Using netcat to send a UDP packet without binding
I am trying to use netcat to simulate a NAT traversal protocol.
I have one instance that is listening for UDP packets on port 6666, as so:
nc -ul 6666
In another terminal window, I am trying to periodically send a UDP packet from port 6666 (to open the return path on my router. this would be in a script that repeats every 20 seconds to re-open the port)
nc -u -p6666 mypinghost.com 4444
The problem is netcat fails on this ping call with the message:
nc: bind failed: Address already in use
Which implies that the listener having bound to port 6666 is blocking another process from sending from that port, or possibly that netcat is trying to bind to 6666 to listen.
Is this just how netcat is written, or can I tickle it some way to let me send a packet without binding to the port to listen?
Answer
nc -ul 6666
Listen at UDP port 6666.
nc -u -p6666 mypinghost.com 4444
Using UDP port 6666 as the source port, send to mypinghost:4444.
nc: bind failed: Address already in use
That would be on the second netcat invocation, where 6666 is already in use by the first one.
Which implies that the listener having bound to port 6666 is blocking another process from sending from that port
Correct.
or possibly that netcat is trying to bind to 6666 to listen.
And definitely that. You told it to do that, so it did it.
What you are trying to do is impossible between two processes in the same host. Only one process can use a specific local UDP port at a time, unless you use SO_REUSEADDRESS, which netcat doesn't appear to implement.
As the other poster has suggested, the solution lies in using a single process.