Last modified on 26 September 2007, at 06:41

LPI Linux Certification/DHCP Configuration

Detailed ObjectiveEdit

Weight: 2

Description:
Candidates should be able to configure a DHCP server. This objective includes setting default and per client options, adding static hosts and BOOTP hosts. Also included is configuring a DHCP relay agent and maintaining the DHCP server.

  • Key knowledge area(s):
    • DHCP configuration files, tterms and utilities
    • Subnet and dynamically-allocated range setup
  • The following is a partial list of the used files, terms and utilities:
    • dhcpd.conf
    • dhcpd.leases

DHCP configuration =Edit

OverviewEdit

Description: The candidate should be able to configure a DHCP server and set default options, create a subnet, and create a dynamically-allocated range. This objective includes adding a static host, setting options for a single host, and adding bootp hosts. Also included is to configure a DHCP relay agent, and reload the DHCP server after making changes.

Key files, terms, and utilities include:

dhcpd.conf 
dhcpd.leases

ExercisesEdit

DHCP?Edit

Most people reading this will already know the DHCP protocol. Just as a quick reminder. DHCP stands for Dynamic Host Configuration Protocol and is commonly used to distribute specific network settings in networks. Settings such as the default gateway, nameservers, IP addresses and much more.

As for a small illustration of the protocol itself.

<insert schematic about DHCP Requests here>

Configuring dhcpdEdit

After the installation of dhcpd the main configuration file can be found at /etc/dhcpd.conf. For Debian installations, one should edit /etc/default/dhcp as soon as the installation is finished and change the following line according to your setup.

INTERFACES="eth1" # or "eth1 eth2", whatever interfaces you wish to serve ip's.

The dhcpd.conf file is divided in global parameters and subnet specific parameters. Each subnet can override the global parameters. The most commonly used parameters are the following.

option domain-name "example.com"; 
option domain-name-servers "192.168.0.1, 193.190.63.172"
option subnet-mask 255.255.255.0; # global Subnet mask
default-lease-time 600; # Seconds each DHCP lease is granted and after which a request for the same ip is launched.
max-lease-time 7200; # If DHCP server does not respond, keep IP till 7200 seconds are passed.

subnet 192.168.0.0 netmask 255.255.255.240 { # Subnet for first 13 devices, 10 of which are servers, 3 printers
 range 192.168.0.10 192.168.0.13;  # Range of IP's for our printers
 option subnet-mask 255.255.255.240;
 option broadcast-address 192.168.0.15; # This is the subnets broadcast address
 option routers 192.168.0.14; # The gateway of this subnet
 option time-servers 192.168.0.14; # Gateway is running a timeserver
 option ntp-servers 192.168.0.14; # Gateway running a timeserver
}
subnet 192.168.0.16 netmask 255.255.255.224 { # Subnet for 29 computers
 range 192.168.0.17 192.168.0.45;
 option subnet-mask 255.255.255.224;
 option broadcast-address 192.168.0.47;
 option routers 192.168.0.46;
}
group {
 host server1 { # the first fixed server for subnet 192.168.0.0/28
  server-name server1;
  hardware ethernet 0f:45:d3:23:11:90;
  fixed-address 192.168.0.1;
 }
 host server2 {
  server-name server2;
  hardware ethernet 0f:45:d3:23:11:91;
  fixed-address 192.168.0.2;
 }
}

This example is just providing a hint about possible options and overrides.

More info can be found on dhcpd.conf and dhcp-options in man pages. Look in those pages too for information about using the DHCP server to serve BOOTP as well, usefull for diskless clients.