i've been tinkering with allowing off-site connections to my Jellyfin server from unknown IP addresses (yet expectedly legitimate users) using 'knockd' ..
the premise is basically that you use an app (i use a Android app for convenience called "Knock on Ports") that sends x amount of tcp connections to a server in a specific order which knockd can watch for. if they're done correctly, and in the right order, knockd runs a firewall command that allows the IP that the 'knock' came from to connect to the Jellyfin server.
for example, let's say Knock on Ports is configured to connect to a server
once on port 12345
once on port 54321
once on port 11111
these aren't actually open ports knockd just sees the failed connections to them.
knockd sees those come in that order (with a set timeout) and runs something like:
iptables -I INPUT -s %IP% -p tcp -j ACCEPT
et voila, the user can connect to the server through the firewall. for a personal implementation you could also do the reverse (the full command but -D INPUT instead) to close it off again, for public wifi or whatever.
but i was thinking this might be useful for BBSes. perhaps come up with a simple algorithm that takes the provided hostname's first three letters (or so) in ascii and converts it:
c - ascii = 099 + 2000 = port 2099
f - " = 102 + 1000 = port 1102
b - " = 098 - 3000 = port 2902 (abs)
b - " = 098 - 2000 = port 1902 (abs)
etc. or just send a syncterm specific set (maybe just 'sync'). probably avoid using subdomains like 'bbs' or in the case of synchro.net, specifically use them. *shrug*
if a BBS in your dialing directory is configured to use it ("knock first" turned on or whatever) then it could send the sequence of connections first to tell the BBS to open the port up.
it looks like there's a Windows implementation that uses python:
https://github.com/KJ-black/windows-knock
so it would seem Windows has gotten past needing npcap for these sorts of things (or hidden in the Windows Python implementation?), or the capability of watching for connections on ports you haven't opened seems to be there already?
the popular linux one is here:
https://github.com/jvinet/knock
--- Mystic BBS v1.12 A47 2021/12/25 (Windows/32)
* Origin: cold fusion - cfbbs.net - grand rapids, mi