Netcat uses different port than requested

I have the following problem. I’m using Debian GNU/Linux Stretch and I am trying to use netcat as a simple server. I start it using following command:

$ netcat -l 127.0.0.1 33333

It starts just fine and accepts connections but on a different port than requested:

$ netstat -tulpn | grep netcat
tcp        0      0 0.0.0.0:38782           0.0.0.0:*               LISTEN      2851/netcat    

This behavior is independent of requested port, user or ufw status. Recently I installed LXC with following packages:

• apparmor
• bridge-utils
• cgmanager
• libapparmor-perl
• lxc

All have been removed later, but somehow I feel like this behavior may be related to some changes in configuration.

Answer

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.