Network Services in Linux

Instructor: Alexis Kypridemos

Alexis is a technical writer for an IT company and has worked in publishing as a writer, editor and web designer. He has a BA in Communication.

This lesson explains what Linux network services are and how they work. Also discussed are network ports and their uses, the extended internet daemon xinetd, and the /etc/services file.

Network Services in Linux

Services in Linux and other operating systems are applications or a set of applications that run in the background, enabling certain capabilities as and when they become necessary. This lesson takes a look at Linux services that deal with network capabilities like connecting to a network and file sharing.

Required Services for Networking

The following three services are required for network activities on a Linux system:

network - This service turns on the network card, or powers the modem.

iptables - The kernel based Packet Filtering firewall service. Various Linux firewalls are based on this service.

xinetd - The server that monitors and controls other servers. This service frees up CPU load by allowing other services like FTP (file transfer) to be available, but only run when needed, as opposed to running continuously and consuming unnecessary processing power.

Other Network Services

arpwatch - Monitors and pairs remote IP addresses with hostnames.

dhcpd - Enables the DHCP Server, to dynamically provide IP addresses to the local network. This is used in situations where the local network is large enough such that static IP addresses are no longer convenient.

iplog - A network monitoring tool that logs TCP, UDP and ICMP connections with hostnames of a remote host.

netplugd - The Network Card Daemon. It monitors the network interface and enables it or disables it depending on whether it detects a signal. It is used mainly for laptops, as they may not always be connected to a network.

nfs - The Network File Share service. Required for computers that are part of a network file sharing system.

nfsfs - The Network File Share Server. This service powers the computer which acts as the server in a network file sharing system.

nfslock - This service allows for NFS file locking in a network file sharing system.

saned - The Network Scanner Service; allows any machine on the network to use the scanner.

snmpd - The Simple Network Management Protocol. This service is used in a small network environment, like a home office network.

How Daemons Provide Network Services

Daemons in Linux and computing in general, are services that run continuously in the background, 'waiting' for certain events to occur before they trigger other services. For example, in relation to network services, the xinetd (extended Internet daemon) launches when the operating system boots up and runs in the background. It 'listens' for activity on network ports. So if, for example, a request to connect to the Internet is made, the xinetd daemon triggers the appropriate service to make this happen.

Ports and the xinetd daemon are explained in more detail in the sections below.

Internet Super Daemon (xinetd)

As mentioned above, the xinetd daemon is a service that starts when the operating system boots, and monitors activity on the various ports, 'waiting' for a request. When a request is made, the xinetd starts the relevant service to accomplish the task. For example, if a request is made on port 25 to send an email, xinetd would then enable the postfix or sendmail service.

Because it is a service that controls other services, the xinetd daemon is frequently referred to as a 'super server'.

xinetd monitors the ports listed in the /etc/services file described below.

Ports and the /etc/services File

Network ports, as opposed to hardware ports like a USB port, allow a computer to perform various network-dependent tasks, like sending email and sharing files, simultaneously while using the same network resources. Ports are identified by their number.

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