Re: Controlling IPv6 MTU

[David Young <dyoung%pobox.com@localhost>]

On Fri, May 25, 2012 at 09:23:08AM +0100, 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.

It sounds to me like you may intend to set an MTU on a per-interface
basis that is actually applicable (or not) on a per-destination basis.

Why don't you use the MTU in the RA to set the initial MTU on the routes
that dhcpcd installs?

Dave

-- 
David Young
dyoung%pobox.com@localhost    Urbana, IL    (217) 721-9981