Kevin Oberman wrote:
Date: Thu, 21 Aug 2008 13:38:38 -0400
From: Mikhail Teterin <[EMAIL PROTECTED]>
Sender: [EMAIL PROTECTED]
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 85.234.158.180
Aug 12 10:21:18 symbion sshd[4335]: Invalid user mythtv from 85.234.158.180
Aug 12 10:21:20 symbion sshd[4337]: Invalid user mythtv from 85.234.158.180
Aug 12 10:21:21 symbion sshd[4339]: Invalid user mythtv from 85.234.158.180
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!
also, if you do this, have a single rule that uses a table
and add the addresses to the table.
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.
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