Specifically:
https://github.com/jamesmcm/basic_tcp_proxy/blob/e3adcff17b46ecd871ff7d6f785de987d4945272/src/lib.rs#L36
fails to bind when a connection is made (even if the connecting process is also running as root).

This seems to be some sort of socket permissions issue.