Configuring the DHCP Server

Configuring the DHCP Server

Introduction

Normally if you have a cable modem or DSL, you get your home PC's IP address dynamically assigned from your service provider. If you install a home cable/DSL router between your modem and home network, your PC will most likely get its IP address at boot time from the home router instead. You can choose to disable the DHCP server feature on your home router and set up a Linux box as the DHCP server.

This chapter covers only the configuration of a DHCP server that provides IP addresses. The configuration of a Linux DHCP client that gets its IP address from a DHCP server is covered in Chapter 3, 'Linux Networking', on Linux Networking.

Download and Install the DHCP Package

Most RedHat and Fedora Linux software products are available in the RPM format. Downloading and installing RPMs aren't hard. If you need a refresher, Chapter 6, 'Installing Linux Software', covers how to do this in detail.

When searching for the file, remember that the DHCP server RPM's filename usually starts with the word dhcp followed by a version number like this: dhcp-3.0.1rc14-1.i386.rpm.

Debian Note: With Debian / Ubuntu the package name may include a version number. Use the dpkg --list | grep dhcp command to get a list of all your dhcp packages and use the output to infer what the DHCP server package name would be. In this case we can guess that the package name should be dhcp3-server. If you need a DEB package installation refresher you can take a look at Chapter 6, 'Installing Linux Software'.

root@u-bigboy:/tmp# dpkg --list | grep dhcp

ii dhcp3-client  3.0.3-6ubuntu7 DHCP Client

ii dhcp3-common  3.0.3-6ubuntu7 Files used by all the dhcp3* packages

root@u-bigboy:/tmp#

The /etc/dhcpd.conf File

When DHCP starts, it reads the file /etc/dhcpd.conf. It uses the commands here to configure your network. The standard DHCP RPM package doesn't automatically install a /etc/dhcpd.conf file, but you can find a sample copy of dhcpd.conf in the following directory which you can always use as a guide.

/usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample

You have to copy the sample dhcpd.conf file to the /etc directory and then you'll have to edit it. Here is the command to do the copying for the version 3.0p11 RPM file:

[root@bigboy tmp]# cp /usr/share/doc/dhcp-3.0pl1/dhcpd.conf.sample /etc/dhcpd.conf

Debian Note: With Debian / Ubuntu the configuration file name is /etc/dhcp*/dhcpd.conf and has the same syntax as that used by Redhat / Fedora.

Here is a quick explanation of the dhcpd.conf file: Most importantly, there must be a subnet section for each interface on your Linux box.

ddns-update-style interim

ignore client-updates

subnet 192.168.1.0 netmask 255.255.255.0

# List an unused interface here

subnet 192.168.2.0 netmask 255.255.255.0

There are many more options statements you can use to configure DHCP. These include telling the DHCP clients where to go for services such as finger and IRC. Check the dhcp-options man page after you do your install:

[root@bigboy tmp]# man dhcp-options

Note: The host statement seen in the sample dhcpd.conf file can be very useful. Some devices such as network printers default to getting their IP addresses using DHCP, but users need to access them by a fixed IP address to print their documents. This statement can be used to always provide specific IP address to DHCP queries from a predefined a NIC MAC address. This can help to reduce systems administration overhead.

How to Get DHCP Started

To get DHCP started:

1) Some older Fedora/RedHat versions of the DHCP server will fail unless there is an existing dhcpd.leases file. Use the command touch /var/lib/dhcp/dhcpd.leases to create the file if it does not exist.

[root@bigboy tmp]# touch /var/lib/dhcp/dhcpd.leases

2) Use the chkconfig command to get DHCP configured to start at boot:

[root@bigboy tmp]# chkconfig dhcpd on

With Debian / Ubuntu the equivalent command for the dhcp3-server package would be:

root@u-bigboy:/tmp# sysv-rc-conf dhcp3-server on

3) Use the service command to instruct the /etc/init.d/dhcpd script to start/stop/restart DHCP after booting

[root@bigboy tmp]# service dhcpd start

[root@bigboy tmp]# service dhcpd stop

[root@bigboy tmp]# service dhcpd restart

With Debian / Ubuntu the equivalent commands would be:

root@u-bigboy:/tmp# /etc/init.d/dhcp*-server start

root@u-bigboy:/tmp# /etc/init.d/dhcp*-server stop

root@u-bigboy:/tmp# /etc/init.d/dhcp*-server restart

4) Remember to restart the DHCP process every time you make a change to the conf file for the changes to take effect on the running process. You also can test whether the DHCP process is running with the following command; you should get a response of plain old process ID numbers:

[root@bigboy tmp]# pgrep dhcpd

5) Finally, always remember to set your PC to get its IP address via DHCP.

DHCP Servers with Multiple NICs

When a DHCP configured PC boots, it requests its IP address from the DHCP server. It does this by sending a standardized DHCP broadcast request packet to the DHCP server with a source IP address of 255.255.255.255.

If your DHCP server has more than one interface, you have to add a route for this 255.255.255.255 address so that it knows the interface on which to send the reply; if not, it sends it to the default gateway. (In both of the next two examples, we assume that DHCP requests will be coming in on interface eth0).

Note: More information on adding Linux routes and routing may be found in Chapter 3, 'Linux Networking'.

Note: You can't run your DHCP sever on multiple interfaces because you can only have one route to network 255.255.255.255. If you try to do it, you'll discover that DHCP serving working on only one interface.

Temporary Solution

You can temporarily add a route to 255.255.255.255 using the route add command as seen below.

[root@bigboy tmp]# route add -host 255.255.255.255 dev eth0

If you want this routing state to be maintained after a reboot, then use the permanent solution that's discussed next.

Permanent Solution

The new Fedora Linux method of adding static routes doesn't seem to support sending traffic out an interface that's not destined for a specific gateway IP address. The DHCP packet destined for address 255.255.255.255 isn't intended to be relayed to a gateway, but it should be sent using the MAC address of the DHCP client in the Ethernet frame. To avoid this problem add the route add command to your /etc/rc.local script.

A better alternative is to create a route file. In Fedora Linux, permanent static routes are added on a per interface basis in files located in the /etc/sysconfig/network-scripts directory. The filename format is route-interface-name so the filename for interface wlan0 would be route-wlan0. In this example the single 255.255.255.255 host address is routed through interface wlan0 via the gateway 192.168.1.254.

# File /etc/sysconfig/network-scripts/route-wlan0

255.255.255.255/32 via 192.168.1.254

Simple Linux routing is covered in Chapter 3, 'Linux Networking' and will add more clarity to adding permanent static routes.

Configuring Linux Clients to Use DHCP

A Linux NIC interface can be configured to obtain its IP address using DHCP with the examples outlined in , 'Chapter 3, Linux Networking'. Please refer to this chapter if you need a quick refresher on how to configure a Linux DHCP client.

Configuring Windows Clients to Use DHCP

Fortunately Windows defaults to using DHCP for all its NIC cards so you don't have to worry about doing any reconfiguration.

Using a Single DHCP Server to Serve Multiple Networks

As stated before, DHCP clients send their requests for IP addresses to a broadcast address which is limited to the local LAN. This would imply that a DHCP server is required on each subnet. Not so. It is possible to configure routers to forward DHCP requests to a DHCP server many hops away. This is done by inserting the IP address of the router's interface on the DHCP client's network into the forwarded packet. To the DHCP server, the non-blank router IP address field takes precedence over the broadcast address and it uses this value to provide a DHCP address that is meaningful to the client. The DHCP server replies with a broadcast packet, and the router, which has kept track of the initial forwarded request, forwards it back towards the client. You can configure this feature on Cisco devices by using the ip helper-address command on all the interfaces on which DHCP clients reside. Here is a configuration sample that points to a DHCP server with the IP address 192.168.36.25:

interface FastEthernet 2/1

ip address 192.168.1.30 255.255.255.0

ip helper-address 192.168.36.25

Simple DHCP Troubleshooting

The most common problems with DHCP usually aren't related to the server; after the server is configured correctly there is no need to change any settings and it therefore runs reliably. The problems usually occur at the DHCP client's end for a variety of reasons. The following sections present simple troubleshooting steps that you can go through to ensure that DHCP is working correctly on your network.

DHCP Clients Obtaining 169.254.0.0 Addresses

Whenever Microsoft DHCP clients are unable to contact their DHCP server they default to selecting their own IP address from the 169.254.0.0 network until the DHCP server becomes available again. This is frequently referred to as Automatic Private IP Addressing (APIPA). Here are some steps you can go through to resolve the problem:

Ensure that your DHCP server is configured correctly and use the pgrep command discussed earlier to make sure the DHCP process is running. Pay special attention to your 255.255.255.255 route, especially if your DHCP server has multiple interfaces.

Give your DHCP client a static IP address from the same range that the DHCP server is supposed to provide. See whether you can ping the DHCP server. If you cannot, double-check your cabling and your NIC cards.

DHCP uses the BOOTP protocol for its communication between the client and server. Make sure there are no firewalls blocking this traffic. DHCP servers expect requests on UDP port 67 and the DHCP clients expect responses on UDP port 68. Use tcpdump on the server's NIC to verify the correct traffic flows.

Other DHCP Failures

If the DHCP server fails to start then use your regular troubleshooting techniques outlined in Chapter 4, 'Simple Network Troubleshooting', to help rectify your problems. Most problems with an initial setup are often due to:

Incorrect settings in the /etc/dhcpd.conf file such as not defining the networks for which the DHCP server is responsible;

Firewall rules that block the DHCP bootp protocol on UDP ports 67 and 68;

Routers failing to forward the bootp packets to the DHCP server when the clients reside on a separate network.

Always check your /var/logs/messages file for dhcpd errors and remember that mandatory keywords in your configuration file may change when you upgrade your operating system. Always read the release notes to be sure.

Conclusion

In most home-based networks, a DHCP server isn't necessary because the DSL router / firewall usually has DHCP capabilities, but it is an interesting project to try. Just remember to make sure that the range of IP addresses issued by all DHCP servers on a network doesn't overlap because it could possibly cause unexpected errors. You might want to disable the router/firewall's DHCP server capabilities to experiment with your new Linux server.

A DHCP server may be invaluable in an office environment where the time and cost of getting a network engineer to get the work done may make it simpler for Linux systems administrators to do it by themselves.

Creating a Linux DHCP server is straightforward and touches all the major themes in the previous chapters. Now it's time to try something harder, but before we do, we'll do a quick refresher on how to create the Linux users who'll be using many of the applications outlined in the rest of the book.