FreeBSD based frewall on ADSL link with /29 subnet

[Raymond Wagner]

For a number of years, I have had an ADSL connection using a Cisco 675 modem
in NAT mode.  My ISP gives me a /29 subnet, which results in 6 available
external addresses.  Since the modem was running NAT, I could only use the
public address attached to the modem.  A few weeks ago, I switched over to a
FreeBSD based router and decided to make use of those extra addresses.  I
added another 4 aliases to my external interface and set natd to redirect
two of my inside machines onto two of the available addresses.  I restarted
ipfw and everything works great.

Now the problem.  I use www.dyndns.org to keep track of my public IPs and I
use ddclient on the firewall to automatically update them if needed.  I can
only locally see 10.xxx address space internal addresses that my ISP
assigns, so I have to use checkip.dyndns.org to figure out what my public IP
is.  The addresses are randomly dynamically assigned from one of 5 class B
subnets, so I can't just count up from the first address.  I can run dyndns
clients on the two internal machines, but I would rather keep all network
related activities contained on the firewall.  ddclient still works fine on
the firewall, but it can only update the primary address.  How do I get
ddclient (or some perl script or other program capable of parsing a website)
to access the internet through one of the aliased addresses, rather than
through the main address?

Thanks,
Raymond Wagner