On the importance of MTU

04 Jul 2009 • 2 min read

So, I'm now finally natviely IPv6 enabled 😃

This led to an interesting debtate about my internet connection. In a nutshell my Drayetk Vigor 100 is broken, but apparently by design. The problem is this - for IPv4 goes through my NAT whose public IP has an MTU of 1492 so Path MTU Discovery works fine. However IPv6 does not need NAT and the PPPoE interface does not have a public IPv6 and nor do the internal clients use it even it there was one. So the path MTU is 1500, which is too big for the PPPoE to handle.

The correct solution is to obviously terminate the IPv6 on a PPPoA connection, but only the Cisco 877 router does this and that's outside of my price range, even on eBay. One solution is to clamp the MSS on the router to 1430 so that all clients work. But this is a hack. The best solution with my hardware is to force the MTU to 1492 for all nodes inside my network, so they share the same MTU as the PPPoE so that Path MTU Discovery works.

You can see this working for yourself on IPv6 now, as this site is IPv6 enabled and sits behind the PPPoE link. You'll need to query IPv4 servers for the address though as I don't yet have a glue record for IPv6, but as I'm changing registrar that should change in a few weeks 😃

Because of this, I've made the dhcpcd default to request and use the MTU value if offered by the DHCP server. You'll be seeing this in dhcpcd-5.0.5 (now out). dhcpcd-5.0.6 will feature restoring the MTU correctly between leases.