Re: [PATCH v3] net: sysctl for RA default route MTU

From: Hannes Frederic Sowa
Date: Tue Apr 07 2015 - 11:58:40 EST

On Do, 2015-04-02 at 21:08 +0300, Roman Gushchin wrote:
> >> The next question I have is about the behavior of the new setting
> >> in the presence of an RA MTU option. It seems like the sysctl
> >> doesn't override that RA MTU option, but rather just clamps it.
> >>
> >> And then if it's in range, this controls only whether the default
> >> route has it's MTU adjusted.
> >>
> >> That doesn't make any sense to me if we then go and do the
> >> rt6_mtu_change() call unconditionally. The route metric update
> >> and the rt6_mtu_change() go hand in hand.
> >
> > Agreed but that gets interesting:
> >
> > I guess during testing the cnf.mtu6 value was equal to the newly
> > announced mtu value, so the rt6_mtu_change call does not happen. We
> > update cnf.mtu6 so a second RA packet would actually bring the system
> > into the desired state but we have a moment where the default route
> > carries a too big MTU. That's not good.
> Agreed.
> > Easiest solution is to reorder those calls but that also leaves us with
> > a time frame where we carry the incorrect MTU on the default route.
> > Otherwise we must conditionally filter out the default routes.
> > Roman, any ideas?
> I think, such approach will work on practise, but looks not very beatiful.
> May be, a better idea is to serarate per-route and per-device MTU,
> so an updating of per-device MTU will not affect per-route MTU.
> Actual MTU can always been calculated as min(route_mtu, device_mtu),
> but we wouldn't need to update mtu on each route on receiving RA MTU option,
> for instance.
> Do you see any problems with such approach?

If I understood you correct this actually seems to be quite an intrusive
change? :/ Can you show me some code how to do this?

I would also dislike adding a filtering capability to the route mtu
updates. Currently I don't have a god idea, sorry.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at