whatismyudp.c

This is a UDP server that simply responds to any packet with the sender's IP address and port number. Internet users can use this to discover their global IP address/port mapping, so they can attempt NAT traversal for P2P applications. This is a very limited subset of the functionality of the STUN protocol, and is intended to be as simple as possible so that it can be used even with just netcat in a terminal. The response packet format is in ASCII: host " " port "\n" - the source IP address followed by a space, followed by the source port, followed by a newline. The protocol supports IPv4 and IPv6.

Public deployments:

• celehner.com:7777
• west.celehner.com:7777

Example session: client runs netcat command, presses enter to send packet, receives reply, and then quits the program:

$ nc -u -p 8888 celehner.com 7777

104.156.229.84 8888
^C

Note: some netcat implementations are IPv4-only, and may have a separate IPv6 version, like netcat6/nc6. Other netcat implementations let you specify one or the other with a -4/-6 option.

#udp #p2p #c

@the Queen of England

Did you try both IPv4 (nc -4) and IPv6 (nc -6)? Also, did you press enter after starting the netcat command? (The server can't write first.) Maybe try pressing enter a few times, as packets could be dropped. Otherwise maybe your network just doesn't allow packets on these UDP ports.

@Arthur did you make sure to press enter after starting netcat -u?
It doesn't send any packet until you give it some data to send - even just one newline byte from pressing enter.