If your Debian system is suddenly very slow to login – like, 25 to 90 seconds of apparent inactivity on every new ssh – it’s not DNS.

Check to see if your elogind or systemd/logind is actually running. Look for a log entry in auth.log along the lines of

dbus-daemon[4501]: [system] Failed to activate service 'org.freedesktop.login1'

right after an SSH attempt.

If so, restart the daemon.

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