Netcat – Listen for Multiple UDP Connections

linux-networkingnetcatnetworkingudp

I'm trying to use netcat to debug SIP calls, which communicate over UDP.

Is there a way to get the same netcat instance to listen to multiple UDP Connections? I've tried connecting to the same netcat session from two different hosts and the second is always unable to connect, even when I send an EOF from the first.

I'm aware that UDP "Connection" is a misnomer and that no such thing exists, am I thinking about this in the wrong way?

I'm version agnostic as I have both OpenBSD and Ubuntu "traditional" (GNU Debian) netcats installed.

I've tried the -k flag in OpenBSD to no avail.

Any ideas are appreciated.

Best Answer

The author of the ncat tool, in his page for netcat users notes that:

By default, Ncat will accept 100 simultaneous connections, instead of 1 like the original netcat. Use -m to specify the maximum number of simultaneous connections.

I rather suspect this means that netcat won't do what you want, but there's a simple drop-in replacement tool which will.

Related Solutions

Listen to UDP data on local port with netcat

To quote the nc man page:

-l Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host. It is an error to use this option in conjunction with the -p, -s, or -z options. Additionally, any timeouts specified with the -w option are ignored.

The key here is that -p cannot be combined with the -l flag. When using the -l flag, any ports specified in the positional arguments are used. So instead, you could use the following:

netcat -ul 2115

Linux – Netcat UDP File Transfer

Try it like this:

nc -u -l 7777 > newfile.jpg #on the destination machine
cat file.jpg | nc -u 192.168.x.x 7777 #on the source machine

Usually you want the machine getting the file to "listen" (run that first), and when it's listening, send the data over udp. UDP does not have a 'handshake' sequence, and packets are sent immediately, even if noone is listening*.

*sometimes you get an ICMP packet, that the port is closed (unreachable), but you cannot depend on that (firewalls etc.)

Best Answer

Related Solutions

Listen to UDP data on local port with netcat

Linux – Netcat UDP File Transfer

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