recv | xmit | via {ifX | if* | ipno | any}
Matches packets received, transmitted or going through, respec-
tively, the interface specified by exact name (ifX), by device
name (if*), by IP address, or through some interface.

The via keyword causes the interface to always be checked. If
recv or xmit is used instead of via, then only the receive or
transmit interface (respectively) is checked. By specifying
both, it is possible to match packets based on both receive and
transmit interface, e.g.:

ipfw add deny ip from any to any out recv ed0 xmit ed1

The recv interface can be tested on either incoming or outgoing
packets, while the xmit interface can only be tested on outgoing
packets. So out is required (and in is invalid) whenever xmit is
used.

A packet might not have a receive or transmit interface: packets
originating from the local host have no receive interface, while
packets destined for the local host have no transmit interface.