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 > ------------------------------------------------------------------- -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
