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user1046658
Asked: 2020-06-23 20:16:18 +0800 CST

Confusion about nc -l and nc -lp

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In the manpage for nc -l, it says:

It is an error to use this option in conjunction with the -p, -s, or -z options.

But most examples I see online using this command use nc -lp [port].

Can you use -p with -l in nc? And with just nc -l [port], is that the correct usage of it because when I connect to it on another terminal with nc [ip address] [port], it works. Also, when I use nc -lp [port] and connect to it with nc [ip address] [port], it also works. Is there a difference between these two commands?

command-line netcat

2 Answers

  1. Best Answer

    Note that there are two netcat packages. And the options are not the same for the nc commands they provide:

    netcat-traditional shows this command as one of the main examples:

    nc -l -p port [-options] [hostname] [port]

    While netcat-openbsd states:

     -l      Listen for an incoming connection rather than initiating a connection to a remote
         host.  The destination and port to listen on can be specified either as non-optional
         arguments, or with options -s and -p respectively.  Cannot be used together with -x
         or -z.  Additionally, any timeouts specified with the -w option are ignored.

    So yes, both commands do the same, but you might run into examples that do not work with your version of netcat. For example -e.

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  2. When you are using the listen mode -l you must specify port. For example:

    nc -l 3000  # Listen at port 3000 to all network interfaces

    nc -l4 3000  # Listen at port 3000 to all IPv4 network interfaces

    nc -l6 3000  # Listen at port 3000 to all IPv6 network interfaces

    So actually you do not need an additional option like -p to instruct the nc command at which port to listen. The examples that uses -lp option probably are based on an old version of nc or the writers didn't read the manual like you :) Also probably when the -lp combination is used the -p option is just omitted.

    In addition, here is how to create a listen service, based on nc: How to listen new port Ubuntu Server from command line?

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