Key Concepts, Class 3 – Part 2: Managing UNIX Network Settings

Overview

As with most UNIX services, network settings are configured by simple text files.
UNIX systems are best suited for Ethernet networks, which is the type of network that is most commonly used in corporate LANs and for high-speed home Internet access like DSL and Cable Modem services.
In order to configure your UNIX system for network access, you need to have the following information:

- IP address

- Subnet Mask

- Broadcast Address, if available

- Default Gateway (sometime referred to as your Default Router)

- One or more DNS server (sometimes called name servers)

This information is usually available from your network administrator or Internet service provider. When you have a dedicated IP address that is assigned to you and never changes, we say that you have a static IP address.

Sometimes the information noted above is not the same each time you connect to a network. Some LANs use a protocol known as DHCP (Dynamic Host Configuration Protocol), which dynamically assigns your computer this information when it reboots. When you dial-up to an ISP using a modem, you will almost always be assigned different network settings every time. Some Cable modem and DSL providers also use DHCP. When you receive different network settings from your provider each time you connect, we say that you have a dynamic IP address. If you have a dynamic IP address, it is not necessary to know the network information specified above.
Yes, it is possible to gain network access by using a standard modem to dial into an ISP. For details on setting up Linux dial-up networking, check out the PPP How-to at http://howto.tucows.com/LDP/HOWTO/PPP-HOWTO.html.
Like hard drives, Linux systems have special names for each network adapter (or NIC – network interface card). The first NIC installed on your system is known as “eth0” (“eth” stands for “Ethernet”). The NIC number will increase for each network adapter installed on the system. For example, the second and third NIC cards will be called eth1 and eth2, respectively. Since most UNIX system have 1 NIC card, you will most often see ‘eth0’ used in configuration examples. BSD systems also use this naming scheme.
Sun Solaris systems use a similar naming convention, but substitute the token “le” for “eth” for 10Megabit NIC adapters and “hme” for “eth” for 100Megabit NIC adapters. For example, the first 100MB NIC adapter on a Sun system is called “hme0”
There are 5 important configuration files used on Linux systems:

Important Network Configuration Files for Linux

/etc/sysconfig/network

The /etc/sysconfig/network file is used to enable networking on bootup, specify your default gateway and the network adapter associated with your default gateway. Here’s a look at the file with comments that explain what each line means.

[chrisjur@ sysconfig]$ more /etc/sysconfig/network

#Tells Linux whether to enable networking on boot

NETWORKING=yes

#Enables IP Forwarding – this line is not #required

FORWARD_IPV4=true

#The hostname of your computer

HOSTNAME=mycomputer

#The domain name associated with your computer, if you have one

DOMAINNAME=localdomain

# The default gateway assigned to you by your ISP

GATEWAY=10.0.0.254

#The name of the NIC card that is attached to your default gateway

GATEWAYDEV=eth0

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

The /etc/sysconfig/network-scripts/ifcfg-eth0 file is used to configure your primary NIC card, eth0. There are similar configuration files for each NIC you have installed on the system (e.g. /etc/sysconfig/network-scripts/ifcfg-eth1 configures the eth1 adapter). This file is where you specify IP address, broadcast address and netmask info. Here is a sample of the file with comments for each line:

[chrisjur@ sysconfig]$ more /etc/sysconfig/network-scripts/ifcfg-eth0

#Specifies the name of the network card you are configuring

DEVICE=eth0

#Tells Linux which boot protocol you are using.

#If you have a static IP address, this is an optional line.

# If you are using DHCP, it should be set to ‘dhcp’

BOOTPROTO=none

#Put your IP address here

IPADDR=145.16.122.254

#Put your netmask here

NETMASK=255.255.255.0

#Put your network address here

NETWORK=145.16.122.0

#Put your broadcast address here

BROADCAST=145.16.122.255

#Tells Linux to activate this card when the machine boots up

ONBOOT=yes

/etc/resolve.com

This file contains the list of your DNS servers. It is in the format:

nameserver <nameserver ip address>

…

e.g.:

nameserver 167.206.112.4

nameserver 167.206.112.3

You can specify as many nameserver IP addresses as you wish within this file. Keep in mind that you can only have one ‘nameserver’ entry per line in this file.

/etc/hosts

The /etc/hosts file contains a list of hostnames (system names) and their associated IP addresses. It is in the format:

127.0.0.1 localhost

IP address system-name system-alias1 system-alias2

…

e.g.

127.0.0.1 localhost

10.0.0.254 your-system-name

192.168.99.1 some-other-system

There should always contain the line:

127.0.0.1 localhost

There must also be a line that contains your system hostname and IP addresses.

/etc/nsswitch.conf

This file tells the system how it should perform name resolution (hostname to IP address translation), among other things. There is one very important line in this file:

hosts: files dns nisplus nis

This line tells the system to search perform hostname resolution in a specific order. First, the system will look in the /etc/hosts file (set by the ‘file’ directive) and then will perform a full DNS lookup if it can’t find the hostname-IP address pair in that file. Finally, the system will attempt nisplus and nis name resolutions, if these services are available. It is not necessary to have ‘nisplus’ and ‘nis’ directives on this line, but it is very important that the ‘files’ and ‘dns’ directives are included.

Most of the time, you do not have to edit this file at all. Solaris systems are famous for shipping nsswitch.conf files that do not contain a ‘hosts:’ line with ‘dns’ and ‘files’ options specified.

Examples

You are going to your new Linux system with the hostname ‘thor’. Your network admin or ISP assigns you the following network settings:

IP Address: 10.221.49.112

Gateway: 10.221.49.254

Netmask: 255.255.255.0

Network Address: 10.221.49.0

Broadcast Address: 10.221.49.255

DNS Server: 206.20.36.1, 206.20.36.150

Using these network specs, your configuration files will look like this:

/etc/sysconfig/network:

NETWORKING=yes

FORWARD_IPV4=true

HOSTNAME=thor

DOMAINNAME=localdomain

GATEWAY=10.221.49.254

GATEWAYDEV=eth0

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

DEVICE=eth0

BOOTPROTO=none

IPADDR=10.221.49.112

NETMASK=255.255.255.0

NETWORK=10.221.49.0

BROADCAST=10.221.49.255

ONBOOT=yes

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

127.0.0.1 localhost

10.221.49.112 thor

/etc/resolve.conf:

nameserver 206.20.36.1

nameserver 206.20.36.150

/etc/nsswitch.conf (hosts: line only showed)

hosts: files dns