You are reading this file because you want to build a new copy of the LwIP library for use in ZeroTier.
Subdirectories:
ports/ -- contains ports for various architectures (for our purposes, unix)
In order for the Network Containers feature to work in ZeroTier, a copy of the LwIP libary is needed since we dynamically load it into memory. You can build a new copy of the libary by going to /contrib/ports/unix/proj/lib and running make.
This will generate: liblwip.so
You can enable LwIP debug traces by adding the flag -DLWIP_DEBUG See additional debug info here: http://lwip.wikia.com/wiki/Debugging_lwIP