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This chapter is from the book

This chapter is from the book

Other DHCP Options

The configuration file in Example 3.9 specifies the configuration parameters that the network architect must define to the DHCP server. Many other options (as specified in RFC 2132) can be provided to a DHCP client. Some of these options, such as domain name, are widely used, and others, such as Impress server, are rarely used.

NOTE

The Impress server option specifies a list of Imagen Impress servers that the DHCP client can use. (Imagen Impress is a type of networked printer that is no longer manufactured.) This option was originally defined as a BOOTP (in RFC 1048 and RFC 951) vendor extension and is included as an option in DHCP for backward compatibility.

However, all these DHCP options are specified in the configuration file for an ISC DHCP server, using the syntax illustrated in Example 3.10.

Example 3.10

option option-name option-value;

Appendix B, "ISC DHCP Server Configuration File Reference," includes a complete list of options that can be specified to the ISC server.

Subnet Options

Some options should apply to all subnets, and others are specific to certain subnets. In the GSI network, all hosts use the same DNS server, but each subnet uses a different default router.

In an ISC DHCP server configuration file, global options are defined at the beginning of the file. These global options apply to each defined subnet so that the definitions need not be repeated in each subnet definition. The configuration file in Example 3.11 specifies that dns.genericstartup.com should be used by all the hosts in the GSI network and that those hosts should use genericstartup.com as their DNS domain. The default routers are specified for each subnet.

Example 3.11

option domain-name-servers "dns1.genericstartup.com", "dns2.genericstartup.com";
option domain-name "genericstartup.com";
# default lease = 30 days, max lease = 45 days
default-lease-time 2592000;
max-lease-time 3888000;
# Server subnet
subnet 192.168.11.0 netmask 255.255.255.0 {
  range 192.168.11.1 192.168.11.251;
  # 192.168.11.252 reserved for DHCP server
  # 192.168.11.253 reserved for DNS server
  # 192.168.11.254 reserved for router interface
  option routers 192.168.11.254;
  option subnet-mask 255.255.255.0;
  # default lease = 90 days, max lease = 120 days
  default-lease-time 7776000;
  max-lease-time 10368000;
}
# Staff subnet 1
subnet 192.168.12.0 netmask 255.255.255.0 {
  range 192.168.12.1 192.168.12.253;
  # 192.168.12.254 reserved for router interface
  option routers 192.168.12.254;
  option subnet-mask 255.255.255.0;
}
# Staff subnet 2
subnet 192.168.13.0 netmask 255.255.255.0 {
  range 192.168.13.1 192.168.13.253;
  # 192.168.13.254 reserved for router interface
  option routers 192.168.13.254;
  option subnet-mask 255.255.255.0;
  }
# Staff subnet 3
subnet 192.168.14.0 netmask 255.255.255.0 {
  range 192.168.14.1 192.168.14.253;
  # 192.168.14.254 reserved for router interface
  option routers 192.168.14.254;
  option subnet-mask 255.255.255.0;
  }
# Staff subnet 4
subnet 192.168.15.0 netmask 255.255.255.0 {
  range 192.168.15.1 192.168.15.253;
  # 192.168.15.254 reserved for router interface
  option routers 192.168.15.254;
  option subnet-mask 255.255.255.0;
  # default lease = 12 hrs, max-lease-time = 24 hrs
  default-lease-time 43200;
  max-lease-time 86400;
 }

Global Values for Options

Example 3.11 demonstrates that options can be specified as DNS names as well as IP addresses. The ISC server resolves any domain names in the configuration file and uses the corresponding IP address as the value for the associated option. If a domain name corresponds to more than one IP address, and the option allows more than one IP address to be sent, all the IP addresses for that domain name are sent. Example 3.11 illustrates that global values for options can be overridden with new values for specific subnets. The global values for default lease time and maximum lease time values are set to 30 days and 45 days, respectively; those values are set to 90 days and 120 days, respectively, within the server subnet declaration.

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