22 *:* users:(("sshd",pid=1176,fd=3))
tcp LISTEN 0 128 127.0.0.1:631 *:* users:(("cupsd",pid=1177,fd=12))
tcp LISTEN 0 100 127.0.0.1:25 *:* users:(("master",pid=1664,fd=13))
...
sctp LISTEN 0 5 *:2500 *:* users:(("sctp_darn",pid=20985,fd=3))
UNCONN
state shows the ports in UDP listening mode.
-6
option for scanning an IPv6 address.
~]# nmap -sT -O 192.168.122.65
Starting Nmap 6.40 ( http://nmap.org ) at 2017-03-27 09:30 CEST
Nmap scan report for 192.168.122.65
Host is up (0.00032s latency).
Not shown: 998 closed ports
PORT STATE SERVICE
22/tcp open ssh
111/tcp open rpcbind
Device type: general purpose
Running: Linux 3.X
OS CPE: cpe:/o:linux:linux_kernel:3
OS details: Linux 3.7 - 3.9
Network Distance: 0 hops
OS detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 1.79 seconds
(-sT)
is the default TCP scan type when the TCP SYN scan (-sS)
is not an option. The -O
option detects the operating system of the host.
Using netstat and ss to Scan for Open SCTP Ports
root
:
~]# netstat -plnS
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
sctp 127.0.0.1:250 LISTEN 4125/sctp_darn
sctp 0 0 127.0.0.1:260 127.0.0.1:250 CLOSE 4250/sctp_darn
sctp 0 0 127.0.0.1:250 127.0.0.1:260 LISTEN 4125/sctp_darn
~]# netstat -nl -A inet,inet6 | grep 2500
sctp 0.0.0.0:2500 LISTEN
~]# ss -an | grep 2500
sctp LISTEN 0 5 *:2500 *:*
4.4.3. Disabling Source Routing
accept_source_route
option causes network interfaces to accept packets with the Strict Source Routing (SSR) or Loose Source Routing (LSR) option set. The acceptance of source routed packets is controlled by sysctl settings. Issue the following command as root to drop packets with the SSR or LSR option set:
~]# /sbin/sysctl -w net.ipv4.conf.all.accept_source_route=0
~]# /sbin/sysctl -w net.ipv4.conf.all.forwarding=0
~]# /sbin/sysctl -w net.ipv6.conf.all.forwarding=0
~]# /sbin/sysctl -w net.ipv4.conf.all.mc_forwarding=0
~]# /sbin/sysctl -w net.ipv6.conf.all.mc_forwarding=0
~]# /sbin/sysctl -w net.ipv4.conf.all.accept_redirects=0
~]# /sbin/sysctl -w net.ipv6.conf.all.accept_redirects=0
~]# /sbin/sysctl -w net.ipv4.conf.all.secure_redirects=0
~]# /sbin/sysctl -w net.ipv4.conf.all.send_redirects=0
Important
0
value for every interface. To automatically disable sending of ICMP requests whenever you add a new interface, enter the following command:
~]# /sbin/sysctl -w net.ipv4.conf.default.send_redirects=0
Note
/etc/sysctl.conf
file. For example, to disable acceptance of all IPv4 ICMP redirected packets on all interfaces, open the /etc/sysctl.conf
file with an editor running as the root
user and add a line as follows: net.ipv4.conf.all.send_redirects=0
sysctl(8)
, for more information. See RFC791 for an explanation of the Internet options related to source based routing and its variants.
Warning
4.4.3.1. Reverse Path Forwarding
IP
addresses from local subnets and reduces the opportunity for DDoS attacks.
Note
Warning
-
rp_filter
- Reverse Path Forwarding is enabled by means of the
rp_filter
directive. Thesysctl
utility can be used to make changes to the running system, and permanent changes can be made by adding lines to the/etc/sysctl.conf
file. Therp_filter
option is used to direct the kernel to select from one of three modes.To make a temporary global change, enter the following commands asroot
:sysctl -w net.ipv4.conf.default.rp_filter=integer sysctl -w net.ipv4.conf.all.rp_filter=integer
where integer is one of the following:0
— No source validation.1
— Strict mode as defined in RFC 3704.2
— Loose mode as defined in RFC 3704.
The setting can be overridden per network interface using thenet.ipv4.conf.interface.rp_filter
command as follows:sysctl -w net.ipv4.conf.interface.rp_filter=integer
Note
To make these settings persistent across reboots, modify the/etc/sysctl.conf
file. For example, to change the mode for all interfaces, open the/etc/sysctl.conf
file with an editor running as theroot
user and add a line as follows:net.ipv4.conf.all.rp_filter=2
-
IPv6_rpfilter
- In case of the
IPv6
protocol the firewalld daemon applies to Reverse Path Forwarding by default. The setting can be checked in the/etc/firewalld/firewalld.conf
file. You can change the firewalld behavior by setting theIPv6_rpfilter
option.If you need a custom configuration of Reverse Path Forwarding, you can perform it without the firewalld daemon by using theip6tables
command as follows:ip6tables -t raw -I PREROUTING -m rpfilter --invert -j DROP
This rule should be inserted near the beginning of the raw/PREROUTING chain, so that it applies to all traffic, in particular before the stateful matching rules. For more information about theiptables
andip6tables
services, see Section 5.13, “Setting and Controlling IP sets usingiptables
”.
Enabling Packet Forwarding
root
and change the line which reads net.ipv4.ip_forward = 0
in the /etc/sysctl.conf
file to the following:
net.ipv4.ip_forward = 1
/etc/sysctl.conf
file, enter the following command:
/sbin/sysctl -p
root
:
/sbin/sysctl net.ipv4.ip_forward
1
, then IP forwarding is enabled. If it returns a 0
, then you can turn it on manually using the following command:
/sbin/sysctl -w net.ipv4.ip_forward=1
4.4.3.2. Additional Resources
- Installed Documentation
/usr/share/doc/kernel-doc-version/Documentation/networking/ip-sysctl.txt
- This file contains a complete list of files and options available in the directory. Before accessing the kernel documentation for the first time, enter the following command asroot
:~]#
yum install kernel-doc
- Online DocumentationSee RFC 3704 for an explanation of Ingress Filtering for Multihomed Networks.
4.5. Securing DNS Traffic with DNSSEC
4.5.1. Introduction to DNSSEC
DNS
client to authenticate and check the integrity of responses from a DNS
nameserver in order to verify their origin and to determine if they have been tampered with in transit.
4.5.2. Understanding DNSSEC
HTTPS
. However, before connecting to an HTTPS
webserver, a DNS
lookup must be performed, unless you enter the IP address directly. These DNS
lookups are done insecurely and are subject to man-in-the-middle attacks due to lack of authentication. In other words, a DNS
client cannot have confidence that the replies that appear to come from a given DNS
nameserver are authentic and have not been tampered with. More importantly, a recursive nameserver cannot be sure that the records it obtains from other nameservers are genuine. The DNS
protocol did not provide a mechanism for the client to ensure it was not subject to a man-in-the-middle attack. DNSSEC was introduced to address the lack of authentication and integrity checks when resolving domain names using DNS
. It does not address the problem of confidentiality.
DNS
resource records as well as distributing public keys in such a way as to enable DNS
resolvers to build a hierarchical chain of trust. Digital signatures for all DNS
resource records are generated and added to the zone as digital signature resource records (RRSIG). The public key of a zone is added as a DNSKEY resource record. To build the hierarchical chain, hashes of the DNSKEY are published in the parent zone as Delegation of Signing (DS) resource records. To facilitate proof of non-existence, the NextSECure (NSEC) and NSEC3 resource records are used. In a DNSSEC signed zone, each resource record set (RRset) has a corresponding RRSIG resource record. Note that records used for delegation to a child zone (NS and glue records) are not signed; these records appear in the child zone and are signed there.
.com
. The root zone also serves NS and glue records for the .com
name servers. The resolver follows this delegation and queries for the DNSKEY record of .com
using these delegated name servers. The hash of the DNSKEY record obtained should match the DS record in the root zone. If so, the resolver will trust the obtained DNSKEY for .com
. In the .com
zone, the RRSIG records are created by the .com
DNSKEY. This process is repeated similarly for delegations within .com
, such as redhat.com
. Using this method, a validating DNS
resolver only needs to be configured with one root key while it collects many DNSKEYs from around the world during its normal operation. If a cryptographic check fails, the resolver will return SERVFAIL to the application.
DNS
answers. DNSSEC protects the integrity of the data between DNS
servers (authoritative and recursive), it does not provide security between the application and the resolver. Therefore, it is important that the applications are given a secure transport to their resolver. The easiest way to accomplish that is to run a DNSSEC capable resolver on localhost
and use 127.0.0.1
in /etc/resolv.conf
. Alternatively a VPN connection to a remote DNS
server could be used.
Understanding the Hotspot Problem
DNS
in order to redirect users to a page where they are required to authenticate (or pay) for the Wi-Fi service. Users connecting to a VPN often need to use an “internal only” DNS
server in order to locate resources that do not exist outside the corporate network. This requires additional handling by software. For example, dnssec-trigger can be used to detect if a Hotspot is hijacking the DNS
queries and unbound
can act as a proxy nameserver to handle the DNSSEC queries.
Choosing a DNSSEC Capable Recursive Resolver
unbound
can be used. Both enable DNSSEC by default and are configured with the DNSSEC root key. To enable DNSSEC on a server, either will work however the use of unbound
is preferred on mobile devices, such as notebooks, as it allows the local user to dynamically reconfigure the DNSSEC overrides required for Hotspots when using dnssec-trigger, and for VPNs when using Libreswan. The unbound
daemon further supports the deployment of DNSSEC exceptions listed in the etc/unbound/*.d/
directories which can be useful to both servers and mobile devices.
4.5.3. Understanding Dnssec-trigger
unbound
is installed and configured in /etc/resolv.conf
, all DNS
queries from applications are processed by unbound
. dnssec-trigger only reconfigures the unbound
resolver when triggered to do so. This mostly applies to roaming client machines, such as laptops, that connect to different Wi-Fi networks. The process is as follows:
- NetworkManager “triggers” dnssec-trigger when a new
DNS
server is obtained throughDHCP
. - Dnssec-trigger then performs a number of tests against the server and decides whether or not it properly supports DNSSEC.
- If it does, then dnssec-trigger reconfigures
unbound
to use thatDNS
server as a forwarder for all queries. - If the tests fail, dnssec-trigger will ignore the new
DNS
server and try a few available fall-back methods. - If it determines that an unrestricted port 53 (
UDP
andTCP
) is available, it will tellunbound
to become a full recursiveDNS
server without using any forwarder. - If this is not possible, for example because port 53 is blocked by a firewall for everything except reaching the network's
DNS
server itself, it will try to useDNS
to port 80, orTLS
encapsulatedDNS
to port 443. Servers runningDNS
on port 80 and 443 can be configured in/etc/dnssec-trigger/dnssec-trigger.conf
. Commented out examples should be available in the default configuration file. - If these fall-back methods also fail, dnssec-trigger offers to either operate insecurely, which would bypass DNSSEC completely, or run in “cache only” mode where it will not attempt new
DNS
queries but will answer for everything it already has in the cache.
dnssec-trigger
daemon continues to probe for DNSSEC resolvers every ten seconds. See Section 4.5.8, “Using Dnssec-trigger” for information on using the dnssec-trigger graphical utility.
4.5.4. VPN Supplied Domains and Name Servers
unbound
, dnssec-trigger, and NetworkManager can properly support domains and name servers provided by VPN software. Once the VPN tunnel comes up, the local unbound
cache is flushed for all entries of the domain name received, so that queries for names within the domain name are fetched fresh from the internal name servers reached using the VPN. When the VPN tunnel is terminated, the unbound
cache is flushed again to ensure any queries for the domain will return the public IP addresses, and not the previously obtained private IP addresses. See Section 4.5.11, “Configuring DNSSEC Validation for Connection Supplied Domains”.
4.5.5. Recommended Naming Practices
DNS
, such as host.example.com
.
.yourcompany
) to the public register. Therefore, Red Hat strongly recommends that you do not use a domain name that is not delegated to you, even on a private network, as this can result in a domain name that resolves differently depending on network configuration. As a result, network resources can become unavailable. Using domain names that are not delegated to you also makes DNSSEC more difficult to deploy and maintain, as domain name collisions require manual configuration to enable DNSSEC validation. See the ICANN FAQ on domain name collision for more information on this issue.
4.5.6. Understanding Trust Anchors
DNS
name and public key (or hash of the public key) associated with that name. It is expressed as a base 64 encoded key. It is similar to a certificate in that it is a means of exchanging information, including a public key, which can be used to verify and authenticate DNS
records. RFC 4033 defines a trust anchor as a configured DNSKEY RR or DS RR hash of a DNSKEY RR. A validating security-aware resolver uses this public key or hash as a starting point for building the authentication chain to a signed DNS response. In general, a validating resolver will have to obtain the initial values of its trust anchors through some secure or trusted means outside the DNS protocol. Presence of a trust anchor also implies that the resolver should expect the zone to which the trust anchor points to be signed.
4.5.7. Installing DNSSEC
4.5.7.1. Installing unbound
DNS
using DNSSEC locally on a machine, it is necessary to install the DNS
resolver unbound
(or bind
). It is only necessary to install dnssec-trigger on mobile devices. For servers, unbound
should be sufficient although a forwarding configuration for the local domain might be required depending on where the server is located (LAN or Internet). dnssec-trigger will currently only help with the global public DNS zone. NetworkManager, dhclient, and VPN applications can often gather the domain list (and nameserver list as well) automatically, but not dnssec-trigger nor unbound.
unbound
enter the following command as the root
user:
~]# yum install unbound
4.5.7.2. Checking if unbound is Running
unbound
daemon is running, enter the following command:
~]$ systemctl status unbound
unbound.service - Unbound recursive Domain Name Server
Loaded: loaded (/usr/lib/systemd/system/unbound.service; disabled)
Active: active (running) since Wed 2013-03-13 01:19:30 CET; 6h ago
systemctl status
command will report unbound
as Active: inactive (dead)
if the unbound
service is not running.
4.5.7.3. Starting unbound
unbound
daemon for the current session, enter the following command as the root
user:
~]# systemctl start unbound
systemctl enable
command to ensure that unbound
starts up every time the system boots:
~]# systemctl enable unbound
unbound
daemon allows configuration of local data or overrides using the following directories:
- The
/etc/unbound/conf.d
directory is used to add configurations for a specific domain name. This is used to redirect queries for a domain name to a specificDNS
server. This is often used for sub-domains that only exist within a corporate WAN. - The
/etc/unbound/keys.d
directory is used to add trust anchors for a specific domain name. This is required when an internal-only name is DNSSEC signed, but there is no publicly existing DS record to build a path of trust. Another use case is when an internal version of a domain is signed using a different DNSKEY than the publicly available name outside the corporate WAN. - The
/etc/unbound/local.d
directory is used to add specificDNS
data as a local override. This can be used to build blacklists or create manual overrides. This data will be returned to clients byunbound
, but it will not be marked as DNSSEC signed.
unbound.conf(5)
man page.
4.5.7.4. Installing Dnssec-trigger
dnssec-triggerd
. To install dnssec-trigger enter the following command as the root
user:
~]# yum install dnssec-trigger
4.5.7.5. Checking if the Dnssec-trigger Daemon is Running
dnssec-triggerd
is running, enter the following command:
~]$ systemctl status dnssec-triggerd
systemctl status dnssec-triggerd.service
dnssec-triggerd.service - Reconfigure local DNS(SEC) resolver on network change
Loaded: loaded (/usr/lib/systemd/system/dnssec-triggerd.service; enabled)
Active: active (running) since Wed 2013-03-13 06:10:44 CET; 1h 41min ago
systemctl status
command will report dnssec-triggerd
as Active: inactive (dead)
if the dnssec-triggerd
daemon is not running. To start it for the current session enter the following command as the root
user:
~]# systemctl start dnssec-triggerd
systemctl enable
command to ensure that dnssec-triggerd
starts up every time the system boots:
~]# systemctl enable dnssec-triggerd
4.5.8. Using Dnssec-trigger
DNSSEC
and then press Enter. An icon resembling a ships anchor is added to the message tray at the bottom of the screen. Press the round blue notification icon in the bottom right of the screen to reveal it. Right click the anchor icon to display a pop-up menu.
resolv.conf
points to 127.0.0.1
. When you click on the Hotspot Sign-On panel this is changed. The DNS
servers are queried from NetworkManager and put in resolv.conf
. Now you can authenticate on the Hotspot's sign-on page. The anchor icon shows a big red exclamation mark to warn you that DNS
queries are being made insecurely. When authenticated, dnssec-trigger should automatically detect this and switch back to secure mode, although in some cases it cannot and the user has to do this manually by selecting Reprobe.
unbound
about changes to the resolv.conf
file.
4.5.9. Using dig With DNSSEC
DNS
utilities nslookup and host are obsolete and should not be used.
+dnssec
is added to the command, for example:
~]$ dig +dnssec whitehouse.gov
; <<>> DiG 9.9.3-rl.13207.22-P2-RedHat-9.9.3-4.P2.el7 <<>> +dnssec whitehouse.gov
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21388
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096
;; QUESTION SECTION:
;whitehouse.gov. IN A
;; ANSWER SECTION:
whitehouse.gov. 20 IN A 72.246.36.110
whitehouse.gov. 20 IN RRSIG A 7 2 20 20130825124016 20130822114016 8399 whitehouse.gov. BB8VHWEkIaKpaLprt3hq1GkjDROvkmjYTBxiGhuki/BJn3PoIGyrftxR HH0377I0Lsybj/uZv5hL4UwWd/lw6Gn8GPikqhztAkgMxddMQ2IARP6p wbMOKbSUuV6NGUT1WWwpbi+LelFMqQcAq3Se66iyH0Jem7HtgPEUE1Zc 3oI=
;; Query time: 227 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Aug 22 22:01:52 EDT 2013
;; MSG SIZE rcvd: 233
In addition to the A record, an RRSIG record is returned which contains the DNSSEC signature, as well as the inception time and expiration time of the signature. The unbound
server indicated that the data was DNSSEC authenticated by returning the ad
bit in the flags:
section at the top.
dig
command would return a SERVFAIL error:
~]$ dig badsign-a.test.dnssec-tools.org
; <<>> DiG 9.9.3-rl.156.01-P1-RedHat-9.9.3-3.P1.el7 <<>> badsign-a.test.dnssec-tools.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 1010
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;badsign-a.test.dnssec-tools.org. IN A
;; Query time: 1284 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Aug 22 22:04:52 EDT 2013
;; MSG SIZE rcvd: 60]
+cd
option to the dig
command:
~]$ dig +cd +dnssec badsign-a.test.dnssec-tools.org
; <<>> DiG 9.9.3-rl.156.01-P1-RedHat-9.9.3-3.P1.el7 <<>> +cd +dnssec badsign-a.test.dnssec-tools.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26065
;; flags: qr rd ra cd; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096
;; QUESTION SECTION:
;badsign-a.test.dnssec-tools.org. IN A
;; ANSWER SECTION:
badsign-a.test.dnssec-tools.org. 49 IN A 75.119.216.33
badsign-a.test.dnssec-tools.org. 49 IN RRSIG A 5 4 86400 20130919183720 20130820173720 19442 test.dnssec-tools.org. E572dLKMvYB4cgTRyAHIKKEvdOP7tockQb7hXFNZKVbfXbZJOIDREJrr zCgAfJ2hykfY0yJHAlnuQvM0s6xOnNBSvc2xLIybJdfTaN6kSR0YFdYZ n2NpPctn2kUBn5UR1BJRin3Gqy20LZlZx2KD7cZBtieMsU/IunyhCSc0 kYw=
;; Query time: 1 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Aug 22 22:06:31 EDT 2013
;; MSG SIZE rcvd: 257
systemctl status unbound
and the unbound
daemon logs these errors to syslog as follows: Aug 22 22:04:52 laptop unbound: [3065:0] info: validation failure badsign-a.test.dnssec-tools.org. A IN
unbound-host
:
~]$ unbound-host -C /etc/unbound/unbound.conf -v whitehouse.gov
whitehouse.gov has address 184.25.196.110 (secure)
whitehouse.gov has IPv6 address 2600:1417:11:2:8800::fc4 (secure)
whitehouse.gov has IPv6 address 2600:1417:11:2:8000::fc4 (secure)
whitehouse.gov mail is handled by 105 mail1.eop.gov. (secure)
whitehouse.gov mail is handled by 110 mail5.eop.gov. (secure)
whitehouse.gov mail is handled by 105 mail4.eop.gov. (secure)
whitehouse.gov mail is handled by 110 mail6.eop.gov. (secure)
whitehouse.gov mail is handled by 105 mail2.eop.gov. (secure)
whitehouse.gov mail is handled by 105 mail3.eop.gov. (secure)
4.5.10. Setting up Hotspot Detection Infrastructure for Dnssec-trigger
- Set up a web server on some machine that is publicly reachable on the Internet. See the Web Servers chapter in the Red Hat Enterprise Linux 7 System Administrator's Guide.
- Once you have the server running, publish a static page with known content on it. The page does not need to be a valid HTML page. For example, you could use a plain-text file named
hotspot.txt
that contains only the stringOK
. Assuming your server is located atexample.com
and you published yourhotspot.txt
file in the web serverdocument_root/static/
sub-directory, then the address to your static web page would beexample.com/static/hotspot.txt
. See theDocumentRoot
directive in the Web Servers chapter in the Red Hat Enterprise Linux 7 System Administrator's Guide. - Add the following line to the
/etc/dnssec-trigger/dnssec-trigger.conf
file:url: "http://example.com/static/hotspot.txt OK"
This command adds a URL that is probed usingHTTP
(port 80). The first part is the URL that will be resolved and the page that will be downloaded. The second part of the command is the text string that the downloaded webpage is expected to contain.
dnssec-trigger.conf(8)
.
4.5.11. Configuring DNSSEC Validation for Connection Supplied Domains
unbound
by dnssec-trigger for every domain provided by any connection, except Wi-Fi connections through NetworkManager. By default, all forward zones added into unbound
are DNSSEC validated.
validate_connection_provided_zones
variable in the dnssec-trigger configuration file /etc/dnssec.conf
. As root
user, open and edit the line as follows: validate_connection_provided_zones=noThe change is not done for any existing forward zones, but only for future forward zones. Therefore if you want to disable DNSSEC for the current provided domain, you need to reconnect.
4.5.11.1. Configuring DNSSEC Validation for Wi-Fi Supplied Domains
add_wifi_provided_zones
variable in the dnssec-trigger configuration file, /etc/dnssec.conf
. As root
user, open and edit the line as follows: add_wifi_provided_zones=yesThe change is not done for any existing forward zones, but only for future forward zones. Therefore, if you want to enable DNSSEC for the current Wi-Fi provided domain, you need to reconnect (restart) the Wi-Fi connection.
Warning
unbound
may have security implications such as:
- A Wi-Fi access point can intentionally provide you a domain through
DHCP
for which it does not have authority and route all yourDNS
queries to itsDNS
servers. - If you have the DNSSEC validation of forward zones turned off, the Wi-Fi provided
DNS
servers can spoof theIP
address for domain names from the provided domain without you knowing it.
4.5.12. Additional Resources
4.5.12.1. Installed Documentation
dnssec-trigger(8)
man page — Describes command options fordnssec-triggerd
, dnssec-trigger-control and dnssec-trigger-panel.dnssec-trigger.conf(8)
man page — Describes the configuration options fordnssec-triggerd
.unbound(8)
man page — Describes the command options forunbound
, theDNS
validating resolver.unbound.conf(5)
man page — Contains information on how to configureunbound
.resolv.conf(5)
man page — Contains information that is read by the resolver routines.
4.5.12.2. Online Documentation
- http://tools.ietf.org/html/rfc4033
- RFC 4033 DNS Security Introduction and Requirements.
- http://www.dnssec.net/
- A website with links to many DNSSEC resources.
- http://www.dnssec-deployment.org/
- The DNSSEC Deployment Initiative, sponsored by the Department for Homeland Security, contains a lot of DNSSEC information and has a mailing list to discuss DNSSEC deployment issues.
- http://www.internetsociety.org/deploy360/dnssec/community/
- The Internet Society's “Deploy 360” initiative to stimulate and coordinate DNSSEC deployment is a good resource for finding communities and DNSSEC activities worldwide.
- http://www.unbound.net/
- This document contains general information about the
unbound
DNS
service. - http://www.nlnetlabs.nl/projects/dnssec-trigger/
- This document contains general information about dnssec-trigger.
4.6. Securing Virtual Private Networks (VPNs) Using Libreswan
IPsec
protocol which is supported by the Libreswan application. Libreswan is a continuation of the Openswan application and many examples from the Openswan documentation are interchangeable with Libreswan. The NetworkManager IPsec
plug-in is called NetworkManager-libreswan. Users of GNOME Shell should install the NetworkManager-libreswan-gnome package, which has NetworkManager-libreswan as a dependency. Note that the NetworkManager-libreswan-gnome package is only available from the Optional channel. See Enabling Supplementary and Optional Repositories.
IPsec
protocol for VPN is itself configured using the Internet Key Exchange (IKE) protocol. The terms IPsec and IKE are used interchangeably. An IPsec VPN is also called an IKE VPN, IKEv2 VPN, XAUTH VPN, Cisco VPN or IKE/IPsec VPN. A variant of an IPsec VPN that also uses the Level 2 Tunneling Protocol (L2TP) is usually called an L2TP/IPsec VPN, which requires the Optional channel xl2tpd application.
IKE
implementation available in Red Hat Enterprise Linux 7. IKE
version 1 and 2 are implemented as a user-level daemon. The IKE protocol itself is also encrypted. The IPsec
protocol is implemented by the Linux kernel and Libreswan configures the kernel to add and remove VPN tunnel configurations.
IKE
protocol uses UDP port 500 and 4500. The IPsec
protocol consists of two different protocols, Encapsulated Security Payload (ESP) which has protocol number 50, and Authenticated Header (AH) which as protocol number 51. The AH
protocol is not recommended for use. Users of AH
are recommended to migrate to ESP
with null encryption.
IPsec
protocol has two different modes of operation, Tunnel Mode
(the default) and Transport Mode
. It is possible to configure the kernel with IPsec without IKE. This is called Manual Keying
. It is possible to configure manual keying using the ip xfrm
commands, however, this is strongly discouraged for security reasons. Libreswan interfaces with the Linux kernel using netlink. Packet encryption and decryption happen in the Linux kernel.
Important
IKE
/IPsec
VPNs, implemented by Libreswan and the Linux kernel, is the only VPN technology recommended for use in Red Hat Enterprise Linux 7. Do not use any other VPN technology without understanding the risks of doing so.
4.6.1. Installing Libreswan
root
:
~]# yum install libreswan
~]$ yum info libreswan
~]#systemctl stop ipsec
~]#rm /etc/ipsec.d/*db
root
:
~]# ipsec initnss
Initializing NSS database
~]# certutil -N -d sql:/etc/ipsec.d
Enter a password which will be used to encrypt your keys.
The password should be at least 8 characters long,
and should contain at least one non-alphabetic character.
Enter new password:
Re-enter password:
ipsec
daemon provided by Libreswan, issue the following command as root
:
~]# systemctl start ipsec
~]$ systemctl status ipsec
* ipsec.service - Internet Key Exchange (IKE) Protocol Daemon for IPsec
Loaded: loaded (/usr/lib/systemd/system/ipsec.service; disabled; vendor preset: disabled)
Active: active (running) since Sun 2018-03-18 18:44:43 EDT; 3s ago
Docs: man:ipsec(8)
man:pluto(8)
man:ipsec.conf(5)
Process: 20358 ExecStopPost=/usr/sbin/ipsec --stopnflog (code=exited, status=0/SUCCESS)
Process: 20355 ExecStopPost=/sbin/ip xfrm state flush (code=exited, status=0/SUCCESS)
Process: 20352 ExecStopPost=/sbin/ip xfrm policy flush (code=exited, status=0/SUCCESS)
Process: 20347 ExecStop=/usr/libexec/ipsec/whack --shutdown (code=exited, status=0/SUCCESS)
Process: 20634 ExecStartPre=/usr/sbin/ipsec --checknflog (code=exited, status=0/SUCCESS)
Process: 20631 ExecStartPre=/usr/sbin/ipsec --checknss (code=exited, status=0/SUCCESS)
Process: 20369 ExecStartPre=/usr/libexec/ipsec/_stackmanager start (code=exited, status=0/SUCCESS)
Process: 20366 ExecStartPre=/usr/libexec/ipsec/addconn --config /etc/ipsec.conf --checkconfig (code=exited, status=0/SUCCESS)
Main PID: 20646 (pluto)
Status: "Startup completed."
CGroup: /system.slice/ipsec.service
└─20646 /usr/libexec/ipsec/pluto --leak-detective --config /etc/ipsec.conf --nofork
root
:
~]# systemctl enable ipsec
ipsec
service. See Chapter 5, Using Firewalls for information on firewalls and allowing specific services to pass through. Libreswan requires the firewall to allow the following packets:
UDP
port 500 and 4500 for theInternet Key Exchange
(IKE) protocol- Protocol 50 for
Encapsulated Security Payload
(ESP)IPsec
packets - Protocol 51 for
Authenticated Header
(AH)IPsec
packets (uncommon)
IPsec
VPN. The first example is for connecting two hosts together so that they may communicate securely. The second example is connecting two sites together to form one network. The third example is supporting remote users, known as road warriors in this context.
4.6.2. Creating VPN Configurations Using Libreswan
- Pre-Shared Keys (PSK) is the simplest authentication method. PSKs should consist of random characters and have a length of at least 20 characters. In FIPS mode, PSKs need to comply to a minimum strength requirement depending on the integrity algorithm used. It is recommended not to use PSKs shorter than 64 random characters.
- Raw RSA keys are commonly used for static host-to-host or subnet-to-subnet
IPsec
configurations. The hosts are manually configured with each other's public RSA key. This method does not scale well when dozens or more hosts all need to setupIPsec
tunnels to each other. - X.509 certificates are commonly used for large-scale deployments where there are many hosts that need to connect to a common
IPsec
gateway. A central certificate authority (CA) is used to sign RSA certificates for hosts or users. This central CA is responsible for relaying trust, including the revocations of individual hosts or users. - NULL Authentication is used to gain mesh encryption without authentication. It protects against passive attacks but does not protect against active attacks. However, since
IKEv2
allows asymmetrical authentication methods, NULL Authentication can also be used for internet scale Opportunistic IPsec, where clients authenticate the server, but servers do not authenticate the client. This model is similar to secure websites usingTLS
(also known as https:// websites).
4.6.3. Creating Host-To-Host VPN Using Libreswan
IPsec
VPN, between two hosts referred to as “left” and “right”, enter the following commands as root
on both of the hosts (“left” and “right”) to create new raw RSA key pairs:
~]# ipsec newhostkey --output /etc/ipsec.d/hostkey.secrets
Generated RSA key pair with CKAID 14936e48e756eb107fa1438e25a345b46d80433f was stored in the NSS database
root
on the host where the new hostkey was added, using the CKAID returned by the “newhostkey” command:
~]# ipsec showhostkey --left --ckaid 14936e48e756eb107fa1438e25a345b46d80433f
# rsakey AQPFKElpV
leftrsasigkey=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
~]# ipsec showhostkey --list
< 1 > RSA keyid: AQPFKElpV ckaid: 14936e48e756eb107fa1438e25a345b46d80433f
/etc/ipsec.d/*.db
.
leftrsasigkey=
and rightrsasigkey=
from above are added to a custom configuration file placed in the /etc/ipsec.d/
directory.
root
, create a file with a suitable name in the following format:
/etc/ipsec.d/my_host-to-host.conf
conn mytunnel leftid=@west.example.com left=192.1.2.23 leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ== rightid=@east.example.com right=192.1.2.45 rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ== authby=rsasig # load and initiate automatically auto=start
IP
address is not known in advance, then on the mobile client use %defaultroute
as its IP
address. This will pick up the dynamic IP
address automatically. On the static server host that accepts connections from incoming mobile hosts, specify the mobile host using %any
for its IP
address.
leftrsasigkey
value is obtained from the “left” host and the rightrsasigkey
value is obtained from the “right” host. The same applies when using leftckaid
and rightckaid
.
ipsec
to ensure it reads the new configuration and if configured to start on boot, to confirm that the tunnels establish:
~]# systemctl restart ipsec
auto=start
option, the IPsec
tunnel should be established within a few seconds. You can manually load and start the tunnel by entering the following commands as root
:
~]#ipsec auto --add mytunnel
~]#ipsec auto --up mytunnel
4.6.3.1. Verifying Host-To-Host VPN Using Libreswan
IKE
negotiation takes place on UDP
ports 500 and 4500. IPsec
packets show up as Encapsulated Security Payload
(ESP) packets. The ESP
protocol has no ports. When the VPN connection needs to pass through a NAT router, the ESP
packets are encapsulated in UDP
packets on port 4500.
root
in the following format:
~]# tcpdump -n -i interface esp or udp port 500 or udp port 4500
00:32:32.632165 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1a), length 132
00:32:32.632592 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1a), length 132
00:32:32.632592 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 7, length 64
00:32:33.632221 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1b), length 132
00:32:33.632731 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1b), length 132
00:32:33.632731 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 8, length 64
00:32:34.632183 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1c), length 132
00:32:34.632607 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1c), length 132
00:32:34.632607 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 9, length 64
00:32:35.632233 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1d), length 132
00:32:35.632685 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1d), length 132
00:32:35.632685 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 10, length 64
Note
IPsec
. It only sees the outgoing encrypted packet, not the outgoing plaintext packet. It does see the encrypted incoming packet, as well as the decrypted incoming packet. If possible, run tcpdump on a router between the two machines and not on one of the endpoints itself. When using the Virtual Tunnel Interface (VTI), tcpdump on the physical interface shows ESP
packets, while tcpdump on the VTI interface shows the cleartext traffic.
root
:
~]# ipsec whack --trafficstatus
006 #2: "mytunnel", type=ESP, add_time=1234567890, inBytes=336, outBytes=336, id='@east'
4.6.4. Configuring Site-to-Site VPN Using Libreswan
IPsec
VPN, joining together two networks, an IPsec
tunnel is created between two hosts, endpoints, which are configured to permit traffic from one or more subnets to pass through. They can therefore be thought of as gateways to the remote portion of the network. The configuration of the site-to-site VPN only differs from the host-to-host VPN in that one or more networks or subnets must be specified in the configuration file.
IPsec
VPN, first configure a host-to-host IPsec
VPN as described in Section 4.6.3, “Creating Host-To-Host VPN Using Libreswan” and then copy or move the file to a file with a suitable name, such as /etc/ipsec.d/my_site-to-site.conf
. Using an editor running as root
, edit the custom configuration file /etc/ipsec.d/my_site-to-site.conf
as follows:
conn mysubnet also=mytunnel leftsubnet=192.0.1.0/24 rightsubnet=192.0.2.0/24 auto=start conn mysubnet6 also=mytunnel connaddrfamily=ipv6 leftsubnet=2001:db8:0:1::/64 rightsubnet=2001:db8:0:2::/64 auto=start conn mytunnel leftid=@west.example.com left=192.1.2.23 leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ== rightid=@east.example.com right=192.1.2.45 rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ== authby=rsasig
root
:
~]# ipsec auto --add mysubnet
~]# ipsec auto --add mysubnet6
~]# ipsec auto --up mysubnet
104 "mysubnet" #1: STATE_MAIN_I1: initiate
003 "mysubnet" #1: received Vendor ID payload [Dead Peer Detection]
003 "mytunnel" #1: received Vendor ID payload [FRAGMENTATION]
106 "mysubnet" #1: STATE_MAIN_I2: sent MI2, expecting MR2
108 "mysubnet" #1: STATE_MAIN_I3: sent MI3, expecting MR3
003 "mysubnet" #1: received Vendor ID payload [CAN-IKEv2]
004 "mysubnet" #1: STATE_MAIN_I4: ISAKMP SA established {auth=OAKLEY_RSA_SIG cipher=aes_128 prf=oakley_sha group=modp2048}
117 "mysubnet" #2: STATE_QUICK_I1: initiate
004 "mysubnet" #2: STATE_QUICK_I2: sent QI2, IPsec SA established tunnel mode {ESP=>0x9414a615 <0x1a8eb4ef xfrm=AES_128-HMAC_SHA1 NATOA=none NATD=none DPD=none}
~]# ipsec auto --up mysubnet6
003 "mytunnel" #1: received Vendor ID payload [FRAGMENTATION]
117 "mysubnet" #2: STATE_QUICK_I1: initiate
004 "mysubnet" #2: STATE_QUICK_I2: sent QI2, IPsec SA established tunnel mode {ESP=>0x06fe2099 <0x75eaa862 xfrm=AES_128-HMAC_SHA1 NATOA=none NATD=none DPD=none}
4.6.4.1. Verifying Site-to-Site VPN Using Libreswan
4.6.5. Configuring Site-to-Site Single Tunnel VPN Using Libreswan
IP
addresses instead of their public IP
addresses. This can be accomplished using a single tunnel. If the left host, with host name west
, has internal IP
address 192.0.1.254
and the right host, with host name east
, has internal IP
address 192.0.2.254
, store the following configuration using a single tunnel to the /etc/ipsec.d/myvpn.conf
file on both servers:
conn mysubnet leftid=@west.example.com leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ== left=192.1.2.23 leftsourceip=192.0.1.254 leftsubnet=192.0.1.0/24 rightid=@east.example.com rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ== right=192.1.2.45 rightsourceip=192.0.2.254 rightsubnet=192.0.2.0/24 auto=start authby=rsasig
4.6.6. Configuring Subnet Extrusion Using Libreswan
IPsec
is often deployed in a hub-and-spoke architecture. Each leaf node has an IP
range that is part of a larger range. Leaves communicate with each other through the hub. This is called subnet extrusion.
Example 4.2. Configuring Simple Subnet Extrusion Setup
10.0.0.0/8
and two branches that use a smaller /24
subnet.
conn branch1 left=1.2.3.4 leftid=@headoffice leftsubnet=0.0.0.0/0 leftrsasigkey=0sA[...] # right=5.6.7.8 rightid=@branch1 rightsubnet=10.0.1.0/24 rightrsasigkey=0sAXXXX[...] # auto=start authby=rsasig conn branch2 left=1.2.3.4 leftid=@headoffice leftsubnet=0.0.0.0/0 leftrsasigkey=0sA[...] # right=10.11.12.13 rightid=@branch2 rightsubnet=10.0.2.0/24 rightrsasigkey=0sAYYYY[...] # auto=start authby=rsasig
conn branch1 left=1.2.3.4 leftid=@headoffice leftsubnet=0.0.0.0/0 leftrsasigkey=0sA[...] # right=10.11.12.13 rightid=@branch2 rightsubnet=10.0.1.0/24 rightrsasigkey=0sAYYYY[...] # auto=start authby=rsasig conn passthrough left=1.2.3.4 right=0.0.0.0 leftsubnet=10.0.1.0/24 rightsubnet=10.0.1.0/24 authby=never type=passthrough auto=route
4.6.7. Configuring IKEv2 Remote Access VPN Libreswan
IP
address, such as laptops. These are authenticated using certificates. To avoid needing to use the old IKEv1 XAUTH protocol, IKEv2 is used in the following example:
conn roadwarriors ikev2=insist # Support (roaming) MOBIKE clients (RFC 4555) mobike=yes fragmentation=yes left=1.2.3.4 # if access to the LAN is given, enable this, otherwise use 0.0.0.0/0 # leftsubnet=10.10.0.0/16 leftsubnet=0.0.0.0/0 leftcert=vpn-server.example.com leftid=%fromcert leftxauthserver=yes leftmodecfgserver=yes right=%any # trust our own Certificate Agency rightca=%same # pick an IP address pool to assign to remote users # 100.64.0.0/16 prevents RFC1918 clashes when remote users are behind NAT rightaddresspool=100.64.13.100-100.64.13.254 # if you want remote clients to use some local DNS zones and servers modecfgdns="1.2.3.4, 5.6.7.8" modecfgdomains="internal.company.com, corp" rightxauthclient=yes rightmodecfgclient=yes authby=rsasig # optionally, run the client X.509 ID through pam to allow/deny client # pam-authorize=yes # load connection, don't initiate auto=add # kill vanished roadwarriors dpddelay=1m dpdtimeout=5m dpdaction=%clear
left=1.2.3.4
- The 1.2.3.4 value specifies the actual IP address or host name of your server.
leftcert=vpn-server.example.com
- This option specifies a certificate referring to its friendly name or nickname that has been used to import the certificate. Usually, the name is generated as a part of a PKCS #12 certificate bundle in the form of a
.p12
file. See thepkcs12(1)
andpk12util(1)
man pages for more information.
conn to-vpn-server ikev2=insist # pick up our dynamic IP left=%defaultroute leftsubnet=0.0.0.0/0 leftcert=myname.example.com leftid=%fromcert leftmodecfgclient=yes # right can also be a DNS hostname right=1.2.3.4 # if access to the remote LAN is required, enable this, otherwise use 0.0.0.0/0 # rightsubnet=10.10.0.0/16 rightsubnet=0.0.0.0/0 # trust our own Certificate Agency rightca=%same authby=rsasig # allow narrowing to the server’s suggested assigned IP and remote subnet narrowing=yes # Support (roaming) MOBIKE clients (RFC 4555) mobike=yes # Initiate connection auto=start
auto=start
- This option enables the user to connect to the VPN whenever the
ipsec
system service is started. Replace it with theauto=add
if you want to establish the connection later.
4.6.8. Configuring IKEv1 Remote Access VPN Libreswan and XAUTH with X.509
IP
address and DNS information to roaming VPN clients as the connection is established by using the XAUTH IPsec
extension. Extended authentication (XAUTH) can be deployed using PSK or X.509 certificates. Deploying using X.509 is more secure. Client certificates can be revoked by a certificate revocation list or by Online Certificate Status Protocol (OCSP). With X.509 certificates, individual clients cannot impersonate the server. With a PSK, also called Group Password, this is theoretically possible.
xauthby=pam
- This uses the configuration in
/etc/pam.d/pluto
to authenticate the user. Pluggable Authentication Modules (PAM) can be configured to use various back ends by itself. It can use the system account user-password scheme, an LDAP directory, a RADIUS server or a custom password authentication module. See the Using Pluggable Authentication Modules (PAM) chapter for more information. xauthby=file
- This uses the
/etc/ipsec.d/passwd
configuration file (it should not be confused with the/etc/ipsec.d/nsspassword
file). The format of this file is similar to the Apache.htpasswd
file and the Apachehtpasswd
command can be used to create entries in this file. However, after the user name and password, a third column is required with the connection name of theIPsec
connection used, for example when using aconn remoteusers
to offer VPN to remove users, a password file entry should look as follows:user1:$apr1$MIwQ3DHb$1I69LzTnZhnCT2DPQmAOK.:remoteusers
Note
When using thehtpasswd
command, the connection name has to be manually added after the user:password part on each line. xauthby=alwaysok
- The server always pretends the XAUTH user and password combination is correct. The client still has to specify a user name and a password, although the server ignores these. This should only be used when users are already identified by X.509 certificates, or when testing the VPN without needing an XAUTH back end.
conn xauth-rsa ikev2=never auto=add authby=rsasig pfs=no rekey=no left=ServerIP leftcert=vpn.example.com #leftid=%fromcert leftid=vpn.example.com leftsendcert=always leftsubnet=0.0.0.0/0 rightaddresspool=10.234.123.2-10.234.123.254 right=%any rightrsasigkey=%cert modecfgdns="1.2.3.4,8.8.8.8" modecfgdomains=example.com modecfgbanner="Authorized access is allowed" leftxauthserver=yes rightxauthclient=yes leftmodecfgserver=yes rightmodecfgclient=yes modecfgpull=yes xauthby=pam dpddelay=30 dpdtimeout=120 dpdaction=clear ike_frag=yes # for walled-garden on xauth failure # xauthfail=soft # leftupdown=/custom/_updown
xauthfail
is set to soft, instead of hard, authentication failures are ignored, and the VPN is setup as if the user authenticated properly. A custom updown script can be used to check for the environment variable XAUTH_FAILED
. Such users can then be redirected, for example, using iptables DNAT, to a “walled garden” where they can contact the administrator or renew a paid subscription to the service.
modecfgdomain
value and the DNS entries to redirect queries for the specified domain to these specified nameservers. This allows roaming users to access internal-only resources using the internal DNS names. Note while IKEv2 supports a comma-separated list of domain names and nameserver IP addresses using modecfgdomains
and modecfgdns
, the IKEv1 protocol only supports one domain name, and libreswan only supports up to two nameserver IP addresses. Optionally, to send a banner text to VPN cliens, use the modecfgbanner
option.
leftsubnet
is not 0.0.0.0/0
, split tunneling configuration requests are sent automatically to the client. For example, when using leftsubnet=10.0.0.0/8
, the VPN client would only send traffic for 10.0.0.0/8
through the VPN.
xauthby=file
- The administrator generated the password and stored it in the
/etc/ipsec.d/passwd
file. xauthby=pam
- The password is obtained at the location specified in the PAM configuration in the
/etc/pam.d/pluto
file. xauthby=alwaysok
- The password is not checked and always accepted. Use this option for testing purposes or if you want to ensure compatibility for xauth-only clients.
Additional Resources
4.6.9. Using the Protection against Quantum Computers
ppk=yes
to the connection definition. To require PPK, add ppk=insist
. Then, each client can be given a PPK ID with a secret value that is communicated out-of-band (and preferably quantum safe). The PPK's should be very strong in randomness and not be based on dictionary words. The PPK ID and PPK data itself are stored in ipsec.secrets
, for example:
@west @east : PPKS "user1" "thestringismeanttobearandomstr"
PPKS
option refers to static PPKs. There is an experimental function to use one-time-pad based Dynamic PPKs. Upon each connection, a new part of a onetime pad is used as the PPK. When used, that part of the dynamic PPK inside the file is overwritten with zeroes to prevent re-use. If there is no more one time pad material left, the connection fails. See the ipsec.secrets(5)
man page for more information.
Warning
4.6.10. Additional Resources
ipsec
daemon.
4.6.10.1. Installed Documentation
ipsec(8)
man page — Describes command options foripsec
.ipsec.conf(5)
man page — Contains information on configuringipsec
.ipsec.secrets(5)
man page — Describes the format of theipsec.secrets
file.ipsec_auto(8)
man page — Describes the use of the auto command line client for manipulating LibreswanIPsec
connections established using automatic exchanges of keys.ipsec_rsasigkey(8)
man page — Describes the tool used to generate RSA signature keys./usr/share/doc/libreswan-version/
4.6.10.2. Online Documentation
- https://libreswan.org
- The website of the upstream project.
- https://libreswan.org/wiki
- The Libreswan Project Wiki.
- https://libreswan.org/man/
- All Libreswan man pages.
- NIST Special Publication 800-77: Guide to IPsec VPNs
- Practical guidance to organizations on implementing security services based on IPsec.
4.7. Using OpenSSL
list-standard-commands
, list-message-digest-commands
, and list-cipher-commands
output a list of all standard commands, message digest commands, or cipher commands, respectively, that are available in the present openssl utility.
list-cipher-algorithms
and list-message-digest-algorithms
list all cipher and message digest names. The pseudo-command list-public-key-algorithms
lists all supported public key algorithms. For example, to list the supported public key algorithms, issue the following command:
~]$ openssl list-public-key-algorithms
4.7.1. Creating and Managing Encryption Keys
~]$ openssl genpkey -algorithm
RSA -out
privkey.pem
rsa_keygen_bits:numbits
— The number of bits in the generated key. If not specified1024
is used.rsa_keygen_pubexp:value
— The RSA public exponent value. This can be a large decimal value, or a hexadecimal value if preceded by0x
. The default value is65537
.
3
as the public exponent, issue the following command:
~]$ openssl genpkey -algorithm
RSA -out
privkey.pem -pkeyopt
rsa_keygen_bits:2048 \ -pkeyopt
rsa_keygen_pubexp:3
~]$ openssl genpkey -algorithm
RSA -out
privkey.pem -aes-128-cbc
-pass
pass:hello
4.7.2. Generating Certificates
4.7.2.1. Creating a Certificate Signing Request
~]$ openssl req -new
-key
privkey.pem -out
cert.csr
cert.csr
encoded in the default privacy-enhanced electronic mail (PEM) format. The name PEM is derived from “Privacy Enhancement for Internet Electronic Mail” described in RFC 1424. To generate a certificate file in the alternative DER format, use the -outform
DER
command option.
- The two letter country code for your country
- The full name of your state or province
- City or Town
- The name of your organization
- The name of the unit within your organization
- Your name or the host name of the system
- Your email address
/etc/pki/tls/openssl.cnf
file. See man openssl.cnf(5)
for more information.
4.7.2.2. Creating a Self-signed Certificate
366
days, issue a command in the following format:
~]$ openssl req -new
-x509
-key
privkey.pem -out
selfcert.pem -days
366
4.7.2.3. Creating a Certificate Using a Makefile
/etc/pki/tls/certs/
directory contains a Makefile
which can be used to create certificates using the make
command. To view the usage instructions, issue a command as follows:
~]$ make -f /etc/pki/tls/certs/Makefile
Alternatively, change to the directory and issue the make
command as follows:
~]$cd /etc/pki/tls/certs/
~]$make
4.7.3. Verifying Certificates
~]$ openssl verify cert1.pem cert2.pem
cert.pem
and the intermediate certificates which you do not trust must be directly concatenated in untrusted.pem
. The trusted root CA certificate must be either among the default CA listed in /etc/pki/tls/certs/ca-bundle.crt
or in a cacert.pem
file. Then, to verify the chain, issue a command in the following format:
~]$ openssl verify -untrusted
untrusted.pem -CAfile
cacert.pem cert.pem
Important
4.7.4. Encrypting and Decrypting a File
pkeyutl
or enc
built-in commands can be used. With pkeyutl
, RSA
keys are used to perform the encrypting and decrypting, whereas with enc
, symmetric algorithms are used.
Using RSA Keys
plaintext
, issue a command as follows:
~]$ openssl pkeyutl -in
plaintext -out
cyphertext -inkey
privkey.pem
-keyform DER
option to specify the DER key format.
-engine
option as follows:
~]$ openssl pkeyutl -in
plaintext -out
cyphertext -inkey
privkey.pem -engine id
~]$ openssl engine -t
~]$ openssl pkeyutl -sign
-in
plaintext -out
sigtext -inkey
privkey.pem
~]$ openssl pkeyutl -verifyrecover
-in
sig -inkey
key.pem
~]$ openssl pkeyutl -verify
-in
file -sigfile sig -inkey
key.pem
Using Symmetric Algorithms
enc
command with an unsupported option, such as -l
:
~]$
openssl enc -l
aes-128-cbc
algorithm, use the following syntax:
openssl enc -aes-128-cbc
plaintext
using the aes-128-cbc
algorithm, enter the following command:
~]$
openssl enc -aes-128-cbc -in plaintext -out plaintext.aes-128-cbc
-d
option as in the following example:
~]$
openssl enc -aes-128-cbc -d -in plaintext.aes-128-cbc -out plaintext
Important
enc
command does not properly support AEAD
ciphers, and the ecb
mode is not considered secure. For best results, do not use other modes than cbc
, cfb
, ofb
, or ctr
.
4.7.5. Generating Message Digests
dgst
command produces the message digest of a supplied file or files in hexadecimal form. The command can also be used for digital signing and verification. The message digest command takes the following form:
openssl dgst algorithm-out
filename-sign
private-key
md5|md4|md2|sha1|sha|mdc2|ripemd160|dss1
. At time of writing, the SHA1 algorithm is preferred. If you need to sign or verify using DSA, then the dss1
option must be used together with a file containing random data specified by the -rand
option.
~]$ openssl dgst sha1 -out
digest-file
~]$ openssl dgst sha1 -out
digest-file -sign
privkey.pem
4.7.6. Generating Password Hashes
passwd
command computes the hash of a password. To compute the hash of a password on the command line, issue a command as follows:
~]$ openssl passwd password
-crypt
algorithm is used by default.
1
, issue a command as follows:
~]$ openssl passwd -1
password
-apr1
option specifies the Apache variant of the BSD algorithm.
Note
openssl passwd -1 password
command only with FIPS mode disabled. Otherwise, the command does not work.
xx
, issue a command as follows:
~]$ openssl passwd -salt
xx
-in
password-file
-out
option to specify an output file. The -table
will generate a table of password hashes with their corresponding clear text password.
4.7.7. Generating Random Data
~]$ openssl rand -out
rand-file -rand
seed-file
:
, as a list separator.
4.7.8. Benchmarking Your System
~]$ openssl speed algorithm
openssl speed
and then press tab.
4.7.9. Configuring OpenSSL
/etc/pki/tls/openssl.cnf
, referred to as the master configuration file, which is read by the OpenSSL library. It is also possible to have individual configuration files for each application. The configuration file contains a number of sections with section names as follows: [ section_name ]
. Note the first part of the file, up until the first [ section_name ]
, is referred to as the default section. When OpenSSL is searching for names in the configuration file the named sections are searched first. All OpenSSL commands use the master OpenSSL configuration file unless an option is used in the command to specify an alternative configuration file. The configuration file is explained in detail in the config(5)
man page.
4.8. Using stunnel
4.8.1. Installing stunnel
root
:
~]# yum install stunnel
4.8.2. Configuring stunnel as a TLS Wrapper
- You need a valid certificate for stunnel regardless of what service you use it with. If you do not have a suitable certificate, you can apply to a Certificate Authority to obtain one, or you can create a self-signed certificate.
Warning
Always use certificates signed by a Certificate Authority for servers running in a production environment. Self-signed certificates are only appropriate for testing purposes or private networks.See Section 4.7.2.1, “Creating a Certificate Signing Request” for more information about certificates granted by a Certificate Authority. On the other hand, to create a self-signed certificate for stunnel, enter the/etc/pki/tls/certs/
directory and type the following command asroot
:certs]#
make stunnel.pem
Answer all of the questions to complete the process. - When you have a certificate, create a configuration file for stunnel. It is a text file in which every line specifies an option or the beginning of a service definition. You can also keep comments and empty lines in the file to improve its legibility, where comments start with a semicolon.The stunnel RPM package contains the
/etc/stunnel/
directory, in which you can store the configuration file. Although stunnel does not require any special format of the file name or its extension, use/etc/stunnel/stunnel.conf
. The following content configures stunnel as a TLS wrapper:cert = /etc/pki/tls/certs/stunnel.pem ; Allow only TLS, thus avoiding SSL sslVersion = TLSv1 chroot = /var/run/stunnel setuid = nobody setgid = nobody pid = /stunnel.pid socket = l:TCP_NODELAY=1 socket = r:TCP_NODELAY=1 [service_name] accept = port connect = port TIMEOUTclose = 0
Alternatively, you can avoid SSL by replacing the line containingsslVersion = TLSv1
with the following lines:options = NO_SSLv2 options = NO_SSLv3
The purpose of the options is as follows:cert
— the path to your certificatesslVersion
— the version of SSL; note that you can useTLS
here even though SSL and TLS are two independent cryptographic protocolschroot
— the changed root directory in which the stunnel process runs, for greater securitysetuid
,setgid
— the user and group that the stunnel process runs as;nobody
is a restricted system accountpid
— the file in which stunnel saves its process ID, relative tochroot
socket
— local and remote socket options; in this case, disable Nagle's algorithm to improve network latency[service_name]
— the beginning of the service definition; the options used below this line apply to the given service only, whereas the options above affect stunnel globallyaccept
— the port to listen onconnect
— the port to connect to; this must be the port that the service you are securing usesTIMEOUTclose
— how many seconds to wait for the close_notify alert from the client;0
instructs stunnel not to wait at alloptions
— OpenSSL library options
Example 4.3. Securing CUPS
To configure stunnel as a TLS wrapper for CUPS, use the following values:[cups] accept = 632 connect = 631
Instead of632
, you can use any free port that you prefer.631
is the port that CUPS normally uses. - Create the
chroot
directory and give the user specified by thesetuid
option write access to it. To do so, enter the following commands asroot
:~]#
mkdir /var/run/stunnel
~]#chown nobody:nobody /var/run/stunnel
This allows stunnel to create the PID file. - If your system is using firewall settings that disallow access to the new port, change them accordingly. See Section 5.6.7, “Opening Ports using GUI” for details.
- When you have created the configuration file and the
chroot
directory, and when you are sure that the specified port is accessible, you are ready to start using stunnel.
4.8.3. Starting, Stopping, and Restarting stunnel
root
:
~]# stunnel /etc/stunnel/stunnel.conf
/var/log/secure
to log its output.
root
:
~]# kill `cat /var/run/stunnel/stunnel.pid`
4.9. Encryption
4.9.1. Using LUKS Disk Encryption
Overview of LUKS
- What LUKS does
- LUKS encrypts entire block devices and is therefore well-suited for protecting the contents of mobile devices such as removable storage media or laptop disk drives.
- The underlying contents of the encrypted block device are arbitrary. This makes it useful for encrypting
swap
devices. This can also be useful with certain databases that use specially formatted block devices for data storage. - LUKS uses the existing device mapper kernel subsystem.
- LUKS provides passphrase strengthening which protects against dictionary attacks.
- LUKS devices contain multiple key slots, allowing users to add backup keys or passphrases.
- What LUKS does not do:
- LUKS is not well-suited for scenarios requiring many (more than eight) users to have distinct access keys to the same device.
- LUKS is not well-suited for applications requiring file-level encryption.
Important
4.9.1.1. LUKS Implementation in Red Hat Enterprise Linux
cryptsetup --help
) is aes-cbc-essiv:sha256 (ESSIV - Encrypted Salt-Sector Initialization Vector). Note that the installation program, Anaconda, uses by default XTS mode (aes-xts-plain64). The default key size for LUKS is 256 bits. The default key size for LUKS with Anaconda (XTS mode) is 512 bits. Ciphers that are available are:
- AES - Advanced Encryption Standard - FIPS PUB 197
- Twofish (a 128-bit block cipher)
- Serpent
- cast5 - RFC 2144
- cast6 - RFC 2612
4.9.1.2. Manually Encrypting Directories
Warning
- Enter runlevel 1 by typing the following at a shell prompt as root:
telinit 1
- Unmount your existing
/home
:umount /home
- If the command in the previous step fails, use
fuser
to find processes hogging/home
and kill them:fuser -mvk /home
- Verify
/home
is no longer mounted:grep home /proc/mounts
- Fill your partition with random data:
shred -v --iterations=1 /dev/VG00/LV_home
This command proceeds at the sequential write speed of your device and may take some time to complete. It is an important step to ensure no unencrypted data is left on a used device, and to obfuscate the parts of the device that contain encrypted data as opposed to just random data. - Initialize your partition:
cryptsetup --verbose --verify-passphrase luksFormat /dev/VG00/LV_home
- Open the newly encrypted device:
cryptsetup luksOpen /dev/VG00/LV_home home
- Make sure the device is present:
ls -l /dev/mapper | grep home
- Create a file system:
mkfs.ext3 /dev/mapper/home
- Mount the file system:
mount /dev/mapper/home /home
- Make sure the file system is visible:
df -h | grep home
- Add the following to the
/etc/crypttab
file:home /dev/VG00/LV_home none
- Edit the
/etc/fstab
file, removing the old entry for/home
and adding the following line:/dev/mapper/home /home ext3 defaults 1 2
- Restore default SELinux security contexts:
/sbin/restorecon -v -R /home
- Reboot the machine:
shutdown -r now
- The entry in the
/etc/crypttab
makes your computer ask yourluks
passphrase on boot. - Log in as root and restore your backup.
4.9.1.3. Add a New Passphrase to an Existing Device
cryptsetup luksAddKey device
4.9.1.4. Remove a Passphrase from an Existing Device
cryptsetup luksRemoveKey device
4.9.1.5. Creating Encrypted Block Devices in Anaconda
Note
Note
kickstart
to set a separate passphrase for each new encrypted block device.
4.9.1.6. Additional Resources
4.9.2. Creating GPG Keys
4.9.2.1. Creating GPG Keys in GNOME
- Install the Seahorse utility, which makes GPG key management easier:
~]#
yum install seahorse
- To create a key, from the Seahorse.→ menu select , which starts the application
- From the PGP Key. Then click .menu select and then
- Type your full name, email address, and an optional comment describing who you are (for example: John C. Smith,
jsmith@example.com
, Software Engineer). Click . A dialog is displayed asking for a passphrase for the key. Choose a strong passphrase but also easy to remember. Click and the key is created.
Warning
0x
to the key ID, as in 0x6789ABCD
. You should make a backup of your private key and store it somewhere secure.
4.9.2.2. Creating GPG Keys in KDE
- Start the KGpg program from the main menu by selecting → → . If you have never used KGpg before, the program walks you through the process of creating your own GPG keypair.
- A dialog box appears prompting you to create a new key pair. Enter your name, email address, and an optional comment. You can also choose an expiration time for your key, as well as the key strength (number of bits) and algorithms.
- Enter your passphrase in the next dialog box. At this point, your key appears in the main KGpg window.
Warning
0x
to the key ID, as in 0x6789ABCD
. You should make a backup of your private key and store it somewhere secure.
4.9.2.3. Creating GPG Keys Using the Command Line
- Use the following shell command:
~]$
gpg2 --gen-key
This command generates a key pair that consists of a public and a private key. Other people use your public key to authenticate and decrypt your communications. Distribute your public key as widely as possible, especially to people who you know will want to receive authentic communications from you, such as a mailing list. - A series of prompts directs you through the process. Press the Enter key to assign a default value if desired. The first prompt asks you to select what kind of key you prefer:
Please select what kind of key you want: (1) RSA and RSA (default) (2) DSA and Elgamal (3) DSA (sign only) (4) RSA (sign only) Your selection?
In almost all cases, the default is the correct choice. An RSA/RSA key allows you not only to sign communications, but also to encrypt files. - Choose the key size:
RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048)
Again, the default, 2048, is sufficient for almost all users, and represents an extremely strong level of security. - Choose when the key will expire. It is a good idea to choose an expiration date instead of using the default, which is
none
. If, for example, the email address on the key becomes invalid, an expiration date will remind others to stop using that public key.Please specify how long the key should be valid. 0 = key does not expire d = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years key is valid for? (0)
Entering a value of 1y, for example, makes the key valid for one year. (You may change this expiration date after the key is generated, if you change your mind.) - Before the gpg2 application asks for signature information, the following prompt appears:
Is this correct (y/N)?
Entery
to finish the process. - Enter your name and email address for your GPG key. Remember this process is about authenticating you as a real individual. For this reason, include your real name. If you choose a bogus email address, it will be more difficult for others to find your public key. This makes authenticating your communications difficult. If you are using this GPG key for self-introduction on a mailing list, for example, enter the email address you use on that list.Use the comment field to include aliases or other information. (Some people use different keys for different purposes and identify each key with a comment, such as "Office" or "Open Source Projects.")
- At the confirmation prompt, enter the letter
O
to continue if all entries are correct, or use the other options to fix any problems. Finally, enter a passphrase for your secret key. The gpg2 program asks you to enter your passphrase twice to ensure you made no typing errors. - Finally, gpg2 generates random data to make your key as unique as possible. Move your mouse, type random keys, or perform other tasks on the system during this step to speed up the process. Once this step is finished, your keys are complete and ready to use:
pub 1024D/1B2AFA1C 2005-03-31 John Q. Doe <jqdoe@example.com> Key fingerprint = 117C FE83 22EA B843 3E86 6486 4320 545E 1B2A FA1C sub 1024g/CEA4B22E 2005-03-31 [expires: 2006-03-31]
- The key fingerprint is a shorthand "signature" for your key. It allows you to confirm to others that they have received your actual public key without any tampering. You do not need to write this fingerprint down. To display the fingerprint at any time, use this command, substituting your email address:
~]$
gpg2 --fingerprint jqdoe@example.com
Your "GPG key ID" consists of 8 hex digits identifying the public key. In the example above, the GPG key ID is1B2AFA1C
. In most cases, if you are asked for the key ID, prepend0x
to the key ID, as in0x6789ABCD
.
Warning
4.9.2.4. About Public Key Encryption
4.9.3. Using openCryptoki for Public-Key Cryptography
4.9.3.1. Installing openCryptoki and Starting the Service
root
:
~]# yum install opencryptoki
pkcsslotd
daemon. Start the daemon for the current session by executing the following command as root
:
~]# systemctl start pkcsslotd
~]# systemctl enable pkcsslotd
4.9.3.2. Configuring and Using openCryptoki
pkcsslotd
daemon reads the /etc/opencryptoki/opencryptoki.conf
configuration file, which it uses to collect information about the tokens configured to work with the system and about their slots.
pkcsslotd
daemon at run time, use the pkcsconf
utility. This tool allows you to show and configure the state of the daemon, as well as to list and modify the currently configured slots and tokens. For example, to display information about tokens, issue the following command (note that all non-root users that need to communicate with the pkcsslotd
daemon must be a part of the pkcs11
system group):
~]$ pkcsconf -t
pkcsconf
tool.
Warning
pkcs11
group, as all members of this group have the right to block other users of the openCryptoki service from accessing configured PKCS#11 tokens. All members of this group can also execute arbitrary code with the privileges of any other users of openCryptoki.
4.9.4. Using Smart Cards to Supply Credentials to OpenSSH
~/.ssh/authorized_keys
file. Install the PKCS#11
library provided by the opensc package on the client. PKCS#11
is a Public-Key Cryptography Standard that defines an application programming interface (API) to cryptographic devices called tokens. Enter the following command as root
:
~]#
yum install opensc
4.9.4.1. Retrieving a Public Key from a Card
ssh-keygen
command. Specify the shared library (OpenSC in the following example) with the -D
directive.
~]$
ssh-keygen -D /usr/lib64/pkcs11/opensc-pkcs11.so
ssh-rsa AAAAB3NzaC1yc[...]+g4Mb9
4.9.4.2. Storing a Public Key on a Server
smartcard.pub
in the following example) and using the ssh-copy-id
command:
~]$
ssh-copy-id -f -i smartcard.pub user@hostname
user@hostname's password: Number of key(s) added: 1 Now try logging into the machine, with: "ssh user@hostname" and check to make sure that only the key(s) you wanted were added.
SSH_COPY_ID_LEGACY=1
environment variable or the -f
option.
4.9.4.3. Authenticating to a Server with a Key on a Smart Card
[localhost ~]$
ssh -I /usr/lib64/pkcs11/opensc-pkcs11.so hostname
Enter PIN for 'Test (UserPIN)':[hostname ~]$
PKCS#11
library in your ~/.ssh/config
file:
Host hostname PKCS11Provider /usr/lib64/pkcs11/opensc-pkcs11.so
ssh
command without any additional options:
[localhost ~]$
ssh hostname
Enter PIN for 'Test (UserPIN)':[hostname ~]$
4.9.4.4. Using ssh-agent
to Automate PIN Logging In
ssh-agent
. You can skip this step in most cases because ssh-agent
is already running in a typical session. Use the following command to check whether you can connect to your authentication agent:
~]$
ssh-add -l
Could not open a connection to your authentication agent.~]$
eval `ssh-agent`
~]$
ssh-add -s /usr/lib64/pkcs11/opensc-pkcs11.so
Enter PIN for 'Test (UserPIN)': Card added: /usr/lib64/pkcs11/opensc-pkcs11.so
ssh-agent
, use the following command:
~]$
ssh-add -e /usr/lib64/pkcs11/opensc-pkcs11.so
Card removed: /usr/lib64/pkcs11/opensc-pkcs11.so
Note
pin_cache_ignore_user_consent = true;
option in the /etc/opensc-x86_64.conf
.
4.9.4.5. Additional Resources
PKCS#11
security tokens, see the pkcs11-tool(1)
man page.
4.9.5. Trusted and Encrypted Keys
RSA
key called the storage root key (SRK).
AES
encryption, which makes them faster than trusted keys. Encrypted keys are created using kernel-generated random numbers and encrypted by a master key when they are exported into user-space blobs. This master key can be either a trusted key or a user key, which is their main disadvantage — if the master key is not a trusted key, the encrypted key is only as secure as the user key used to encrypt it.
4.9.5.1. Working with keys
- For RHEL kernels with the
x86_64
architecture, the TRUSTED_KEYS and ENCRYPTED_KEYS code is built in as a part of the core kernel code. As a result, thex86_64
system users can use these keys without loading the trusted and encrypted-keys modules. - For all other architectures, it is necessary to load the trusted and encrypted-keys kernel modules before performing any operations with the keys. To load the kernel modules, execute the following command:
~]#
modprobe trusted encrypted-keys
Note
tpm_setactive
command from the tpm-tools package of utilities. Also, the TrouSers application needs to be installed (the trousers package), and the tcsd
daemon, which is a part of the TrouSers suite, running to communicate with the TPM.
keyctl
command with the following syntax:
~]$ keyctl add trusted name "new keylength [options]" keyring
~]$ keyctl add trusted kmk "new 32" @u
642500861
kmk
with the length of 32 bytes (256 bits) and places it in the user keyring (@u
). The keys may have a length of 32 to 128 bytes (256 to 1024 bits). Use the show
subcommand to list the current structure of the kernel keyrings:
~]$ keyctl show
Session Keyring
-3 --alswrv 500 500 keyring: _ses
97833714 --alswrv 500 -1 \_ keyring: _uid.1000
642500861 --alswrv 500 500 \_ trusted: kmk
print
subcommand outputs the encrypted key to the standard output. To export the key to a user-space blob, use the pipe
subcommand as follows:
~]$ keyctl pipe 642500861 > kmk.blob
add
command again with the blob as an argument:
~]$ keyctl add trusted kmk "load `cat kmk.blob`" @u
268728824
~]$ keyctl add encrypted name "new [format] key-type:master-key-name keylength" keyring
~]$ keyctl add encrypted encr-key "new trusted:kmk 32" @u
159771175
~]$ keyctl add user kmk-user "`dd if=/dev/urandom bs=1 count=32 2>/dev/null`" @u
427069434
~]$ keyctl add encrypted encr-key "new user:kmk-user 32" @u
1012412758
list
subcommand can be used to list all keys in the specified kernel keyring:
~]$ keyctl list @u
2 keys in keyring:
427069434: --alswrv 1000 1000 user: kmk-user
1012412758: --alswrv 1000 1000 encrypted: encr-key
Important
4.9.5.2. Additional Resources
Installed Documentation
- keyctl(1) — Describes the use of the keyctl utility and its subcommands.
Online Documentation
- Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide — The SELinux User's and Administrator's Guide for Red Hat Enterprise Linux 7 describes the basic principles of SELinux and documents in detail how to configure and use SELinux with various services, such as the Apache HTTP Server.
- https://www.kernel.org/doc/Documentation/security/keys-trusted-encrypted.txt — The official documentation about the trusted and encrypted keys feature of the Linux kernel.
See Also
- Section A.1.1, “Advanced Encryption Standard — AES” provides a concise description of the
Advanced Encryption Standard
. - Section A.2, “Public-key Encryption” describes the public-key cryptographic approach and the various cryptographic protocols it uses.
4.9.6. Using the Random Number Generator
rngd
daemon, which is a part of the rng-tools package, is capable of using both environmental noise and hardware random number generators for extracting entropy. The daemon checks whether the data supplied by the source of randomness is sufficiently random and then stores it in the random-number entropy pool of the kernel. The random numbers it generates are made available through the /dev/random
and /dev/urandom
character devices.
/dev/random
and /dev/urandom
is that the former is a blocking device, which means it stops supplying numbers when it determines that the amount of entropy is insufficient for generating a properly random output. Conversely, /dev/urandom
is a non-blocking source, which reuses the entropy pool of the kernel and is thus able to provide an unlimited supply of pseudo-random numbers, albeit with less entropy. As such, /dev/urandom
should not be used for creating long-term cryptographic keys.
root
user:
~]# yum install rng-tools
rngd
daemon, execute the following command as root
:
~]# systemctl start rngd
~]# systemctl status rngd
rngd
daemon with optional parameters, execute it directly. For example, to specify an alternative source of random-number input (other than /dev/hwrandom
), use the following command:
~]# rngd --rng-device=/dev/hwrng
rngd
daemon with /dev/hwrng
as the device from which random numbers are read. Similarly, you can use the -o
(or --random-device
) option to choose the kernel device for random-number output (other than the default /dev/random
). See the rngd(8) manual page for a list of all available options.
root
:
~]# rngd -vf
Unable to open file: /dev/tpm0
Available entropy sources:
DRNG
Note
rngd -v
command, the according process continues running in background. The -b, --background
option (become a daemon) is applied by default.
~]$ cat /proc/cpuinfo | grep rdrand
Note
/dev/random
, use the rngtest tool as follows:
~]$ cat /dev/random | rngtest -c 1000
rngtest 5
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
rngtest: starting FIPS tests...
rngtest: bits received from input: 20000032
rngtest: FIPS 140-2 successes: 998
rngtest: FIPS 140-2 failures: 2
rngtest: FIPS 140-2(2001-10-10) Monobit: 0
rngtest: FIPS 140-2(2001-10-10) Poker: 0
rngtest: FIPS 140-2(2001-10-10) Runs: 0
rngtest: FIPS 140-2(2001-10-10) Long run: 2
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=1.171; avg=8.453; max=11.374)Mibits/s
rngtest: FIPS tests speed: (min=15.545; avg=143.126; max=157.632)Mibits/s
rngtest: Program run time: 2390520 microseconds
/dev/random
), and QEMU will use /dev/random
as the source for entropy requested by guests.
Figure 4.1. The virtio RNG device
Important
4.10. Configuring Automated Unlocking of Encrypted Volumes using Policy-Based Decryption
4.10.1. Network-Bound Disk Encryption
Figure 4.2. The Network-Bound Disk Encryption using Clevis and Tang
/tmp
, /var
, and /usr/local/
directories, that contain a file system requiring to start before the network connection is established are considered to be root volumes. Additionally, all mount points that are used by services run before the network is up, such as /var/log/
, var/log/audit/
, or /opt
, also require to be mounted early after switching to a root device. You can also identify a root volume by not having the _netdev
option in the /etc/fstab
file.
4.10.2. Installing an Encryption Client - Clevis
root
:
~]# yum install clevis
clevis decrypt
command and provide the cipher text (JWE):
~]$ clevis decrypt < JWE > PLAINTEXT
~]$clevis
Usage: clevis COMMAND [OPTIONS] clevis decrypt Decrypts using the policy defined at encryption time clevis encrypt http Encrypts using a REST HTTP escrow server policy clevis encrypt sss Encrypts using a Shamir's Secret Sharing policy clevis encrypt tang Encrypts using a Tang binding server policy clevis encrypt tpm2 Encrypts using a TPM2.0 chip binding policy ~]$clevis decrypt
Usage: clevis decrypt < JWE > PLAINTEXT Decrypts using the policy defined at encryption time ~]$clevis encrypt tang
Usage: clevis encrypt tang CONFIG < PLAINTEXT > JWE Encrypts using a Tang binding server policy This command uses the following configuration properties: url: <string> The base URL of the Tang server (REQUIRED) thp: <string> The thumbprint of a trusted signing key adv: <string> A filename containing a trusted advertisement adv: <object> A trusted advertisement (raw JSON) Obtaining the thumbprint of a trusted signing key is easy. If you have access to the Tang server's database directory, simply do: $ jose jwk thp -i $DBDIR/$SIG.jwk Alternatively, if you have certainty that your network connection is not compromised (not likely), you can download the advertisement yourself using: $ curl -f $URL/adv > adv.jws
4.10.3. Deploying a Tang Server with SELinux in Enforcing Mode
tangd_port_t
SELinux type, and a Tang server can be deployed as a confined service in SELinux enforcing mode.
Prerequisites
- The policycoreutils-python-utils package and its dependencies are installed.
Procedure
- To install the tang package and its dependencies, enter the following command as
root
:~]#
yum install tang
- Pick an unoccupied port, for example, 7500/tcp, and allow the tangd service to bind to that port:
~]#
semanage port -a -t tangd_port_t -p tcp 7500
Note that a port can be used only by one service at a time, and thus an attempt to use an already occupied port implies theValueError: Port already defined
error message. - Open the port in the firewall:
~]#
firewall-cmd --add-port=7500/tcp
~]#firewall-cmd --runtime-to-permanent
- Enable the
tangd
service using systemd:~]#
systemctl enable tangd.socket
Created symlink from /etc/systemd/system/multi-user.target.wants/tangd.socket to /usr/lib/systemd/system/tangd.socket. - Create an override file:
~]#
systemctl edit tangd.socket
- In the following editor screen, which opens an empty
override.conf
file located in the/etc/systemd/system/tangd.socket.d/
directory, change the default port for the Tang server from 80 to the previously picked number by adding the following lines:[Socket] ListenStream= ListenStream=7500
Save the file and exit the editor. - Reload the changed configuration and start the
tangd
service:~]#
systemctl daemon-reload
- Check that your configuration is working:
~]#
systemctl show tangd.socket -p Listen
Listen=[::]:7500 (Stream) - Start the
tangd
service:~]#
systemctl start tangd.socket
tangd
uses the systemd
socket activation mechanism, the server starts as soon as the first connection comes in. A new set of cryptographic keys is automatically generated at the first start.
jose
utility. Enter the jose -h
command or see the jose(1)
man pages for more information.
Example 4.4. Rotating Tang Keys
/var/db/tang
. For example, you can create new signature and exchange keys with the following commands:
~]#DB=/var/db/tang
~]#jose jwk gen -i '{"alg":"ES512"}' -o $DB/new_sig.jwk
~]#jose jwk gen -i '{"alg":"ECMR"}' -o $DB/new_exc.jwk
.
to hide them from advertisement. Note that the file names in the following example differs from real and unique file names in the key database directory.
~]#mv $DB/old_sig.jwk $DB/.old_sig.jwk
~]#mv $DB/old_exc.jwk $DB/.old_exc.jwk
Warning
4.10.3.1. Deploying High-Availability Systems
- Client Redundancy (Recommended)Clients should be configured with the ability to bind to multiple Tang servers. In this setup, each Tang server has its own keys and clients are able to decrypt by contacting a subset of these servers. Clevis already supports this workflow through its
sss
plug-in.For more information about this setup, see the following man pages:tang(8)
, section High Availabilityclevis(1)
, section Shamir's Secret Sharingclevis-encrypt-sss(1)
Red Hat recommends this method for a high-availability deployment. - Key SharingFor redundancy purposes, more than one instance of Tang can be deployed. To set up a second or any subsequent instance, install the tang packages and copy the key directory to the new host using
rsync
over SSH. Note that Red Hat does not recommend this method because sharing keys increases the risk of key compromise and requires additional automation infrastructure.
4.10.4. Deploying an Encryption Client for an NBDE system with Tang
Prerequisites
- The Clevis framework is installed. See Section 4.10.2, “Installing an Encryption Client - Clevis”
- A Tang server or its downloaded advertisement is available. See Section 4.10.3, “Deploying a Tang Server with SELinux in Enforcing Mode”
Procedure
clevis encrypt tang
sub-command:
~]$ clevis encrypt tang '{"url":"http://tang.srv"}' < PLAINTEXT > JWE
The advertisement contains the following signing keys:
_OsIk0T-E2l6qjfdDiwVmidoZjA
Do you wish to trust these keys? [ynYN] y
clevis decrypt
command and provide the cipher text (JWE):
~]$ clevis decrypt < JWE > PLAINTEXT
clevis-encrypt-tang(1)
man page or use the built-in CLI help:
~]$clevis
Usage: clevis COMMAND [OPTIONS] clevis decrypt Decrypts using the policy defined at encryption time clevis encrypt http Encrypts using a REST HTTP escrow server policy clevis encrypt sss Encrypts using a Shamir's Secret Sharing policy clevis encrypt tang Encrypts using a Tang binding server policy clevis luks bind Binds a LUKSv1 device using the specified policy clevis luks unlock Unlocks a LUKSv1 volume ~]$clevis decrypt
Usage: clevis decrypt < JWE > PLAINTEXT Decrypts using the policy defined at encryption time ~]$clevis encrypt tang
Usage: clevis encrypt tang CONFIG < PLAINTEXT > JWE Encrypts using a Tang binding server policy This command uses the following configuration properties: url: <string> The base URL of the Tang server (REQUIRED) thp: <string> The thumbprint of a trusted signing key adv: <string> A filename containing a trusted advertisement adv: <object> A trusted advertisement (raw JSON) Obtaining the thumbprint of a trusted signing key is easy. If you have access to the Tang server's database directory, simply do: $ jose jwk thp -i $DBDIR/$SIG.jwk Alternatively, if you have certainty that your network connection is not compromised (not likely), you can download the advertisement yourself using: $ curl -f $URL/adv > adv.jws
4.10.5. Deploying an Encryption Client with a TPM 2.0 Policy
clevis encrypt tpm2
sub-command with the only argument in form of the JSON configuration object:
~]$ clevis encrypt tpm2 '{}' < PLAINTEXT > JWE
~]$ clevis encrypt tpm2 '{"hash":"sha1","key":"rsa"}' < PLAINTEXT > JWE
~]$ clevis decrypt < JWE > PLAINTEXT
~]$ clevis encrypt tpm2 '{"pcr_bank":"sha1","pcr_ids":"0,1"}' < PLAINTEXT > JWE
clevis-encrypt-tpm2(1)
man page.
4.10.6. Configuring Manual Enrollment of Root Volumes
clevis luks bind
command:
~]# yum install clevis-luks
~]# clevis luks bind -d /dev/sda tang '{"url":"http://tang.srv"}'
The advertisement contains the following signing keys:
_OsIk0T-E2l6qjfdDiwVmidoZjA
Do you wish to trust these keys? [ynYN] y
You are about to initialize a LUKS device for metadata storage.
Attempting to initialize it may result in data loss if data was
already written into the LUKS header gap in a different format.
A backup is advised before initialization is performed.
Do you wish to initialize /dev/sda? [yn] y
Enter existing LUKS password:
- Creates a new key with the same entropy as the LUKS master key.
- Encrypts the new key with Clevis.
- Stores the Clevis JWE object in the LUKS header with LUKSMeta.
- Enables the new key for use with LUKS.
clevis-luks-bind(1)
man page.
Note
clevis luks bind
command takes one of the slots.
luksmeta show
command:
~]# luksmeta show -d /dev/sda
0 active empty
1 active cb6e8904-81ff-40da-a84a-07ab9ab5715e
2 inactive empty
3 inactive empty
4 inactive empty
5 inactive empty
6 inactive empty
7 inactive empty
~]#yum install clevis-dracut
~]#dracut -f --regenerate-all
Important
~]# dracut -f --regenerate-all --kernel-cmdline "ip=192.0.2.10 netmask=255.255.255.0 gateway=192.0.2.1 nameserver=192.0.2.45"
/etc/dracut.conf.d/
directory with the static network information. For example:
~]# cat /etc/dracut.conf.d/static_ip.conf
kernel_cmdline="ip=10.0.0.103 netmask=255.255.252.0 gateway=10.0.0.1 nameserver=10.0.0.1"
~]# dracut -f --regenerate-all
dracut.cmdline(7)
man page for more information.
4.10.7. Configuring Automated Enrollment Using Kickstart
- Instruct Kickstart to partition the disk such that LUKS encryption has enabled for all mount points, other than
/boot
, with a temporary password. The password is temporary for this step of the enrollment process.part /boot --fstype="xfs" --ondisk=vda --size=256 part / --fstype="xfs" --ondisk=vda --grow --encrypted --passphrase=temppass
Note that OSPP-complaint systems require a more complex configuration, for example:part /boot --fstype="xfs" --ondisk=vda --size=256 part / --fstype="xfs" --ondisk=vda --size=2048 --encrypted --passphrase=temppass part /var --fstype="xfs" --ondisk=vda --size=1024 --encrypted --passphrase=temppass part /tmp --fstype="xfs" --ondisk=vda --size=1024 --encrypted --passphrase=temppass part /home --fstype="xfs" --ondisk=vda --size=2048 --grow --encrypted --passphrase=temppass part /var/log --fstype="xfs" --ondisk=vda --size=1024 --encrypted --passphrase=temppass part /var/log/audit --fstype="xfs" --ondisk=vda --size=1024 --encrypted --passphrase=temppass
- Install the related Clevis packages by listing them in the
%packages
section:%packages clevis-dracut %end
- Call
clevis luks bind
to perform binding in the%post
section. Afterward, remove the temporary password:%post clevis luks bind -f -k- -d /dev/vda2 \ tang '{"url":"http://tang.srv","thp":"_OsIk0T-E2l6qjfdDiwVmidoZjA"}' \ <<< "temppass" cryptsetup luksRemoveKey /dev/vda2 <<< "temppass" %end
In the above example, note that we specify the thumbprint that we trust on the Tang server as part of our binding configuration, enabling binding to be completely non-interactive.You can use an analogous procedure when using a TPM 2.0 policy instead of a Tang server.
4.10.8. Configuring Automated Unlocking of Removable Storage Devices
~]# yum install clevis-udisks2
clevis luks bind
command as described in Section 4.10.6, “Configuring Manual Enrollment of Root Volumes”, for example:
~]# clevis luks bind -d /dev/sdb1 tang '{"url":"http://tang.srv"}'
clevis luks unlock
command:
~]# clevis luks unlock -d /dev/sdb1
4.10.9. Configuring Automated Unlocking of Non-root Volumes at Boot Time
- Install the clevis-systemd package:
~]#
yum install clevis-systemd
- Enable the Clevis unlocker service:
~]#
systemctl enable clevis-luks-askpass.path
Created symlink from /etc/systemd/system/remote-fs.target.wants/clevis-luks-askpass.path to /usr/lib/systemd/system/clevis-luks-askpass.path. - Perform the binding step using the
clevis luks bind
command as described in Section 4.10.6, “Configuring Manual Enrollment of Root Volumes”. - To set up the encrypted block device during system boot, add the corresponding line with the
_netdev
option to the/etc/crypttab
configuration file. See thecrypttab(5)
man page for more information. - Add the volume to the list of accessible filesystems in the
/etc/fstab
file. Use the_netdev
option in this configuration file, too. See thefstab(5)
man page for more information.
4.10.10. Deploying Virtual Machines in a NBDE Network
clevis luks bind
command does not change the LUKS master key. This implies that if you create a LUKS-encrypted image for use in a virtual machine or cloud environment, all the instances that run this image will share a master key. This is extremely insecure and should be avoided at all times.
4.10.11. Building Automatically-enrollable VM Images for Cloud Environments using NBDE
Important
4.10.12. Additional Resources
tang(8)
clevis(1)
jose(1)
clevis-luks-unlockers(1)
tang-nagios(1)
4.11. Checking Integrity with AIDE
4.11.1. Installing AIDE
root
:
~]# yum install aide
root
:
~]# aide --init
AIDE, version 0.15.1
### AIDE database at /var/lib/aide/aide.db.new.gz initialized.
Note
aide --init
command checks just a set of directories and files defined in the /etc/aide.conf
file. To include additional directories or files in the AIDE database, and to change their watched parameters, edit /etc/aide.conf
accordingly.
.new
substring from the initial database file name:
~]# mv /var/lib/aide/aide.db.new.gz /var/lib/aide/aide.db.gz
/etc/aide.conf
file and modify the DBDIR
value. For additional security, store the database, configuration, and the /usr/sbin/aide
binary file in a secure location such as a read-only media.
Important
4.11.2. Performing Integrity Checks
root
:
~]# aide --check
AIDE 0.15.1 found differences between database and filesystem!!
Start timestamp: 2017-03-30 14:12:56
Summary:
Total number of files: 147173
Added files: 1
Removed files: 0
Changed files: 2
...
cron
(see the Automating System Tasks chapter in the System Administrator's Guide), add the following line to /etc/crontab
:
05 4 * * * root /usr/sbin/aide --check
4.11.3. Updating an AIDE Database
~]# aide --update
aide --update
command creates the /var/lib/aide/aide.db.new.gz
database file. To start using it for integrity checks, remove the .new
substring from the file name.
4.11.4. Additional Resources
aide(1)
man pageaide.conf(5)
man page
4.12. Using USBGuard
- The daemon component with an inter-process communication (IPC) interface for dynamic interaction and policy enforcement.
- The command-line interface to interact with a running USBGuard instance.
- The rule language for writing USB device authorization policies.
- The C++ API for interacting with the daemon component implemented in a shared library.
4.12.1. Installing USBGuard
root
:
~]# yum install usbguard
root
:
~]# usbguard generate-policy > /etc/usbguard/rules.conf
Note
/etc/usbguard/rules.conf
file. See the usbguard-rules.conf(5)
man page for more information. Additionally, see Section 4.12.3, “Using the Rule Language to Create Your Own Policy” for examples.
root
:
~]#systemctl start usbguard.service
~]#systemctl status usbguard
● usbguard.service - USBGuard daemon Loaded: loaded (/usr/lib/systemd/system/usbguard.service; disabled; vendor preset: disabled) Active: active (running) since Tue 2017-06-06 13:29:31 CEST; 9s ago Docs: man:usbguard-daemon(8) Main PID: 4984 (usbguard-daemon) CGroup: /system.slice/usbguard.service └─4984 /usr/sbin/usbguard-daemon -k -c /etc/usbguard/usbguard-daem...
root
:
~]# systemctl enable usbguard.service
Created symlink from /etc/systemd/system/basic.target.wants/usbguard.service to /usr/lib/systemd/system/usbguard.service.
root
:
~]# usbguard list-devices
1: allow id 1d6b:0002 serial "0000:00:06.7" name "EHCI Host Controller" hash "JDOb0BiktYs2ct3mSQKopnOOV2h9MGYADwhT+oUtF2s=" parent-hash "4PHGcaDKWtPjKDwYpIRG722cB9SlGz9l9Iea93+Gt9c=" via-port "usb1" with-interface 09:00:00
...
6: block id 1b1c:1ab1 serial "000024937962" name "Voyager" hash "CrXgiaWIf2bZAU+5WkzOE7y0rdSO82XMzubn7HDb95Q=" parent-hash "JDOb0BiktYs2ct3mSQKopnOOV2h9MGYADwhT+oUtF2s=" via-port "1-3" with-interface 08:06:50
allow-device
option:
~]# usbguard allow-device 6
reject-device
option. To just deauthorize a device, use the usbguard
command with the block-device
option:
~]# usbguard block-device 6
- block - do not talk to this device for now
- reject - ignore this device as if did not exist
usbguard
command, enter it with the --help
directive:
~]$ usbguard --help
4.12.2. Creating a White List and a Black List
usbguard-daemon.conf
file is loaded by the usbguard
daemon after it parses its command-line options and is used to configure runtime parameters of the daemon. To override the default configuration file (/etc/usbguard/usbguard-daemon.conf
), use the -c
command-line option. See the usbguard-daemon(8)
man page for further details.
usbguard-daemon.conf
file and use the following options:
USBGuard configuration file
RuleFile=
<path>- The
usbguard
daemon use this file to load the policy rule set from it and to write new rules received through the IPC interface. IPCAllowedUsers=
<username> [<username> ...]- A space-delimited list of user names that the daemon will accept IPC connections from.
IPCAllowedGroups=
<groupname> [<groupname> ...]- A space-delimited list of group names that the daemon will accept IPC connections from.
IPCAccessControlFiles=
<path>- Path to a directory holding the IPC access control files.
ImplicitPolicyTarget=
<target>- How to treat devices that do not match any rule in the policy. Accepted values: allow, block, reject.
PresentDevicePolicy=
<policy>- How to treat devices that are already connected when the daemon starts:
- allow - authorize every present device
- block - deauthorize every present device
- reject - remove every present device
- keep - just sync the internal state and leave it
- apply-policy - evaluate the ruleset for every present device
PresentControllerPolicy=
<policy>- How to treat USB controllers that are already connected when the daemon starts:
- allow - authorize every present device
- block - deauthorize every present device
- reject - remove every present device
- keep - just sync the internal state and leave it
- apply-policy - evaluate the ruleset for every present device
Example 4.5. USBGuard configuration
usbguard
daemon to load rules from the /etc/usbguard/rules.conf
file and it allows only users from the usbguard
group to use the IPC interface:
RuleFile=/etc/usbguard/rules.conf IPCAccessControlFiles=/etc/usbguard/IPCAccessControl.d/
usbguard add-user
or usbguard remove-user
commands. See the usbguard(1)
for more details. In this example, to allow users from the usbguard
group to modify USB device authorization state, list USB devices, listen to exception events, and list USB authorization policy, enter the following command as root
:
~]# usbguard add-user -g usbguard --devices=modify,list,listen --policy=list --exceptions=listen
Important
root
user only. Consider setting either the IPCAccessControlFiles
option (recommended) or the IPCAllowedUsers
and IPCAllowedGroups
options to limit access to the IPC interface. Do not leave the ACL unconfigured as this exposes the IPC interface to all local users and it allows them to manipulate the authorization state of USB devices and modify the USBGuard policy.
usbguard-daemon.conf(5)
man page.
4.12.3. Using the Rule Language to Create Your Own Policy
usbguard
daemon decides whether to authorize a USB device based on a policy defined by a set of rules. When a USB device is inserted into the system, the daemon scans the existing rules sequentially and when a matching rule is found, it either authorizes (allows), deauthorizes (blocks) or removes (rejects) the device, based on the rule target. If no matching rule is found, the decision is based on an implicit default target. This implicit default is to block the device until a decision is made by the user.
rule ::= target device_id device_attributes conditions. target ::= "allow" | "block" | "reject". device_id ::= "*:*" | vendor_id ":*" | vendor_id ":" product_id. device_attributes ::= device_attributes | attribute. device_attributes ::= . conditions ::= conditions | condition. conditions ::= .
usbguard-rules.conf(5)
man page.
Example 4.6. USBguard example policies
- Allow USB mass storage devices and block everything else
- This policy blocks any device that is not just a mass storage device. Devices with a hidden keyboard interface in a USB flash disk are blocked. Only devices with a single mass storage interface are allowed to interact with the operating system. The policy consists of a single rule:
allow with-interface equals { 08:*:* }
The blocking is implicit because there is no block rule. Implicit blocking is useful to desktop users because a desktop applet listening to USBGuard events can ask the user for a decision if an implicit target was selected for a device. - Allow a specific Yubikey device to be connected through a specific port
- Reject everything else on that port.
allow 1050:0011 name "Yubico Yubikey II" serial "0001234567" via-port "1-2" hash "044b5e168d40ee0245478416caf3d998" reject via-port "1-2"
- Reject devices with suspicious combination of interfaces
- A USB flash disk which implements a keyboard or a network interface is very suspicious. The following set of rules forms a policy which allows USB flash disks and explicitly rejects devices with an additional and suspicious interface.
allow with-interface equals { 08:*:* } reject with-interface all-of { 08:*:* 03:00:* } reject with-interface all-of { 08:*:* 03:01:* } reject with-interface all-of { 08:*:* e0:*:* } reject with-interface all-of { 08:*:* 02:*:* }
Note
Blacklisting is the wrong approach and you should not just blacklist a set of devices and allow the rest. The policy above assumes that blocking is the implicit default. Rejecting a set of devices considered as "bad" is a good approach how to limit the exposure of the system to such devices as much as possible. - Allow a keyboard-only USB device
- The following rule allows a keyboard-only USB device only if there is not a USB device with a keyboard interface already allowed.
allow with-interface one-of { 03:00:01 03:01:01 } if !allowed-matches(with-interface one-of { 03:00:01 03:01:01 })
usbguard generate-policy
command, edit the /etc/usbguard/rules.conf
to customize the USBGuard policy rules.
~]$usbguard generate-policy > rules.conf
~]$vim rules.conf
~]# install -m 0600 -o root -g root rules.conf /etc/usbguard/rules.conf
4.12.4. Additional Resources
usbguard(1)
man pageusbguard-rules.conf(5)
man pageusbguard-daemon(8)
man pageusbguard-daemon.conf(5)
man page
4.13. Hardening TLS Configuration
TLS
(Transport Layer Security
) is a cryptographic protocol used to secure network communications. When hardening system security settings by configuring preferred key-exchange protocols, authentication methods, and encryption algorithms, it is necessary to bear in mind that the broader the range of supported clients, the lower the resulting security. Conversely, strict security settings lead to limited compatibility with clients, which can result in some users being locked out of the system. Be sure to target the strictest available configuration and only relax it when it is required for compatibility reasons.
TLS
implementations use secure algorithms where possible while not preventing connections from or to legacy clients or servers. Apply the hardened settings described in this section in environments with strict security requirements where legacy clients or servers that do not support secure algorithms or protocols are not expected or allowed to connect.
4.13.1. Choosing Algorithms to Enable
Protocol Versions
TLS
provides the best security mechanism. Unless you have a compelling reason to include support for older versions of TLS
(or even SSL
), allow your systems to negotiate connections using only the latest version of TLS
.
SSL
version 2 or 3. Both of those versions have serious security vulnerabilities. Only allow negotiation using TLS
version 1.0 or higher. The current version of TLS
, 1.2, should always be preferred.
Note
TLS
depends on the use of TLS
extensions, specific ciphers (see below), and other workarounds. All TLS
connection peers need to implement secure renegotiation indication (RFC 5746), must not support compression, and must implement mitigating measures for timing attacks against CBC
-mode ciphers (the Lucky Thirteen attack). TLS 1.0
clients need to additionally implement record splitting (a workaround against the BEAST attack). TLS 1.2
supports Authenticated Encryption with Associated Data (AEAD) mode ciphers like AES-GCM
, AES-CCM
, or Camellia-GCM
, which have no known issues. All the mentioned mitigations are implemented in cryptographic libraries included in Red Hat Enterprise Linux.
Protocol Version | Usage Recommendation |
---|---|
SSL v2 |
Do not use. Has serious security vulnerabilities.
|
SSL v3 |
Do not use. Has serious security vulnerabilities.
|
TLS 1.0 |
Use for interoperability purposes where needed. Has known issues that cannot be mitigated in a way that guarantees interoperability, and thus mitigations are not enabled by default. Does not support modern cipher suites.
|
TLS 1.1 |
Use for interoperability purposes where needed. Has no known issues but relies on protocol fixes that are included in all the
TLS implementations in Red Hat Enterprise Linux. Does not support modern cipher suites.
|
TLS 1.2 |
Recommended version. Supports the modern
AEAD cipher suites.
|
TLS 1.0
even though they provide support for TLS 1.1
or even 1.2
. This is motivated by an attempt to achieve the highest level of interoperability with external services that may not support the latest versions of TLS
. Depending on your interoperability requirements, enable the highest available version of TLS
.
Important
SSL v3
is not recommended for use. However, if, despite the fact that it is considered insecure and unsuitable for general use, you absolutely must leave SSL v3
enabled, see Section 4.8, “Using stunnel” for instructions on how to use stunnel to securely encrypt communications even when using services that do not support encryption or are only capable of using obsolete and insecure modes of encryption.
Cipher Suites
RC4
or HMAC-MD5
, which have serious shortcomings, should also be disabled. The same applies to the so-called export cipher suites, which have been intentionally made weaker, and thus are easy to break.
3DES
ciphers advertise the use of 168 bits, they actually offer 112 bits of security.
RSA
key exchange, but allows for the use of ECDHE
and DHE
. Of the two, ECDHE
is the faster and therefore the preferred choice.
AEAD
ciphers, such as AES-GCM
, before CBC
-mode ciphers as they are not vulnerable to padding oracle attacks. Additionally, in many cases, AES-GCM
is faster than AES
in CBC
mode, especially when the hardware has cryptographic accelerators for AES
.
ECDHE
key exchange with ECDSA
certificates, the transaction is even faster than pure RSA
key exchange. To provide support for legacy clients, you can install two pairs of certificates and keys on a server: one with ECDSA
keys (for new clients) and one with RSA
keys (for legacy ones).
Public Key Length
RSA
keys, always prefer key lengths of at least 3072 bits signed by at least SHA-256, which is sufficiently large for true 128 bits of security.
Warning
4.13.2. Using Implementations of TLS
TLS
. In this section, the configuration of OpenSSL and GnuTLS is described. See Section 4.13.3, “Configuring Specific Applications” for instructions on how to configure TLS
support in individual applications.
TLS
implementations offer support for various cipher suites that define all the elements that come together when establishing and using TLS
-secured communications.
Important
TLS
implementation you use or the applications that utilize that implementation. New versions may introduce new cipher suites that you do not want to have enabled and that your current configuration does not disable.
4.13.2.1. Working with Cipher Suites in OpenSSL
SSL
and TLS
protocols. On Red Hat Enterprise Linux 7, a configuration file is provided at /etc/pki/tls/openssl.cnf
. The format of this configuration file is described in config(1). See also Section 4.7.9, “Configuring OpenSSL”.
openssl
command with the ciphers
subcommand as follows:
~]$ openssl ciphers -v 'ALL:COMPLEMENTOFALL'
ciphers
subcommand to narrow the output. Special keywords can be used to only list suites that satisfy a certain condition. For example, to only list suites that are defined as belonging to the HIGH
group, use the following command:
~]$ openssl ciphers -v 'HIGH'
~]$ openssl ciphers -v 'kEECDH+aECDSA+AES:kEECDH+AES+aRSA:kEDH+aRSA+AES' | column -t
ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(256) Mac=AEAD
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA384
ECDHE-ECDSA-AES256-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA1
ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(128) Mac=AEAD
ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA256
ECDHE-ECDSA-AES128-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA1
ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(256) Mac=AEAD
ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA384
ECDHE-RSA-AES256-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA1
ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(128) Mac=AEAD
ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA256
ECDHE-RSA-AES128-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA1
DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES256-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(256) Mac=SHA256
DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1
DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(128) Mac=AEAD
DHE-RSA-AES128-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(128) Mac=SHA256
DHE-RSA-AES128-SHA SSLv3 Kx=DH Au=RSA Enc=AES(128) Mac=SHA1
ephemeral elliptic curve Diffie-Hellman
key exchange and ECDSA
ciphers, and omits RSA
key exchange (thus ensuring perfect forward secrecy).
4.13.2.2. Working with Cipher Suites in GnuTLS
SSL
and TLS
protocols and related technologies.
Note
gnutls-cli
command with the -l
(or --list
) option to list all supported cipher suites:
~]$ gnutls-cli -l
-l
option, pass one or more parameters (referred to as priority strings and keywords in GnuTLS documentation) to the --priority
option. See the GnuTLS documentation at http://www.gnutls.org/manual/gnutls.html#Priority-Strings for a list of all available priority strings. For example, issue the following command to get a list of cipher suites that offer at least 128 bits of security:
~]$ gnutls-cli --priority SECURE128 -l
~]$ gnutls-cli --priority SECURE256:+SECURE128:-VERS-TLS-ALL:+VERS-TLS1.2:-RSA:-DHE-DSS:-CAMELLIA-128-CBC:-CAMELLIA-256-CBC -l
Cipher suites for SECURE256:+SECURE128:-VERS-TLS-ALL:+VERS-TLS1.2:-RSA:-DHE-DSS:-CAMELLIA-128-CBC:-CAMELLIA-256-CBC
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384 0xc0, 0x2c TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA384 0xc0, 0x24 TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA1 0xc0, 0x0a SSL3.0
TLS_ECDHE_ECDSA_AES_128_GCM_SHA256 0xc0, 0x2b TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA256 0xc0, 0x23 TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA1 0xc0, 0x09 SSL3.0
TLS_ECDHE_RSA_AES_256_GCM_SHA384 0xc0, 0x30 TLS1.2
TLS_ECDHE_RSA_AES_256_CBC_SHA1 0xc0, 0x14 SSL3.0
TLS_ECDHE_RSA_AES_128_GCM_SHA256 0xc0, 0x2f TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA256 0xc0, 0x27 TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA1 0xc0, 0x13 SSL3.0
TLS_DHE_RSA_AES_256_CBC_SHA256 0x00, 0x6b TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA1 0x00, 0x39 SSL3.0
TLS_DHE_RSA_AES_128_GCM_SHA256 0x00, 0x9e TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA256 0x00, 0x67 TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA1 0x00, 0x33 SSL3.0
Certificate types: CTYPE-X.509
Protocols: VERS-TLS1.2
Compression: COMP-NULL
Elliptic curves: CURVE-SECP384R1, CURVE-SECP521R1, CURVE-SECP256R1
PK-signatures: SIGN-RSA-SHA384, SIGN-ECDSA-SHA384, SIGN-RSA-SHA512, SIGN-ECDSA-SHA512, SIGN-RSA-SHA256, SIGN-DSA-SHA256, SIGN-ECDSA-SHA256
RSA
key exchange and DSS
authentication.
4.13.3. Configuring Specific Applications
TLS
. This section describes the TLS
-related configuration files employed by the most commonly used server applications and offers examples of typical configurations.
4.13.3.1. Configuring the Apache HTTP Server
TLS
needs. Depending on your choice of the TLS
library, you need to install either the mod_ssl or the mod_nss module (provided by eponymous packages). For example, to install the package that provides the OpenSSL mod_ssl module, issue the following command as root:
~]# yum install mod_ssl
/etc/httpd/conf.d/ssl.conf
configuration file, which can be used to modify the TLS
-related settings of the Apache HTTP Server. Similarly, the mod_nss package installs the /etc/httpd/conf.d/nss.conf
configuration file.
TLS
configuration. The directives available in the /etc/httpd/conf.d/ssl.conf
configuration file are described in detail in /usr/share/httpd/manual/mod/mod_ssl.html
. Examples of various settings are in /usr/share/httpd/manual/ssl/ssl_howto.html
.
/etc/httpd/conf.d/ssl.conf
configuration file, be sure to consider the following three directives at the minimum:
-
SSLProtocol
- Use this directive to specify the version of
TLS
(orSSL
) you want to allow. -
SSLCipherSuite
- Use this directive to specify your preferred cipher suite or disable the ones you want to disallow.
-
SSLHonorCipherOrder
- Uncomment and set this directive to
on
to ensure that the connecting clients adhere to the order of ciphers you specified.
SSLProtocol all -SSLv2 -SSLv3 SSLCipherSuite HIGH:!aNULL:!MD5 SSLHonorCipherOrder on
/etc/httpd/conf.d/nss.conf
configuration file. The mod_nss module is derived from mod_ssl, and as such it shares many features with it, not least the structure of the configuration file, and the directives that are available. Note that the mod_nss directives have a prefix of NSS
instead of SSL
. See https://git.fedorahosted.org/cgit/mod_nss.git/plain/docs/mod_nss.html for an overview of information about mod_nss, including a list of mod_ssl configuration directives that are not applicable to mod_nss.
4.13.3.2. Configuring the Dovecot Mail Server
TLS
, modify the /etc/dovecot/conf.d/10-ssl.conf
configuration file. You can find an explanation of some of the basic configuration directives available in that file in /usr/share/doc/dovecot-2.2.10/wiki/SSL.DovecotConfiguration.txt
(this help file is installed along with the standard installation of Dovecot).
/etc/dovecot/conf.d/10-ssl.conf
configuration file, be sure to consider the following three directives at the minimum:
-
ssl_protocols
- Use this directive to specify the version of
TLS
(orSSL
) you want to allow. -
ssl_cipher_list
- Use this directive to specify your preferred cipher suites or disable the ones you want to disallow.
-
ssl_prefer_server_ciphers
- Uncomment and set this directive to
yes
to ensure that the connecting clients adhere to the order of ciphers you specified.
ssl_protocols = !SSLv2 !SSLv3 ssl_cipher_list = HIGH:!aNULL:!MD5 ssl_prefer_server_ciphers = yes
4.13.4. Additional Information
Installed Documentation
- config(1) — Describes the format of the
/etc/ssl/openssl.conf
configuration file. - ciphers(1) — Includes a list of available OpenSSL keywords and cipher strings.
/usr/share/httpd/manual/mod/mod_ssl.html
— Contains detailed descriptions of the directives available in the/etc/httpd/conf.d/ssl.conf
configuration file used by the mod_ssl module for the Apache HTTP Server./usr/share/httpd/manual/ssl/ssl_howto.html
— Contains practical examples of real-world settings in the/etc/httpd/conf.d/ssl.conf
configuration file used by the mod_ssl module for the Apache HTTP Server./usr/share/doc/dovecot-2.2.10/wiki/SSL.DovecotConfiguration.txt
— Explains some of the basic configuration directives available in the/etc/dovecot/conf.d/10-ssl.conf
configuration file used by the Dovecot mail server.
Online Documentation
- Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide — The SELinux User's and Administrator's Guide for Red Hat Enterprise Linux 7 describes the basic principles of SELinux and documents in detail how to configure and use SELinux with various services, such as the Apache HTTP Server.
- http://tools.ietf.org/html/draft-ietf-uta-tls-bcp-00 — Recommendations for secure use of
TLS
andDTLS
.
See Also
- Section 4.7, “Using OpenSSL” describes, among other things, how to use OpenSSL to create and manage keys, generate certificates, and encrypt and decrypt files.
4.15. Using MACsec
Media Access Control Security
(MACsec
, IEEE 802.1AE) encrypts and authenticates all traffic in LANs with the GCM-AES-128 algorithm. MACsec
can protect not only IP
but also Address Resolution Protocol (ARP), Neighbor Discovery (ND), or DHCP
. While IPsec
operates on the network layer (layer 3) and SSL
or TLS
on the application layer (layer 7), MACsec
operates in the data link layer (layer 2). Combine MACsec
with security protocols for other networking layers to take advantage of different security features that these standards provide.
MACsec
network, use case scenarios, and configuration examples.
4.16. Removing Data Securely Using scrub
scrub
command, install the scrub package:
~]# yum install scrub
-
Character or Block Device
- The special file corresponding to a whole disk is scrubbed and all data on it, is destroyed. This is the most effective method.
scrub
[OPTIONS] special file -
File
- A regular file is scrubbed and only the data in the file is destroyed.
scrub
[OPTIONS] file -
Directory
- With the
-X
option, a directory is created and filled with files until the file system is full. Then, the files are scrubbed as in file mode.scrub
-X
[OPTIONS] directory
Example 4.7. Scrubbing a Raw Device
~]# scrub
/dev/sdf1
scrub: using NNSA NAP-14.1-C patterns
scrub: please verify that device size below is correct!
scrub: scrubbing /dev/sdf1 1995650048 bytes (~1GB)
scrub: random |................................................|
scrub: random |................................................|
scrub: 0x00 |................................................|
scrub: verify |................................................|
Example 4.8. Scrubbing a File
- Create a 1MB file:
~]$
base64
/dev/urandom | head -c $[ 1024*1024 ] > file.txt - Show the file size:
~]$
ls -lh
total 1.0M -rw-rw-r--. 1 username username 1.0M Sep 8 15:23 file.txt - Show the contents of the file:
~]$
head
file.txt JnNpaTEveB/IYsbM9lhuJdw+0jKhwCIBUsxLXLAyB8uItotUlNHKKUeS/7bCRKDogEP+yJm8VQkL-1
- Scrub the file:
~]$
scrub
file.txt scrub: using NNSA NAP-14.1-C patterns scrub: scrubbing file.txt 1048576 bytes (~1024KB) scrub: random |................................................| scrub: random |................................................| scrub: 0x00 |................................................| scrub: verify |................................................| - Verify that the file contents have been scrubbed:
~]$ cat file.txt SCRUBBED!
- Verify that the file size remains the same:
~]$
ls -lh
total 1.0M -rw-rw-r--. 1 username username 1.0M Sep 8 15:24 file.txt
scrub
modes, options, methods, and caveats, see the scrub(1) man page.
Chapter 5. Using Firewalls
5.1. Getting Started with firewalld
firewalld
is a firewall service daemon that provides a dynamic customizable host-based firewall with a D-Bus
interface. Being dynamic, it enables creating, changing, and deleting the rules without the necessity to restart the firewall daemon each time the rules are changed.
firewalld
uses the concepts of zones and services, that simplify the traffic management. Zones are predefined sets of rules. Network interfaces and sources can be assigned to a zone. The traffic allowed depends on the network your computer is connected to and the security level this network is assigned. Firewall services are predefined rules that cover all necessary settings to allow incoming traffic for a specific service and they apply within a zone.
firewalld
blocks all traffic on ports that are not explicitly set as open. Some zones, such as trusted, allow all traffic by default.
Figure 5.1. The Firewall Stack
5.1.1. Zones
firewalld
can be used to separate networks into different zones according to the level of trust that the user has decided to place on the interfaces and traffic within that network. A connection can only be part of one zone, but a zone can be used for many network connections.
firewalld
of the zone of an interface. You can assign zones to interfaces with NetworkManager, with the firewall-config tool, or the firewall-cmd
command-line tool. The latter two only edit the appropriate NetworkManager configuration files. If you change the zone of the interface using firewall-cmd
or firewall-config, the request is forwarded to NetworkManager and is not handled by firewalld
.
/usr/lib/firewalld/zones/
directory and can be instantly applied to any available network interface. These files are copied to the /etc/firewalld/zones/
directory only after they are modified. The following table describes the default settings of the predefined zones:
block
- Any incoming network connections are rejected with an icmp-host-prohibited message for
IPv4
and icmp6-adm-prohibited forIPv6
. Only network connections initiated from within the system are possible. dmz
- For computers in your demilitarized zone that are publicly-accessible with limited access to your internal network. Only selected incoming connections are accepted.
-
drop
- Any incoming network packets are dropped without any notification. Only outgoing network connections are possible.
external
- For use on external networks with masquerading enabled, especially for routers. You do not trust the other computers on the network to not harm your computer. Only selected incoming connections are accepted.
home
- For use at home when you mostly trust the other computers on the network. Only selected incoming connections are accepted.
internal
- For use on internal networks when you mostly trust the other computers on the network. Only selected incoming connections are accepted.
public
- For use in public areas where you do not trust other computers on the network. Only selected incoming connections are accepted.
trusted
- All network connections are accepted.
work
- For use at work where you mostly trust the other computers on the network. Only selected incoming connections are accepted.
firewalld
is set to be the public
zone. The default zone can be changed.
Note
5.1.2. Predefined Services
firewalld.service(5)
man page. The services are specified by means of individual XML configuration files, which are named in the following format: service-name.xml
. Protocol names are preferred over service or application names in firewalld
.
5.1.3. Runtime and Permanent Settings
firewalld
is running. When firewalld
is restarted, the settings revert to their permanent values.
--permanent
option. Alternatively, to make changes persistent while firewalld
is running, use the --runtime-to-permanent
firewall-cmd
option.
firewalld
is running using only the --permanent
option, they do not become effective before firewalld
is restarted. However, restarting firewalld
closes all open ports and stops the networking traffic.
5.1.4. Modifying Settings in Runtime and Permanent Configuration using CLI
--permanent
option with the firewall-cmd
command.
~]# firewall-cmd --permanent <other options>
- Change runtime settings and then make them permanent as follows:
~]#
firewall-cmd <other options>
~]#firewall-cmd --runtime-to-permanent
- Set permanent settings and reload the settings into runtime mode:
~]#
firewall-cmd --permanent <other options>
~]#firewall-cmd --reload
Note
--timeout
option. After a specified amount of time, any change reverts to its previous state. Using this options excludes the --permanent
option.
SSH
service for 15 minutes:
~]# firewall-cmd --add-service=ssh --timeout 15m
5.2. Installing the firewall-config GUI configuration tool
root
:
~]# yum install firewall-config
Software
to launch the Software Sources application. Type firewall
to the search box, which appears after selecting the search button in the top-right corner. Select the item from the search results, and click on the button.
firewall-config
command or press the Super key to enter the , type firewall
, and press Enter.
5.3. Viewing the Current Status and Settings of firewalld
5.3.1. Viewing the Current Status of firewalld
firewalld
, is installed on the system by default. Use the firewalld
CLI interface to check that the service is running.
~]# firewall-cmd --state
systemctl status
sub-command:
~]# systemctl status firewalld
firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor pr
Active: active (running) since Mon 2017-12-18 16:05:15 CET; 50min ago
Docs: man:firewalld(1)
Main PID: 705 (firewalld)
Tasks: 2 (limit: 4915)
CGroup: /system.slice/firewalld.service
└─705 /usr/bin/python3 -Es /usr/sbin/firewalld --nofork --nopid
firewalld
is set up and which rules are in force before you try to edit the settings. To display the firewall settings, see Section 5.3.2, “Viewing Current firewalld
Settings”
5.3.2. Viewing Current firewalld
Settings
5.3.2.1. Viewing Allowed Services using GUI
firewall
, and press Enter. The firewall-config tool appears. You can now view the list of services under the tab.
~]$ firewall-config
Figure 5.2. The Services tab in firewall-config
5.3.2.2. Viewing firewalld
Settings using CLI
--list-all
option shows a complete overview of the firewalld
settings.
firewalld
uses zones to manage the traffic. If a zone is not specified by the --zone
option, the command is effective in the default zone assigned to the active network interface and connection.
~]# firewall-cmd --list-all
public
target: default
icmp-block-inversion: no
interfaces:
sources:
services: ssh dhcpv6-client
ports:
protocols:
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:
Note
--zone=zone-name
argument to the firewall-cmd --list-all
command, for example:
~]# firewall-cmd --list-all --zone=home
home
target: default
icmp-block-inversion: no
interfaces:
sources:
services: ssh mdns samba-client dhcpv6-client
... [output truncated]
firewalld
manual pages or get a list of the options using the command help:
~]# firewall-cmd --help
Usage: firewall-cmd [OPTIONS...]
General Options
-h, --help Prints a short help text and exists
-V, --version Print the version string of firewalld
-q, --quiet Do not print status messages
Status Options
--state Return and print firewalld state
--reload Reload firewall and keep state information
... [output truncated]
~]# firewall-cmd --list-services
ssh dhcpv6-client
SSH
service and firewalld
opens the necessary port (22) for the service. Later, if you list the allowed services, the list shows the SSH
service, but if you list open ports, it does not show any. Therefore, it is recommended to use the --list-all
option to make sure you receive a complete information.
5.4. Starting firewalld
firewalld
, enter the following command as root
:
~]#systemctl unmask firewalld
~]#systemctl start firewalld
firewalld
starts automatically at system start, enter the following command as root
:
~]# systemctl enable firewalld
5.5. Stopping firewalld
firewalld
, enter the following command as root
:
~]# systemctl stop firewalld
firewalld
from starting automatically at system start, enter the following command as root
:
~]# systemctl disable firewalld
firewalld
D-Bus
interface and also if other services require firewalld
, enter the following command as root
:
~]# systemctl mask firewalld
5.6. Controlling Traffic
5.6.1. Predefined Services
firewall-cmd
, and firewall-offline-cmd
.
/etc/firewalld/services/
directory. If a service is not added or changed by the user, then no corresponding XML file is found in /etc/firewalld/services/
. The files in the /usr/lib/firewalld/services/
directory can be used as templates if you want to add or change a service.
5.6.2. Disabling All Traffic in Case of Emergency using CLI
~]# firewall-cmd --panic-on
~]# firewall-cmd --panic-off
~]# firewall-cmd --query-panic
5.6.3. Controlling Traffic with Predefined Services using CLI
firewalld
. This opens all necessary ports and modifies other settings according to the service definition file.
- Check that the service is not already allowed:
~]#
firewall-cmd --list-services
ssh dhcpv6-client - List all predefined services:
~]#
firewall-cmd --get-services
RH-Satellite-6 amanda-client amanda-k5-client bacula bacula-client bitcoin bitcoin-rpc bitcoin-testnet bitcoin-testnet-rpc ceph ceph-mon cfengine condor-collector ctdb dhcp dhcpv6 dhcpv6-client dns docker-registry ... [output truncated] - Add the service to the allowed services:
~]#
firewall-cmd --add-service=<service-name>
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.6.4. Controlling Traffic with Predefined Services using GUI
IPv4
or IPv6
).
Note
5.6.5. Adding New Services
firewall-cmd
, and firewall-offline-cmd
. Alternatively, you can edit the XML files in /etc/firewalld/services/
. If a service is not added or changed by the user, then no corresponding XML file are found in /etc/firewalld/services/
. The files /usr/lib/firewalld/services/
can be used as templates if you want to add or change a service.
firewall-cmd
, or firewall-offline-cmd
in case of not active firewalld
. enter the following command to add a new and empty service:
~]$ firewall-cmd --new-service=service-name
~]$ firewall-cmd --new-service-from-file=service-name.xml
--name=service-name
option.
/etc/firewalld/services/
.
root
, you can enter the following command to copy a service manually:
~]# cp /usr/lib/firewalld/services/service-name.xml /etc/firewalld/services/service-name.xml
firewalld
loads files from /usr/lib/firewalld/services
in the first place. If files are placed in /etc/firewalld/services
and they are valid, then these will override the matching files from /usr/lib/firewalld/services
. The overriden files in /usr/lib/firewalld/services
are used as soon as the matching files in /etc/firewalld/services
have been removed or if firewalld
has been asked to load the defaults of the services. This applies to the permanent environment only. A reload is needed to get these fallbacks also in the runtime environment.
5.6.6. Controlling Ports using CLI
httpd
daemon, for example, listens on port 80. However, system administrators by default configure daemons to listen on different ports to enhance security or for other reasons.
Opening a Port
- List all allowed ports:
~]#
firewall-cmd --list-ports
- Add a port to the allowed ports to open it for incoming traffic:
~]#
firewall-cmd --add-port=port-number/port-type
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
tcp
, udp
, sctp
, or dccp
. The type must match the type of network communication.
Closing a Port
firewalld
. It is highly recommended to close all unnecessary ports as soon as they are not used because leaving a port open represents a security risk.
- List all allowed ports:
~]#
firewall-cmd --list-ports
[WARNING] ==== This command will only give you a list of ports that have been opened as ports. You will not be able to see any open ports that have been opened as a service. Therefore, you should consider using the --list-all option instead of --list-ports. ==== - Remove the port from the allowed ports to close it for the incoming traffic:
~]#
firewall-cmd --remove-port=port-number/port-type
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.6.7. Opening Ports using GUI
5.6.8. Controlling Traffic with Protocols using GUI
5.6.9. Opening Source Ports using GUI
5.7. Working with Zones
5.7.1. Listing Zones
~]# firewall-cmd --get-zones
firewall-cmd --get-zones
command displays all zones that are available on the system, but it does not show any details for particular zones.
~]# firewall-cmd --list-all-zones
~]# firewall-cmd --zone=zone-name --list-all
5.7.2. Modifying firewalld
Settings for a Certain Zone
--zone=zone-name
option. For example, to allow the SSH
service in the zone public:
~]# firewall-cmd --add-service=ssh --zone=public
5.7.3. Changing the Default Zone
firewalld
service, firewalld
loads the settings for the default zone and makes it active.
- Display the current default zone:
~]#
firewall-cmd --get-default-zone
- Set the new default zone:
~]#
firewall-cmd --set-default-zone zone-name
Note
--permanent
option.
5.7.4. Assigning a Network Interface to a Zone
- List the active zones and the interfaces assigned to them:
~]#
firewall-cmd --get-active-zones
- Assign the interface to a different zone:
~]#
firewall-cmd --zone=zone-name --change-interface=<interface-name>
Note
--permanent
option to make the setting persistent across restarts. If you set a new default zone, the setting becomes permanent.
5.7.5. Assigning a Default Zone to a Network Connection
/etc/sysconfig/network-scripts/ifcfg-connection-name
file and add a line that assigns a zone to this connection:
ZONE=zone-name
5.7.6. Creating a New Zone
Note
--permanent
option, otherwise the command does not work.
- Create a new zone:
~]#
firewall-cmd --permanent --new-zone=zone-name
- Reload the new zone:
~]#
firewall-cmd --reload
- Check if the new zone is added to your permanent settings:
~]#
firewall-cmd --get-zones
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.7.7. Creating a New Zone using a Configuration File
firewalld
zone configuration file contains the information for a zone. These are the zone description, services, ports, protocols, icmp-blocks, masquerade, forward-ports and rich language rules in an XML file format. The file name has to be zone-name.xml
where the length of zone-name is currently limited to 17 chars. The zone configuration files are located in the /usr/lib/firewalld/zones/
and /etc/firewalld/zones/
directories.
SSH
) and one port range, for both the TCP
and UDP
protocols.:
<?xml version="1.0" encoding="utf-8"?> <zone> <short>My zone</short> <description>Here you can describe the characteristic features of the zone.</description> <service name="ssh"/> <port port="1025-65535" protocol="tcp"/> <port port="1025-65535" protocol="udp"/> </zone>
firewalld.zone
manual pages.
5.7.8. Using Zone Targets to Set Default Behavior for Incoming Traffic
default
, ACCEPT
, REJECT
, and DROP
. By setting the target to ACCEPT
, you accept all incoming packets except those disabled by a specific rule. If you set the target to REJECT
or DROP
, you disable all incoming packets except those that you have allowed in specific rules. When packets are rejected, the source machine is informed about the rejection, while there is no information sent when the packets are dropped.
- List the information for the specific zone to see the default target:
~]$
firewall-cmd --zone=zone-name --list-all
- Set a new target in the zone:
~]#
firewall-cmd --zone=zone-name --set-target=<default|ACCEPT|REJECT|DROP>
5.8. Using Zones to Manage Incoming Traffic Depending on Source
5.8.1. Adding a Source
- To set the source in the current zone:
~]#
firewall-cmd --add-source=<source>
- To set the source IP address for a specific zone:
~]#
firewall-cmd --zone=zone-name --add-source=<source>
trusted
zone:
- List all available zones:
~]#
firewall-cmd --get-zones
- Add the source IP to the trusted zone in the permanent mode:
~]#
firewall-cmd --zone=trusted --add-source=192.168.2.15
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.8.2. Removing a Source
- List allowed sources for the required zone:
~]#
firewall-cmd --zone=zone-name --list-sources
- Remove the source from the zone permanently:
~]#
firewall-cmd --zone=zone-name --remove-source=<source>
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.8.3. Adding a Source Port
--add-source-port
option. You can also combine this with the --add-source
option to limit the traffic to a certain IP address or IP range.
~]# firewall-cmd --zone=zone-name --add-source-port=<port-name>/<tcp|udp|sctp|dccp>
5.8.4. Removing a Source Port
~]# firewall-cmd --zone=zone-name --remove-source-port=<port-name>/<tcp|udp|sctp|dccp>
5.8.5. Using Zones and Sources to Allow a Service for Only a Specific Domain
192.0.2.0/24
network while any other traffic is blocked.
Warning
default
target. Using a zone that has the target set to ACCEPT
is a security risk, because for traffic from 192.0.2.0/24
, all network connections would be accepted.
- List all available zones:
~]#
firewall-cmd --get-zones
block dmz drop external home internal public trusted work - Add the IP range to the
internal
zone to route the traffic originating from the source through the zone:~]#
firewall-cmd --zone=internal --add-source=192.0.2.0/24
- Add the http service to the
internal
zone:~]#
firewall-cmd --zone=internal --add-service=http
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
- Check that the
internal
zone is active and that the service is allowed in it:~]#
firewall-cmd --zone=internal --list-all
internal (active) target: default icmp-block-inversion: no interfaces: sources: 192.0.2.0/24 services: dhcpv6-client mdns samba-client ssh http ...
5.8.6. Configuring Traffic Accepted by a Zone Based on Protocol
Adding a Protocol to a Zone
~]# firewall-cmd --zone=zone-name --add-protocol=port-name/tcp|udp|sctp|dccp|igmp
Note
igmp
value with the --add-protocol
option.
Removing a Protocol from a Zone
~]# firewall-cmd --zone=zone-name --remove-protocol=port-name/tcp|udp|sctp|dccp|igmp
5.9. Port Forwarding
firewalld
, you can set up ports redirection so that any incoming traffic that reaches a certain port on your system is delivered to another internal port of your choice or to an external port on another machine.
5.9.1. Adding a Port to Redirect
~]# firewall-cmd --add-forward-port=port=port-number:proto=tcp|udp|sctp|dccp:toport=port-number
- Add the port to be forwarded:
~]#
firewall-cmd --add-forward-port=port=port-number:proto=tcp|udp:toport=port-number:toaddr=IP
- Enable masquerade:
~]#
firewall-cmd --add-masquerade
Example 5.1. Redirecting TCP Port 80 to Port 88 on the Same Machine
- Redirect the port 80 to port 88 for TCP traffic:
~]#
firewall-cmd --add-forward-port=port=80:proto=tcp:toport=88
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
- Check that the port is redirected:
~]#
firewall-cmd --list-all
5.9.2. Removing a Redirected Port
~]# firewall-cmd --remove-forward-port=port=port-number:proto=<tcp|udp>:toport=port-number:toaddr=<IP>
- Remove the forwarded port:
~]#
firewall-cmd --remove-forward-port=port=port-number:proto=<tcp|udp>:toport=port-number:toaddr=<IP>
- Disable masquerade:
~]#
firewall-cmd --remove-masquerade
Note
Example 5.2. Removing TCP Port 80 forwarded to Port 88 on the Same Machine
- List redirected ports:
~]#
firewall-cmd --list-forward-ports
port=80:proto=tcp:toport=88:toaddr= - Remove the redirected port from the firewall::
~]#
firewall-cmd --remove-forward-port=port=80:proto=tcp:toport=88:toaddr=
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.10. Configuring IP Address Masquerading
external
zone), enter the following command as root
:
~]# firewall-cmd --zone=external --query-masquerade
yes
with exit status 0
if enabled. It prints no
with exit status 1
otherwise. If zone
is omitted, the default zone will be used.
root
:
~]# firewall-cmd --zone=external --add-masquerade
--permanent
option.
root
:
~]# firewall-cmd --zone=external --remove-masquerade
--permanent
option.
5.11. Managing ICMP
Requests
Internet Control Message Protocol
(ICMP
) is a supporting protocol that is used by various network devices to send error messages and operational information indicating a connection problem, for example, that a requested service is not available. ICMP
differs from transport protocols such as TCP and UDP because it is not used to exchange data between systems.
ICMP
messages, especially echo-request
and echo-reply
, to reveal information about your network and misuse such information for various kinds of fraudulent activities. Therefore, firewalld
enables blocking the ICMP
requests to protect your network information.
5.11.1. Listing ICMP
Requests
ICMP
requests are described in individual XML files that are located in the /usr/lib/firewalld/icmptypes/
directory. You can read these files to see a description of the request. The firewall-cmd
command controls the ICMP
requests manipulation.
ICMP
types:
~]# firewall-cmd --get-icmptypes
ICMP
request can be used by IPv4, IPv6, or by both protocols. To see for which protocol the ICMP
request is used:
~]# firewall-cmd --info-icmptype=<icmptype>
ICMP
request shows yes
if the request is currently blocked or no
if it is not. To see if an ICMP
request is currently blocked:
~]# firewall-cmd --query-icmp-block=<icmptype>
5.11.2. Blocking or Unblocking ICMP
Requests
ICMP
requests, it does not provide the information that it normally would. However, that does not mean that no information is given at all. The clients receive information that the particular ICMP
request is being blocked (rejected). Blocking the ICMP
requests should be considered carefully, because it can cause communication problems, especially with IPv6 traffic.
ICMP
request is currently blocked:
~]# firewall-cmd --query-icmp-block=<icmptype>
ICMP
request:
~]# firewall-cmd --add-icmp-block=<icmptype>
ICMP
request:
~]# firewall-cmd --remove-icmp-block=<icmptype>
5.11.3. Blocking ICMP
Requests without Providing any Information at All
ICMP
requests, clients know that you are blocking it. So, a potential attacker who is sniffing for live IP addresses is still able to see that your IP address is online. To hide this information completely, you have to drop all ICMP
requests.
ICMP
requests:
- Set the target of your zone to
DROP
:~]#
firewall-cmd --set-target=DROP
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
ICMP
requests, is dropped, except traffic which you have explicitly allowed.
ICMP
requests and allow others:
- Set the target of your zone to
DROP
:~]#
firewall-cmd --set-target=DROP
- Add the ICMP block inversion to block all
ICMP
requests at once:~]#
firewall-cmd --add-icmp-block-inversion
- Add the ICMP block for those
ICMP
requests that you want to allow:~]#
firewall-cmd --add-icmp-block=<icmptype>
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
ICMP
requests blocks, so all requests, that were not previously blocked, are blocked. Those that were blocked are not blocked. Which means that if you need to unblock a request, you must use the blocking command.
- Set the target of your zone to
default
orACCEPT
:~]#
firewall-cmd --set-target=default
- Remove all added blocks for
ICMP
requests:~]#
firewall-cmd --remove-icmp-block=<icmptype>
- Remove the
ICMP
block inversion:~]#
firewall-cmd --remove-icmp-block-inversion
- Make the new settings persistent:
~]#
firewall-cmd --runtime-to-permanent
5.11.4. Configuring the ICMP
Filter using GUI
ICMP
filter, start the firewall-config tool and select the network zone whose messages are to be filtered. Select the ICMP Filter tab and select the check box for each type of ICMP
message you want to filter. Clear the check box to disable a filter. This setting is per direction and the default allows everything.
ICMP
types are now accepted, all other are rejected. In a zone using the DROP target, they are dropped.
5.12. Setting and Controlling IP sets using firewalld
firewalld
, enter the following command as root.
~]# firewall-cmd --get-ipset-types
hash:ip hash:ip,mark hash:ip,port hash:ip,port,ip hash:ip,port,net hash:mac hash:net hash:net,iface hash:net,net hash:net,port hash:net,port,net
5.12.1. Configuring IP Set Options with the Command-Line Client
firewalld
zones as sources and also as sources in rich rules. In Red Hat Enterprise Linux 7, the preferred method is to use the IP sets created with firewalld
in a direct rule.
firewalld
in the permanent environment, use the following command as root
:
~]# firewall-cmd --permanent --get-ipsets
root
:
~]# firewall-cmd --permanent --new-ipset=test --type=hash:net
success
hash:net
type for IPv4
. To create an IP set for use with IPv6
, add the --option=family=inet6
option. To make the new setting effective in the runtime environment, reload firewalld
. List the new IP set with the following command as root
:
~]# firewall-cmd --permanent --get-ipsets
test
root
:
~]# firewall-cmd --permanent --info-ipset=test
test
type: hash:net
options:
entries:
root
:
~]# firewall-cmd --permanent --ipset=test --add-entry=192.168.0.1 success
root
:
~]# firewall-cmd --permanent --ipset=test --get-entries
192.168.0.1
~]# cat > iplist.txt <<EOL
192.168.0.2
192.168.0.3
192.168.1.0/24
192.168.2.254
EOL
root
:
~]# firewall-cmd --permanent --ipset=test --add-entries-from-file=iplist.txt success
root
:
~]# firewall-cmd --permanent --ipset=test --get-entries 192.168.0.1 192.168.0.2 192.168.0.3 192.168.1.0/24 192.168.2.254
root
:
~]# firewall-cmd --permanent --ipset=test --remove-entries-from-file=iplist.txt
success
~]# firewall-cmd --permanent --ipset=test --get-entries
192.168.0.1
root
:
~]# firewall-cmd --permanent --zone=drop --add-source=ipset:test
success
ipset:
prefix in the source shows firewalld
that the source is an IP set and not an IP address or an address range.
--permanent
option.
5.12.2. Configuring a Custom Service for an IP Set
firewalld
starts:
- Using an editor running as
root
, create a file as follows:~]#
vi /etc/systemd/system/ipset_name.service
[Unit] Description=ipset_name Before=firewalld.service [Service] Type=oneshot RemainAfterExit=yes ExecStart=/usr/local/bin/ipset_name.sh start ExecStop=/usr/local/bin/ipset_name.sh stop [Install] WantedBy=basic.target - Use the IP set permanently in firewalld:
~]# vi
/etc/firewalld/direct.xml
<?xml version="1.0" encoding="utf-8"?> <direct> <rule ipv="ipv4" table="filter" chain="INPUT" priority="0">-m set --match-set <replaceable>ipset_name</replaceable> src -j DROP</rule> </direct> - A
firewalld
reload is required to activate the changes:~]#
This reloads the firewall without losing state information (TCP sessions will not be terminated), but service disruption is possible during the reload.firewall-cmd --reload
Warning
firewalld
. To use such IP sets, a permanent direct rule is required to reference the set, and a custom service must be added to create these IP sets. This service needs to be started before firewalld starts, otherwise firewalld
is not able to add the direct rules using these sets. You can add permanent direct rules with the /etc/firewalld/direct.xml
file.
5.13. Setting and Controlling IP sets using iptables
firewalld
and the iptables (and ip6tables) services are:
- The iptables service stores configuration in
/etc/sysconfig/iptables
and/etc/sysconfig/ip6tables
, whilefirewalld
stores it in various XML files in/usr/lib/firewalld/
and/etc/firewalld/
. Note that the/etc/sysconfig/iptables
file does not exist asfirewalld
is installed by default on Red Hat Enterprise Linux. - With the iptables service, every single change means flushing all the old rules and reading all the new rules from
/etc/sysconfig/iptables
, while withfirewalld
there is no recreating of all the rules. Only the differences are applied. Consequently,firewalld
can change the settings during runtime without existing connections being lost.
iptables
and ip6tables
services instead of firewalld
, first disable firewalld
by running the following command as root
:
~]#systemctl disable firewalld
~]#systemctl stop firewalld
root
:
~]# yum install iptables-services
The iptables-services package contains the iptables
service and the ip6tables
service.
iptables
and ip6tables
services, enter the following commands as root
:
~]#To enable the services to start on every system start, enter the following commands:systemctl start iptables
~]#systemctl start ip6tables
~]#systemctl enable iptables
~]#systemctl enable ip6tables
~]#The set is created as follows:iptables -A INPUT -s 10.0.0.0/8 -j DROP
~]#iptables -A INPUT -s 172.16.0.0/12 -j DROP
~]#iptables -A INPUT -s 192.168.0.0/16 -j DROP
~]#The set is then referenced in an iptables command as follows:ipset create my-block-set hash:net
~]#ipset add my-block-set 10.0.0.0/8
~]#ipset add my-block-set 172.16.0.0/12
~]#ipset add my-block-set 192.168.0.0/16
~]# iptables -A INPUT -m set --set my-block-set src -j DROP
If the set is used more than once a saving in configuration time is made. If the set contains many entries a saving in processing time is made.
5.14. Using the Direct Interface
--direct
option with the firewall-cmd tool. A few examples are presented here. See the firewall-cmd(1)
man page for more information.
--permanent
option using the firewall-cmd --permanent --direct
command or by modifying /etc/firewalld/direct.xml
. See man firewalld.direct(5)
for information on the /etc/firewalld/direct.xml
file.
5.14.1. Adding a Rule using the Direct Interface
root
:
~]#firewall-cmd --direct --add-rule ipv4 filter IN_public_allow \
0 -m tcp -p tcp --dport 666 -j ACCEPT
--permanent
option to make the setting persistent.
5.14.2. Removing a Rule using the Direct Interface
root
:
~]#firewall-cmd --direct --remove-rule ipv4 filter IN_public_allow \
0 -m tcp -p tcp --dport 666 -j ACCEPT
--permanent
option to make the setting persistent.
5.14.3. Listing Rules using the Direct Interface
root
:
~]# firewall-cmd --direct --get-rules ipv4 filter IN_public_allow
--get-rules
option) only lists rules previously added using the --add-rule
option. It does not list existing iptables rules added by other means.
5.15. Configuring Complex Firewall Rules with the "Rich Language" Syntax
5.15.1. Formatting of the Rich Language Commands
root
. The format of the command to add a rule is as follows:
firewall-cmd [--zone=zone] --add-rich-rule='rule' [--timeout=timeval]
s
(seconds), m
(minutes), or h
(hours) to specify the unit of time. The default is seconds.
firewall-cmd [--zone=zone] --remove-rich-rule='rule'
firewall-cmd [--zone=zone] --query-rich-rule='rule'
yes
with exit status 0
if enabled. It prints no
with exit status 1
otherwise. If the zone is omitted, the default zone is used.
5.15.2. Understanding the Rich Rule Structure
rule [family="rule family"] [ source [NOT] [address="address"] [mac="mac-address"] [ipset="ipset"] ] [ destination [NOT] address="address" ] [ element ] [ log [prefix="prefix text"] [level="log level"] [limit value="rate/duration"] ] [ audit ] [ action ]
Note
NOT
keyword to invert the sense of the source and destination address commands, but the command line uses the invert
="true" option.
5.15.3. Understanding the Rich Rule Command Options
family
- If the rule family is provided, either
ipv4
oripv6
, it limits the rule toIPv4
orIPv6
, respectively. If the rule family is not provided, the rule is added for bothIPv4
andIPv6
. If source or destination addresses are used in a rule, then the rule family needs to be provided. This is also the case for port forwarding.
Source and Destination Addresses
source
- By specifying the source address, the origin of a connection attempt can be limited to the source address. A source address or address range is either an IP address or a network IP address with a mask for
IPv4
orIPv6
. ForIPv4
, the mask can be a network mask or a plain number. ForIPv6
, the mask is a plain number. The use of host names is not supported. It is possible to invert the sense of the source address command by adding theNOT
keyword; all but the supplied address matches.A MAC address and also an IP set with typecan be added forIPv4
andIPv6
if nofamily
is specified for the rule. Other IP sets need to match thefamily
setting of the rule. destination
- By specifying the destination address, the target can be limited to the destination address. The destination address uses the same syntax as the source address for IP address or address ranges. The use of source and destination addresses is optional, and the use of a destination addresses is not possible with all elements. This depends on the use of destination addresses, for example, in service entries. You can combine
destination
andaction
.
Elements
service
, port
, protocol
, masquerade
, icmp-block
, forward-port
, and source-port
.
service
- The
service
element is one of the firewalld provided services. To get a list of the predefined services, enter the following command:~]$
If a service provides a destination address, it will conflict with a destination address in the rule and will result in an error. The services using destination addresses internally are mostly services using multicast. The command takes the following form:firewall-cmd --get-services
service name=service_name
port
- The
port
element can either be a single port number or a port range, for example,5060-5062
, followed by the protocol, either astcp
orudp
. The command takes the following form:port port=number_or_range protocol=protocol
protocol
- The
protocol
value can be either a protocol ID number or a protocol name. For allowedprotocol
entries, see/etc/protocols
. The command takes the following form:protocol value=protocol_name_or_ID
icmp-block
- Use this command to block one or more
ICMP
types. TheICMP
type is one of theICMP
types firewalld supports. To get a listing of supported