Networking - Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) is a network service that enables host computers to be automatically assigned settings from a server as opposed to manually configuring each network host. The most common settings provided by a DHCP server to DHCP clients include:

• IP-Address and Netmask
• DNS
• WINS
• Host Name
• Domain Name
• Default Gateway
• Time Server
• Print Server

A DHCP server can provide configuration settings using two methods:

MAC Address 

This method entails using DHCP to identify the unique hardware address of each network card connected to the network and then continually supplying a constant configuration each time the DHCP client makes a request to the DHCP server using that network device.  

Address Pool 

This method entails defining a pool (sometimes also called a range or scope) of IP addresses from which DHCP clients are supplied their configuration properties dynamically and on a fist come first serve basis. When a DHCP client is no longer on the network for a specified period, the configuration is expired and released back to the address pool for use by other DHCP Clients.

Follow the below stages to install and configure DHCP:

Stage 1]  Execute the following command to install DHCP server from internet in server computer.

susanta@admin:~$ sudo   apt-get   install   dhcp3-server

Stage 2]   If we have two network cards in your ubuntu server we need to select which interface you want to use for  DHCP server listening.By default it listens to eth0. To select another interface, edit the /etc/default/dhcp3-server file

susanta@admin:~$ sudo gedit /etc/default/dhcp3-server

 Defaults for dhcp initscript
# sourced by /etc/init.d/dhcp
# installed at /etc/default/dhcp3-server by the maintainer scripts

#
# This is a POSIX shell fragment
#

# On what interfaces should the DHCP server (dhcpd) serve DHCP requests?
#    Separate multiple interfaces with spaces, e.g. "eth0 eth1".
# INTERFACES="eth0"
INTERFACES="eth1"  




Stage 3]    Before editing the main configuration file of DHCP, take a backup of the file.

susanta@admin:~$ sudo  cp  /etc/dhcp3/dhcpd.conf   /etc/dhcp3/dhcpd.conf.back

Stage 4]   Now, edit the configuration file of DHCP server,

susanta@admin:~$ sudo   gedit   /etc/dhcp3/dhcpd.conf

#
# Sample configuration file for ISC dhcpd for Debian
#
# Attention: If /etc/ltsp/dhcpd.conf exists, that will be used as
# configuration file instead of this file.
#
# $Id: dhcpd.conf,v 1.1.1.1 2002/05/21 00:07:44 peloy Exp $
#

# The ddns-updates-style parameter controls whether or not the server will
# attempt to do a DNS update when a lease is confirmed. We default to the
# behavior of the version 2 packages ('none', since DHCP v2 didn't
# have support for DDNS.)
ddns-update-style none;

# option definitions common to all supported networks...
# option domain-name "example.org";
# option domain-name-servers ns1.example.org, ns2.example.org;

# default-lease-time 600;
# max-lease-time 7200;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
#authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.

#subnet 10.152.187.0 netmask 255.255.255.0 {
#}

# This is a very basic subnet declaration.

#subnet 10.254.239.0 netmask 255.255.255.224 {
#  range 10.254.239.10 10.254.239.20;
#  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
#}

# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.

#subnet 10.254.239.32 netmask 255.255.255.224 {
#  range dynamic-bootp 10.254.239.40 10.254.239.60;
#  option broadcast-address 10.254.239.31;
#  option routers rtr-239-32-1.example.org;
#}

# A slightly different configuration for an internal subnet.
#subnet 10.5.5.0 netmask 255.255.255.224 {
#  range 10.5.5.26 10.5.5.30;
#  option domain-name-servers ns1.internal.example.org;
#  option domain-name "internal.example.org";
#  option routers 10.5.5.1;
#  option broadcast-address 10.5.5.31;
#  default-lease-time 600;
#  max-lease-time 7200;
#}

# Hosts which require special configuration options can be listed in
# host statements.   If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.

#host passacaglia {
#  hardware ethernet 0:0:c0:5d:bd:95;
#  filename "vmunix.passacaglia";
#  server-name "toccata.fugue.com";
#}

# Fixed IP addresses can also be specified for hosts.   These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP.   Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
#host fantasia {
#  hardware ethernet 08:00:07:26:c0:a5;
#  fixed-address fantasia.fugue.com;
#}

# You can declare a class of clients and then do address allocation
# based on that.   The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.

#class "foo" {
#  match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
#}

#shared-network 224-29 {
#  subnet 10.17.224.0 netmask 255.255.255.0 {
#    option routers rtr-224.example.org;
#  }
#  subnet 10.0.29.0 netmask 255.255.255.0 {
#    option routers rtr-29.example.org;
#  }
#  pool {
#    allow members of "foo";
#    range 10.17.224.10 10.17.224.250;
#  }
#  pool {
#    deny members of "foo";
#    range 10.0.29.10 10.0.29.230;
#  }
#}

default-lease-time  600;
max-lease-time  7200;

option  subnet-mask  255.255.255.0;
option  broadcast-address  192.168.1.255;
option  routers  192.168.1.254;
option  domain-name-servers  192.168.1.1, 192.168.1.2;
option  domain-name  “yourdomainname.com”;

# Using address pool method

subnet  192.168.1.0  netmask  255.255.255.0 {
range  192.168.1.10  192.168.1.90;
}

# Using MAC address method

host  dns_server {
  hardware  ethernet  00:16:76:1e:6a:60;
  fixed-address  192.168.1.100;
}

Note: Must select subnet IP address is the starting IP address of DHCP server's IP address (Example: 192.168.1.2). And DHCP server's IP address is always a Static IP address.

Note: To get the MAC address of the Network Interface Card, execute the below command,

susanta@admin:~$ ifconfig

eth0     Link encap:Ethernet  HWaddr 00:16:76:1e:6a:60  
         UP BROADCAST MULTICAST  MTU:1500  Metric:1
         RX packets:0 errors:0 dropped:0 overruns:0 frame:0
         TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
         collisions:0 txqueuelen:1000
         RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
         Interrupt:21 Base address:0xde00

Stage 5]  Now, we need to restart the DHCP server restart to get effect of the change in configuration file.

susanta@admin:~$ sudo /etc/init.d/dhcp3-server start

 * Starting DHCP server dhcpd3                            [ OK ]

Note: If the service is unable to restart and get a error message as like "check syslog for diagnostics", then check the syslog file which contents DHCP error related info,

susanta@admin:~$ tail -n 100 /var/log/syslog

Note: Check the few lines of the file, because it is very big file.

Stage 5] Now we have to configure our DHCP client computer, just by editing the /etc/network/interfaces file,

susanta@admin:~$ sudo gedit /etc/network/interfaces

Note: The DHCP server writes all current IP address "leases" to the file /var/lib/dhcp3/dhcpd,

susanta@admin:~$ vi /var/lib/dhcp3/dhcpd.leases
 

Note: To check DHCP status, execute the below command,

susanta@admin:~$ netstat -uap