ipfw2/dummynet set new mss on thy fly

Louis A. Mamakos louie at TransSys.COM
Fri Feb 6 17:11:35 PST 2004

Can't you write a rule that only diverts packets with the SYN bit set?
It's the only time a TCP MSS option is allowed to be set.  It doesn't
seem like there's any reason to send all the traffic through tcpmssd.

(And the problem isn't a "wrong" windows implementation, it's broken
Path MTU discovery due to overzelous filtering of ICMP.)


> Hi
> Is it possible to modify the mss with ipfw2/dummynet? This is necessary 
> to fix wrong window implementations for example for adsl providers.
> Something like "modify mss=1452 tcp from any to any out recv em0 xmit 
> em1 tcpflags syn". (Of course this string is wrong, it's just to give 
> you an impression).
> I know there is tcpmssd port but it uses divert. I've to modify every 
> packet on gigabit lan (at least 200mb/s traffic). So tcpmssd is a bit to 
> slow.
> We already have a ipfw1 patch and It's not that difficult to port it to 
> ipfw2. But I'm not sure if ipfw2/dummynet already have something similar 
> implemented by default.
> Any hints?
> cheers,
> Thomas
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