If you’ve been playing in your virtual server you might notice this type of entry in /var/log/messages
Feb 11 12:21:10 load1 sshd(pam_unix)[397]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Feb 11 12:21:10 load1 sshd(pam_unix)[398]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Feb 11 12:21:11 load1 sshd(pam_unix)[402]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Feb 11 12:21:11 load1 sshd(pam_unix)[404]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Feb 11 12:21:12 load1 sshd(pam_unix)[406]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Feb 11 12:21:13 load1 sshd(pam_unix)[412]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=mail.kwpskt.edu.hk user=root
Those lines are bad guys trying to get into your machine via brute force dictionary attacks.
If you maintain good passwords there’s little to worry about here. However, if like me these things annoy you there are a variety of ways to get rid of them.