Find external router IP from Arduino Yun inside router network I want two Arduino Yuns in different private networks to communicate, i.e., behind two distinct routers of which at least one does not have dynamic DNS, so its external IP may change from time to time. 

To be reachable, this Yun could forward its router's external IP to the other Yun which has a fixed DNS name. But for this to work, it must be able to determine its router's external IP. 


How can the Yun find out that the external IP of the router has changed? The Yun will be runnning all the time. 
If the external router IP has changed (i.e., my Fritz.Box disconnects in the night and may receive a different IP when it reconnects), how can the Yun determine the new external IP? 


If this is not the right place to ask, I'm happy for pointers to somewhere else (Server Fault SE forum?).
You can write a program that updates its own external IP every couple of minutes/hours, or when it doesn't get a response from another Yún/PC/server.

You can use this link (or find another site of the web that gives you your external IP address): http://www.realip.info/api/p/realip.php

This link has a beautifully formatted information that you can easily extract. I have a similar solution on one of my servers that have a dynamic IP...