I update my DynDNS.org IP from my Linux box which knows nothing about
the external IP address and it works just fine. I use the packed
version of ddclient in debian. In /etc/ddclient.conf, all you need to
do is add a line like:

use=web, web=checkip.dyndns.org 

instead of the default. The checkip server is similar to
whatismyip.com...it returns the IP it is getting a connection from.
The debconf configuration doesn't quite get the config file correct
when you use the web interface thing, so edit the conf file yourself.

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
 A password is like your underwear; Change it
 frequently, don't share it with others, and
     don't ask to borrow someone else's.

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