Re: DDoS attack causing bad effect on conntrack searches

Jesper Dangaard Brouer
2010-04-22T20:38:02Z

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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.

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

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