Re: IPv6 and PPPoE with MSSFIX

[Hugo Slabbert <hugo.slabbert@menlosecurity.com>]

> In my case, the PPPoE interface got MTU=1480. They might be stacking
> something else on top of it or PPPoE might have optional fields. I
> read somewhere that PPPoE might use either 8 or 20 bytes, but I'm not
> an expert on PPPoE.

For ref, an L2TP + PPPoE stack isn't too uncommon, and gives you 20
bytes overhead from the 12 bytes L2TP + 8 bytes PPPoE.


On Wed, Aug 23, 2023 at 12:02 PM Luiz Angelo Daros de Luca
<luizluca@gmail.com> wrote:
>
> > Hi,
>
> Hi Daniel,
>
> > On Wed, Aug 23, 2023 at 04:58:40PM +0200, Marek Küthe wrote:
> > > PPPoE adds 8 bytes of overhead so that an MTU of 1432 can be used. I
> > > also have to do this at home with my DSL line for example.
> > > The MTU should be set on each side (on both peers) for this to work.
> >
> > Oh, I just realized I used the 1432 MTU in my earlier reply based on
> > Marek's math but since Luiz's underlay network is IPv6 this is not actually
> > correct. MTU=1440 is only correct on top of IPv4, for IPv6 the "optimal"
> > MTU is 1420 so with PPPoE involved that's MTU=1412.
> >
> >   1500 Ethernet payload
> >    -40 IPv6 header
> >     -8 UDP header
> >    -32 Wg header
> >     -8 PPPoE
> > ===================
> >   1412 wg tunnel MTU
>
> In my case, the PPPoE interface got MTU=1480. They might be stacking
> something else on top of it or PPPoE might have optional fields. I
> read somewhere that PPPoE might use either 8 or 20 bytes, but I'm not
> an expert on PPPoE. If I don't control both sides, I would use 1400 by
> default.
>
> > --Daniel