I have a router running Linux that acts as the central gateway to the Internet for a network of embedded devices that use IPv6 to communicate. I would like to intercept, inspect, and rewrite every packet that goes in and out from the Internet to this subnetwork. The reason I need to do this is that I want to create a transparent (to both ends) proxy service that will redirect packets, ingoing and outgoing, to a cache that lives elsewhere depending on the packet content.
These devices communicate using their own application protocol that runs over UDP so I can't re-use something like Squid straight out of the box. Is the best place to start somewhere like libpcap? Or should I start from the top and rewrite Squid? This is the first time I've had to really dive deep into networking, and I'm unsure where to begin.