1. netstat – a command-line tool that displays network connections, routing tables, and a number of network interface statistics.
2. fuser – a command line tool to identify processes using files or sockets.
3. lsof – a command line tool to list open files under Linux / UNIX to report a list of all open files and the processes that opened them.
4. /proc/$pid/ file system – Under Linux /proc includes a directory for each running process (including kernel processes) at /proc/PID, containing information about that process, notably including the processes name that opened port.

Examples

Open a terminal and then type the following command as root user:

netstat command find out what is using port 80

Type the following command
# netstat -tulpn | grep :80
OR pass the –color option to grep command as follows:
# netstat -tulpn | grep --color :80
Sample outputs:

tcp        0      0 0.0.0.0:80                  0.0.0.0:*                   LISTEN      1215/nginx

Where,

1. 0 0.0.0.0:80 – Source IP:Port
2. 1215/nginx – PID/Process name

The tcp port 80 is opened and used by nginx web server. Type the following command to find out more about nginx:

whatis nginx
whereis nginx

Note: You may need to install lsof and fuser command.

Use /proc/$pid/exec file find out what is using port 80

First, find out the processes PID that opened tcp port 90, enter:
# fuser 80/tcp
Sample outputs:

80/tcp:              12161 21776 25250 25393

Finally, find out process name associated with PID # 3813, enter:
# ls -l /proc/12161/exe
Sample outputs:

lrwxrwxrwx. 1 root root 0 Aug  9 13:28 /proc/12161/exe -> /usr/sbin/lighttpd

Find out more about lighttpd:

man lighttpd
whatis lighttpd
whereis lighttpd

Sample outputs:

lighttpd             (8)  - a fast, secure and flexible web server
lighttpd: /usr/sbin/lighttpd /usr/lib64/lighttpd /usr/share/man/man8/lighttpd.8.gz

You can use package manager to dig into lighttpd:
# rpm -qa | grep lighttpd
Sample outputs:

lighttpd-1.4.32-1.el6.x86_64

To find out more about lighttpd-1.4.32-1.el6.x86_64 package, type:
# yum info lighttpd-1.4.32-1.el6.x86_64
Sample outputs:

Loaded plugins: auto-update-debuginfo, protectbase, rhnplugin, security
This system is receiving updates from RHN Classic or RHN Satellite.
0 packages excluded due to repository protections
Installed Packages
Name        : lighttpd
Arch        : x86_64
Version     : 1.4.32
Release     : 1.el6
Size        : 664 k
Repo        : installed
Summary     : A web server more optimized for speed-critical environments.
URL         : http://lighttpd.net/
License     : Revised BSD
Description : It is a secure and fast web server a very low memory footprint compared
            : to other webservers and takes care of cpu-load.

OR use rpm command:
# rpm -qi lighttpd
Sample outputs:

Name        : lighttpd                     Relocations: (not relocatable)
Version     : 1.4.32                            Vendor: nixCraft
Release     : 1.el6                         Build Date: Sun 03 Feb 2013 03:22:08 AM CST
Install Date: Mon 04 Feb 2013 04:44:26 AM CST      Build Host: rhel6.nixcraft.net.in
Group       : System Environment/Daemons    Source RPM: lighttpd-1.4.32-1.el6.src.rpm
Size        : 680402                           License: Revised BSD
Signature   : (none)
URL         : http://lighttpd.net/
Summary     : A web server more optimized for speed-critical environments.
Description :
It is a secure and fast web server a very low memory footprint compared
to other webservers and takes care of cpu-load.

Debian / Ubuntu Linux user can use the following commands:
# dpkg --list | grep lighttpd
# apt-cache search lighttpd
# apt-cache show lighttpd
Sample outputs from the last command:

Package: lighttpd
Priority: optional
Section: universe/web
Installed-Size: 841
Maintainer: Ubuntu Developers 
Original-Maintainer: Debian lighttpd maintainers 
Architecture: amd64
Version: 1.4.28-2ubuntu4
Provides: httpd, httpd-cgi
Depends: libattr1 (>= 1:2.4.46-5), libbz2-1.0, libc6 (>= 2.4), libgamin0 | libfam0, libldap-2.4-2 (>= 2.4.7), libpcre3 (>= 8.10), libssl1.0.0 (>= 1.0.0), zlib1g (>= 1:1.1.4), lsb-base (>= 3.2-14), mime-support, libterm-readline-perl-perl
Recommends: spawn-fcgi
Suggests: openssl, rrdtool, apache2-utils, ufw
Conflicts: cherokee (<= 0.6.1-1)
Filename: pool/universe/l/lighttpd/lighttpd_1.4.28-2ubuntu4_amd64.deb
Size: 279838
MD5sum: 65aedfd0e0ab6d3ee28e7b394567ed22
SHA1: 34a9156fa3d23635eb24efb436de585c0594f046
SHA256: 751d6f8309d249740d7aab240a74b6bae713e524cf6815544b6cdbb6107fded2
Description-en: A fast webserver with minimal memory footprint
 lighttpd is a small webserver and fast webserver developed with
 security in mind and a lot of features.
 It has support for
   * CGI, FastCGI and SSI
   * virtual hosts
   * URL rewriting
   * authentication (plain files, htpasswd, ldap)
   * transparent content compression
   * conditional configuration
 and configuration is straight-forward and easy.
Homepage: http://www.lighttpd.net
Description-md5: 267ee2989b526d8253e822e7d8244ccd
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu

lsof command find out what is using port 80

Type the following command
# lsof -i :80 | grep LISTEN
Sample outputs:

apache2   1607     root    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1616 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1617 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)

See also

• Linux: Find Out Which Process Is Listening Upon a Port
• ss: Display Linux TCP / UDP Network and Socket Information
• See man pages for more info lsof(8),fuser(1),proc(5),netstat(8),ss(8)