Check Port

netcat

The syntax of the command is as follows.

nc [-options] host-ip-adress port-number

Let’s try to use it on a remote computer.

$ nc -zvw10 192.168.0.1 22

As you can see, the connection succeeded. This means that port 22 is open. If the connection fails, then you will get an error message of “failed: Connection refused”

In the above command, we also used different options. Let’s list them below.

z: zero-I/O mode which is used for scanning v: for verbose output w10: timeout wait seconds

nmap

Let’s see the nmap syntax below.

nmap [-options] [IP or Hostname] [-p] [PortNumber]

As you can see, its syntax matches that of the nc command. Let’s run it to get a better understanding.

nmap 192.168.0.1 -p 22

If the port is closed, then it will show status is closed

telnet

The syntax of the command is as below.

$ telnet [IP or Hostname] [PortNumber]

If the connection fails, then the port is not open, and you will get the following output.

echo > /dev/tcp/...

The syntax of the command is as below

echo > /dev/tcp/[host]/[port] && echo "Port is open"

or

echo > /dev/udp/[host]/[port] && echo "Port is open"

netstat -tuplen

It will output the whole list of the IP addresses. The entries that have “Listen” in the “State” column are the open ports.