ipfw -- selecting locally generated packets

[Jeff Kletsky]

 From time to time, I rewrite my firewall rules to take advantages of 
the ever-improving set of features that ipfw provides. One of the 
challenges I have faced in the past was selecting packets that are 
generated on the firewall host itself, as opposed to those that it 
received through an interface.

While I find most of the Linux firewall implementations untenable for a 
variety of reasons, it does provide differentiation between what they 
call "OUTPUT" and "FORWARD". I'm looking to see if there is a "better" 
way to implement this kind of selection with the 11.1 version of ipfw.

"out and not in" may years ago seemed an obvious selector, and it's good 
to see that it is now clearly documented that it doesn't work in "man 
ipfw" with "(in fact, out is implemented as not in)".

"not recv any" doesn't seem to be helpful either

     $ sudo ipfw add 64000 count ip from any to any out xmit any not 
recv any
     64000 count ip from any to any out

In the past, I've tagged all incoming packets and used that tag to 
differentiate between the two.

Is there something "cleaner" (or perhaps clearer) that using a tag in 
that way?


TIA,

Jeff