How to find open sockets on your Linux server

Here we will look into lsof - list open file, and Nmap (“Network Mapper”)

Nmap is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts.

There are a number of methods that you can use to show open sockets at least:

lsof -U will list open sockets

nmap -sT -sU localhost will scan your local machine for open TCP or UDP ports

$ sudo nmap -sT -sU localhost

Starting Nmap 4.68 ( ) at 2010-11-15 06:54 IST
Interesting ports on localhost (
Not shown: 3201 closed ports
123/udp open|filtered ntp
5353/udp open|filtered zeroconf

Nmap done: 1 IP address (1 host up) scanned in 4.003 seconds

netstat -a | grep LISTEN will show all listening sockets.

Nmap has lots of options, so we are going to focus on only some of them.

sudo nmap -sS -O

TCP SYN scan
Enable Operating System detection

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