A trick I learned a long time ago for linux and other *nix systems is to use fuser to check what’s listening. It’s actually a more generic file based command but most versions also allow you to check out sockets too. The standard syntax is
fuser -v 81/tcp. That will display a list of the processes listening to the port.
It’s actually a part of the
lsof list of open files functionality generally useful on *nix.