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I upgraded a system today from Ubuntu 9.10 to 10.04, everything was fine until I rebooted after the upgrade, since then I've been receiving SSH Connection Refused messages from putty. I presume a firewall or IPtable setting has been edited / set during the upgrade, but I'm unsure as to how to fix this issue as we're currently locked out.

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Obvious missing information: Is sshd running (/etc/init.d/sshd status)? And what are your firewall rules (iptables -L -nv)? – Juliano Sep 10 '10 at 15:41
You should still be able to sign in via the console when sshd isn't working. – EEAA Sep 10 '10 at 15:42
Until you've sorted out the access problem, the reason you can't connect via ssh is academic. – symcbean Sep 10 '10 at 15:42
How about /var/log/auth.log? What's in it? – azghash Aug 1 '11 at 9:11

If you're locked out, with no physical access, there's little you can do.
Can you remotely reboot it? Might be worth a try.
Are you using keys? The permissions may have changed, try using passwords if you can.
Can you use a different user?
All in all, if you have no access to the server you can't fix it. Get someone who does have physical access to go and have a look.

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Connection refused would indicate that the firewall is allowing you through to the port, but, I would suspect that your ssh daemon isn't running.

Remote hands/KVM is probably the only way to log into the console to restart the ssh daemon and figure out why it didn't restart on reboot.

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-j REJECT would give connection refused on IPTables, and I think Cisco firewalls can do the same. – James L Sep 10 '10 at 15:43

Most likely in the process of the upgrade the keys were changed/replaced.

PuTTy caches host keys, so may be complaining that the key signature no longer matches.

Or OpenSSH may not be running.

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