Host parameter does not match hashed host field in supplied key

While using the known_hosts Ansible module, I stumbled upon the error message that gives name to this blog post. All I could find was this GitHub issue, which unfortunately didn’t help me much. I wasn’t hashing the hostname (hash_host: false), and the key had a valid format (according to the SSH_KNOWN_HOSTS FILE FORMAT in the sshd(8) man page).

My problem was that I was dealing with an SSH server that listened on a port other than 22. The documentation for the known_host module says:

For custom SSH port, name needs to specify port as well. See example section.

The format is a bit special. Example:

- name: Add public key
  known_hosts:
    name: "[{{ my_host }}]:{{ my_custom_port }}"
    key: "{{ lookup('pipe', 'ssh-keyscan -p {{ my_custom_port }} {{ my_host }}') }}"

Notice the square brackets around the hostname.