How to Check Remote Ports are Reachable Using ‘nc’ Command

 How to determine if ports on a remote host are reachable/open using simple netcat (in short nc) command.

netcat (or nc in short) is a powerful and easy-to-use utility that can be employed for just about anything in Linux in relation to TCP, UDP, or UNIX-domain sockets.

# yum install nc                  [On CentOS/RHEL]
# dnf install nc                  [On Fedora 22+]
$ sudo apt-get install netcat     [On Debian/Ubuntu]

We can use it to: open TCP connections, listen on arbitrary TCP and UDP ports, send UDP packets, do port scanning under both IPv4 and IPv6 and beyond.

Using netcat, you can check if a single or multiple or a range of open ports as follows. The command below will help us see if the port 22 is open on the host 192.168.56.10:

$ nc -zv 192.168.1.15 22

In the command above, the flag:

1. -z – sets nc to simply scan for listening daemons, without actually sending any data to them.
2. -v – enables verbose mode.

The next command will check if ports 80, 22 and 21 are open on the remote host 192.168.5.10 (we can use the hostname as well):
nc -zv 192.168.56.10 80 22 21

It is also possible to specify a range of ports to be scanned:’

$ nc -zv 192.168.56.10 20-80