Subject: UDP checksum broken since 2.6.18?

From: "Thomas B. Rücker" <[email-masked]>
Date: Tue, 22 May 2007 21:47:22 +0000

hi,

a friend of mine recently contacted me about what he at first thought
were IPv6 issues with some java software.

As it turns out it probably is a general IP issue with the Linux kernel:

He wrote this piece of c which sends an UDP packet to 127.28.50.50 -
http://www2.futureware.at/~philipp/udp-problem.c
Packets generated by this code were captured by tcpdump and wireshark.
When feeding the dump into wireshark it says:
"Checksum: 0x62fd [incorrect, should be 0xe4f3]" for the udp packet.

We've tested this on several kernel versions.
Wireshark reports checksum broken:
Linux version 2.6.18-4-vserver-686 (Debian 2.6.18.dfsg.1-12)
([email-masked]) (gcc version 4.1.2 20061115 (prerelease) (Debian
4.1.1-21)) #1 SMP Mon Mar 26 19:55:22 UTC 2007
Linux version 2.6.19-dm8tbr-1 (root@oberon) (gcc version 4.1.2 20061028
(prerelease) (Debian 4.1.1-19)) #3 SMP PREEMPT Sun Dec 3 18:31:00 CET
2006 - (that's vanilla)
Linux version 2.6.21.1-dm8tbr-1 (root@oberon) (gcc version 4.1.2
20061115 (prerelease) (Debian 4.1.1-21)) #3 SMP Fri May 18 09:04:55 CEST
2007 - (that's vanilla + dscape patch)

Wireshark reports checksum ok:
Linux version 2.6.16.13-4-default (geeko@buildhost) (gcc version 4.1.0
(SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006
Linux version 2.6.17-11-386 (root@terranova) (gcc version 4.1.2 20060928
(prerelease) (Ubuntu 4.1.1-13ubuntu5)) #2 Tue Mar 13 23:30:30 UTC 2007
(Ubuntu 2.6.17-11.37-386)

So my guess is something between 2.6.17 and 2.6.18 broke.

Second option is: The way you are supposed to send UDP packets changed
in 2.6.18 and sun javavm and that piece of c are broken for the same reason.

Third option: everything is perfectly ok, the UDP checksum is computed
in a different way since 2.6.18 - due to some reason I don't know - and
Wireshark is broken.

We'd be grateful for some enlightment.

Cheers

Thomas

PS: please keep me CCed - I'm not subscribed to the netdev ml.