RE: [cobalt-security] SYN attacks killing me! Please HELP!

["E.B. Dreger" <eddy+public+spam@xxxxxxxxxxxxxxxxx>]

GF> Date: Tue, 23 Jul 2002 15:04:40 +0100
GF> From: Graeme Fowler


GF> To answer everyone else's points too; it _is_ possible to
GF> effectively squash TCP SYN flood attacks without needing to
GF> enable something as resource-intensive as TCP Intercept at
GF> your network boundary. A similar effect can be gained by
GF> rate-limiting SYN packets to a predetermined percentage of
GF> your line speed, and permitting them to burst to a slightly
GF> higher rate.

Except this blocks valid SYN requests, too.  If you normally get
50 kbps of SYN and set a limit of 250 kbps, one easily can drown
you with 5000 kbps.  The box doesn't crash, but you still have a
DoS due to dropped packets.

Yes, I've used rate-limiting when no better alternative was
available.  I'd consider it a last resort, along with per-IP
blocking.  (Note that, when the TCP handshake is incomplete or
gets RST, there's a good chance you're blocking someone you
shouldn't.)


GF> It still means it has to be done at your network edge,
GF> though, so if have no control over your router you'll have to
GF> ask whoever does.

Or run a TCP stack that isn't as vulnerable to this sort of
thing.  *shrug*  People demand Linux, they get Linux.[1]

Without suggesting that Cobalt should consider a BSD-based
product *cough*, you might look at:

http://www.cymru.com/~robt/Docs/Articles/ip-stack-tuning.html

Try increasing the queue depth.  If you're CPU-bound, this will
make things _worse_, however.

How to harden a stack against SYN floods has been discussed on
many mailing lists.  It seems that some sort of drop algorithm,
perhaps coupled with a known-{good | bad} cache, is the best way
to deal with SYN floods.

[1] Disclaimer:  I'm a BSD bigot, and don't know how Linux 2.4
    handles this.  No, the BSD family isn't perfect.


Eddy
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