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

On Wed, Apr 1, 2015, at 19:55, David Miller wrote:
> From: Roman Gushchin <>
> Date: Wed, 01 Apr 2015 12:58:50 +0300
> > 31.03.2015, 23:49, "David Miller" <>:
> >> From: Hannes Frederic Sowa <>
> >> Date: Tue, 31 Mar 2015 22:35:48 +0200
> >>>  Could you quickly comment on what you had in mind? I guess it is about
> >>>  handling RA in user space on the end hosts and overwriting MTU during
> >>>  insertion of the routes?
> >>
> >> Even after reading your email I have no idea why you can't just have
> >> RA provide a 1500 byte MTU, everything else uses the device's 9000
> >> MTU, problem solved?
> >
> > Because the MTU (provided by RA) is assigned to the device.
> Ok, that severely limits the usefulness of this option I guess.
> 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.

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?


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