How to Listen for Multiple Tcp Connection Using Nc

How to listen for multiple tcp connection using nc

Simultaneous connections are not possible with netcat. You should use something like ucspi-tcp's tcpserver tool or leverage xinetd since you're on Linux.

See: https://superuser.com/questions/232747/netcat-as-a-multithread-server

Consecutive connections could be handled through a shell script that restarts netcat after it finishes.

Linux - Receive data via UDP; accept TCP connections, pass the data to the clients

If it is not bi-directional traffic you could use the following:
nc -l -u -p 20001 | ncat -k -l -p 30001

Learning from this post: How to listen for multiple tcp connection using nc it is not possible to connect multiple TCP streams to nc so they advice to use ncat.

How netcat can listen to the same port on the same host from two different terminals?

The -p option specifies a source port, not a listening port.

The -l option puts netcat into listening mode.

In your example, 1234 is the input value for the -p option, not the -l option, which means there is no explicit listening port being specified. If netcat is not failing, then most likely netcat is binding to port 0 instead, which tells the listening socket to bind to a random available ephemeral port. As such, your two netcat instances would actually be listening on different ports. Use netstat to verify.

According to the Linux manpage for netcat:

-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.

-p source_port

Specifies the source port nc should use, subject to privilege restrictions and availability. It is an error to use this option in conjunction with the -l option.

So technically, your example may not be valid to begin with.

However, on some systems, including some Debian installs, depending on which flavor of netcat you use (in particular, the traditional flavor), you actually may need to use -l and -p together, but you need to swap their order to specify a listening port correctly, eg:

nc -l -p 1234

Read and write to the same netcat tcp connection

The correct way to do this in UNIX is to make use of a back pipe. You can do so as follows:

First, create a pipe: mknod bkpipe p

This creates a file named bkpipe of type pipe.

Next, figure out what you need to do. Here are two useful scenarios. In these, replace the hosts/addresses and port numbers with the appropriate ports for your relay.

To forward data sent to a local port to a remote port on another machine:

 nc -l -p 9999 0<bkpipe | nc remotehost 7000 | tee bkpipe

To connect to another machine and then relay data in that connection to another:

 nc leftHost 6000 0<bkpipe | nc rightHost 6000 | tee bkpipe

If you simply need to handle basic IPC within a single host, however, you can do away with netcat completely and just use the FIFO pipe that mknod creates. If you stuff things into the FIFO with one process, they will hang out there until something else reads them out.

Using BusyBox version of netcat for listening tcp port

Here's the manual page for busybox's nc implementation.

The correct syntax is

nc -l -p <port>

The issue is, I think, that your version of busybox is compiled without nc listening capabilities. Indeed there's a config option at build time, NC_SERVER, that needs to be enabled to turn that feature on.

Can you build another nc, perhaps from this version, and copy the binary onto your embedded host? You may need to build a cross-compiler environment.

How can I modify the netcat command line to send many requests at once

No need to modify nc; just pipe your input to it: yes hello | head -n 1000 | nc localhost:8080.

Netcat uses different port than requested

It looks like you are using traditional netcat which requires providing -p argument for the listening port:

netcat -l 127.0.0.1 -p 33333

From nc -h:

   -p port                 local port number

Syntax you use would work with OpenBSD netcat.

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