D.3.1 Topology Statements

server-identifier hostname ;

The server-identifier statement documents the IP address of the server. It is sometimes used at the start of the file as the first statement of a group of parameter statements and option statements that apply to every network served by this server. The documentation calls these "global parameters."

group {[ parameters ] [ options ] }

The group statement groups together shared-network , subnet , host , or other group statements to apply a set of parameters or options to all members of the group.

shared-network name { [ parameters ] [ options ] }

The shared-network statement is used only if more than one IP subnet shares the same physical network. In most cases, different subnets are on different physical networks. The name , which must be provided, can be any descriptive name. It is used only in debugging messages. Parameters and options associated with the shared network are declared within the curly braces and apply to all subnets in the shared network. The subnets in a shared network must be defined within the curly braces of the shared-network statement. It is assumed that each shared-network statement contains at least two subnet statements; otherwise there is no need to use the shared-subnet statement. dhcpd cannot tell on which subnet of a shared network a client should boot. Therefore, dynamically allocated addresses are taken from the available range of all subnets on the shared network and assigned as needed.

subnet address mask netmask { [ parameters ] [ options ] }

The subnet statement defines the IP address and address mask of every subnet the daemon will serve. The address and mask are used to identify the clients that belong to the subnet. The parameters and options defined within the curly braces apply to every client on the subnet. Every subnet physically connected to the server must have a subnet statement even if the subnet does not have any clients.

host hostname {[ parameters ] [ options ] }

The host statement defines parameters and options for individual clients. Every BOOTP client must have a host statement in the dhcpd.conf file. For DHCP clients, the host statement is optional. It is matched to an actual DHCP or BOOTP clients by matching the dhcp-client-identifier provided by the client or by matching the hardware parameter to the hardware address of the client. BOOTP clients do not provide a dhcp-client-identifier, so use the hardware address for BOOTP clients. DHCP clients can be identified by either the dhcp-client-identifier or the hardware address.

D.3.2 Configuration Parameters

The parameter statements defined in this section control the operation of the DHCP server and the DHCP protocol. The standard DHCP configuration values that are passed to clients are defined in option statements, which are covered in the next section. Some parameter statements can be associated with any of the topology statements discussed above. Others can only be used with specific statements. These are noted in the description of the parameter.

range [ dynamic-bootp ] low-address [ high-address ] ;

The range parameter defines the scope of addresses that are available for dynamic assignment by defining the lowest and highest IP addresses available for assignment. The range parameter must be associated with a subnet statement. All addresses in the scope of the range parameter must be in the subnet in which the range parameter is declared. The dynamic-bootp flag is specified if addresses may be automatically assigned to BOOTP clients as well as DHCP clients. The range parameter must be defined if you intend to use dynamic address assignment. If the subnet statement does not include a range parameter, dynamic address assignments are not made to clients on the subnet.

default-lease-time seconds ;

The life of an address lease in seconds that is used if the client does not request a specific lease length.

max-lease-time seconds ;

The maximum life of an address lease in seconds regardless of the lease length the client requests.

hardware type address ;

Defines a client's hardware address. At present, type must be either ethernet or token-ring . address must be an appropriate physical address for the type of hardware. The hardware parameter must be associated with a host statement. It is required for a BOOTP client to be recognized. It is optional for DHCP clients for which it is an alternative to the dhcp-client-identifier option.

filename file ;

Identifies the boot file for diskless clients. file is an ASCII string enclosed in quotation marks.

server-name name ;

The hostname of the DHCP server that is provided to the client. name is an ASCII string enclosed in quotation marks.

next-server name ;

The hostname or address of the server from which the boot file is to be loaded.

fixed-address address [ , address ... ] ;

Assigns one or more fixed IP addresses to a host. The fixed-address parameter is valid only when associated with a host statement. If more than one address is supplied, the client is assigned the address that is valid for the subnet on which it is booting. If none of the addresses is valid for the subnet, no configuration data is sent to the client.

dynamic-bootp-lease-cutoff date ;

Sets a termination date for addresses assigned to BOOTP clients. BOOTP clients do not have a way of renewing leases and don't know that address leases expire. By default, dhcpd assigns permanent address to BOOTP clients. This parameter changes that behavior. It is used only in special circumstances where the life of all systems is known in advance - for example, on a college campus where it is known that all student systems will be removed by June.

dynamic-bootp-lease-length seconds ;

Defines the life of an address lease in seconds for an address automatically assigned to a BOOTP client. As noted above, BOOTP clients do not understand address leases. This parameter is used only in special circumstances where clients use a BOOTP boot PROM and run an operating system that supports DHCP. During the boot the client acts as a BOOTP client, but once it boots the client runs DHCP and knows how to renew a lease. Use this parameter, and the previous one, with caution.

boot-unknown-clients flag ;

Tells dhcpd whether or not to dynamically assign addresses to unknown clients. If flag is "false," addresses are provided only to clients that have a host statement in the configuration file. By default, the flag is "true" and addresses are dynamically assigned to any client on a valid subnet.

get-lease-hostnames flag ;

Tells dhcpd if it should send a DNS hostname to the client when it dynamically assigns it an IP address. If flag is "true," dhcpd uses DNS to look up the hostnames for all dynamically assigned addresses, which dramatically slows DHCP performance. By default the flag is "false" and no lookups are done.

use-host-decl-names flag ;

Causes the name provided on the host statement to be supplied to the client as its hostname.

D.3.3 DHCP Options

The option statements available with dhcpd cover all of the standard DHCP configuration options currently defined in the RFCs. Furthermore the syntax of the dhcpd.conf option statement is extensible. A new option can be identified by its decimal option code. All options are assigned a decimal option code, either in the RFC that describes the option or in the vendor documentation if it is vendor-specific. The value assigned to the new option can be expressed as a string enclosed in quotes or as a colon-separated list of hexadecimal numbers. Imagine that a new DHCP option is created and assigned an option code of 133. Further, imagine that the value carried by this option is a 16-bit binary mask and that you want your clients to "turn on" the high-order 4-bit and "turn off" all other bits in the mask. You could add the following option to your configuration:

option option-133 F0:00

All option statements begin with the keyword option . The keyword is then followed by the name of the option and the value assigned to the option, in that order. In the example above, the option name is in the form option- nnn , where nnn is the decimal option code assigned to the option. In this manner any new option that appears can be added to dhcpd.conf file. The value assigned to this imaginary option is F000.

Looking at the huge list of standard options, you may well wonder if they will ever need to be extended. The standard options are listed in the following section. The types of values that are assigned to options are:

Address

An IP address written in dotted decimal notation or a host name that resolves to an address

String

A series of characters enclosed in quotation marks

Number

A numeric value

Flag

A switch containing either 1 or 0

In this book, the list of options is divided into "Commonly used options" and "Other options."