In message <m0zhM7C-0009LRC@most.weird.com>Greg A. Woods writes
>I currently cannot get e-mail from at least one known
>site unless either they fix their firewall or I find some way to ensure
>the path MTU is 1500 bytes all the way through.  Since I can't control
>their firewall, and I have other reasons not to want to change the path
>MTU in this case, I want to find some way to disable PMTUD on my end.

You can turn off path MTU discovery on NetBSD with the sysctl
(this defaults to off as shipped, anyway):

	% sysctl -w net.inet.ip.mtudisc=0

If you just want to turn off PMTUD to a single host, we can do that, too:

	% route add 198.41.0.6 204.92.254.x -lock -mtu 0

I realize that the problem is on the other end, and that you really
can't do much to turn off PMTUD on the connecting host, alas.
Perhaps we could come up with a way to force the NetBSD system to
send a small MSS (536 was the pre-PMTUD default) you'd be able to 
ensure that the other side would send reasonable packets.

It still seems stunningly lame for a site to turn on PMTUD when it
is behind firewalls that default PMTU!  Yeesh.

Kevin
kml@nas.nasa.gov