Understanding /etc/services file in Linux

The article helps you to understand /etc/services file in Linux. Learn about content, format & importance of this file.

Internet daemon is an important service in the Linux world. It takes care of all network services with the help of /etc/services file. In this article, we will walk you through the content, format of this file, and what it means to a Linux system.

/etc/services file contains a list of network services and ports mapped to them. inetd or xinetd looks at these details so that it can call a particular program when a packet hits respective port and demand for service.

As a normal user, you can view this file since the file is world-readable. To edit this file you need to have root privileges.

$ ll /etc/services
-rw-r--r--. 1 root root 670293 Jun  7  2013 /etc/services

/etc/services file format

service-name    port/protocol   [aliases..]  [#comment]

Last two fields are optional hence denoted in [ ] where –

• service-name is the name of the network service. e.g. telnet, FTP, etc.
• port/protocol is the port being used by that network service (numerical value) and protocol (TCP/UDP) used for communication by service.
• alias is an alternate name for service.
• a comment is a note or description you can add to service. Starts with # mark

Sample /etc/services file

# Each line describes one service, and is of the form:
#
# service-name  port/protocol  [aliases ...]   [# comment]

tcpmux          1/tcp                           # TCP port service multiplexer
rje             5/tcp                           # Remote Job Entry
echo            7/udp
discard         9/udp           sink null

Here, you can see the use of optional last two fields as well. discard service has an alternate name as sink or null.