Dynamic Host Configuration Protocol (DHCP)
1. Introduction
The Dynamic Host Configuration Protocol
(DHCP) provides configuration parameters to the Internet hosts. On a typical
LAN, a DHCP server allocates network addresses and delivers network
configuration parameters such as the gateway, netmask, DNS, etc. DHCP supports
three mechanisms to allocate IP addresses:
- Automatic allocation: A permanent IP address is allocated to a client.
- Dynamic allocation: An IP address is assigned for a limited period of time by the DHCP server.
- Manual allocation: The network administrator explicitly assigns an IP address to a particular client and DHCP is used to convey the assigned address.
This guide will demonstrate how to set up a
DHCP server and client. More information regarding the protocol can be found in
the “References” section.
2. Installtion
Many distributions come with a dhcpd
server and client. In case you would like to install it manually, however, you
can use rpm -ivh dhcp-VERSION.rpm to install the rpm. The DHCP server
can be downloaded from "http://people.redhat.com/~jvdias/DHCP/". DHCP
reads from a configuration /etc/dhcpd.conf. The rpm package does not install this file,
but you can use the sample DHCP configuration file that can be found under /usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample.
Copy this file to /etc and rename it to dhcpd.conf.
# cp
/usr/share/doc/dhcp-3.0.1/dhcpd.conf.sample /etc/dhcpd.conf
2.1. Server Configuration
The sample dhcpd.conf file is shown
below:
|
ddns-update-style interim;
ignore client-updates;
subnet 192.168.0.0 netmask 255.255.255.0 {
# --- default gateway
option routers
192.168.0.1;
option subnet-mask
255.255.255.0;
option nis-domain
"domain.org";
option domain-name "domain.org";
option domain-name-servers
192.168.1.1;
option time-offset
-18000; # Eastern Standard Time
#
option ntp-servers
192.168.1.1;
#
option netbios-name-servers
192.168.1.1;
# --- Selects point-to-point node (default
is hybrid). Don't change this unless
# -- you understand Netbios very well
#
option netbios-node-type 2;
range dynamic-bootp 192.168.0.128 192.168.0.254;
default-lease-time 21600;
max-lease-time 43200;
# we want the nameserver to appear at a fixed address
host ns {
next-server
marvin.redhat.com;
hardware ethernet
12:34:56:78:AB:CD;
fixed-address 207.175.42.254;
}
}
|
A DHCP configuration file contains the
configuration parameters for the clients. The important section in a DHCP
client is "subnet 192.168.0.0 netmask 255.255.255.0" that specifies
the subnet and netmask for which the address will be allocated. Other important
options are:
option routers ---> specifies
default gateway for clients
option subnet-mask --->
netmask for clients
option domain-name-servers --->
DNS server to be used by the clients
range ---> Range of IP addresses that will be allocated to
clients
The working DHCP configuration file is given
below.
|
ddns-update-style interim;
ignore client-updates;
subnet 10.0.0.0 netmask 255.0.0.0 {
option routers
10.10.10.1;
option subnet-mask 255.0.0.0;
option broadcast-address
10.255.255.255;
option domain-name-servers
10.10.10.10;
range 10.10.10.10 10.10.10.50;
default-lease-time 21600;
max-lease-time 43200;
}
|
Make sure to include "option broadcast-address".
The above configuration file specifies that
the clients will be given an IP address in the range 10.10.10.10 to
10.10.10.50. The default lease time, in case the client does not explicitly ask
for it, will be 21600 seconds; otherwise the maximum allowed lease time is
43200 sconds. The server also specifies that the client should use the subnet
mask 255.0.0.0, 10.255.255.255 as its broadcast address, 10.10.10.1 as the
default gateway and 10.10.10.10 as the DNS server.
The network diagram
for this scenario is illustrated below:
Once you have edited the dhcpd.conf
file, start the server. If you get a failure message regarding lease file being
missing, create the following file:
# touch /var/lib/dhcp/dhcpd.leases
# service dhcpd start
You can check whether DHCP is running by pgrep
command
# pgrep dhcpd
The above configuration file dynamically
allocates an IP address. DHCP can also be configured as a mixed environment
that can allocate IP addresses dynamically and statically. The "host"
section in the sample configuration file allocates an IP, based on the hardware
address. Please see the “References” section for more advanced DHCP configurations. While
running,
DHCP server creates the client entries in /var/lib/dhcp/dhcpd.leases.
2.2. Client Configuration
The client side requires very little
configuration. Most distributions come with a pre-installed version of a DHCP
client. As an alternative, you can install rpm package, dhclient from
"http://people.redhat.com/~jvdias/DHCP/".
For RedHat, you can use netconfig to
configure the DHCP host.
# netconfig
This will ask for a couple of options. Click
on "Use dynamic IP configuration (BOOTP/DHCP)". Select "OK"
and restart the network services.
# service network
restart
If the DHCP server has been configured
properly, the client should be able to get an IP address. Use ipconfig
to check the IP.
# ifconfig
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