Network Administration

Linux manages services through the configuration files. This tutorial explains configuration files which are used to manage network in detail.

This article is the part of article series. Previous articles of this series.

IP address
How to assign IP address in Linux
Networking tools

From previous articles you have learned the basic of IP address and their associated commands. In this article we addressing associated configuration files.

This article is written for RHCE exam prospective so we are including simplified configuration files.

These configuration files determine whether networking is started during the system boot process or not. Check these files when you have trouble with network configuration in Exam.

  • /etc/init.d/network
  • /etc/sysconfig/network
  • /etc/sysconfig/network-scripts
  • /etc/sysconfig/network-scripts/ifcfg-eth0
  • /etc/nsswitch.conf
  • /etc/hosts
  • /etc/resolv.conf

/etc/init.d/network

/etc/init.d directory store script files. You can run the scripts directly, or use the service command to start/stop. We will demonstrate both.

Sometimes by mistake you may deactivate the network interface. Or there is an unknown error in configuration. In trouble this should be first point to check.

Execute script directly

#/etc/init.d/network status

network-status

Use service command to execute script

#service network status

service-network-status

Output will be same in both case it should list configured and active devices. Output will give you a clue for future steps. If a main device such as eth0 is not listed as active, that explains why the network seems to be down. Try to restart the network service.

#service network restart

service-network-restart

Any change made in IP configuration will not take place until you restart the network interface. This script will restart the network interface.

#/etc/init.d/network restart

network-restart

If a restart of networking services does not solve the issue move to the next steps. Next step is to get into the configuration files.

/etc/sysconfig/network

This file contains global configuration settings.

etc-sysconfig-network

This file specifies routing and host information for all network interfaces.

It is used to contain directives which are to have global effect and not to be interface specific.

Directive that you should check here for is NETWORKING=[yes][no].

If directive NETWORKING is set to no then the /etc/init.d/network script doesn’t activate any network devices. It must be set to yes in order to start networking.

Directive HOSTNAME controls computer name. If you want to change computer name, change it here.

If networking still not start check the status of networking service.

If the settings next to runlevels 3 and 5 are off, that’s a problem.

chkconfig-list

To make sure a service is active in appropriate runlevels, run the chkconfig network on command.

chkconfig-network-on

/etc/sysconfig/network-scripts

/etc/sysconfig/network-scripts directory contain executable files based on a series of text commands. These executable files are actually scripts based on the ifup and ifdown commands customized for the network device type.

ls-network-scripts

service network restart command may return with error like it in above example for eth0. In such a situation you should try to reactivate particular network device with ifup or ifdown command.

ifup-ifdown

/etc/sysconfig/network-scripts/ifcfg-eth0

This file contain configuration for eth0 first network adaptor. What you see in the ifcfg-eth0 file depends on how that first Ethernet network adapter was configured.

ifcfg-eth0

For exam you should understand following directive

DEVICE Type of device. for example lo=loopback adaptor, eth0= first Ethernet, eth1 second Ethernet etc.
IPADDR Static IP Address
NETMASK Network mask or subnet mask
NETWORK Network ID or network address
BROADCAST Broadcast address
GATEWAY IP address of the default gateway
ONBOOT specify whether the device is activated during the boot process
NAME common name for the device
HWADDR hardware address of network interface also known as MAC address
NM_CONTROLLED Specify that network card should be controlled by the Network Manager service or not. If it is set to yes be sure that Network Manager running
BOOTPROTO How IP address would be configured. dhcp for dynamic configuration, static for static ip configuration

/etc/nsswitch.conf

This file includes database search entries. It include from authentication to name services.

nsswitch

It includes the following entry which determines what database is search first.

nsswitch1

When a system gets a request to search for a hostname, the preceding directive means the /etc/hosts file is searched first.

If that name is not found in /etc/hosts, the next step is to search available configured DNS servers, normally using that configured in the /etc/resolv.conf file.

/etc/hosts

This file is used to resolve hostnames that cannot be resolved any other way. It can also be used to resolve hostnames on small networks with no DNS server.

This file by default contains a line specifying the IP address of the loopback device (127.0.0.1) as localhost.localdomain.

etc-hosts

/etc/resolv.conf

This file specifies the IP addresses of DNS servers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file.

etc-reslov

These are the files which you need to be familiar with. So go through these files again and again until you feel comfortable.