Linux Remove Previous nc Directive

Question

I need to use nc -lp4 5432 --allow 192.168.1.24 but I have a previous nc directive that only has it listening on localhost. It throws an error when I try overwriting it:

$ nc -lp4 5432 --allow 192.168.1.24
Ncat: Got more than one port specification: 4 5432. QUITTING.

How can I overwrite the previous instruction, or delete that listener? I've had a look around and I can't find how to get this removed.

Kamil Maciorowski · Answer 1 · 2022-07-11T05:03:31.273

2

$ nc -lp4 5432 --allow 192.168.1.24
Ncat: Got more than one port specification: 4 5432. QUITTING.

This has nothing to do with "previous" whatever. The error states that 4 (the option-argument for -p) and 5432 both specify the port, but nc that is about to listen (i.e. with -l) expects at most one port specified in the command line.

If you want to use 5432 and not 4 then either use -p 5432 or don't use -p at all. Any of these will work (one at a time), they are equivalent:

nc -l -p 5432 --allow 192.168.1.24
nc -l -p5432 --allow 192.168.1.24
nc -lp 5432 --allow 192.168.1.24
nc -lp5432 --allow 192.168.1.24
nc -l 5432 --allow 192.168.1.24

edited Jul 11 '22 at 05:03

answered Jul 11 '22 at 04:25

Kamil Maciorowski

74,068

[Mon Jul 11 02:17:11 root@server_n ~] nc -l -p 5432 --allow 192.168.1.24 Ncat: bind to :::5432: Address already in use. QUITTING. The port seems already bound. – Rich_F Jul 11 '22 at 06:18
1

@Rich_F How do I find (and kill) process running on a certain port? – Kamil Maciorowski Jul 11 '22 at 06:21
Interesting there is no command to overwrite or delete it. Thank you. – Rich_F Jul 11 '22 at 06:25
1

@Rich_F A new nc is a standalone process, not a command line interface for some "nc service" that could be reconfigured. – Kamil Maciorowski Jul 11 '22 at 06:44
Mine is up and down. I get connected, then disconnected on another try. That did kill the process though. Is there a way to make this permanent? – Rich_F Jul 11 '22 at 06:46
2

@Rich_F Let's not go chameleon. Ask a new question if you need. I'm not sure what you're describing, maybe this will be a helpful start: your listening nc accepts and processes exactly one connection, then it exits, it's by design. – Kamil Maciorowski Jul 11 '22 at 06:50
OK, that's fine. I thought this was a permanent listener. – Rich_F Jul 11 '22 at 06:51
@Rich_F Oh, this. There is nc -k. See man 1 nc. – Kamil Maciorowski Jul 11 '22 at 06:55
@Rich_F If you need something more complex, then use socat. See this and this. – Kamil Maciorowski Jul 11 '22 at 07:05
Just trying to get a postgresql port alive and listening 24/7. I don't think nc is an option. I'm sure there's a true way to get this to work. I will keep socat in mind. – Rich_F Jul 11 '22 at 07:16

Linux Remove Previous nc Directive

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