Subject: Re: [PATCH] Add IPv6 support to TCP SYN cookies

From: Ross Vandegrift <[email-masked]>
Date: Tue, 5 Feb 2008 16:23:35 -0500

On Tue, Feb 05, 2008 at 09:11:06PM +0100, Andi Kleen wrote:
> > The problem is that any reasonably recent PC can generate enough
> > forged SYN packets to overwhelm reasonable SYN queues on a much more
> > powerful server.
> 
> Have you actually seen this with a recent kernel in the wild or are
> you just talking theoretically?
> 
> Linux uses some heuristics to manage the syn queue that should
> still ensure reasonable service even without cookies under attack.
> Also SYN-RECV sockets are stored in a special data structure optimized
> to use minimal resources.
> 
> It is far from the classical head drop method that was so vunerable
> to syn flooding.

I work at a hosting company and we see these kinds of issues in the
real world fairly frequently.  I would guess maybe a monthly basis.
The servers where we have seen this are typically running RHEL 4 or 5
kernels, so I can't really speak to how recent the kernel is in this
specific term.

If I can find a box that we could temporary get a kernel.org kernel
on, I'll see if I can get a real comparison together.  We have
collected a few of the more effective attack tools that have been left
on compromised systems, so it wouldn't be too difficult to get some
numbers.

-- 
Ross Vandegrift
[email-masked]

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
	--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37