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SubjectRe: test SYN cookies (was Re: SYN cookies security bugfix?)
Thank you much for the reply.

Alan Cox <> writes:

> > Is there anyone who has any evidence that SYN cookies do anything in
> > kernel 2.2.x? If so, how did you get that evidence, because I would
> > like to reproduce it.
> They work fine for me in 2.2.19/2.2.20.

That was reassuring enough that I persisted and found that the problem
was this: my home-spun SYN-flooder wasn't changing the TCP sequence
number, and so the "victim" was discarding the packets.

The three-second pause I observed previously was a red herring that
went away when I started using separate hosts for flooding and

Now I see a night-and-day difference between with and without SYN
cookies (although when tcp_max_syn_backlog is set to more than a five
it takes a long time to fill the queue).

Thanks again.

--Ed Cashin PGP public key:

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