Hi
I have a need to get the MTU of the outgoing interface given an IPv4 or IPv6 destination address.
I have had several attempts at getting this information using different hacks.
The first hack I tried was using RTNETLINK. However, given the destination address, I was given the Path MTU which might be less than the MTU of the outgoing interface. The code is at
http://www.wand.net.nz/~mjl12/netlink-mtu.c
The caveat with that code is that IPv6 routes seem to have an expiry set that is not related to any PMTU caching. I can get the interface's MTU using an IPv4 address flushing the route cache first, and then quering for the destination.
The second attempt I had was using a similar technique to tracepath. That too returned the path MTU given a destination.
http://www.wand.net.nz/~mjl12/udp-mtu.c
The third attempt was to use SIOCGIFCONF to get a list of interfaces and addresses, and then match the interface address with whatever was returned by getsockname. I found that I was only returned AF_INET addresses, which does not fulfill my desire to have both AF_INET and AF_INET6.
http://www.wand.net.nz/~mjl12/ioctl-mtu.c
What is the suggested way to get the outgoing interface's MTU under Linux?
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