Heckroth Industries

IPv6 Routing issue

Recently I have been noticing an issue with my IPv6 networking. Everything was working fine locally but if I connected from outside the network then connections would lock up when they got busy.

It took a bit of investigating but I discovered that the IPv6 MTU size wasn't being detected correctly by some devices. The reason it would work locally was that the local network's MTU was 1500, but for IPv6 packets leaving the network the routing goes through an IPv4 tunnel, which results in it having a lower MTU size.

The solution was to add a line to the radvd.conf file (used by the daemon that handle the router advertisements). The line that I added was

AdvLinkMTU 1280;

Which results in those devices that configure their IPv6 details from the router advertisement setting their MTU to 1280 which is the minimum datagram size for IPv6.

IPv6 Networking
Jason — 2013-04-24