sshd: unknown [priv] and sshd: unknown [net] is when someone trying to attempt to login to your system, but there is no account under the username.
mostly such log entries will be followed by something like below in /var/log/messages
Mar 26 10:41:33 athena sshd(pam_unix): check pass; user unknown
Mar 26 10:41:33 athena sshd(pam_unix): authentication failure; logname= uid=0 euid=0 tty=NODEVssh ruser= rhost=126.96.36.199
where you can block the IP 188.8.131.52.
Meanwhile you should edit /etc/ssh/sshd_config to read the config as below,
Protocol 2 (only SSH2 should be allowed, SSH1 is vulnerable)
Port 22 (22 can be changed to something else, preferably a number 10000+, but you have to know that once changed and restarted the sshd, you won't be able to connect with default port 22, instead specifically mention the port number given here in your SSH client to connect to SSH. Don't change it unless you know what you are doing)
and follow the steps at http://www.lunarforums.com/viewtopic.php?t=26253