Chris Tyler - Fedora Linux

DHCP can assign any combination of two address types:

static

Addresses that are always assigned to a specific computer or network device and never change. Even though these do not change, they are still communicated to the device using the DHCP protocol. Static addresses should be used for any host that other users will need to connect to, such as a web server or printer.

dynamic

Addresses assigned from a pool on a first-come, first-serve basis. Dynamic addresses are appropriate for computers, such as desktop systems, which will be connecting to remote hosts but will never (or rarely) be a destination for network connections.

Table 7-2 shows a possible network configuration for a home or small office network that will use the network prefix 192.168.1. In this example, available addresses have been divided into four ranges, one each for servers, network devices, desktop and laptop systems, and network infrastructure.

Table 7-2. Example of a small-office network configuration

DHCP is configured through the text file /etc/dhcpd.conf , which contains configuration statements and comments. Configuration statements are case-insensitive and are separated by semicolons (;) whitespace doesn't matter. Some statements create blocks, delimited with curly braces ({}), that contain other statements. Comments start with # and continue to the end of the line.

The dhcpd.conf file starts out with global statements; only one is required:

ddns-update-style none;

This prevents the DHCP server from attempting to update records on the DNS server (which is prohibited by Fedora's default SELinux configuration).

The rest of the configuration statements are placed in a block as part of a subnet statement:

subnet 192.168.1.0 netmask 255.255.255.0 {

# Statements that apply only to this subnet...

}

These are the most commonly used configuration statements:

option routers 192.168.1.254

The default gateway. Packets destined for a host that is not in your local network are sent to this gateway for forwarding.

option subnet-mask 255.255.255.0

The subnet mask, which is used to determine whether an IP address is on the local network (which determines routing).

option domain-name-servers 192.168.1.1

Nameservers for this subnet (they may be in the subnet, or they may be external). If there is more than one, list them all, separating the IP addresses or hostnames with commas.

option domain-name " fedorabook.com "

The domain name for machines on this subnet. This is used as the default domain for hostname lookup, so that if a user types a command such as telnet server42 , the hostname will be looked up (using a nameserver) as server42.fedorabook.com .

option time-offset -21600

The difference (in seconds) between the local time zone and Coordinated Universal Time (UTC). -21600 indicates a time zone that is six hours behind Greenwich, England (Eastern Standard Time in North America).

option ntp-servers pool.ntp.org

The hostnames or addresses of any available network time protocol servers. The hostname pool.ntp.org accesses a server randomly drawn from a pool of publicly accessible timeservers. You can prepend your ISO country code to select only timeservers in your country; for example, ca.pool.ntp.org would randomly select a Canadian timeserver.

range 192.168.1.128 192.168.1.191

The range of address from which dynamic IP addresses will be assigned.

default-lease-time 86400

The normal lease time in seconds. 86,400 seconds corresponds to one day.

max-lease-time 172800

The maximum lease time, in case the client requests a lease that is longer than the default.

To configure static hosts, statements are placed in the block of a host statement:

host hostname {

# Statements that apply only to this host...

}

These are the statements that are most commonly used in a host block:

hardware ethernet aa:bb:cc:dd:ee:ff

Determines which Ethernet hardware MAC address will match this host block. This block will be selected if the hostname sent by the DHCP client matches the hostname in the host statement, or if the client's Ethernet card has the same MAC address as the hardware statement.

fixed-address 192.168.1.1

Specifies the static address for this host.

To configure a network that uses the layout shown in Table 7-2 , where the devices have the MAC addresses shown in Table 7-3 , you would write this /etc/dhcpd.conf file:

# Sample /etc/dhcpd.conf file

# Don't update DNS

ddns-update-style none;

# The local network is 192.168.1.X

subnet 192.168.1.0 netmask 255.255.255.0 {

option routers 192.168.1.254; # Default gateway

option subnet-mask 255.255.255.0; # Client netmask

option domain-name "fedorabook.com"; # Domain

option domain-name-servers 172.16.97.1; # Nameserver is .1

option time-offset -21600; # Eastern Standard Time

option ntp-servers pool.ntp.org; # Timeservers

default-lease-time 86400; # 1 day

max-lease-time 172800; # 2 days

# Dynamic configuration

range 192.168.1.128 192.168.1.191

# Static configuration for various hosts

host prime {

hardware ethernet 00:0c:0d:99:99:99 ;

fixed-address 192.168.1.1 ;

}

host cabinet {

hardware ethernet 00:0c:0d:aa:aa:aa ;

fixed-address 192.168.1.2 ;

}

host chatterbox {

hardware ethernet 00:0c:0d:bb:bb:bb ;

fixed-address 192.168.1.3 ;

}

host laser1 {

hardware ethernet 00:0c:0d:cc:cc:cc ;

fixed-address 192.168.1.64 ;

}

host multifunction1 {

hardware ethernet 00:0c:0d:dd:dd:dd ;