netcat - keep listening for connection in Debian

Question

There's -k option in OS X (BSD) version of netcat to keep listening after current connection is completed. However in Debian (GNU?) version this option is missing.

There's -q -1 option to listen forever after EOF appears on stdin but this doesn't do the trick and connections close anyway.

Is there any way to force Debian's netcat to keep listening indefinitely?

Solution 1

Debian's implementation of Netcat does have the -k option. However, it's not documented in the manual because it doesn't work in a reliable manner, for some unknown reason.

Luckily, there's ncat, which is yet another implementation of Netcat and is part of the nmap package. This one has a working -k. You can get it by installing nmap. ncat's options and usage are more or less similar to other implementations of Netcat, so your knowledge of other implementations should transfer very well to ncat.

Solution 2

I have the same problem if netcat gets a rst or fin packet, I think.

You could simply call netcat again as soon as it closes using a bash loop.

while true; do nc -lv <listeningport> ; done

Comments

[Aalex Gabi]

Thank you! Thanks to you I found best version of netcat ever.

[Andrew Savinykh]

openbsd-netcat appears to have the -k option as well

[Robin Thoni]

Package is netcat-openbsd, not openbsd-netcat for those who might not click @AndrewSavinykh link

[peterh]

Welcome on the SU! Please explain, what your code is doing and how does it work.

[Jens Timmerman]

This is different from ncat or opensbd-ncat -k option, which will actually allow multiple clients to connect at the same time; your solution will only allow one after the other

[Vishrant]

ncat worked for me in macos

[Tinmarino]

@peterh-ReinstateMonica it is doing a loop that starts a new netcat command when the previous one has returned. While true for infinite loop.

[bmaupin]

This doesn't work for me. After the message is sent, netcat just hangs until I press Enter. Maybe because I'm listening on UDP? Adding -w0 to the netcat command fixes it, e.g. while true; do nc -luv 5140 -w0; done

[Abdull]

ncat, UDP and -k don't work together: ncat -u -l -k 1900 results in Ncat: UDP mode does not support the -k or --keep-open options, except with --exec or --sh-exec. QUITTING.