Section 15.1. Delivering IP Addresses with DHCP

15.1. Delivering IP Addresses with DHCP

Networks of all sizes use DHCP to simplify configuration of most computers on the network. A computer configured to use DHCP for basic network configuration tries to locate a DHCP server at startup and, if one is found, sets its basic network options to those specified by the DHCP server. This task may sound impossible because basic network configuration information is delivered via the network before the DHCP client is configured, but it is possible, and understanding the basics of how it's possible is the first order of business. Next up is an overview of DHCP configuration files. You can use DHCP to assign addresses either so that the same computer is or isn't guaranteed the same address on each boot, and understanding how to do both is important. Finally, knowing how to tell clients to use DHCP is critically important to this protocol's successful deployment.

15.1.1. The Role of DHCP

DHCP is an unusual protocol because it uses very low-level network protocols to help a computer "bootstrap" a more sophisticated configuration. When a DHCP client boots or starts its TCP/IP network stack, the client has no IP address, but it does have working hardware, and it can send data over the network wire. The DHCP client therefore sends a network broadcast with a query directed at any DHCP server that might be listening. Although the DHCP client has no IP address, it does have a working low-level hardware address, and the DHCP server is able to direct its reply to this address. The DHCP server's reply includes information on basic network settingsmost importantly, the client's IP address and network mask, as well as the IP addresses of the subnet's router and the DNS servers the client should use. Additional information can be delivered, as well, such as the IP addresses of NBNS computers, but this information isn't delivered by all DHCP servers or used by all clients.

DHCP works by issuing leases on IP addresses. The idea is that, should the client crash or otherwise become inaccessible, the IP address will revert back to the pool of available addresses after a limited time. In this way, a DHCP server can continue delivering addresses from a fixed set of addresses. Clients can also give up their leases voluntarilysay, when they're shut down. Not all clients do this, though. Leases have fixed terms, and when a lease expires, a client must renew that lease. (Typically, clients attempt to renew their leases halfway through their terms. If the initial renewal attempt fails, subsequent attempts are made.)

Considered on the network as a whole, DHCP simplifies configuration. Instead of entering several IP addresses on each client (for the client itself, including its netmask, the router, DNS servers, and perhaps more), DHCP enables you to set a single option to use DHCP on each client. The details of IP address assignment can then be handled by the server, either automatically or by assigning one address to each computer. This centralized control has a certain appeal by itself, but it's also helpful because it reduces the risk of a typo causing problems, and it makes it easier to change your network's configuration. For instance, if you need to change your network's DNS server address for some reason, you can do so by implementing a change in the DHCP server configuration.

Because DHCP works by providing leases of fixed length, changes to network features such as DNS and router IP addresses won't immediately propagate to all DHCP clients. Most clients should update their settings by half the DHCP lease time, though. You can adjust the lease time from the DHCP server.

Because DHCP can assign IP addresses arbitrarily to any computer that asks for one, it can be particularly handy on networks on which computers are frequently added and removed. For instance, if your office hosts a number of notebook computers that are frequently removed and taken on business trips, these computers might not have permanent IP address assignments. A DHCP server can assign addresses to them as they're used. Furthermore, if a notebook computer is used on two networks, and if both networks use DHCP, the notebook computer can adjust itself automatically to the settings needed by both networks.

Although enabling DHCP to deliver IP addresses to any new computer it sees, and using notebook computers with any network's DHCP server are both useful procedures, they're also both potentially risky. If a new computer on your network is infected with a worm or virus, it might be able to spread locally, bypassing any firewall controls you have on your router. Likewise, a notebook computer can come under attack from worms or viruses if connected to a foreign network. To reduce risk in a security-conscious environment, you might configure DHCP to deliver IP addresses only to known computers. You can do so by referencing specific hardware addresses, as described in the Section

In the Linux world, the Internet Software Consortium's (ISC; DHCP server is the standard one. Most Linux distributions ship with this server under the package name dhcpd or dhcp-server. ISC also produces a DHCP client, which is frequently installed as dhcpcd. Other DHCP clients, such as pump and dhcp-client, are also available.

15.1.2. Kernel and Routing Requirements for DHCP

Because the DHCP server must communicate with clients that are not yet fully configured for TCP/IP, the server needs support for packet sockets. This is a way for programs to communicate over the network without using an intervening network protocol, such as TCP/IP. It's usually enabled by default, but if you've rebuilt your kernel yourself, you might want to check for it. In 2.6.x kernels, check Networking Support Networking Options Packet Socket. If this option isn't enabled, enable it and recompile your kernel. This option is also needed by some DHCP clients.

DHCP servers send their responses to clients using a broadcast address. This can be either a global broadcast ( or a broadcast that reaches all the computers on a particular subnet (such as to reach all the computers on the subnet). Unfortunately, some DHCP clients (such as some versions of Windows) expect DHCP servers to deliver global broadcasts, but some Linux configurations convert global broadcasts into local broadcasts. The usual symptom of this problem is that some Windows clients (particularly Windows 9x/Me systems) can't obtain IP addresses via DHCP, but others (such as Linux clients) can. If this happens, try typing this command on the Linux DHCP server:

# route add -host dev eth0

If the DHCP server is delivering addresses on a device other than eth0, change that part of the command appropriately. After making this change, try restarting an affected Windows system; it should now work. If so, add this line to a local Linux startup script or to the SysV startup script for your DHCP server.

This route command adds an explicit route for traffic destined to; without it, Linux changes the IP address to conform to existing routes. You can check your existing routing table by typing route -n. After adding the route as just described, it should appear as the first entry in the routing table.

15.1.3. DHCP Configuration Files

The DHCP server's configuration file is /etc/dhcpd.conf. Some package distributions include this file in this location, but some packages instead provide a sample file in another location. For instance, SuSE's dhcp-server package stores a sample file in /usr/share/doc/packages/dhcp-server/. The ISC DHCP server also creates a file to track its leases, often called /var/lib/dhcp/dhcpd.leases. You shouldn't need to adjust this file, although examining it can sometimes be a good way to check the server's operation.

Don't confuse the DHCP server's dhcpd.conf file with dhcpcd.conf, which some DHCP client packages use as a configuration file!

Aside from comments, which are denoted by hash marks (#), dhcpd.conf contains two broad types of statements:


These are relatively simple statements that provide information for clients (such as a router's IP address) or that tell the server whether or how to do something (such as how long leases should be). Many parameters begin with the keyword option.


These statements describe the network topology, assign IP addresses to clients, or create a grouping of related parameters. Declarations often span multiple lines, in which case the grouped lines are denoted by curly braces ({ }).

Most dhcpd.conf files begin with a set of parameters and then move on to declarations. A simple network may contain just one declaration, which in turn contains multiple parameters. A more complex network might contain multiple declarations.

15.1.4. Assigning Addresses

The bulk of DHCP server configuration is in assigning IP addresses. A full configuration file, though, sets assorted other parameters, and it's best to consider them together. I therefore describe two complete configurations as examples: assigning dynamic IP addresses and assigning fixed IP addresses. Dynamic address assignment

In a dynamic IP address assignment configuration, the DHCP server hands out IP addresses to any computer that asks for them but doesn't go to any great lengths to track computers and match them with the same IP address every time they boot. Thus, a single computer might receive four different IP addresses over the course of a week, depending on factors such as the lease time, the number of addresses available, the number of computers on the network, and how often the computer reboots or is shut down for an extended period.

Even when a network uses dynamic IP addresses, a computer's IP address is unlikely to change unless the computer is shut down for an extended period of time, such that its lease expires and another computer grabs the address. Even when a computer reboots, it typically asks for its old address back, and will probably receive that address if the lease hasn't expired yet. When the computer stays up continuously, it renews its lease before the lease expires and so keeps the same IP address continuously.

Example 15-1 shows a typical dhcpd.conf file for assigning dynamic IP addresses. Although this example is simple, something like it should be adequate for many small networks or even simple large networks. Most lines in this file end in semicolons (;), and omitting that character is an easy way to create a nonfunctional configuration file, so be sure to check that detail. Multiline declarations, however, don't use a semicolon on lines that end in either an open or a close curly brace.

Example 15-1. Sample dynamic DHCP server configuration
allow bootp; default-lease-time 86400; max-lease-time 172800; option subnet-mask; option domain-name-servers,; option domain-name ""; option netbios-node-type 8; option netbios-name-servers option ntp-servers; option x-display-manager; option tftp-server-name ""; subnet netmask {    option routers;    range; }

This example begins with several lines that set assorted parameters. Most of these have fairly descriptive names, such as default-lease-time or option subnet-mask. However, some options require some elaboration:

allow bootp

This option tells DHCP to respond to BootP requests, which are similar to DHCP requests but designed to be used by computers that boot off the network. Such configurations are often used with thin clients, as described in Chapter 12.


You set the default lease time, in seconds, with this option. The lease time is actually determined by negotiation between the client and the server, so you may end up delivering longer leases than you specify with this file, depending on the client's options.


This parameter sets the maximum lease time the server delivers. Even if the client requests a longer time, the server won't comply.

When testing a DHCP configuration, or sometime before making important changes such as altering a router or NTP server IP address, you may want to reduce the default-lease-time and max-lease-time values to something shortpossibly as short as a few minutes (say, 300five minutes). This action reduces the amount of time that clients have invalid information, which can speed up DHCP testing. Using lease times in the range of several minutes to a few hours can be good on networks that see lots of coming and going, such as a network to which users frequently attach laptops for a few minutes at a time. Note that using short lease times will increase network traffic and load on the DHCP server. Setting lease times in the range of many hours or days (such as the 86400, or one day, default lease time in Example 15-1), is a better policy for a stable and working DHCP server on a network that sees few changes to its clients.

option subnet-mask

As you might guess, this option sets the subnet mask (a.k.a. the network mask or netmask).

option domain-name-servers

Point to your network's DNS servers with this option. You can specify multiple servers by separating their values with commas. Most clients accept up to three DNS server addresses, but you can deliver fewer than this number if you like. Example 15-1 specifies DNS servers by IP address, but you can specify hostnames for the DHCP server to resolve when it starts up. (The DHCP server delivers IP addresses to clients in both cases.)

option domain-name

You can tell clients what domain name to use with this option. Not all clients use this information, but for those that do, it can be a handy feature.

option netbios-node-type

This option tells Windows clients how they should attempt to resolve nameswhether to use a NBNS system, broadcast resolution, or both. A value of 8 tells the system to use an NBNS system if possible but to fall back on broadcast name resolution. Chapter 5 describes NBNS configuration in more detail.

option netbios-name-servers

This option is the NetBIOS equivalent of option domain-name-servers and works in a similar way. Linux clients ignore this information, though; it's useful only for Windows clients.

option ntp-servers

You can point clients at an NTP server with this option; however, many clients ignore this information. Section 15.3, describes NTP in more detail.

option x-display-manager

This option can point certain X terminal thin clients at an XDMCP server, as described in Chapter 12. Some thin clients ignore this information, however.

option tftp-server-name

A TFTP server delivers files to computers, such as some thin clients, that boot off of the network. This option points such clients to the TFTP server.

option routers

Although Example 15-1 shows this option within a declaration block, it can appear outside such a block. It points the clients at the network's router (a.k.a. the gateway).

DHCP supports many additional parameters, but most of them are quite obscure. Consult the dhcpd.conf manpage or a book on DHCP for more information.

The core of the DHCP configuration, and the part that sets the dynamic addresses it can deliver, is the subnet declaration in Example 15-1. This declaration begins with a specification of the numeric subnet it serves192.168.1.0 netmask The curly braces then set off several lines that define features unique to this declaration. In Example 15-1, the declaration contains just two parameter lines, but you can add more. These lines override parameters set outside the declaration body or set new options. The key to defining a dynamic DHCP definition is the range parameter:


This line tells dhcpd what IP addresses it may deliver. In this case, the range includes 205 addresses, from through The server won't deliver addresses outside of this range, even within the subnet. Thus, you can assign IP addresses for some systems without using DHCP. For instance, the DHCP server itself typically has a static IP address. You might also want to give a router or other important server a static IP address outside the DHCP server's control.

Once you make changes to the DHCP server's configuration, you must restart the server. Typically, you do this using the server's SysV startup script, as in /etc/init.d/dhcpd restart. If the server isn't already running, you can start it the same way. Consult distribution-specific documentation if you need help with SysV startup scripts. Fixed address assignment

The leap from dynamic IP address assignment to fixed address assignment is a matter of adding new declarations, one for each computer to which you want to assign a fixed address. These host declarations appear within the declaration for the dynamic subnet:

subnet netmask {    option routers;    range;    host gingko {       hardware ethernet 00:0C:76:96:A3:73;       fixed-address;    } }

Each host declaration begins with a hostname (gingko in this case). This hostname may be passed to the client if you set the use-host-decl-names true parameter, but most clients ignore the name. The declaration then contains two lines. The first sets the client's media type (ethernet in this case, although token-ring is appropriate for Token Ring networks) and media access control (MAC) address, a.k.a. the hardware address. The DHCP server then knows to assign the specified fixed-address as the IP address whenever the computer with the specified MAC address requests an address.

The IP address you specify with fixed-address must be outside the range specified with range but within the range determined by the subnet IP address and netmask declaration.

Of course, on a large network, managing all the computers' MAC and IP addresses can be tedious. For this reason, assigning fixed IP addresses in this way is best reserved for small networks or for a limited number of computers (say, important servers) on large networks. This type of configuration is also often used when configuring thin clients, which may need to be told on a client-by-client basis what files to use as a boot OS. This topic is described in Chapter 12.

One critical detail about this configuration is locating the target computer's MAC address. Several approaches to doing so exist:

Examining board markings

Some network card manufacturers mark the MAC address on their boards, usually by affixing a sticker. This can be a good way to get a MAC address if you're installing a card, but it can be inconvenient if the computer's already assembled and has the network card in it. Also, not all manufacturers label their cards this way.

Client configuration tools

Most clients enable you to view the MAC address in one way or another. On Linux and most other Unix-like clients, you can find the address using ifconfig, as in ifconfig eth0. The MAC address is called the HWaddr in the output. Typing IPCONFIG /ALL at a DOS prompt on Windows NT/200x/XP systems also displays this information, on a line labelled Physical Address. Windows 9x/Me systems provide a GUI tool, WINIPCFG, which delivers this information.

Using arp

You can give the client an IP address, either by configuring it statically or by using DHCP without a static IP address defined for the system. You can then use the Linux arp command to find the MAC address. For instance, arp finds the MAC address (and some other information) for You may need to direct some other traffic at the computer first, though; using ping should do the job.

Examining DHCP logs

If you have the client obtain an IP address from the DHCP server's pool of dynamic addresses, you can then examine the /var/lib/dhcp/dhcpd.leases file to locate the MAC address. This appears in a block named after the IP address given to the client. Similarly, the /var/log/messages or other system logfile on the DHCP server usually records the MAC address of clients and the assigned IP address. These techniques are both simplest to use on a network with little DHCP activity, such as a small network. Locating the correct entry on a larger network can be trickier.

In all these cases, the exact form of the MAC address can vary. For inclusion in the dhcpd.conf file's host declaration, the address appears as a series of hexadecimal (base 16) numbers separated by colons (:). Some utilities may show the address in another form, such as separated by dashes. If necessary, make the change. For hexadecimal values between A and F, the DHCP server doesn't care about case. For instance, 5a is exactly equivalent to 5A.

15.1.5. Telling Clients to Use DHCP

Like any server, the ISC DHCP server is useful only if it has clients. These clients can be Windows, Linux, Mac OS, or just about any other system. The DHCP client sets its own basic TCP/IP features by consulting the DHCP server, so the DHCP client program must run before most other network-related programs on the client.

A DHCP client can be a server for other protocols. For instance, you might use DHCP to assign an IP address to an SMB/CIFS file or printer server. Some servers work best with fixed IP addresses simply because this makes it easier to enter a name-to-IP-address mapping in your DNS server. For them, you can assign a fixed IP address to the server via DHCP, as just described.

To tell a computer to use a DHCP server, you must typically run the computer's basic network configuration tools. For instance, you can do this job on a Windows XP system that's already configured with a static IP address:

  1. Open the Windows Control Panel.

  2. Double-click the Network Connections icon in the Control Panel. This action opens a Network Connections window.

  3. In the Network Connections window, right-click the Local Area Connection icon and select Properties from the resulting context menu. This action opens a Local Area Connection Properties dialog box.

  4. In the Local Area Connection Properties dialog box, select the Internet Protocol (TCP/IP) item and click Properties. This action yields a dialog box called Internet Protocol (TCP/IP) Properties, as shown in Figure 15-1.

    Figure 15-1. The TCP/IP Properties dialog box lets you set basic network features

  5. In the Internet Protocol (TCP/IP) Properties dialog box, click "Obtain an IP address automatically". If any are set, the values in the "Use the following IP address" fields should disappear.

  6. If your DHCP server delivers DNS server addresses, click "Obtain DNS server address automatically". Any addresses you've set manually should disappear. (You can continue to set DNS server addresses manually, though, which can be handy if you want to use a nonstandard DNS server for some reason.)

  7. Click OK. This should activate your changes. You then need to close the remaining open dialog boxes and network option windows.

In many cases, you'll configure the DHCP client when you first set up its networking features. For instance, you might double-click the Network Setup Wizard in Windows XP. This setup procedure will guide you through various network settings, but in all cases, you should be presented with a choice to use DHCP or to set the IP address manually, as in Figure 15-1. (Windows often doesn't use the term DHCP, though, instead referring to this option as "obtain IP address automatically," as in Figure 15-1, or something similar.)

Precise details vary from one operating system to another, as well. Most versions of Windows, Mac OS, and other GUI-oriented OSs provide a procedure similar to that just described for Windows XP, but many details differ. Consult the OS's documentation, or browse the system configuration options for ones relating to network configuration. Linux distributions with GUI system administration tools can usually be configured to use DHCP in much this way, as well. You can also do so using text-mode tools: install a DHCP client (usually in a package called dhcpcd, dhcp-client, or pump), and use chkconfig or other tools to enable this package in your default runlevel. Consult distribution-specific documentation if you need help with this task.

    Linux in a Windows World
    Linux in a Windows World
    ISBN: 0596007582
    EAN: 2147483647
    Year: 2005
    Pages: 152

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