I have setup the ssh between two servers, server A has generated a public key and copy it to server B , and have setup the remote host , all work well , but now when use "ssh serverB -l user" , then pop "Connection closed by serverB" , when use "Connection closed by serverB ,lost connection" , can advise what is the possible reason ? thx.

1. try to connect from serverB to A.
2. what is on the logs of serverB (in your example)?

When does "Connection closed" pop up? Immediately, or after you've started some authentication?

As keyF suggested, check the logs on serverB. Location may vary by distro. Mine are in /var/log/auth.log, but yours could be elsewhere depending on your syslog setup.

Also, from serverA try "ssh serverB -vv -l user" (add a "double v" option to the command line to indicate "very verbose"). You should be able to see what it's doing when serverB closes the connection.

If you can't interpret the verbose output from ssh, post it here and maybe somebody can help debug it for you.

Also check /etc/hosts.allow and /etc/hosts.deny for anything that might be denying the connection.

Also, "iptables --list" for firewall things that might be blocking you.

Also, is sshd even running on serverB? On the port you are expecting it on? Check with "ps -ef | grep sshd" and "netstat -anp | grep LISTEN | grep sshd"

You will need to be root to run the -p option for netstat, and to run the iptables command. Probably to look at auth.log too (if your system is configured securely).