Droms R. The DHCP handbook
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FIGURE 13.16 Configuring the Routers scope option.
Server Options
You can also define server options, which are returned to clients from any scope that
is managed by the server. To modify the server options, click on Server Options and
select Actions, Configure Options. Figure 13.17 shows the configuration of the DNS
Servers option as a server option.
CHAPTER 13 The Microsoft DHCP Server216
FIGURE 13.17 Configuring the DNS Servers server option.
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Client Options
The Microsoft DHCP manager allows you to specify options for a client that has a
reserved address. By selecting the specific reservation under Reservations in the scope
that contains the reservation, you can add options and values, and you can override
scope and server options. Click on the reservation you want to change and then
select Action, Configure Options to specify the options that are associated with the
reservation.
Vendor-Specific Options
You can configure the Microsoft DHCP server to send vendor-specific options to
clients that identify themselves with a vendor-class identifier. When a client includes
a
vendor-class-identifier option in a message to the server, the Microsoft DHCP
server matches the value of the option against its list of vendor classes. If the server
finds a match, it includes the specified options in a
vendor-specific-information
option in the message the server returns to the client.
The Microsoft DHCP server is configured with four vendor classes:
• The default DHCP Standard Options class, which is used for any client that
does not send a vendor-class-identifier option
• The Microsoft Windows 2000 Options class, which is used for Windows 2000
clients
• The Microsoft Windows 98 Options class, which is used for Windows 98 clients
• The Microsoft Option class, which is used for other Microsoft clients
You configure a
vendor-class-identifier option through the same Scope Options
window you use for other options. When you click on the Advanced tab, you should
see a window in which you can select the vendor class and option, as shown in
Figure 13.18.
You can define a new vendor class and options for that class by selecting a server and
choosing Action, Define Vendor Classes. To add a new vendor class, click the Add
button and fill in the requested information, as shown in Figure 13.19. The display
name is the identifier used in the vendor class list when you’re configuring options,
and the ID is the value used to match with the
vendor-class-identifier option
sent by the client. You can enter the identifier value in either ASCII or hexadecimal.
Configuring DHCP Servers 217
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FIGURE 13.18 Configuring vendor-specific options.
CHAPTER 13 The Microsoft DHCP Server218
FIGURE 13.19 Adding a new vendor class.
User Class Options
You can also configure the Microsoft DHCP server to send options to a client based
on its user class. When you do so, the server examines the contents of the
user-
class option (RFC 3004) and sends to the client any options that have been selected
for that class. To configure the server to send options based on the client’s user class,
you need to configure the server with an option and select the User Class in the
Advanced tab of the Scope Options dialog box.
You can define a new user class by selecting Action, Define User Classes and filling
in the dialog box that appears, as shown in Figure 13.20.
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FIGURE 13.20 Adding a new user class.
Defining New Options
The Microsoft DHCP server is configured with all the options defined in RFC 2132.
You can define new options in a server by selecting the server and choosing Action,
Set Predefined Options. You need to click the Add button in order to display a dialog
box for the new option (see Figure 13.21). The Name field defines the name, and the
Code field defines the option code for the new option.
Configuring DHCP Servers 219
FIGURE 13.21 Defining a new option.
You can use the Data Type drop-down list box to select the type of data that can be
carried in the newly defined option. The following data types are allowed:
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Data Type Description
Byte 8-bit integer
Word 16-bit integer
Long 32-bit integer
Long Integer 32-bit integer
IP Address 32-bit IP address
String String of ASCII characters
Binary Arbitrary binary value
Encapsulated 32-bit integer
Selecting the Array checkbox defines the option to carry a list of data values in the
way that the Routers option carries a list of IP addresses.
When a new option is defined, it appears in the Available Options dialog box for
defining scopes or server options. Selecting the option in this dialog box displays the
current value of the option and enables you to change the value. If the option carries
a list of data values, you can add, delete, or reorder the list of values through the
Data Entry dialog box.
To define a new vendor-specific option for a vendor class, select the option class and
then click the Add button. Then, you can enter the information for the new option.
New options you define for an option class appear in the Advanced dialog box when
you configure new options. Figure 13.22 shows a new option for the Generic Startup
vendor class.
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FIGURE 13.22 A new option in the Generic Startup vendor class.
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Controlling the Windows DHCP Server
You can control the DHCP service through three different methods: DHCP manager,
Services control panel, and Network control panel. Each control function is covered
in the next sections of this chapter.
Activating Scope
The DHCP manager can activate or deactivate individual scopes. You should
completely configure each scope before activating it. When you first create a new
scope, the DHCP manager asks whether it should be activated; click No and config-
ure any options and reserved addresses before activating the scope.
After you configure the scope, highlight the scope in the main server manager
window and select Action, Activate.
Starting the DHCP Server Automatically and Manually
You can start and stop the DHCP server and arrange for the server to start automati-
cally when Windows 2000 is started. To control these server functions, click Start
and then select Settings, Control Panel. Double-click the Services icon in the Control
Panel and then select DHCP Server from the list of services. You can click the Stop or
Start button in the menu bar to stop or start the DHCP service manually. Select
Action, Properties to display a window through which you can disable or enable
automatic initiation of the DHCP service at system startup.
Uninstalling the DHCP Service
To uninstall the DHCP service, click Start and then select Settings, Control Panel.
Double-click the Network icon in the Control Panel. Select Services, select Microsoft
DHCP Server from the list of services, and click the Delete button. The DHCP server
and DHCP server manager are removed from your system.
Summary
The Microsoft Windows 2000 Server distribution includes a DHCP server. The
Microsoft DHCP software includes two components: a DHCP server and a DHCP
manager. The server runs as an independent process and is responsible for all interac-
tions with DHCP clients. The DHCP manager is a GUI for configuration and manage-
ment of DHCP servers. You specify the configuration of a network to the DHCP
server through the DHCP manager.
Each range of available addresses is defined as a scope. The definition for a scope can
include option values for that scope; any client assigned an address from that scope
also receives the option values that are defined for the scope. Options can also be
Summary 221
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defined to apply to all scopes managed by a server. The Microsoft DHCP manager
can assign values to most of the options defined in Appendix D, and you can define
your own options to accommodate newly standardized DHCP options or your own
locally defined options.
The Microsoft DHCP manager uses superscopes to define the configuration of
network segments that are assigned to more than one IP subnet. Each superscope
consists of a set of scopes, which are then assumed to be assigned to a common
network segment. The Microsoft DHCP server can choose from the available
addresses of any of the scopes in a superscope when assigning a new address to a
DHCP client.
The Microsoft DHCP server can differentiate clients based on vendor class or user
class. The server can be configured to send specific options and parameter values
based on the client’s vendor and user class. The server is distributed with several
predefined vendor and user classes, and you can also define your own classes and
options through the DHCP manager.
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IN THIS CHAPTER
• Obtaining the ISC DHCP
Server
•Prerequisites to Operation of
the ISC DHCP Server
• Configuring the ISC DHCP
Server
•Invoking the ISC DHCP Server
• Server Operation
14
The ISC DHCP Server
This chapter discusses how the Internet Software
Consortium (ISC) DHCP server operates. Specifically, it
describes the following:
• How to get the ISC DHCP server
• How to configure the ISC DHCP server
•What you need to do before you can start the ISC
DHCP server
• How you can configure the ISC DHCP server to run
in a production environment
The complete set of ISC DHCP server configuration
commands is documented in Appendix B, “ISC DHCP
Server Configuration File Reference.” Although this
chapter briefly describes what you must do to configure
the ISC DHCP server, you should also read the other chap-
ters in this book that describe in detail the theory behind
setting up a successful DHCP service on a production
network.
Obtaining the ISC DHCP Server
The ISC DHCP server is one of the three components of
the ISC DHCP distribution. The other two components are
a DHCP client and a DHCP relay agent. When you install
the ISC DHCP distribution, you install all three compo-
nents. Chapter 15, “Configuring a DHCP Server,” provides
an example of how to use the ISC DHCP relay agent, and
Chapter 21, “DHCP Clients,” describes in detail how to use
the ISC DHCP client.
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CHAPTER 14 The ISC DHCP Server224
The ISC DHCP distribution is an open-source product; that is, the source is provided
free of charge, and you can modify it as needed. The ISC does not charge to use the
software. Because the software is normally provided in source form, that is the form
described in this chapter.
The ISC DHCP distribution is available from the ISC Web site, at
www.isc.org/dhcp.html. You can also find it at ftp://ftp.isc.org/isc/dhcp. The
DHCP distribution is updated from time to time, and the version number is encoded
in the name of the file you download. The format of the filename depends on
whether a released version is available or is in alpha or beta testing. Released
versions have filenames similar to
dhcp-3.0pl2.tar.gz, in which 3.0 is the version
number,
pl2 is Patchlevel 2, and .tar.gz is appended because the distribution is a
Unix Tape Archive (tar) file that is compressed with the GNU zip utility (
gzip).
Beta releases have filenames similar to dhcp-2.0b1pl27.tar.gz, in which 2.0 is the
version to be used when the software is released from beta,
b1 indicates Beta 1, and
pl27 stands for Patchlevel 27. Alpha releases have filenames similar to dhcp-3.0.1-
20020724, in which 3.0 is the version number under which the software will eventu-
ally be released and
20020724 is the date of the snapshot. The first four digits
represent the year, the next two digits represent the month, and the final two digits
represent the day of the month.
When a new version of the ISC DHCP distribution is being released, the ISC issues
release candidates. A release candidate should not contain any bugs that would
prevent a release from happening. When a release candidate is issued, users who
want the new features or bug fixes in the candidate try it out to see if it works for
them. If bugs are found that the ISC isn’t willing to allow into the release (we call
these bugs “showstoppers”), the release is delayed until the bugs can be fixed,
and then the ISC issues a new release candidate. If no showstopper bugs are found in
the release candidate, that release candidate becomes the new release. Release candi-
date versions consist of the intended release version, followed by rc, followed by a
number (for example, 3.0.1rc8). When the release is made, it is released with the
same version number, but no rc (in this case, 3.0.1).
Support for the ISC DHCP Server
Support for the ISC DHCP server is provided free of charge (as time allows, of course)
on the ISC DHCP server mailing list; information about the mailing list is available
at
www.isc.org/dhcp.html. To get help, just send an email to the mailing list,
describing your problem in detail and asking for help with it.
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NOTE
Although the e-mail address of the author and maintainers of the ISC DHCP distribution is
readily available, he would appreciate it if you would not contact him directly for support, or
even to ask about the appropriateness of asking for support. If you need help with the ISC
DHCP server, ask for help on the ISC DHCP server mailing list. If you don’t get a reply when
you ask for help, it’s not because the folks at the ISC didn’t see your message—it’s because
they don’t have time to answer your question or because it wasn’t a good question. If you
send mail to the author or maintainers of the ISC DHCP server, they will simply tell you to ask
the question again on the mailing list.
Information about an archive of the mailing list and a FAQ are also available at
www.isc.org/dhcp.html.
Requests for help
When asking for help with the ISC DHCP server, be sure to provide complete infor-
mation to the ISC. Following is a list of the information to include in your request:
• The name and exact version number of the operating system that is running
on the computer on which the DHCP server or client is running (for example,
Solaris 2.8).
• The name and exact version number of the operating system that is running
on the computer on which the client is running, if you have trouble getting a
client working with the server (for example, Windows 98 SE). If you are
running on an open-source operating system such as Linux, you need to also
specify which DHCP client you are running because at least three different
DHCP clients run on Linux.
• If you’re running Linux, the version of the kernel, C library, and distribution
(for example, Linux 2.2.35, libc 5, Red Hat 7.2).
• The specific version of the DHCP distribution you’re running (for example,
3.0.1b1pl19, not 3.0).
•Adetailed explanation of the problem that is written so that someone who has
absolutely no knowledge of your situation will have enough information to
understand the problem.
•Your
dhcpd.conf and dhcpd.leases files, if they’re not huge (if they are huge,
the ISC might need them anyway, but don’t send them until you’re asked).
•Alog of your server or client running until it encounters the problem (for
example, if you have trouble getting a client to acquire an address, restart the
server with the
-d flag and then restart the client, and send the resulting
output to the support team).
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