Re: DDoS attack causing bad effect on conntrack searches

Eric Dumazet
2010-04-22T21:03:19Z

Le jeudi 22 avril 2010 à 22:38 +0200, Jesper Dangaard Brouer a écrit :
> On Thu, 22 Apr 2010, Eric Dumazet wrote:
>
> > Le jeudi 22 avril 2010 à 08:51 -0700, Paul E. McKenney a écrit :
> >> On Thu, Apr 22, 2010 at 04:53:49PM +0200, Eric Dumazet wrote:
> >>> Le jeudi 22 avril 2010 à 16:36 +0200, Eric Dumazet a écrit :
> >>>
> >>> If we can do the 'retry' a 10 times, it means the attacker was really
> >>> clever enough to inject new packets (new conntracks) at the right
> >>> moment, in the right hash chain, and this sounds so higly incredible
> >>> that I cannot believe it at all :)
> >>
> >> Or maybe the DoS attack is injecting so many new conntracks that a large
> >> fraction of the hash chains are being modified at any given time?
> >>
>
> I think its plausable, there is a lot of modification going on.
> Approx 40.000 deletes/sec and 40.000 inserts/sec.
> The hash bucket size is 300032, and with 80000 modifications/sec, we are
> (potentially) changing 26.6% of the hash chains each second.
>

OK but a lookup last a fraction of a micro second, unless interrupted by
hard irq.

Probability of a change during a lookup should be very very small.

Note that the scenario for a restart is :

The lookup go through the chain.
While it is examining one object, this object is deleted.
The object is re-allocated by another cpu and inserted to a new chain.

What exact version of kernel are you running ?

> As can be seen from the graphs:
> http://people.netfilter.org/hawk/DDoS/2010-04-12__001/list.html
>
> Notice that primarily CPU2 is doing the 40k deletes/sec, while CPU1 is
> caught searching...
>
>
> > maybe hash table has one slot :)
>
> Guess I have to reproduce the DoS attack in a testlab (I will first have
> time Tuesday). So we can determine if its bad hashing or restart of the
> search loop.
>
>
> The traffic pattern was fairly simple:
>
> 200 bytes UDP packets, comming from approx 60 source IPs, going to one
> destination IP. The UDP destination port number was varied in the range
> of 1 to 6000. The source UDP port was varied a bit more, some ranging
> from 32768 to 61000, and some from 1028 to 5000.
>
>
> Cheers,
> Jesper Brouer
>
> --
> -------------------------------------------------------------------
> MSc. Master of Computer Science
> Dept. of Computer Science, University of Copenhagen
> Author of http://www.adsl-optimizer.dk
> -------------------------------------------------------------------

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