Recent discovery: forwarding serial ports over the network is pretty easy. Useful for embedded development!
For example, if you're working with an ESP8266 SDK in a Linux VM, but the device is attached to a FreeBSD machine: run something like
doas python2 ~/rfc2217_server.py -p 8266 /dev/cuaU0 on the host and use something like
rfc2217://192.168.1.2:8266 as the
--port argument to esptool.
(Yeah, use that script. Other RFC2217 implementations don't seem to support binary mode for some reason.)