Re: [PATCH] fix small DoS on connect() (was Re: BUG: Unusual TCP Connect() results.)

[Willy Tarreau <willy@w.ods.org>]

On Mon, Jun 13, 2005 at 02:48:10PM +1000, Herbert Xu wrote:
> On Sun, Jun 12, 2005 at 04:24:01PM +0200, Willy Tarreau wrote:
> >
> > 1) no firewall in front of A
> >   - C spoofs A and sends a fake SYN to B
> >   - B responds to A with a SYN-ACK
> >   - A sends an RST to B, which clears the session
> >   - A wants to connect and sends its SYN to B which accepts it.
> 
> Well the attacker simply has to keep sending the same SYN packet
> over and over again until A runs out of SYN retries.
> 
> What I really don't like about your patch is the fact that it is
> trying to impose a policy decision (that of forbidding all
> simultaneous connection initiations) inside the TCP stack.

It's the same for ECN or SYN cookies.

> A much better place to do that is netfilter.  If you do it there
> then not only will your protect all Linux machines from this attack,
> but you'll also protect all the other BSD-derived TCP stacks.

Netfilter already blocks simultaneous connection. A SYN in return to
a SYN produces an INVALID state.

Cheers,
Willy