Subject: PPPoE and the MTU/MSS problem
To: None <>
From: Michael Kukat <>
List: netbsd-users
Date: 07/09/2001 18:56:10
Hallo all,

hope, this is the right place to ask it. I have the following problem:
My firewall at home is an old 486 running NetBSD-1.5. For now, it had an ISDN
router connected, but some weeks ago i switched this over to DSL. Worked fine,
but i wondered about several server never sending me a reply. So i searched
the web about this and found out the reason for this in the setting of the
MSS option in my TCP packets. tcpdump told me what i thought, it's really 1460
bytes, even if the MTU of ppp0 is set to 1492, so MSS should be 1452. Looking
into the kernel sources told me, the MTU is set to the highest MTU of all
interfaces in the system, if no interface is given (as it seems to be here).

Ok, so i can't do something against this. The other problem will be packets
originating from other machines in my network, i tested it on my FreeBSD box,
which will be switched off soon, the MTU of 1492 works very fine there over my

I heard of the rp-pppoe being very unstable, and so i use the mouse-pppoe,
another reason for this is the fact that i just like simple programs not being
bloated by unneccessary functions :)
And today i saw, there is PPPoE support coming into the current kernels.

So, what to do best?

a) switch over to rp-pppoe
b) implement MSS rewriting into mouse-pppoe
c) other nice hacks

Did someone had the opportunity to collect experiences for this thing, and
maybe even found a good solution?
tcpmssd didn't compile because of the missing divert support in the network
sources of the kernel.

Any hints welcome


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