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

Ubuntu IPv6 Only Torrents

If you didn’t notice, Ubuntu 8.10 was released today. As usual the mirrors and torrents were flooded with eager downloaders.

Canonical was nice enough to setup an IPv6 only tracker (

Unfortunately it looks like only 27 downloads have completed in the 12+ hours it’s been released.

It’s a start, but when the first response to any networking problem on the support forum is to "DISABLE IPv6" I tend to think we have a long road left to travel.

tags: IPv6 Linux Ubuntu
Cody Lerum - October 30, 2008
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