Managing DHCP
DHCP Overview
• Is a protocol that allows client computers to
automatically receive an IP address and TCP/IP
settings from a Server
• Reduces the amount of time you spend configuring
computers on your network
• Is the default configuration for clients.
• The ipconfig /all command will indicate whether the
configuration came from a DHCP server computer
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DHCP Overview (continued)
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DHCP Overview (continued)
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Leasing an IP Address
• An IP address is leased during the boot process
• The overall process is composed of four broadcast
packets:
•
•
•
•
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
DHCPACK
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Leasing an IP Address
(continued)
• Any DHCP server that receives the DHCPDISCOVER
packet responds with a DHCPOFFER packet
• The DHCP client responds to the DHCPOFFER
packet it receives with a DHCPREQUEST packet
• A DHCPACK packet indicates confirmation that the
client can use the lease
• Once DHCPACK is received, the client can start using
the IP address and options in the lease
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Leasing an IP Address
(continued)
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Renewing an IP Address
• The IP address can either be permanent or timed
• A permanent address is never reused for another
client
• Timed leases expire after a certain amount of time
• Windows clients attempt to renew their lease after
50% of the lease time has expired. If the renewal
process fails, it attempts again after 87.5% of the
lease time has expired.
• Renewing the lease involves the client sending a
DHCP Request packet to DHCP Server
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Renewing an IP Address
(continued)
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More on the Renewal Process…
• DHCP Client, at startup, attempts to reach the DHCP
Server
Server Available:.
• If the server is available and the lease has not yet
expired, the client retains the IP address
• If the server is available and the lease has expired,
the client attempts to renew the lease.
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More on the Renewal Process…
• DHCP Client, at startup, attempts to reach the DHCP
Server
Server Unavailable:
• If the server is unavailable, the client will ping the
previously assigned default gateway to determine
if it’s on the same network.
• If the gateway responds and the lease hasn’t expired, the
client retains the IP address
• If the gateway doesn’t respond the client will send a
DISCOVER packet to begin the lease process over
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Autoconfiguration
• When a DHCP Server does not respond to a Client’s
call for an IP Address, the client will autoconfigure
itself
• The client selects an IP address from the
169.254.0.0 subnet
• The client will attempt to contact a DHCP server
using DISCOVER packets every 5 minutes
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Installing the DHCP Service
• When placing a DHCP Service on a Server in a Domain:
• Install the DHCP Server Service
• Authorize DHCP Server in Active Directory
• Configure DHCP Server with appropriate scopes,
exclusions, reservations and options
• Activate the DHCP Server’s Scopes
• When placing a DHCP Service on a Server in a Workgroup:
• Install the DHCP Server Service
• Configure DHCP Server with appropriate scopes,
exclusions, reservations and options
• Activate the DHCP Server’s Scopes
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Authorizing the DHCP Service
• A server that is a member of a domain can be authorized
• During the installation of the service: the Install Wizard
provides an option to authorize the server
• Using the DHCP management snap-in
• Only members of the Enterprise Admins group can
authorize a server
• A server that is a member of a workgroup does not need to be
authorized.
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Configuring DHCP Scopes
• Scope defines a range of IP addresses
• Each scope is configured with:
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•
•
•
•
•
Description
Starting IP address
Ending IP address
Subnet mask
Exclusions
Lease duration
• Two strategies exist for defining the starting and
ending IP addresses
• Allow all and exclude the few static addresses
• Reserve a range of addresses at beginning or end of range
that can be used for static addresses
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Configuring DHCP Scopes
(continued)
• Lease duration defines how long client computers are
allowed to use an IP address
• Default lease duration varies based on the network
type and the DHCP Server version
• A scope must be activated before the DHCP service
can begin using it
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Creating DHCP Reservations
• Reservations are used to hand out a specific IP
address to a particular client
• Useful when delivering IP addresses to devices that
would normally use static addresses
• Reservations are created based on MAC addresses
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Creating DHCP Exclusions
• Exclusions are IP Addresses that are within the
subnet defined within the scope but that should not be
assigned to a dhcp client
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Configuring DHCP Options
• DHCP can hand out a variety of other IP configuration
options
• It is common that all workstations within an entire
organization use the same DNS servers
• DNS is often configured at the server level
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DHCP Relay Agent
• DHCP packets cannot travel across a router
• A relay agent is necessary in order to have a single DHCP
server handle all leases on both network segments
• This can be a Windows 2003/2008 server with DHCP
Relay Agent protocol installed or a router that is configured
as a relay
• Relay agents receive broadcast DHCP packets and forward
them as unicast packets to a DHCP server
• The relay agent must be configured with the IP address of the
DHCP server
• The DHCP relay cannot be installed on the same server as the
DHCP service
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Configuring a DHCP Relay
(continued)
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Superscopes
• Used to combine multiple scopes into a single logical
scope
• Allows multiple scopes to be treated as a single scope
• Useful when a single physical network segment
contains more than one logical subnet
• If a superscope is used, then the DHCP server offers
only one lease as opposed to multiple leases
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Example 1: No Superscope
• One physical network
segment
• One logical subnet
(192.168.1)
• One DHCP Server
• Single scope is used to
service all DHCP
clients on Subnet A
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Example 2: Superscope
• One physical network segment
• Multiple logical subnets
•
•
•
192.168.1
192.168.2
192.168.3
• Three single scopes created and
joined into one superscope
• One DHCP Server services all
clients on Subnet A with an IP
address from the superscope
• Router configured with multiple
addresses to allow packets to
move from one logical network
to another
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Example 3: Superscope
Implemented across a Router
• Two physical network
segments: Subnet A and
Subnet B
• One DHCP Server
• Router configured with
Relay Agent
• Something that will pass
Discover Packets back
and forth from DHCP
Clients and DHCP Server
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Example 3: Superscope
Implemented across a Router
• Subnet A:
• One physical segment
• One logical subnet (192.168.1)
• One single scope defined
• DHCP server distributes
addresses to clients on Subnet
A using addresses in single
scope
• Subnet B:
• One physical segment
• Two logical subnets (192.168.2
& 192.168.3)
• Two single scopes defined and
joined into one Superscope
• DHCP server distributes
address to clients on Subnet B
using addresses in superscope
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Vendor and User Classes
• Used to differentiate between clients within a scope
• Vendor classes are based on the operating system
• User classes are defined based on network
connectivity or the administrator
• You can use the ipconfig /setclassid command to set
the DHCP user class ID
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DHCP Audit Logging
• DHCP audit logs keep detailed information about
DHCP server activity
• The logs are used to troubleshoot a DHCP server
• They are stored in the C:\WINDOWS\system32\dhcp
directory. There’s a file for each day of the week.
• Each line contains an event ID that states the nature
of the event
• The Header of the log file provides a summary of
events and their meanings
• Auditing can be disabled
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Configuring DHCP Bindings
• The DHCP service will bind automatically to the first
network card on the server
• You can choose which network card the DHCP
Service is bound to
• The server only hands out IP addresses through a
network card that has the DHCP Service bound
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Integrating DHCP and DNS
• DNS Dynamic Update protocol allows clients
running Windows 2000 or later to automatically
update records in the DNS database
• The default DHCP configuration has this protocol
enabled and will update clients only if requested
• DHCP server can be configured to dynamically
update older clients
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Conflict Detection
• Using DHCP does not prevent static IP configuration
• A DHCP server may hand out an IP address that was
already statically assigned
• Conflict detection prevents a DHCP server from
creating IP address conflicts
• A DHCP server pings an IP address before it is leased
to a client computer
• This can be configured from the GUI as well as well
as with the netsh command
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Saving and Restoring DHCP
Configuration
• DHCP Server configurations can be saved to a file
• These saved settings can then be used to restore the
server to a known state OR to use the same settings
on another server
• To store the configuration while logged on locally:
netsh dhcp server dump > filename
• To restore the configuration:
Netsh exec filename
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Managing and Maintaining the
DHCP Database
• The default location of the DHCP database is
%systemroot%\system32\dhcp
• The DHCP server service performs 2 routine actions to
maintain the database. The actions are performed every 60
minutes:
• Checks and cleans up expired leases and leases that no
longer apply
• Database backup – the backup files are automatically stored
in the %systemroot%\system32\dhcp\backup directory
• To view the current configuration:
netsh dhcp server show dbproperties
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Managing and Maintaining the
DHCP Database
• The netsh command can be used to change the values of the
database properties
Netsh dhcp server set PropertyName NewPropertyValue
• When changing the database name or folder locations you must
stop and start the dhcp server service
Net stop “dhcp server”
Net start “dhcp server”
• The database can be manually backed up and/or restored
• The database files can be moved to another server
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Viewing DHCP Statistics
• Windows Server 2008 DHCP Service automatically
tracks statistics
• Statistics are viewable as a whole or by scope
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DHCP Availability and Fault Tolerance
• Multiple DHCP servers on the network increases
reliability and allows fault tolerance
• In a server cluster DHCP server service can be failed
over to another server – this is costly
• Simpler and less expensive approaches
• 50/50 failover approach
• 80/20 failover approach
• 100/100 failover approach
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