[Firehol-support] PPPoE TCP MSS clamping / match

Daniel Pittman daniel at rimspace.net
Wed Dec 10 13:00:19 GMT 2003

In order to use my ADSL connection, I need to clamp the MSS value on TCP
connections to something slightly lower than a standard Ethernet value.

Anyway, the "standard" way of achieving this for the in-kernel PPPoE
driver is to issue the following iptables command:

    iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
             -j TCPMSS --clamp-mss-to-pmtu

Now, I can obviously stick that in using the 'iptables' command, but
that is a bit less selective than I will want to be in the near future.

So, any chance of seeing an action to implement this?  I would suggest:

    tcpmss ( 'pmtu' | <number> ) <optional rule parameters>

If the argument is 'pmtu', pass the '--clamp-mss-to-pmtu' option,
otherwise pass '--set-mss <number>'.

Having it match on the 'router' statement, etc, would be ideal.

That way I don't need to fiddle with the interface matching and so on
when I have a PPPoE link and an 802.11 link heading out of the same box,
and I need to clamp the MSS on only one of them.


Reality is not as strong as perception. Perception all too often swallows
reality and spits it out in a new, unrecognizable form.
        -- Maytee Aspuro

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