ipfw and divert and trying to do something clever

[Bruce Campbell]

I have some machines behind a freebsd firewall, and I'm using ipfw.

Presently, I reset attempts to smtp past the firewall:

  reset tcp from [subnet] to any 25

but I'd like to divert them to my own smtp server, so it doesn't
matter what the clients try to use.

I thought this would be easy.  Maybe it is.

The "fwd" feature doesn't seem to do it, as it just forwards a
specific ipaddr[,port] (no subnet/mask)

"divert" looks like the way to do it, and after a few hours of
fiddling with a program that opens a divert socket, I can watch
all manner of traffic going back and forth, but each time
I attempt to send it elsewhere, I get nowhere.  I am duly
setting both the ip and tcp checksum, before re-injection.

Somebody else must have done this, and/or I must be doing it
the wrong way.

Any suggestions ?  Please e-mail me directly also as I am
not on this list.  A code snippet using divert would
be excellent.

-- 
Bruce Campbell
Engineering Computing
CPH-2374B
University of Waterloo
(519)888-4567 ext 5889

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