[PLUG] How to detect ppp address change remotely?

[Tom Sharples]

Hello all,

We have several installations of embedded wireless routers (running 
slack on a 2.4 kernel) in which the gateway router_1 uses a Verizon 3G 
USB modem (ppp0) as its connection to the outside world, and one or more 
downstream router_2 have 5.8Ghz wireless links to router_1, and thus 
route traffic to the outside world through the same ppp0 3G connection.

Verizon has a habit of constantly changing the ppp0 IP address often 
sometime several time per hour. I would like to figure out a way that 
the downstream router_2 can be made aware of that ppp0 address change at 
router_1 within a minute or two of it happening. The obvious way seemed 
to be running a cronjob traceroute from the router_2 to a known outside 
IP, with a limit of two of three hops, and grepping the first public IP 
and looking for the change. However, for some reason that shows the 
Verizon gateway router on their tower as the first public IP rather that 
the locally assigned ppp0 public IP address. Any suggestions for how to 
determine that ppp0 IP using standard tools and pref. without writing 
e.g. inter-router communications scripts etc. would be welcome.

verizon_tower------------------[3G_modem(ppp0)]-[router_1]---------------[5.8Ghz_link]----------------[router_2]

Thanks,

Tom S.