Networking Puzzle

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Here is one for those of you that like a challenge:
I have a freebsd 7-current box, it has two interfaces rl0 (connected  
to wireless link) and rl1 (LAN)
rl0 has no addresses
I run PPPoE on rl0 which gives me an static IP address (lets call  
this 1.2.3.4) and Default Gateway.
I also have a /29 of public IP's which are routed through this address
the first address x.x.x.1 is assigned to rl1

The normal setup is a cisco router on the wireless link, and all  
computers route through it (but my cisco router is broken).

Any traffic originating from 1.2.3.4 and going to the outside world  
is blocked by an upstream firewall that I have no control over,  
anything in my public range has no upstream firewalling.

Sysctl is set to forward packets, and machines on the LAN with public  
ips in my range work as expected.

however if i do something like this:
ping freebsd.org
it fails because the packets automatically originate from 1.2.3.4

if I do this:
ping -S x.x.x.1 freebsd.org (thus setting the src address to a non- 
firewalled IP)
it all goes fine and the packets return.

Inbound connections (eg ssh) from the internet to x.x.x.1 work, but  
obviously any web access from my freebsd box fails.

My Question: How do i set the src address for all outbound packets  
originating on my machine to x.x.x.1 instead of 1.2.3.4 when they are  
passing through my pppoe tunnel?

BTW this is not a show stopper for me, I have placed an old PII  
machine between my server and the pppoe tunnel, which solves it. I'm  
just curious as to whether or not there is a solution.

Regards, Cian.
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