Default public listening port amounts to an open proxy

[rvalles]

By default, even if just a serial port is specified, tcpser will offer its services at 0.0.0.0:6400.

This is not the best default, from a security concern, as the service offered is equivalent to an open proxy.

Default behavior could be improved by e.g. listening only on the loopback interface or even not listening at all.

[go4retro]

Given how long the tool has been used and expectations, I think changing default behavior would break quite a few things. But, I think adding some verbiage to use -p0.0.0.0:6400 to minimize security for outbound only uses would be fine.