Subject: can't raise in kernel pppoe0 with mtu > 1460
To: None <,>
From: Alicia da Conceicao <>
List: port-i386
Date: 03/21/2005 03:58:46

For some reason, whenever I attempt to increase the MTU on my in kernel
pppoe interface beyond 1460, it fails:

        # ifconfig pppoe0 mtu 1460
        # ifconfig pppoe0 mtu 1461
        ifconfig: SIOCSIFMTU: Invalid argument

Note that pppoe0 is tried to physical ethernet interface fxp0, which
itself has a MTU of 1500.

However, other PPPoE implimentations can use MTU values as high as 1492.
Even the NetBSD PPPoE documentation at:

uses 1492 as an example MTU value for in kernel PPPoE.

Any ideas as to why I can't use the higher MTU value for the NetBSD in
kernel PPPoE?  The reason why this is a problem is that Windows clients
(running WinXP or Win98SE) behind a NetBSD NAT firewall are unable to
use Windows Update or down load large files from some websites.  In my

map pppoe0 -> portmap tcp/udp 20000:40000 mssclamp 1420
map pppoe0 -> mssclamp 1420
map pppoe0 ->

Unfortunately, higher mssclamp values (including 1430 & 1440) do not
work with my pppoe.

Thanks in advance.