11. Configuring dhcp-client

11.1 The Configuration Files

dhcp-client will look for two files for a configuration information. Both these files should be located in the system configuration directory. By default this is defined as `/usr/local/etc' You can change this definition at build time by specifying a different prefix to the configure script.

The first file that is checked for begins with the name of the interface the client is configuring. For example, if the name of our interface is "xl0" then the configuration file to be checked will be `/usr/local/etc/dhcp-agent/dhcp-client/xl0.conf'

If this file does not exist then the default configuration file is `/usr/local/etc/dhcp-agent/dhcp-client/default.conf'

This allows dhcp-client to accept configuration per interface, and use a default file without forcing the user to name interfaces in the actual configuration file. It is believed that configuration files are more portable this way and are not dependant on interface names by avoiding having to specify interface names in the configuration file.

11.2 Configuration Directives

DHCP Option Handling

The following directives affect DHCP option handling by the client

Client Configuration Directive: request

This directive instructs the client to request a set of dhcp options. The request directive accepts a list of strings which name the options to be requested:

request subnet-mask, ip-address-lease-time, renewal-time, rebinding-time, interface-mtu, domain-name, domain-name-servers;

Client Configuration Directive: require

This directive instructs the client to require a set of dhcp options to be passed before accepting the lease. This is useful to make sure you receive a minimum of desired options. If these options are never met the client will eventually timeout. If this directive is not set, the first DHCP lease offer will be accepted.

require subnet-mask, ip-address-lease-time, renewal-time, rebinding-time, interface-mtu;

Client Configuration Directive: configure

This directive instructs the client to only configure a set of dhcp options. This is useful if the server is passing you options you would rather not configure but implicitly claim to configure by accepting the lease. This directive does not define how options are configured. In order to define how configuration is done see "Writing Client Extensions," for more info.

configure subnet-mask, ip-address-lease-time, renewal-time, rebinding-time, interface-mtu, domain-name, domain-name-servers;

Client Configuration Directive: append

This directive instructs the client to append a value to a DHCP option prior to system configuration. If this option is a list of values, the value specified is merely appended. If this value is a string the value is appended to the string. If this option is a single datum then you should be using the override option instead.

append domain-name-servers = 127.0.0.1, 192.168.0.1;

Client Configuration Directive: prepend

This directive works the same as the append directive only the values are placed at the beginning of the list or string. You should not use prepend on single datum options and should use override instead.

prepend domain-name-servers = 127.0.0.1, 192.168.0.1;

Client Configuration Directive: override

This directive instructs the client to overwrite a DHCP option with a specified value prior to system configuration. This is useful to for clients that wish to use an alternate value for a DHCP option than the one passed by the DHCP server.

override domain-name = "example.com";

Variable Settings

Client Configuration Directive: set

This directive instructs the client to set a variable to a defined value.

set default-subnet-mask = 255.255.255.0;

Client Configuration Directive: enable

This directive instructs the client to set a boolean variable to a defined value. The boolean variable only accepts "yes" or "no" for values.

enable do-measure-router-latency = no;

Configurable Variables

dhcp-client accepts a number of variables to configure default values, values passed to the server, and control some of its behaviour.

Client Configuration Variable: hostname

This variable can be set to a string which is passed as the hostname DHCP option to the server. This allows servers to pass configuration based on a hostname passed by the client. It will also instruct the client to set the hostname to that variable. If you want to pass a hostname to the server you should set this variable and not use the "override" directive because the "override" directive only affects system configuration.

set hostname = "foo.example.com";

Client Configuration Variable: dhcp-discovery-retries

This variable can be set to an integer value which instructs the client on how many times it should retry a DHCP discover before it gives up.

set dhcp-discovery-retries = 3;

Client Configuration Variable: icmp-retries

This variable can be set to an integer value which instructs the client on how many times it should retry an ICMP operation. This affects operations such as router latency discovery, ICMP netmask discovery etc.

set icmp-retries = 3;

Client Configuration Variable: default-interface-mtu

This variable can be set to an integer value which instructs the client on what it's default interface MTU should be. This MTU is used during the initial DHCP discovery messages, and later used to configure the interface MTU if no MTU was specified by the DHCP server.

set default-interface-mtu = 1500;

Client Configuration Variable: default-subnet-mask

This variable can be set to a netmask value to specify a default subnet mask in case none is provided by the DHCP operation. Unlike the "override" directive this provides a fallback in case no subnet-mask is provided by the server.

set default-subnet-mask = 255.255.255.0;

Client Configuration Variable: do-measure-router-latency

This variable can be set to a boolean value to indicate whether or not the client should attempt to send ICMP ECHO requests to routers passed by the DHCP server and determine which is one has the least latency to become the default route. Disabling this variable stops this operation from taking place, and the first router is used as the default route. This is useful if the routers cannot be reached by ICMP ECHO requests.

enable do-measure-router-latency = yes;