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

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

On Sun, Jun 12, 2005 at 11:13:23PM +1000, Herbert Xu wrote:
> On Sun, Jun 12, 2005 at 02:32:53PM +0200, Willy Tarreau wrote:
> >
> > but it's not the case (although the naming is not clear). So if the remote
> > end was the one which sent the SYN-ACK, it will clear its session. If it has
> > been spoofed, it will ignore the RST because in turn, the SEQ will not be
> > within its window.
> 
> This is what should happen:
> 
> 1) client A sends SYN to server B.
> 2) attcker C sends spoofed SYN-ACK to client A purporting to be server B.
> 3) client A sends RST to server B.

Agreed till here.

> The RST packet is sent by client A using its sequence numbers.  Therefore
> it will pass the sequence number check on server B.
>
> 4) server B resets the connection.

No, precisely the RST sent by A will take its SEQ from C's ACK number.
This is why B will *not* reset the connection (again, tested) if C's ACK
was not within B's window.

Cheers,
Willy