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[fw@xxxxxxxxxxxxx: Route cache performance under stress]

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Subject: [fw@xxxxxxxxxxxxx: Route cache performance under stress]
From: bert hubert <ahu@xxxxxxx>
Date: Sat, 5 Apr 2003 18:50:16 +0200
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----- Forwarded message from Florian Weimer <fw@xxxxxxxxxxxxx> -----

To: linux-kernel@xxxxxxxxxxxxxxx
Subject: Route cache performance under stress
From: Florian Weimer <fw@xxxxxxxxxxxxx>
Date: Sat, 05 Apr 2003 18:37:43 +0200
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Please read the following paper:

<http://www.cs.rice.edu/~scrosby/tr/HashAttack.pdf>

Then look at the 2.4 route cache implementation.

Short summary: It is possible to freeze machines with 1 GB of RAM and
more with a stream of 400 packets per second with carefully chosen
source addresses.  Not good.

The route cache is a DoS bottleneck in general (that's why I started
looking at it).  You have to apply rate-limits in the PREROUTING
chain, otherwise a modest packet flood will push the machine off the
network (even with truly random source addresses, not triggering hash
collisions).  The route cache partially defeats the purpose of SYN
cookies, too, because the kernel keeps (transient) state for spoofed
connection attempts in the route cache.

The following patch can be applied in an emergency, if you face the
hash collision DoS attack.  It drastically limits the size of the
cache (but not the bucket count), and decreases performance in some
applications, but 

--- route.c     2003/04/05 12:41:51     1.1
+++ route.c     2003/04/05 12:42:42
@@ -2508,8 +2508,8 @@
                rt_hash_table[i].chain = NULL;
        }
 
-       ipv4_dst_ops.gc_thresh = (rt_hash_mask + 1);
-       ip_rt_max_size = (rt_hash_mask + 1) * 16;
+       ipv4_dst_ops.gc_thresh = 512;
+       ip_rt_max_size = 2048;
 
        devinet_init();
        ip_fib_init();


(Yeah, I know, it's stupid, but it might help in an emergency.)

I wonder why the route cache is needed at all for hosts which don't
forward any IP packets, and why it has to include the source addresses
and TOS (for policy-based routing, probably).  Most hosts simply don't
face such complex routing decisions to make the cache a win.

If you don't believe me, hook a Linux box to a packet generator
(generating packets with random source addresses) and use iptables to
drop the packets, in a first test run in the INPUT chain (after route
cache), and in a second one in the PREROUTING chain (before route
cache).  I've observed an incredible difference (not in laboratory
tests, but during actual DoS attacks).

Netfilter ip_conntrack support might have similar issues, but you
can't use it in a uncooperative environment anyway, at least in my
experience.  (Note that there appears to be no way to disable
connection tracking while the code is in the kernel.)
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