mjhjmtu

There are two flavors of netcat: netcat-openbsd and netcat-traditional.

How to know which flavor of netcat I'm using? I've tried man nc but it doesn't say what flavor it is.

First of all, you can install multiple flavors in your machine. So the answer depends on how many flavors you've installed and what command you type.

netcat-traditional and netcat-openbsd is available to be install via package manager apt in Ubuntu. In my case I also build from source to install GNU netcat flavor via this official website.

For "openbsd" flavor, you can figure out location binary name with dpkg -L <package-name> (Googling yourself to find equivalent of dpkg L if yor package manager is not apt):

$ dpkg -L netcat-openbsd | grep /bin
/bin
/bin/nc.openbsd

Then use type -a to confirm that binary name nc.openbsd is searchable in $PATH and interpreted as command:

$ type -a nc.openbsd
nc.openbsd is /bin/nc.openbsd
nc.openbsd is /bin/nc.openbsd

For "traditional" flavor is same:

$ dpkg -L netcat-traditional | grep /bin
/bin
/bin/nc.traditional
$ type -a nc.traditional
nc.traditional is /bin/nc.traditional
nc.traditional is /bin/nc.traditional

That's means I can issue command nc.openbsd to run netcat-openbsd tool, and also command nc.traditional to run netcat-traditional tool. (There may confuse where the command contains '.' but package name contains '-' )

Seems like there are 3 flavors to install via apt:

$ apt-cache search netcat --names-only
netcat-openbsd - TCP/IP swiss army knife
netcat - TCP/IP swiss army knife -- transitional package
netcat-traditional - TCP/IP swiss army knife

But actually netcat is dummy package only:

$ apt-cache show netcat | grep Description-en -A 2
Description-en: TCP/IP swiss army knife -- transitional package
 This is a "dummy" package that depends on lenny's default version of
 netcat, to ease upgrades. It may be safely removed.

So you can only install netcat-openbsd and netcat-traditional via apt if you want:

sudo apt-get install netcat-openbsd
sudo apt-get install netcat-traditional

How about commands nc and netcat ? They can tied to multiple flavors searchable by $PATH, one of the path will run if you type nc or netcat. Again, you can use type -a to check, whereas priority is the first line (as bold below):

$ type -a nc 
nc is /usr/local/bin/nc
nc is /bin/nc
nc is /usr/local/bin/nc
nc is /bin/nc
$ type -a netcat
netcat is /usr/local/bin/netcat
netcat is /bin/netcat
netcat is /usr/local/bin/netcat
netcat is /bin/netcat

You can use realpath to figure out resolved path of them:

$ realpath /usr/local/bin/netcat 
/usr/local/bin/netcat
$ realpath /bin/netcat 
/bin/nc.openbsd
$ realpath /usr/local/bin/nc
/usr/local/bin/netcat
$ realpath /bin/nc
/bin/nc.openbsd

4 of them only 2 paths is unique in my system, one is "GNU", and the other one is "openbsd":

$ /usr/local/bin/netcat --version | head -1
netcat (The GNU Netcat) 0.7.1
$ /bin/nc.openbsd -h |& head -1
OpenBSD netcat (Debian patchlevel 1.130-3)

That's means if I type nc OR netcat, it will execute /usr/local/bin/netcat which is "GNU Netcat".

You can try update-alternatives to adjust the resolved symlink path:

$ realpath /bin/nc
/bin/nc.openbsd
$ realpath /bin/netcat
/bin/nc.openbsd
$ sudo update-alternatives --config nc
There are 2 choices for the alternative nc (providing /bin/nc).

 Selection Path Priority Status
------------------------------------------------------------
 0 /bin/nc.openbsd 50 auto mode
* 1 /bin/nc.openbsd 50 manual mode
 2 /bin/nc.traditional 10 manual mode

Press <enter> to keep the current choice[*], or type selection number: 2
update-alternatives: using /bin/nc.traditional to provide /bin/nc (nc) in manual mode
$ realpath /bin/nc
/bin/nc.traditional
$ realpath /bin/netcat
/bin/nc.traditional

It changed both /bin/nc and /bin/netcat resolved symlink¹ to /bin/nc.traditional, but still it doesn't changed the flavor if I type nc OR netcat since /usr/local/bin/ still has higher precedence over /bin in my $PATH:

$ /bin/nc -h |& head -1
[v1.10-41]
$ nc -h |& head -1
GNU netcat 0.7.1, a rewrite of the famous networking tool.
$ type -a nc | head -1
nc is /usr/local/bin/nc

Note that there are more flavors of netcat, e.g. ncat, socat, sbd, netcat6, pnetcat, and cryptcat.

¹ _{The actual symlink updated were /etc/alternatives/nc and /etc/alternatives/netcat, which /bin/nc and /bin/netcat were already symlink to /etc/alternatives/nc and /etc/alternatives/netcat respectively.}

When I run nc --version, I get:

netcat (The GNU Netcat) 0.7.1
Copyright (C) 2002 - 2003 Giovanni Giacobbi

This program comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of this program under the terms of
the GNU General Public License.
For more information about these matters, see the file named COPYING.

Original idea and design by Avian Research <hobbit@avian.org>,
Written by Giovanni Giacobbi <giovanni@giacobbi.net>.

Maybe the BSD version will say specifically as well.

On a Mac (though it presents itself as the GNU version):

$ nc -h
GNU netcat 0.7.1, a rewrite of the famous networking tool.
[ further output snipped ]

On a Linux box (specifically Ubuntu):

$ nc -h
[v1.10-41]
[ further output snipped ]

netcat --version, suggested in another answer, threw an 'invalid option' error to --version, so -h seems to potentially be a universal test.