Use getsockname() to get port number assigned by the kernel

[dxdxdt]

Describe the bug
ncat accepts zero for listen port number. This is a reasonable behaviour. According to ip(4):

  IP_PORTRANGE may	be used	to set the port	range used for selecting a lo-
  cal port	number on a socket with	an unspecified (zero) port number.  It
  has the following possible values:

When sin_port is zero, the kernel will pick a port from the ephemeral range. This behaviour is observed on Windows, BSD and Linux. The problem is that ncat does not use getsockname() to get the actual port assigned. Instead, the program reports the local port number being zero.

To Reproduce

$ nc -lv 0 & ss -tlnp                                                                                                           
[1] 214046      
Ncat: Version 7.92 ( https://nmap.org/ncat )
Ncat: Listening on :::0
Ncat: Listening on 0.0.0.0:0
State        Recv-Q       Send-Q             Local Address:Port               Peer Address:Port       Process                               
LISTEN       0            10                       0.0.0.0:42767                   0.0.0.0:*           users:(("nc",pid=214046,fd=4))                                            
LISTEN       0            10                          [::]:44911                      [::]:*           users:(("nc",pid=214046,fd=3))                                            

Expected behavior
ncat should report the actual local port.

[rmhowe425]

And just to clarify here, the command works as intended for nc. The problem is with ncat

[dxdxdt]

@rmhowe425

And just to clarify here, the command works as intended for nc. The problem is with ncat

nc or ncat executable is symlinked on most platforms. Dead giveaway you should be questioning whether you know what you're doing.

My piece of advice to you when it comes to contributing to FOSS projects: NEVER jump the gun. You can make pretty good/bad code and submit PR or patches. But never expect that you'll get any reply at all. They just straight up ignore you most of the time. Doesn't matter how good or bad your idea is. The maintainers are the ones who are responsible for steering the project, not random bystanders like us who show brief interest in their projects. You have to respect that. Get their opinion/permission before working on your idea any further.

See how many unattended issues on this repo. They'll never care. That's how FOSS usually works, unfortunately.

Godspeed.

[rmhowe425]

Interesting. If nc and ncat point to the same binary then I wonder why the output is slightly different in my screenshots above. The screenshots are taken from compiled code based on the master branch.

I hear you though. This is my 1st contribution to Nmap, although I am an experienced open source contributor to other repos. Everyone has to start somewhere though.