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So this computer is reachable on port 22 (from everywhere).
Since messages indicating failed login attempts (usernames like root, cgi, bash, production...) have been flooding /var/log/auth.log, I have disabled password authentication from external IPs (using public key authentication only).
And this works, when trying to ssh into that machine from an external IP (without key) I don't even get the username prompt:

Permission denied (publickey).

So how do all those fake usernames still end up in auth.log?

  1 Aug  4 17:02:48 host sshd[17190]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=217.116.204.99  user=root
  2 Aug  4 17:02:48 host sshd[17190]: pam_winbind(sshd:auth): getting password (0x00000388)  
  3 Aug  4 17:02:48 host sshd[17190]: pam_winbind(sshd:auth): pam_get_item returned a password  
  4 Aug  4 17:02:48 host sshd[17190]: pam_winbind(sshd:auth): request wbcLogonUser failed: WBC_ERR_AUTH_ERROR, PAM error:

PAM_USER_UNKNOWN (10), NTSTATUS: NT_STATUS_NO_SUCH_USER, Error mess 4 age was: No such user
5 Aug 4 17:02:50 host sshd[17190]: Failed password for root from 217.116.204.99 port 40054 ssh2
6 Aug 4 17:02:50 host sshd[17190]: Received disconnect from 217.116.204.99: 11: Bye Bye [preauth]
...
513322 Apr 7 19:45:40 host sshd[15986]: input_userauth_request: invalid user cgi [preauth]
...

http://paste.debian.net/92403/

1 Answer 1

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While you don't enter a username, if you are connecting from a linux/osx/bsd workstation the username is implicit (it defaults to the username you are logged in as), if you have windows and use putty try connecting without setting the Auto-login username, and present a key, it will ask for a username to try and match the pair.

Keys only replace passwords, each one is associated with a user (and therefor username), which is why you'll find the authorized_keys file under ~/.ssh/ .

What you are probably seeing are attackers doing something similar to ssh bash@<your.server.ip> . The server sees a username, but since they don't present a key, they are denied access.

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  • Right, that's it! So security is working fine then. Thanks.
    – basic6
    Commented Apr 8, 2014 at 11:25

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