How to check for open ports in Ubuntu (in general – any Linux based operating system)?

There are multiple ways to check for open ports in Ubuntu or in general – any Linux based operating system. Let me describe two of the most commonly used methods (at least by myself) in this article.

Method 1: Using the ‘netstat’ command

We can utilize the ‘netstat’ command as illustrated below to check for open ports in Ubuntu or in general any Linux based operating system.

 

netstat –lp --inet

For example:

When I executed the above mentioned command on my machine, it displayed the below list of open ports.

user@ubuntu:~/Desktop$ sudo netstat -lp --inet
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address  State   PID/Program name
tcp        0      0 localhost.localdo:50001 *:*              LISTEN  3738/firefox-bin
tcp        0      0 localhost.localdoma:ipp *:*              LISTEN  1243/cupsd
udp        0      0 192.168.1.2:ntp         *:*                      2655/ntpd
udp        0      0 localhost.localdoma:ntp *:*                      2655/ntpd
udp        0      0 *:ntp                   *:*                      2655/ntpd

 
Method 2: Using ‘nmap’

Nmap is an abbreviated name for ‘Network Mapper’. It supports both CLI and GUI front end. In this article, I will show you how to check for open ports utilizing the nmap CLI and will allow you to research further on other possibilities of this beautiful multi-purpose utility.

sudo apt-get install nmap

The command listed above allows you to install ‘nmap’ in a Ubuntu based operating system. Let us start with a simple scan option of ‘nmap’.

nmap -v <hostname>

Be sure to substitute the right host name of the machine against the <hostname> attribute above. The above command scans all the reserved TCP ports on the machine pointed by the <hostname> value and displays verbose output (due to the usage of the ‘-v’ option).

nmap -sS -O myhost.com/24

The above command launches a stealth SYN scan (due to the usage of ‘-sS’ option) against 256 IP’s (Class C network) where myhost.com resides. The usage of ‘-O’ option enables operating system detection on each of those machines.

nmap -sS -O -p 2000-3000 myhost.com

The above command scans ports from 2000 to 3000 on the machine that hosts myhost.com. The ‘-p’ option used above allows us to specify either a single port or a range of ports or distinct individual ports separated by commas.

Note: You can always consider specifying the ‘-v’ option for verbose output.


References:

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