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SubjectRe: Probably bug in netfilter hashlimit extension
(Cc'ing netdev and netfilter-devel)

On Mon, May 11, 2015 at 2:29 AM, Klaus Ethgen <> wrote:
> Hash: SHA512
> 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
> pub 4096R/4E20AF1C 2011-05-16 Klaus Ethgen <>
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