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Securing networks

Red Hat Enterprise Linux 9

Configuring secured networks and network communication

Red Hat Customer Content Services

Abstract

Learn the tools and techniques to improve the security of your networks and lower the risks of data breaches and intrusions.

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SSH (Secure Shell) is a protocol which provides secure communications between two systems using a client-server architecture and allows users to log in to server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, which prevents intruders from collecting unencrypted passwords from the connection.

1.1. SSH and OpenSSH
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SSH (Secure Shell) is a program for logging into a remote machine and executing commands on that machine. The SSH protocol provides secure encrypted communications between two untrusted hosts over an insecure network. You can also forward X11 connections and arbitrary TCP/IP ports over the secure channel.

The SSH protocol mitigates security threats, such as interception of communication between two systems and impersonation of a particular host, when you use it for remote shell login or file copying. This is because the SSH client and server use digital signatures to verify their identities. Additionally, all communication between the client and server systems is encrypted.

A host key authenticates hosts in the SSH protocol. Host keys are cryptographic keys that are generated automatically when OpenSSH is started for the first time or when the host boots for the first time.

OpenSSH is an implementation of the SSH protocol supported by Linux, UNIX, and similar operating systems. It includes the core files necessary for both the OpenSSH client and server. The OpenSSH suite consists of the following user-space tools:

  • ssh is a remote login program (SSH client).
  • sshd is an OpenSSH SSH daemon.
  • scp is a secure remote file copy program.
  • sftp is a secure file transfer program.
  • ssh-agent is an authentication agent for caching private keys.
  • ssh-add adds private key identities to ssh-agent.
  • ssh-keygen generates, manages, and converts authentication keys for ssh.
  • ssh-copy-id is a script that adds local public keys to the authorized_keys file on a remote SSH server.
  • ssh-keyscan gathers SSH public host keys.
Note

In RHEL 9, the Secure copy protocol (SCP) is replaced with the SSH File Transfer Protocol (SFTP) by default. This is because SCP has already caused security issues, for example CVE-2020-15778.

If SFTP is unavailable or incompatible in your scenario, you can use the scp command with the -O option to force the use of the original SCP/RCP protocol.

For additional information, see the OpenSSH SCP protocol deprecation in Red Hat Enterprise Linux 9 article.

The OpenSSH suite in RHEL supports only SSH version 2. It has an enhanced key-exchange algorithm that is not vulnerable to exploits known in the older version 1.

Red Hat Enterprise Linux includes the following OpenSSH packages: the general openssh package, the openssh-server package, and the openssh-clients package. The OpenSSH packages require the OpenSSL package openssl-libs, which installs several important cryptographic libraries that enable OpenSSH to provide encrypted communications.

OpenSSH, as one of core cryptographic subsystems of RHEL, uses system-wide crypto policies. This ensures that weak cipher suites and cryptographic algorithms are disabled in the default configuration. To modify the policy, the administrator must either use the update-crypto-policies command to adjust the settings or manually opt out of the system-wide crypto policies. See the Excluding an application from following system-wide crypto policies section for more information.

The OpenSSH suite uses two sets of configuration files: one for client programs (that is, ssh, scp, and sftp), and another for the server (the sshd daemon).

System-wide SSH configuration information is stored in the /etc/ssh/ directory. The /etc/ssh/ssh_config file contains the client configuration, and the /etc/ssh/sshd_config file is the default OpenSSH server configuration file.

User-specific SSH configuration information is stored in ~/.ssh/ in the user’s home directory. For a detailed list of OpenSSH configuration files, see the FILES section in the sshd(8) man page on your system.

1.2. Generating SSH key pairs
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You can log in to an OpenSSH server without entering a password by generating an SSH key pair on a local system and copying the generated public key to the OpenSSH server. Each user who wants to create a key must run this procedure.

To preserve previously generated key pairs after you reinstall the system, back up the ~/.ssh/ directory before you create new keys. After reinstalling, copy it back to your home directory. You can do this for all users on your system, including root.

Prerequisites

  • You are logged in as a user who wants to connect to the OpenSSH server by using keys.
  • The OpenSSH server is configured to allow key-based authentication.

Procedure

  1. Generate an ECDSA key pair: 

    $ ssh-keygen -t ecdsa
Generating public/private ecdsa key pair.
Enter file in which to save the key (/home/<username>/.ssh/id_ecdsa):
Enter passphrase (empty for no passphrase): <password>
Enter same passphrase again: <password>
Your identification has been saved in /home/<username>/.ssh/id_ecdsa.
Your public key has been saved in /home/<username>/.ssh/id_ecdsa.pub.
The key fingerprint is:
SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI <username>@<localhost.example.com>
The key's randomart image is:
+---[ECDSA 256]---+
|.oo..o=++        |
|.. o .oo .       |
|. .. o. o        |
|....o.+...       |
|o.oo.o +S .      |
|.=.+.   .o       |
|E.*+.  .  . .    |
|.=..+ +..  o     |
|  .  oo*+o.      |
+----[SHA256]-----+
    Copy to Clipboard Toggle word wrap

    You can also generate an RSA key pair by using the ssh-keygen command without any parameter or an Ed25519 key pair by entering the ssh-keygen -t ed25519 command. Note that the Ed25519 algorithm is not FIPS-140-compliant, and OpenSSH does not work with Ed25519 keys in FIPS mode.

  2. Copy the public key to a remote machine: 

    $ ssh-copy-id <username>@<ssh-server-example.com>
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
<username>@<ssh-server-example.com>'s password:
…
Number of key(s) added: 1

Now try logging into the machine, with: "ssh '<username>@<ssh-server-example.com>'" and check to make sure that only the key(s) you wanted were added.
    Copy to Clipboard Toggle word wrap

    Replace <username>@<ssh-server-example.com> with your credentials.

    If you do not use the ssh-agent program in your session, the previous command copies the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. To specify another public-key file or to prioritize keys in files over keys cached in memory by ssh-agent, use the ssh-copy-id command with the -i option.

Verification

  • Log in to the OpenSSH server by using the key file: 

    $ ssh -o PreferredAuthentications=publickey <username>@<ssh-server-example.com>
    Copy to Clipboard Toggle word wrap

To improve system security, enforce key-based authentication by disabling password authentication on your OpenSSH server.

Prerequisites

  • The openssh-server package is installed.
  • The sshd daemon is running on the server.

  • You can already connect to the OpenSSH server by using a key.

    See the Generating SSH key pairs section for details.

Procedure

  1. Open the /etc/ssh/sshd_config configuration in a text editor, for example: 

    # vi /etc/ssh/sshd_config
    Copy to Clipboard Toggle word wrap

  2. Change the PasswordAuthentication option to no: 

    PasswordAuthentication no
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  3. On a system other than a new default installation, check that the PubkeyAuthentication parameter is either not set or set to yes.

  4. Set the KbdInteractiveAuthentication directive to no.

    Note that the corresponding entry is commented out in the configuration file and the default value is yes.

  5. To use key-based authentication with NFS-mounted home directories, enable the use_nfs_home_dirs SELinux boolean: 

    # setsebool -P use_nfs_home_dirs 1
    Copy to Clipboard Toggle word wrap
  6. If you are connected remotely, not using console or out-of-band access, test the key-based login process before disabling password authentication.

  7. Reload the sshd daemon to apply the changes: 

    # systemctl reload sshd
    Copy to Clipboard Toggle word wrap

To avoid entering a passphrase each time you initiate an SSH connection, you can use the ssh-agent utility to cache the private SSH key for a login session. If the agent is running and your keys are unlocked, you can log in to SSH servers by using these keys but without having to enter the key’s password again. The private key and the passphrase remain secure.

Prerequisites

  • You have a remote host with the SSH daemon running and reachable through the network.
  • You know the IP address or hostname and credentials to log in to the remote host.

  • You have generated an SSH key pair with a passphrase and transferred the public key to the remote machine.

    See the Generating SSH key pairs section for details.

Procedure

  1. Add the command for automatically starting ssh-agent in your session to the ~/.bashrc file:

    1. Open ~/.bashrc in a text editor of your choice, for example: 

      $ vi ~/.bashrc
      Copy to Clipboard Toggle word wrap

    2. Add the following line to the file: 

      eval $(ssh-agent)
      Copy to Clipboard Toggle word wrap
    3. Save the changes, and quit the editor.

  2. Add the following line to the ~/.ssh/config file: 

    AddKeysToAgent yes
    Copy to Clipboard Toggle word wrap

    With this option and ssh-agent started in your session, the agent prompts for a password only for the first time when you connect to a host.

Verification

  • Log in to a host which uses the corresponding public key of the cached private key in the agent, for example: 

    $ ssh <example.user>@<ssh-server@example.com>
    Copy to Clipboard Toggle word wrap

    Note that you did not have to enter the passphrase.

You can create and store ECDSA and RSA keys on a smart card and authenticate by the smart card on an OpenSSH client. Smart-card authentication replaces the default password authentication.

Prerequisites

  • On the client side, the opensc package is installed and the pcscd service is running.

Procedure

  1. List all keys provided by the OpenSC PKCS #11 module including their PKCS #11 URIs and save the output to the keys.pub file: 

    $ ssh-keygen -D pkcs11: > keys.pub
    Copy to Clipboard Toggle word wrap

  2. Transfer the public key to the remote server. Use the ssh-copy-id command with the keys.pub file created in the previous step: 

    $ ssh-copy-id -f -i keys.pub <username@ssh-server-example.com>
    Copy to Clipboard Toggle word wrap

  3. Connect to <ssh-server-example.com> by using the ECDSA key. You can use just a subset of the URI, which uniquely references your key, for example: 

    $ ssh -i "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" <ssh-server-example.com>
Enter PIN for 'SSH key':
[ssh-server-example.com] $
    Copy to Clipboard Toggle word wrap

    Because OpenSSH uses the p11-kit-proxy wrapper and the OpenSC PKCS #11 module is registered to the p11-kit tool, you can simplify the previous command: 

    $ ssh -i "pkcs11:id=%01" <ssh-server-example.com>
Enter PIN for 'SSH key':
[ssh-server-example.com] $
    Copy to Clipboard Toggle word wrap

    If you skip the id= part of a PKCS #11 URI, OpenSSH loads all keys that are available in the proxy module. This can reduce the amount of typing required: 

    $ ssh -i pkcs11: <ssh-server-example.com>
Enter PIN for 'SSH key':
[ssh-server-example.com] $
    Copy to Clipboard Toggle word wrap

  4. Optional: You can use the same URI string in the ~/.ssh/config file to make the configuration permanent: 

    $ cat ~/.ssh/config
IdentityFile "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so"
$ ssh <ssh-server-example.com>
Enter PIN for 'SSH key':
[ssh-server-example.com] $
    Copy to Clipboard Toggle word wrap

    The ssh client utility now automatically uses this URI and the key from the smart card.

1.6. Making OpenSSH more secure
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You can tweak the system to increase security when using OpenSSH.

Note that changes in the /etc/ssh/sshd_config OpenSSH server configuration file require reloading the sshd daemon to take effect: 

# systemctl reload sshd
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Warning

The majority of security hardening configuration changes reduce compatibility with clients that do not support up-to-date algorithms or cipher suites.

Disabling insecure connection protocols
To make SSH truly effective, prevent the use of insecure connection protocols that are replaced by the OpenSSH suite. Otherwise, a user’s password might be protected using SSH for one session only to be captured later when logging in using Telnet.
Disabling password-based authentication
Disabling passwords for authentication and allowing only key pairs reduces the attack surface. See the Setting key-based authentication as the only method on an OpenSSH server section for more information.
Stronger key types

Although the ssh-keygen command generates a pair of RSA keys by default, you can instruct it to generate Elliptic Curve Digital Signature Algorithm (ECDSA) or Edwards-Curve 25519 (Ed25519) keys by using the -t option. The ECDSA offers better performance than RSA at the equivalent symmetric key strength. It also generates shorter keys. The Ed25519 public-key algorithm is an implementation of twisted Edwards curves that is more secure and also faster than RSA, DSA, and ECDSA.

OpenSSH creates RSA, ECDSA, and Ed25519 server host keys automatically if they are missing. To configure the host key creation in RHEL, use the sshd-keygen@.service instantiated service. For example, to disable the automatic creation of the RSA key type: 

# systemctl mask sshd-keygen@rsa.service
# rm -f /etc/ssh/ssh_host_rsa_key*
# systemctl restart sshd
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Note

In images with the cloud-init method enabled, the ssh-keygen units are automatically disabled. This is because the ssh-keygen template service can interfere with the cloud-init tool and cause problems with host key generation. To prevent these problems the etc/systemd/system/sshd-keygen@.service.d/disable-sshd-keygen-if-cloud-init-active.conf drop-in configuration file disables the ssh-keygen units if cloud-init is running.

To allow only a particular key type for SSH connections, remove a comment out at the beginning of the relevant line in /etc/ssh/sshd_config, and reload the sshd service. For example, to allow only Ed25519 host keys, the corresponding lines must be as follows: 

# HostKey /etc/ssh/ssh_host_rsa_key
# HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
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Important

The Ed25519 algorithm is not FIPS-140-compliant, and OpenSSH does not work with Ed25519 keys in FIPS mode.

Non-default port

By default, the sshd daemon listens on TCP port 22. Changing the port reduces the exposure of the system to attacks based on automated network scanning on the default port and therefore increases security through obscurity. You can specify the port using the Port directive in the /etc/ssh/sshd_config configuration file.

You also have to update the default SELinux policy to allow the use of a non-default port. To do so, use the semanage tool from the policycoreutils-python-utils package: 

# semanage port -a -t ssh_port_t -p tcp <port-number>
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Furthermore, update firewalld configuration: 

# firewall-cmd --add-port <port-number>/tcp
# firewall-cmd --remove-port=22/tcp
# firewall-cmd --runtime-to-permanent
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In the previous commands, replace <port-number> with the new port number specified using the Port directive.

Root login

PermitRootLogin is set to prohibit-password by default. This enforces the use of key-based authentication instead of the use of passwords for logging in as root and reduces risks by preventing brute-force attacks.

Warning

Enabling logging in as the root user is not a secure practice because the administrator cannot audit which users run which privileged commands. For using administrative commands, log in and use sudo instead.

Using the X Security extension

The X server in Red Hat Enterprise Linux clients does not provide the X Security extension. Therefore, clients cannot request another security layer when connecting to untrusted SSH servers with X11 forwarding. Most applications are not able to run with this extension enabled anyway.

By default, the ForwardX11Trusted option in the /etc/ssh/ssh_config.d/50-redhat.conf 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.

If your scenario does not require the X11 forwarding feature at all, set the X11Forwarding directive in the /etc/ssh/sshd_config configuration file to no.

Restricting SSH access to specific users, groups, or IP ranges

The AllowUsers and AllowGroups directives in the /etc/ssh/sshd_config configuration file server enable you to permit only certain users, domains, or groups to connect to your OpenSSH server. You can combine AllowUsers and AllowGroups to restrict access more precisely, for example: 

AllowUsers *@192.168.1.* *@10.0.0.* !*@192.168.1.2
AllowGroups example-group
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This configuration allows only connections if all of the following conditions meet:

  • The connection’s source IP is within the 192.168.1.0/24 or 10.0.0.0/24 subnet.
  • The source IP is not 192.168.1.2.
  • The user is a member of the example-group group.

The OpenSSH server permits only connections that pass all Allow and Deny directives in /etc/ssh/sshd_config. For example, if the AllowUsers directive lists a user that is not part of a group listed in the AllowGroups directive, then the user cannot log in.

Note that using allowlists (directives starting with Allow) is more secure than using blocklists (options starting with Deny) because allowlists block also new unauthorized users or groups.

Changing system-wide cryptographic policies

OpenSSH uses RHEL system-wide cryptographic policies, and the default system-wide cryptographic policy level offers secure settings for current threat models. To make your cryptographic settings more strict, change the current policy level: 

# update-crypto-policies --set FUTURE
Setting system policy to FUTURE
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Warning

If your system communicates with legacy systems, you might face interoperability problems due to the strict setting of the FUTURE policy.

You can also disable only specific ciphers for the SSH protocol through the system-wide cryptographic policies. See the Customizing system-wide cryptographic policies with subpolicies section in the Security hardening document for more information.

Opting out of system-wide cryptographic policies

To opt out of the system-wide cryptographic policies for your OpenSSH server, specify the cryptographic policy in a drop-in configuration file located in the /etc/ssh/sshd_config.d/ directory, with a two-digit number prefix smaller than 50, so that it lexicographically precedes the 50-redhat.conf file, and with a .conf suffix, for example, 49-crypto-policy-override.conf.

See the sshd_config(5) man page for more information.

To opt out of system-wide cryptographic policies for your OpenSSH client, perform one of the following tasks:

  • For a given user, override the global ssh_config with a user-specific configuration in the ~/.ssh/config file.
  • For the entire system, specify the cryptographic policy in a drop-in configuration file located in the /etc/ssh/ssh_config.d/ directory, with a two-digit number prefix smaller than 50, so that it lexicographically precedes the 50-redhat.conf file, and with a .conf suffix, for example, 49-crypto-policy-override.conf.

You can connect from your local system to a remote server through an intermediary server, also called jump host. A jump server bridges hosts from different security zones and can manage multiple client-server connections.

Prerequisites

  • A jump host accepts SSH connections from your local system.
  • A remote server accepts SSH connections from the jump host.

Procedure

  1. If you connect through a jump server or more intermediary servers once, use the ssh -J command and specify the jump servers directly, for example: 

    $ ssh -J <jump-1.example.com>,<jump-2.example.com>,<jump-3.example.com> <target-server-1.example.com>
    Copy to Clipboard Toggle word wrap

    Change the host name-only notation in the previous command if the user names or SSH ports on the jump servers differ from the names and ports on the remote server, for example: 

    $ ssh -J <example.user.1>@<jump-1.example.com>:<75>,<example.user.2>@<jump-2.example.com>:<75>,<example.user.3>@<jump-3.example.com>:<75> <example.user.f>@<target-server-1.example.com>:<220>
    Copy to Clipboard Toggle word wrap

  2. If you connect to a remote server through jump servers regularly, store the jump-server configuration in your SSH configuration file:

    1. Define the jump host by editing the ~/.ssh/config file on your local system, for example: 

      Host <jump-server-1>
  HostName <jump-1.example.com>
      Copy to Clipboard Toggle word wrap
      • The Host parameter defines a name or alias for the host you can use in ssh commands. The value can match the real host name, but can also be any string.
      • The HostName parameter sets the actual host name or IP address of the jump host.

    2. Add the remote server jump configuration with the ProxyJump directive to ~/.ssh/config file on your local system, for example: 

      Host <remote-server-1>
  HostName <target-server-1.example.com>
  ProxyJump <jump-server-1>
      Copy to Clipboard Toggle word wrap

    3. Use your local system to connect to the remote server through the jump server: 

      $ ssh <remote-server-1>
      Copy to Clipboard Toggle word wrap

      This command is equivalent to the ssh -J jump-server1 remote-server command if you omit the previous configuration steps.

As an administrator, you can use the sshd system role to configure SSH servers and the ssh system role to configure SSH clients consistently on any number of RHEL systems at the same time using the Ansible Core package.

In the sshd RHEL system role playbook, you can define the parameters for the server SSH configuration file.

If you do not specify these settings, the role produces the sshd_config file that matches the RHEL defaults.

In all cases, booleans correctly render as yes and no in the final configuration on your managed nodes. You can use lists to define multi-line configuration items. For example: 

sshd_ListenAddress:
  - 0.0.0.0
  - '::'
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renders as: 

ListenAddress 0.0.0.0
ListenAddress ::
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You can use the sshd RHEL system role to configure multiple OpenSSH servers. These ensure secure communication environment for remote users by providing namely:

  • Management of incoming SSH connections from remote clients
  • Credentials verification
  • Secure data transfer and command execution
Note

You can use the sshd RHEL system role alongside with other RHEL system roles that change SSHD configuration, for example the Identity Management RHEL system roles. To prevent the configuration from being overwritten, ensure the sshd RHEL system role uses namespaces (RHEL 8 and earlier versions) or a drop-in directory (RHEL 9).

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: SSH server configuration
  hosts: managed-node-01.example.com
  tasks:
    - name: Configure sshd to prevent root and password login except from particular subnet
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.sshd
      vars:
        sshd_config:
          PermitRootLogin: no
          PasswordAuthentication: no
          Match:
            - Condition: "Address 192.0.2.0/24"
              PermitRootLogin: yes
              PasswordAuthentication: yes
    Copy to Clipboard Toggle word wrap

    The settings specified in the example playbook include the following:

    PasswordAuthentication: yes|no
    Controls whether the OpenSSH server (sshd) accepts authentication from clients that use the username and password combination.
    Match:
    The match block allows the root user login using password only from the subnet 192.0.2.0/24.

    For details about the role variables and the OpenSSH configuration options used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.sshd/README.md file and the sshd_config(5) manual page on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
    Copy to Clipboard Toggle word wrap

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
    Copy to Clipboard Toggle word wrap

Verification

  1. Log in to the SSH server: 

    $ ssh <username>@<ssh_server>
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  2. Verify the contents of the sshd_config file on the SSH server: 

    $ cat /etc/ssh/sshd_config.d/00-ansible_system_role.conf
#
# Ansible managed
#
PasswordAuthentication no
PermitRootLogin no
Match Address 192.0.2.0/24
  PasswordAuthentication yes
  PermitRootLogin yes
    Copy to Clipboard Toggle word wrap

  3. Check that you can connect to the server as root from the 192.0.2.0/24 subnet:

    1. Determine your IP address: 

      $ hostname -I
192.0.2.1
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      If the IP address is within the 192.0.2.1 - 192.0.2.254 range, you can connect to the server.

    2. Connect to the server as root: 

      $ ssh root@<ssh_server>
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In the ssh RHEL system role playbook, you can define the parameters for the client SSH configuration file.

If you do not specify these settings, the role produces a global ssh_config file that matches the RHEL defaults.

In all the cases, booleans correctly render as yes or no in the final configuration on your managed nodes. You can use lists to define multi-line configuration items. For example: 

LocalForward:
  - 22 localhost:2222
  - 403 localhost:4003
Copy to Clipboard Toggle word wrap

renders as: 

LocalForward 22 localhost:2222
LocalForward 403 localhost:4003
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Note

The configuration options are case sensitive.

You can use the ssh RHEL system role to configure multiple OpenSSH clients. These enable the local user to establish a secure connection with the remote OpenSSH server by ensuring namely:

  • Secure connection initiation
  • Credentials provision
  • Negotiation with the OpenSSH server on the encryption method used for the secure communication channel
  • Ability to send files securely to and from the OpenSSH server
Note

You can use the ssh RHEL system role alongside with other system roles that change SSH configuration, for example the Identity Management RHEL system roles. To prevent the configuration from being overwritten, make sure that the ssh RHEL system role uses a drop-in directory (default in RHEL 8 and later).

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: SSH client configuration
  hosts: managed-node-01.example.com
  tasks:
    - name: Configure ssh clients
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.ssh
      vars:
        ssh_user: root
        ssh:
          Compression: true
          GSSAPIAuthentication: no
          ControlMaster: auto
          ControlPath: ~/.ssh/.cm%C
          Host:
            - Condition: example
              Hostname: server.example.com
              User: user1
        ssh_ForwardX11: no
    Copy to Clipboard Toggle word wrap

    The settings specified in the example playbook include the following:

    ssh_user: root
    Configures the root user’s SSH client preferences on the managed nodes with certain configuration specifics.
    Compression: true
    Compression is enabled.
    ControlMaster: auto
    ControlMaster multiplexing is set to auto.
    Host
    Creates alias example for connecting to the server.example.com host as a user called user1.
    ssh_ForwardX11: no
    X11 forwarding is disabled.

    For details about the role variables and the OpenSSH configuration options used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.ssh/README.md file and the ssh_config(5) manual page on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
    Copy to Clipboard Toggle word wrap

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
    Copy to Clipboard Toggle word wrap

Verification

  • Verify that the managed node has the correct configuration by displaying the SSH configuration file: 

    # cat ~/root/.ssh/config
# Ansible managed
Compression yes
ControlMaster auto
ControlPath ~/.ssh/.cm%C
ForwardX11 no
GSSAPIAuthentication no
Host example
  Hostname example.com
  User user1
    Copy to Clipboard Toggle word wrap

By default, applying the sshd RHEL system role overwrites the entire configuration. This may be problematic if you have previously adjusted the configuration, for example, with a different RHEL system role or a playbook. To apply the sshd RHEL system role for only selected configuration options while keeping other options in place, you can use the non-exclusive configuration.

You can apply a non-exclusive configuration:

  • In RHEL 8 and earlier by using a configuration snippet.
  • In RHEL 9 and later by using files in a drop-in directory. The default configuration file is already placed in the drop-in directory as /etc/ssh/sshd_config.d/00-ansible_system_role.conf.

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content:

    • For managed nodes that run RHEL 8 or earlier: 

      ---
- name: Non-exclusive sshd configuration
  hosts: managed-node-01.example.com
  tasks:
    - name: Configure SSHD to accept environment variables
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.sshd
      vars:
        sshd_config_namespace: <my-application>
        sshd_config:
          # Environment variables to accept
          AcceptEnv:
            LANG
            LS_COLORS
            EDITOR
      Copy to Clipboard Toggle word wrap

    • For managed nodes that run RHEL 9 or later: 

      - name: Non-exclusive sshd configuration
  hosts: managed-node-01.example.com
  tasks:
    - name: Configure sshd to accept environment variables
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.sshd
      vars:
        sshd_config_file: /etc/ssh/sshd_config.d/<42-my-application>.conf
        sshd_config:
          # Environment variables to accept
          AcceptEnv:
            LANG
            LS_COLORS
            EDITOR
      Copy to Clipboard Toggle word wrap

      The settings specified in the example playbooks include the following:

      sshd_config_namespace: <my-application>
      The role places the configuration that you specify in the playbook to configuration snippets in the existing configuration file under the given namespace. You need to select a different namespace when running the role from different context.
      sshd_config_file: /etc/ssh/sshd_config.d/<42-my-application>.conf
      In the sshd_config_file variable, define the .conf file into which the sshd system role writes the configuration options. Use a two-digit prefix, for example 42- to specify the order in which the configuration files will be applied.
      AcceptEnv:

      Controls which environment variables the OpenSSH server (sshd) will accept from a client:

      • LANG: defines the language and locale settings.
      • LS_COLORS: defines the displaying color scheme for the ls command in the terminal.
      • EDITOR: specifies the default text editor for the command-line programs that need to open an editor.

      For details about the role variables and the OpenSSH configuration options used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.sshd/README.md file and the sshd_config(5) manual page on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
    Copy to Clipboard Toggle word wrap

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
    Copy to Clipboard Toggle word wrap

Verification

  • Verify the configuration on the SSH server:

    • For managed nodes that run RHEL 8 or earlier: 

      # cat /etc/ssh/sshd_config
...
# BEGIN sshd system role managed block: namespace <my-application>
Match all
  AcceptEnv LANG LS_COLORS EDITOR
# END sshd system role managed block: namespace <my-application>
      Copy to Clipboard Toggle word wrap

    • For managed nodes that run RHEL 9 or later: 

      # cat /etc/ssh/sshd_config.d/42-my-application.conf
# Ansible managed
#
AcceptEnv LANG LS_COLORS EDITOR
      Copy to Clipboard Toggle word wrap

You can encrypt communication transmitted between two systems by using the TLS (Transport Layer Security) protocol. This standard uses asymmetric cryptography with private and public keys, digital signatures, and certificates.

2.1. TLS certificates
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TLS (Transport Layer Security) is a protocol that enables client-server applications to pass information securely. TLS uses a system of public and private key pairs to encrypt communication transmitted between clients and servers. TLS is the successor protocol to SSL (Secure Sockets Layer).

TLS uses X.509 certificates to bind identities, such as hostnames or organizations, to public keys using digital signatures. X.509 is a standard that defines the format of public key certificates.

Authentication of a secure application depends on the integrity of the public key value in the application’s certificate. If an attacker replaces the public key with its own public key, it can impersonate the true application and gain access to secure data. To prevent this type of attack, all certificates must be signed by a certification authority (CA). A CA is a trusted node that confirms the integrity of the public key value in a certificate.

A CA signs a public key by adding its digital signature and issues a certificate. A digital signature is a message encoded with the CA’s private key. The CA’s public key is made available to applications by distributing the certificate of the CA. Applications verify that certificates are validly signed by decoding the CA’s digital signature with the CA’s public key.

To have a certificate signed by a CA, you must generate a public key, and send it to a CA for signing. This is referred to as a certificate signing request (CSR). A CSR contains also a distinguished name (DN) for the certificate. The DN information that you can provide for either type of certificate can include a two-letter country code for your country, a full name of your state or province, your city or town, a name of your organization, your email address, and it can also be empty. Many current commercial CAs prefer the Subject Alternative Name extension and ignore DNs in CSRs.

RHEL provides two main toolkits for working with TLS certificates: GnuTLS and OpenSSL. You can create, read, sign, and verify certificates using the openssl utility from the openssl package. The certtool utility provided by the gnutls-utils package can do the same operations using a different syntax and above all a different set of libraries in the back end.

2.2. Creating a private CA using OpenSSL
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Private certificate authorities (CA) are useful when your scenario requires verifying entities within your internal network. For example, use a private CA when you create a VPN gateway with authentication based on certificates signed by a CA under your control or when you do not want to pay a commercial CA. To sign certificates in such use cases, the private CA uses a self-signed certificate.

Prerequisites

  • You have root privileges or permissions to enter administrative commands with sudo. Commands that require such privileges are marked with #.

Procedure

  1. Generate a private key for your CA. For example, the following command creates a 256-bit Elliptic Curve Digital Signature Algorithm (ECDSA) key: 

    $ openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:P-256 -out <ca.key>
    Copy to Clipboard Toggle word wrap

    The time for the key-generation process depends on the hardware and entropy of the host, the selected algorithm, and the length of the key.

  2. Create a certificate signed using the private key generated in the previous command: 

    $ openssl req -key <ca.key> -new -x509 -days 3650 -addext keyUsage=critical,keyCertSign,cRLSign -subj "/CN=<Example_CA>" -out <ca.crt>
    Copy to Clipboard Toggle word wrap

    The generated ca.crt file is a self-signed CA certificate that you can use to sign other certificates for ten years. In the case of a private CA, you can replace <Example_CA> with any string as the common name (CN).

  3. Set secure permissions on the private key of your CA, for example: 

    # chown <root>:<root> <ca.key>
# chmod 600 <ca.key>
    Copy to Clipboard Toggle word wrap

Next steps

  • To use a self-signed CA certificate as a trust anchor on client systems, copy the CA certificate to the client and add it to the clients' system-wide truststore as root: 

    # trust anchor <ca.crt>
    Copy to Clipboard Toggle word wrap

    See Chapter 3, Using shared system certificates for more information.

Verification

  1. Create a certificate signing request (CSR), and use your CA to sign the request. The CA must successfully create a certificate based on the CSR, for example: 

    $ openssl x509 -req -in <client-cert.csr> -CA <ca.crt> -CAkey <ca.key> -CAcreateserial -days 365 -extfile <openssl.cnf> -extensions <client-cert> -out <client-cert.crt>
Signature ok
subject=C = US, O = Example Organization, CN = server.example.com
Getting CA Private Key
    Copy to Clipboard Toggle word wrap

    See Section 2.5, “Using a private CA to issue certificates for CSRs with OpenSSL” for more information.

  2. Display the basic information about your self-signed CA: 

    $ openssl x509 -in <ca.crt> -text -noout
Certificate:
…
        X509v3 extensions:
            …
            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: critical
                Certificate Sign, CRL Sign
…
    Copy to Clipboard Toggle word wrap

  3. Verify the consistency of the private key: 

    $ openssl pkey -check -in <ca.key>
Key is valid
-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgcagSaTEBn74xZAwO
18wRpXoCVC9vcPki7WlT+gnmCI+hRANCAARb9NxIvkaVjFhOoZbGp/HtIQxbM78E
lwbDP0BI624xBJ8gK68ogSaq2x4SdezFdV1gNeKScDcU+Pj2pELldmdF
-----END PRIVATE KEY-----
    Copy to Clipboard Toggle word wrap

You can use TLS-encrypted communication channels only if you have a valid TLS certificate from a certificate authority (CA). To obtain the certificate, you must create a private key and a certificate signing request (CSR) for your server first.

Procedure

  1. Generate a private key on your server system, for example: 

    $ openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:P-256 -out <server-private.key>
    Copy to Clipboard Toggle word wrap

  2. Optional: Use a text editor of your choice to prepare a configuration file that simplifies creating your CSR, for example: 

    $ vim <example_server.cnf>
[server-cert]
keyUsage = critical, digitalSignature, keyEncipherment, keyAgreement
extendedKeyUsage = serverAuth
subjectAltName = @alt_name

[req]
distinguished_name = dn
prompt = no

[dn]
C = <US>
O = <Example Organization>
CN = <server.example.com>

[alt_name]
DNS.1 = <example.com>
DNS.2 = <server.example.com>
IP.1 = <192.168.0.1>
IP.2 = <::1>
IP.3 = <127.0.0.1>
    Copy to Clipboard Toggle word wrap

    The extendedKeyUsage = serverAuth option limits the use of a certificate.

  3. Create a CSR using the private key you created previously: 

    $ openssl req -key <server-private.key> -config <example_server.cnf> -new -out <server-cert.csr>
    Copy to Clipboard Toggle word wrap

    If you omit the -config option, the req utility prompts you for additional information, for example: 

    You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]: <US>
State or Province Name (full name) []: <Washington>
Locality Name (eg, city) [Default City]: <Seattle>
Organization Name (eg, company) [Default Company Ltd]: <Example Organization>
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []: <server.example.com>
Email Address []: <server@example.com>
    Copy to Clipboard Toggle word wrap

Next steps

Verification

  1. After you obtain the requested certificate from the CA, check that the human-readable parts of the certificate match your requirements, for example: 

    $ openssl x509 -text -noout -in <server-cert.crt>
Certificate:
…
        Issuer: CN = Example CA
        Validity
            Not Before: Feb  2 20:27:29 2023 GMT
            Not After : Feb  2 20:27:29 2024 GMT
        Subject: C = US, O = Example Organization, CN = server.example.com
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
…
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment, Key Agreement
            X509v3 Extended Key Usage:
                TLS Web Server Authentication
            X509v3 Subject Alternative Name:
                DNS:example.com, DNS:server.example.com, IP Address:192.168.0.1, IP
…
    Copy to Clipboard Toggle word wrap

You can use TLS-encrypted communication channels only if you have a valid TLS certificate from a certificate authority (CA). To obtain the certificate, you must create a private key and a certificate signing request (CSR) for your client first.

Procedure

  1. Generate a private key on your client system, for example: 

    $ openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:P-256 -out <client-private.key>
    Copy to Clipboard Toggle word wrap

  2. Optional: Use a text editor of your choice to prepare a configuration file that simplifies creating your CSR, for example: 

    $ vim <example_client.cnf>
[client-cert]
keyUsage = critical, digitalSignature, keyEncipherment
extendedKeyUsage = clientAuth
subjectAltName = @alt_name

[req]
distinguished_name = dn
prompt = no

[dn]
CN = <client.example.com>

[clnt_alt_name]
email= <client@example.com>
    Copy to Clipboard Toggle word wrap

    The extendedKeyUsage = clientAuth option limits the use of a certificate.

  3. Create a CSR using the private key you created previously: 

    $ openssl req -key <client-private.key> -config <example_client.cnf> -new -out <client-cert.csr>
    Copy to Clipboard Toggle word wrap

    If you omit the -config option, the req utility prompts you for additional information, for example: 

    You are about to be asked to enter information that will be incorporated
into your certificate request.
…
Common Name (eg, your name or your server's hostname) []: <client.example.com>
Email Address []: <client@example.com>
    Copy to Clipboard Toggle word wrap

Next steps

Verification

  1. Check that the human-readable parts of the certificate match your requirements, for example: 

    $ openssl x509 -text -noout -in <client-cert.crt>
Certificate:
…
            X509v3 Extended Key Usage:
                TLS Web Client Authentication
            X509v3 Subject Alternative Name:
                email:client@example.com
…
    Copy to Clipboard Toggle word wrap

To enable systems to establish a TLS-encrypted communication channel, a certificate authority (CA) must provide valid certificates to them. If you have a private CA, you can create the requested certificates by signing certificate signing requests (CSRs) from the systems.

Prerequisites

Procedure

  1. Optional: Use a text editor of your choice to prepare an OpenSSL configuration file for adding extensions to certificates, for example: 

    $ vim <openssl.cnf>
[server-cert]
extendedKeyUsage = serverAuth

[client-cert]
extendedKeyUsage = clientAuth
    Copy to Clipboard Toggle word wrap

    Note that the previous example illustrates only the principle and openssl does not add all extensions to the certificate automatically. You must add the extensions you require either to the CNF file or append them to parameters of the openssl command.

  2. Use the x509 utility to create a certificate based on a CSR, for example: 

    $ openssl x509 -req -in <server-cert.csr> -CA <ca.crt> -CAkey <ca.key> -days 365 -extfile <openssl.cnf> -extensions <server-cert> -out <server-cert.crt>
Signature ok
subject=C = US, O = Example Organization, CN = server.example.com
Getting CA Private Key
    Copy to Clipboard Toggle word wrap

2.6. Creating a private CA using GnuTLS
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Private certificate authorities (CA) are useful when your scenario requires verifying entities within your internal network. For example, use a private CA when you create a VPN gateway with authentication based on certificates signed by a CA under your control or when you do not want to pay a commercial CA. To sign certificates in such use cases, the private CA uses a self-signed certificate.

Prerequisites

  • You have root privileges or permissions to enter administrative commands with sudo. Commands that require such privileges are marked with #.

  • You have already installed GnuTLS on your system. If you did not, you can use this command: 

    $ dnf install gnutls-utils
    Copy to Clipboard Toggle word wrap

Procedure

  1. Generate a private key for your CA. For example, the following command creates a 256-bit ECDSA (Elliptic Curve Digital Signature Algorithm) key: 

    $ certtool --generate-privkey --sec-param High --key-type=ecdsa --outfile <ca.key>
    Copy to Clipboard Toggle word wrap

    The time for the key-generation process depends on the hardware and entropy of the host, the selected algorithm, and the length of the key.

  2. Create a template file for a certificate.

    1. Create a file with a text editor of your choice, for example: 

      $ vi <ca.cfg>
      Copy to Clipboard Toggle word wrap

    2. Edit the file to include the necessary certification details: 

      organization = "Example Inc."
state = "Example"
country = EX
cn = "Example CA"
serial = 007
expiration_days = 365
ca
cert_signing_key
crl_signing_key
      Copy to Clipboard Toggle word wrap

  3. Create a certificate signed using the private key generated in step 1:

    The generated <ca.crt> file is a self-signed CA certificate that you can use to sign other certificates for one year. <ca.crt> file is the public key (certificate). The loaded file <ca.key> is the private key. You should keep this file in safe location. 

    $ certtool --generate-self-signed --load-privkey <ca.key> --template <ca.cfg> --outfile <ca.crt>
    Copy to Clipboard Toggle word wrap

  4. Set secure permissions on the private key of your CA, for example: 

    # chown <root>:<root> <ca.key>
# chmod 600 <ca.key>
    Copy to Clipboard Toggle word wrap

Next steps

  • To use a self-signed CA certificate as a trust anchor on client systems, copy the CA certificate to the client and add it to the clients' system-wide truststore as root: 

    # trust anchor <ca.crt>
    Copy to Clipboard Toggle word wrap

    See Chapter 3, Using shared system certificates for more information.

Verification

  1. Display the basic information about your self-signed CA: 

    $ certtool --certificate-info --infile <ca.crt>
Certificate:
…
    	X509v3 extensions:
        	…
        	X509v3 Basic Constraints: critical
            	CA:TRUE
        	X509v3 Key Usage: critical
            	Certificate Sign, CRL Sign
    Copy to Clipboard Toggle word wrap

  2. Create a certificate signing request (CSR), and use your CA to sign the request. The CA must successfully create a certificate based on the CSR, for example:

    1. Generate a private key for your CA: 

      $ certtool --generate-privkey --outfile <example-server.key>
      Copy to Clipboard Toggle word wrap

    2. Open a new configuration file in a text editor of your choice, for example: 

      $ vi <example-server.cfg>
      Copy to Clipboard Toggle word wrap

    3. Edit the file to include the necessary certification details: 

      signing_key
encryption_key
key_agreement

tls_www_server

country = "US"
organization = "Example Organization"
cn = "server.example.com"

dns_name = "example.com"
dns_name = "server.example.com"
ip_address = "192.168.0.1"
ip_address = "::1"
ip_address = "127.0.0.1"
      Copy to Clipboard Toggle word wrap

    4. Generate a request with the previously created private key: 

      $ certtool --generate-request --load-privkey <example-server.key> --template <example-server.cfg> --outfile <example-server.crq>
      Copy to Clipboard Toggle word wrap

    5. Generate the certificate and sign it with the private key of the CA: 

      $ certtool --generate-certificate --load-request <example-server.crq> --load-ca-certificate <ca.crt> --load-ca-privkey <ca.key> --outfile <example-server.crt>
      Copy to Clipboard Toggle word wrap

To obtain the certificate, you must create a private key and a certificate signing request (CSR) for your server first.

Procedure

  1. Generate a private key on your server system, for example: 

    $ certtool --generate-privkey --sec-param High --outfile <example-server.key>
    Copy to Clipboard Toggle word wrap

  2. Optional: Use a text editor of your choice to prepare a configuration file that simplifies creating your CSR, for example: 

    $ vim <example_server.cnf>
signing_key
encryption_key
key_agreement

tls_www_server

country = "US"
organization = "Example Organization"
cn = "server.example.com"

dns_name = "example.com"
dns_name = "server.example.com"
ip_address = "192.168.0.1"
ip_address = "::1"
ip_address = "127.0.0.1"
    Copy to Clipboard Toggle word wrap

  3. Create a CSR using the private key you created previously: 

    $ certtool --generate-request --template <example-server.cfg> --load-privkey <example-server.key> --outfile <example-server.crq>
    Copy to Clipboard Toggle word wrap

    If you omit the --template option, the certool utility prompts you for additional information, for example: 

    You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Generating a PKCS #10 certificate request...
Country name (2 chars): <US>
State or province name: <Washington>
Locality name: <Seattle>
Organization name: <Example Organization>
Organizational unit name:
Common name: <server.example.com>
    Copy to Clipboard Toggle word wrap

Next steps

Verification

  1. After you obtain the requested certificate from the CA, check that the human-readable parts of the certificate match your requirements, for example: 

    $ certtool --certificate-info --infile <example-server.crt>
Certificate:
…
        Issuer: CN = Example CA
        Validity
            Not Before: Feb  2 20:27:29 2023 GMT
            Not After : Feb  2 20:27:29 2024 GMT
        Subject: C = US, O = Example Organization, CN = server.example.com
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
…
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment, Key Agreement
            X509v3 Extended Key Usage:
                TLS Web Server Authentication
            X509v3 Subject Alternative Name:
                DNS:example.com, DNS:server.example.com, IP Address:192.168.0.1, IP
…
    Copy to Clipboard Toggle word wrap

To obtain the certificate, you must create a private key and a certificate signing request (CSR) for your client first.

Procedure

  1. Generate a private key on your client system, for example: 

    $ certtool --generate-privkey --sec-param High --outfile <example-client.key>
    Copy to Clipboard Toggle word wrap

  2. Optional: Use a text editor of your choice to prepare a configuration file that simplifies creating your CSR, for example: 

    $ vim <example_client.cnf>
signing_key
encryption_key

tls_www_client

cn = "client.example.com"
email = "client@example.com"
    Copy to Clipboard Toggle word wrap

  3. Create a CSR using the private key you created previously: 

    $ certtool --generate-request --template <example-client.cfg> --load-privkey <example-client.key> --outfile <example-client.crq>
    Copy to Clipboard Toggle word wrap

    If you omit the --template option, the certtool utility prompts you for additional information, for example: 

    Generating a PKCS #10 certificate request...
Country name (2 chars): <US>
State or province name: <Washington>
Locality name: <Seattle>
Organization name: <Example Organization>
Organizational unit name:
Common name: <server.example.com>
    Copy to Clipboard Toggle word wrap

Next steps

Verification

  1. Check that the human-readable parts of the certificate match your requirements, for example: 

    $ certtool --certificate-info --infile <example-client.crt>
Certificate:
…
            X509v3 Extended Key Usage:
                TLS Web Client Authentication
            X509v3 Subject Alternative Name:
                email:client@example.com
…
    Copy to Clipboard Toggle word wrap

To enable systems to establish a TLS-encrypted communication channel, a certificate authority (CA) must provide valid certificates to them. If you have a private CA, you can create the requested certificates by signing certificate signing requests (CSRs) from the systems.

Prerequisites

Procedure

  1. Optional: Use a text editor of your choice to prepare an GnuTLS configuration file for adding extensions to certificates, for example: 

    $ vi <server-extensions.cfg>
honor_crq_extensions
ocsp_uri = "http://ocsp.example.com"
    Copy to Clipboard Toggle word wrap

  2. Use the certtool utility to create a certificate based on a CSR, for example: 

    $ certtool --generate-certificate --load-request <example-server.crq> --load-ca-privkey <ca.key> --load-ca-certificate <ca.crt> --template <server-extensions.cfg> --outfile <example-server.crt>
    Copy to Clipboard Toggle word wrap

Chapter 3. Using shared system certificates
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The shared system certificate storage enables NSS, GnuTLS, OpenSSL, and Java to share a default source for retrieving system certificate anchors and blocklist information.

3.1. The system-wide truststore
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Red Hat Enterprise Linux provides a centralized system for managing TLS certificates. By default, the truststore contains the Mozilla CA list, which includes both positive and negative trust. The system enables you to update the core Mozilla CA list.

The consolidated system-wide truststore is located in the /etc/pki/ca-trust/ and /usr/share/pki/ca-trust-source/ directories. The trust settings in /usr/share/pki/ca-trust-source/ have lower priority than settings in /etc/pki/ca-trust/.

The system treats certificate files based on the subdirectory to which you install them:

  • Trust anchors belong to

    • /usr/share/pki/ca-trust-source/anchors/ or
    • /etc/pki/ca-trust/source/anchors/.

  • Distrusted certificates are stored in

    • /usr/share/pki/ca-trust-source/blocklist/ or
    • /etc/pki/ca-trust/source/blocklist/.

  • Certificates in the extended BEGIN TRUSTED file (OpenSSL trust certificate) format are located in

    • /usr/share/pki/ca-trust-source/ or
    • /etc/pki/ca-trust/source/.

To add a new certificate to the truststore, you can copy the file containing your certificate to the corresponding directory and use the update-ca-trust command to apply the changes. Alternatively, you can use the trust anchor sub-command.

Note

In a hierarchical cryptographic system, a trust anchor is an authoritative entity that other parties consider trustworthy. In the X.509 architecture, a root certificate is a trust anchor from which a chain of trust is derived. To enable chain validation, the trusting party must first have access to the trust anchor.

To acknowledge applications on your system with a new source of trust, add the corresponding certificate to the system-wide store and use the update-ca-trust command.

Prerequisites

  • The ca-certificates package is present on the system.

Procedure

  1. Add a certificate in the simple PEM or DER file formats to the list of CAs trusted on the system, copy the certificate file to the /usr/share/pki/ca-trust-source/anchors/ or /etc/pki/ca-trust/source/anchors/ directory, for example: 

    # cp <~/certificate-trust-examples/Cert-trust-test-ca.pem> /usr/share/pki/ca-trust-source/anchors/
    Copy to Clipboard Toggle word wrap

  2. Update the system-wide truststore configuration, use the update-ca-trust command: 

    # update-ca-trust extract
    Copy to Clipboard Toggle word wrap
Note

Even though the Firefox browser can use an added certificate without a prior execution of update-ca-trust, enter the update-ca-trust command after every CA change. Also note that browsers, such as Firefox and Chromium, cache files, and you might have to clear your browser’s cache or restart your browser to load the current system certificate configuration.

You can add or remove certificates from system-wide truststore by using either basic file operations with the corresponding files and by using the update-ca-trust command as described in the Adding new certificates to the system-wide truststore section or the trust command.

The trust command provides a way for managing certificates in the shared system-wide truststore. You can use its sub-commands to list, extract, add, remove, or change trust anchors.

  • To see the built-in help for the trust command, enter it without any arguments or with the --help directive. Also, all sub-commands of the trust commands provide a detailed built-in help, for example: 

    $ trust list --help
usage: trust list --filter=<what>
…
    Copy to Clipboard Toggle word wrap

  • To list all system trust anchors and certificates, use the trust list command, for example: 

    $ trust list
…
pkcs11:id=%DD%04%09%07%A2%F5%7A%7D%52%53%12%92%95%EE%38%80%25%0D%A6%59;type=cert
    type: certificate
    label: SSL.com Root Certification Authority RSA
    trust: anchor
    category: authority
…
    Copy to Clipboard Toggle word wrap

  • To store a trust anchor into the system-wide truststore, use the trust anchor sub-command and specify a path to a certificate. Replace <path.to/certificate.crt> by a path to your certificate and its file name: 

    # trust anchor <path.to/certificate.crt>
    Copy to Clipboard Toggle word wrap

  • To remove a certificate, use either a path to a certificate or the ID of a certificate: 

    # trust anchor --remove <path.to/certificate.crt>
# trust anchor --remove "pkcs11:id=<%AA%BB%CC%DD%EE>;type=cert"
    Copy to Clipboard Toggle word wrap

Chapter 4. Planning and implementing TLS
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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.

4.1. SSL and TLS protocols
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The Secure Sockets Layer (SSL) protocol was originally developed by Netscape Corporation to provide a mechanism for secure communication over the Internet. Subsequently, the protocol was adopted by the Internet Engineering Task Force (IETF) and renamed to Transport Layer Security (TLS).

The TLS protocol sits between an application protocol layer and a reliable transport layer, such as TCP/IP. It is independent of the application protocol and can thus be layered underneath many different protocols, for example: HTTP, FTP, SMTP, and so on.

Expand
Protocol versionUsage recommendation

SSL v2

Do not use. Has serious security vulnerabilities. Removed from the core crypto libraries since RHEL 7.

SSL v3

Do not use. Has serious security vulnerabilities. Removed from the core crypto libraries since RHEL 8.

TLS 1.0

Not recommended to use. Has known issues that cannot be mitigated in a way that guarantees interoperability, and does not support modern cipher suites. In RHEL 9, disabled in all cryptographic policies.

TLS 1.1

Use for interoperability purposes where needed. Does not support modern cipher suites. In RHEL 9, disabled in all cryptographic policies.

TLS 1.2

Supports the modern AEAD cipher suites. This version is enabled in all system-wide crypto policies, but optional parts of this protocol contain vulnerabilities and TLS 1.2 also allows outdated algorithms.

TLS 1.3

Recommended version. TLS 1.3 removes known problematic options, provides additional privacy by encrypting more of the negotiation handshake and can be faster thanks usage of more efficient modern cryptographic algorithms. TLS 1.3 is also enabled in all system-wide cryptographic policies.

4.2. Security considerations for TLS in RHEL 9
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In RHEL 9, TLS configuration is performed using the system-wide cryptographic policies mechanism. TLS versions below 1.2 are not supported anymore. DEFAULT, FUTURE, and LEGACY cryptographic policies allow only TLS 1.2 and 1.3. See Using system-wide cryptographic policies for more information.

The default settings provided by libraries included in RHEL 9 are secure enough for most deployments. The TLS implementations use secure algorithms where possible while not preventing connections from or to legacy clients or servers. Apply hardened settings 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.

The most straightforward way to harden your TLS configuration is switching the system-wide cryptographic policy level to FUTURE using the update-crypto-policies --set FUTURE command.

Warning

Algorithms disabled for the LEGACY cryptographic policy do not conform to Red Hat’s vision of RHEL 9 security, and their security properties are not reliable. Consider moving away from using these algorithms instead of re-enabling them. If you do decide to re-enable them, for example for interoperability with old hardware, treat them as insecure and apply extra protection measures, such as isolating their network interactions to separate network segments. Do not use them across public networks.

If you decide to not follow RHEL system-wide crypto policies or create custom cryptographic policies tailored to your setup, use the following recommendations for preferred protocols, cipher suites, and key lengths on your custom configuration:

4.2.1. Protocols
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The latest version of TLS provides the best security mechanism. TLS 1.2 is now the minimum version even when using the LEGACY cryptographic policy. Re-enabling older protocol versions is possible through either opting out of cryptographic policies or providing a custom policy, but the resulting configuration will not be supported.

Note that even though that RHEL 9 supports TLS version 1.3, not all features of this protocol are fully supported by RHEL 9 components. For example, the 0-RTT (Zero Round Trip Time) feature, which reduces connection latency, is not yet fully supported by the Apache web server.

Warning

A RHEL 9.2 and later system running in FIPS mode enforces that any TLS 1.2 connection must use the Extended Master Secret (EMS) extension (RFC 7627) as requires the FIPS 140-3 standard. Thus, legacy clients not supporting EMS or TLS 1.3 cannot connect to RHEL 9 servers running in FIPS mode, RHEL 9 clients in FIPS mode cannot connect to servers that support only TLS 1.2 without EMS. For more information, see the Red Hat Knowledgebase solution TLS Extension "Extended Master Secret" enforced with Red Hat Enterprise Linux 9.2

4.2.2. Cipher suites
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Modern, more secure cipher suites should be preferred to old, insecure ones. Always disable the use of eNULL and aNULL cipher suites, which do not offer any encryption or authentication at all. If at all possible, ciphers suites based on 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.

While not immediately insecure, cipher suites that offer less than 128 bits of security should not be considered for their short useful life. Algorithms that use 128 bits of security or more can be expected to be unbreakable for at least several years, and are thus strongly recommended. Note that while 3DES ciphers advertise the use of 168 bits, they actually offer 112 bits of security.

Always prefer cipher suites that support (perfect) forward secrecy (PFS), which ensures the confidentiality of encrypted data even in case the server key is compromised. This rules out the fast RSA key exchange, but allows for the use of ECDHE and DHE. Of the two, ECDHE is the faster and therefore the preferred choice.

You should also prefer AEAD ciphers, such as AES-GCM, over 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.

Note also that when using the ECDHE key exchange with ECDSA certificates, the transaction is even faster than a 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).

4.2.3. Public key length
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When using 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

The security of your system is only as strong as the weakest link in the chain. For example, a strong cipher alone does not guarantee good security. The keys and the certificates are just as important, as well as the hash functions and keys used by the Certification Authority (CA) to sign your keys.

4.3. Hardening TLS configuration in applications
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In RHEL, system-wide crypto policies provide a convenient way to ensure that your applications that use cryptographic libraries do not allow known insecure protocols, ciphers, or algorithms.

If you want to harden your TLS-related configuration with your customized cryptographic settings, you can use the cryptographic configuration options described in this section, and override the system-wide crypto policies just in the minimum required amount.

Regardless of the configuration you choose to use, always ensure that your server application enforces server-side cipher order, so that the cipher suite to be used is determined by the order you configure.

The Apache HTTP Server can use both OpenSSL and NSS libraries for its TLS needs. RHEL 9 provides the mod_ssl functionality through eponymous packages: 

# dnf install mod_ssl
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The mod_ssl package installs the /etc/httpd/conf.d/ssl.conf configuration file, which can be used to modify the TLS-related settings of the Apache HTTP Server.

Install the httpd-manual package to obtain complete documentation for the Apache HTTP Server, including TLS configuration. The directives available in the /etc/httpd/conf.d/ssl.conf configuration file are described in detail in the /usr/share/httpd/manual/mod/mod_ssl.html file. Examples of various settings are described in the /usr/share/httpd/manual/ssl/ssl_howto.html file.

When modifying the settings in the /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 or SSL 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.

For example, to use only the TLS 1.2 and 1.3 protocol: 

SSLProtocol             all -SSLv3 -TLSv1 -TLSv1.1
Copy to Clipboard Toggle word wrap

See the Configuring TLS encryption on an Apache HTTP Server chapter in the Deploying web servers and reverse proxies document for more information.

To enable TLS 1.3 support in Nginx, add the TLSv1.3 value to the ssl_protocols option in the server section of the /etc/nginx/nginx.conf configuration file: 

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    ....
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers
    ....
}
Copy to Clipboard Toggle word wrap

See the Adding TLS encryption to an Nginx web server chapter in the Deploying web servers and reverse proxies document for more information.

To configure your installation of the Dovecot mail server to use 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 the /usr/share/doc/dovecot/wiki/SSL.DovecotConfiguration.txt file, which is installed along with the standard installation of Dovecot.

When modifying the settings in the /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 or SSL you want to allow or disable.
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.

For example, the following line in /etc/dovecot/conf.d/10-ssl.conf allows only TLS 1.1 and later: 

ssl_protocols = !SSLv2 !SSLv3 !TLSv1
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You can enable encrypted DNS to secure DNS communication that uses DNS-over-TLS (DoT) protocol. Encrypted DNS (eDNS) encrypts all DNS traffic end-to-end, with no fallback to insecure protocols, and aligns with the principles of zero trust architecture (ZTA).

The current implementation of eDNS in RHEL uses only the DoT protocol. There are two primary methods to install RHEL with eDNS enabled. You can perform an interactive installation from local media, or you can build a custom bootable ISO to ensure eDNS is configured with an enforce policy during and after installation. Alternatively, you can convert an existing RHEL installation to use eDNS.

Important

Encrypted DNS is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.

5.1. Overview of components for eDNS in RHEL
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The following components comprise the eDNS setup in RHEL and interact in a layered fashion:

NetworkManager
NetworkManager enables eDNS and enforces the use of encrypted DNS protocols based on the configured policy. It is set to use dnsconfd as its backend DNS resolver.
dnsconfd
dnsconfd is a local DNS cache configuration daemon. It simplifies the setup of DNS caching, split DNS, and DNS over TLS (DoT).
unbound
unbound is a validating, recursive, and caching DNS resolver. In the eDNS setup, it serves as the runtime cache service for dnsconfd. unbound uses TLS for upstream DNS queries, which is essential for encrypting DNS traffic to external DoT servers. unbound also manages various caches to store DNS responses, which reduces the need for repeated external queries and improves performance.
  1. An application requests to resolve a hostname.
  2. The system reads the /etc/resolv.conf file and sends the query to the local unbound service.
  3. unbound first checks its internal caches for a valid, cached response.
  4. If the request record is not found, unbound encrypts the DNS query by using TLS and sends it to the configured upstream DoT enabled DNS server.
  5. The upstream DoT server processes the query and sends an encrypted DNS response back to unbound.
  6. unbound decrypts, validates, and caches the response.
  7. Finally, unbound sends the resolved DNS response back to the application.

Install a RHEL system with an eDNS enforce policy that ensures that all DNS queries are private and secure during and after installation. If you require a custom CA certificate bundle, you must install it by using the %certificate section in the Kickstart file.

During the installation, you must provide both the RHEL installation content and the Kickstart file from local media. You cannot download the Kickstart file from a remote HTTP server because the installer would need to use DNS to resolve the server’s hostname. If your environment allows a fallback to unencrypted DNS, you can perform a standard RHEL installation and configure eDNS afterwards.

Prerequisites

  • Commands that start with the # command prompt require administrative privileges provided by sudo or root user access. For information on how to configure sudo access, see Enabling unprivileged users to run certain commands.
  • You have the RHEL installation media available locally.
  • If you require a custom CA bundle, have your Kickstart file with a %certificate section available locally.

Procedure

  1. Optional: Create a Kickstart file with a %certificate section. Ensure the certificate is saved in a file named tls-ca-bundle.pem. 

    %certificate --dir /etc/pki/dns/extracted/pem/ --filename tls-ca-bundle.pem
-----BEGIN CERTIFICATE-----
<Base64-encoded_certificate_content>
-----END CERTIFICATE-----
%end
    Copy to Clipboard Toggle word wrap
  2. Prepare your bootable installation media, and include the Kickstart file if you need a custom CA bundle.
  3. Boot the installation media.
  4. From the boot menu window, select the required option and press the e key to edit the boot parameters.

  5. Add the eDNS kernel arguments: 

    linux ($root)/vmlinuz-6.12.0-0.el10_0.x86_64 root=/dev/mapper/rhel-root ro crashkernel=2G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet emergency ip=dhcp rd.net.dns=dns+tls://<server_ip>#<dns_server_hostname> rd.net.dns-resolve-mode=exclusive rd.net.dns-backend=dnsconfd inst.ks=hd:/dev/sdb1/mykickstart.ks
    Copy to Clipboard Toggle word wrap
  6. When you finish editing, press Ctrl+X to start the installation using the specified options.

Verification

  • Verify your eDNS configuration: 

    $ dnsconfd status
    Copy to Clipboard Toggle word wrap

    Expected output: 

    Running cache service:
unbound
Resolving mode: exclusive
Config present in service:
{
    ".": [
        "dns+tls://198.51.100.143#dot.dns.example.com"
    ]
}
State of Dnsconfd:
RUNNING
Info about servers: [
    {
        "address": "198.51.100.143",
        "port": 853,
        "name": "dot.dns.example.com",
        "routing_domains": [
            "."
        ],
        "search_domains": [],
        "interface": null,
        "protocol": "dns+tls",
        "dnssec": true,
        "networks": [],
        "firewall_zone": null
    }
]
    Copy to Clipboard Toggle word wrap

  • Verify that DNS server is responsive by using nslookup: 

    $ nslookup <domain_name>
    Copy to Clipboard Toggle word wrap

    Replace the <domain_name> with the domain that you want to query.

Troubleshooting

  • Enable detailed logging in unbound: 

    # unbound-control verbosity 5
    Copy to Clipboard Toggle word wrap

  • Review logs for the relevant service: 

    $ journalctl -xe -u <service_name>
    Copy to Clipboard Toggle word wrap

    Replace <service_name> with NetworkManager, dnsconfd, or unbound.

Create a custom bootable ISO to install RHEL with an eDNS enforce policy that ensures that all DNS queries are private and secure during and after installation. If you require a custom CA certificate bundle, you must install it by using the %certificate section in the Kickstart file. You then reference this Kickstart file in a script to build a new ISO, which includes kernel arguments to enforce a strict DoT policy. If your environment allows a fallback to unencrypted DNS, you can perform a standard RHEL installation and configure eDNS afterwards.

Prerequisites

  • Commands that start with the # command prompt require administrative privileges provided by sudo or root user access. For information on how to configure sudo access, see Enabling unprivileged users to run certain commands.
  • You have downloaded the full installation DVD ISO or minimal installation boot ISO image from the Product Downloads page.
  • You have a Kickstart file ready with a %certificate section if you need a custom CA bundle.
  • The lorax package is installed.

Procedure

  1. Optional: If you use the minimal installation boot ISO image, configure the installation source in the Kickstart file by using the rhsm command: 

    # Register the system and use CDN as the installation source
rhsm --organization=<org_id> --activation-key=<activation_key>
    Copy to Clipboard Toggle word wrap
    Note

    Alternatively, you can create an installation source and make it available over HTTP, FTP, or NFS. For more information, see Preparing network-based repositories.

  2. Optional: Create a Kickstart file with a %certificate section. Ensure the certificate is saved in a file named tls-ca-bundle.pem. 

    %certificate --dir /etc/pki/dns/extracted/pem/ --filename tls-ca-bundle.pem
-----BEGIN CERTIFICATE-----
<Base64-encoded_certificate_content>
-----END CERTIFICATE-----
%end
    Copy to Clipboard Toggle word wrap

  3. Add the Kickstart file and kernel arguments into the ISO:

    The following script example demonstrates how to create a custom bootable ISO with eDNS enabled. You must create a script file to automate this process. 

    !/bin/bash set -ex KERNELARGS="" # Enable network KERNELARGS+="ip=dhcp " # Set DoT DNS server KERNELARGS+="rd.net.dns=dns+tls://<server_ip>!/bin/bash set -ex KERNELARGS="" # Enable network KERNELARGS+="ip=dhcp " # Set DoT DNS server KERNELARGS+="rd.net.dns=dns+tls://<server_ip><dns_server_hostname> "

# Set to 'exclusive' to disable fallback to unencrypted DNS. Other values: 'backup', 'prefer'.
KERNELARGS+="rd.net.dns-resolve-mode=exclusive "

# Set the dnsconfd plugin for NetworkManager
KERNELARGS+="rd.net.dns-backend=dnsconfd "

# Remove any existing ISO to prevent conflicts with the new build
rm -f <output_iso_filename>

# Create a new bootable ISO with the Kickstart config file and kernel arguments
mkksiso --ks <kickstart_file> --cmdline "$KERNELARGS" <input_iso_filename> <output_iso_filename>
    Copy to Clipboard Toggle word wrap

  4. Run the script. 

    # sh <script_filename>
    Copy to Clipboard Toggle word wrap
  5. Install RHEL using the customized ISO file.

Verification

  • Verify your eDNS configuration: 

    $ dnsconfd status
    Copy to Clipboard Toggle word wrap

    Expected output: 

    Running cache service:
unbound
Resolving mode: exclusive
Config present in service:
{
    ".": [
        "dns+tls://198.51.100.143#dot.dns.example.com"
    ]
}
State of Dnsconfd:
RUNNING
Info about servers: [
    {
        "address": "198.51.100.143",
        "port": 853,
        "name": "dot.dns.example.com",
        "routing_domains": [
            "."
        ],
        "search_domains": [],
        "interface": null,
        "protocol": "dns+tls",
        "dnssec": true,
        "networks": [],
        "firewall_zone": null
    }
]
    Copy to Clipboard Toggle word wrap

  • Verify that DNS server is responsive by using nslookup: 

    $ nslookup <domain_name>
    Copy to Clipboard Toggle word wrap

    Replace the <domain_name> with the domain that you want to query.

Troubleshooting

  • Enable detailed logging in unbound: 

    # unbound-control verbosity 5
    Copy to Clipboard Toggle word wrap

  • Review logs for the relevant service: 

    $ journalctl -xe -u <service_name>
    Copy to Clipboard Toggle word wrap

    Replace <service_name> with NetworkManager, dnsconfd, or unbound.

You can enable encrypted DNS (eDNS) on an existing RHEL installation to handle all DNS traffic by using DNS-over-TLS.

Prerequisites

  • Have an existing RHEL installation.

  • The following packages are installed on your system:

    • dnsconfd
    • dnsconfd-dracut
    • grubby
  • If on an IBM Z system, the zipl utility is installed.

Procedure

  1. Configure NetworkManager in the /etc/NetworkManager/conf.d/global-dot.conf file: 

    [main]
dns=dnsconfd

[global-dns]
resolve-mode=exclusive

[global-dns-domain-*]
servers=dns+tls://<server_ip_1><dns_server_hostname_1>,dns+tls://<server_ip_2><dns_server_hostname_1>,dns+tls://<server_ip_2><dns_server_hostname_1>,dns+tls://<server_ip_2><dns_server_hostname_2>
    Copy to Clipboard Toggle word wrap

  2. Optional: To use a custom CA bundle for validating upstream DoT servers, copy the PEM-formatted file to the /etc/pki/dns/extracted/pem/tls-ca-bundle.pem file.

    Note

    After adding or removing certificates in /etc/pki/dns/extracted/pem, restart the dnsconfd service to apply the changes.

  3. Enable the dnsconfd service: 

    # systemctl enable --now dnsconfd
    Copy to Clipboard Toggle word wrap

  4. Reload NetworkManager: 

    # systemctl reload NetworkManager
    Copy to Clipboard Toggle word wrap

  5. Regenerate initramfs for all installed kernels to include dnsconfd and its configuration: 

    # for kernel in `rpm -q kernel --qf '%{VERSION}-%{RELEASE}.%{ARCH}\n'`; do
    dracut -f --kver="$kernel"
done
    Copy to Clipboard Toggle word wrap

  6. Set kernel arguments to the current and newly installed kernel version: 

    # grubby --args="rd.net.dns=dns+tls://<server_ip>#<dns_server_hostname> rd.net.dns-resolve-mode=exclusive rd.net.dns-backend=dnsconfd" --update-kernel=ALL
    Copy to Clipboard Toggle word wrap

    • If on IBM Z, update the boot menu: 

      # zipl
      Copy to Clipboard Toggle word wrap

Verification

  • Verify your eDNS configuration: 

    $ dnsconfd status
    Copy to Clipboard Toggle word wrap

    Expected output: 

    Running cache service:
unbound
Resolving mode: exclusive
Config present in service:
{
    ".": [
        "dns+tls://198.51.100.143#dot.dns.example.com"
    ]
}
State of Dnsconfd:
RUNNING
Info about servers: [
    {
        "address": "198.51.100.143",
        "port": 853,
        "name": "dot.dns.example.com",
        "routing_domains": [
            "."
        ],
        "search_domains": [],
        "interface": null,
        "protocol": "dns+tls",
        "dnssec": true,
        "networks": [],
        "firewall_zone": null
    }
]
    Copy to Clipboard Toggle word wrap

  • Verify that the DNS server is responsive by using nslookup: 

    $ nslookup <domain_name>
    Copy to Clipboard Toggle word wrap

    Replace the <domain_name> with the domain that you want to query.

Troubleshooting

  • Enable detailed logging in unbound: 

    # unbound-control verbosity 5
    Copy to Clipboard Toggle word wrap

  • Review logs for the relevant service: 

    $ journalctl -xe -u <service_name>
    Copy to Clipboard Toggle word wrap

    Replace <service_name> with NetworkManager, dnsconfd, or unbound.

5.5. Kernel parameters for DNS configuration
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You can use kernel arguments to enable DNS over TLS (DoT) at boot time and set DNS resolution behavior for your system.

rd.net.dns-resolve-mode

Defines how DNS servers from global configuration are used during resolution. The following modes are relevant for both kernel arguments and NetworkManager.conf global configuration:

exclusive
Uses only the DNS servers specified by kernel arguments or in NetworkManager.conf. Forbids fallback to DNS servers retrieved from connections. This mode is currently relevant only for dnsconfd plugin.
prefer
Forbids using DNS servers from connections for general queries unless the queries are subdomains of domains set by connection.
backup
Merges and uses DNS servers from both the global configuration and network connections for the same purposes.

rd.net.dns-servers

Configure the list of DNS servers to use. To define multiple DNS servers, set rd.net.dns multiple times: 

rd.net.dns=dns+tls://<server_ip_1>#<dns_server_hostname_1> rd.net.dns=dns+tls://<server_ip_2>#<dns_server_hostname_2>
Copy to Clipboard Toggle word wrap

For example: 

rd.net.dns=dns+tls://198.51.100.143#dot.dns.example.com rd.net.dns=dns+tls://203.0.113.1#dot.dns.example.net
Copy to Clipboard Toggle word wrap

rd.net.dns-backend

Specifies the backend DNS resolver. When set to dnsconfd, the system uses dnsconfd as a local DNS cache configuration daemon.

Chapter 6. Setting up an IPsec VPN
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Configure and manage a secure Virtual Private Network (VPN) by using the Libreswan implementation of the IPsec protocol suite to create encrypted tunnels for secure data transmission over the internet.

IPsec tunnels ensure the confidentiality and integrity of data in transit. Common use cases include connecting branch offices to headquarters or providing remote users with secure access to a corporate network.

RHEL provides different options to configure Libreswan:

  • Manually edit the Libreswan configuration files for granular control over advanced options.
  • Use the vpn RHEL system role to automate the process of creating Libreswan VPN configurations.
  • Use Nmstate to configure a Libreswan connection through a declarative API.

Libreswan does not use terms such as "client" and "server". Instead, IPsec refers to endpoints as "left" and "right". This design often enables you to use the same configuration on both hosts because Libreswan dynamically determines which role to adopt. As a convention, administrators typically use "left" for the local host and "right" for the remote host.

Note

Libreswan is the only supported VPN technology in RHEL.

IPsec relies on standardized protocols, such as Internet Key Exchange (IKE), to ensure that different systems can communicate effectively. However, in practice, minor differences in how vendors implement these standards can lead to compatibility problems. If you encounter such interoperability issues when connecting Libreswan to a third-party IPsec peer, contact Red Hat Support.

6.1. Libreswan as an IPsec VPN implementation
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In RHEL, you can configure a Virtual Private Network (VPN) by using the 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 IPsec protocol for a VPN is 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 Layer 2 Tunneling Protocol (L2TP) is usually called an L2TP/IPsec VPN, which requires the xl2tpd package provided by the optional repository.

Libreswan is an open-source, user-space IKE implementation. IKE v1 and v2 are implemented as a user-level daemon. The IKE protocol is also encrypted. The IPsec protocol is implemented by the Linux kernel, and Libreswan configures the kernel to add and remove VPN tunnel configurations.

The IKE protocol uses UDP port 500 and 4500. The IPsec protocol consists of two protocols:

  • Encapsulated Security Payload (ESP), which has protocol number 50.
  • Authenticated Header (AH), which has protocol number 51.

The AH protocol is not recommended for use. Users of AH are recommended to migrate to ESP with null encryption.

The IPsec protocol provides two modes of operation:

  • Tunnel Mode (the default)
  • Transport Mode

You can configure the kernel with IPsec without IKE. This is called manual keying. You can also configure manual keying using the ip xfrm commands, however, this is strongly discouraged for security reasons. Libreswan communicates with the Linux kernel using the Netlink interface. The kernel performs packet encryption and decryption.

Libreswan uses the Network Security Services (NSS) cryptographic library. NSS is certified for use with the Federal Information Processing Standard (FIPS) Publication 140-2.

Important

IKE/IPsec VPNs, implemented by Libreswan and the Linux kernel, is the only VPN technology recommended for use in RHEL. Do not use any other VPN technology without understanding the risks of doing so.

In RHEL, Libreswan follows system-wide cryptographic policies by default. This ensures that Libreswan uses secure settings for current threat models including IKEv2 as a default protocol. See Using system-wide crypto policies for more information.

Libreswan does not use the terms "source" and "destination" or "server" and "client" because IKE/IPsec are peer to peer protocols. Instead, it uses the terms "left" and "right" to refer to end points (the hosts). This also allows you to use the same configuration on both end points in most cases. However, administrators usually choose to always use "left" for the local host and "right" for the remote host.

The leftid and rightid options serve as identification of the respective hosts in the authentication process. See the ipsec.conf(5) man page for more information.

6.2. Authentication methods in Libreswan
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Libreswan supports several authentication methods, each of which fits a different scenario.

Pre-Shared key (PSK)

Pre-Shared Key (PSK) is the simplest authentication method. For security reasons, do not use PSKs shorter than 64 random characters. In FIPS mode, PSKs must comply with a minimum-strength requirement depending on the integrity algorithm used. You can set PSK by using the authby=secret connection.

Raw RSA keys

Raw RSA keys are commonly used for static host-to-host or subnet-to-subnet IPsec configurations. Each host is manually configured with the public RSA keys of all other hosts, and Libreswan sets up an IPsec tunnel between each pair of hosts. This method does not scale well for large numbers of hosts.

You can generate a raw RSA key on a host using the ipsec newhostkey command. You can list generated keys by using the ipsec showhostkey command. The leftrsasigkey= line is required for connection configurations that use CKA ID keys. Use the authby=rsasig connection option for raw RSA keys.

X.509 certificates

X.509 certificates are commonly used for large-scale deployments with hosts that connect to a common IPsec gateway. A central certificate authority (CA) signs RSA certificates for hosts or users. This central CA is responsible for relaying trust, including the revocations of individual hosts or users.

For example, you can generate X.509 certificates using the openssl command and the NSS certutil command. Because Libreswan reads user certificates from the NSS database using the certificates' nickname in the leftcert= configuration option, provide a nickname when you create a certificate.

If you use a custom CA certificate, you must import it to the Network Security Services (NSS) database. You can import any certificate in the PKCS #12 format to the Libreswan NSS database by using the ipsec import command.

Warning

Libreswan requires an Internet Key Exchange (IKE) peer ID as a subject alternative name (SAN) for every peer certificate as described in section 3.1 of RFC 4945. Disabling this check by setting the require-id-on-certificate=no connection option can make the system vulnerable to man-in-the-middle attacks.

Use the authby=rsasig connection option for authentication based on X.509 certificates using RSA with SHA-2. You can further limit it for ECDSA digital signatures using SHA-2 by setting authby= to ecdsa and RSA Probabilistic Signature Scheme (RSASSA-PSS) digital signatures based authentication with SHA-2 through authby=rsa-sha2. The default value is authby=rsasig,ecdsa.

The certificates and the authby= signature methods should match. This increases interoperability and preserves authentication in one digital signature system.

NULL authentication

NULL authentication is used to gain mesh encryption without authentication. It protects against passive attacks but not against active attacks. However, because IKEv2 allows asymmetric authentication methods, NULL authentication can also be used for internet-scale opportunistic IPsec. In this model, clients authenticate the server, but servers do not authenticate the client. This model is similar to secure websites using TLS. Use authby=null for NULL authentication.

Protection against quantum computers

In addition to the previously mentioned authentication methods, you can use the Post-quantum Pre-shared Key (PPK) method to protect against possible attacks by quantum computers. Individual clients or groups of clients can use their own PPK by specifying a PPK ID that corresponds to an out-of-band configured pre-shared key.

Using IKEv1 with pre-shared keys protects against quantum attackers. The redesign of IKEv2 does not offer this protection natively. Libreswan offers the use of a Post-quantum Pre-shared Key (PPK) to protect IKEv2 connections against quantum attacks.

To enable optional PPK support, add 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 based on dictionary words. The PPK ID and PPK data are stored in the ipsec.secrets file, for example: 

@west @east : PPKS "user1" "thestringismeanttobearandomstr"
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The PPKS option refers to static PPKs. This experimental function uses one-time-pad-based Dynamic PPKs. Upon each connection, a new part of the one-time pad is used as the PPK. When used, that part of the dynamic PPK inside the file is overwritten with zeros 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

The implementation of dynamic PPKs is provided as an unsupported Technology Preview. Use with caution.

6.3. Installing Libreswan
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Before you can set a VPN through the Libreswan IPsec/IKE implementation, you must install the corresponding packages, start the ipsec service, and allow the service in your firewall.

Prerequisites

  • The AppStream repository is enabled.

Procedure

  1. Install the libreswan packages: 

    # dnf install libreswan
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  2. If you are re-installing Libreswan, remove its old database files and create a new database: 

    # systemctl stop ipsec
# rm /var/lib/ipsec/nss/*db
# ipsec initnss
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  3. Start the ipsec service, and enable the service to be started automatically on boot: 

    # systemctl enable ipsec --now
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  4. Configure the firewall to allow 500 and 4500/UDP ports for the IKE, ESP, and AH protocols by adding the ipsec service: 

    # firewall-cmd --add-service="ipsec"
# firewall-cmd --runtime-to-permanent
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6.4. Creating a host-to-host VPN
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You can configure Libreswan to create a host-to-host IPsec VPN between two hosts referred to as left and right using authentication by raw RSA keys.

Prerequisites

  • Libreswan is installed and the ipsec service is started on each node.

Procedure

  1. Generate a raw RSA key pair on each host: 

    # ipsec newhostkey
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  2. The previous step returned the generated key’s ckaid. Use that ckaid with the following command on left, for example: 

    # ipsec showhostkey --left --ckaid 2d3ea57b61c9419dfd6cf43a1eb6cb306c0e857d
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    The output of the previous command generated the leftrsasigkey= line required for the configuration. Do the same on the second host (right): 

    # ipsec showhostkey --right --ckaid a9e1f6ce9ecd3608c24e8f701318383f41798f03
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  3. In the /etc/ipsec.d/ directory, create a new my_host-to-host.conf file. Write the RSA host keys from the output of the ipsec showhostkey commands in the previous step to the new file. For example: 

    conn mytunnel
    leftid=@west
    left=192.1.2.23
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightid=@east
    right=192.1.2.45
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
    authby=rsasig
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  4. After importing keys, restart the ipsec service: 

    # systemctl restart ipsec
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  5. Load the connection: 

    # ipsec auto --add mytunnel
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  6. Establish the tunnel: 

    # ipsec auto --up mytunnel
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  7. To automatically start the tunnel when the ipsec service is started, add the following line to the connection definition: 

    auto=start
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  8. If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.

6.5. Configuring a site-to-site VPN
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To create a site-to-site IPsec VPN, by joining two networks, an IPsec tunnel between the two hosts is created. The hosts thus act as the end points, which are configured to permit traffic from one or more subnets to pass through. Therefore you can think of the host 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.

Prerequisites

Procedure

  1. Copy the file with the configuration of your host-to-host VPN to a new file, for example: 

    # cp /etc/ipsec.d/my_host-to-host.conf /etc/ipsec.d/my_site-to-site.conf
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  2. Add the subnet configuration to the file created in the previous step, for example: 

    conn mysubnet
     also=mytunnel
     leftsubnet=192.0.1.0/24
     rightsubnet=192.0.2.0/24
     auto=start

conn mysubnet6
     also=mytunnel
     leftsubnet=2001:db8:0:1::/64
     rightsubnet=2001:db8:0:2::/64
     auto=start

# the following part of the configuration file is the same for both host-to-host and site-to-site connections:

conn mytunnel
    leftid=@west
    left=192.1.2.23
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightid=@east
    right=192.1.2.45
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
    authby=rsasig
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  3. If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.

6.6. Configuring a remote access VPN
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Road warriors are traveling users with mobile clients and a dynamically assigned IP address. The mobile clients authenticate using X.509 certificates.

The following example shows configuration for IKEv2, and it avoids using the IKEv1 XAUTH protocol.

On the server: 

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=gw.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 or deny client
    # pam-authorize=yes
    # load connection, do not initiate
    auto=add
    # kill vanished roadwarriors
    dpddelay=1m
    dpdtimeout=5m
    dpdaction=clear
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On the mobile client, the road warrior’s device, use a slight variation of the previous configuration: 

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
    fragmentation=yes
    # 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
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Note

If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.

6.7. Configuring a mesh VPN
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A mesh VPN network, which is also known as an any-to-any VPN, is a network where all nodes communicate using IPsec. The configuration allows for exceptions for nodes that cannot use IPsec. The mesh VPN network can be configured in two ways:

  • To require IPsec.
  • To prefer IPsec but allow a fallback to clear-text communication.

Authentication between the nodes can be based on X.509 certificates or on DNS Security Extensions (DNSSEC).

You can use any regular IKEv2 authentication method for opportunistic IPsec, because these connections are regular Libreswan configurations, except for the opportunistic IPsec that is defined by right=%opportunisticgroup entry. A common authentication method is for hosts to authenticate each other based on X.509 certificates using a commonly shared certification authority (CA). Cloud deployments typically issue certificates for each node in the cloud as part of the standard procedure.

Important

Do not use PreSharedKey (PSK) authentication because one compromised host would result in the group PSK secret being compromised as well.

You can use NULL authentication to deploy encryption between nodes without authentication, which protects only against passive attackers.

The following procedure uses X.509 certificates. You can generate these certificates by using any kind of CA management system, such as the Dogtag Certificate System. Dogtag assumes that the certificates for each node are available in the PKCS #12 format (.p12 files), which contain the private key, the node certificate, and the Root CA certificate used to validate other nodes' X.509 certificates.

Each node has an identical configuration with the exception of its X.509 certificate. This allows for adding new nodes without reconfiguring any of the existing nodes in the network. The PKCS #12 files require a "friendly name", for which we use the name "node" so that the configuration files referencing the friendly name can be identical for all nodes.

Prerequisites

  • Libreswan is installed, and the ipsec service is started on each node.

  • A new NSS database is initialized.

    1. If you already have an old NSS database, remove the old database files: 

      # systemctl stop ipsec
# rm /var/lib/ipsec/nss/*db
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    2. You can initialize a new database with the following command: 

      # ipsec initnss
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Procedure

  1. On each node, import PKCS #12 files. This step requires the password used to generate the PKCS #12 files: 

    # ipsec import nodeXXX.p12
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  2. Create the following three connection definitions for the IPsec required (private), IPsec optional (private-or-clear), and No IPsec (clear) profiles: 

    # cat /etc/ipsec.d/mesh.conf
conn clear
	auto=ondemand
	type=passthrough
	authby=never
	left=%defaultroute
	right=%group

conn private
	auto=ondemand
	type=transport
	authby=rsasig
	failureshunt=drop
	negotiationshunt=drop
	ikev2=insist
	left=%defaultroute
	leftcert=nodeXXXX
	leftid=%fromcert
	rightid=%fromcert
	right=%opportunisticgroup

conn private-or-clear
	auto=ondemand
	type=transport
	authby=rsasig
	failureshunt=passthrough
	negotiationshunt=passthrough
	# left
	left=%defaultroute
	leftcert=nodeXXXX
	leftid=%fromcert
	leftrsasigkey=%cert
	# right
	rightrsasigkey=%cert
	rightid=%fromcert
	right=%opportunisticgroup
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    where:

    auto=<option>

    Specifies how Libreswan handles connection initiation. Valid options:

    • ondemand: Initiates a connection automatically when the kernel receives the first packet that matches a trap XFRM policy. Use this option for opportunistic IPsec or for connections that do not need to be active continuously.
    • add: Prepares the connection and accepts incoming connection requests. To initiate the connection from the local peer, use the ipsec auto --up command.
    • start: Automatically activates the connection when the Libreswan service starts.
    leftid=%fromcert and rightid=%fromcert
    Configures Libreswan to retrieve the identity from the distinguished name (DN) field of the certificate.
    leftcert="<server_certificate_nickname>"
    Specifies the nickname of the server’s certificate used in the NSS database.

  3. Add the IP address of the network to the corresponding category. For example, if all nodes reside in the 10.15.0.0/16 network, and all nodes must use IPsec encryption: 

    # echo "10.15.0.0/16" >> /etc/ipsec.d/policies/private
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  4. To allow certain nodes, for example, 10.15.34.0/24, to work with and without IPsec, add those nodes to the private-or-clear group: 

    # echo "10.15.34.0/24" >> /etc/ipsec.d/policies/private-or-clear
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  5. To define a host, for example, 10.15.1.2, which is not capable of IPsec into the clear group, use: 

    # echo "10.15.1.2/32" >> /etc/ipsec.d/policies/clear
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    You can create the files in the /etc/ipsec.d/policies directory from a template for each new node, or you can provision them by using Puppet or Ansible.

    Note that every node has the same list of exceptions or different traffic flow expectations. Two nodes, therefore, might not be able to communicate because one requires IPsec and the other cannot use IPsec.

  6. Restart the node to add it to the configured mesh: 

    # systemctl restart ipsec
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  7. If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.

Verification

  1. Open an IPsec tunnel by using the ping command: 

    # ping <nodeYYY>
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  2. Display the NSS database with the imported certification: 

    # certutil -L -d sql:/etc/ipsec.d

Certificate Nickname    Trust Attributes
                        SSL,S/MIME,JAR/XPI

west                    u,u,u
ca                      CT,,
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  3. See which tunnels are open on the node: 

    # ipsec trafficstatus
006 #2: "private#10.15.0.0/16"[1] ...<nodeYYY>, type=ESP, add_time=1691399301, inBytes=512, outBytes=512, maxBytes=2^63B, id='C=US, ST=NC, O=Example Organization, CN=east'
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6.8. Deploying a FIPS-compliant IPsec VPN
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You can deploy a FIPS-compliant IPsec VPN solution with Libreswan. To do so, you can identify which cryptographic algorithms are available and which are disabled for Libreswan in FIPS mode.

Prerequisites

  • The AppStream repository is enabled.

Procedure

  1. Install the libreswan packages: 

    # dnf install libreswan
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  2. If you are re-installing Libreswan, remove its old NSS database: 

    # systemctl stop ipsec

# rm /var/lib/ipsec/nss/*db
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  3. Start the ipsec service, and enable the service to be started automatically on boot: 

    # systemctl enable ipsec --now
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  4. Configure the firewall to allow 500 and 4500 UDP ports for the IKE, ESP, and AH protocols by adding the ipsec service: 

    # firewall-cmd --add-service="ipsec"
# firewall-cmd --runtime-to-permanent
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  5. Switch the system to FIPS mode: 

    # fips-mode-setup --enable
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  6. Restart your system to allow the kernel to switch to FIPS mode: 

    # reboot
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Verification

  1. Confirm Libreswan is running in FIPS mode: 

    # ipsec whack --fipsstatus
000 FIPS mode enabled
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  2. Alternatively, check entries for the ipsec unit in the systemd journal: 

    $ journalctl -u ipsec
...
Jan 22 11:26:50 localhost.localdomain pluto[3076]: FIPS Mode: YES
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  3. To see the available algorithms in FIPS mode: 

    # ipsec pluto --selftest 2>&1 | head -6
Initializing NSS using read-write database "sql:/var/lib/ipsec/nss"
FIPS Mode: YES
NSS crypto library initialized
FIPS mode enabled for pluto daemon
NSS library is running in FIPS mode
FIPS HMAC integrity support [disabled]
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  4. To query disabled algorithms in FIPS mode: 

    # ipsec pluto --selftest 2>&1 | grep disabled
Encryption algorithm CAMELLIA_CTR disabled; not FIPS compliant
Encryption algorithm CAMELLIA_CBC disabled; not FIPS compliant
Encryption algorithm NULL disabled; not FIPS compliant
Encryption algorithm CHACHA20_POLY1305 disabled; not FIPS compliant
Hash algorithm MD5 disabled; not FIPS compliant
PRF algorithm HMAC_MD5 disabled; not FIPS compliant
PRF algorithm AES_XCBC disabled; not FIPS compliant
Integrity algorithm HMAC_MD5_96 disabled; not FIPS compliant
Integrity algorithm HMAC_SHA2_256_TRUNCBUG disabled; not FIPS compliant
Integrity algorithm AES_XCBC_96 disabled; not FIPS compliant
DH algorithm MODP1536 disabled; not FIPS compliant
DH algorithm DH31 disabled; not FIPS compliant
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  5. To list all allowed algorithms and ciphers in FIPS mode: 

    # ipsec pluto --selftest 2>&1 | grep ESP | grep FIPS | sed "s/^.*FIPS//"
aes_ccm, aes_ccm_c
aes_ccm_b
aes_ccm_a
NSS(CBC)  3des
NSS(GCM)  aes_gcm, aes_gcm_c
NSS(GCM)  aes_gcm_b
NSS(GCM)  aes_gcm_a
NSS(CTR)  aesctr
NSS(CBC)  aes
aes_gmac
NSS       sha, sha1, sha1_96, hmac_sha1
NSS       sha512, sha2_512, sha2_512_256, hmac_sha2_512
NSS       sha384, sha2_384, sha2_384_192, hmac_sha2_384
NSS       sha2, sha256, sha2_256, sha2_256_128, hmac_sha2_256
aes_cmac
null
NSS(MODP) null, dh0
NSS(MODP) dh14
NSS(MODP) dh15
NSS(MODP) dh16
NSS(MODP) dh17
NSS(MODP) dh18
NSS(ECP)  ecp_256, ecp256
NSS(ECP)  ecp_384, ecp384
NSS(ECP)  ecp_521, ecp521
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By default, the IPsec service creates its Network Security Services (NSS) database with an empty password during the first start. To enhance security, you can add password protection.

Prerequisites

  • The /var/lib/ipsec/nss/ directory contains NSS database files.

Procedure

  1. Enable password protection for the NSS database for Libreswan: 

    # certutil -N -d sql:/var/lib/ipsec/nss
Enter Password or Pin for "NSS Certificate DB":
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:
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  2. Create the /etc/ipsec.d/nsspassword file that contains the password you have set in the previous step, for example: 

    # cat /etc/ipsec.d/nsspassword
NSS Certificate DB:_<password>_
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    The nsspassword file use the following syntax: 

    <token_1>:<password1>
<token_2>:<password2>
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    The default NSS software token is NSS Certificate DB. If your system is running in FIPS mode, the name of the token is NSS FIPS 140-2 Certificate DB.

  3. Depending on your scenario, either start or restart the ipsec service after you finish the nsspassword file: 

    # systemctl restart ipsec
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Verification

  1. Check that the ipsec service is running after you have added a non-empty password to its NSS database: 

    # systemctl status ipsec
● ipsec.service - Internet Key Exchange (IKE) Protocol Daemon for IPsec
   Loaded: loaded (/usr/lib/systemd/system/ipsec.service; enabled; vendor preset: disable>
   Active: active (running)...
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  2. Check that the Journal log contains entries that confirm a successful initialization: 

    # journalctl -u ipsec
...
pluto[6214]: Initializing NSS using read-write database "sql:/var/lib/ipsec/nss"
pluto[6214]: NSS Password from file "/etc/ipsec.d/nsspassword" for token "NSS Certificate DB" with length 20 passed to NSS
pluto[6214]: NSS crypto library initialized
...
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6.10. Configuring an IPsec VPN to use TCP
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Libreswan supports TCP encapsulation of IKE and IPsec packets as described in RFC 8229. With this feature, you can establish IPsec VPNs on networks that prevent traffic transmitted via UDP and Encapsulating Security Payload (ESP). You can configure VPN servers and clients to use TCP either as a fallback or as the main VPN transport protocol. Because TCP encapsulation has bigger performance costs, use TCP as the main VPN protocol only if UDP is permanently blocked in your scenario.

Prerequisites

Procedure

  1. Add the following option to the /etc/ipsec.conf file in the config setup section: 

    listen-tcp=yes
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  2. To use TCP encapsulation as a fallback option when the first attempt over UDP fails, add the following two options to the client’s connection definition: 

    enable-tcp=fallback
tcp-remoteport=4500
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    Alternatively, if you know that UDP is permanently blocked, use the following options in the client’s connection configuration: 

    enable-tcp=yes
tcp-remoteport=4500
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Offloading Encapsulating Security Payload (ESP) to the hardware accelerates IPsec connections over Ethernet. By default, Libreswan detects if hardware supports this feature and, as a result, enables ESP hardware offload. In case that the feature was disabled or explicitly enabled, you can switch back to automatic detection.

Prerequisites

  • The network card supports ESP hardware offload.
  • The network driver supports ESP hardware offload.
  • The IPsec connection is configured and works.

Procedure

  1. Edit the Libreswan configuration file in the /etc/ipsec.d/ directory of the connection that should use automatic detection of ESP hardware offload support.
  2. Ensure the nic-offload parameter is not set in the connection’s settings.

  3. If you removed nic-offload, restart the ipsec service: 

    # systemctl restart ipsec
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Verification

  1. Display the tx_ipsec and rx_ipsec counters of the Ethernet device the IPsec connection uses: 

    # ethtool -S enp1s0 | grep -E "_ipsec"
     tx_ipsec: 10
     rx_ipsec: 10
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  2. Send traffic through the IPsec tunnel. For example, ping a remote IP address: 

    # ping -c 5 remote_ip_address
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  3. Display the tx_ipsec and rx_ipsec counters of the Ethernet device again: 

    # ethtool -S enp1s0 | grep -E "_ipsec"
     tx_ipsec: 15
     rx_ipsec: 15
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    If the counter values have increased, ESP hardware offload works.

Offloading Encapsulating Security Payload (ESP) to the hardware accelerates IPsec connections. If you use a network bond for fail-over reasons, the requirements and the procedure to configure ESP hardware offload are different from those using a regular Ethernet device. For example, in this scenario, you enable the offload support on the bond, and the kernel applies the settings to the ports of the bond.

Prerequisites

  • All network cards in the bond support ESP hardware offload. Use the ethtool -k <interface_name> | grep "esp-hw-offload" command to verify whether each bond port supports this feature.
  • The bond is configured and works.
  • The bond uses the active-backup mode. The bonding driver does not support any other modes for this feature.
  • The IPsec connection is configured and works.

Procedure

  1. Enable ESP hardware offload support on the network bond: 

    # nmcli connection modify bond0 ethtool.feature-esp-hw-offload on
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    This command enables ESP hardware offload support on the bond0 connection.

  2. Reactivate the bond0 connection: 

    # nmcli connection up bond0
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  3. Edit the Libreswan configuration file in the /etc/ipsec.d/ directory of the connection that should use ESP hardware offload, and append the nic-offload=yes statement to the connection entry: 

    conn example
    ...
    nic-offload=yes
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  4. Restart the ipsec service: 

    # systemctl restart ipsec
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Verification

The verification methods depend on various aspects, such as the kernel version and driver. For example, certain drivers provide counters, but their names can vary. See the documentation of your network driver for details.

The following verification steps work for the ixgbe driver on Red Hat Enterprise Linux 9:

  1. Display the active port of the bond: 

    # grep "Currently Active Slave" /proc/net/bonding/bond0
Currently Active Slave: enp1s0
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  2. Display the tx_ipsec and rx_ipsec counters of the active port: 

    # ethtool -S enp1s0 | grep -E "_ipsec"
     tx_ipsec: 10
     rx_ipsec: 10
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  3. Send traffic through the IPsec tunnel. For example, ping a remote IP address: 

    # ping -c 5 remote_ip_address
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  4. Display the tx_ipsec and rx_ipsec counters of the active port again: 

    # ethtool -S enp1s0 | grep -E "_ipsec"
     tx_ipsec: 15
     rx_ipsec: 15
    Copy to Clipboard Toggle word wrap

    If the counter values have increased, ESP hardware offload works.

Configure IPsec VPN connections to establish encrypted tunnels over untrusted networks and ensure the integrity of data in transit. By using the RHEL system roles, you can automate the setup for use cases, such as connecting branch offices to headquarters.

Note

The vpn RHEL system role can only create VPN configurations that use pre-shared keys (PSKs) or certificates to authenticate peers to each other.

A host-to-host VPN establishes an encrypted connection between two devices, allowing applications to communicate safely over an insecure network. By using the vpn RHEL system role, you can automate the process of creating IPsec host-to-host connections.

For authentication, a pre-shared key (PSK) is a straightforward method that uses a single, shared secret known only to the two peers. This approach is simple to configure and ideal for basic setups where ease of deployment is a priority. However, you must keep the key strictly confidential. An attacker with access to the key can compromise the connection.

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: Configuring VPN
  hosts: managed-node-01.example.com, managed-node-02.example.com
  tasks:
    - name: IPsec VPN with PSK authentication
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.vpn
      vars:
        vpn_connections:
          - hosts:
              managed-node-01.example.com:
              managed-node-02.example.com:
            auth_method: psk
            auto: start
        vpn_manage_firewall: true
        vpn_manage_selinux: true
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    The settings specified in the example playbook include the following:

    hosts: <list>

    Defines a YAML dictionary with the peers between which you want to configure a VPN. If an entry is not an Ansible managed node, you must specify its fully-qualified domain name (FQDN) or IP address in the hostname parameter, for example: 

              ...
          - hosts:
              ...
              external-host.example.com:
                hostname: 192.0.2.1
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    The role configures the VPN connection on each managed node. The connections are named <peer_A>-to-<peer_B>, for example, managed-node-01.example.com-to-managed-node-02.example.com. Note that the role cannot configure Libreswan on external (unmanaged) nodes. You must manually create the configuration on these peers.

    auth_method: psk
    Enables PSK authentication between the peers. The role uses openssl on the control node to create the PSK.
    auto: <startup_method>
    Specifies the startup method of the connection. Valid values are add, ondemand, start, and ignore. For details, see the ipsec.conf(5) man page on a system with Libreswan installed. The default value of this variable is null, which means no automatic startup operation.
    vpn_manage_firewall: true
    Defines that the role opens the required ports in the firewalld service on the managed nodes.
    vpn_manage_selinux: true
    Defines that the role sets the required SELinux port type on the IPsec ports.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.vpn/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
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    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
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Verification

  • Confirm that the connections are successfully started, for example: 

    # ansible managed-node-01.example.com -m shell -a 'ipsec trafficstatus | grep "managed-node-01.example.com-to-managed-node-02.example.com"'
...
006 #3: "managed-node-01.example.com-to-managed-node-02.example.com", type=ESP, add_time=1741857153, inBytes=38622, outBytes=324626, maxBytes=2^63B, id='@managed-node-02.example.com'
    Copy to Clipboard Toggle word wrap

    Note that this command only succeeds if the VPN connection is active. If you set the auto variable in the playbook to a value other than start, you might need to manually activate the connection on the managed nodes first.

Use the vpn RHEL system role to automate the process of creating an IPsec host-to-host VPN. To enhance security by minimizing the risk of control messages being intercepted or disrupted, configure separate connections for both the data traffic and the control traffic.

A host-to-host VPN establishes a direct, secure, and encrypted connection between two devices, allowing applications to communicate safely over an insecure network, such as the internet.

For authentication, a pre-shared key (PSK) is a straightforward method that uses a single, shared secret known only to the two peers. This approach is simple to configure and ideal for basic setups where ease of deployment is a priority. However, you must keep the key strictly confidential. An attacker with access to the key can compromise the connection.

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: Configuring VPN
  hosts: managed-node-01.example.com, managed-node-02.example.com
  tasks:
    - name: IPsec VPN with PSK authentication
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.vpn
      vars:
        vpn_connections:
          - name: control_plane_vpn
            hosts:
              managed-node-01.example.com:
                hostname: 203.0.113.1  # IP address for the control plane
              managed-node-02.example.com:
                hostname: 198.51.100.2 # IP address for the control plane
            auth_method: psk
            auto: start
          - name: data_plane_vpn
            hosts:
              managed-node-01.example.com:
                hostname: 10.0.0.1   # IP address for the data plane
              managed-node-02.example.com:
                hostname: 172.16.0.2 # IP address for the data plane
            auth_method: psk
            auto: start
        vpn_manage_firewall: true
        vpn_manage_selinux: true
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    The settings specified in the example playbook include the following:

    hosts: <list>

    Defines a YAML dictionary with the hosts between which you want to configure a VPN. The connections are named <name>-<IP_address_A>-to-<IP_address_B>, for example control_plane_vpn-203.0.113.1-to-198.51.100.2.

    The role configures the VPN connection on each managed node. Note that the role cannot configure Libreswan on external (unmanaged) nodes. You must manually create the configuration on these hosts.

    auth_method: psk
    Enables PSK authentication between the hosts. The role uses openssl on the control node to create the pre-shared key.
    auto: <startup_method>
    Specifies the startup method of the connection. Valid values are add, ondemand, start, and ignore. For details, see the ipsec.conf(5) man page on a system with Libreswan installed. The default value of this variable is null, which means no automatic startup operation.
    vpn_manage_firewall: true
    Defines that the role opens the required ports in the firewalld service on the managed nodes.
    vpn_manage_selinux: true
    Defines that the role sets the required SELinux port type on the IPsec ports.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.vpn/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
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    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
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Verification

  • Confirm that the connections are successfully started, for example: 

    # ansible managed-node-01.example.com -m shell -a 'ipsec trafficstatus | grep "control_plane_vpn-203.0.113.1-to-198.51.100.2"'
...
006 #3: "control_plane_vpn-203.0.113.1-to-198.51.100.2", type=ESP, add_time=1741860073, inBytes=0, outBytes=0, maxBytes=2^63B, id='198.51.100.2'
    Copy to Clipboard Toggle word wrap

    Note that this command only succeeds if the VPN connection is active. If you set the auto variable in the playbook to a value other than start, you might need to manually activate the connection on the managed nodes first.

A site-to-site VPN establishes an encrypted tunnel between two distinct networks, seamlessly linking them across an insecure public network. By using the vpn RHEL system role, you can automate the process of creating IPsec site-to-site VPN connections.

A site-to-site VPN enables devices in a branch office to access resources at a corporate headquarters just as if they were all part of the same local network.

For authentication, a pre-shared key (PSK) is a straightforward method that uses a single, shared secret known only to the two peers. This approach is simple to configure and ideal for basic setups where ease of deployment is a priority. However, you must keep the key strictly confidential. An attacker with access to the key can compromise the connection.

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: Configuring VPN
  hosts: managed-node-01.example.com, managed-node-02.example.com
  tasks:
    - name: IPsec VPN with PSK authentication
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.vpn
      vars:
        vpn_connections:
          - hosts:
              managed-node-01.example.com:
                subnets:
                  - 192.0.2.0/24
              managed-node-02.example.com:
                subnets:
                  - 198.51.100.0/24
                  - 203.0.113.0/24
            auth_method: psk
            auto: start
        vpn_manage_firewall: true
        vpn_manage_selinux: true
    Copy to Clipboard Toggle word wrap

    The settings specified in the example playbook include the following:

    hosts: <list>

    Defines a YAML dictionary with the gateways between which you want to configure a VPN. If an entry is not an Ansible-managed node, you must specify its fully-qualified domain name (FQDN) or IP address in the hostname parameter, for example: 

              ...
          - hosts:
              ...
              external-host.example.com:
                hostname: 192.0.2.1
    Copy to Clipboard Toggle word wrap

    The role configures the VPN connection on each managed node. The connections are named <gateway_A>-to-<gateway_B>, for example, managed-node-01.example.com-to-managed-node-02.example.com. Note that the role cannot configure Libreswan on external (unmanaged) nodes. You must manually create the configuration on these peers.

    subnets: <yaml_list_of_subnets>
    Defines subnets in classless inter-domain routing (CIDR) format that are connected through the tunnel.
    auth_method: psk
    Enables PSK authentication between the peers. The role uses openssl on the control node to create the PSK.
    auto: <startup_method>
    Specifies the startup method of the connection. Valid values are add, ondemand, start, and ignore. For details, see the ipsec.conf(5) man page on a system with Libreswan installed. The default value of this variable is null, which means no automatic startup operation.
    vpn_manage_firewall: true
    Defines that the role opens the required ports in the firewalld service on the managed nodes.
    vpn_manage_selinux: true
    Defines that the role sets the required SELinux port type on the IPsec ports.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.vpn/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
    Copy to Clipboard Toggle word wrap

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
    Copy to Clipboard Toggle word wrap

Verification

  • Confirm that the connections are successfully started, for example: 

    # ansible managed-node-01.example.com -m shell -a 'ipsec trafficstatus | grep "managed-node-01.example.com-to-managed-node-02.example.com"'
...
006 #3: "managed-node-01.example.com-to-managed-node-02.example.com", type=ESP, add_time=1741857153, inBytes=38622, outBytes=324626, maxBytes=2^63B, id='@managed-node-02.example.com'
    Copy to Clipboard Toggle word wrap

    Note that this command only succeeds if the VPN connection is active. If you set the auto variable in the playbook to a value other than start, you might need to manually activate the connection on the managed nodes first.

An IPsec mesh creates a fully interconnected network where every server can communicate securely and directly with every other server. By using the vpn RHEL system role, you can automate configuring a VPN mesh with certificate-based authentication among managed nodes.

An IPsec mesh is ideal for distributed database clusters or high-availability environments that span multiple data centers or cloud providers. Establishing a direct, encrypted tunnel between each pair of servers ensures secure communication without a central bottleneck.

For authentication, using digital certificates managed by a Certificate Authority (CA) offers a highly secure and scalable solution. Each host in the mesh presents a certificate signed by a trusted CA. This method provides strong, verifiable authentication and simplifies user management. Access can be granted or revoked centrally at the CA, and Libreswan enforces this by checking each certificate against a certificate revocation list (CRL), denying access if a certificate appears on the list.

Prerequisites

  • You have prepared the control node and the managed nodes.
  • You are logged in to the control node as a user who can run playbooks on the managed nodes.
  • The account you use to connect to the managed nodes has sudo permissions on them.

  • You prepared a PKCS #12 file for each managed node:

    • Each file contains:

      • The private key of the server
      • The server certificate
      • The CA certificate
      • If required, intermediate certificates
    • The files are named <managed_node_name_as_in_the_inventory>.p12.
    • The files are stored in the same directory as the playbook.

    • The server certificate contains the following fields:

      • Extended Key Usage (EKU) is set to TLS Web Server Authentication.
      • Common Name (CN) or Subject Alternative Name (SAN) is set to the fully-qualified domain name (FQDN) of the host.
      • X509v3 CRL distribution points contains URLs to Certificate Revocation Lists (CRLs).

Procedure

  1. Edit the ~/inventory file, and append the cert_name variable: 

    managed-node-01.example.com cert_name=managed-node-01.example.com
managed-node-02.example.com cert_name=managed-node-02.example.com
managed-node-03.example.com cert_name=managed-node-03.example.com
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    Set the cert_name variable to the value of the common name (CN) field used in the certificate for each host. Typically, the CN field is set to the fully-qualified domain name (FQDN).

  2. Store your sensitive variables in an encrypted file:

    1. Create the vault: 

      $ ansible-vault create ~/vault.yml
New Vault password: <vault_password>
Confirm New Vault password: <vault_password>
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    2. After the ansible-vault create command opens an editor, enter the sensitive data in the <key>: <value> format: 

      pkcs12_pwd: <password>
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    3. Save the changes, and close the editor. Ansible encrypts the data in the vault.

  3. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    - name: Configuring VPN
  hosts: managed-node-01.example.com, managed-node-02.example.com, managed-node-03.example.com
  vars_files:
    - ~/vault.yml
  tasks:
    - name: Install LibreSwan
      ansible.builtin.package:
        name: libreswan
        state: present

    - name: Identify the path to IPsec NSS database
      ansible.builtin.set_fact:
        nss_db_dir: "{{ '/etc/ipsec.d/' if
          ansible_distribution in ['CentOS', 'RedHat']
          and ansible_distribution_major_version is version('8', '=')
          else '/var/lib/ipsec/nss/' }}"

    - name: Locate IPsec NSS database files
      ansible.builtin.find:
        paths: "{{ nss_db_dir }}"
        patterns: "*.db"
      register: db_files

    - name: Initialize IPsec NSS database if not initialized
      ansible.builtin.command:
        cmd: ipsec initnss
      when: db_files.matched == 0

    - name: Copy PKCS #12 file to the managed node
      ansible.builtin.copy:
        src: "~/{{ inventory_hostname }}.p12"
        dest: "/etc/ipsec.d/{{ inventory_hostname }}.p12"
        mode: 0600

    - name: Import PKCS #12 file in IPsec NSS database
      ansible.builtin.shell:
        cmd: 'pk12util -d {{ nss_db_dir }} -i /etc/ipsec.d/{{ inventory_hostname }}.p12 -W "{{ pkcs12_pwd }}"'

    - name: Remove PKCS #12 file
      ansible.builtin.file:
        path: "/etc/ipsec.d/{{ inventory_hostname }}.p12"
        state: absent

    - name: Opportunistic mesh IPsec VPN with certificate-based authentication
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.vpn
      vars:
        vpn_connections:
          - opportunistic: true
            auth_method: cert
            policies:
              - policy: private
                cidr: default
              - policy: private
                cidr: 192.0.2.0/24
              - policy: clear
                cidr: 192.0.2.1/32
        vpn_manage_firewall: true
        vpn_manage_selinux: true
    Copy to Clipboard Toggle word wrap

    The settings specified in the example playbook include the following:

    opportunistic: true
    Enables an opportunistic mesh among multiple hosts. The policies variable defines for which subnets and hosts traffic must or can be encrypted and which of them should continue using plain text connections.
    auth_method: cert
    Enables certificate-based authentication. This requires that you specify the nickname of each managed node’s certificate in the inventory.
    policies: <list_of_policies>

    Defines the Libreswan policies in YAML list format.

    The default policy is private-or-clear. To change it to private, the above playbook contains an according policy for the default cidr entry.

    To prevent a loss of the SSH connection during the execution of the playbook if the Ansible control node is in the same IP subnet as the managed nodes, add a clear policy for the control node’s IP address. For example, if the mesh should be configured for the 192.0.2.0/24 subnet and the control node uses the IP address 192.0.2.1, you require a clear policy for 192.0.2.1/32 as shown in the playbook.

    For details about policies, see the ipsec.conf(5) man page on a system with Libreswan installed.

    vpn_manage_firewall: true
    Defines that the role opens the required ports in the firewalld service on the managed nodes.
    vpn_manage_selinux: true
    Defines that the role sets the required SELinux port type on the IPsec ports.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.vpn/README.md file on the control node.

  4. Validate the playbook syntax: 

    $ ansible-playbook --ask-vault-pass --syntax-check ~/playbook.yml
    Copy to Clipboard Toggle word wrap

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  5. Run the playbook: 

    $ ansible-playbook --ask-vault-pass ~/playbook.yml
    Copy to Clipboard Toggle word wrap

Verification

  1. On a node in the mesh, ping another node to activate the connection: 

    [root@managed-node-01]# ping managed-node-02.example.com
    Copy to Clipboard Toggle word wrap

  2. Confirm that the connection is active: 

    [root@managed-node-01]# ipsec trafficstatus
006 #2: "private#192.0.2.0/24"[1] ...192.0.2.2, type=ESP, add_time=1741938929, inBytes=372408, outBytes=545728, maxBytes=2^63B, id='CN=managed-node-02.example.com'
    Copy to Clipboard Toggle word wrap

Overriding system-wide crypto-policies for a connection

The RHEL system-wide cryptographic policies create a special connection called %default. This connection contains the default values for the ikev2, esp, and ike options. However, you can override the default values by specifying the mentioned option in the connection configuration file.

For example, the following configuration allows connections that use IKEv1 with AES and SHA-1 or SHA-2, and IPsec (ESP) with either AES-GCM or AES-CBC: 

conn MyExample
  ...
  ikev2=never
  ike=aes-sha2,aes-sha1;modp2048
  esp=aes_gcm,aes-sha2,aes-sha1
  ...
Copy to Clipboard Toggle word wrap

Note that AES-GCM is available for IPsec (ESP) and for IKEv2, but not for IKEv1.

Disabling system-wide crypto policies for all connections

To disable system-wide crypto policies for all IPsec connections, comment out the following line in the /etc/ipsec.conf file: 

include /etc/crypto-policies/back-ends/libreswan.config
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Then add the ikev2=never option to your connection configuration file.

6.15. Troubleshooting IPsec configurations
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Diagnosing IPsec configuration failures can be challenging, because issues can be caused by mismatched settings, firewall rules, and kernel-level errors. The following information provides a systematic approach to resolving common problems with IPsec VPN connections.

6.15.1. Basic connection issues
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Problems with VPN connections often occur due to mismatched configurations between the endpoints.

To confirm that an IPsec connection is established, enter: 

# ipsec trafficstatus
006 #8: "vpn.example.com"[1] 192.0.2.1, type=ESP, add_time=1595296930, inBytes=5999, outBytes=3231, id='@vpn.example.com', lease=198.51.100.1/32
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For a successful connection, the command shows an entry with the connection’s name and details. If the output is empty, the tunnel is not established.

6.15.3. Mismatched Configurations
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VPN connections fail if the endpoints are not configured with matching Internet Key Exchange (IKE) versions, algorithms, IP address ranges, or pre-shared keys (PSK). If you identify a mismatch, you must align the settings on both endpoints to resolve the issue.

Remote Peer Not Running IKE/IPsec

If the connection was refused, an ICMP error is displayed: 

# ipsec up vpn.example.com
...
000 "vpn.example.com"[1] 192.0.2.2 #16: ERROR: asynchronous network error report on wlp2s0 (192.0.2.2:500), complainant 198.51.100.1: Connection refused [errno 111, origin ICMP type 3 code 3 (not authenticated)]
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Mismatched IKE Algorithms

The connection fails with a NO_PROPOSAL_CHOSEN notification during the initial setup: 

# ipsec up vpn.example.com
...
003 "vpn.example.com"[1] 193.110.157.148 #3: dropping unexpected IKE_SA_INIT message containing NO_PROPOSAL_CHOSEN notification; message payloads: N; missing payloads: SA,KE,Ni
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Mismatched IPsec Algorithms

The connection fails with a NO_PROPOSAL_CHOSEN error after the initial exchange: 

# ipsec up vpn.example.com
...
182 "vpn.example.com"[1] 193.110.157.148 #5: STATE_PARENT_I2: sent v2I2, expected v2R2 {auth=IKEv2 cipher=AES_GCM_16_256 integ=n/a prf=HMAC_SHA2_256 group=MODP2048}
002 "vpn.example.com"[1] 193.110.157.148 #6: IKE_AUTH response contained the error notification NO_PROPOSAL_CHOSEN
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Mismatched IP Address Ranges (IKEv2)

The remote peer responds with a TS_UNACCEPTABLE error: 

# ipsec up vpn.example.com
...
1v2 "vpn.example.com" #1: STATE_PARENT_I2: sent v2I2, expected v2R2 {auth=IKEv2 cipher=AES_GCM_16_256 integ=n/a prf=HMAC_SHA2_512 group=MODP2048}
002 "vpn.example.com" #2: IKE_AUTH response contained the error notification TS_UNACCEPTABLE
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Mismatched IP Address Ranges (IKEv1)

The connection times out during quick mode, with a message indicating the peer did not accept the proposal: 

# ipsec up vpn.example.com
...
031 "vpn.example.com" #2: STATE_QUICK_I1: 60 second timeout exceeded after 0 retransmits.  No acceptable response to our first Quick Mode message: perhaps peer likes no proposal
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Mismatched PSK (IKEv2)

The peer rejects the connection with an AUTHENTICATION_FAILED error: 

# ipsec up vpn.example.com
...
003 "vpn.example.com" #1: received Hash Payload does not match computed value
223 "vpn.example.com" #1: sending notification INVALID_HASH_INFORMATION to 192.0.2.23:500
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Mismatched PSK (IKEv1)

The hash payload does not match, making the IKE message unreadable and resulting in an INVALID_HASH_INFORMATION error: 

# ipsec up vpn.example.com
...
002 "vpn.example.com" #1: IKE SA authentication request rejected by peer: AUTHENTICATION_FAILED
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6.15.4. MTU issues
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Diagnose intermittent IPsec connection failures caused by Maximum Transmission Unit (MTU) issues. Encryption increases packet size, leading to fragmentation and lost data when packets exceed the network’s MTU, often seen with larger data transfers.

A common symptom is that small packets, for example pings, work correctly, but larger packets, such as an SSH session, freeze after the login. To fix the problem, lower the MTU for the tunnel by adding the mtu=1400 option to the configuration file.

6.15.5. NAT conflicts
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Resolve NAT conflicts that occur when an IPsec host also acts as a NAT router. Incorrect NAT application can translate source IP addresses before encryption, causing packets to be sent unencrypted over the network.

For example, if the source IP address of the packet is translated by a masquerade rule before IPsec encryption is applied, the packet’s source no longer matches the IPsec policy, and Libreswan sends it unencrypted over the network.

To solve this problem, add a firewall rule that excludes traffic between the IPsec subnets from NAT. This rule should be inserted at the beginning of the POSTROUTING chain to ensure it is processed before the general NAT rule.

Example 6.1. Solution by using the nftables framework

The following example uses nftables to set up a basic NAT environment that excludes traffic between the 192.0.2.0/24 and 198.51.100.0/24 subnets from address translation: 

# nft add table ip nat
# nft add chain ip nat postrouting { type nat hook postrouting priority 100 \; }
# nft add rule ip nat postrouting ip saddr 192.0.2.0/24 ip daddr 198.51.100.0/24 return
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6.15.6. Kernel-level IPsec issues
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Troubleshoot kernel-level IPsec issues when a VPN tunnel appears established but no traffic flows. In this case, inspect the kernel’s IPsec state to check if the tunnel policies and cryptographic keys were correctly installed.

This process involves checking two components:

  • The Security Policy Database (SPD): The rule that instructs the kernel what traffic to encrypt.
  • The Security Association Database (SAD): The keys that instruct the kernel how to encrypt that traffic.

First, check if the correct policy exists in the SPD: 

# ip xfrm policy
src 192.0.2.1/32 dst 10.0.0.0/8
	dir out priority 666 ptype main
	tmpl src 198.51.100.13 dst 203.0.113.22
		proto esp reqid 16417 mode tunnel
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The output should contain the policies matching your leftsubnet and rightsubnet parameters with both in and out directions. If you do not see a policy for your traffic, Libreswan failed to create the kernel rule, and traffic is not encrypted.

If the policy exists, check if it has a corresponding set of keys in the SAD: 

# ip xfrm state
src 203.0.113.22 dst 198.51.100.13
	proto esp spi 0xa78b3fdb reqid 16417 mode tunnel
	auth-trunc hmac(sha1) 0x3763cd3b... 96
	enc cbc(aes) 0xd9dba399...
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Warning

This command displays private cryptographic keys. Do not share this output, because attackers can use it to decrypt your VPN traffic.

If a policy exists but you see no corresponding state with the same reqid, it typically means the Internet Key Exchange (IKE) negotiation failed. The two VPN endpoints could not agree on a set of keys.

For more detailed diagnostics, use the -s option with either of the commands. This option adds traffic counters, which can help you identify if the kernel processes packets by a specific rule.

6.15.7. Kernel IPsec subsystem bugs
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Under rare conditions, a defect in the IPsec subsystem of the kernel can cause it to lose synchronization with the Internet Key Exchange (IKE) user-space daemon. This de-synchronization can result in discrepancies between negotiated security associations and the actual enforcement of IPsec policies within the kernel, potentially disrupting secure network communication. To check for kernel-level errors, display the transform (XFRM) statistics: 

# cat /proc/net/xfrm_stat
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If any of the counters in the output, such as XfrmInError, show a nonzero value, it indicates a problem with the kernel subsystem. In this case, open a support case, and attach the output of the command along with the corresponding IKE logs.

6.15.8. Displaying Libreswan logs
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Display Libreswan logs to diagnose and troubleshoot IPsec service events and issues. Access the journal for the ipsec service to gain insights into connection status and potential problems.

To display the journal, enter: 

# journalctl -xeu ipsec
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If the default logging level does not provide enough details, enable comprehensive debug logging by adding the following settings to the config setup section in the /etc/ipsec.conf file: 

plutodebug=all
logfile=/var/log/pluto.log
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Because debug logging can produce many entries, redirecting the messages to a dedicated log file can prevent the journald and systemd services from rate-limiting the messages.

If you use Red Hat Enterprise Linux with a graphical interface, you can configure a VPN connection in the GNOME control-center.

Prerequisites

  • The NetworkManager-libreswan-gnome package is installed.

Procedure

  1. Press the Super key, type Settings, and press Enter to open the control-center application.
  2. Select the Network entry on the left.
  3. Click the + icon.
  4. Select VPN.

  5. Select the Identity menu entry to see the basic configuration options:

    General

    Gateway - The name or IP address of the remote VPN gateway.

    Authentication

    Type

    • IKEv2 (Certificate)- client is authenticated by certificate. It is more secure (default).

    • IKEv1 (XAUTH) - client is authenticated by user name and password, or a pre-shared key (PSK).

      The following configuration settings are available under the Advanced section:

      Figure 6.1. Advanced options of a VPN connection

      networking vpn advanced options
      View larger image
      networking vpn advanced options
      Warning

      When configuring an IPsec-based VPN connection using the gnome-control-center application, the Advanced dialog displays the configuration, but it does not allow any changes. As a consequence, users cannot change any advanced IPsec options. Use the nm-connection-editor or nmcli tools instead to perform configuration of the advanced properties.

      Identification

    • Domain - If required, enter the Domain Name.

      Security

    • Phase1 Algorithms - corresponds to the ike Libreswan parameter - enter the algorithms to be used to authenticate and set up an encrypted channel.

    • Phase2 Algorithms - corresponds to the esp Libreswan parameter - enter the algorithms to be used for the IPsec negotiations.

      Check the Disable PFS field to turn off Perfect Forward Secrecy (PFS) to ensure compatibility with old servers that do not support PFS.

    • Phase1 Lifetime - corresponds to the ikelifetime Libreswan parameter - how long the key used to encrypt the traffic will be valid.

    • Phase2 Lifetime - corresponds to the salifetime Libreswan parameter - how long a particular instance of a connection should last before expiring.

      Note that the encryption key should be changed from time to time for security reasons.

    • Remote network - corresponds to the rightsubnet Libreswan parameter - the destination private remote network that should be reached through the VPN.

      Check the narrowing field to enable narrowing. Note that it is only effective in IKEv2 negotiation.

    • Enable fragmentation - corresponds to the fragmentation Libreswan parameter - whether or not to allow IKE fragmentation. Valid values are yes (default) or no.
    • Enable Mobike - corresponds to the mobike Libreswan parameter - whether or not to allow Mobility and Multihoming Protocol (MOBIKE, RFC 4555) to enable a connection to migrate its endpoint without needing to restart the connection from scratch. This is used on mobile devices that switch between wired, wireless, or mobile data connections. The values are no (default) or yes.

  6. Select the IPv4 menu entry:

    IPv4 Method

    • Automatic (DHCP) - Choose this option if the network you are connecting to uses a DHCP server to assign dynamic IP addresses.
    • Link-Local Only - Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be assigned as per RFC 3927 with prefix 169.254/16.
    • Manual - Choose this option if you want to assign IP addresses manually.

    • Disable - IPv4 is disabled for this connection.

      DNS

      In the DNS section, when Automatic is ON, switch it to OFF to enter the IP address of a DNS server you want to use separating the IPs by comma.

      Routes

      Note that in the Routes section, when Automatic is ON, routes from DHCP are used, but you can also add additional static routes. When OFF, only static routes are used.

    • Address - Enter the IP address of a remote network or host.
    • Netmask - The netmask or prefix length of the IP address entered above.
    • Gateway - The IP address of the gateway leading to the remote network or host entered above.

    • Metric - A network cost, a preference value to give to this route. Lower values will be preferred over higher values.

      Use this connection only for resources on its network

      Select this check box to prevent the connection from becoming the default route. Selecting this option means that only traffic specifically destined for routes learned automatically over the connection or entered here manually is routed over the connection.

  7. To configure IPv6 settings in a VPN connection, select the IPv6 menu entry:

    IPv6 Method

    • Automatic - Choose this option to use IPv6 Stateless Address AutoConfiguration (SLAAC) to create an automatic, stateless configuration based on the hardware address and Router Advertisements (RA).
    • Automatic, DHCP only - Choose this option to not use RA, but request information from DHCPv6 directly to create a stateful configuration.
    • Link-Local Only - Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be assigned as per RFC 4862 with prefix FE80::0.
    • Manual - Choose this option if you want to assign IP addresses manually.

    • Disable - IPv6 is disabled for this connection.

      Note that DNS, Routes, Use this connection only for resources on its network are common to IPv4 settings.

  8. Once you have finished editing the VPN connection, click the Add button to customize the configuration or the Apply button to save it for the existing one.
  9. Switch the profile to ON to activate the VPN connection.
  10. If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.

If you use Red Hat Enterprise Linux with a graphical interface, you can configure a VPN connection in the nm-connection-editor application.

Prerequisites

  • The NetworkManager-libreswan-gnome package is installed.

  • If you configure an Internet Key Exchange version 2 (IKEv2) connection:

    • The certificate is imported into the IPsec network security services (NSS) database.
    • The nickname of the certificate in the NSS database is known.

Procedure

  1. Open a terminal, and enter: 

    $ nm-connection-editor
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  2. Click the + button to add a new connection.
  3. Select the IPsec based VPN connection type, and click Create.

  4. On the VPN tab:

    1. Enter the host name or IP address of the VPN gateway into the Gateway field, and select an authentication type. Based on the authentication type, you must enter different additional information:

      • IKEv2 (Certifiate) authenticates the client by using a certificate, which is more secure. This setting requires the nickname of the certificate in the IPsec NSS database

      • IKEv1 (XAUTH) authenticates the user by using a user name and password (pre-shared key). This setting requires that you enter the following values:

        • User name
        • Password
        • Group name
        • Secret

    2. If the remote server specifies a local identifier for the IKE exchange, enter the exact string in the Remote ID field. In the remote server runs Libreswan, this value is set in the server’s leftid parameter.

      nm connection editor vpn tab

    3. Optional: Configure additional settings by clicking the Advanced button. You can configure the following settings:

      • Identification

        • Domain - If required, enter the domain name.

      • Security

        • Phase1 Algorithms corresponds to the ike Libreswan parameter. Enter the algorithms to be used to authenticate and set up an encrypted channel.

        • Phase2 Algorithms corresponds to the esp Libreswan parameter. Enter the algorithms to be used for the IPsec negotiations.

          Check the Disable PFS field to turn off Perfect Forward Secrecy (PFS) to ensure compatibility with old servers that do not support PFS.

        • Phase1 Lifetime corresponds to the ikelifetime Libreswan parameter. This parameter defines how long the key used to encrypt the traffic is valid.
        • Phase2 Lifetime corresponds to the salifetime Libreswan parameter. This parameter defines how long a security association is valid.

      • Connectivity

        • Remote network corresponds to the rightsubnet Libreswan parameter and defines the destination private remote network that should be reached through the VPN.

          Check the narrowing field to enable narrowing. Note that it is only effective in the IKEv2 negotiation.

        • Enable fragmentation corresponds to the fragmentation Libreswan parameter and defines whether or not to allow IKE fragmentation. Valid values are yes (default) or no.
        • Enable Mobike corresponds to the mobike Libreswan parameter. The parameter defines whether or not to allow Mobility and Multihoming Protocol (MOBIKE) (RFC 4555) to enable a connection to migrate its endpoint without needing to restart the connection from scratch. This is used on mobile devices that switch between wired, wireless or mobile data connections. The values are no (default) or yes.

  5. On the IPv4 Settings tab, select the IP assignment method and, optionally, set additional static addresses, DNS servers, search domains, and routes.

    IPsec IPv4 tab

  6. Save the connection.
  7. Close nm-connection-editor.
  8. If you use this host in a network with DHCP or Stateless Address Autoconfiguration (SLAAC), the connection can be vulnerable to being redirected. For details and mitigation steps, see Assigning a VPN connection to a dedicated routing table to prevent the connection from bypassing the tunnel.
Note

When you add a new connection by clicking the + button, NetworkManager creates a new configuration file for that connection and then opens the same dialog that is used for editing an existing connection. The difference between these dialogs is that an existing connection profile has a Details menu entry.

Configure IPsec VPN connections to establish encrypted tunnels over untrusted networks and ensure the integrity of data in transit. By using Nmstate, you can create IPsec VPN connections by using a declarative API.

You can use the nmstatectl utility to configure Libreswan IPsec VPN connections through the Nmstate API. The nmstatectl utility is a command-line tool to manage host networking through the declarative Nmstate API. Instead of running multiple imperative commands to configure an interface, you define the expected state in a YAML file. Nmstate then takes this definition and applies it to the system. A key advantage of this approach is an atomic result. Nmstate ensures that the resulting configuration precisely matches your YAML definition. If any part of the configuration fails to apply, it automatically rolls back all changes and prevents the system from entering an incorrect or broken network state.

Note

Due to the design of the NetworkManager-libreswan plugin, you can use nmstatectl only on one peer and must manually configure Libreswan on the other peer.

You can use the declarative Nmstate API to configure a host-to-host VPN between two devices to communicate safely over an insecure network. Nmstate ensures that the result matches the configuration file or rolls back the changes.

For authentication, RSA keys are more secure than pre-shared keys (PSKs) because their asymmetric encryption eliminates the risk of a shared secret. Using RSA keys also simplifies deployment by avoiding the need for a certificate authority (CA), while still providing strong peer-to-peer authentication.

Note

In general, the choice of which host is named left and right is arbitrary. However, NetworkManager always uses the term left for the local host and right for the remote host.

Prerequisites

  • The remote peer runs Libreswan IPsec and is prepared for a host-to-host connection.

    Due to the design of the NetworkManager-libreswan plugin, Nmstate cannot communicate with other peers that also use this plugin for the same connection.

Procedure

  1. If Libreswan is not yet installed, perform the following steps:

    1. Install the required packages: 

      # dnf install nmstate libreswan NetworkManager-libreswan
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    2. Restart the NetworkManager service: 

      # systemctl restart NetworkManager
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    3. Initialize the Network Security Services (NSS) database: 

      # ipsec initnss
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      The command creates the database in the /var/lib/ipsec/nss/ directory.

    4. Open the IPsec ports and protocols in the firewall: 

      # firewall-cmd --permanent --add-service="ipsec"
# firewall-cmd --reload
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  2. Create an RSA key pair: 

    # ipsec newhostkey
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    The ipsec utility stores the key pair in the NSS database.

  3. Display the Certificate Key Attribute ID (CKAID) on both the left and right peers: 

    # ipsec showhostkey --list
< 1> RSA keyid: <key_id> ckaid: <ckaid>
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    You require the CKAIDs of both peers in the next steps.

  4. Display the public keys:

    1. On the left peer, enter: 

      # ipsec showhostkey --left --ckaid <ckaid_of_left_peer>
        # rsakey AwEAAdKCx
        leftrsasigkey=0sAwEAAdKCxpc9db48cehzQiQD...
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    2. On the right peer, enter: 

      # ipsec showhostkey --right --ckaid <ckaid_of_right_peer>
        # rsakey AwEAAcNWC
        rightrsasigkey=0sAwEAAcNWCzZO+PR1j8WbO8X...
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    The commands display the public keys with the corresponding parameters that you must use in the configuration file.

  5. Create a YAML file, for example ~/ipsec-host-to-host-rsa-auth.yml, with the following content: 

    ---
interfaces:
- name: '<connection_name>'
  type: ipsec
  libreswan:
    ikev2: insist

    left: <ip_address_or_fqdn_of_left_peer>
    leftid: peer_b
    leftrsasigkey: <public_key_of_left_peer>
    leftmodecfgclient: false

    right: <ip_address_or_fqdn_of_right_peer>
    rightid: peer_a
    rightrsasigkey: <public_key_of_right_peer>
    rightsubnet: <ip_address_of_right_peer>/32
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    The settings specified in the example include the following:

    ikev2: insist
    Defines the modern IKEv2 protocol as the only allowed protocol without fallback to IKEv1. This setting is mandatory in a host-to-host configuration with Nmstate.
    left=<ip_address_or_fqdn_of_left_peer> and right=<ip_address_or_fqdn_of_right_peer>
    Defines the IP address or DNS name of the peers.
    leftid=<id> and rightid=<id>
    Defines how each peer is identified during the Internet Key Exchange (IKE) negotiation process. This can be an IP address or a literal string. Note that NetworkManager interprets all values other than IP addresses as a literal string and internally adds a leading @ sign. This requires that the Libreswan peer also uses literal strings as IDs or authentication fails.
    leftrsasigkey=<public_key> and rightrsasigkey=<public_key>
    Specifies the public key of the peers. Use the values displayed by the ipsec showhostkey command in a previous step.
    leftmodecfgclient: false
    Disables dynamic configuration on this host. This setting is mandatory in a host-to-host configuration with Nmstate.
    rightsubnet: <ip_address_of_right_peer>/32
    Defines that the host can only access this peer. This setting is mandatory in a host-to-host configuration with Nmstate.

  6. Apply the settings to the system: 

    # nmstatectl apply ~/ipsec-host-to-host-rsa-auth.yml
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Verification

  • Display the IPsec status: 

    # ipsec status
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    If the connection is successfully established, the output contains lines as follows:

    • Phase 1 of an Internet Key Exchange version 2 (IKEv2) negotiation has been successfully completed: 

      000 #1: "<connection_name>":500 STATE_V2_ESTABLISHED_IKE_SA (established IKE SA); REKEY in 27935s; REPLACE in 28610s; newest; idle;
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      The Security Association (SA) is now ready to negotiate the actual data encryption tunnels, known as child SAs or Phase 2 SAs.

    • A child SA has been established: 

      000 #2: "<connection_name>":500 STATE_V2_ESTABLISHED_CHILD_SA (established Child SA); REKEY in 27671s; REPLACE in 28610s; IKE SA #1; idle;
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      This is the actual tunnel that your data traffic flows through.

Troubleshooting

  • To display the actual configuration NetworkManager passes to Libreswan, enter: 

    # nmcli connection export <connection_name>
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    The output can help to identify deviating settings, such as IDs and keys, when you compare them with the Libreswan configuration on the remote host.

Next steps

You can use the declarative Nmstate API to configure a site-to-site VPN between two distinct networks, seamlessly linking them across an insecure network. Nmstate ensures that the result matches the configuration file or rolls back the changes.

For authenticating the gateway devices, RSA keys are more secure than pre-shared keys (PSKs) because their asymmetric encryption eliminates the risk of a shared secret. Using RSA keys also simplifies deployment by avoiding the need for a certificate authority (CA), while still providing strong peer-to-peer authentication.

Note

In general, the choice which host is named left and right is arbitrary. However, NetworkManager always uses the term left for the local host and right for the remote host.

Prerequisites

  • The remote gateway runs Libreswan IPsec and is prepared for a site-to-site connection.

    Due to the design of the NetworkManager-libreswan plugin, Nmstate cannot communicate with other peers that also use this plugin for the same connection.

Procedure

  1. If Libreswan is not yet installed, perform the following steps:

    1. Install the required packages: 

      # dnf install nmstate libreswan NetworkManager-libreswan
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    2. Restart the NetworkManager service: 

      # systemctl restart NetworkManager
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    3. Initialize the Network Security Services (NSS) database: 

      # ipsec initnss
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      The command creates the database in the /var/lib/ipsec/nss/ directory.

    4. Open the IPsec ports and protocols in the firewall: 

      # firewall-cmd --permanent --add-service="ipsec"
# firewall-cmd --reload
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  2. Create an RSA key pair: 

    # ipsec newhostkey
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    The ipsec utility stores the key pair in the NSS database.

  3. Display the Certificate Key Attribute ID (CKAID) on both the left and right peer: 

    # ipsec showhostkey --list
< 1> RSA keyid: <key_id> ckaid: <ckaid>
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    You require the CKAIDs of both peers in the following steps.

  4. Display the public keys:

    1. On the left peer, enter: 

      # ipsec showhostkey --left --ckaid <ckaid_of_left_peer>
        # rsakey AwEAAdKCx
        leftrsasigkey=0sAwEAAdKCxpc9db48cehzQiQD...
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    2. On the right peer, enter: 

      # ipsec showhostkey --right --ckaid <ckaid_of_right_peer>
        # rsakey AwEAAcNWC
        rightrsasigkey=0sAwEAAcNWCzZO+PR1j8WbO8X...
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    The commands display the public keys with the corresponding parameters that you must use in the configuration file.

  5. Create a YAML file, for example ~/ipsec-site-to-site-rsa-auth.yml, with the following content: 

    ---
interfaces:
- name: '<connection_name>'
  type: ipsec
  libreswan:
    ikev2: insist

    left: <ip_address_or_fqdn_of_left_peer>
    leftid: peer_b
    leftrsasigkey: <public_key_of_left_peer>
    leftmodecfgclient: false
    leftsubnet: 198.51.100.0/24

    right: <ip_address_or_fqdn_of_right_peer>
    rightid: peer_a
    rightrsasigkey: <public_key_of_right_peer>
    rightsubnet: 192.0.2.0/24
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    The settings specified in the example include the following:

    ikev2: insist
    Defines the modern IKEv2 protocol as the only allowed protocol without fallback to IKEv1. This setting is mandatory in a site-to-site configuration with Nmstate.
    left=<ip_address_or_fqdn_of_left_peer> and right=<ip_address_or_fqdn_of_right_peer>
    Defines the IP address or DNS name of the peers.
    leftid=<id> and rightid=<id>
    Defines how each peer is identified during the Internet Key Exchange (IKE) negotiation process. This can be an IP address or a literal string. Note that NetworkManager interprets all values other than IP addresses as a literal string and internally adds a leading @ sign. This requires that the Libreswan peer also uses literal strings as IDs or authentication fails.
    leftrsasigkey=<public_key> and rightrsasigkey=<public_key>
    Specifies the public key of the peers. Use the values displayed by the ipsec showhostkey command in a previous step.
    leftmodecfgclient: false
    Disables dynamic configuration on this host. This setting is mandatory in a site-to-site configuration with Nmstate.
    leftsubnet=<subnet> and rightsubnet=<subnet>
    Defines subnets in classless inter-domain routing (CIDR) format that are connected through the tunnel.

  6. Enable packet forwarding: 

    # echo "net.ipv4.ip_forward=1" > /etc/sysctl.d/95-IPv4-forwarding.conf
# sysctl -p /etc/sysctl.d/95-IPv4-forwarding.conf
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  7. Apply the settings to the system: 

    # nmstatectl apply ~/ipsec-site-to-site-rsa-auth.yml
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Verification

  1. Display the IPsec status: 

    # ipsec status
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    If the connection is successfully established, the output contains lines as follows:

    • Phase 1 of an Internet Key Exchange version 2 (IKEv2) negotiation has been successfully completed: 

      000 #1: "<connection_name>":500 STATE_V2_ESTABLISHED_IKE_SA (established IKE SA); REKEY in 27935s; REPLACE in 28610s; newest; idle;
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      The Security Association (SA) is now ready to negotiate the actual data encryption tunnels, known as child SAs or Phase 2 SAs.

    • A child SA has been established: 

      000 #2: "<connection_name>":500 STATE_V2_ESTABLISHED_CHILD_SA (established Child SA); REKEY in 27671s; REPLACE in 28610s; IKE SA #1; idle;
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      This is the actual tunnel that your data traffic flows through.

  2. From a client in the local subnet, ping a client in the remote subnet.

Troubleshooting

  • To display the actual configuration NetworkManager passes to Libreswan, enter: 

    # nmcli connection export <connection_name>
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    The output can help to identify deviating settings, such as IDs and keys, when you compare them with the Libreswan configuration on the remote host.

Next steps

To access resources on a remote private network, users must first configure an IPsec VPN connection. By using Nmstate, you can create the connection with an existing Libreswan IPsec gateway by using a declarative API.

Note

In general, the choice of which host is named left and right is arbitrary. However, NetworkManager always uses the term left for the local host and right for the remote host.

Prerequisites

  • The remote gateway runs Libreswan IPsec and is prepared for a host-to-site connection with certificate-based authentication.

    Due to the design of the NetworkManager-libreswan plugin, Nmstate cannot communicate with other peers that also use this plugin for the same connection.

  • The PKCS#12 file ~/file.p12 exists on the client with the following contents:

    • The private key of the user
    • The user certificate
    • The CA certificate
    • If required, intermediate certificates

    For details about creating a private key and certificate signing request (CSR), as well as about requesting a certificate from a CA, see your CA’s documentation.

  • The Extended Key Usage (EKU) in the certificate is set to TLS Web Client Authentication.

Procedure

  1. If Libreswan is not yet installed:

    1. Install the required packages: 

      # dnf install nmstate libreswan NetworkManager-libreswan
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    2. Restart the NetworkManager service: 

      # systemctl restart NetworkManager
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    3. Initialize the Network Security Services (NSS) database: 

      # ipsec initnss
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      The command creates the database in the /var/lib/ipsec/nss/ directory.

    4. Open the IPsec ports and protocols in the firewall: 

      # firewall-cmd --permanent --add-service="ipsec"
# firewall-cmd --reload
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  2. Import the PKCS #12 file into the NSS database: 

    # ipsec import ~/file.p12
Enter password for PKCS12 file: <password>
pk12util: PKCS12 IMPORT SUCCESSFUL
correcting trust bits for Example-CA
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  3. Display the nicknames of the user and CA certificates: 

    # certutil -L -d /var/lib/ipsec/nss/
Certificate Nickname     Trust Attributes
                         SSL,S/MIME,JAR/XPI

user                     u,u,u
Example-CA               CT,,
...
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    You require this information in the Nmstate YAML file.

  4. Create a YAML file, for example, ~/ipsec-host-to-site-cert-auth.yml, with the following content: 

    ---
interfaces:
- name: '<connection_name>'
  type: ipsec
  libreswan:
    ikev2: insist

    left: <ip_address_or_fqdn_of_left_peer>
    leftid: '%fromcert'
    leftcert: <user_certificate_nickname>

    right: <ip_address_or_fqdn_of_right_peer>
    rightid: '%fromcert'
    rightsubnet: 192.0.2.0/24
    Copy to Clipboard Toggle word wrap

    The settings specified in the example include the following:

    ikev2: insist
    Defines the modern IKEv2 protocol as the only allowed protocol without fallback to IKEv1. This setting is mandatory in a host-to-site configuration with Nmstate.
    left=<ip_address_or_fqdn_of_left_peer> and right=<ip_address_or_fqdn_of_right_peer>
    Defines the IP address or DNS name of the peers.
    leftid=%fromcert and rightid=%fromcert
    Configures Libreswan to retrieve the identity from the distinguished name (DN) field of the certificate.
    leftcert="<server_certificate_nickname>"
    Sets the nickname of the server’s certificate used in the NSS database.
    rightsubnet: <subnet>
    Defines the subnet in classless inter-domain routing (CIDR) format that is connected to the gateway.

  5. Apply the settings to the system: 

    # nmstatectl apply ~/ipsec-host-to-site-cert-auth.yml
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Verification

  • Establish a connection to a host in the remote network or ping it.

Troubleshooting

  • To display the actual configuration NetworkManager passes to Libreswan, enter: 

    # nmcli connection export <connection_name>
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    The output can help to identify deviating settings, such as IDs and keys, when you compare them with the Libreswan configuration on the remote host.

Next steps

To protect your VPN connection from traffic redirection attacks, assign it to a dedicated routing table. This prevents malicious network servers from bypassing the secure tunnel and compromising data integrity.

Both a DHCP server and Stateless Address Autoconfiguration (SLAAC) can add routes to a client’s routing table. For example, a malicious DHCP server can use this feature to force a host with VPN connection to redirect traffic through a physical interface instead of the VPN tunnel. This vulnerability is also known as TunnelVision and described in the CVE-2024-3661 vulnerability article.

To mitigate this vulnerability, you can assign the VPN connection to a dedicated routing table. This prevents the DHCP configuration or SLAAC from manipulating routing decisions for network packets intended for the VPN tunnel.

Follow the steps if at least one of the conditions applies to your environment:

  • At least one network interface uses DHCP or SLAAC.
  • Your network does not use mechanisms, such as DHCP snooping, that prevent a rogue DHCP server.
Important

Routing the entire traffic through the VPN prevents the host from accessing local network resources.

Prerequisites

  • You use NetworkManager 1.48.10-5 or later.

Procedure

  1. Decide which routing table you want to use. The following steps use table 75. By default, RHEL does not use the tables 1-254, and you can use any of them.

  2. Configure the VPN connection profile to place the VPN routes in a dedicated routing table: 

    # nmcli connection modify <vpn_connection_profile> ipv4.route-table 75 ipv6.route-table 75
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  3. Set a low priority value for the table you used in the previous command: 

    # nmcli connection modify <vpn_connection_profile> ipv4.routing-rules "priority 32345 from all table 75" ipv6.routing-rules "priority 32345 from all table 75"
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    The priority value can be any value between 1 and 32766. The lower the value, the higher the priority.

  4. Reconnect the VPN connection: 

    # nmcli connection down <vpn_connection_profile>
# nmcli connection up <vpn_connection_profile>
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Verification

  1. Display the IPv4 routes in table 75: 

    # ip route show table 75
...
192.0.2.0/24 via 192.0.2.254 dev vpn_device proto static metric 50
default dev vpn_device proto static scope link metric 50
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    The output confirms that both the route to the remote network and the default gateway are assigned to routing table 75 and, therefore, all traffic is routed through the tunnel. If you set ipv4.never-default true in the VPN connection profile, a default route is not created and, therefore, not visible in this output.

  2. Display the IPv6 routes in table 75: 

    # ip -6 route show table 75
...
2001:db8:1::/64 dev vpn_device proto kernel metric 50 pref medium
default dev vpn_device proto static metric 50 pref medium
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    The output confirms that both the route to the remote network and the default gateway are assigned to routing table 75 and, therefore, all traffic is routed through the tunnel. If you set ipv6.never-default true in the VPN connection profile, a default route is not created and, therefore, not visible in this output.

Chapter 7. Securing network services
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Red Hat Enterprise Linux 9 supports many different types of network servers. Their network services can expose the system security to risks of various types of attacks, such as denial of service attacks (DoS), distributed denial of service attacks (DDoS), script vulnerability attacks, and buffer overflow attacks.

To increase the system security against attacks, it is important to monitor active network services that you use. For example, when a network service is running on a machine, its daemon listens for connections on network ports, and this can reduce the security. To limit exposure to attacks over the network, all services that are unused should be turned off.

7.1. Securing the rpcbind service
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The rpcbind service is a dynamic port-assignment daemon for remote procedure calls (RPC) services such as Network Information Service (NIS) and Network File System (NFS). Because it has weak authentication mechanisms and can assign a wide range of ports for the services it controls, it is important to secure rpcbind.

You can secure rpcbind by restricting access to all networks and defining specific exceptions using firewall rules on the server.

Note
  • The rpcbind service is required on NFSv3 servers.
  • The rpcbind service is not required on NFSv4.

Prerequisites

  • The rpcbind package is installed.
  • The firewalld package is installed and the service is running.

Procedure

  1. Add firewall rules, for example:

    • Limit TCP connection and accept packages only from the 192.168.0.0/24 host via the 111 port: 

      # firewall-cmd --add-rich-rule='rule family="ipv4" port port="111" protocol="tcp" source address="192.168.0.0/24" invert="True" drop'
      Copy to Clipboard Toggle word wrap

    • Limit TCP connection and accept packages only from local host via the 111 port: 

      # firewall-cmd --add-rich-rule='rule family="ipv4" port port="111" protocol="tcp" source address="127.0.0.1" accept'
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    • Limit UDP connection and accept packages only from the 192.168.0.0/24 host via the 111 port: 

      # firewall-cmd --permanent --add-rich-rule='rule family="ipv4" port port="111" protocol="udp" source address="192.168.0.0/24" invert="True" drop'
      Copy to Clipboard Toggle word wrap

      To make the firewall settings permanent, use the --permanent option when adding firewall rules.

  2. Reload the firewall to apply the new rules: 

    # firewall-cmd --reload
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Verification

  • List the firewall rules: 

    # firewall-cmd --list-rich-rule
rule family="ipv4" port port="111" protocol="tcp" source address="192.168.0.0/24" invert="True" drop
rule family="ipv4" port port="111" protocol="tcp" source address="127.0.0.1" accept
rule family="ipv4" port port="111" protocol="udp" source address="192.168.0.0/24" invert="True" drop
    Copy to Clipboard Toggle word wrap

7.2. Securing the rpc.mountd service
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The rpc.mountd daemon implements the server side of the NFS mount protocol. The NFS mount protocol is used by NFS version 3 (RFC 1813).

You can secure the rpc.mountd service by adding firewall rules to the server. You can restrict access to all networks and define specific exceptions using firewall rules.

Prerequisites

  • The rpc.mountd package is installed.
  • The firewalld package is installed and the service is running.

Procedure

  1. Add firewall rules to the server, for example:

    • Accept mountd connections from the 192.168.0.0/24 host: 

      # firewall-cmd --add-rich-rule 'rule family="ipv4" service name="mountd" source address="192.168.0.0/24" invert="True" drop'
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    • Accept mountd connections from the local host: 

      # firewall-cmd --permanent --add-rich-rule 'rule family="ipv4" source address="127.0.0.1" service name="mountd" accept'
      Copy to Clipboard Toggle word wrap

      To make the firewall settings permanent, use the --permanent option when adding firewall rules.

  2. Reload the firewall to apply the new rules: 

    # firewall-cmd --reload
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Verification

  • List the firewall rules: 

    # firewall-cmd --list-rich-rule
rule family="ipv4" service name="mountd" source address="192.168.0.0/24" invert="True" drop
rule family="ipv4" source address="127.0.0.1" service name="mountd" accept
    Copy to Clipboard Toggle word wrap

7.3. Securing the NFS service
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You can secure Network File System version 4 (NFSv4) by authenticating and encrypting all file system operations using Kerberos. When using NFSv4 with Network Address Translation (NAT) or a firewall, you can turn off the delegations by modifying the /etc/default/nfs file. Delegation is a technique by which the server delegates the management of a file to a client.

In contrast, NFSv3 do not use Kerberos for locking and mounting files.

The NFS service sends the traffic using TCP in all versions of NFS. The service supports Kerberos user and group authentication, as part of the RPCSEC_GSS kernel module.

NFS allows remote hosts to mount file systems over a network and interact with those file systems as if they are mounted locally. You can merge the resources on centralized servers and additionally customize NFS mount options in the /etc/nfsmount.conf file when sharing the file systems.

7.3.1. Export options for securing an NFS server
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The NFS server determines a list structure of directories and hosts about which file systems to export to which hosts in the /etc/exports file.

You can use the following export options on the /etc/exports file:

ro
Exports the NFS volume as read-only.
rw
Allows read and write requests on the NFS volume. Use this option cautiously because allowing write access increases the risk of attacks. If your scenario requires mounting the directories with the rw option, make sure they are not writable for all users to reduce possible risks.
root_squash
Maps requests from uid/gid 0 to the anonymous uid/gid. This does not apply to any other uids or gids that might be equally sensitive, such as the bin user or the staff group.
no_root_squash
Turns off root squashing. By default, NFS shares change the root user to the nobody user, which is an unprivileged user account. This changes the owner of all the root created files to nobody, which prevents the uploading of programs with the setuid bit set. When using the no_root_squash option, remote root users can change any file on the shared file system and leave applications infected by trojans for other users.
secure
Restricts exports to reserved ports. By default, the server allows client communication only through reserved ports. However, it is easy for anyone to become a root user on a client on many networks, so it is rarely safe for the server to assume that communication through 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.
Warning

Extra spaces in the syntax of the /etc/exports file can lead to major changes in the configuration.

In the following example, the /tmp/nfs/ directory is shared with the bob.example.com host and has read and write permissions. 

/tmp/nfs/     bob.example.com(rw)
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The following example is the same as the previous one but shares the same directory to the bob.example.com host with read-only permissions and shares it to the world with read and write permissions due to a single space character after the hostname. 

/tmp/nfs/     bob.example.com (rw)
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You can check the shared directories on your system by entering the showmount -e <hostname> command.

Additionally, consider the following best practices when exporting an NFS server:

  • Exporting home directories is a risk because some applications store passwords in plain text or in a weakly encrypted format. You can reduce the risk by reviewing and improving the application code.
  • Some users do not set passwords on SSH keys which again leads to risks with home directories. You can reduce these risks by enforcing the use of passwords or using Kerberos.
  • Restrict the NFS exports only to required clients. Use the showmount -e command on the NFS server to review what the server is exporting. Do not export anything that is not specifically required.
  • Do not allow unnecessary users to log in to a server to reduce the risk of attacks. You can periodically check who and what can access the server.
Warning

Export an entire file system because exporting a subdirectory of a file system is not secure. An attacker might access the unexported part of a partially-exported file system.

7.3.2. Mount options for securing an NFS client
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You can pass the following options to the mount command to increase the security of NFS-based clients:

nosuid
Use the nosuid option to disable the set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program and you can use this option opposite to setuid option.
noexec
Use the noexec option to disable all executable files on the client. Use this to prevent users from accidentally executing files placed in the shared file system.
nodev
Use the nodev option to prevent the client’s processing of device files as a hardware device.
resvport
Use the resvport option to restrict communication to a reserved port and you can use a privileged source port to communicate with the server. The reserved ports are reserved for privileged users and processes such as the root user.
sec
Use the sec option on the NFS server to choose the RPCGSS security flavor for accessing files on the mount point. Valid security flavors are none, sys, krb5, krb5i, and krb5p.
Important

The MIT Kerberos libraries provided by the krb5-libs package do not support the Data Encryption Standard (DES) algorithm in new deployments. DES is deprecated and disabled by default in Kerberos libraries because of security and compatibility reasons. Use newer and more secure algorithms instead of DES, unless your environment requires DES for compatibility reasons.

7.3.3. Securing NFS with firewall
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To secure the firewall on an NFS server, keep only the required ports open. Do not use the NFS connection port numbers for any other service.

Prerequisites

  • The nfs-utils package is installed.
  • The firewalld package is installed and running.

Procedure

  • On NFSv4, the firewall must open TCP port 2049.

  • On NFSv3, open four additional ports with 2049:

    1. rpcbind service assigns the NFS ports dynamically, which might cause problems when creating firewall rules. To simplify this process, use the /etc/nfs.conf file to specify which ports to use:

      1. Set TCP and UDP port for mountd (rpc.mountd) in the [mountd] section in port=<value> format.
      2. Set TCP and UDP port for statd (rpc.statd) in the [statd] section in port=<value> format.

    2. Set the TCP and UDP port for the NFS lock manager (nlockmgr) in the /etc/nfs.conf file:

      1. Set TCP port for nlockmgr (rpc.statd) in the [lockd] section in port=value format. Alternatively, you can use the nlm_tcpport option in the /etc/modprobe.d/lockd.conf file.
      2. Set UDP port for nlockmgr (rpc.statd) in the [lockd] section in udp-port=value format. Alternatively, you can use the nlm_udpport option in the /etc/modprobe.d/lockd.conf file.

Verification

  • List the active ports and RPC programs on the NFS server: 

    $ rpcinfo -p
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7.4. Securing the FTP service
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You can use the File Transfer Protocol (FTP) to transfer files over a network. Because all FTP transactions with the server, including user authentication, are unencrypted, make sure it is configured securely.

RHEL 9 provides two FTP servers:

Red Hat Content Accelerator (tux)
A kernel-space web server with FTP capabilities.
Very Secure FTP Daemon (vsftpd)
A standalone, security-oriented implementation of the FTP service.

The following security guidelines are for setting up the vsftpd FTP service.

7.4.1. Securing the FTP greeting banner
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When a user connects to the FTP service, FTP shows a greeting banner, which by default includes version information. Attackers might use this information to identify weaknesses in the system. You can hide this information by changing the default banner.

You can define a custom banner by editing the /etc/banners/ftp.msg file to either directly include a single-line message, or to refer to a separate file, which can contain a multi-line message.

Procedure

  • To define a single line message, add the following option to the /etc/vsftpd/vsftpd.conf file: 

    ftpd_banner=Hello, all activity on ftp.example.com is logged.
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  • To define a message in a separate file:

    • Create a .msg file which contains the banner message, for example /etc/banners/ftp.msg: 

      ######### Hello, all activity on ftp.example.com is logged. #########
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      To simplify the management of multiple banners, place all banners into the /etc/banners/ directory.

    • Add the path to the banner file to the banner_file option in the /etc/vsftpd/vsftpd.conf file: 

      banner_file=/etc/banners/ftp.msg
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Verification

  • Display the modified banner: 

    $ ftp localhost
Trying ::1…
Connected to localhost (::1).
Hello, all activity on ftp.example.com is logged.
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By default, installing the vsftpd package creates the /var/ftp/ directory and a directory tree for anonymous users with read-only permissions on the directories. Because anonymous users can access the data, do not store sensitive data in these directories.

To increase the security of the system, you can configure the FTP server to allow anonymous users to upload files to a specific directory and prevent anonymous users from reading data. In the following procedure, the anonymous user must be able to upload files in the directory owned by the root user but not change it.

Procedure

  • Create a write-only directory in the /var/ftp/pub/ directory: 

    # 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
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  • Add the following lines to the /etc/vsftpd/vsftpd.conf file: 

    anon_upload_enable=YES
anonymous_enable=YES
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  • Optional: If your system has SELinux enabled and enforcing, enable SELinux boolean attributes allow_ftpd_anon_write and allow_ftpd_full_access.
Warning

Allowing anonymous users to read and write in directories might lead to the server becoming a repository for stolen software.

7.4.3. Securing user accounts for FTP
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FTP transmits usernames and passwords unencrypted over insecure networks for authentication. You can improve the security of FTP by denying system users access to the server from their user accounts.

Perform as many of the following steps as applicable for your configuration.

Procedure

  • Disable all user accounts in the vsftpd server, by adding the following line to the /etc/vsftpd/vsftpd.conf file: 

    local_enable=NO
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  • Disable FTP access for specific accounts or specific groups of accounts, such as the root user and users with sudo privileges, by adding the usernames to the /etc/pam.d/vsftpd PAM configuration file.
  • Disable user accounts, by adding the usernames to the /etc/vsftpd/ftpusers file.

7.5. Securing HTTP servers
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7.5.1. Security enhancements in httpd.conf
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You can enhance the security of the Apache HTTP server by configuring security options in the /etc/httpd/conf/httpd.conf file.

Always verify that all scripts running on the system work correctly before putting them into production.

Ensure that only the root user has write permissions to any directory containing scripts or Common Gateway Interfaces (CGI). To change the directory ownership to root with write permissions, enter the following commands: 

# chown root <directory_name>
# chmod 755 <directory_name>
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In the /etc/httpd/conf/httpd.conf file, you can configure the following options:

FollowSymLinks
This directive is enabled by default and follows symbolic links in the directory.
Indexes
This directive is enabled by default. Disable this directive to prevent visitors from browsing files on the server.
UserDir
This directive is disabled by default because it can confirm the presence of a user account on the system. To activate user directory browsing for all user directories other than /root/, use the UserDir enabled and UserDir disabled root directives. To add users to the list of disabled accounts, add a space-delimited list of users on the UserDir disabled line.
ServerTokens

This directive controls the server response header field which is sent back to clients. You can use the following parameters to customize the information:

ServerTokens Full

Provides all available information such as web server version number, server operating system details, installed Apache modules, for example: 

Apache/2.4.37 (Red Hat Enterprise Linux) MyMod/1.2
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ServerTokens Full-Release

Provides all available information with release versions, for example: 

Apache/2.4.37 (Red Hat Enterprise Linux) (Release 41.module+el8.5.0+11772+c8e0c271)
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ServerTokens Prod / ServerTokens ProductOnly

Provides the web server name, for example: 

Apache
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ServerTokens Major

Provides the web server major release version, for example: 

Apache/2
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ServerTokens Minor

Provides the web server minor release version, for example: 

Apache/2.4
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ServerTokens Min / ServerTokens Minimal

Provides the web server minimal release version, for example: 

Apache/2.4.37
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ServerTokens OS

Provides the web server release version and operating system, for example: 

Apache/2.4.37 (Red Hat Enterprise Linux)
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Use the ServerTokens Prod option to reduce the risk of attackers gaining any valuable information about your system.

Important

Do not remove the IncludesNoExec directive. By default, the Server Side Includes (SSI) module cannot execute commands. Changing this can allow an attacker to enter commands on the system.

Removing httpd modules

You can remove the httpd modules to limit the functionality of the HTTP server. To do so, edit configuration files in the /etc/httpd/conf.modules.d/ or /etc/httpd/conf.d/ directory. For example, to remove the proxy module: 

echo '# All proxy modules disabled' > /etc/httpd/conf.modules.d/00-proxy.conf
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7.5.2. Securing the Nginx server configuration
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Nginx is a high-performance HTTP and proxy server. You can harden your Nginx configuration with the following configuration options.

Procedure

  • To disable version strings, modify the server_tokens configuration option: 

    server_tokens off;
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    This option stops displaying additional details such as server version number. This configuration displays only the server name in all requests served by Nginx, for example: 

    $ curl -sI http://localhost | grep Server
Server: nginx
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  • Add extra security headers that mitigate certain known web application vulnerabilities in specific /etc/nginx/ conf files:

    • For example, the X-Frame-Options header option denies any page outside of your domain to frame any content served by Nginx, mitigating clickjacking attacks: 

      add_header X-Frame-Options "SAMEORIGIN";
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    • For example, the x-content-type header prevents MIME-type sniffing in certain older browsers: 

      add_header X-Content-Type-Options nosniff;
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    • For example, the X-XSS-Protection header enables Cross-Site Scripting (XSS) filtering, which prevents browsers from rendering potentially malicious content included in a response by Nginx: 

      add_header X-XSS-Protection "1; mode=block";
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  • You can limit the services exposed to the public and limit what they do and accept from the visitors, for example: 

    limit_except GET {
    allow 192.168.1.0/32;
    deny  all;
}
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    The snippet will limit access to all methods except GET and HEAD.

  • You can disable HTTP methods, for example: 

    # Allow GET, PUT, POST; return "405 Method Not Allowed" for all others.
if ( $request_method !~ ^(GET|PUT|POST)$ ) {
    return 405;
}
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  • You can configure SSL to protect the data served by your Nginx web server, consider serving it over HTTPS only. Furthermore, you can generate a secure configuration profile for enabling SSL in your Nginx server using the Mozilla SSL Configuration Generator. The generated configuration ensures that known vulnerable protocols (for example, SSLv2 and SSLv3), ciphers, and hashing algorithms (for example, 3DES and MD5) are disabled. You can also use the SSL Server Test to verify that your configuration meets modern security requirements.

PostgreSQL is an object-relational database management system (DBMS). In Red Hat Enterprise Linux, PostgreSQL is provided by the postgresql-server package.

You can reduce the risks of attacks by configuring client authentication. The pg_hba.conf configuration file stored in the database cluster’s data directory controls the client authentication. Follow the procedure to configure PostgreSQL for host-based authentication.

Procedure

  1. Install PostgreSQL: 

    # yum install postgresql-server
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  2. Initialize a database storage area using one of the following options:

    1. Using the initdb utility: 

      $ initdb -D /home/postgresql/db1/
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      The initdb command with the -D option creates the directory you specify if it does not already exist, for example /home/postgresql/db1/. This directory then contains all the data stored in the database and also the client authentication configuration file.

    2. Using the postgresql-setup script: 

      $ postgresql-setup --initdb
      Copy to Clipboard Toggle word wrap

      By default, the script uses the /var/lib/pgsql/data/ directory. This script helps system administrators with basic database cluster administration.

  3. To allow any authenticated local users to access any database with their usernames, modify the following line in the pg_hba.conf file: 

    local   all             all                                     trust
    Copy to Clipboard Toggle word wrap

    This can be problematic when you use layered applications that create database users and no local users. If you do not want to explicitly control all user names on the system, remove the local line entry from the pg_hba.conf file.

  4. Restart the database to apply the changes: 

    # systemctl restart postgresql
    Copy to Clipboard Toggle word wrap

    The previous command updates the database and also verifies the syntax of the configuration file.

7.7. Securing the Memcached service
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Memcached is an open source, high-performance, distributed memory object caching system. It can improve the performance of dynamic web applications by lowering database load.

Memcached is an in-memory key-value store for small chunks of arbitrary data, such as strings and objects, from results of database calls, API calls, or page rendering. Memcached allows assigning memory from underutilized areas to applications that require more memory.

In 2018, vulnerabilities of DDoS amplification attacks by exploiting Memcached servers exposed to the public internet were discovered. These attacks took advantage of Memcached communication using the UDP protocol for transport. The attack was effective because of the high amplification ratio where a request with the size of a few hundred bytes could generate a response of a few megabytes or even hundreds of megabytes in size.

In most situations, the memcached service does not need to be exposed to the public Internet. Such exposure may have its own security problems, allowing remote attackers to leak or modify information stored in Memcached.

7.7.1. Hardening Memcached against DDoS
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To mitigate security risks, perform as many of the following steps as applicable for your configuration.

Procedure

  • Configure a firewall in your LAN. If your Memcached server should be accessible only in your local network, do not route external traffic to ports used by the memcached service. For example, remove the default port 11211 from the list of allowed ports: 

    # firewall-cmd --remove-port=11211/udp
# firewall-cmd --runtime-to-permanent
    Copy to Clipboard Toggle word wrap

  • If you use a single Memcached server on the same machine as your application, set up memcached to listen to localhost traffic only. Modify the OPTIONS value in the /etc/sysconfig/memcached file: 

    OPTIONS="-l 127.0.0.1,::1"
    Copy to Clipboard Toggle word wrap

  • Enable Simple Authentication and Security Layer (SASL) authentication:

    1. Modify or add the /etc/sasl2/memcached.conf file: 

      sasldb_path: /path.to/memcached.sasldb
      Copy to Clipboard Toggle word wrap

    2. Add an account in the SASL database: 

      # saslpasswd2 -a memcached -c cacheuser -f /path.to/memcached.sasldb
      Copy to Clipboard Toggle word wrap

    3. Ensure that the database is accessible for the memcached user and group: 

      # chown memcached:memcached /path.to/memcached.sasldb
      Copy to Clipboard Toggle word wrap

    4. Enable SASL support in Memcached by adding the -S value to the OPTIONS parameter in the /etc/sysconfig/memcached file: 

      OPTIONS="-S"
      Copy to Clipboard Toggle word wrap

    5. Restart the Memcached server to apply the changes: 

      # systemctl restart memcached
      Copy to Clipboard Toggle word wrap
    6. Add the username and password created in the SASL database to the Memcached client configuration of your application.

  • Encrypt communication between Memcached clients and servers with TLS:

    1. Enable encrypted communication between Memcached clients and servers with TLS by adding the -Z value to the OPTIONS parameter in the /etc/sysconfig/memcached file: 

      OPTIONS="-Z"
      Copy to Clipboard Toggle word wrap
    2. Add the certificate chain file path in the PEM format using the -o ssl_chain_cert option.
    3. Add a private key file path using the -o ssl_key option.

You can use MACsec to secure the communication between two devices (point-to-point). For example, your branch office is connected over a Metro-Ethernet connection with the central office, you can configure MACsec on the two hosts that connect the offices to increase the security.

8.1. How MACsec increases security
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Media Access Control security (MACsec) is a layer-2 protocol that secures different traffic types over the Ethernet links, including:

  • Dynamic host configuration protocol (DHCP)
  • address resolution protocol (ARP)
  • IPv4 and IPv6 traffic
  • Any traffic over IP such as TCP or UDP

MACsec encrypts and authenticates all traffic in LANs, by default with the GCM-AES-128 algorithm, and uses a pre-shared key to establish the connection between the participant hosts. To change the pre-shared key, you must update the NM configuration on all network hosts that use MACsec.

A MACsec connection uses an Ethernet device, such as an Ethernet network card, VLAN, or tunnel device, as a parent. You can either set an IP configuration only on the MACsec device to communicate with other hosts only by using the encrypted connection, or you can also set an IP configuration on the parent device. In the latter case, you can use the parent device to communicate with other hosts using an unencrypted connection and the MACsec device for encrypted connections.

MACsec does not require any special hardware. For example, you can use any switch, except if you want to encrypt traffic only between a host and a switch. In this scenario, the switch must also support MACsec.

In other words, you can configure MACsec for two common scenarios:

  • Host-to-host
  • Host-to-switch and switch-to-other-hosts
Important

You can use MACsec only between hosts being in the same physical or virtual LAN.

Using the MACsec security standard for securing communication at the link layer, also known as layer 2 of the Open Systems Interconnection (OSI) model provides the following notable benefits:

  • Encryption at layer 2 eliminates the need for encrypting individual services at layer 7. This reduces the overhead associated with managing a large number of certificates for each endpoint on each host.
  • Point-to-point security between directly connected network devices such as routers and switches.
  • No changes needed for applications and higher-layer protocols.

You can use the nmcli utility to configure Ethernet interfaces to use MACsec. For example, you can create a MACsec connection between two hosts that are connected over Ethernet.

Procedure

  1. On the first host on which you configure MACsec:

    • Create the connectivity association key (CAK) and connectivity-association key name (CKN) for the pre-shared key:

      1. Create a 16-byte hexadecimal CAK: 

        # dd if=/dev/urandom count=16 bs=1 2> /dev/null | hexdump -e '1/2 "%04x"'
50b71a8ef0bd5751ea76de6d6c98c03a
        Copy to Clipboard Toggle word wrap

      2. Create a 32-byte hexadecimal CKN: 

        # dd if=/dev/urandom count=32 bs=1 2> /dev/null | hexdump -e '1/2 "%04x"'
f2b4297d39da7330910a74abc0449feb45b5c0b9fc23df1430e1898fcf1c4550
        Copy to Clipboard Toggle word wrap
  2. On both hosts you want to connect over a MACsec connection:

  3. Create the MACsec connection: 

    # nmcli connection add type macsec con-name macsec0 ifname macsec0 connection.autoconnect yes macsec.parent enp1s0 macsec.mode psk macsec.mka-cak 50b71a8ef0bd5751ea76de6d6c98c03a macsec.mka-ckn f2b4297d39da7330910a74abc0449feb45b5c0b9fc23df1430e1898fcf1c4550
    Copy to Clipboard Toggle word wrap

    Use the CAK and CKN generated in the previous step in the macsec.mka-cak and macsec.mka-ckn parameters. The values must be the same on every host in the MACsec-protected network.

  4. Configure the IP settings on the MACsec connection.

    1. Configure the IPv4 settings. For example, to set a static IPv4 address, network mask, default gateway, and DNS server to the macsec0 connection, enter: 

      # nmcli connection modify macsec0 ipv4.method manual ipv4.addresses '192.0.2.1/24' ipv4.gateway '192.0.2.254' ipv4.dns '192.0.2.253'
      Copy to Clipboard Toggle word wrap

    2. Configure the IPv6 settings. For example, to set a static IPv6 address, network mask, default gateway, and DNS server to the macsec0 connection, enter: 

      # nmcli connection modify macsec0 ipv6.method manual ipv6.addresses '2001:db8:1::1/32' ipv6.gateway '2001:db8:1::fffe' ipv6.dns '2001:db8:1::fffd'
      Copy to Clipboard Toggle word wrap

  5. Activate the connection: 

    # nmcli connection up macsec0
    Copy to Clipboard Toggle word wrap

Verification

  1. Verify that the traffic is encrypted: 

    # tcpdump -nn -i enp1s0
    Copy to Clipboard Toggle word wrap

  2. Optional: Display the unencrypted traffic: 

    # tcpdump -nn -i macsec0
    Copy to Clipboard Toggle word wrap

  3. Display MACsec statistics: 

    # ip macsec show
    Copy to Clipboard Toggle word wrap

  4. Display individual counters for each type of protection: integrity-only (encrypt off) and encryption (encrypt on) 

    # ip -s macsec show
    Copy to Clipboard Toggle word wrap

You can configure Ethernet interfaces to use MACsec through the nmstatectl utility in a declarative way. For example, in a YAML file, you describe the desired state of your network, which is supposed to have a MACsec connection between two hosts connected over Ethernet. The nmstatectl utility interprets the YAML file and deploys persistent and consistent network configuration across the hosts.

Prerequisites

  • A physical or virtual Ethernet Network Interface Controller (NIC) exists in the server configuration.
  • The nmstate package is installed.

Procedure

  1. On the first host on which you configure MACsec, create the connectivity association key (CAK) and connectivity-association key name (CKN) for the pre-shared key:

    1. Create a 16-byte hexadecimal CAK: 

      # dd if=/dev/urandom count=16 bs=1 2> /dev/null | hexdump -e '1/2 "%04x"'
50b71a8ef0bd5751ea76de6d6c98c03a
      Copy to Clipboard Toggle word wrap

    2. Create a 32-byte hexadecimal CKN: 

      # dd if=/dev/urandom count=32 bs=1 2> /dev/null | hexdump -e '1/2 "%04x"'
f2b4297d39da7330910a74abc0449feb45b5c0b9fc23df1430e1898fcf1c4550
      Copy to Clipboard Toggle word wrap

  2. On both hosts that you want to connect over a MACsec connection, complete the following steps:

    1. Create a YAML file, for example create-macsec-connection.yml, with the following settings: 

      ---
routes:
  config:
  - destination: 0.0.0.0/0
    next-hop-interface: macsec0
    next-hop-address: 192.0.2.2
    table-id: 254
  - destination: 192.0.2.2/32
    next-hop-interface: macsec0
    next-hop-address: 0.0.0.0
    table-id: 254
dns-resolver:
  config:
    search:
    - example.com
    server:
    - 192.0.2.200
    - 2001:db8:1::ffbb
interfaces:
- name: macsec0
  type: macsec
  state: up
  ipv4:
    enabled: true
    address:
    - ip: 192.0.2.1
      prefix-length: 32
  ipv6:
    enabled: true
    address:
    - ip: 2001:db8:1::1
      prefix-length: 64
  macsec:
    encrypt: true
    base-iface: enp0s1
    mka-cak: 50b71a8ef0bd5751ea76de6d6c98c03a
    mka-ckn: f2b4297d39da7330910a74abc0449feb45b5c0b9fc23df1430e1898fcf1c4550
    port: 0
    validation: strict
    send-sci: true
      Copy to Clipboard Toggle word wrap
    2. Use the CAK and CKN generated in the previous step in the mka-cak and mka-ckn parameters. The values must be the same on every host in the MACsec-protected network.

    3. Optional: In the same YAML configuration file, you can also configure the following settings:

      • A static IPv4 address - 192.0.2.1 with the /32 subnet mask
      • A static IPv6 address - 2001:db8:1::1 with the /64 subnet mask
      • An IPv4 default gateway - 192.0.2.2
      • An IPv4 DNS server - 192.0.2.200
      • An IPv6 DNS server - 2001:db8:1::ffbb
      • A DNS search domain - example.com

  3. Apply the settings to the system: 

    # nmstatectl apply create-macsec-connection.yml
    Copy to Clipboard Toggle word wrap

Verification

  1. Display the current state in YAML format: 

    # nmstatectl show macsec0
    Copy to Clipboard Toggle word wrap

  2. Verify that the traffic is encrypted: 

    # tcpdump -nn -i enp0s1
    Copy to Clipboard Toggle word wrap

  3. Optional: Display the unencrypted traffic: 

    # tcpdump -nn -i macsec0
    Copy to Clipboard Toggle word wrap

  4. Display MACsec statistics: 

    # ip macsec show
    Copy to Clipboard Toggle word wrap

  5. Display individual counters for each type of protection: integrity-only (encrypt off) and encryption (encrypt on) 

    # ip -s macsec show
    Copy to Clipboard Toggle word wrap

Chapter 9. Securing the Postfix service
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Postfix is a mail transfer agent (MTA) that uses the Simple Mail Transfer Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although MTAs can encrypt traffic between one another, they might not do so by default. You can also mitigate risks to various attacks by changing setting to more secure values.

An attacker can flood the server with traffic, or send information that triggers a crash, causing a denial of service (DoS) attack. You can configure your system to reduce the risk of such attacks by setting limits in the /etc/postfix/main.cf file. You can change the value of the existing directives or you can add new directives with custom values in the <directive> = <value> format.

Use the following list of directives for limiting a DoS attack:

smtpd_client_connection_rate_limit
Limits the maximum number of connection attempts any client can make to this service per time unit. The default value is 0, which means a client can make as many connections per time unit as Postfix can accept. By default, the directive excludes clients in trusted networks.
anvil_rate_time_unit
Defines a time unit to calculate the rate limit. The default value is 60 seconds.
smtpd_client_event_limit_exceptions
Excludes clients from the connection and rate limit commands. By default, the directive excludes clients in trusted networks.
smtpd_client_message_rate_limit
Defines the maximum number of message deliveries from client to request per time unit (regardless of whether or not Postfix actually accepts those messages).
default_process_limit
Defines the default maximum number of Postfix child processes that provide a given service. You can ignore this rule for specific services in the master.cf file. By default, the value is 100.
queue_minfree
Defines the minimum amount of free space required to receive mail in the queue file system. The directive is currently used by the Postfix SMTP server to decide if it accepts any mail at all. By default, the Postfix SMTP server rejects MAIL 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
Defines the maximum amount of memory in bytes for storing a message header. If a header is large, it discards the excess header. By default, the value is 102400 bytes.
message_size_limit
Defines the maximum size of a message including the envelope information in bytes. By default, the value is 10240000 bytes.

9.3. Configuring Postfix to use SASL
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Postfix supports Simple Authentication and Security Layer (SASL) based SMTP Authentication (AUTH). SMTP AUTH is an extension of the Simple Mail Transfer Protocol. Currently, the Postfix SMTP server supports the SASL implementations in the following ways:

Dovecot SASL
The Postfix SMTP server can communicate with the Dovecot SASL implementation using either a UNIX-domain socket or a TCP socket. Use this method if Postfix and Dovecot applications are running on separate machines.
Cyrus SASL
When enabled, SMTP clients must authenticate with the SMTP server using an authentication method supported and accepted by both the server and the client.

Prerequisites

  • The dovecot package is installed on the system

Procedure

  1. Set up Dovecot:

    1. Include the following lines in the /etc/dovecot/conf.d/10-master.conf file: 

      service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}
      Copy to Clipboard Toggle word wrap

      The previous example uses UNIX-domain sockets for communication between Postfix and Dovecot. The example also assumes default Postfix SMTP server settings, which include the mail queue located in the /var/spool/postfix/ directory, and the application running under the postfix user and group.

    2. Optional: Set up Dovecot to listen for Postfix authentication requests through TCP: 

      service auth {
  inet_listener {
      port = <port-number>
  }
}
      Copy to Clipboard Toggle word wrap

    3. Specify the method that the email client uses to authenticate with Dovecot by editing the auth_mechanisms parameter in /etc/dovecot/conf.d/10-auth.conf file: 

      auth_mechanisms = plain login
      Copy to Clipboard Toggle word wrap

      The auth_mechanisms parameter supports different plaintext and non-plaintext authentication methods.

  2. Set up Postfix by modifying the /etc/postfix/main.cf file:

    1. Enable SMTP Authentication on the Postfix SMTP server: 

      smtpd_sasl_auth_enable = yes
      Copy to Clipboard Toggle word wrap

    2. Enable the use of Dovecot SASL implementation for SMTP Authentication: 

      smtpd_sasl_type = dovecot
      Copy to Clipboard Toggle word wrap

    3. 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 chroot or not: 

      smtpd_sasl_path = private/auth
      Copy to Clipboard Toggle word wrap

      This step uses UNIX-domain sockets for communication between Postfix and Dovecot.

      To configure Postfix to look for Dovecot on a different machine in case you use TCP sockets for communication, use configuration values similar to the following: 

      smtpd_sasl_path = inet: <IP_address> : <port_number>
      Copy to Clipboard Toggle word wrap

      In the previous example, replace the ip-address with the IP address of the Dovecot machine and port-number with the port number specified in Dovecot’s /etc/dovecot/conf.d/10-master.conf file.

    4. Specify SASL mechanisms that the Postfix SMTP server makes available to clients. Note that you can specify different mechanisms for encrypted and unencrypted sessions. 

      smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_tls_security_options = noanonymous
      Copy to Clipboard Toggle word wrap

      The previous directives specify that during unencrypted sessions, no anonymous authentication is allowed and no mechanisms that transmit unencrypted user names or passwords are allowed. For encrypted sessions that use TLS, only non-anonymous authentication mechanisms are allowed.

Legal Notice
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The text of and illustrations in this document are licensed by Red Hat under a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version.
Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.
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