own ddns server, need help with dynamic client behind nat

[mouse]

I have my own public available dns server on a static IP.  I can configue
it to allow updates from a dynamic client.

Where I am having difficulty is where the dynamic client is attached to a
dsl adaptor which acts as a gateway for the client.  The dsl obtains a
dynamic address on it's WAN side, and issues IPs to the LAN.

updating the client ip address to my ddns server shows that it updates
with it's RFC 1918 address.  How do I get the client to recognize the
internet routable dynamic WAN IP of the dsl apaptor and update to ddns
server with that address?  what tool on the client will "get" the real
WAN address?

Both client and server are running debian.
The dsl adaptor is an actiontech.
The client will host email and web services.

Thanks in advance,
mouse