Dynamic DNS update with m0n0wall behind router

Hello all,

Two questions about the behavior of RFC 2136 Dynamic DNS updates:

1. Does m0n0wall use the WAN interface address when sending the
update, or does it perform a check of the WAN IP using some external
service?

2. Are updates sent on a regular schedule or only when the WAN
interface address changes?

My m0n0wall gateway is located behind the ISP's router (Verizon FiOS).
It's a DMZ host, the second router just provides the physical
connection to the coax cable, but the m0n0wall WAN interface still
receives a local 192.168.100.x/24 IP. As far as I know, there is no
way for the m0n0wall to get a public IP assigned to the WAN interface.

The updates would only work for me if m0n0wall sends them on a regular
schedule (e.g. every 1 hour). Checking what the actual public IP is
first would make things easier, but since I'm writing my own RFC 2136
implementation anyway, I can get around this.

- Max