How can I force SSH to give an RSA key instead of ECDSA?

By removing the ECDSA algorithms from the HostKeyAlgorithms configuration variable.

ssh -o [email protected],[email protected],[email protected],[email protected],ssh-rsa,ssh-dss user@server

I've simply removed all the ECDSA algorithms from the default list.

You can, of course, put that in your .ssh/config for that machine:

Host: server
    HostKeyAlgorithms [email protected],[email protected],[email protected],[email protected],ssh-rsa,ssh-dss

Don't use RSA since ECDSA is the new default.

On the server do this: ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub and record that number.

On the client you can SSH to the host and if and when you see that same number, you can answer the prompt Are you sure you want to continue connecting (yes/no)? affirmatively. Then the ECDSA key will get recorded on the client for future use.

Yes, OK switch to ECDSA soon, but in the meantime try this:

ssh -o HostKeyAlgorithms=ssh-rsa -o FingerprintHash=md5 [email protected]