Monday, April 10, 2006

DHCP hows to

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.
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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 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, the RPM chapter, 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.

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.


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

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 netmask {

# The range of IP addresses the server

# will issue to DHCP enabled PC clients

# booting up on the network


# Set the amount of time in seconds that

# a client may keep the IP address

default-lease-time 86400;
max-lease-time 86400;

# Set the default gateway to be used by

# the PC clients

option routers;

# Don't forward DHCP requests from this

# NIC interface to any other NIC

# interfaces

option ip-forwarding off;

# Set the broadcast address and subnet mask

# to be used by the DHCP clients

option broadcast-address;
option subnet-mask;

# Set the DNS server to be used by the

# DHCP clients

option domain-name-servers;

# Set the NTP server to be used by the

# DHCP clients

option nntp-server;

# If you specify a WINS server for your Windows clients,

# you need to include the following option in the dhcpd.conf file:

option netbios-name-servers;

# You can also assign specific IP addresses based on the clients'

# ethernet MAC address as follows (Host's name is "laser-printer":

host laser-printer {

hardware ethernet 08:00:2b:4c:59:23;




# List an unused interface here

subnet netmask {

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

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

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

If your DHCP server has more than one interface, you have to add a route for this 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 on Linux Networking.

Note: You can't run your DHCP sever on multiple interfaces because you can only have one route to network 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 using the route add command as seen below.

[root@bigboy tmp]# route add -host 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 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.

You have one of two choices. Add the route add command to your /etc/rc.local script, or add an entry like this to your /etc/sysconfig/static-routes file.


# File /etc/sysconfig/static-routes


eth0 host

Note: The /etc/sysconfig/static-routes file is a deprecated feature and Fedora support for it will eventually be removed.

Now that you have configured your server, it's time to take a look at the DHCP clients.

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. 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.

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 Addresses

Whenever Microsoft DHCP clients are unable to contact their DHCP server they default to selecting their own IP address from the 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:

o 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 route, especially if your DHCP server has multiple interfaces.

o 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.


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.

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