Last week, Oskar Stenberg asked on Unix & Linux if it were possible to configure connectivity between two networks, both using the same address range, without involving network namespaces. That is, given this high level view of the network… …can we set things up so that hosts on the “inner” network can communicate with hosts on the “outer” network using the range 192.168.3.0/24, and similarly for communication in the other direction?

In this question, August Vrubel has some C code that sets up a tun interface and then injects a packet, but the packet seemed to disappear into the ether. In this post, I’d like to take a slightly extended look at my answer because I think it’s a great opportunity for learning a bit more about performing network diagnostics. The original code looked like this: c original sendpacket.c #include <arpa/inet.h> #include <fcntl.