Fixing slow SSH logins

On a fresh VPS, logging in over SSH took around ten seconds before the password prompt appeared. Once connected, everything was fast. That pattern almost always means the server is waiting on a lookup that times out.

Find where it stalls

Run the client in verbose mode and watch where the output pauses:

ssh -vvv user@host

In my case the pause came right after SSH2_MSG_SERVICE_ACCEPT, while it was trying gssapi-with-mic. That's the client attempting Kerberos authentication, which nobody uses on a hobby box.

The two usual culprits

GSSAPI. Turn it off on the client, for all hosts or per host, in ~/.ssh/config:

Host *
    GSSAPIAuthentication no

Reverse DNS on the server. If sshd tries to resolve the client's IP and the resolver is slow, every login waits for it. In /etc/ssh/sshd_config:

UseDNS no

Recent OpenSSH already defaults to no, but older images and some provider templates still set it to yes. Check with:

sshd -T | grep -i usedns

Then reload:

systemctl reload ssh

Result

Login dropped from about ten seconds to well under one. If neither of these helps, check /etc/resolv.conf on the server. A dead nameserver listed first makes everything that does DNS slow, not only SSH.