Setting firewall symbolic constants

[Walter]

In the example firewall rule set in rc.firewall, there are
the following lines:

# set these to your outside interface network
    oif="$firewall_simple_oif"
    onet="$firewall_simple_onet"

# set these to your inside interface network
    iif="$firewall_simple_iif"
    inet="$firewall_simple_inet"

Can these be set by the system automatically?  Specifically
$firewall_simple_onet?

When the IP changes on the ISP's side, I'd like to
have this detected and updated in the rules without my
manual intervention.  Do I need to write a utility and
run in crontab?  Or is there a better way?

I'm off-list, so please reply directly to this e-mail addy.

TIA.

Walter