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List:       openbsd-misc
Subject:    Firewall MTU with PPPoE/DSL
From:       Dan Weeks <danimal () danimal ! org>
Date:       2001-01-31 18:33:14
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I have a firewall running OpenBSD.  Unfortunatley I have to connect to the
DSL service using PPPoE.  The problem is that all TCP traffic has to be
encapsulated in a PPP datagram and then put into the Ethernet packet, thus
causing more overhead and fragmented packets if you have too high of an MTU
on an internal host.  Well, I can adjust the MTU for all my internal hosts
manually to be the correct value, but what I am looking for is a way to
have the firewall handle such operations.  

I see that FreeBSD has something called "tcpmssd" that corrects the MSS of
outgoing packets via the firewall rules (see
http://www.daemonnews.org/200101/pppoe.html section 6.3).

What I would like to know is if there is something like that in OpenBSD.
Will I have to attempt to port this from FreeBSD? (using FreeBSD is not an
option).

If anyone has any suggestions on ipf/ipnat rules to set this to work please
let me know.  I will summarize to the list later.

-danimal

-- 
dan weeks - propellerhead - http://danimal.org

Build a man a fire, and he's warm for a day.
Set a man on fire, and he'll be warm for the rest of his life.

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