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Guide to TCP/IP, Third Edition
Chapter 8:pThe Dynamic Host Configuration
Protocol
1
Protocol
CISNTWK-11PerformanceObjectives
• Understand the basic services DHCP offers to its clients and explain its backgroundclients and explain its background
• Understand the specifics of IP address management using DHCPg g
• Explain the DHCP Discovery, renewal, and release processesU d t d th b i DHCP k t t t • Understand the basic DHCP packet structure and types of DHCP messages in use
2IP Addressing and Related Topics 2
CISNTWK-11PerformanceObjective (continued)
• Describe broadcast and unicast addressingD ib l t i ti• Describe relay agent communications
• Discuss Microsoft DHCP scopes and classes• Use DHCP troubleshooting utilities• Use DHCP troubleshooting utilities
3IP Addressing and Related Topics 3
CISNTWK-11PerformanceIntroducing DHCP
• DHCP Service that provides a way for client computer to – Service that provides a way for client computer to request IP address assignment
– Delivers necessary configuration information to clients – Traces its origins back to an earlier protocol named
BOOTP– Servers can manage one or more ranges of IP
addresses
4IP Addressing and Related Topics 4
CISNTWK-11PerformanceHow DHCP Works
• When TCP/IP is configured on client computer The Obtain an IP address automatically option is the – The Obtain an IP address automatically option is the only necessary set-up element
• Next time workstation attempts to access network– It broadcasts a DHCP address request to the network
because it has no IP address• All DHCP servers present on same broadcast
domainR i t d d b k th t – Receive request and send back message that indicates a willingness to grant an address lease
5IP Addressing and Related Topics 5
CISNTWK-11PerformanceHow DHCP Works (continued)
• Client Accepts address lease offer and sends packet to server – Accepts address lease offer and sends packet to server that extended offer
• In reply– Server proffers an IP address for a specific period of
time that the client uses thereafter• When half the lease period expiresWhen half the lease period expires
– Client attempts to renew the lease
6IP Addressing and Related Topics 6
CISNTWK-11Performance
7IP Addressing and Related Topics 7
CISNTWK-11PerformanceThe Role of Leases
• Leases “Loans” of an address for a specific amount of time– “Loans” of an address for a specific amount of time
• Leases that range from one to three weeks – Typical on networks in which machines seldom move yp
and the workforce is stable• Lease periods
Average between one and three days for networks on – Average between one and three days for networks on which roving workers come and go regularly
• Lease periods of four to eight hours – Common on ISP networks
8IP Addressing and Related Topics 8
CISNTWK-11PerformanceDHCP Software Elements
• DHCP clientEnabled at client machine when you select the Obtain – Enabled at client machine when you select the Obtain an IP address automatically option
• DHCP server– Manages address pools and configuration data
• DHCP relay agentIntercepts address requests on local cable segment – Intercepts address requests on local cable segment
– Repackages requests as a unicast to one or more DHCP servers
9IP Addressing and Related Topics 9
CISNTWK-11PerformanceDHCP Lease Types
• Manual address leaseAdministrator assigns an IP address manually – Administrator assigns an IP address manually
• DHCP server – Assigns addresses for specific periods of timeg p p
10IP Addressing and Related Topics 10
CISNTWK-11PerformanceMore About DHCP Leases
• How DHCP integrates with DNSServer addresses are advertised using DNS– Server addresses are advertised using DNS
– All address updates must be entered manually– Client addresses usually come into play only when
• e-mail addresses of the form [email protected] must be resolved
11IP Addressing and Related Topics 11
CISNTWK-11Performance
Understanding IP Address Management with DHCPManagement with DHCP
• DHCP Discovery– Occurs when DHCP client broadcasts a request for an – Occurs when DHCP client broadcasts a request for an
IP address• DHCP Discovery
Relies on an initial DHCP broadcast– Relies on an initial DHCP broadcast• DHCP servers
– PING an address before offering it to the client• Windows clustering
– Allows two or more servers to be managed as a single system
12IP Addressing and Related Topics 12
CISNTWK-11PerformanceDHCP Address Discovery
• Uses four packetsDHCP Discover packet– DHCP Discover packet
– DHCP Offer packet– DHCP Request packet– DHCP Acknowledgment packet
13IP Addressing and Related Topics 13
CISNTWK-11Performance
14IP Addressing and Related Topics 14
CISNTWK-11PerformanceThe Discover Packet
• During DHCP Discovery processClient broadcasts Discover packet that identifies – Client broadcasts Discover packet that identifies client’s hardware address
– Header containsS IP dd 0 0 0 0 • Source IP address 0.0.0.0
• Preferred address– Typically the last address the client usedyp y
15IP Addressing and Related Topics 15
CISNTWK-11PerformanceThe Offer Packet
• Send by DHCP server to offer IP address to DHCP clientclient
• Offer packet – Includes IP address that is offered to the client and, – Sometimes, answers to the requested options in the
DHCP Discover packet
16IP Addressing and Related Topics 16
CISNTWK-11Performance
17IP Addressing and Related Topics 17
CISNTWK-11PerformanceThe Request Packet
• Once Offer packet is receivedClient can either accept offer by issuing a DHCP – Client can either accept offer by issuing a DHCP Request packet, or
– Reject offer by sending a DHCP Decline packet• Typically
– Client sends a Decline only if it receives more than one Offer
18IP Addressing and Related Topics 18
CISNTWK-11PerformanceThe Acknowledgment Packet
• Sent from server to client to indicate the completion of the four packet DHCP Discovery completion of the four-packet DHCP Discovery process
• Response p– Contains answers to any configuration options
requested by client in the previous Request packet
19IP Addressing and Related Topics 19
CISNTWK-11PerformanceThe Address Renewal Process
• When DHCP client receives an address from a DHCP serverDHCP server– Client also receives a lease time and notes the time
that the address was received• Lease time
– Defines how long the client can keep the address• In middle of the lease period• In middle of the lease period
– Client starts a renewal process
20IP Addressing and Related Topics 20
CISNTWK-11PerformanceThe Renewal Time (T1)
• T1Defined as the time that the client tries to renew its – Defined as the time that the client tries to renew its network address
• The Renewal packet– Is unicast directly to the DHCP server
• Default value for T10 5 * duration of lease (i e lease time)– 0.5 duration_of_lease (i.e., lease time)
21IP Addressing and Related Topics 21
CISNTWK-11PerformanceThe Rebinding Time (T2)
• The time that client begins to Broadcast a renewal request for an extended lease – Broadcast a renewal request for an extended lease time from another DHCP server
• DHCP specification, RFC 2131, defines default value for T2 as– 0.875 * duration_of_lease
22IP Addressing and Related Topics 22
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23IP Addressing and Related Topics 23
CISNTWK-11Performance
24IP Addressing and Related Topics 24
CISNTWK-11Performance
The DHCP Address Release ProcessProcess
• Client should release its address by Sending a DHCP Release packet to the server – Sending a DHCP Release packet to the server
• DHCP Release packet– Sent over UDP– DHCP server does not send any acknowledgment
• If client does not send the DHCP Release packetDHCP t ti ll l th dd t th – DHCP server automatically releases the address at the lease expiration time
25IP Addressing and Related Topics 25
CISNTWK-11PerformanceDHCP Packet Structures
• FieldsOperation Code (OPCODE) or Message Type Field– Operation Code (OPCODE) or Message Type Field
– Hardware Type Field– Hardware Length Field– Hops Field– Transaction ID Number Field– Seconds Since Boot or Seconds Elapsed FieldSeconds Since Boot or Seconds Elapsed Field– Flags Field
26IP Addressing and Related Topics 26
CISNTWK-11Performance
DHCP Packet Structures (continued)(continued)
• FieldsClient IP Address Field– Client IP Address Field
– Your IP Address Field– Server IP Address Field– Gateway or Relay Agent IP Address Field– Client Hardware Address Field– Server Host Name FieldServer Host Name Field– Boot File Field
27IP Addressing and Related Topics 27
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28IP Addressing and Related Topics 28
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29IP Addressing and Related Topics 29
CISNTWK-11Performance
DHCP Option 53: Message TypeType
• Only DHCP option required in all DHCP packetsI di t l f DHCP • Indicates general purpose of any DHCP message
• DHCP boot sequence uses these message types:DHCP boot sequence uses these message types:– DHCP MessageType 1: Discover (client to server)– DHCP MessageType 2: Offer (server to client)
DHCP M T 3 R t ( li t t )– DHCP MessageType 3: Request (client to server)– DHCP MessageType 5: ACK (server to client)
30IP Addressing and Related Topics 30
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31IP Addressing and Related Topics 31
CISNTWK-11Performance
Broadcast and UNICAST in DHCPDHCP
• DHCP clients Must broadcast service requests until they obtain IP – Must broadcast service requests until they obtain IP addresses
• DHCP clients – Use unicast addressing after they obtain an address for
a local DHCP server or relay agent• DHCP servers DHCP servers
– Examine DHCP packets coming from clients
32IP Addressing and Related Topics 32
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33IP Addressing and Related Topics 33
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Communications with a DHCP Relay AgentRelay Agent
• DHCP boot up process rRelies heavily on broadcasts but most routers do not – Relies heavily on broadcasts, but most routers do not forward broadcasts
• Relay agent function – Typically loaded on a router connected to the
segment containing DHCP clients• Relay agent device Relay agent device
– Configured with the address of the DHCP server– Can communicate using unicast packets directly with
that serverthat server
34IP Addressing and Related Topics 34
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35IP Addressing and Related Topics 35
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36IP Addressing and Related Topics 36
CISNTWK-11Performance
Microsoft DHCP Scopes and ClassesClasses
• Address scopesDefine a set of addresses that a DHCP server can – Define a set of addresses that a DHCP server can assign to clients
• Superscopes– A collection of scopes that contain sets of non-
consecutive IP addresses
37IP Addressing and Related Topics 37
CISNTWK-11Performance
Setting up a Simple DHCP ServerServer
• Field values in the Dynamic Pool SettingsIP Assigned From– IP Assigned From
– The Range of Pool (1-255):– SubMask– Gateway– Wins
38IP Addressing and Related Topics 38
CISNTWK-11Performance
Setting up a Simple DHCP Server (continued)Server (continued)
• Field values in the Dynamic Pool SettingsDNS– DNS
– Domain Name– Lease Time (60 – 31536000 sec)– Status
39IP Addressing and Related Topics 39
CISNTWK-11Performance
40IP Addressing and Related Topics 40
CISNTWK-11PerformanceTroubleshooting DHCP
• One good way to troubleshoot DHCPUse a protocol analyzer such as Ethereal– Use a protocol analyzer, such as Ethereal
• Analyzer – Can display the sequence of messages that occurs on p y q g
a network• Commands to release and/or renew an IP
addressaddress– ipconfig /release– ipconfig /renew
41IP Addressing and Related Topics 41
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42IP Addressing and Related Topics 42
CISNTWK-11PerformanceSummary
• DHCP Provides a way for computers to obtain usable unique – Provides a way for computers to obtain usable, unique IP addresses and necessary TCP/IP configurations
• From the administrative side– DHCP makes it easy to define and manage pools of IP
addresses• DHCP’s origins DHCP s origins
– Lie in an earlier TCP/IP Application layer protocol, called BOOTP
43IP Addressing and Related Topics 43
CISNTWK-11PerformanceSummary (continued)
• DHCP – Supports manual or static address allocation– Supports manual or static address allocation
• When a DHCP client starts– It begins the process of DHCP Discovery, during which
the client receives an IP address and leasethe client receives an IP address and lease• DHCP
– Supports a variety of message types and options– Only MessageType 53 (DHCP Message) is mandatory
for any given DHCP message
44IP Addressing and Related Topics 44
CISNTWK-11PerformanceSummary (continued)
• Because DHCP can ferry a surprisingly large range of configuration informationrange of configuration information– The protocol makes use of several message options
• A protocol analyzer is especially effective when p y p ydiagnosing DHCP difficulties
45IP Addressing and Related Topics 45