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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MSc. Master of Computer Science
Dept. of Computer Science, University of Copenhagen
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