How can I configure netcat (or some other stock linux utility) to listen on a specific port on a secondary IP address?

Question

CentOS 5.9

For testing purposes, I want my CentOS server to listen on a secondary virtual IP (eth0:0). I'm familiar with nc -l -p <port> but it only listens on the primary. Is there a way I can specify a specific IP for the listener to use?

If not, is there another "stock" utility in CentOS 5.9 that can do this?

Hauke Laging · Accepted Answer · 2020-05-13T20:02:47.740

40

The syntax depends on the netcat package.

netcat-openbsd

nc -l 192.168.2.1 3000

netcat-traditional

nc -l -p 3000 -s 192.168.2.1

A simple way (at least in bash) for telling them apart in scripts is:

if ldd $(type -P nc) | grep -q libbsd; then
    nc -l 192.168.2.1 3000
else
    nc -l -p 3000 -s 192.168.2.1
fi

edited May 13 '20 at 20:02

answered May 31 '13 at 16:34

Hauke Laging

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This is exactly what the man page says. – Michael Hampton May 31 '13 at 16:36
@MichaelHampton I am not sure whether I understand correctly what you mean. – Hauke Laging May 31 '13 at 16:39
1

I think in netcat-traditional you can do `-l -p` – user May 31 '13 at 16:39
It means you will get lots of upvotes. :) – Michael Hampton May 31 '13 at 16:40
(*sigh) Thanks @HaukeLaging. That's exactly what I was looking for. – Mike B May 31 '13 at 16:41
1

@MichaelHampton I guess I will take quite a while yet until I understand the practice rules of upvoting. If I have a look at what I get upvotes for (and much worse: What not for!) then I could cry. But it looks like this big problem's solution kicks me above the desired 3,000 points... (I noticed that on Unix & Linux my interest in rep dropped a lot after that) – Hauke Laging May 31 '13 at 16:43
This answer is incorrect, I don't know if this was ok in the past but now @schlicht's answer is correct. – SAMPro May 13 '20 at 17:09
1

@SAMPro The answer was not incorrect but incomplete. – Hauke Laging May 13 '20 at 20:11

score 9 · Answer 2 · answered Mar 23 '16 at 15:23

9

For completion:

nc -l -p port -s ip

should work too. Works with nc6 version 1.0 and netcat (The GNU Netcat) 0.7.1.

The command from Laging doesn't work with nc6 (used in debian).

answered Mar 23 '16 at 15:23

schlicht

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The only way I can get to specify the listening interface on Ubuntu's `nc`. – djule5 Aug 05 '16 at 15:33
2

nc says that you cannot use both -s and -l together – SunSparc Oct 11 '16 at 22:42
1

@SunSparc yet it is the only way to go for me to specify both local ip and local port in server mode - no other combination of options works here (nc v1.10-38, debian) – Antony Hatchkins Mar 22 '17 at 18:01
I think the issue could be that different netcat versions behave differently. I can't seem to get a version string out of mine, but there are at least two packages netcat-traditional vs netcat-openbsd. `netcat -vvlp 4445 -s 172.17.0.1` gives me `listening on [172.17.0.1] 4445 ...` vs `netcat -vvl 172.17.0.1 4445` gives me `listening on [any] 36539 ...` – relet Oct 03 '17 at 08:30
I had to specify the -s flag on debian buster. – Karl Pokus Feb 07 '20 at 07:33

How can I configure netcat (or some other stock linux utility) to listen on a specific port on a secondary IP address?

2 Answers2