machine hangs on occasion - correlated with ssh break-in attempts

Kevin Oberman oberman at
Thu Aug 21 18:49:50 UTC 2008

> Date: Thu, 21 Aug 2008 13:38:38 -0400
> From: Mikhail Teterin <mi+mill at>
> Sender: owner-freebsd-stable at
> Hello!
> A machine I manage remotely for a friend comes under a distributed ssh 
> break-in attack every once in a while. Annoyed (and alarmed) by the 
> messages like:
> Aug 12 10:21:17 symbion sshd[4333]: Invalid user mythtv from
> Aug 12 10:21:18 symbion sshd[4335]: Invalid user mythtv from
> Aug 12 10:21:20 symbion sshd[4337]: Invalid user mythtv from
> Aug 12 10:21:21 symbion sshd[4339]: Invalid user mythtv from
> I wrote an awk-script, which adds a block of the attacking IP-address to 
> the ipfw-rules after three such "invalid user" attempts with:
>     ipfw add 550 deny ip from ip
> The script is fed by syslogd directly -- through a syslog.conf rule 
> ("|/opt/sbin/auth-log-watch").
> Once in a while I manually flush these rules... I this a good (safe) 
> reaction?
> I'm asking, because the machine (currently running 7.0 as of July 7) 
> hangs solid once every few weeks... My only guess is that a spike in 
> attacks causes "too many" ipfw-entries created, which paralyzes the 
> kernel due to some bug -- the machine is running natd and is the gateway 
> for the rest of the network...
> The hangs could, of course, be caused by something else entirely, but my 
> self-defense mechanism is my first suspect...
> Any comments? Thanks!

Looks remarkably like sshguard (ports/security/sshguard-*). It does almost
exactly what you are doing but is written in C and has command-line
switches to set how long a system is blocked, how many attempts
constitute an attack and how long it should remember failed attempts. It
also allows the use of back-end scripts if you want it to do something
else such as generate reports (beyond an entry in /var/log/messages).

As far as the hangs, I don't believe it is from the large nu,ber of
brute force attempts as they will stop for a given host as soon as the
firewall is updated. I seldom see more than a handful of attack sources
over any short period.

Should you want to continue with your own tool, at least for IPv4,
consider using tables rather than a raft of rules. With tables, you need
only a single rule and it is there at boot time.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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