1. Week #3: Configuring and Troubleshooting DHCP
Course 6421A
Week #3: Configuring and Troubleshooting DHCP
Module 4: Configuring and Troubleshooting DHCP
Overview of the DHCP Server Role
Configuring DHCP Scopes and Options
Managing a DHCP Database
Monitoring and Troubleshooting DHCP
Securing DHCP

2. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
Benefits of Using DHCP
Module 4: Configuring and Troubleshooting DHCP
DHCP reduces the complexity and amount of administrative work by using automatic TCP/IP configuration
DHCP reduces the complexity and amount of administrative work by using automatic Transmission Control Protocol/Internet Protocol (TCP/IP) configuration.
Manual TCP/IP Configuration
Automatic TCP/IP Configuration
IP addresses are entered manually
IP address could be entered incorrectly
Communication and network issues can result
Frequent computer moves increase administrative effort
IP addresses are supplied automatically
Correct configuration information is ensured
Client configuration is updated automatically
A common source of network problems is eliminated

3. New DHCP Features in Windows Server 2008
Course 6421A
New DHCP Features in Windows Server 2008
Module 4: Configuring and Troubleshooting DHCP
New DHCP features include:
The DHCP role on Microsoft Windows® Server 2008 supports several new features:
Supports DHCPv6 stateful and stateless configuration for configuring clients in an IPv6 environment.
Helps isolate potentially malware-infected computers from the corporate network via Network Access Protection (NAP) with DHCP.
Enables DHCP installation as a role on a Windows Server® 2008 Server Core installation.
Windows Server 2008 Support for DHCPv6
Support for advanced network security configuration using NAP
DHCP on Server Core

4. How DHCP Allocates IP Addresses
Course 6421A
How DHCP Allocates IP Addresses
Module 4: Configuring and Troubleshooting DHCP
DHCP Client2: IP configuration from DHCP server
Non-DHCP Client: Static IP configuration
DHCP allocates IP addresses on a dynamic basis, called a lease. You can set the lease value to unlimited. However, the value typically is not more than a few hours or days.
Two methods for obtaining a lease are to request a new lease or to renew an existing lease.
DHCP uses IP broadcasts to initiate communications. Therefore, DHCP servers are limited to communication within their IP subnet. This means that in many networks, there is a DHCP server for each IP subnet. When this is not feasible, either for cost or management reasons, you can use a DHCP relay agent.
The DHCP relay agent allows DHCP broadcast packets to be relayed into another IP subnet across a router. This makes it possible to maintain a singe DHCP server that services multiple IP subnets. DHCP packets also may be relayed into other subnets using a router that is compatible with RFC 1531.
Lease Renewal
Lease Generation
DHCP Server
DHCP
Database
DHCP Client1: IP configuration from DHCP server
IP Address1: Leased to DHCP Client1
IP Address2: Leased to DHCP Client2
IP Address3: Available to be leased

5. How DHCP Lease Generation Works
Course 6421A
How DHCP Lease Generation Works
Module 4: Configuring and Troubleshooting DHCP
DHCP client broadcasts a DHCPDISCOVER packet 1
DHCP servers broadcast a DHCPOFFER packet 2
DHCP client broadcasts a DHCPREQUEST packet 3
DHCP Server1 broadcasts a DHCPACK packet 4
DHCP Client
DHCP Server1
DHCP Server2
DHCP Server2
1. The DHCP client broadcasts a DHCPDISCOVER packet. This is a message that is broadcast to every computer in the subnet. The only computer that will respond is the computer that has the DHCP server role or, if the computer is running the DHCP server agent. In the latter case, the agent will forward the message to the DHCP server with which it is configured.
2. Any DHCP Server in the subnet will respond by broadcasting a DHCPOFFER packet. This packet will provide the client with a potential address.
3. The client receives the DHCPOFFER packet. It may receive packets from multiple servers. If the client receives offers from more than one server, it usually will choose the server that made the fastest response to its DHCPDISCOVER. This typically is the DHCP server closest to the client. The client then will broadcast a DHCPREQUEST. The DHCPREQUEST contains a server identifier. This informs the DHCP servers that receive the broadcast which server the client has chosen to accept the DHCPOFFER.
4. The DHCP servers receive the DHCPREQUEST. Those servers that the DHCPREQUEST message does not accept use the message as notification that the client has declined that server’s offer. The chosen server stores the IP address client information in the DHCP database and responds with a DHCPACK message. If for some reason the DHCP server cannot provide the address that was offered in the initial DHCPOFFER, the DHCP server will send a DHCPNAK message.
DHCP Server1
DHCP Client
DHCP client broadcasts a DHCPDISCOVER packet 1
DHCP servers broadcast a DHCPOFFER packet 2
DHCP client broadcasts a DHCPREQUEST packet 3
DHCP Server1 broadcasts a DHCPACK packet 4

6. How DHCP Lease Renewal Works
Course 6421A
How DHCP Lease Renewal Works
Module 4: Configuring and Troubleshooting DHCP
DHCP Client
DHCP Server1
DHCP Server2
DHCP client sends a DHCPREQUEST packet 1
DHCP Server1 sends a DHCPACK packet 2
50% of lease duration has expired
DHCP Client
DHCP Server1
DHCP Server2
DHCP Renewal occurs when 50% of lease duration has expired.
Describe the DHCP lease-renewal process by referring to the slide’s illustration.
100% of lease duration has expired
50% of lease duration has expired
87.5% of lease duration has expired
If the client fails to renew its lease, after 50% of the lease duration has expired, then the DHCP lease renewal process will begin again after 87.5% of the lease duration has expired
If the client fails to renew it’s lease, after 87.5% of the lease has expired, then the DHCP lease generation process starts over again with a DHCP client broadcasting a DHCPDISCOVER
DHCP Client sends a DHCPREQUEST packet 1
DHCP Server1 sends a DHCPACK packet 2

7. DHCP Server Authorization
Course 6421A
DHCP Server Authorization
Module 4: Configuring and Troubleshooting DHCP
DHCP authorization is the process of registering the DHCP Server service in the Active Directory domain to support DHCP clients
DHCP Server1 checks with the domain controller to obtain a list of authorized DHCP servers
If DHCP Server1 finds its IP address on the list, the service starts and supports DHCP clients
A rogue DHCP server can cause problems in a network. Incorrectly configured clients can cause numerous issues. When you install a DHCP role in a domain, an Enterprise Administrator must authorize it because several domains can exist in the same IP subnet.
Although it is not recommended, you can use a stand-alone server as a DHCP server, provided that it is not on a subnet with any authorized DHCP servers. When a stand-alone DHCP server detects an authorized server on the same subnet, it automatically stops leasing IP addresses to DHCP clients.
It also is important to note that other network devices may run DHCP servers. These devices do not comply with the notion of being authorized, and therefore they may cause issues in a networked environment.
Domain
Controller
Active
Directory
DHCP Client
DHCP Server1
Authorized
Services DHCP requests
DHCP Server2
Unauthorized
Does not service DHCP requests
If DHCP Server2 does not find its IP address on the list, the service does not start and support DHCP clients
DHCP client receives IP address
from authorized DHCP Server1
DHCP Server2 checks with the
domain controller to obtain a list of
authorized DHCP servers

8. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
What Are DHCP Scopes?
Module 4: Configuring and Troubleshooting DHCP
A scope is a range of IP addresses that are available to be leased
DHCP Server
Administrators must create a DHCP scope before leasing IP addresses to a client. A DHCP scope is a range of IP addresses that are available for lease.
Scope properties contain data about the scope, such as the scope range, the lease duration, Domain Name System (DNS) update settings, NAP Configuration, and DHCP/BOOTP configuration options.
LAN A
LAN B
Scope A
Scope B
Scope Properties
Network ID
Subnet mask
Scope name
Exclusion range
Lease duration
Network IP address range

9. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
What Are DHCP Options?
Module 4: Configuring and Troubleshooting DHCP
DHCP options are values for common configuration data that applies to the server, scopes, reservations, and class options
DHCP options allow you to apply common settings to computers that you define in scopes.
Common scope options are:
DNS Servers
DNS Name
Default Gateway
WINS Servers
WINS Servers

10. What Are DHCP Class-Level Options?
Course 6421A
What Are DHCP Class-Level Options?
Module 4: Configuring and Troubleshooting DHCP
DHCP class-level options are scope options that apply to a specific type of device
Vendor-class
Vendors specify vendor classes internally You cannot change the vendor class. Microsoft’s DHCP server role offers special options based on the vendor class. An example is disabling NetBIOS over TCP/IP for clients with a vendor class matching Windows 2000 or Windows XP.
User-class
You can specify user class as needed when you want to set options for a certain class of users (for example, users from a particular physical location).
DHCP class-level option
Description
Vendor-class
Configured by vendors such as Microsoft, HP, and Sun
User-class
Set and viewed by the user

11. What Is a DHCP Reservation?
Course 6421A
What Is a DHCP Reservation?
Module 4: Configuring and Troubleshooting DHCP
A reservation is a specific IP address, within a scope, that is reserved permanently for lease to a specific DHCP client
A DHCP reservation is when an IP address within a scope is set aside for use with a specific DHCP client.
Why to use a DHCP reservation.
If you intend to have servers or printers, it often is desirable to provide them with a fixed address. This ensures that IP addresses in a predefined scope will not be assigned inadvertently to another device. This also will ensure that should a scope be depleted of addresses, the devices with reservations will be guaranteed to have an IP address.
The process for configuring a DHCP reservation:
Open the DHCP Server role.
Expand the DHCP scope, and then click Reservations.
Click More Actions, and then click New Reservation.
To configure a reservation, you must know the media access control (MAC) or physical address of the device. This is how the DHCP server knows that the device should have a reservation.
Workstation 1
File and Print Server
Subnet A
Subnet B
DHCP Server
Workstation 2
IP Address1: Leased to Workstation 1
IP Address2: Leased to Workstation 2
IP Address3: Reserved for File and Print Server

12. DHCP Sizing and Availability
Course 6421A
DHCP Clients
DHCP Server1
DHCP Server2
DHCP Sizing and Availability
Module 4: Configuring and Troubleshooting DHCP
For balancing DHCP server usage, a best practice is to use the “80/20” rule to divide the scope addresses between the two DHCP servers. If you configure Server 1 to make available most (approximately 80%) of the addresses, then you can configure Server 2 to make the other addresses (approximately 20%) available to clients.
This also adds a degree of fault tolerance to the DHCP servers by increasing their availability. If one server fails, the second server can continue to renew and provide leases for the clients.
DHCP Server1 has 20% of addresses as follows:
Scope range:
Excluded addresses:
DHCP Server2 has 80% of addresses as follows:
Excluded addresses: 12

13. How DHCP Options Are Applied
Course 6421A
How DHCP Options Are Applied
Module 4: Configuring and Troubleshooting DHCP
DHCP options can be applied at various levels:
DHCP applies options to client computers in a specific order:
Server level
Scope level
Class level
Reserved-client level
Scope options will override server options. Class options will override both scope and server options. Reserved-client options apply to devices that have a DHCP reservation.
Server
Scope
Class
Reserved client

14. Overview of DHCP Management Scenarios
Course 6421A
Overview of DHCP Management Scenarios
Module 4: Configuring and Troubleshooting DHCP
The DHCP service needs to be managed to respond to network changes
Scenarios for managing DHCP:
DHCP management scenarios include:
Managing DHCP database growth. The DHCP database is based on a Microsoft Jet database. You need to compact Jet databases on a regular basis.
Backup and restore. Information in the DHCP database is important to maintain. If the DHCP server database becomes corrupt or gets lost, it could lead to significant IP configuration issues.
DHCP database consistency. The database needs to be accurate. If lease data in the DHCP database does not match the lease information on the client, issues such as duplicate IP addresses can occur on the network.
Moving the DHCP database. If the database is very large, it may need to be moved to a larger partition or a better performing volume.
Adding clients.
Adding new network service servers.
Adding new subnets. Adding clients, servers, and subnets can lead to changes in the way the DHCP database is used. These changes require database monitoring and may require new maintenance actions.
Managing DHCP database growth
Protecting the DHCP database
Ensuring DHCP database consistency
Adding clients
Adding new network service servers
Adding new subnets

15. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
What Is a DHCP Database?
Module 4: Configuring and Troubleshooting DHCP
The DHCP database is a dynamic database that contains configuration information
The DHCP database contains DHCP configuration data such as:
Scopes
Address leases
Reservations
The J50.log file, J50#####.log file, Dhcp.mdb file, and Dhcp.tmp file should not be removed or altered.
Jet databases do not recover space automatically when records are erased. Thus, the database is compacted periodically. If the database has an increased amount of usage, it may be necessary to compact the database manually. Starting with Windows NT Server 4.0, dynamic database compaction occurs on DHCP servers as an automatic background process during idle time or after a database update.
Windows Server 2003 stores the DHCP database in the %Systemroot%\System32\Dhcp folder
The DHCP database files include:
Dhcp.mdb
Tmp.edb
J50.log and J50*.log
Res*.log
J50.chk

16. How a DHCP Database Is Backed Up and Restored
Course 6421A
How a DHCP Database Is Backed Up and Restored
Module 4: Configuring and Troubleshooting DHCP
DHCP Server
DHCP
Offline Storage
Restore
Automatic backup (synchronous backup)
Occurs every 60 minutes.
Best practice: Ensure that an offsite backup of the database is kept.
Best practice: Make sure that your automatic backup is to a different volume than that on which your DHCP server is running.
Manual backup (asynchronous backup)
Requires administrative-level permissions. Can also be a member of the DHCP administrators group.
What is backed up:
All scopes, including superscopes and multicast scopes.
Reservations.
Leases.
All options, including server options, scope options, reservation options, and class options.
All registry keys and other configuration settings (for example, audit log settings and folder location settings) set in DHCP server properties. These settings are stored in the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer\Parameters
 To back up this subkey, open Registry Editor, and save the specified key to a text file
Backup Security:
Best Practice: Backups stored on another volume should grant permissions only to the administrative and DHCP administrator groups.
Restore process
Back up
Restore
Back up
The DHCP service automatically backs up the DHCP database to the backup directory on the local drive
If the original database is unable to load, the DHCP service automatically restores from the backup directory on the local drive
In the event that the server hardware fails, the administrator can restore only from the offline storage location
The administrator moves a copy of the backed up DHCP database to an offline storage location

17. How a DHCP Database Is Reconciled
Course 6421A
How a DHCP Database Is Reconciled
Module 4: Configuring and Troubleshooting DHCP
DHCP
Database
Detailed IP address lease information
Compares and reconciles inconsistencies in the DHCP Database
Reconciling scopes can fix inconsistencies, such as incorrect or missing information for client IP addresses that are stored in scope-lease information. The DHCP Server service stores the Scope IP address-lease information:
Detailed IP address-lease information, stored in the DHCP database.
Summary IP address-lease information, stored in the DHCP database.
When reconciling scopes, the detail and summary entries are compared to find inconsistencies.
To correct and repair these inconsistencies, you need to reconcile any scope inconsistencies found when performing this operation. Once you select and reconcile scope inconsistencies, the DHCP service either restores those IP addresses to the original owner or creates a temporary reservation for them. These reservations are valid for the lease time assigned to the scope. When the lease time expires, the addresses are recovered for future use.
Registry
Summary IP address lease information
DHCP Server
Example
Registry
DHCP Database
After Reconciliation
Client has IP address
IP address
is available
Lease entry is created in DHCP Database

18. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
Moving a DHCP Database
Module 4: Configuring and Troubleshooting DHCP
DHCP
Database
You can move a DHCP database from one server to another by using the normal backup and restore procedure.
Steps for moving a DHCP database:
Back up database on old server.
Stop the old DHCP server.
Copy database to the new server and, if necessary, install the DHCP server role.
Restore the database.
Start the DHCP server role.
Backup
Media
DHCP
Database
Old DHCP Server
New DHCP Server

19. DHCP Server Configuration Options
Course 6421A
DHCP Server Configuration Options
Module 4: Configuring and Troubleshooting DHCP
DHCP server configuration options are the global settings that you define at the server level.
General options
Allow the administrator to set DHCP statistic for debugging and troubleshooting.
DNS options
This is an important panel to configure if there are devices or operating systems that do not update their DNS information automatically. You can configure the DHCP server to update the DNS server if the client is unable to do so.
Network Access Protection options
This panel enables you to enforce NAP for one or more scopes. NAP allows administrators to validate that machines requesting an IP address have been patched to their operating system’s latest version, and that they are running an updated anti-virus program.
Advanced options
Allows the administrator to force the DHCP server to check for IP conflicts when a DHCP client requests a particular IP address. Older clients that do not perform their own check benefit from this. However, this also can cause some overhead. The recommended configuration is to turn this setting off.
The IP binding allows the administrator to specify on which IP address the DHCP server should listen for requests. 19

20. Overview of Monitoring DHCP
Course 6421A
Overview of Monitoring DHCP
Module 4: Configuring and Troubleshooting DHCP
Why monitor DHCP?
To observe the dynamic DHCP environment
To determine DHCP server performance
To facilitate planning for current and future needs
Methods of, and the reasons for, monitoring DHCP:
To ensure the DHCP service is performing at an acceptable level.
To ensure the DHCP service has sufficient IP addresses to provide to all clients.
To anticipate future growth and proactively address possible issues.
Monitoring tasks include:
DHCP statistics
DHCP events
DHCP performance data
DHCP data includes:
DHCP statistics
DHCP events
DHCP performance data

21. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
Common DHCP Issues
Module 4: Configuring and Troubleshooting DHCP
Address conflicts
Failure to obtain a DHCP address
Address obtained from incorrect scope
DHCP database suffered data corruption or loss
DHCP server has exhausted its IP address pool

22. What Are DHCP Statistics?
Course 6421A
What Are DHCP Statistics?
Module 4: Configuring and Troubleshooting DHCP
DHCP statistics are collected at either the server level or scope level
DHCP statistics provide a general view of DHCP activity and usage.
You can configure the refresh rate for the statistics in the server properties General tab.
How to monitor DHCP statistics
Show students the statistics panel in the DHCP Server.
DHCP server statistics
Provide an overview of DHCP server usage. You can use this data to understand the DHCP server’s state quickly.
DHCP scope statistics
Provides basic data about the leases in the DHCP scope.
DHCP Server

23. What Is a DHCP Audit Log File?
Course 6421A
What Is a DHCP Audit Log File?
Module 4: Configuring and Troubleshooting DHCP
A DHCP audit log is a log of service-related events
DHCP audit log.
The audit log provides a traceable log of DHCP server activity that you can use to track lease requests, and grants and denials, and to troubleshoot DHCP server issues.
The audit file is stored in systemroot\system32\dhcp.
The name of the audit file is based on the weekday it was created. For example, if the day of the week is Monday, then the file name is DhcpSrvLog-Mon.log.
Fields that make up a DHCP audit log:
Field Description
ID A DHCP server event ID code
Date The date on which this entry was logged on the DHCP server
Time The time at which this entry was logged on the DHCP server
Description A description of this DHCP server event
IP Address The IP address of the DHCP client
Host Name The host name of the DHCP client
MAC Address The MAC address that the network adapter hardware of the client uses
Common Event ID codes
ID,Date,Time,Description,IP Address,Host Name,MAC Address
00,06/08/03,22:35:10,Started,,,,
56,06/08/03,22:35:10,Authorization failure, stopped servicing,,domain1.local,,
55,06/08/03,22:45:38,Authorized(servicing),,domain1.local

24. Monitoring DHCP Server Performance
Course 6421A
Monitoring DHCP Server Performance
Module 4: Configuring and Troubleshooting DHCP
Performance counters
What to look for after a baseline is established
Packets received/second
Monitor for sudden increases or decreases, which could reflect network problems
Requests/second
Active queue length
Monitor for both sudden and gradual increases, which could reflect increased load or decreased server capacity
Duplicates dropped/second
Monitor for any activity that could indicate that more than one request is being transmitted on behalf of clients
Create a DHCP performance baseline
The DHCP performance counters become available after you install the DHCP Server role. Once available, you can load the performance counters using the performance monitor.
A DHCP server typically should not come under a heavy network load. However, if you notice the queue lengths are logging consistently high values, you should check the server for bottlenecks that could be slowing DHCP performance.
Common performance counters include:
Packets received/second
Packets expired/second
Requests/second
Milliseconds per packet
Active queue length
Duplicates dropped/second
Check the standard counters for server performance
Review DHCP server counters for significant changes in DHCP traffic

25. Module 4: Configuring and Troubleshooting DHCP
Course 6421A
Securing DHCP
Module 4: Configuring and Troubleshooting DHCP
Preventing an unauthorized user from obtaining a lease
Enable and monitor Audit log
Configure NAP
Use x LAN switch or Wireless Access Point
Control Physical Access as well as Wireless Access
Reasons for securing DHCP:
Preventing an unauthorized user from obtaining a lease
Restricting unauthorized, non-Microsoft DHCP servers from leasing IP addresses
Restricting DHCP administration
Restricting unauthorized DHCP servers from leasing
IP addresses
Use only Microsoft DHCP Server in order to be authorized
by Active Directory
Restricting DHCP administration
DHCP Users Group: Read-only access to DHCP Console
DHCP Administrators Group: Limit members