Udprelay

Udprelay implements a trivial symmetrical UDP relay. After start, it "opens" two UDP ports and waits for clients. When clients are connected, the tool sends data received from one of them to another and vice versa. Typical use case is to allow UDP clients behind different NATs to talk to each other.

One (and maybe the only) application is to help Mosh client to connect to Mosh server behind NAT while staying behind another NAT. Details are described in this StackOverflow answer.

Usage

To build, just make install. The only mode of operation is:

$ udprelay bind-ip port-a port-b

where bind-ip is an IP to bind both ports to (like 0.0.0.0), port-a and port-b are UDP ports to wait messages from.

Limitations

• No security. The encription is up to user.
• Absolutely no security. Application doesn't attempt to track port-a and port-b remote credentials. Instead, it just sends one hand's data to the last seen address of another hand.
• Udprelay drops packets from one hand before it knows the remote credentials of the other hand.

Debugging

The following tools may be usful:

• strace
• tcpdump -i IFACE udp portrange PORTA-PORTB -vv -X -n

Credits

Based on udp_redirect C program by Ivan Tikhonov