PPPoE mtu overwrites/ignores

[Valdrin MUJA]

Hello misc,

I try to change mtu size of my pppoe client but somehow that value returns to 1492 after getting ip address from ISP.
I've opened a ticket them and got replied as '' you could use mtu up to 1600.'' So no limitation at their side...

I have simple pppoe config:

inet 0.0.0.0 255.255.255.255 NONE mtu 1550 \
pppoedev em0 authproto pap \
authname '***@intisp' authkey 's3cr4ty3s' up
dest 0.0.0.1
!/sbin/route add default -ifp pppoe0 0.0.0.1

Em0 mtu size is 1558

As a grumpy person, I didn't believe at them and quickly installed npppd into another computer and used it as pppoe-server but nothing changed. (I've set mru as 1550 at npppd.conf)

I use OpenBSD 6.9 with 19 April iso image.

Is this a bug or am i missing something?

Thanks in advance.

[Todd C. Miller]

Note that pppoe caches the MTU value of the parent device (em0 in
this case) so if you increased the MTU of em0 after pppoe0 has been
configured it probably didn't have an effect. You can tell this
is what happened by ifconfig failing with an invalid argument error.

You can also check your kernel messages (dmesg) to see if there is
an error like "No valid PPP-Max-Payload tag received in PADO". If
you see this, then the ISP or telco's equipment probably doesn't
support RFC 4638 and you can't do baby jumbos with pppoe.

- todd

[Stuart Henderson]

Post by Valdrin MUJA
As a grumpy person, I didn't believe at them and quickly installed npppd into another computer and used it as pppoe-server but nothing changed. (I've set mru as 1550 at npppd.conf)

npppd isn't a valid test as it does not support RFC 4638.