What to Expect with Your First Login

The first time you log in, you get a warning message saying that the remote host doesn't know about your machine and prompting you to store a copy of the remote host's SSH identification keys on your local machine. It will look something like this:

     [root@bigboy tmp]# ssh smallfry
     The authenticity of host 'smallfry (smallfry)' can't be established.
     RSA key fingerprint is
     5d:d2:f5:21:fa:07:64:0d:63:1b:3b:ee:a6:58:58:bb.
     Are you sure you want to continue connecting (yes/no)? yes
     Warning: Permanently added 'smallfry' (RSA) to the list of known
     hosts. root@smallfry's password:
     Last login: Thu Nov 14 10:18:45 2002 from 192.168.1.98
     No mail.
     [root@smallfry tmp]#

The key is stored in your ~/.ssh/known_hosts file and you should never be prompted for this again.

SSH Failures Due to Linux Reinstallations

If Linux or SSH is reinstalled on the remote server, then the keys are regenerated and your SSH client will detect that this new key doesn't match the saved value in the known_hosts file. The SSH client will fail, erring on the side of caution to alert you to the possibility of a form of hacking attack:

     [root@bigboy tmp]# ssh 192.168.1.102
     @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
     @    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
     @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
     IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
     Someone could be eavesdropping on you right now (man-in-the-middle
     attack)!
     It is also possible that the RSA host key has just been changed.
     The fingerprint for the RSA key sent by the remote host is
     5d:d2:f5:21:fa:07:64:0d:63:1b:3b:ee:a6:58:58:bb.
     Please contact your system administrator.
     Add correct host key in /root/.ssh/known_hosts to get rid of this
     message.
     Offending key in /root/.ssh/known_hosts:2
     RSA host key for 192.168.1.102 has changed and you have requested
     strict checking.
     Host key verification failed.
     [root@bigboy tmp]#

If you are confident that the error is due to a reinstallation, then edit your ~/.ssh/known_hosts text file, removing the entry for the offending remote server. When you try connecting via SSH again, you'll be prompted to add the new key to your ~/.ssh/known_hosts file and the login session should proceed as normal after that.