Finding the external IP on a DSL router?

[Edward]

> #!/bin/bash
> #
> # 11-01 Pete Nesbitt
> # script to check if eth0's IP has changed.
> #
> 
> DATE=`/bin/date`
> 
> # set variables for email
> EMAIL="user at domain.com user at otherdomain.com"
> 
> # a file to hold the previous IP
> OLDIP="/usr/local/scripts/check_ip/oldip.txt"
> 
> # check for the current IP
> CURRENT_IP="`/sbin/ifconfig -a|/bin/grep -A 2 eth0|/bin/awk '/inet/ { print $2 
> }'|/bin/sed -e s/addr://`"
> 
> # compare to new IP to last IP
> if [ "`/bin/cat $OLDIP`" != "$CURRENT_IP" ]; then
> 
> # next part is all one line
>   /bin/echo "The IP was reset at $DATE (ea 20 min.). The OLD IP was `cat  
> $OLDIP`. The New IP is $CURRENT_IP. You must update
> yi.org and hostname."|/bin/mail -s "New IP Address" $EMAIL
> # end if the long line
> 
> # copy the new IP to the old IP
>   /bin/echo "$CURRENT_IP" > $OLDIP
> fi
> 
> exit

Just a side note Pete - I'm by no means a scripter, but where does the 
above script find the router's external IP address?

What you said before holds true - if you set up dyndns or equivalent, 
you can ping the address to get the outside ip, but your script (to my 
untrained eyes), seem to grab the IP information from ifconfig.

If I run ifconfig -a, it shows a local address of my gateway as the inet 
address on eth0, and my outside address on ppp0.

But if he's using a router, would his gateway not be a local address?

You're probably right though, as I don't know much about these things, 
but I just want to make sure.

Regards,
Ed.