Week #3: 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

Benefits of Using DHCP

DHCP reduces the complexity and amount of administrative work by using automatic TCP/IP configurationDHCP reduces the complexity and amount of administrative work by using automatic TCP/IP configuration

Manual 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

Automatic TCP/IP Configuration

• IP addresses are supplied automatically

• Correct configuration information is ensured

• Client configuration is updated automatically

• A common source of network problems is eliminated

New DHCP Features in Windows Server 2008

New DHCP features include:

• Windows Server 2008 Support for DHCPv6

• Support for advanced network security configuration using NAP

• DHCP on Server Core

How DHCP Allocates IP Addresses

DHCP Server

DHCP Databas

e

IP Address1: Leased to DHCP Client1

IP Address2: Leased to DHCP Client2

IP Address3: Available to be leased

DHCP Client2:IP configuration

from DHCP server

Non-DHCP Client:Static IP

configuration

DHCP Client1:IP configuration

from DHCP server

Lease Renewal

Lease Generation

DHCP client broadcasts a DHCPDISCOVER packet1

DHCP servers broadcast a DHCPOFFER packet2

DHCP client broadcasts a DHCPREQUEST packet3

DHCP Server1 broadcasts a DHCPACK packet4

How DHCP Lease Generation Works

DHCP Client

DHCP Server1

DHCP Server2

DHCP client broadcasts a DHCPDISCOVER packet1

DHCP servers broadcast a DHCPOFFER packet2

DHCP client broadcasts a DHCPREQUEST packet3

DHCP Server1 broadcasts a DHCPACK packet4

DHCP Client

DHCP Server1

DHCP Server2

DHCP Client sends a DHCPREQUEST packetDHCP Client sends a DHCPREQUEST packet11

DHCP Server1 sends a DHCPACK packetDHCP Server1 sends a DHCPACK packet22

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 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

How DHCP Lease Renewal Works

DHCP ClientDHCP ClientDHCP

Server1DHCP

Server1

DHCP Server2DHCP

Server2

50% of lease duration has

expired

50% of lease duration has

expired

87.5% of lease

duration has expired

87.5% of lease

duration has expired

100% of lease

duration has expired

DHCP ClientDHCP

Server1

DHCP Server2

DHCP client sends a DHCPREQUEST packet1

DHCP Server1 sends a DHCPACK packet2

50% of lease duration has expired

DHCP Server2 checks with the domain controller to obtain a list of

authorized DHCP servers

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 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

DomainControllerDomain

Controller

Active DirectoryActive Directory

DHCP Client DHCP Client

DHCP Server Authorization

Unauthorized

Does not service DHCP requests

Authorized

Services DHCP requests

DHCP Server1DHCP Server1

DHCP Server2DHCP Server2

DHCP authorization is the process of registering the DHCP Server service in the Active Directory domain to support DHCP clientsDHCP authorization is the process of registering the DHCP Server service in the Active Directory domain to support DHCP clients

What Are DHCP Scopes?

A scope is a range of IP addresses that are available to be leasedA scope is a range of IP addresses that are available to be leased

Scope Properties

• Scope name

• Exclusion range

• Lease duration

• Network IP address range

• Network ID

• Subnet mask

LAN A LAN B

DHCP ServerDHCP Server

Scope BScope BScope AScope A

• WINS Servers

Common scope options are:

What Are DHCP Options?

DHCP options are values for common configuration data that applies to the server, scopes, reservations, and class options

DHCP options are values for common configuration data that applies to the server, scopes, reservations, and class options

• DNS Servers

• DNS Name

• WINS Servers

• Default Gateway

What Are DHCP Class-Level Options?

DHCP class-level options are scope options that apply to a specific type of deviceDHCP class-level options are scope options that apply to a specific type of device

DHCP class-level option Description

Vendor-class Configured by vendors such as Microsoft, HP, and Sun

User-class Set and viewed by the user

What Is a DHCP Reservation?

A reservation is a specific IP address, within a scope, that is reserved permanently for lease to a specific DHCP client A reservation is a specific IP address, within a scope, that is reserved permanently for lease to a specific DHCP client

Subnet ASubnet A Subnet BSubnet B

Workstation 1

DHCP ServerWorkstation 2

File and Print Server

IP Address1: Leased to Workstation 1IP Address2: Leased to Workstation 2 IP Address3: Reserved for File and Print Server

DHCP Sizing and Availability

DHCP Clients

DHCP Server1

192.168.1.2

DHCP Server2

192.168.1.1

DHCP Clients

DHCP Server1 has 20% of addresses as follows:

•Scope range: 192.168.1.10-192.168.1.254

•Excluded addresses: 192.168.1.10-192.168.1.205

DHCP Server2 has 80% of addresses as follows:

•Scope range: 192.168.1.10-192.168.1.254

•Excluded addresses: 192.168.1.206-192.168.1.254

How DHCP Options Are Applied

DHCP options can be applied at various levels:

• Server

• Scope

• Reserved client

• Class

Scenarios for managing DHCP:

Overview of DHCP Management Scenarios

The DHCP service needs to be managed to respond to network changesThe DHCP service needs to be managed to respond to network changes

• Managing DHCP database growth

• Protecting the DHCP database

• Ensuring DHCP database consistency

• Adding clients

• Adding new network service servers

• Adding new subnets

What Is a DHCP Database?

• 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

The DHCP database is a dynamic database that contains configuration informationThe DHCP database is a dynamic database that contains configuration information

• The DHCP database contains DHCP configuration data such as:

• Scopes

• Address leases

• Reservations

DHCP Server

DHCP

DHCP

Offline Storage

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

The administrator moves a copy of the backed up DHCP database to an offline storage location

In the event that the server hardware fails, the administrator can restore only from the offline storage location

How a DHCP Database Is Backed Up and Restored

Back up Restore

Back up

Restore

How a DHCP Database Is Reconciled

Example

Registry DHCP Database After Reconciliation

Client has IP address 192.168.1.34

IP address 192.168.1.34 is available

Lease entry is created in DHCP Database

DHCP Server

DHCPDatabase

Registry Summary IP address lease information

Detailed IP address lease information

Compares and reconciles

inconsistencies in the DHCP Database

Moving a DHCP Database

DHCPDatabase

Old DHCP Server

New DHCP Server

DHCPDatabase

BackupMedia

DHCP Server Configuration Options

Overview of Monitoring DHCP

Why monitor DHCP?

• To observe the dynamic DHCP environment

• To determine DHCP server performance

• To facilitate planning for current and future needs

DHCP data includes:

• DHCP statistics

• DHCP events

• DHCP performance data

Common DHCP Issues

• 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

What Are DHCP Statistics?

DHCP statistics are collected at either the server level or scope levelDHCP statistics are collected at either the server level or scope level

DHCP Server

What Is a DHCP Audit Log File?

A DHCP audit log is a log of service-related eventsA DHCP audit log is a log of service-related events

• Create a DHCP performance baseline

• Check the standard counters for server performance

• Review DHCP server counters for significant changes in DHCP traffic

Monitoring DHCP Server Performance

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 Monitor for sudden increases or decreases, which could reflect network problems

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

Securing DHCP

• Preventing an unauthorized user from obtaining a lease

• Enable and monitor Audit log

• Configure NAP

• Use 802.11x LAN switch or Wireless Access Point

• Control Physical Access as well as Wireless Access

• 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