How to use DHCP in Linux

How to use DHCP in Linux

DHCP is the Dynamic Host Configuration Protocol, which allows a networked machine to obtain its IP address and other information automatically from a DHCP server.

There are two methods through which a Linux system can obtain an IP address using DHCP:
1. Kernel level configuration.
2. User level configuration.

Kernel level auto-configuration:

To use kernel level auto configurtion select below options in the Linux kernel configuration.

-> Networking support (NET [=y])
-> Networking options
-> TCP/IP networking (INET [=y])
[*]   IP: kernel level autoconfiguration
[*]     IP: DHCP support

Advantage:
In case of your root file system in NFS and wanted to have a IP dynamicaly configirued though DHCP, kernel level DHCP will come to rescue you. Using kernel level DHCP you will have have a valid IP address by the time the Linux kernel have completed the booting and can immediately mount an NFS root file system.

Disadvantage:
1. Can only configure basic network parameters, which do not include items such as DNS servers.

User level auto-configuration

I will use DHCP client applet that comes Busybox named udhcpc

As per Busybox:

udhcpc is a DHCP client geared primarily toward embedded systems, while striving to be fully functional and RFC compliant. The udhcp client negotiates a lease with the DHCP server and runs a script when a lease is obtained or lost.

User level auto-configuration is much more flexible, but has the does require that there is a root file system available when the system has finished booting which does not depend on the network being configured. This file system is usually on a Flash, SD/MMC, etc.,

In order to use udhcpc, the Linux kernel must be configured with packet socket support (CONFIG_PACKET):

-> Networking support (NET [=y])
-> Networking options
<*> Packet socket

Once the udchpc obtained or lost lease it will run a script. This script should configure the system ip, gateway, and resolv.conf etc. The busybox provides sample scripts, which is placed in the examples/udhcp/ directory.

copy the sample scripts to /etc/udhcpc/ of your device.

edit the file /etc/udhcpc/sample.script changed the path where the scripts are located.

exec /etc/udhcpc/sample.$1

Create a dummy script named sample.fail

$ touch /etc/udhcpc/sample.fail

Now start the DHCP client.

$ udhcpc -i wlan0 -s /etc/udhcpc/sample.script
Configuring IP using DHCP
udhcpc (v1.18.3) started
Sending discover...
Sending select for 192.168.7.10...
Lease of 192.168.7.10 obtained, lease time 604800
deleting routers
route: ioctl 0x890c failed: No such process
adding dns 192.168.7.1
Initializing completed

That’s it.