Connect direct and fallback

[Brian Candler]

On 17/03/2022 11:57, Chris Mitchell wrote:
> Can I instruct ssh to
> (for example) initiate a connection with 192.168.1.1, authenticate the
> remote host's identity against a custom known_hosts file, hang up
> without trying to log in or anything, and return a simple yes/no — or
> better, on success return the line number of the matching known_hosts
> entry?

You can determine the key(s) which a remote host presents, without 
logging into it, using ssh-keyscan.

$ ssh-keyscan github.com
# github.com:22 SSH-2.0-babeld-4f04c79d
github.com ssh-rsa 
AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==
# github.com:22 SSH-2.0-babeld-4f04c79d
github.com ecdsa-sha2-nistp256 
AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg=
# github.com:22 SSH-2.0-babeld-4f04c79d
github.com ssh-ed25519 
AAAAC3NzaC1lZDI1NTE5AAAAIOMqqnkVzrm0SdG6UOoqKLsabgH5C9okWi0dh2l9GKJl
# github.com:22 SSH-2.0-babeld-4f04c79d
# github.com:22 SSH-2.0-babeld-4f04c79d

If you know what key type you're looking for:

$ ssh-keyscan -t rsa github.com
# github.com:22 SSH-2.0-babeld-4f04c79d
github.com ssh-rsa 
AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==