<blockquote>Tailscale is a mesh VPN alternative, based on WireGuard, that connects your computers, databases, and services together securely without any proxies.</blockquote>

(Possibly private) API to get requesting IP and location

The service implements a multi-purpose channel via HTTP. The sender send any data through a POST request and the receiver can GET it. Pubsub is also supported.

Provide an IP and a port, this service try to access that port and report if that port is open (like nc from public internet).

A database about required open ports for various applications. Useful for setting up firewall and port forwarding rules.

ftp ipv4.cat
telnet ipv4.cat
ncat ipv4.cat
curl ipv4.cat
http://ipv4.cat

Vendor <=> MAC address