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We're using SSH with public key authentication.

The user can connect from one machine, but not from another machine on a different network.

He was previously able to connect.

When we run ssh with -vvv we can see that our server is reached, but the ssh connection times out before he can do anything:

debug1: connect to address 1.2.3.4 port 30000: Operation timed out

(I've disguised our ip address above)

We turned on password authentication, and he can't connect with that either, from one machine,

We figure there is some sort of auto-generated blacklist, but don't know where to check.

EDIT: Well, magically, when we left it alone for a few minutes and tried again, the connection worked...

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Have you tried waiting a few minutes and trying again? :-) –  Bart Silverstrim Jan 12 '10 at 12:44

2 Answers 2

That sounds like there was no TCP connection, rather than an authentication failure. Have you tried looking for his traffic by running tcpdump on the server?

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where do I find tcpdump? –  Steve McLeod Jan 12 '10 at 11:22
    
when the connection is not established to the server, you will not see anything in tcpdump on the server. –  Christian Jan 12 '10 at 11:30
    
Sure, but that's what you want to find out... if there's no packets getting to the server, you're wasting your time fiddling with ssh authentication when you should be figuring out what is going on with the network. apt-get install tcpdump should get you a tcpdump, and tcpdump -n 'port 22 and tcp[tcpflags] & (tcp-syn|tcp-fin) != 0' is the command you want to use to see if the TCP SYN packets are in fact arriving. –  Andrew McGregor Jan 13 '10 at 6:22

try to telnet to your ssh server: telnet ssh-server 30000. When this does not work it is most likely that you have a firewall between the two networks. do you have a local firewall on the client or the server? is there a central firewall between the two networks?

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