network computers by assigning addresses from a pool (called a scope) and reclaiming them
when their leases expire.
Aside from being a time-consuming chore, manually configuring TCP/IP clients can result
in typographical errors that cause addressing conflicts that interrupt network communications.
DHCP prevents these errors and provides many other advantages, including automatic
assignment of new addresses when computers are moved from one subnet to another and
automatic reclamation of addresses that are no longer in use.
DHCP consists of three components, as follows:
-A DHCP service, which responds to client requests for TCP/IP configuration settings
-A DHCP client, which issues requests to servers and applies the TCP/IP configuration
settings it receives to the local computer
-A DHCP communications protocol, which defines the formats and sequences of the
messages exchanged by DHCP clients and servers
All the Microsoft Windows operating systems include DHCP client capabilities, and all the
server operating systems (including Windows Server 2012 R2) include the Microsoft DHCP
Server role.
The DHCP standards define three different IP address allocation methods:
- Dynamic allocation The DHCP server assigns an IP address to a client computer
from a scope for a specified length of time. Each client must periodically renew the
lease to continue using the address. If the client allows the lease to expire, the address
is returned to the scope for reassignment to another client.
- Automatic allocation The DHCP server permanently assigns an IP address to a client
computer from a scope. Once the DHCP server assigns the address to the client, the
only way to change it is to manually reconfigure the computer.
- Manual allocation The DHCP server permanently assigns a specific IP address to a
specific computer on the network. In the Microsoft DHCP server, manually allocated
addresses are called reservations.
In addition to IP addresses, DHCP can provide clients with values for the other parameters
needed to configure a TCP/IP client, including a subnet mask, default gateway, and DNS
server addresses. The object is to eliminate the need for any manual TCP/IP configuration on
a client system. For example, the Microsoft DHCP server includes more than 50 configuration
parameters, which it can deliver along with the IP address, even though Windows clients can
only use a subset of those parameters.
DHCP communications use eight types of messages, each of which uses the same basic
packet format. DHCP traffic is carried within standard UDP/IP datagrams, using port 67 at the
server and port 68 at the client.
DHCP options
All DHCP messages include an options field, which is a catch-all area designed to carry the
various parameters (other than the IP address) used to configure the client system’s TCP/IP
stack. Some of the most commonly-used options are described in the following sections.
THE DHCP MESSAGE TYPE OPTION
The DHCP Message Type option identifies the overall function of the DHCP message and
is required in all DHCP packets. The DHCP communication protocol defines eight message
types, as follows:
- DHCPDISCOVER Used by clients to request configuration parameters from a DHCP
server
- DHCPOFFER Used by servers to offer IP addresses to requesting clients
-DHCPREQUEST Used by clients to accept or renew an IP address assignment
- DHCPDECLINE Used by clients to reject an offered IP address
- DHCPACK Used by servers to acknowledge a client’s acceptance of an offered IP
address
- DHCPNAK Used by servers to reject a client’s acceptance of an offered IP address
- DHCPRELEASE Used by clients to terminate an IP address lease
- DHCPINFORM Used by clients to obtain additional TCP/IP configuration parameters
from a server
BOOTP VENDOR INFORMATION EXTENSIONS
These options include many of the basic TCP/IP configuration parameters used by most client
systems, such as the following:
- Subnet Mask Specifies which bits of the IP address identify the host system and
which bits identify the network where the host system resides
- Router Specifies the IP address of the router (or default gateway) on the local network
segment the client should use to transmit to systems on other network segments
- Domain Name Server Specifies the IP addresses of the servers the client will use for
DNS name resolution
- Host Name Specifies the DNS host name the client will use
- Domain Name Specifies the name of the DNS domain on which the system will
reside
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NOTE: BOOTP
The Bootstrap Protocol (BOOTP) is the predecessor to DHCP. The two are largely
compatible; the primary difference is that BOOTP allocates IP addresses permanently,
and not by leasing them.
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DHCP EXTENSIONS
These options are used to provide parameters that govern the DHCP lease negotiation and
renewal processes.
- Requested IP Address Used by the client to request a particular IP address from the
server
- IP Address Lease Time Specifies the duration of a dynamically allocated IP address
lease
- Server Identifier Specifies the IP address of the server involved in a DHCP
transaction; used by the client to address unicasts to the server
- Parameter Request List Used by the client to send a list of requested configuration
options (identified by their code numbers) to the server
- Message Used to carry an error message from the server to the client in a DHCPNAK
message
- Renewal (T1) time value Specifies the time period that must elapse before an IP
address lease enters the renewing state
- Rebinding (T2) time value Specifies the time period that must elapse before an IP
address lease enters the rebinding state
DHCP communications
To design a DHCP strategy for an enterprise network and deploy it properly requires an
understanding of the communications that occur between DHCP clients and servers. In
Windows computers, the DHCP client is enabled by default, although it is not mentioned by
name in the interface. The Obtain An IP Address Automatically option in the Internet Protocol
Version 4 (TCP/IPv4) Properties sheet and the Obtain An IPv6 Address Automatically option in
the Internet Protocol Version 6 (TCP/IPv6) Properties sheet control the activation of the client
for IPv4 and IPv6, respectively.
DHCP LEASE NEGOTIATION
DHCP communication is always initiated by the client, as shown in Figure 4-6, and proceeds
as follows:
1. When a computer boots for the first time with the DHCP client active, the client
generates a series of DHCPDISCOVER messages to solicit an IP address assignment
from a DHCP server and broadcasts them on the local network.
2. All DHCP servers receiving the DHCPDISCOVER broadcast messages generate
DHCPOFFER messages containing an IP address and other TCP/IP configuration
parameters and transmit them to the client.
3. After a specified period, the client accepts one of the offered addresses by
broadcasting a DHCPREQUEST message containing the address of the offering server.
4. When the offering server receives the DHCPREQUEST message, it adds the offered IP
address and other settings to its database.
5. The server transmits a DHCPACK message to the client, acknowledging the completion
of the process. If the server cannot complete the assignment, it transmits a DHCPNAK
message to the client and the process restarts.
6. As a final test, the client broadcasts the offered IP address using the Address Resolution
Protocol (ARP) to ensure that no other system on the network is using it. If the
client receives no response to the ARP broadcast, the DHCP transaction is completed.
If another system responds to the ARP message, the client discards the IP address and
transmits a DHCPDECLINE message to the server, nullifying the transaction. The client
then restarts the process.
DHCP LEASE RENEWAL
By default, the DHCP Server service in Windows Server 2012 R2 uses dynamic allocation,
leasing IP addresses to clients for eight-day periods. At periodic intervals during the lease, the
client attempts to contact the server to renew the lease, as shown in Figure 4-7, by using the
following procedure:
1. When the DHCP client reaches the 50 percent point of the lease’s duration (called the
renewal time value or T1 value), the client begins generating unicast DHCPREQUEST
messages and transmitting them to the DHCP server holding the lease.
2. If the server does not respond by the time the client reaches the 87.5 percent point
of the lease’s duration (called the rebinding time value or T2 value), the client begins
transmitting its DHCPREQUEST messages as broadcasts in an attempt to solicit an IP
address assignment from any DHCP server on the network.
3. If the server receives the DHCPREQUEST message from the client, it responds with
either a DHCPACK message, which approves the lease renewal request, or a DHCPNAK
message, which terminates the lease. If the client receives no responses to its
DHCPREQUEST messages by the time the lease expires, or if it receives a DHCPNAK
message, the client releases its IP address. All TCP/IP communication then ceases,
except for the transmission of DHCPDISCOVER broadcasts.
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