Chapter 5, Implementing the Dynamic Host Configuration Protocol

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Note This chapter examines the Dynamic Host Configuration Protocol (DHCP) service, a Microsoft Windows 2000 service that automatically configures the Transmission Control Protocol/Internet Protocol (TCP/IP) client of the computers on your network.

Lesson 1, “Introducing DHCP,” introduces DHCP, its message format, and its communications procedures. It’s important for the you to understand the interaction between a DHCP client and server, and one of the best ways to do this is to capture DHCP traffic and analyze it. Lesson 2, Exercise 1 of the textbook covers the process of capturing DHCP traffic.

Lesson 2, “Using the DHCP Server,” discusses the process of installing and configuring the Windows 2000 DHCP server. This discussion can serve as preparation for Lab 5, in which the you will install, configure, and use your own DHCP servers.

Lesson 3, “Administering DHCP,” covers some of the tasks involved in maintaining a DHCP server.

Lesson 4, “Troubleshooting DHCP,” covers some of the most common problems that affect DHCP clients and servers.

 

 

Chapter 5, Lesson 1

Introducing DHCP

       1.    Understanding the Origins of DHCP

|1|               A.      The Reverse Address Resolution Protocol (RARP) is standardized in Request for Comments (RFC) 903, published by the Internet Engineering Task Force (IETF).

                           1.       RARP was the first attempt to create a protocol that would automatically configure TCP/IP clients on a network.

                           2.       RARP is closely related to the Address Resolution Protocol (ARP) used on TCP/IP networks, but with a difference.

                                     a.      With RARP, a computer transmits a message containing its data-link layer hardware address to a server and receives a message containing an Internet Protocol (IP) address for it to use.

                                     b.      By contrast, ARP resolves IP addresses into hardware addresses.

                           3.       An RARP server uses the same message format as ARP and is a simple mechanism for assigning IP addresses over the network.

                           4.       RARP is insufficient for the configuration of a modern TCP/IP client for several reasons:

                                     a.      RARP assigns a workstation an IP address only.

                                     b.      RARP cannot assign a subnet mask or other TCP/IP configuration parameters.

                                     c.       An administrator must configure an RARP server with the individual IP address assignments, so it provides no help in tracking address assignments.

|2|               B.      BOOTP

                           1.       The next step in the development of an automated TCP/IP configuration solution is called the Bootstrap Protocol (BOOTP).

                           2.       BOOTP is based on RFC 951, with extensions in RFC 1533 and RFC 1542.

                           3.       BOOTP offers improvements over RARP, including the ability to supply clients with subnet masks, router addresses, and other TCP/IP parameters in addition to IP addresses.

                  C.      DHCP

                           1.       The Dynamic Host Configuration Protocol (DHCP) is defined in RFC 2131.

|3|                        2.       DHCP design goals

                                     a.      DHCP client computers and DHCP clients should require no manual configuration.

                                     b.      DHCP should not require a server on each subnet.

                                     c.       DHCP clients must be able to handle multiple responses to a request for configuration parameters.

                                     d.      DHCP must coexist with statically configured, nonparticipating hosts and with existing network protocol implementations.

|4|                                  e.      DHCP must be able to provide service to existing BOOTP clients.

                                     f.       DHCP must ensure that only one DHCP client at a time will use any specific network address.

                                     g.      DHCP clients must retain their configurations across a DHCP client reboot or a DHCP server reboot.

                                     h.      DHCP must support fixed or permanent allocation of configuration parameters to specific clients.

|5|               D.      Assigning IP addresses

                           1.       When you configure the DHCP server, you create a scope consisting of a range of IP addresses for each of your subnets.

                           2.       When a DHCP client requests an address, the DHCP server assigns an address from the appropriate scope and updates its internal database to show that address as being in use.

                           3.       A DHCP server cannot assign the same IP address to two different computers, so it maintains a record of addresses currently in use.

|6|                        4.       Methods of assigning IP addresses

                                     a.      Manual allocation. The assignment of a single predetermined IP address to a specific DHCP client, identified by its data-link layer hardware address

                                     b.      Automatic allocation. The permanent assignment of IP addresses from a scope

                                     c.       Dynamic allocation. The assignment of IP addresses from a scope on a leased basis

                  E.      Configuring TCP/IP parameters

                           1.       To support large network installations, TCP/IP client workstations have to be assigned a subnet mask and usually other settings, such as default gateway and Domain Name System (DNS) server addresses.

                                     a.      DHCP provides support for many options that correspond to the configuration parameters for a wide variety of DHCP client configurations.

                                     b.      DHCP also supports a vendor-specific information option that enables third-party developers and network administrators to create customized options that they can apply to specific clients.

       2.    DHCP Messaging

|7|               A.      DHCP consists of the following three parts:

                           1.       A service that responds to DHCP requests from clients and maintains records of IP address assignments

                           2.       A client that generates requests for TCP/IP parameters and configures the TCP/IP client

                           3.       The protocol that the clients and server use to communicate

                  B.      DHCP is an application layer protocol that generates messages that are transmitted using User Datagram Protocol (UDP).

|8|               C.      DHCP clients and servers use one basic message format for their communications. The functions of the message fields are as follows:

|9|                        1.       Op (1 byte). Specifies whether the message originated at a client or a server

                           2.       Htype (1 byte). Specifies the type of hardware address in the Chaddr field

                           3.       Hlen (1 byte). Specifies the length of the hardware address in the Chaddr field, in bytes

                           4.       Hops (1 byte). Specifies the number of routers in the path between the client and the server

                           5.       Xid (4 bytes). Contains a transaction identifier used to associate requests and replies

                           6.       Secs (2 bytes). Specifies the elapsed time (in seconds) since the beginning of an address allocation or lease renewal process

|10|                       7.       Flags (2 bytes). Indicates whether DHCP servers and relay agents should use broadcast transmissions instead of unicast transmissions to communicate with a client

                           8.       Ciaddr (4 bytes). Contains the client computer’s IP address when it is in the bound, renewal, or rebinding state

                           9.       Yiaddr (4 bytes). Contains the IP address being offered to a client by a server

                           10.     Siaddr (4 bytes). Specifies the IP address of the next server in a bootstrap sequence

                                     a.      Used only when the DHCP server supplies an executable boot file to a diskless workstation

                           11.     Giaddr (4 bytes). Contains the IP address of a DHCP relay agent located on a different network, when necessary

|11|                       12.     Chaddr (16 bytes). Contains the hardware address of the client system, using the type and length specified in the Htype and Hlen fields

                           13.     Sname (64 bytes). Contains either the host name of the DHCP server or overflow data from the Options field

                           14.     File (128 bytes). Contains the name and path to an executable boot file for diskless workstations

                           15.     Options (variable). Contains a series of DHCP options, which specify the configuration parameters for the client computer

       3.    DHCP Options

                  A.      DHCP messages can carry many options in the Options field, some of which are listed below.

                  B.      The Message Type option

                           1.       The DHCP Message Type option contains a code that specifies the function of each message.

|12|                       2.       This option has eight possible values:

                                     a.      1—DHCPDISCOVER. Used by clients to request configuration parameters from a DHCP server

                                     b.      2—DHCPOFFER. Used by servers to offer IP addresses to requesting clients

                                     c.       3—DHCPREQUEST. Used by clients to accept or renew an IP address assignment

                                     d.      4—DHCPDECLINE. Used by clients to reject an offered IP address

                                     e.      5—DHCPACK. Used by servers to acknowledge a client’s acceptance of an offered IP address

                                     f.       6—DHCPNAK. Used by servers to reject a client’s acceptance of an offered IP address

                                     g.      7—DHCPRELEASE. Used by clients to terminate an IP address lease

                                     h.      8—DHCPINFORM. Used by clients to obtain additional TCP/IP configuration parameters from a server

                  C.      The Option Overload option

                           1.       When a DHCP message includes many options, it can exceed the maximum size of the Options field, which is limited by the 576-byte maximum size for an IP datagram.

                           2.       To carry the maximum amount of option information in a message, the transmitting system can use the Sname and File fields to carry additional options.

                           3.       By including the Option Overload option, the transmitting computer can specify whether the Sname field, the File field, or both, carry option information.

                  D.      Other DHCP options

|13|                       1.       The other options supported by DHCP are grouped into the following different categories:

                                     a.      Host-specific IP layer parameters. Configure various IP settings on the client computer, such as the default Time to Live settings

                                     b.      Interface-specific IP layer parameters. Configure settings that are particular to the network interface over which the DHCP message arrives

                                     c.       Link layer parameters. Configure settings specific to the link (or data-link) layer protocol running on the client computer

                                     d.      TCP parameters. Configure TCP-specific settings on the client, such as the TCP Time to Live value

                                     e.      Application and service parameters. Configure settings for specific application layer processes

                                     f.       DHCP extensions. Configure settings that control the behavior of the DHCP client itself, such as the length of the DHCP address lease and the renewal and rebinding time values

                           2.       Most DHCP server implementations support these options, but not all DHCP clients can process them.

       4.    Understanding DHCP Communications

|14|              A.      Activating a DHCP client causes it to initiate communications with DHCP servers when the computer starts.

|15|                       1.       The client uses the DHCPDISCOVER message type to generate a series of DHCP packets, which it transmits as broadcasts.

                           2.       When a DHCP server receives a DHCPDISCOVER message from a client, it uses the DHCPOFFER message type to generate a response containing an IP address and any other DHCP options the server is configured to supply.

                           3.       After a specified period of time, the client stops its broadcasting and accepts one of the offered IP addresses.

                                     a.      To signal its acceptance, the client generates a DHCPREQUEST message.

                                     b.      If the client does not receive a DHCPOFFER message in response to a DHCPDISCOVER message, it times out and repeats the DHCPDISCOVER broadcast.

                           4.       On receipt of the DHCPREQUEST message, the server whose offer the client is accepting commits the offered IP address and other settings to its database.

                           5.       The server then sends a DHCPACK message to the client, containing the same offered IP address and other options that were in the DHCPOFFER message.

                                     a.      If the server cannot complete the assignment (because it has already assigned the offered IP address to another system, for example), it transmits a DHCPNAK message to the client and the whole process begins again with DHCPDISCOVER broadcasts.

                           6.       After receiving a DHCPACK message from the server, the client performs a final test by generating a series of ARP broadcasts to ensure that no other system on the network is using the assigned IP address.

                                     a.      If another computer on the network does respond to the ARP broadcasts, the client cannot use the offered IP address and transmits a DHCPDECLINE message to the server, nullifying the transaction.

                                     b.      The client can then reissue a series of DHCPDISCOVER messages, restarting the whole process.

                  B.      DHCP leasing

                           1.       The process by which a DHCP server assigns configuration parameters to a client is the same whether the server uses manual, automatic, or dynamic allocation.

                                     a.      With manual and automatic allocation, the process described in the previous section is the end of the automated DHCP client/server communications.

                                     b.      When the server uses dynamic allocation, the client leases its IP address for a certain period of time (configured at the server) and must periodically renew the lease to continue using it.

|16|                       2.       The lease renewal process begins when a bound client reaches what is known as the renewal time value, or T1 value, of its lease, which by default is 50 percent of the lease period.

|17|                                a.      When a client reaches the T1 value, it enters the renewing state and begins generating DHCPREQUEST messages.

                                     b.      The client transmits the messages to the server that holds the lease as unicasts, unlike the broadcast DHCPREQUEST messages the client generates while in the init state.

                                     c.       If the server is available to receive the message, it responds with one of the following:

                                              (1)     A DHCPACK message, which renews the lease and restarts the lease time clock
                                              (2)     A DHCPNAK message, which terminates the lease and forces the client to begin the address assignment process again

                                     d.      If the server does not respond to the DHCPREQUEST unicast message, the client continues to send requests at regular intervals until it reaches the rebinding time value, or T2 value, which defaults to 87.5 percent of the lease period.

                                     e.      At this point, the client enters the rebinding state and begins transmitting DHCPREQUEST messages as broadcasts, soliciting an address assignment from any DHCP server on the network.

                                              (1)     A server can respond with either a DHCPACK or DHCPNAK message.

                           3.       If the lease time expires with no response from any DHCP server, the client’s IP address is released and all its TCP/IP communication ceases, except for the transmission of DHCPDISCOVER broadcasts.

 

Chapter 5, Lesson 2

Using the DHCP Server

       1.    Introduction

                  A.      All Windows operating systems include a DHCP client, which Windows uses by default.

                  B.      A DHCP server is included with all the Windows 2000 Server and Microsoft Windows NT Server products.

       2.    Implementing DHCP

|18|              A.      When planning a DHCP implementation, you must consider many factors, including the following:

                           1.       The number of clients you intend to support

                                     a.      A single DHCP server can theoretically support 1000 scopes with a total of 10,000 clients, depending on the other factors listed here.

                                     b.      To provide fault tolerance, you should divide the range of IP addresses you plan to assign on each subnet between two DHCP servers in an 80/20 split.

                                     c.       DHCP servers do not work together; each server has its own range of addresses to assign.

                           2.       The configuration of your internetwork

                                     a.      The number of networks that make up your internetwork and the technologies used to connect them can affect your DHCP implementation.

                                     b.      Using DHCP relay agents prevents you from having to install a DHCP server on every network segment.

                           3.       The performance capabilities of your servers

                                     a.      DHCP is a highly disk-intensive service. If you plan to have DHCP servers with heavy client loads, you should make sure that your server hardware is fast enough to avoid becoming a performance bottleneck.

       3.    Installing Windows 2000 DHCP Server

                  A.      The DHCP client is installed automatically with the TCP/IP protocols on every computer running Windows.

                           1.       Installation of the Windows 2000 DHCP Server service is optional.

|19|              B.      The Windows 2000 DHCP Server service installation process

                           1.       Install the Windows 2000 DHCP Server service software.

                           2.       Authorize the server in the Active Directory service.

                           3.       Create one or more scopes on the server.

                           4.       Configure DHCP options.

                           5.       Activate the scope or scopes.

|20|              C.      To install the DHCP Server service:

                           1.       Click Start, point to Settings, and then select Control Panel.

                           2.       Double-click the Add/Remove Programs icon in Control Panel to open the Add/Remove Programs dialog box.

                           3.       Click Add/Remove Windows Components to open the Windows Components Wizard.

                           4.       In the Components list, select Networking Services, and then click Details to open the Networking Services dialog box.

                           5.       In the Subcomponents Of Networking Services list, select the check box next to the Dynamic Host Configuration Protocol (DHCP) entry, and then click OK to return to the Windows Components Wizard.

6.      Click Next to install the DHCP Server service.

                                     a.      You might have to insert the Microsoft Windows 2000 Server CD‑ROM (the distribution disk).

                           7.       Click Finish to complete the installation and close the Windows Components Wizard.

                           8.       In the System Settings Change message box, click Yes to restart the computer.

       4.    Authorizing a DHCP Server

                  A.      The Windows 2000 DHCP Server service includes a feature that enables the computer to detect unauthorized DHCP servers, called rogues, and prevent them from starting.

|21|              B.      To authorize a DHCP server:

                           1.       Log on to the network, using an account that is a member of the Enterprise Admins group.

                           2.       Click Start, point to Programs, and then select DHCP from the Administrative Tools program group to open the DHCP console.

                           3.       In the scope pane, select the server you installed on the local computer, and then select Authorize from the Action menu.

                           4.       Press the F5 key to refresh the server display, and then click the DHCP header at the root of the scope tree.

                                     a.      The Status of the server has changed from Not Authorized to Running.

       5.    Creating a Scope

|22|              A.      When creating scopes, consider the following restrictions:

                           1.       You can create only one scope per subnet on a single DHCP server.

                           2.       Reservations must be included in a scope.

                           3.       Statically configured IP addresses must not be included in a scope.

|23|              B.      To create a scope:

                           1.       Click Start, point to Programs, and then select DHCP from the Administrative Tools program group to open the DHCP console.

                           2.       Select the entry for your DHCP server in the scope pane, and then select New Scope from the Action menu to open the New Scope Wizard.

                           3.       Click Next to bypass the Welcome page and proceed to the Scope Name dialog box.

                           4.       In the Name and Description boxes, type a name and (optionally) a descriptive string for the scope you are creating, and then click Next to proceed to the IP Address Range page.

                           5.       In the Start IP Address and End IP Address fields, type the beginning and ending IP addresses of the address range you want the DHCP server to assign.

                           6.       Specify the subnet mask for the addresses that the server will assign, either by indicating the number of bits in the network identifier in the Length selector or by typing the mask in the Subnet Mask box, using dotted decimal notation; then click Next to proceed to the Add Exclusions dialog box.

|24|                       7.       (Optional) Type a range of addresses that you want to exclude from the range you specified in the IP Address Range page in the Start IP Address and End IP Address box, and then click Add to add the range to the Excluded Address Range List.

                                     a.      Repeat this step to exclude additional address ranges.

                           8.       Click Next to proceed to the Lease Duration page.

                           9.       Specify the time interval (in days, hours, or minutes, or all three) for the IP address leases the server will assign for this scope, and then click Next to proceed to the Configure DHCP Options page.

                           10.     Click No, I Will Configure These Options Later, and then click Next to proceed to the Completing The New Scope Wizard page.

                           11.     Click Finish to close the wizard and create the scope.

                                     a.      The scope now appears in the DHCP console, subordinate to the server on which you created it.

       6.    Activating a Scope

                  A.      After you have created a scope, you must activate it before the server can use it to assign IP addresses to clients.

                  B.      To activate a scope:

                           1.       Click Start, point to Programs, and then select DHCP from the Administrative Tools program group to open the DHCP console.

                           2.       Select the scope you just created, and then select Activate from the Action menu.

                                     a.      The status of the scope has changed to “**Active**”.

       7.    Configuring DHCP Options

                  A.      Creating a scope enables a DHCP server to assign IP addresses and a subnet mask to the clients on a particular subnet.

                           1.       However, the clients’ TCP/IP configuration process will not be complete until they receive other parameters, such as a default gateway.

                  B.      The Windows 2000 DHCP Server service enables you to configure DHCP options at the server level or the scope level.

|25|              C.      To configure the most commonly used DHCP options:

                           1.       Click Start, point to Programs, and then select DHCP from the Administrative Tools program group to open the DHCP console.

                           2.       Select the Server Options heading or the Scope Options heading under a particular scope, and then select Configure Options from the Action menu to display the Server Options or Scope Options dialog box.

                           3.       Highlight the 003 Router entry in the Available Options list. Notice that additional controls for this particular option appear in the bottom half of the dialog box.

                           4.       Select the check box next to the 003 Router entry.

                                     a.      The Server Name and IP Address fields in the bottom half of the dialog box are now activated.

                           5.       In the Server Name box, type the name of the router that you want the DHCP clients to use as their default gateway.

                                     a.      If you already know the IP address of the router, type it in the IP Address box and proceed to step 8.

                           6.       Click Resolve to resolve the name you supplied into an IP address and add the address to the IP Address box.

|26|                       7.       Click Add to add the IP address to the list of default gateway addresses.

                           8.       Repeat steps 5–7 to add additional router addresses to the list.

                           9.       Select the check box next to the 003 DNS Servers entry in the Available Options list.

                           10.     Repeat steps 5–7 to specify the names or IP addresses of the DNS servers that you want your DHCP clients to use.

                           11.     Scroll down in the Available Options list and select the check box next to the 044 WINS/NBNS Servers option.

                           12.     Repeat steps 5–7 to specify the names of IP addresses of the Windows Internet Name Service (WINS) server that you want your DHCP clients to use.

13.    Click OK to apply the options you have configured.

                                     a.      The configured options now appear in the detail pane of the DHCP console.

       8.    Creating a Reservation

                  A.      Manual IP address allocation is implemented in the DHCP Server service in the form of reservations, which you create to assign a specific IP address to a specific computer.

                           1.       When you create a reservation, the IP address you specify is always assigned to the same DHCP client.

|27|              B.      To create a reservation:

                           1.       Click Start, point to Programs, and then select DHCP from the Administrative Tools program group to open the DHCP console.

                           2.       In the scope pane, expand the scope in which you want to create the reservation.

                           3.       Select the Reservations heading, and then select New Reservation from the Action menu to display the New Reservation dialog box.

                           4.       Type a name for the reservation in the Reservation Name box.

                           5.       Type the IP address you want to assign in the IP Address box.

                           6.       In the MAC Address box, type the data-link layer hardware address of the computer to which you want to assign the IP address.

                           7.       Click Add to create the reservation.

                           8.       Repeat steps 4–7 to create additional reservations for the scope, or click Close to close the New Reservation dialog box.

                                     a.      The reservations you created now appear in the detail pane of the DHCP console.

Chapter 5, Lesson 3

Administering DHCP

       1.    Monitoring DHCP Activity

                  A.      Keeping track of the DHCP servers’ status should be a regular concern of a network administrator.

                           1.       Using the DHCP console, you can monitor the address leases and the server status for DHCP servers anywhere on the network.

                  B.      When you install the DHCP Server service on a computer running Windows 2000 Server, the DHCP console is installed as well.

                  C.      To view addresses currently leased by a DHCP server:

                           1.       Select a scope under one of the servers listed in the scope pane and expand it to display the icons beneath it.

                           2.       Click the Address Leases icon.

                                     a.      A list of the current leases appears in the detail pane.

                           3.       From the Address Leases list, you can delete any or all of the current leases in order to free up IP addresses for use by other clients.

                  D.      To display a statistics window for a DHCP server:

|28|                       1.       In the scope pane, click the name of the DHCP server

                                    2.       Select Display Statistics from the Action menu.

                  E.      Using DHCP audit logging

                           1.       Another form of DHCP server monitoring that you can control from a server’s Properties dialog box is the DHCP audit logging.

                                     a.      By default, Windows 2000 DHCP servers have audit logging enabled.

                                     b.      The server maintains a rotating series of log files, named using the days of the week, in the \Winnt\System32\dhcp folder by default.

                  F.      DHCP performance counters

                           1.       The Windows 2000 DHCP Server service also includes a set of performance counters that you can use to monitor various types of server activity.

                           2.       By default, these counters are available in the Performance console after you install the DHCP Server service.

       2.    Compacting the DHCP Database

                  A.      The Windows 2000 DHCP Server service uses the Jet storage engine to maintain its database of lease information.

                           1.       The database files are located in the \Winnt\System32\dhcp folder, by default.

                           2.       You can periodically compact the DHCP database using the Jetpack.exe command-line utility included with Windows 2000.

       3.    Using DHCP Relay Agents

                  A.      A DHCP relay agent is a small program that enables a DHCP server to assign TCP/IP configuration settings to clients on another network.

                           1.       Most routers have relay agent capabilities built into them, enabling you to configure them to access your DHCP servers.

                           2.       The standard that defines functionality of a DHCP relay agent is no different from the BOOTP relay agent standard.

                  B.      A relay agent works by monitoring the network on which it is located for DHCPDISCOVER broadcasts generated by the DHCP client.

                           1.       The relay agent itself is configured with the IP addresses of DHCP servers located on other networks, so it can transmit the DHCPDISCOVER messages to the servers as unicasts.

                                     a.      Because unicasts are not limited to the local network as broadcasts are, the relay agent can interact with a DHCP server located anywhere on the network.

                           2.       When the server responds to the DHCPDISCOVER messages, it sends its DHCPOFFER messages back to the relay agent, which forwards them to the client as broadcasts.

                           3.       The agent continues in this manner throughout the lease negotiation process, until the client enters the bound state and can communicate with the DHCP server directly.

                  C.      Windows 2000 includes a DHCP relay agent as part of the Routing and Remote Access (RRAS) service.

                           1.       You must configure the relay agent before it will function.

|29|              D.      To configure the Windows 2000 DHCP relay agent:

                           1.       Open the Routing And Remote Access console.

                           2.       Expand the icon for the server you want to configure, expand the IP Routing icon, and then click the DHCP Relay Agent icon.

                           3.       Select Properties from the Action menu to display the DHCP Relay Agent Properties dialog box.

                           4.       In the Server Address box, type the IP address of the DHCP server you want the relay agent to use, and then click Add.

                           5.       Repeat step 4 to add additional DHCP servers to the list, and then click OK.

       4.    Integrating DHCP with DNS

                  A.      One problem with using a service such as DHCP is that a client’s IP address can change, rendering the information in IP address–based services such as DNS obsolete.

                           1.       To overcome this problem, the Windows 2000 DHCP Server service and the Windows 2000 DNS Server service both support the dynamic DNS update standard.

                  B.      Dynamic DNS is an extension to the DNS standard that enables a service such as DHCP to send messages to a DNS server that cause it to update the information in its resource records.

                           1.       Dynamic DNS is defined in RFC 2136, “Dynamic Updates in the Domain Name System.”

                           2.       Not all DHCP and DNS implementations support dynamic DNS updates.

                  C.      In addition to client-supplied instructions, you can configure the DHCP server to process the client updates in any of the following ways:

                           1.       The DHCP server updates the client’s information with the DNS servers only when the client requests it.

                           2.       The DHCP server always updates the client’s information with the DNS servers, whether the client requests it or not.

                           3.       The DHCP server never updates the client’s information with the DNS servers.

|30|              D.      To configure the behavior of a DHCP server regarding dynamic DNS updates, open a server’s Properties dialog box in the DHCP console and click the DNS tab.

                           1.       To configure the server to perform dynamic updates based on the client’s requests:

                                     a.      Select the Automatically Update DHCP Client Information In DNS check box.

                                     b.      Click Update DNS Only If DHCP Client Requests.

                           2.       To configure the server to always perform dynamic updates:

                                     a.      Select the Automatically Update DHCP Client Information In DNS check box.

                                     b.      Click Always Update DNS.

                           3.       To prevent the server from performing any dynamic updates, clear the Automatically Update DHCP Client Information In DNS check box.

 

Chapter 5, Lesson 4

Troubleshooting DHCP

       1.    Introduction

                  A.      The most common DHCP client problem is a failure to obtain an IP address or other configuration parameters from the DHCP server during system startup.

B.            The most common DHCP server problems

                           1.       The inability to start the service on the network in a Windows 2000 or Active Directory environment

                           2.       The failure of clients to obtain TCP/IP configuration parameters from a working server

       2.    Preventing DHCP Problems

                  A.      Many DHCP problems involve incorrect or missing configuration details.

|31|              B.      To help prevent the most common types of problems, you should do the following:

                           1.       Use the 80/20 design rule for balancing scope distribution of addresses where multiple DHCP servers are used to service the same scope.

                           2.       Use server-side conflict detection on DHCP servers only when it is needed.

                           3.       Create reservations on all DHCP servers that can potentially service the reserved client.

                           4.       For server performance, remember that DHCP is disk-intensive and purchase hardware with optimal disk performance characteristics.

                           5.       Keep audit logging enabled for use in troubleshooting.

|32|    3.    Troubleshooting DHCP Clients

                  A.      Most DHCP-related problems start as a failed IP configuration attempt at a client, so it is good practice to start there.

                           1.       If a DHCP-related problem does not originate at the client, check the system event log and DHCP server audit logs for clues.

                           2.       The following sections describe common symptoms of DHCP client problems. When a client fails to obtain a TCP/IP configuration, you can use this information to quickly identify the source of the problem.

                  B.      Invalid IP address configuration

                           1.       If a DHCP client does not have an IP address configured or has an IP address configured as 169.254.x.x, that means that the client was not able to contact a DHCP server and obtain an IP address lease.

                           2.       One method for checking the networking capabilities of the client computer is to install the NetBIOS Extended User Interface (NetBEUI) protocol module.

                                     a.      If the computer can communicate over the network using NetBEUI, you know that the problem lies in the TCP/IP implementation or configuration.

                                     b.      If the computer cannot communicate using NetBEUI (assuming that there are other NetBEUI systems on the LAN), you know that the problem lies elsewhere.

                  C.      Missing configuration settings

                           1.       If a DHCP client is missing configuration settings, the client might be missing DHCP options in its leased configuration for one of the following reasons:

                                     a.      The DHCP server is not configured to supply those options.

                                     b.      The client does not support the options distributed by the server.

                           2.       If this problem occurs on Windows 2000 DHCP clients, verify that the most commonly used and supported options have been configured at either the server or scope level of option assignment.

                           3.       Check the DHCP option settings on the server and make sure that you have selected the appropriate options for your clients.

                  D.      DHCP servers do not provide IP addresses.

                           1.       If DHCP clients can access the network but cannot obtain IP addresses from a DHCP server, there are several possible causes.

                           2.       One possible cause: The IP address of the DHCP server might have changed.

                                     a.      A DHCP server can only service requests for a scope that has a network identifier that is the same as the network identifier of its own IP address.

                                     b.      Make sure that the DHCP server IP address falls in the same network range as the scope it is servicing.

                           3.       Another possible cause: The DHCP clients are located on a different local area network (LAN) from the DHCP server and must go through a router to obtain IP addresses.

                                     a.      Completing the following steps might correct this problem: Configure a DHCP/BOOTP relay agent on the LAN where the clients are located.

                                              (1)     The relay agent can be on the router itself or on a computer running Windows 2000 Server and the RRAS service.

                                     b.      At the DHCP server, configure a scope to match the network address on the other side of the router where the affected clients are located.

                                              (1)     In the scope, make sure that the subnet mask is correct for the remote network.

                                     c.       Do not include this scope (that is, the one for the remote network) in superscopes configured for use on the same local subnet or segment where the DHCP server resides.

                           4.       Another possibile cause: Multiple DHCP servers exist on the same LAN.

                                     a.      Make sure that you do not configure multiple DHCP servers on the same LAN with scopes that contain the same addresses.

|33|    4.    Troubleshooting DHCP Servers

                  A.      When a DHCP server fails to provide leases to its clients, clients often discover the failure in one of the following ways:

                           1.       The client might be configured to use an IP address not provided by the server.

                           2.       The server sends a negative response to the client, and the client displays an error message indicating that a DHCP server could not be found.

                           3.       The server leases an address to the client but the client appears to have other network configuration–based problems, such as the inability to register or resolve DNS or NetBIOS names or to access computers beyond its local network.

                  B.      The first troubleshooting task is to make sure that the DHCP Server service is running.

                           1.       You can verify this by opening the DHCP console and attempting to access the server or by opening the Computer Management console and looking at the Services list.

                           2.       If the DHCP Server service is not started, you can start it manually using the Start Service button in the console toolbar.

                           3.       Check the logs in Event Viewer to determine whether the server failed to start or whether it stopped because of a problem elsewhere in the computer, such as a memory shortage.

                  C.      Problem: The DHCP console incorrectly reports lease expirations.

                           1.       When the DHCP console displays the lease expiration time for reserved clients for a scope, it indicates one of the following:

                                     a.      If the scope lease time is set to an infinite lease time, the reserved client’s lease is also shown as infinite.

                                     b.      If the scope lease time is set to a finite length of time (such as 8 days), the reserved client’s lease uses this same lease time.

                           2.       The lease term of a DHCP reserved client is determined by the lease assigned to the reservation.

                           3.       To create reserved clients with unlimited lease durations, create a scope with an unlimited lease duration and add reservations to that scope.

                  D.      Problem: The DHCP server uses broadcasts to respond to all client messages.

                           1.       The DHCP server uses broadcast transmissions to respond to all client configuration request messages, regardless of how each DHCP client has set the broadcast bit flag.

                           2.       By default, the DHCP server in Windows NT Server 3.51 and earlier versions ignored the broadcast flag in DHCPDISCOVER messages and sent all DHCPOFFER replies as broadcasts.

                           3.       Starting with Windows NT Server 4, the DHCP Server service still attempts to transmit all DHCP responses to the limited broadcast address, unless support for unicast responses is explicitly enabled by setting the value of the IgnoreBroadcastFlag registry entry to 1.

                                     a.      This registry entry is located in HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\DHCPServer\Parameters\
IgnoreBroadcastFlag.

                                     b.      When the broacast flag is set to 1, the computer ignores the broadcast flag in client requests and broadcasts all DHCPOFFER responses.

                                     c.       When the registry entry is set to 0, the server adjusts its transmission behavior (whether to broadcast or not) based on the value of the broadcast bit flag in the client’s DHCPDISCOVER request.

                                     d.      If this flag is set in the request, the server transmits its responses to the limited local broadcast address.

                                     e.      If the flag is not set in the request, the server transmits its responses directly to the client as unicasts.

                  E.      Problem: The DHCP server fails to issue address leases for a new scope.

                           1.       In some situations, you might want to assign new IP addresses to all the DHCP clients on a particular network.

                                     a.      You might have obtained a registered class of IP addresses for your network, or you might be changing the address class to accommodate more computers or more networks.

                                              (1)     To do this, you create a new scope on your DHCP server containing a range of new addresses.
                                              (2)     In this type of situation, you want clients to obtain leases in the new scope instead of using the earlier scope to obtain or renew their leases.
                                              (3)     When all clients are actively obtaining leases in the new scope, you intend to remove the existing scope.
                                              (4)     However, when you activate the new scope, you find that the DHCP clients do not obtain leases from the newly defined scope.

                           2.       When superscopes are not available or not used, only a single DHCP scope can be active on the network at one time.

                                     a.      If more than one scope is defined and activated on the DHCP server, the server uses only one scope to provide leases to clients.

                                     b.      The active scope that the DHCP server used for distributing leases is determined by the network identifier in the first IP address assigned to the DHCP server’s network interface adapter.

                                     c.       The DHCP server always uses the scope with the same network identifier as its own IP address.

                                     d.      You can configure additional IP addresses for a network interface by using the IP Settings tab in the Advanced TCP/IP Properties dialog box, but these addresses do not affect the DHCP server’s scope selection.

                           3.       You can resolve this problem in the following ways:

                                     a.      Configure the DHCP server to use a superscope that includes the earlier scope and the new scope.

                                     b.      Change the primary IP address—that is, the address assigned in the Internet Protocol (TCP/IP) Properties dialog box for the DHCP server’s network adapter—to an IP address that has the same network identifier as the new scope.

                                              (1)     You can maintain the prior address that was first assigned as an active IP address for the server computer by moving it to the list of multiple IP addresses maintained in the Advanced TCP/IP Properties tab.