LD_PRELOAD hack to redirect connections to other ports. When started with otherport, binaries will believe they are connecting to OLD_PORT, while actually connecting to NEW_PORT.
Currently it rewrites only sendto and recvfrom.
$ make
$ OLD_PORT=53 NEW_PORT=10053 LD_PRELOAD=$PWD/otherport.so ...
Note: all packets received on NEW_PORT will be rewritten to look like they were received from OLD_PORT, so NEW_PORT becomes effectively unusable.
For example, it lets you test DNS resolvers against servers running on high ports instead of port 53.
$ OLD_PORT=53 NEW_PORT=10053 LD_PRELOAD=$PWD/otherport.so unbound-host google.com
In this case, unbound-host will transparently connect to all DNS servers on the port 10053, allowing root-less tests.