Lowe D. Networking For Dummies

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Book IX

Chapter 3

Basic Linux

Network

Configuration

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Restarting Your Network

Redhat-config-network:

Changes are saved.

You may want to restart

the network and network services

or restart the computer.

10. Click OK.

You can now close the Network Configuration window.

11. Restart the network services.

To find out how to restart, see the next section, “Restarting Your

Network.”

Restarting Your Network

Whenever you make a configuration change to your network, you must

restart Linux networking services in order for the change to take effect. If

you find that annoying, just be thankful that you don’t have to restart the

entire computer. Simply restarting the network services is sufficient.

You can restart the network services from a GNOME desktop by following

these steps:

1. Choose System➪Administration➪Services.

The Service Configuration window appears, as shown in Figure 3-5.

Figure 3-5:

The Service

Config-

uration

window.

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2. Select the Network service.

You have to scroll down the list of services to find it.

3. Click the Restart button.

The service is stopped and then started again. When finished, a small

dialog box appears, displaying the message, “Network restart successful.”

4. Click OK.

You return to the Service Configuration window.

5. Close the Service Configuration window.

That’s all!

If you prefer working in a command shell, you can restart the network by

entering the command service network restart. This results in a dis-

play similar to the following:

Shutting down interface eth0: [ OK ]

Shutting down loopback interface: [ OK ]

Setting network parameters: [ OK ]

Bringing up loopback interface: [ OK ]

Bringing up interface eth0: [ OK ]

Working with Network Configuration Files

Like other operating system services, the Linux network is configured by

settings that are specified in configuration files that you can find in the /etc

directory or in one of its subdirectories. Graphical configuration programs

such as Red Hat Network Configuration are actually little more than glori-

fied text editors that enable you to select network configuration options

from user-friendly screens and then save your configuration changes to the

standard configuration files. If you prefer to do the grunt work yourself, you

can open the configuration files in a text editor and make changes to them

directly.

Any time you open a configuration file in a text editor, you run the risk of

messing up your system’s configuration. So be careful!

Table 3-1 lists the main Linux network configuration files and describes what

each file does. The details of these files are described in the sections that

follow.

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Working with Network Configuration Files

Table 3-1 Linux Network Configuration Files

File Location Description

network /etc/sysconfig

Basic network

configuration.

hostname /etc

Specifies the host name

(obsolete, but should still be

present).

ifcfg-eth0 -/etc/sysconfig/

IP settings for the first

Ethernet adapter network-

scripts.

ifcfg-eth1 -/etc/sysconfig/

IP settings for the second

Ethernet adapter network-

scripts.

hosts /etc

Lists host address

mappings.

resolv.conf /etc

Lists DNS nameservers.

nsswitch.conf /etc

Specifies the name search

order.

xinetd.conf /etc

Specifies which network

services are started

automatically.

The Network file

The Network file, which lives in /etc/sysconfig, specifies basic configura-

tion settings for your network. Here’s a typical Network file:

NETWORKING=yes

HOSTNAME=LSERVER

GATEWAY=192.168.1.1

This file specifies that networking is enabled, the computer’s host name is

LSERVER, and the default gateway address is 192.168.1.1.

The following paragraphs describe all the settings that are valid for this file:

✦ NETWORKING: Specifies yes or no to enable or disable networking for

the computer.

✦ HOSTNAME: Specifies the host name for this computer. You should also

specify this name in /etc/hostname, although that file is considered

obsolete and is used only by some old programs. Note that this can be a

simple host name (like LSERVER) or a fully qualified domain name (like

Lserver.LoweWriter.com).

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✦ FORWARD_IPV4: Specifies YES or NO to enable or disable IP forwarding.

Specify FORWARD_IPV4=YES to set up a router.

✦ GATEWAY: Specifies the IP address of the computer’s Default Gateway. If

the network has a gateway router, specify its address here. If this com-

puter is the network’s gateway router, specify the gateway IP address

provided by your ISP.

✦ GATEWAYDEV: Specifies the interface (such as eth0) that should be used

to reach the gateway.

The ifcfg files

Each network interface has an ifcfg configuration file located in /etc/

sysconfig/network-scripts. The device name is added to the end of

the filename. So, for example, the configuration file for the first Ethernet

interface is called ifcfg-eth0.

This file is created and updated by the Network Configuration program, so

you don’t have to edit it directly (if you don’t want to).

Here’s a typical ifcfg file for an interface that has a static IP address:

DEVICE=eth0

BOOTPROTO=none

ONBOOT=yes

USERCTL=no

IPADDR=192.168.1.200

NETMASK=255.255.255.0

BROADCAST=192.168.1.255

NETWORK=192.168.1.0

Here’s an example for an interface that uses DHCP:

DEVICE=eth0

BOOTPROTO=dhcp

ONBOOT=yes

USERCTL=no

Here, the ifcfg file doesn’t have to specify the IP address information

because the interface gets that information from a DHCP server.

The following paragraphs describe the settings that you’re most likely to see

in this file:

✦ DEVICE: The name of the device, such as eth0 or eth1.

✦ USERCTL: Specifies YES or NO to indicate whether local users are

allowed to start or stop the network.

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✦ ONBOOT: Specifies YES or NO to indicate whether the device should be

enabled when Linux boots up.

✦ BOOTPROTO: Specifies how the device gets its IP address. Possible values

are NONE for static assignment, DHCP, or BOOTP.

✦ BROADCAST: The broadcast address used to send packets to everyone

on the subnet. For example: 192.168.1.255.

✦ NETWORK: The network address. For example: 192.168.1.0.

✦ NETMASK: The subnet mask. For example: 255.255.255.0.

✦ IPADDR: The IP address for the adapter.

The Hosts file

The Hosts file is a simple list of IP addresses and the host names associated

with each address. You can think of the Hosts file as a local DNS database

of sorts. Whenever Linux needs to resolve a DNS name, it first looks for the

name in the Hosts file. If Linux finds the name there, it doesn’t have to do a

DNS lookup; it simply uses the IP address found in the Hosts file.

For small networks, common practice is to list the host name for each com-

puter on the network in the Hosts file on each computer. Then, whenever

you add a new computer to the network, you just update each computer’s

Hosts file to include the new computer. That’s not so bad if the network has

just a few computers, but you wouldn’t want to do it that way for a network

with 1,000 hosts. That’s why other name resolution systems are more popu-

lar for larger networks.

The default Linux Hosts file looks something like this:

# Do not remove the following line, or various programs

# that require network functionality will fail.

127.0.0.1 localhost.localdomain localhost

Here, the names localhost.localdomain and localhost both resolve to

127.0.0.1, which is the standard local loopback address.

Here’s an example of a Hosts file that has some additional entries:

# Do not remove the following line, or various programs

# that require network functionality will fail.

127.0.0.1 LServer localhost.localdomain localhost

192.168.1.1 linksys

192.168.1.100 ward.cleaver.com ward

192.168.1.101 june.cleaver.com june

192.168.1.102 wally.cleaver.com wally

192.168.1.103 theodore.cleaver.com theodore beaver

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Here, I’ve defined host names for each of the Cleaver family’s four comput-

ers and their Linksys router. Each computer can be accessed by using one of

two names (for example, ward.cleaver.com or just ward), except the last

one, which has three names.

The resolv.conf file

The resolv.conf file lists the DNS nameservers that can be consulted to

perform DNS lookups. A typical resolv.conf file looks like this:

nameserver 192.168.1.110

nameserver 204.127.198.19

nameserver 63.249.76.19

If you have set up a nameserver on your own network, its IP address should

be the first one listed.

The nsswitch.conf file

This configuration file controls how name resolution works when looking up

various types of objects, such as host addresses and passwords. Listing 3-1

shows the sample nsswitch.conf file that comes with Fedora Linux. As

you can see, this file is loaded with comments that explain what the various

settings do.

You can use the files, db, and dns keywords to specify how objects

should be retrieved. files specifies that the local file should be used, db

specifies a database lookup, and dns specifies that a DNS server should be

consulted.

The order in which you list these keywords determines the order in which

the data sources are searched. Thus, if you want host names to be resolved

first by the local Hosts file and then by DNS, you should include the follow-

ing line in nsswitch:

hosts: files dns

Listing 3-1: A Sample /etc/nsswitch.conf File

# /etc/nsswitch.conf

# An example Name Service Switch config file. This file should be

# sorted with the most-used services at the beginning.

# The entry ‘[NOTFOUND=return]’ means that the search for an

# entry should stop if the search in the previous entry turned

# up nothing. Note that if the search failed due to some other reason

# (like no NIS server responding) then the search continues with the

# next entry.

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# Legal entries are:

# nisplus or nis+ Use NIS+ (NIS version 3)

# nis or yp Use NIS (NIS version 2), also called YP

# dns Use DNS (Domain Name Service)

# files Use the local files

# db Use the local database (.db) files

# compat Use NIS on compat mode

# hesiod Use Hesiod for user lookups

# [NOTFOUND=return] Stop searching if not found so far

# To use db, put the “db” in front of “files” for entries you want to be

# looked up first in the databases

# Example:

#passwd: db files nisplus nis

#shadow: db files nisplus nis

#group: db files nisplus nis

passwd: files

shadow: files

group: files

#hosts: db files nisplus nis dns

hosts: files dns

# Example - obey only what nisplus tells us...

#services: nisplus [NOTFOUND=return] files

#networks: nisplus [NOTFOUND=return] files

#protocols: nisplus [NOTFOUND=return] files

#rpc: nisplus [NOTFOUND=return] files

#ethers: nisplus [NOTFOUND=return] files

#netmasks: nisplus [NOTFOUND=return] files

bootparams: nisplus [NOTFOUND=return] files

ethers: files

netmasks: files

networks: files

protocols: files

rpc: files

services: files

netgroup: files

publickey: nisplus

automount: files

aliases: files nisplus

The xinetd.conf file

Xinetd is a service that oversees a variety of networking services, such as

Telnet or Finger. Xinetd listens for requests on the ports on which these ser-

vices talk and automatically starts the service when a connection is made.

Xinetd is controlled by the configuration file xinetd.conf, which is found

in the /etc directory, and each of the services controlled by xinetd is in

turn controlled by a configuration file found in the /etc/xinet.d directory.

You should leave most of the settings in these configuration files alone

unless you’ve studied up on xinetd. (You can find out more about it at www.

xinetd.org.) However, you may want to modify the configuration files in

order to enable or disable specific services.

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Displaying Your Network Configuration with the ifconfig Command

Each of the services controlled by xinetd has a configuration file in the /

etc/xinet.d directory. Each of these configuration files ends with a line

that enables or disables the service. For example, here’s the configuration

file for Telnet, /etc/xinet.d/telnet:

# default: on

# description: The telnet server serves telnet sessions; it uses \

# unencrypted username/password pairs for authentication.

service telnet

{

Flags = REUSE

socket_type = stream

wait = no

user = root

server = /usr/sbin/in.telnetd

log_on_failure += USERID

disable = yes

}

Here, the last line disables Telnet. You can enable the Telnet service by

changing the last line to disable = no.

Displaying Your Network Configuration

with the ifconfig Command

Linux doesn’t have an ipconfig command like Windows. Instead, the com-

mand that you use to display information about your network configuration

is ifconfig. You can also use this command to set network configuration

options, but in most cases, using the Network Configuration program or

directly editing the network configuration files is easier.

If you enter ifconfig without any parameters, you get output similar to the

following:

Eth0 Link encap:Ethernet HWaddr 00:40:05:80:51:F3

inet addr:192.168.1.200 Bcast:192.168.1.255 Mask:255.255.255.0

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:17293 errors:0 dropped:0 overruns:0 frame:0

TX packets:334 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:100

RX bytes:1124153 (1.0 Mb) TX bytes:45726 (44.6 Kb)

Interrupt:3 Base address:0xc000

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:202939 errors:0 dropped:0 overruns:0 frame:0

TX packets:202939 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:13856758 (13.2 Mb) TX bytes:13856758 (13.2 Mb)

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Displaying Your Network Configuration with the ifconfig Command

From this output, you can tell that the IP address of the Ethernet adapter

(eth0) is 192.168.1.200, the broadcast address is 192.168.1.255, and the

netmask is 255.255.255.0. You can also see transmit and receive statistics

as well as information about the hardware configuration, such as the MAC

address and the adapter’s interrupt and memory base address assignments.

Linux offers many other commands that can help you configure and trouble-

shoot a network. Many of these commands are described in detail in Chapter 9

of this minibook.

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Book IX: Managing Linux Systems

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