ssh-hardening (Ansible Role)

Description

This role provides secure ssh-client and ssh-server configurations.

Requirements

• Ansible

Role Variables

• network_ipv6_enable - true if IPv6 is needed
• ssh_client_cbc_required - true if CBC for ciphers is required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure ciphers enabled. CBC is a weak alternative. Anything weaker should be avoided and is thus not available.
• ssh_server_cbc_required - true if CBC for ciphers is required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure ciphers enabled. CBC is a weak alternative. Anything weaker should be avoided and is thus not available.
• ssh_client_weak_hmac - true if weaker HMAC mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure HMACs enabled.
• ssh_server_weak_hmac - true if weaker HMAC mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure HMACs enabled.
• ssh_client_weak_kex - true if weaker Key-Exchange (KEX) mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure KEXs enabled.
• ssh_server_weak_kex - true if weaker Key-Exchange (KEX) mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure KEXs enabled.
• ssh_ports: ['22'] - ports to which ssh-server should listen to and ssh-client should connect to
• ssh_listen_to: ['0.0.0.0'] - one or more ip addresses, to which ssh-server should listen to. Default is empty, but should be configured for security reasons!
• ssh_host_key_files: ['/etc/ssh/ssh_host_rsa_key', '/etc/ssh/ssh_host_dsa_key', '/etc/ssh/ssh_host_ecdsa_key'] - Host keys to look for when starting sshd.
• ssh_client_alive_interval: 600
• ssh_client_alive_count: 3
• ssh_remote_hosts: [] - one or more hosts, to which ssh-client can connect to. Default is empty, but should be configured for security reasons!
• ssh_allow_root_with_key - false to disable root login altogether. Set to true to allow root to login via key-based mechanism.
• ssh_allow_tcp_forwarding false to disable TCP Forwarding. Set to true to allow TCP Forwarding.
• ssh_allow_agent_forwarding false to disable Agent Forwarding. Set to true to allow Agent Forwarding.
• ssh_use_pam: false - false to disable pam authentication.
• ssh_deny_users: '' - if specified, login is disallowed for user names that match one of the patterns.
• ssh_allow_users: '' - if specified, login is allowed only for user names that match one of the patterns.
• ssh_deny_groups: '' - if specified, login is disallowed for users whose primary group or supplementary group list matches one of the patterns.
• ssh_allow_groups: '' - if specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns.
• ssh_print_motd - false to disable printing of the MOTD
• ssh_print_last_log - false to disable display of last login information

Example Playbook

- hosts: localhost
  roles:
    - ansible-ssh-hardening

Local Testing

For local testing you can use vagrant and Virtualbox of VMWare to run tests locally. You will have to install Virtualbox and Vagrant on your system. See Vagrant Downloads for a vagrant package suitable for your system. For all our tests we use test-kitchen. If you are not familiar with test-kitchen please have a look at their guide.

Next install test-kitchen:

# Install dependencies
gem install bundler
bundle install

# Fetch tests
bundle exec thor kitchen:fetch-remote-tests

# fast test on one machine
bundle exec kitchen test default-ubuntu-1204

# test on all machines except Debian-based machines
bundle exec kitchen test

# test on Debian-based machines
KITCHEN_YAML=".kitchen_debian.yml" bundle exec kitchen test

# for development
bundle exec kitchen create default-ubuntu-1204
bundle exec kitchen converge default-ubuntu-1204

For more information see test-kitchen

FAQ / Pitfalls

I can't log into my account. I have registered the client key, but it still doesn't let me it.

If you have exhausted all typical issues (firewall, network, key missing, wrong key, account disabled etc.), it may be that your account is locked. The quickest way to find out is to look at the password hash for your user:

sudo grep myuser /etc/shadow

If the hash includes an !, your account is locked:

myuser:!:16280:7:60:7:::

The proper way to solve this is to unlock the account (passwd -u myuser). If the user doesn't have a password, you should can unlock it via:

usermod -p "*" myuser

Alternatively, if you intend to use PAM, you enabled it via ssh_use_pam: true. PAM will allow locked users to get in with keys.

Why doesn't my application connect via SSH anymore?

Always look into log files first and if possible look at the negotation between client and server that is completed when connecting.

We have seen some issues in applications (based on python and ruby) that are due to their use of an outdated crypto set. This collides with this hardening module, which reduced the list of ciphers, message authentication codes (MACs) and key exchange (KEX) algorithms to a more secure selection.

If you find this isn't enough, feel free to activate the attributes cbc_requires for ciphers, weak_hmac for MACs and weak_kexfor KEX in the variables ssh_client or ssh_server based on where you want to support them.

Contributing

See contributor guideline.

License and Author

• Author:: Sebastian Gumprich sebastian.gumprich@38.de
• Author:: Christoph Hartmann chris@lollyrock.com

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.