Steve Bellovin <smb@research.att.com> writes:

> Is there any easy way to change the MTU and the TCP MSS? I'm
> (temporarily) stuck behind a broken gateway that is dropping "large"
> packets, so I want to crank down the TCP MSS. I can change the
> interface mtu via ifconfig, but that doesn't seem to change TCP's
> idea of how large a segment it wants.

Just a "me too" - we're seeing MTU blackholing due to a gif tunnel
(MTU 1280) carrying iBGP, and the usual external factors.

(from memory - this was a couple weeks ago)

The obvious convenient workaround seemed to be to set the MTU on 
Ethernet perimeter interfaces to that of the tunnel, but as noted, the
MSS hint sent during TCP setup by our NetBSD-1.5 zebra routers did not
decrease after the change. It did not help to delete cloned routes or
to try to set a per-route MTU.