Re: test SYN cookies (was Re: SYN cookies security bugfix?)

[Ed L Cashin <ecashin@terry.uga.edu>]

Thank you much for the reply.

Alan Cox <alan@lxorguk.ukuu.org.uk> 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
connection-testing. 

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:
  ecashin@terry.uga.edu       http://www.terry.uga.edu/~ecashin/pgp/