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sshd_config

Setup OpenSSH service from a template. A rollback feature ensures the Ansible Controller will not be locked out the target host.

SUMMARY

Description

This role populates target's sshd_config from a template, and reloads ssh daemon with this new configuration. If the next task fails, meaning that the target is not reachable anymore, the daemon is restarted with its initial configuration, so the ansible controller, at least, is not locked out of its target.

When reconfiguring SSH remotely, you'll have to restart it. Common cases of lock out may appear when you want to harden the service:

  • the Port value has changed and
    • the firewall is not aware of that and the new port is blocked;
    • the new port is already in use and SSH died at reload/restart;
  • the ansible_user is missing in the new list AllowUsers;
  • a Match directive with restrictive options unfortunately applies to the ansible_user;
  • ...

Some of these cases may still apply to this role, except that the lock out is temporary (defaults to 20 seconds). As the sshd_config is not overwritten before reloading the daemon, even if the rollback fails (that is a very bad situation and shouldn't happen), rebooting the host will restore its initial ssh configuration.

The template provided by this role exhaustively supports all options of the server. See the Template Variables section below.

Requirements

Both ssh and paramiko clients must be supported by the controller. This role doesn't work in Cygwin environment.

Role Variables

This role can be set using the following variables:

  • Whether or not to make currently applied configuration persistent. If False, the sshd configuration set by the role will still apply after the role has been played, but this configuration will not be reapplied after reboot.
sshd_config__save_state: yes
  • Port to switch on to confirm applied/current/runtime configuration. May be yes (try to guess port number from new configuration), no, or a port number.
sshd_config__switch_port: yes
  • Path of a template file that once evaluated is used as configuration to be loaded by sshd.
sshd_config__template: sshd_config.j2
  • The delay, in seconds, after what the initial ssh server's configuration is reloaded if the confirmation file is missing.
sshd_config__timeout: 20

Template Variables

The template provided with this role supports all sshd_config's options as variable names with sshd__ as prefix followed by the name of the option, in lowercase. For example:

sshd__port: 2222
sshd__usepam: yes
sshd__permitrootlogin: forced-commands-only
sshd__passwordauthentication: no

will give:

Port 2222
UsePAM yes
PermitRootLogin forced-commands-only
PasswordAuthentication no

booleans

Even if ssh booleans only accept yes or no values, the template accepts all ansible's booleans and translates them to yes or no. NOTE that it is a feature of the template provided by the role, not a feature of the role itself. If you point sshd_config__template to your own, ensure to do the same, otherwise something like:

sshd__passwordauthentication: no

would end up in your sshd_config as:

PasswordAuthentication False

and fail.

This translation to yes/no values also applies to mixed-type options that accept either a boolean or a keyword, as do PermitRootLogin or PermitTunnel.

lists

Some sshd_config's options may accept a comma-separated or a space-separated list of values in a single call. For example:

AllowUsers alice bob
Ciphers [email protected],aes256-ctr,[email protected]

although other options have to be specified as many times as needed, such as:

Port 22
Port 2222
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

For first options (AllowUsers, Ciphers ...) template variables accept both a string that will be passed as is to the option, or a yaml list that will be processed to be passed to the option as a comma-separated or space-separated list, depending on the option.

For second options (Port, ListenAddress ...) template variables accept both a string that will be passed as is to the option, or a yaml list that will be processed to be passed to a list of the same option, one value per line.

For example:

  • These are correct declarations:
sshd__allowusers: alice bob
sshd__allowusers:
  - alice
  - bob
sshd__ciphers: [email protected],aes256-ctr,[email protected]
sshd__ciphers:
  - [email protected]
  - aes256-ctr
  - [email protected]
  • These are incorrect declarations:
sshd__allowusers: alice,bob
sshd__ciphers: [email protected] aes256-ctr [email protected]
  • These are correct declarations:
sshd__port: 2222
sshd__port: [ 2222 ]
sshd__port: [ 22, 2222 ]
sshd__port:
  - 22
  - 2222
  • These are incorrect declarations:
sshd__port: 22 2222
sshd__port: 22,2222

If you are in doubt about what inline list format is expected (spaces or commas) or if it is a multi-decarative option instead, just use a yaml list.

objects

As a Match directive:

  • accepts two parameters, a criteria and a series of patterns matching this criteria;
  • applies a subset of global options to objects caught by the matching rule;
  • may appear more than once;

the sshd__match is a complex object whose values are passed verbatim by the template to sshd_config.

sshd__match:
  - criteria: CRITERIA
    patterns: PATTERN1,PATTERN2
    options:
      - "OptionOne value1"
      - "OptionTwo value2"
      - "OptionSix value6"
  - criteria: CRITERIA
    patterns:
      - PATTERNx
      - PATTERNy
      - PATTERNz
    options:
      - "OptionFour value4"
      - "OptionFive value5"

For example:

sshd__match:
  - criteria: Group
    patterns: sftp-only
    options:
      - "AuthorizedKeysFile /etc/sftp-only/authorized_keys/%u"
      - "ChrootDirectory /srv/sftp-only/%u"
      - "ForceCommand internal-sftp -u 077"
      - "AllowTcpForwarding no"
  - criteria: User
    patterns:
      - root
      - admin
    options:
      - "PermitRootLogin yes"
      - "AuthenticationMethods publickey,password"

That will result in:

Match Group sftp-only
    AuthorizedKeysFile /etc/sftp-only/authorized_keys/%u
    ChrootDirectory /srv/sftp-only/%u
    ForceCommand internal-sftp -u 077
    AllowTcpForwarding no

Match User root,admin
    PermitRootLogin yes
    AuthenticationMethods publickey,password

Dependencies

This role has no dependency.

Example Playbook

Configure sshd_config from scratch.

- hosts: servers
  roles:
    - role: sshd_config
      sshd__passwordauthentication: no
      sshd__permitrootlogin: forced-commands-only
      sshd__usepam: yes
      sshd__acceptenv:
        - LANG
        - LC_*

Proof Of Concept: trigger a rollback due to misconfiguration. The file is valid regarding its syntax (empty) but ansible needs sftp subsystem enabled to work.

- hosts: servers
  roles:
    - role: sshd_config
      sshd_config__template: /dev/null

Galaxy

To make use of this role as a galaxy role, put the following lines in requirements.yml:

- name: sshd_config
  src: https://github.com/quidame/ansible-role-sshd_config.git
  version: 0.2.0
  scm: git

and then

ansible-galaxy install -r requirements.yml

License

GPLv3

Author Information

[email protected]