Dynamic Host Configuration Protocol (DHCP) on Raspberry Pi

Eric Internet of Things, ,

https://www.raspberrypi.org/learning/networking-lessons/lesson-3/plan/

In this lesson, students will learn how the Raspberry Pi can be used to demonstrate Dynamic Host Configuration Protocol on an isolated network.

By now it will be clear that repeatedly changing the /etc/network/interfaces file is time-consuming and laborious. There are a number of disadvantages to giving static IP addresses to all computers on the network. Consider what would happen when you want to add even more computers to your network.

  • Users must manually allocate IP addresses
  • Users must ensure that no two computers have the same address
  • It is time-consuming to edit the configuration file on every computer
  • It is not ideal for mobile devices like laptops, which frequently join and leave the network

How can we make this easier?

For the majority of the lesson, it is suggested that work is carried out by students in pairs. The Ethernet hub or switch should remain completely isolated, without any Ethernet cables connecting it into the main school network.

Learning objectives

  • Understand what Dynamic Host Configuration Protocol (DHCP) is
  • Know the role it plays in the overall structure of a computer network

Learning outcomes

All students are able to:

  • Understand the need for DHCP in a computer network
  • Use a DHCP server to acquire an IP address for a Raspberry Pi

Most students are able to:

  • Understand the internal logic of a DHCP server

Lesson summary

  • A discussion of the logical process followed by a DHCP service
  • Setting up one Raspberry Pi to be a DHCP server
  • Use other Raspberry Pis to get IP addresses from the server
  • Testing the network

Lesson introduction

Firstly, go over the concept of a computer server. A server is essentially a computer whose main purpose is to provide a service. A web server, for instance, provides the service of transmitting web pages, images and files to you over the Internet. A Minecraft server provides the service of preserving the 3D world, remembering what blocks are where and allowing the players to see each other. Servers are computers that are dedicated to a task (but they can be dedicated to more than one).

A computer or application wishing to use a server is often called a client because it is like a customer to the server. Web browsers are sometimes called web clients because they work like a customer to a web server. You have probably heard your copy of Minecraft referred to as the game client for the same reason.

Wouldn’t it be great if we could have a server that would take care of allocating IP addresses on our network and remembering who owns what address?

This is exactly what DHCP is for. DHCP stands for Dynamic Host Configuration Protocol; Dynamic means constantly changing, Host is just another word for a computer, Configuration refers to configuring your network settings, and Protocol means a set of rules that define how to do things.

Starter activity

A computing unplugged activity is quite good to get across the logical process followed by the DHCP service.

Begin by nominating one student to be the DHCP server; they own the set of cards, paper, and pen/pencil. The remaining students are now going to be the dynamic hosts/clients (constantly changing computers) on the network.

The DHCP server has a set of rules that must be followed; this is the protocol part of the name. One of the hosts/clients now wants to join the network; their name is Dave. This is how the conversation should go:

  • HOST: “Hello I am Dave, is there a DHCP server out there?”
  • DHCP: “Yes I am here, Dave. I can offer you address X.”
  • HOST: “DHCP server, can I take address X please.”
  • DHCP: “Dave, here is address X. You may keep it for 12 hours.”

The DHCP server hands over an address card to Dave and writes his name on the paper, along with the address that was given, the time when it was given, and the length of the lease (12 hours). The time it was given is used to keep track of the age of the lease.

Stop for a moment and consider if there was any part of this conversation that was unexpected. When a computer joins a network it has no way of knowing if a DHCP server is available, so it sends out a broadcast signal to the whole network asking if one is there. If one is available it will reply to the host offering an address; the host then officially requests the address. The part you might not have expected is that the address is given with a lease time, in this case 12 hours. Consider why this might be and continue below.

Now let’s suppose Dave wants to leave the network or is shutting down. The conversation would go like this:

  • HOST: “DHCP server, I am Dave and I am giving my IP address back to you.”
  • DHCP: “Thank you Dave, goodbye.”

Dave then hands his address card back to the DHCP server. The DHCP server puts the card back with the others and crosses his name out from the piece of paper. That address card could now be given out to another computer/host that joins the network.

Now consider what might happen if Dave didn’t shut down cleanly. Suppose the power cable was suddenly unplugged and he didn’t get a chance to neatly give his address back to the DHCP server; or perhaps he decided to just run off with the address! What would happen then? The DHCP server won’t give out the same address twice, so would the address be forever lost in limbo?

This is where the lease time comes in! The address will be in limbo but only until the lease time expires. After 12 hours goes by this might happen:

  • HOST: “Hello I am Fred, is there a DHCP server out there?”
  • DHCP: “Yes I am here Fred. It has been over 12 hours since I last heard from Dave so I can can offer you his old address X.”
  • HOST: “DHCP server, can I take address X please.”
  • DHCP: “Fred, here is address X. You may keep it for 12 hours.”

The DHCP server makes a new card with that address on, hands it to Fred, crosses the name Dave off the list, and replaces it with Fred.

So that is how this problem is dealt with; all addresses are given out with a time limit attached to them so that in the event of hosts crashing or suddenly leaving the network, the DHCP server will slowly repossess those addresses as their lease times expire.

An alternative version of this conversation might be:

  • HOST: “DHCP server I am Dave, 12 hours have gone by so can I renew address X please?”
  • DHCP: Dave you may keep the address for another 12 hours.

The DHCP server then updates the time at which the address was given to Dave on the paper. Note that not all DHCP servers will use a 12 hour lease; it can be either longer or shorter depending on the server in question. Now if Fred wants to join the network, he will be given a different address to Dave.

Main practical activity

Firstly, select one Raspberry Pi to act as the DHCP server. It can be a good idea to either put a sticker on it or move it to a more prominent place to avoid any confusion later on. We’ll need to install some software on this Pi, so for this first part you’ll need to connect it to another LAN for internet access.

On the server Pi only

Note: Because only one Raspberry Pi will be the DHCP server, this part of the activity is best carried out by one person with all the other students observing. We do not need more than one DHCP server; in fact, more than one can cause problems!

Enter the following commands:

sudo apt-get update
sudo apt-get install dnsmasq

Once that has finished you can disconnect from the LAN with internet access and return to the original practice hub/switch.

By convention, most DHCP servers have a static IP address which will be the first or lowest number in the IP address space for the network. For example, most private networks use a local IP address space of 192.168.0.X, where X is a number that is different for each device. Following this convention, our DHCP server will have a static IP address of 192.168.0.1; note the .1 at the end. The IP addresses it can serve out will then range from 192.168.0.2, .3, .4, and so on up to .254.

So first, let’s make the DHCP server Raspberry Pi have a static IP address as per this convention. To configure this we must edit the network interfaces file again. Enter the following command:

sudo nano /etc/network/interfaces

In this file eth0 refers to the Raspberry Pi Ethernet port and wlan0 refers to a wireless dongle if you are using one. Find the following line:

iface eth0 inet dhcp

This line tells the Raspberry Pi to try and get an IP address from a DHCP server for the interface eth0. So essentially this is making it a DHCP client, but we want to make this a DHCP server so this line must be disabled. Put a hash # character at the start of the line and add the following four lines below to configure the static IP address, just as you did in previous exercises:

# iface eth0 inet dhcp
auto eth0
iface eth0 inet static
address 192.168.0.1
netmask 255.255.255.0

Press Ctrl – O then Enter to save followed by Ctrl – X to quit nano. Now enter the following command to restart the networking service on the Raspberry Pi:

sudo service networking restart

This Raspberry Pi will now always have the IP address 192.168.0.1. You can double-check this by entering the command ifconfig; the IP address should be shown on the second line just after inet addr.

Next we need to configure the DHCP server software, dnsmasq, that was installed earlier. We are going to explicitly specify a configuration file for the dnsmasq service, so let’s first make a backup of the default config file and then save our one in its place. Enter the following commands:

cd /etc
sudo mv dnsmasq.conf dnsmasq.default
sudo nano dnsmasq.conf

You should now be editing a blank file. Copy and paste the following into it:

interface=eth0
dhcp-range=192.168.0.2,192.168.0.254,255.255.255.0,12h

The first line tells dnsmasq to listen for DHCP requests on the Ethernet port of the Pi. The second line specifies the range of IP addresses that can be given out; notice the 12h at the end of the line which specifies the lease time.

Press Ctrl – O then Enter to save followed by Ctrl – X to quit nano. Before we activate the server, make sure the DHCP server Pi is the only device connected to the practice hub/switch; unplug all other Ethernet connections. Enter the following command to restart the dnsmasq service:

sudo service dnsmasq restart

The DHCP service is now active and listening for requests from client host computers.

On all the remaining client Pis

Before reconnecting any remaining client Pis to the hub/switch, check that their /etc/network/interfaces files are configured to get an IP address from a DHCP server. Enter the following command:

sudo nano /etc/network/interfaces

Ensure that a static IP address is not specified and check the iface eth0 inet dhcp line is there; an example is below.

iface eth0 inet dhcp
# auto eth0
# iface eth0 inet static
# address 192.168.0.1
# netmask 255.255.255.0

Press Ctrl – O then Enter to save followed by Ctrl – X to quit nano.

Restart the networking service on the clients with the command sudo service networking restart; you can then go ahead and start reconnecting them to the hub/switch. They should immediately acquire an IP address from the DHCP server.

Check this by using the command ifconfig again; the IP addresses given out should be randomly selected from the range specified on the server.

Test the network

Once everyone has an IP address the network should work as expected. Test it using your chat program or by playing Minecraft together. Ensure that everyone can successfully ping the DHCP server with the command ping 192.168.0.1, and that they can ping each other with the command ping 192.168.0.X (where X is the fourth part of their IP address). The server should also be able to ping the clients.

One step further

If you want to take it one step further and observe the communication between the DHCP server and the clients, the following commands can be used on the client Pis.

Firstly, to shut down the Ethernet interface and give back your IP address to the DHCP server, enter this command:

sudo ifdown eth0

You should see output similar to the text below. Note the DHCPRELEASE line; this is the IP address being surrendered to the server.

Listening on LPF/eth0/b8:27:eb:aa:bb:cc
Sending on   LPF/eth0/b8:27:eb:aa:bb:cc
Sending on   Socket/fallback
DHCPRELEASE on eth0 to 192.168.0.1 port 67

Next, use the following command to start up the Ethernet interface and get an IP address from the DHCP server:

sudo ifup eth0

You should see output similar to the text below. Note the DHCPDISCOVER, DHCPREQUEST, DHCPOFFER, and DHCPACK lines. See how they correspond to what was being spoken during the starter activity?

Listening on LPF/eth0/b8:27:eb:aa:bb:cc
Sending on   LPF/eth0/b8:27:eb:aa:bb:cc
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 7
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPOFFER from 192.168.0.1
DHCPACK from 192.168.0.1
bound to 192.168.0.X -- renewal in 40000 seconds.

In normal practice you don’t need to keep using these commands because the same thing automatically happens when the Raspberry Pi boots up, shuts down or has its Ethernet port connected to another device.

Plenary

Students can now be invited to discuss similarities in the practical exercise to the starter activity.

One question that should be brought up is how the DHCP server can identify each computer that talks to it. In the starter activity the client host computer said “I am Dave” to the DHCP server before it had been given an IP address. The DHCP server then wrote Dave on the piece of paper against the IP address that was given. What is the equivalent of this for a real computer?

The answer is the MAC address (sometimes called the physical address). MAC stands for Media Access Control; it is a unique ID that is burnt into the hardware of an Ethernet device by the manufacturer. All network devices, including those using WiFi and Bluetooth, have a MAC. A MAC address is six bytes long and is often shown as six hexadecimal numbers separated by colons or dashes like this: 01:23:45:67:89:ab.

The MAC address of a Raspberry Pi can be shown using the ifconfig command; look under eth0 and on the first line just after HWaddr (hardware address). The MAC address will be something like b8:27:eb:aa:bb:cc. A Raspberry Pi MAC address always starts with b8:27:eb. So it’s actually the MAC address of the client host computer that the DHCP server stores to keep a record of who owns which IP address.

Take another look at the ifup and ifdown command output from earlier!