linux-kernel - Probably bug in netfilter hashlimit extension

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Message-ID: <20150511092941.GA8739@ikki.ethgen.ch>
Date:	Mon, 11 May 2015 10:29:41 +0100
From:	Klaus Ethgen <Klaus+lkml@...gen.de>
To:	linux-kernel@...r.kernel.org
Subject: Probably bug in netfilter hashlimit extension

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Recently I tried to mitigate some slow attacks via netfilter rule
utilizing hashlimit target. I used the following specification:

   -A DETECT_INVALID -m hashlimit --hashlimit-upto 10/hour --hashlimit-mode srcip --hashlimit-name attack_invalid -j RETURN

Now I seen some strange stuff. The counter in
/proc/net/ipt_hashlimit/attack_invalid only counts from 60 back to 0 and
then the entry disappears. Than means that a rate of 10/hour will never
ever be detected at all.

On that box I use kernel 3.16.0 from debian backport to oldstable Which
seems to be somewhat equal to 3.16.7. So maybe that bug has beed find
earlier or is even fixed upstream. I have no easy way to upgrade that
kernel short term as the box is productive.

Shorter times like 30/hour with a slightly bigger burst (10 instead of
the default 5) seems to work as expected but is not able to detect the
attacks due to the slow rate.

Am I the only who seen that behaviour or is that a known limitation? I
find no such notes anywhere that there is a limit here. (Although I
would believe that there is a high limit somewhere. But then I would
expect them to be returend with some errno when trying to set a to high
value.)

Please keep me in Cc as I do not monitor this List that often.

Regards
   Klaus
- -- 
Klaus Ethgen                              http://www.ethgen.ch/
pub  4096R/4E20AF1C 2011-05-16   Klaus Ethgen <Klaus@...gen.de>
Fingerprint: 85D4 CA42 952C 949B 1753  62B3 79D0 B06F 4E20 AF1C
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