unexpected HostKeyAgent and HostKeyAlgorithms interaction

[Igor Bukanov]

Hi,

I use HostKeyAgent in sshd_config for sshd 7.1. If I have HostKeyAgent
without any HostKey directives, then ssh 7.1 client, after connecting
to the server using client ED25519 key, stores in ~/.ssh/known_hosts :

host ecdsa-sha2-nistp256 public-key

If I add then to sshd_config HostKey pointing to the ED25519 public
key to restrict the list of keys from the ssh-agent that the server
uses, then the client stores in ~/.ssh/known_hosts:

host ssh-ed25519 ...

As the result, if after adding HostKey I connect to the server, the
client reports remote host identification change and refuse the
connection as it thinks it gets a different server key.

I can workaround that with a recently added explicit HostKeyAlgorithms
that just lists ssh-ed25519,ssh-rsa. Then known_hosts always uses host
ssh-ed25519 even without HostKey in the config.  Still I am puzzled
why presence of HostKey influences HostKeyAlgorithms at all? I really
expected that it should just restrict the keys from the agent.

Regards, Igor