Lowe D. Networking For Dummies

Подождите немного. Документ загружается.

Chapter 4: Running

DHCP and DNS

In This Chapter

✓ Dealing with DHCP

✓ Running a DNS server

✓ Understanding BIND configuration files

ne of the main reasons that many network administrators add Linux

servers to their networks is to run Internet services such as DHCP and

DNS. These services were originally developed for the Unix environment, so

they tend to run better under Linux than they do under Windows.

Well, that’s the theory, at least. The most recent versions of Windows are

probably just as good at running these services as Linux. Still, if you prefer

to set up these services on a Linux server, this chapter is for you.

Running a DHCP Server

DHCP is the TCP/IP protocol that automatically assigns IP addresses to

hosts as they come on the network. (DHCP stands for Dynamic Host Control

Protocol, but that won’t be on the test.) For a very small network (say, fewer

than ten hosts), you don’t really need DHCP: You can just configure each

computer to have a static IP address. For larger networks, however, DHCP

is almost a must. Without DHCP, you have to manually plan your entire IP

address scheme and manually configure every computer with its IP informa-

tion. Then, if a critical address such as your Internet gateway router or your

DNS server address changes, you have to manually update each computer

on the network. As you can imagine, DHCP can save you a lot of time.

Even for small networks, however, DHCP can be a timesaver. For example,

suppose that you have a notebook computer that you take back and forth

between your home and office. If you don’t set up a DHCP server at home,

you have to change the computer’s static IP address each time you move the

computer. With DHCP, the computer can change IP addresses automatically.

66_625873-bk09ch04.indd 70766_625873-bk09ch04.indd 707 9/21/10 10:46 PM9/21/10 10:46 PM

708

Running a DHCP Server

For the complete lowdown on DHCP, please refer to Book IV, Chapter 3. In

the following sections, I show you how to install and configure a DHCP

server on the Fedora 12 Linux distribution.

Installing DHCP

You can quickly find out whether DHCP is installed on your system by enter-

ing the following command from a shell prompt:

rpm -q dhcp

If DHCP has been installed, the package version is displayed. If not, the mes-

sage package dhcp is not installed is displayed.

If DHCP isn’t installed on your Linux server, you can install it by following

these steps:

1. Choose System➪Administration➪Add/Remove Software.

This summons the Add/Remove Software program, shown in Figure 4-1.

Figure 4-1:

The Add/

Remove

Software

program.

2. Select the Servers option in the list box on the left side of the Add/

Remove Software window.

A list of server packages appears in the window’s main list box.

3. Select the Dynamic Host Configuration Protocol Software package.

66_625873-bk09ch04.indd 70866_625873-bk09ch04.indd 708 9/21/10 10:46 PM9/21/10 10:46 PM

Book IX

Chapter 4

Running DHCP

and DNS

709

Running a DHCP Server

4. Click Apply.

The Add/Remove Software program grinds and whirs for a moment and

then installs the package you selected.

5. Close the Add/Remove Software program.

You’re done! DHCP is now installed.

Configuring DHCP

You configure DHCP settings through a file called dhcpd.conf that lives

in the /etc directory. Fedora provides you with a sample configuration file

located in the directory /usr/share/doc/dhcp-version/dhcpd.conf.

sample. Open this file in the text editor and then save it to the /etc direc-

tory, changing its name from dhcpd.conf.sample to just dhcpd.conf.

Then, edit the file to reflect the settings that you want to use.

Listing 4-1 shows the sample configuration file that comes with Fedora. (The

exact contents of this file vary from release to release and include additional

comments that I’ve removed for the sake of brevity.)

Listing 4-1: A Sample dhcpd.conf File

ddns-update-style interim;

ignore client-updates;

subnet 192.168.0.0 netmask 255.255.255.0 {

# --- default gateway

option routers 192.168.0.1;

option subnet-mask 255.255.255.0;

option nis-domain “domain.org”;

option domain-name “domain.org”;

option domain-name-servers 192.168.1.1;

option time-offset -18000; # Eastern Standard Time

# option ntp-servers 192.168.1.1;

# option netbios-name-servers 192.168.1.1;

# --- Selects point-to-point node (default is hybrid). Don’t change this

# -- unless you understand Netbios very well

# option netbios-node-type 2;

range dynamic-bootp 192.168.0.128 192.168.0.255;

default-lease-time 21600;

max-lease-time 43200;

# we want the nameserver to appear at a fixed address

host ns {

next-server marvin.redhat.com;

hardware ethernet 12:34:56:78:AB:CD;

fixed-address 207.175.42.254;

}

66_625873-bk09ch04.indd 70966_625873-bk09ch04.indd 709 9/21/10 10:46 PM9/21/10 10:46 PM

710

Running a DHCP Server

The following paragraphs describe some of the key points of this file:

✦ ddns-update-style: The DHCP standards group is in the midst

of deciding exactly how DHCP will handle changes to DNS data. This

option specifies that the interim method should be used. This line is

required — so don’t mess with it.

✦ subnet: This line specifies a subnet that’s managed by this DHCP

server. Following the subnet ID and netmask is an opening bracket; all

the options that appear between this bracket and the closing bracket in

the last line of the file belong to this subnet. In some cases, your DHCP

server may dole out IP configuration information for two or more subnet

groups. In that case, you need additional subnet groups in the configura-

tion file.

✦ option routers: This line provides the IP address of the Default

Gateway.

✦ option subnet mask: This line provides the subnet mask for the

subnet.

✦ option nis-domain: This line provides the NIS domain name. This

line is important only if you’ve set up one or more NIS servers.

✦ option domain-name: This line provides the domain name for the

network.

✦ option domain-name-servers: This line provides the IP addresses

of your DNS servers.

✦ range: This line specifies the range of addresses that the DHCP server

will assign for this subnet.

✦ default-lease-time: This line determines the default lease time in

seconds.

✦ max-lease-time: This line determines the maximum life of a lease.

✦ host: This line specifies a reservation. The host group specifies the

MAC address for the host and the fixed IP address to be assigned.

Starting DHCP

After you’ve set up the configuration file, you can start DHCP by opening a

terminal window or virtual console and entering the following command:

dhcpd start

If an error exists in the configuration file, a message to that effect is dis-

played. You have to edit the file in order to correct the error and then start

the DHCP service again.

66_625873-bk09ch04.indd 71066_625873-bk09ch04.indd 710 9/21/10 10:46 PM9/21/10 10:46 PM

Book IX

Chapter 4

Running DHCP

and DNS

711

Running a DNS Server

You should also restart the service whenever you make a change to the con-

figuration file. To restart DHCP, enter this command:

dhcpd restart

To automatically start DHCP whenever you start the computer, run this

command:

chkconfig –level 35 dhcpd on

Running a DNS Server

Linux comes with BIND, the best DNS server that money can buy. BIND is

an extremely powerful program. Some people make entire careers of setting

up and configuring BIND. In these few short pages, I just touch on the very

basics of setting up a DNS server on your network.

You can find plenty of details about DNS in Book IV, Chapter 4. Please review

that chapter before playing with BIND on your Linux system.

Installing BIND

You can quickly find out whether BIND is installed on your system by enter-

ing the following command from a shell prompt:

rpm -q bind

If BIND has been installed, the package version is displayed. If not, the mes-

sage package bind is not installed is displayed.

BIND is usually installed by default when you install Linux as a network

server. If it isn’t, you can easily install it by following these steps:

1. Choose System➪Administration➪Add/Remove Software.

This summons the Add/Remove Software program.

2. Type Bind in the text box and click the Find button.

The BIND package will be located.

3. Select the Berkeley Internet Name Domain (BIND) DNS (Domain Name

Server) package.

4. Click Apply.

The Add/Remove Software program grinds and whirs for a moment and

then installs the package you selected.

66_625873-bk09ch04.indd 71166_625873-bk09ch04.indd 711 9/21/10 10:46 PM9/21/10 10:46 PM

712

Running a DNS Server

5. Close the Add/Remove Software program.

You’re done! BIND is now installed.

Looking at BIND configuration files

Although Fedora Linux includes a handy BIND configuration tool, you still

need to know the location and purpose of each of BIND’s basic configuration

files. These files are described in the following sections.

named.conf

This file, found in the /etc directory, is the basic BIND configuration file.

This file contains global properties and links to the other configuration files.

Because the Fedora BIND configuration tool edits this file, you shouldn’t

edit this file directly. If you need to set your own configuration options, use

named.custom instead.

Here’s a typical named.conf file:

## named.conf - configuration for bind

# Generated automatically by redhat-config-bind, alchemist et al.

# Any changes not supported by redhat-config-bind should be put

# in /etc/named.custom

controls {

inet 127.0.0.1 allow { localhost; } keys { rndckey; };

};

include “/etc/named.custom”;

include “/etc/rndc.key”;

zone “0.0.127.in-addr.arpa” {

type master;

file “0.0.127.in-addr.arpa.zone”;

};

zone “localhost” {

type master;

file “localhost.zone”;

};

zone “lowewriter.com” {

type master;

file “lowewriter.com.zone”;

};

The line include “/etc/named.custom”; is what causes the named.

custom file to be read in. The zone lines name the zone files for each

domain for which the server is responsible.

66_625873-bk09ch04.indd 71266_625873-bk09ch04.indd 712 9/21/10 10:46 PM9/21/10 10:46 PM

Book IX

Chapter 4

Running DHCP

and DNS

713

Running a DNS Server

By default, this file always includes two zones: 0.0.127.in-addr.arpa,

which is the reverse-lookup zone for localhost, and localhost, the zone

file for the local computer. Any other zones that you’ve added through the

Fedora BIND configuration tool appear in this file as well.

named.custom

This file, also found in /etc, lets you add information to the named.conf

file. Here’s a typical named.custom file:

## named.custom - custom configuration for bind

# Any changes not currently supported by redhat-config-bind should be put

# in this file.

zone “.” {

type hint;

file “named.ca”;

};

options {

directory “/var/named/”;

};

One reason to use this file is if you want to include zone files that you create

yourself without the aid of the Fedora BIND configuration program. If you

want to include your own zone file, just add a zone statement that names

the zone file. For example, suppose that you want to add a zone named

cleaver.com, and you’ve manually created the cleaver.com.zone. To

include this zone, add these lines to the named.custom file:

zone “cleaver.com” {

type master;

file “cleaver.com.zone”;

};

named.ca

This file, located in the /var/named directory, lists the names and

addresses of the Internet’s root servers. It’s a fascinating file to look at

because it helps to unveil the mystery of how the Internet really works. You

shouldn’t change it, however, unless, of course, you happen to be the admin-

istrator of one of the Internet’s root servers — in which case, I hope you’re

not reading this book to learn how BIND works.

Here’s the named.ca file that ships with Fedora 12:

; This file holds the information on root name servers needed to

; initialize cache of Internet domain name servers

; (e.g. reference this file in the “cache . <file>”

; configuration file of BIND domain name servers).

;

66_625873-bk09ch04.indd 71366_625873-bk09ch04.indd 713 9/21/10 10:46 PM9/21/10 10:46 PM

714

Running a DNS Server

; This file is made available by InterNIC

; under anonymous FTP as

; file /domain/named.cache

; on server FTP.INTERNIC.NET

; -OR- RS.INTERNIC.NET

;

; last update: Jan 29, 2004

; related version of root zone: 2004012900

;

; formerly NS.INTERNIC.NET

;

. 3600000 IN NS A.ROOT-SERVERS.NET.

A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4

;

; formerly NS1.ISI.EDU

;

. 3600000 NS B.ROOT-SERVERS.NET.

B.ROOT-SERVERS.NET. 3600000 A 192.228.79.201

;

; formerly C.PSI.NET

;

. 3600000 NS C.ROOT-SERVERS.NET.

C.ROOT-SERVERS.NET. 3600000 A 192.33.4.12

;

; formerly TERP.UMD.EDU

;

. 3600000 NS D.ROOT-SERVERS.NET.

D.ROOT-SERVERS.NET. 3600000 A 128.8.10.90

;

; formerly NS.NASA.GOV

;

. 3600000 NS E.ROOT-SERVERS.NET.

E.ROOT-SERVERS.NET. 3600000 A 192.203.230.10

;

; formerly NS.ISC.ORG

;

. 3600000 NS F.ROOT-SERVERS.NET.

F.ROOT-SERVERS.NET. 3600000 A 192.5.5.241

;

; formerly NS.NIC.DDN.MIL

;

. 3600000 NS G.ROOT-SERVERS.NET.

G.ROOT-SERVERS.NET. 3600000 A 192.112.36.4

;

; formerly AOS.ARL.ARMY.MIL

;

. 3600000 NS H.ROOT-SERVERS.NET.

H.ROOT-SERVERS.NET. 3600000 A 128.63.2.53

;

; formerly NIC.NORDU.NET

;

. 3600000 NS I.ROOT-SERVERS.NET.

I.ROOT-SERVERS.NET. 3600000 A 192.36.148.17

;

; operated by VeriSign, Inc.

;

66_625873-bk09ch04.indd 71466_625873-bk09ch04.indd 714 9/21/10 10:46 PM9/21/10 10:46 PM

Book IX

Chapter 4

Running DHCP

and DNS

715

Running a DNS Server

. 3600000 NS J.ROOT-SERVERS.NET.

J.ROOT-SERVERS.NET. 3600000 A 192.58.128.30

;

; operated by RIPE NCC

;

. 3600000 NS K.ROOT-SERVERS.NET.

K.ROOT-SERVERS.NET. 3600000 A 193.0.14.129

;

; operated by ICANN

;

. 3600000 NS L.ROOT-SERVERS.NET.

L.ROOT-SERVERS.NET. 3600000 A 198.32.64.12

;

; operated by WIDE

;

. 3600000 NS M.ROOT-SERVERS.NET.

M.ROOT-SERVERS.NET. 3600000 A 202.12.27.33

; End of File

An organization named InterNIC keeps the named.ca file up to date. You can

download the most current version of named.ca from InterNIC’s FTP site at

ftp.internic.net. Every once in awhile, InterNIC publishes a new version

of this file, so you should check now and then to make sure that your file is

current.

named.local

This file, located in /var/named, is a zone file for your local computer —

that is, for the localhost domain. Rarely (if ever) do you need to modify it.

It typically looks like this:

$TTL 86400

@ IN SOA localhost. root.localhost. (

1997022700 ; Serial

28800 ; Refresh

14400 ; Retry

3600000 ; Expire

86400 ) ; Minimum

IN NS localhost.

1 IN PTR localhost.

Zone files

Each zone for which your DNS server is authoritative should have a zone file,

named domain.zone and located in the /var/named directory. If you like

to edit DNS records directly, you can create this file yourself. Or you can use

the point-and-click interface of the Fedora BIND configuration tool to auto-

matically create the file.

66_625873-bk09ch04.indd 71566_625873-bk09ch04.indd 715 9/21/10 10:46 PM9/21/10 10:46 PM

716

Here’s a typical zone file, named lowewriter.com.zone:

$TTL 86400

@ IN SOA ns207.pair.com. root.localhost (

2 ; serial

28800 ; refresh

7200 ; retry

604800 ; expire

86400 ; ttl

)

IN NS ns000.ns0.com.

IN NS ns207.pair.com.

@ IN MX 1 sasi.pair.com.

www IN A 209.68.34.15

Table 4-1 lists the most common types of records that appear in zone files.

For a complete description of each of these record types, refer to Book IV,

Chapter 4.

Table 4-1 Common Resource Record Types

Type Name Description

SOA Start Of Authority Identifies a zone.

NS Name Server Identifies a name server that is authoritative

for the zone.

A Address Maps a fully qualified domain name to an IP

address.

CNAME Canonical Name Creates an alias for a fully qualified domain

name.

MX Mail Exchange Identifies the mail server for a domain.

PTR Pointer Maps an IP address to a fully qualified domain

name for reverse lookups.

Restarting BIND

BIND runs as a service called named. As a result, when you make changes to

your DNS configuration, you have to restart the named service to apply the

changes. To do that, use this command:

service named restart

You can also restart the named service from the Service Configuration tool.

Choose Main Menu➪System Settings➪Server Settings➪Services. This brings

up a dialog box that lists all of the running services. Scroll down the list to

find named, select it, and then click the Restart button.

Running a DNS Server

66_625873-bk09ch04.indd 71666_625873-bk09ch04.indd 716 9/21/10 10:46 PM9/21/10 10:46 PM