Re: Bug in TCP stack: RST has wrong sequence number?
Re: Bug in TCP stack: RST has wrong sequence number?
- Subject: Re: Bug in TCP stack: RST has wrong sequence number?
- From: Axel Rau <email@hidden>
- Date: Mon, 19 Feb 2007 21:26:04 +0100
Am 19.02.2007 um 20:43 schrieb Justin C. Walker:
On Feb 19, 2007, at 09:31 , Axel Rau wrote:
Hi,
while trying to analyze a problem with hanging Mail.app, which
talks to an IMAP server through an OpenBSD firewall, I got some
interesting diagnose from Daniel Hartmeier (email@hidden)
from the OpenBSD project:
I quote from his posting in the OpenBSD pf list:
http://marc.theaimsgroup.com/?l=openbsd-pf&m=117176062511692&w=2
and sends a RST. But the sequence number (th_seq) used in the
RST is not valid. It should be 2634574290 (past the last segment the
client sent), not 2634574252 (past the second-to-last segment,
not including
the payload in the FIN+PSH).
This looks like a bug in the client's TCP/IP stack, and how to
fix it
depends on the client's OS. IMO, the RST is invalid, and pf is
right in
blocking it.
This is perhaps a corner case in the protocol spec. I'm not sure I
buy the conclusion above, but it would be good to hear from the
experts.
As I read 793, the reset is generated with the current state of the
window as the sequence number. From your trace, it looks like the
sender (.106) sent data and then immediately reset the connection.
The last ack from the receiver (.20) was ...252, so using ...252
seems right to me (the sender can't assume that the ...290 segment
was/will be received). Judging from the following resets, the
stack will use the last-acked sequence number.
Any comments, Daniel?
I'd file a bug report, but I'm not convinced the behavior you see
is incorrect (from the standard).
Bug ID# 5008062.
Axel
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Axel Rau, ☀Frankfurt , Germany +49 69 9514 18 0
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