Set default gateway

DHCP also allows for client's gateway configuration.To set any client on the local network to use default gateway 10.1.1.1, add line "option routers 10.1.1.1" into dhcpd.conf file as demonstrated below:

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

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
  option domain-name-servers 10.1.1.1, 8.8.8.8;
  option routers 10.1.1.1;
}

subnet 192.168.0.0 netmask 255.255.0.0 {
}

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
  option routers 10.1.1.1;
}

DHCP will now set DHCP client with gateway 10.1.1.1.

Define DNS server

Another configuration parameter possible to be set by DHCP server to its client is a definition of DNS server. If you want your clients to use DNS server with an IP address 8.8.8.8 and 10.1.1.1 you can do it by including an option "domain-name-servers" to DHCP's configuration file.

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

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
  option domain-name-servers 10.1.1.1, 8.8.8.8;
}

subnet 192.168.0.0 netmask 255.255.0.0 {
}

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
  option routers 10.1.1.1;
}

DHCP default and max lease time

At this point we can add feww aditional settings to our DHCP configuration, namely the default and max lease time expiry.

• default-lease-time is a value in seconds in which a leased IP address expiry will be set to if DHCP client does not ask for any other specific expiry lease time
• max-lease-time is a value in seconds which defines a maximum expiry time for an IP address leased by DHCP server

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

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
}

subnet 192.168.0.0 netmask 255.255.0.0 {
}

Basic DHCP Configuration

By default, DHCP server configuration does not include any subnets on which DHCP server should lease IP addresses. Therefore, depends on your Linux system you may get the following error message when you attempt to start DHCP with default dhcpd.conf configuration file.

Starting ISC DHCP server: dhcpdcheck syslog for diagnostics. ... failed!

Examining log files such as /var/log/syslog reveals more details:

No subnet declaration for eth0 (some IP address). 

Your server may be connected to multiple network subnets. To start DHCP server, at least one subnet must be defined within the DHCP configuration file /etc/dhcp/dhcpd.conf. 

NOTE: if your server has access to more than one subnet, DHCP requires all subnets to be defined even though there isn't immediate intention to enable DHCP service on that subnet. 

Below is the simplest example of DHCP configuration file:

subnet 10.1.1.0 netmask 255.255.255.0 {
  range 10.1.1.3 10.1.1.254;
}

subnet 192.168.0.0 netmask 255.255.0.0 {
}

This configuration file instructs DHCP server to listen for DHCP client requests on subnet 10.1.1.0 with netmask 255.255.255.0. Furthermore, it will assign IP addresses in range 10.1.1.3 - 10.1.1.254. It also defines an empty definition for subnet with network ID 192.168.0.0. 

Alter the above code with your subnet and insert it into /etc/dhcp/dhcpd.conf. When ready, restart your DHCP server with ( restart command may vary ) :

# service isc-dhcp-server restart

How to install DHCP server in linux

Standard DHCP server implementation available in various Linux distributions is an Open source version maintained by ISC ( Internet System Consortium ).

Use the following commands to install DHCP on your Linux server:

Debian and Ubuntu:

# apt-get install isc-dhcp-server

Redhat & Fedora:

# yum install dhcp

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