Subject: Re: RST business

From: Steve Modica <[email-masked]>
Date: Thu, 22 Apr 2004 09:11:42 -0500

If one of the potential causes for RST is that SEQ/ACK synchronization 
has been lost, then you can't do this.

Alex Pankratov wrote:
> 
> Looking at the hype around 'TCP vulnerability' the following
> occured to me, and I wonder if it makes any sense -
> 
> A host may recieve legitimate RST packet only in response to
> something that it has previously sent (let's call it a 'trigger').
> 
> SEQ/ACK values in RST packet are correlated to SEQ/ACK of the
> trigger. If the correlation is not there, then RST packet is
> most certainly spoofed and should be dropped even if its SEQ
> falls into host's rcpt window.
> 
> In other words, it seems to be possible to stregthen ingress
> RST checking (and thus better protect against blind RST attacks)
> while maintaining _full RFC compliance_. Here's a how-to sketch.
> 
>     RFC 793 (page 35) states that for the connection in
>     non-established state -
> 
>     If the incoming segment has an ACK field, the reset takes its
>     sequence number from the ACK field of the segment, otherwise
>     the reset has sequence number zero and the ACK field is set to
>     the sum of the sequence number and segment length of the incoming
>     segment.
> 
> Hence the second RST check (after standard window check) is
> 
>     if (! pkt->seq)
>         check if we've recently sent a segment without
>         an ACK with (pkt->ack - pkt->seq) bytes in it
>     else
>         check if we've recently sent a segment with ACK
>         of (pkt->seq) and with (pkt->ack - pkt->seq)
>         bytes in it
> 
> If RST passes the check, it's accepted. Otherwise checks continue.
> 
>     RFC 793 (page 36) states that for the connection in
>     established state -
> 
>     .. elicit only an empty
>     acknowledgment segment containing the current send-sequence number
>     and an acknowledgment indicating the next sequence number expected
>     to be received ..
> 
> At this point seeing a RST means that
> (a) remote host is an ESTABLISHED state
> (b) we sent a segment that it considers not to be a part of the
>     current connection
> 
> And (b) is something that we can always check since we're now sure
> about (a).
> 
> The above obviously requires keeping some sort of 'outbound history',
> plus (b) involves some non-trivial logic, which however seems to be
> doable from the first glance.
> 
> Comments ?
> 
> Alex