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 PROTECTED]) (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 ([EMAIL PROTECTED]) (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 ([EMAIL PROTECTED]) (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 ([EMAIL PROTECTED]) (gcc version 4.1.0 (SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006 Linux version 2.6.17-11-386 ([EMAIL PROTECTED]) (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. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
