Linux IPv6 MTU issues

I’m seeing an annoying issue with Ubuntu 8.10 and IPv6. It appears that the Linux OS is sending packets larger than 1500 bytes out of a gigibit Ethernet interfaces which is configured for a 1500 byte MTU.

This is easy to reproduce with a dual stack machine. Just SCP/FTP a file down via IPv4 and one via IPv6. You will only see about 10% of the IPv4 throughput on the v6 transfer. A tcpdump or wireshark will confirm that packets sourced from the Linux machine are larger than 1500 bytes…​.sometimes 10x the size. When the network connecting these two machines only supports 1500 bytes, all these oversized packets get dropped.

I’ve opened a case with the ubuntu guys —

However, I’m seeing similar results on CENTOS 5.2 so I’m thinking it’s less distro specific.

I haven’t really heard of anyone else noticing this which makes we wonder if I’m missing something painfully simple in the configuration, or nobody is really serving any IPv6 content off a server with 2.6 kernel.

Anyone else seeing this?

Cody Lerum - November 30, 2008
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