DHCP Dynamic Host Configuration Protocol
Resource: Windows 2000 Server Resource kit TCP/IP Core Networking Guide

Winteacher.com > Part2 > DHCP > Administering > Setting up Options

 

Assigning options

You can manage DHCP options at several distinct levels:

  • Predefined options.  At this level, you control which types of options are predefined for the DHCP server to expose as available options for assigning from any of the option configuration dialog boxes (such as Server Options, Scope Options, or Reservation Options) available through the DHCP console. You can add or remove options to and from the predefined list of standard options as needed. Although options are made available in this way, they are not assigned values until administratively configured at either the server, scope, or reservation.
  • Server options.  Assign values here (using the General tab) for options that should apply to or be inherited by all scopes and clients of the DHCP server as defaults. Options configured here can have their values overridden by different values if those values are set at either a scope, options class, or reserved client level.
  • Scope options.  Assign values here (using the General tab) for options that should apply only to clients of an applicable scope selected in the DHCP console tree. Options configured here can have their values overridden by different values if those values are set at either an options class or reserved client level.
  • Reservation options.   Assign values for options that should apply only to a specific reserved DHCP client. To use this level of assignment, you must first add a reservation for the applicable client to the applicable DHCP server and scope where the client is to obtain its IP address. These options are set for an individual DHCP client configured with an address reservation in a scope. Only properties manually configured at the client computer can override options assigned at this level.
  • Class options.  When using any of the option configuration dialog boxes (Server Options, Scope Options or Reservation Options), you can click the Advanced tab to configure and enable options for assignment to identifying member clients of a specified user or vendor class.

    Depending on the context, only those DHCP clients that identify themselves according to the selected class are distributed options data you have configured specifically for that class. For example, if a class-assigned option is set at a scope, only clients of that scope that indicate class membership during leasing activity are configured with class-assigned option values. Other non-member clients are configured using scope option values set from the General tab.

    Options configured here can override values assigned and set at the same context (either server, scope, or reservation options) or values inherited from options configured at a higher context. However, the ability of the client to indicate membership in a specific options class is typically the decisive criteria for using this level of options assignment. For more information, see Using options classes.

Guidelines for assigning options

The following guidelines can help you determine what level to assign the options you use for clients on your network.

  • Add or define new custom option types only if you have new software or applications that require a nonstandard DHCP option.
  • If your DHCP server manages many scopes for a large network, be selective when assigning Server Options. These options apply by default to all clients of a DHCP server computer, unless otherwise overridden.
  • Use Scope Options for assigning most options that clients use. In most networks, this level is typically preferred for assigning and enabling the use of DHCP options.
  • Use Class Options if you have a mixture of DHCP clients with diverse needs that are able to identify a specific class on the DHCP server when obtaining a lease. For example, if you have a limited number of DHCP client computers running Windows 2000, these clients can be configured to receive vendor-specific options that other clients do not use.
  • Use Reservation Options for individual DHCP clients in your network that have special configuration requirements.

    For any hosts (that is, computers or other networked devices) that do not support DHCP or are not recommended to use it, you can also consider excluding IP addresses for those computers and devices and manually setting the IP address configuration directly at the applicable host. For example, you often need to statically configure the IP address for routers.

Commonly used options

After you set basic TCP/IP configuration settings (such as IP address, subnet mask, and default gateway) for clients, most clients also need the DHCP server to provide other information through DHCP options. The most common of these include the following:

  • Routers.  A preferred list of IP addresses for routers on the same subnet as DHCP clients. The client can then contact these routers as needed to forward IP packets destined for remote hosts.
  • DNS servers.  IP addresses for DNS name servers that DHCP clients can contact and use to resolve a domain host name query.
  • DNS domain.  Specifies the domain name that DHCP clients should use when resolving unqualified names during DNS domain name resolution.
  • WINS node type.  A preferred NetBIOS name resolution method for the DHCP client to use (such as b-node for broadcast only or h-node for a hybrid of point-to-point and broadcast methods).
  • WINS server.  IP addresses of primary and secondary WINS servers for the DHCP client to use.

 Note

  • When the DHCP service is installed, no DHCP option definitions are created. Option definitions are created only when the DHCP console is opened for the first time. If you want to configure the value of an option on a DHCP server on which the console has never been opened, you must first use the Netsh DHCP context command server add optiondef (at a netsh dhcp> command prompt or in a batch file or script) to create the option definition. To use Netsh to assign a value to the option definition you have created, use the Netsh DHCP context command server set optionvalue. For more information, see Netsh commands for DHCP.

 

 

Using option classes

Option classes offer an additional method for grouping DHCP-provided configuration details for clients within a scope. For computers running Windows Server 2003 operating systems, there are two types of option classes that can be used for submanaging options:

  • User classes, for assigning options to clients identified as sharing a common need for similar DHCP options configuration.
  • Vendor classes, for assigning vendor-specific options to clients identified as sharing a commonly defined vendor type.

For more information, see Understanding user and vendor classes.

How user classes work

User classes allow DHCP clients to differentiate themselves by specifying a User Class option. When available for client use, this option includes a user-determined class ID that can help to group clients of similar configuration needs within a scope. For example, you might support users and computers with mobile computing needs by configuring a user class at the DHCP server and setting the related class ID at the client computers.

A user class is useful when you need to keep separate options that cover the special needs of identifying client computers, such as providing a shorter lease time for portable computers that move frequently or use remote access often. In this example, you could configure the DHCP server to distribute different options that are specific to the needs of clients.

For instance, shorter leases could be assigned to mobile clients. For more information, see To change the lease time for remote access clients.

The user class feature gives you greater flexibility in configuring DHCP clients on your network, but is not required for standard DHCP use. If user-defined option classes are not configured, options are provided through the applicable server, scope, or client option settings instead. For more information, see Assigning options.

DHCP servers running Windows Server 2003, which support recognition of user class IDs, perform the following added steps to lease clients identifying themselves according to this process:

  1. The server determines whether the user class identified by the client in its lease request is a recognized class, previously defined on the server.

    If a predefined user classes exists at the server and is configured, class-based assignment is enabled. For other user classes, you must first add and configure them at the server before they are available for use.

  2. If the user class is recognized, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.
  3. If the scope or reservation options are configured to provide options for the user-defined class of the client, the server returns those options to the client as part of its DHCP acknowledgment message (DHCPACK), which is sent to confirm the lease.

Additional user class examples

You can define specific user class identifiers to convey information about client software configuration, its physical location in a building, or about its user preferences. For example, an identifier can specify that DHCP clients are members of a user-defined class called "2nd floor, West," which has need for a special set of router, DNS, and WINS server settings.

You can also use the Microsoft predefined user classes for isolating configuration details specific for clients with special needs, such as BOOTP or Routing and Remote Access service. For more information, see To assign an advanced (class-based) option.

 Notes

  • In addition to configuring or adding user classes at the DHCP server, at computers running Windows 2000, or Windows XP, you can set a DHCP class identifier with the ipconfig /setclassid command. For more information, see To set DHCP class ID information at a client computer.
  • When you configure user classes at the DHCP server, be certain that the class identifying data you set at both the DHCP server and member client computers is identically matching binary or ASCII data. For more information, see To create a new user or vendor class.
  • The user class option only permits one class ID to be used for identifying clients. Each client computer can only be identified as a member of a single user class at the DHCP server. You can use additional user classes and make new hybrids from your other user classes, as needed.

How vendor classes work

Vendor-defined option classes can be used by DHCP clients that are configured to optionally identify themselves by their vendor type to the DHCP server when obtaining a lease. For a client to identify its membership in a vendor class, the client provides a value in the Vendor class identifier option when it requests or selects a lease from the server.

The vendor class identifier information is a string of character data interpreted by a DHCP server. Microsoft supports vendor class identification for its DHCP clients running Windows 98 or Windows XP.

Most vendor types are derived from standard reserved hardware and operating system type abbreviation codes listed in Request for Comments (RFC) 1700. For more information, see TCP/IP RFCs.

Other vendors might choose to define their own specific vendor class IDs for either conveying vendor-specific information or handling special needs for their DHCP clients.

DHCP servers running Windows Server 2003, which support recognition of vendor class IDs, perform the following additional steps to lease the clients that identify themselves according to this process:

  1. The server determines whether the vendor class identified by the client in its lease request is a recognized class, previously defined on the server.

    Only Microsoft vendor classes are predefined at the server. For other vendor classes, you must manually add and configure these options at your DHCP servers running Windows Server 2003 before they are available for use.

  2. If the vendor class is a recognized one, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.
  3. If the scope or reservation options are configured to provide options for the vendor-defined class of the client, the server returns them using the Vendor specific information option as part of its DHCP acknowledgment message (DHCPACK), which is sent to confirm the lease.

Vendor classes permit other system vendors to support custom applications for DHCP in mixed vendor environments. Vendor-specific options, when provided, are used in addition to any of the standard DHCP options assigned or required for DHCP.

For more information about DHCP, see "Dynamic Host Configuration Protocol" at the Microsoft Windows Resource Kits Web site(http://www.microsoft.com/) or refer directly to RFC 2132.

 Notes

  • For more information about DHCP options, see "DHCP Options" at the Microsoft Windows Resource Kits Web site. (http://www.microsoft.com/)
  • In most cases, the default (or unspecified) vendor class, DHCP standard options, can provide a useful default for grouping any DHCP clients that do not specify or recognize vendor class IDs.
  • When you add new vendor classes at the DHCP server, be sure that the vendor class ID data you set at the server matches the actual vendor class ID used by clients for your vendor. For more information, contact the appropriate vendor.
DHCP Dynamic Host Configuration Protocol

LastUpdate:2005/04/05

Winteacher.com > Part2 > DHCP > Administering > Setting up Options