Re: Controlling IPv6 MTU

[Matt Thomas <matt%3am-software.com@localhost>]

On May 25, 2012, at 1:23 AM, Roy Marples wrote:

> Hi List
> 
> Currently, userland can only set the interface MTU SIOCSIFMTU.
> IPv6 has it's own idea of what the MTU should be according to Router Adverts,
> but there is no way of setting this outside of the RA itself.
> 
> I'm currently debugging dhcpcd with it's own RA implementation of handling
> addresses and routes and this is one wall I'm hitting as my local setup
> requires a lower MTU than 1500 and forcing a lower MTU for IPv4 isn't 
> desirable.
> 
> Attached is a patch which adds SIOCSIFMTU_IN6 which I have not tested a lot
> as I only just got home to a workable IPv6 network from a nice holiday but
> I'd like eyes on it to spot anything wrong as I've never had to extend a 
> kernel
> struct or add a new ioctl before.
> 
> Thanks
> 
> Roy<ipv6-mtu.diff>

I'd prefer to see the mtu be unsigned in the ioctl.
You should also make sure the mtu for ipv6 is not greater than the interface's 
mtu.

I'd personally add a inline to get do if_to_in6_extra(ifp) instead of having 
that ugly cast everywhere.