IPFW via command problem

[Ian Moore]

Hi,
I'm setting up a firewall on a dual homed host for an ADSL connection.
I'm using 4.8-RELEASE, cvsupped to the latest RELENG_4_8 as of late October 
2003.
I've followed the instructions at 
http://www.mostgraveconcern.com/freebsd/ipfw.html for setting up the 
firewall, which supposedly work, but seem to be having a problem with the via 
command
eg lines like 
# Allow outgoing pings
${fwcmd} add pass icmp from any to any icmptypes 8 out via ${oif}
${fwcmd} add pass icmp from any to any icmptypes 0 in via ${oif}

where I have defined ${oif} as
oif="xl1"
where xl1 is my external interface

The above lines don't allow pings to the outside world, but if I comment out
via ${oif} then it does allow them.
This seems to be true of all the lines with via in them - they don't work.
I tried substituting the actual IP address for the interface name, but that 
didn't work either.
I can't figure out why via doesn't work - anyone have any clues as to why it 
wouldn't function?

Cheers,
Ian