Subject: RST business

From: Alex Pankratov <[email-masked]>
Date: Wed, 21 Apr 2004 22:59:11 -0700

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