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

Neil Neely neil at
Thu Aug 21 20:00:19 UTC 2008

I haven't explored this issue enough to speak with any authority - but  
once upon a time I had an app doing tons of ipfw rule add/removes all  
the time and we had no end of performance and stability problems on  
that box (this would have been in 4.x or so timeline I expect).  As  
that approach wasn't really critical we abandoned it without really  
digging into the details.

Years later a need for lots of rapid firewall changes came up again  
and I drilled into it and found the use of tables was excellent for  
doing this and it does the job very well.  This is approach is on a  
FreeBSD 6.3 box.

ipfw add 00550 deny ip from 'table(1)' to any

Then just add remove entries to table 1 via:
ipfw table 1 add
ipfw table 1 delete

show all entries in table 1 with:
ipfw table 1 list

Clear out the whole of table 1
ipfw table 1 flush

I can't be sure if this relates to your particular issue, but I would  
recommend trying it out.

Neil Neely

On Aug 21, 2008, at 11:38 AM, Mikhail Teterin wrote:

> 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!
>   -mi
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