ssh_hardening

dev-sec.ssh_hardening

Description

This role provides secure ssh-client and ssh-server configurations. It is intended to be compliant with the DevSec SSH Baseline.

Warning: This role disables root-login on the target server! Please make sure you have another user with su or sudo permissions that can login into the server.

Requirements

• Ansible >= 2.9
• root-privileges on the target system

As this role requires root-privileges, we added become: true to all tasks. So please make sure you run the role as root or as a user with become-privileges.

Role Variables

• network_ipv6_enable
  • Default: true
  • Description: false if IPv6 is not needed. ssh_listen_to must also be set to listen to IPv6 addresses (for example [::]).
• ssh_client_config_file
  • Default: '/etc/ssh/ssh_config'
  • Description: path of the ssh client configuration file, e.g. /etc/ssh/ssh_config.d/custom.conf.
• ssh_server_config_file
  • Default: '/etc/ssh/sshd_config'
  • Description: path of the ssh server configuration file, e.g. /etc/ssh/sshd_config.d/custom.conf.
• ssh_server_ports
  • Default: ['22']
  • Description: ports on which ssh-server should listen.
• ssh_client_port
  • Default: '22'
  • Description: Specifies the port number to connect on the remote host.
• ssh_listen_to
  • Default: ['0.0.0.0']
  • Description: one or more ip addresses, to which ssh-server should listen to. Default is all IPv4 adresses, but should be configured to specific addresses for security reasons!
• ssh_host_key_files
  • Default: []
  • Description: Host keys for sshd. If empty ['/etc/ssh/ssh_host_rsa_key', '/etc/ssh/ssh_host_ecdsa_key', '/etc/ssh/ssh_host_ed25519_key'] will be used, as far as supported by the installed sshd version.
• ssh_host_rsa_key_size
  • Default: 4096
  • Description: Specifies the number of bits in the private host RSA key to create.
• ssh_host_key_algorithms
  • Default: []
  • Description: Host key algorithms that the server offers. If empty the default list will be used. Otherwise overrides the setting with specified list of algorithms. Check man sshd_config, ssh -Q HostKeyAlgorithms or other sources for supported algorithms - make sure you check the correct version!
• ssh_client_host_key_algorithms
  • Default: []
  • Description: Specifies the host key algorithms that the client wants to use in order of preference. If empty the default list will be used. Otherwise overrides the setting with specified list of algorithms. Check man ssh_config, ssh -Q HostKeyAlgorithms or other sources for supported algorithms - make sure you check the correct version!.
• ssh_client_alive_interval
  • Default: 600
  • Description: specifies an interval for sending keepalive messages.
• ssh_client_alive_count
  • Default: 3
  • Description: Defines the number of acceptable unanswered client alive messages before disconnecting clients.
• ssh_permit_tunnel
  • Default: false
  • Description: true if SSH Port Tunneling is required.
• ssh_remote_hosts
  • Default: []
  • Description: one or more hosts and their custom options for the ssh-client. Default is empty. See examples in defaults/main.yml.
• ssh_permit_root_login
  • Default: no
  • Description: Disable root-login. Set to 'without-password' or 'yes' to enable root-login - The quotes are required!
• ssh_allow_tcp_forwarding
  • Default: no
  • Description: 'no' or False to disable TCP Forwarding. Set to 'yes' or True to allow TCP Forwarding. If you are using OpenSSH >= 6.2 version, you can specify 'yes', 'no', 'all', 'local'or'remote'.
• ssh_gateway_ports
  • Default: false
  • Description: false to disable binding forwarded ports to non-loopback addresses. Set to true to force binding on wildcard address. Set to clientspecified to allow the client to specify which address to bind to.
• ssh_allow_agent_forwarding
  • Default: false
  • Description: false to disable Agent Forwarding. Set to true to allow Agent Forwarding.
• ssh_x11_forwarding
  • Default: false
  • Description: false to disable X11 Forwarding. Set to true to allow X11 Forwarding.
• ssh_pam_support
  • Default: true
  • Description: true if SSH has PAM support.
• ssh_use_pam
  • Default: true
  • Description: false to disable pam authentication.
• ssh_gssapi_support
  • Default: false
  • Description: Set to true to enable GSSAPI authentication (both client and server).
• ssh_gssapi_delegation
  • Default: false
  • Description: Set to true to enable GSSAPI credential forwarding.
• ssh_kerberos_support
  • Default: true
  • Description: true if SSH has Kerberos support.
• ssh_deny_users
  • Default: ''
  • Description: if specified, login is disallowed for user names that match one of the patterns.
• ssh_allow_users
  • Default: ''
  • Description: if specified, login is allowed only for user names that match one of the patterns.
• ssh_deny_groups
  • Default: ''
  • Description: if specified, login is disallowed for users whose primary group or supplementary group list matches one of the patterns.
• ssh_allow_groups
  • Default: ''
  • Description: if specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns.
• ssh_authorized_keys_file
  • Default: ''
  • Description: change default file that contains the public keys that can be used for user authentication.
• ssh_trusted_user_ca_keys_file
  • Default: ''
  • Description: specifies the file containing trusted certificate authorities public keys used to sign user certificates.
• ssh_trusted_user_ca_keys
  • Default: []
  • Description: set the trusted certificate authorities public keys used to sign user certificates. Only used if ssh_trusted_user_ca_keys_file is set.
• ssh_authorized_principals_file
  • Default: ''
  • Description: specifies the file containing principals that are allowed. Only used if ssh_trusted_user_ca_keys_file is set.
• ssh_authorized_principals
  • Default: []
  • Description: list of hashes containing file paths and authorized principals, see default_custom.yml for all options. Only used if ssh_authorized_principals_file is set.
• ssh_print_motd
  • Default: false
  • Description: false to disable printing of the MOTD.
• ssh_print_pam_motd
  • Default: false
  • Description: false to disable printing of the MOTD via pam (Debian and Ubuntu).
• ssh_print_last_log
  • Default: false
  • Description: false to disable display of last login information.
• sftp_enabled
  • Default: true
  • Description: false to disable sftp configuration.
• sftp_umask
  • Default: '0027'
  • Description: Specifies the umask for sftp.
• sftp_chroot
  • Default: true
  • Description: false to disable chroot for sftp.
• sftp_chroot_dir
  • Default: /home/%u
  • Description: change default sftp chroot location.
• ssh_client_roaming
  • Default: false
  • Description: enable experimental client roaming.
• sshd_moduli_file
  • Default: '/etc/ssh/moduli'
  • Description: path to the SSH moduli file.
• sshd_moduli_minimum
  • Default: 2048
  • Description: remove Diffie-Hellman parameters smaller than the defined size to mitigate logjam.
• ssh_challengeresponseauthentication
  • Default: false
  • Description: Specifies whether challenge-response authentication is allowed (e.g. via PAM).
• ssh_client_password_login
  • Default: false
  • Description: true to allow password-based authentication with the ssh client.
• ssh_server_password_login
  • Default: false
  • Description: true to allow password-based authentication to the ssh server. You probably also need to change sshd_authenticationmethods to include password if you set ssh_server_password_login: true.
• ssh_banner
  • Default: false
  • Description: true to print a banner on login.
• ssh_banner_path
  • Default: '/etc/sshd/banner.txt'
  • Description: path to the SSH banner file.
• ssh_client_hardening
  • Default: true
  • Description: false to stop harden the client.
• ssh_client_compression
  • Default: false
  • Description: Specifies whether the client requests compression.
• ssh_compression
  • Default: false
  • Description: Specifies whether server-side compression is enabled after the user has authenticated successfully.
• ssh_login_grace_time
  • Default: 30s
  • Description: specifies the time allowed for successful authentication to the SSH server.
• ssh_max_auth_retries
  • Default: 2
  • Description: Specifies the maximum number of authentication attempts permitted per connection.
• ssh_max_sessions
  • Default: 10
  • Description: Specifies the maximum number of open sessions permitted from a given connection.
• ssh_print_debian_banner
  • Default: false
  • Description: true to print debian specific banner.
• ssh_server_enabled
  • Default: true
  • Description: false to disable the opensshd server.
• ssh_server_hardening
  • Default: true
  • Description: false to stop harden the server.
• ssh_server_match_address
  • Default: ''
  • Description: Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
• ssh_server_match_group
  • Default: ''
  • Description: Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
• ssh_server_match_user
  • Default: ''
  • Description: Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
• ssh_server_match_local_port
  • Default: ''
  • Description: Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
• ssh_server_permit_environment_vars
  • Default: no
  • Description: yes to specify that ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd. With openssh version 7.8 it is possible to specify a whitelist of environment variable names in addition to global "yes" or "no" settings.
• ssh_server_accept_env_vars
  • Default: ''
  • Description: Specifies what environment variables sent by the client will be copied into the session's environment, multiple environment variables may be separated by whitespace.
• ssh_use_dns
  • Default: false
  • Description: Specifies whether sshd should look up the remote host name, and to check that the resolved host name for the remote IP address maps back to the very same IP address.
• ssh_server_revoked_keys
  • Default: []
  • Description: a list of revoked public keys that the ssh server will always reject, useful to revoke known weak or compromised keys.
• ssh_max_startups
  • Default: '10:30:60'
  • Description: Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.
• ssh_macs
  • Default: []
  • Description: Change this list to overwrite macs. Defaults found in defaults/main.yml.
• ssh_kex
  • Default: []
  • Description: Change this list to overwrite kexs. Defaults found in defaults/main.yml.
• ssh_ciphers
  • Default: []
  • Description: Change this list to overwrite ciphers. Defaults found in defaults/main.yml.
• ssh_custom_options
  • Default: []
  • Description: Custom lines for SSH client configuration.
• sshd_custom_options
  • Default: []
  • Description: Custom lines for SSH daemon configuration.
• sshd_syslog_facility
  • Default: 'AUTH'
  • Description: The facility code that is used when logging messages from sshd.
• sshd_log_level
  • Default: 'VERBOSE'
  • Description: the verbosity level that is used when logging messages from sshd.
• sshd_strict_modes
  • Default: true
  • Description: Check file modes and ownership of the user's files and home directory before accepting login.
• sshd_authenticationmethods
  • Default: publickey
  • Description: Specifies the authentication methods that must be successfully completed for a user to be granted access. Make sure to set all required variables for your selected authentication method. Defaults found in defaults/main.yml.

Example Playbook

- hosts: localhost
  collections:
    - devsec.hardening
  roles:
    - ssh_hardening

Configuring settings not listed in role-variables

If you want to configure ssh options that are not listed above, you can use ssh_custom_options (for /etc/ssh/ssh_config) or sshd_custom_options (for /etc/ssh/sshd_config) to set them. These options will be set on the beginning of the file so you can override options further down in the file.

Example playbook:

- hosts: localhost
  collections:
    - devsec.hardening
  roles:
    - ssh_hardening
  vars:
    ssh_custom_options:
      - "Include /etc/ssh/ssh_config.d/*"
    sshd_custom_options:
      - "AcceptEnv LANG"

After using the role Ansibles template/copy/file module does not work anymore

If you use this role to disable SFTP (sftp_enabled: false) you will get errors. Ansible uses by default SFTP to transfer files to the remote hosts. You have to set scp_if_ssh = True in your ansible.cfg. This way Ansible uses SCP to copy files. If your control node uses OpenSSH version 9.0 or above, you also need to set scp_extra_args = "-O", since starting with that version the scp utility also defaults to using SFTP.

Changing the default port and idempotency

This role uses the default port 22 or the port configured in the inventory to connect to the server. If the default ssh port is changed via ssh_server_ports, once the ssh server is restarted, it will still try to connect using the previous port. In order to run this role again on the same server the inventory will have to be updated to use the new ssh port.

If idempotency is important, please consider using role ssh-hardening-fallback, which is a wrapper around this role that falls back to port 22 if the configured port is unreachable.