linux-kernel - Re: incorrect checksum on sent TCP-MD5 packets (2.6.20-rc5)

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Date:	Tue, 16 Jan 2007 11:40:48 +0100
From:	Torsten Lüttgert <t.luettgert@...net.de>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Stephen Hemminger <shemminger@...l.org>, yoshfuji@...ux-ipv6.org
Subject: Re: incorrect checksum on sent TCP-MD5 packets (2.6.20-rc5)

On Mo, 2007-01-15 at 10:15 -0800, Stephen Hemminger wrote:

> Are you running over a device that does checksum offload?

Ooh, I'm feeling stupid now. Yes, I was. Turns out the problem
are the md5 checksums after all. Crypto-enabled tcpdump says:

11:05:42.856702 IP (tos 0x0, ttl  64, id 35129, offset 0, flags [DF],
proto: TCP (6), length: 80) 212.87.33.4.60565 > 212.87.49.254.bgp: S,
cksum 0x4a03 (correct), 1122127063:1122127063(0) win 5840 <mss
1460,sackOK,timestamp 63686126 0,nop,wscale 5,nop,nop,md5:valid>

11:05:42.871809 IP (tos 0x0, ttl 253, id 0, offset 0, flags [none],
proto: TCP (6), length: 64) 212.87.49.254.bgp > 212.87.33.4.60565: S,
cksum 0x0cc9 (correct), 2943414712:2943414712(0) ack 1122127064 win
16384 <mss 516,md5:valid,eol>

11:05:42.872085 IP (tos 0x0, ttl  64, id 35130, offset 0, flags [DF],
proto: TCP (6), length: 60) 212.87.33.4.60565 > 212.87.49.254.bgp: .,
cksum 0x4160 (correct), ack 1 win 5840 <nop,nop,md5:valid>

11:05:42.872150 IP (tos 0x0, ttl  64, id 35131, offset 0, flags [DF],
proto: TCP (6), length: 105) 212.87.33.4.60565 > 212.87.49.254.bgp: P,
cksum 0x54ec (correct), 1:46(45) ack 1 win 5840 <nop,nop,md5:invalid>:
BGP, length: 45
...

- Torsten


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