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Transcript
Allocating IP Addressing by Using Dynamic Host Configuration Protocol Overview The Role of DHCP in the Network Infrastructure Adding and Authorizing the DHCP Server Service Configuring a DHCP Scope Configuring DHCP Reservations and Options Configuring a DHCP Relay Agent Dynamic Assignment of IP addresses Dynamic assignment of IP addresses is desirable for several reasons: IP addresses are assigned on-demand Avoid manual IP configuration Support mobility of laptops 3 Solutions for dynamic assignment of IP addresses Reverse Address Resolution Protocol (RARP) Works similar to ARP Broadcast a request for the IP address associated with a given MAC address RARP server responds with an IP address Only assigns IP address (not the default router and subnetmask) IP address (32 bit) ARP RARP Ethernet MAC address (48 bit) 4 BOOTP BOOTstrap Protocol (BOOTP) From 1985 Host can configure its IP parameters at boot time. 3 services. IP address assignment. Detection of the IP address for a serving machine. The name of a file to be loaded and executed by the client machine (boot file name) Not only assign IP address, but also default router, network mask, etc. Sent as UDP messages (UDP Port 67 (server) and 68 (host)) Use limited broadcast address (255.255.255.255): These addresses are never forwarded 5 DHCP Dynamic Host Configuration Protocol (DHCP) From 1993 An extension of BOOTP, very similar to DHCP Same port numbers as BOOTP Extensions: Supports temporary allocation (“leases”) of IP addresses DHCP client can acquire all IP configuration parameters needed to operate DHCP is the preferred mechanism for dynamic assignment of IP addresses DHCP can interoperate with BOOTP clients. 6 BOOTP Interaction (b) (a) Argon 00:a0:24:71:e4:44 BOOTP Server Argon 128.143.137.144 00:a0:24:71:e4:44 DHCP Server BOOTP Response: IP address: 128.143.137.144 Server IP address: 128.143.137.100 Boot file name: filename BOOTP Request 00:a0:24:71:e4:44 Sent to 255.255.255.255 (c) BOOTP can be used for downloading memory image for diskless workstations Assignment of IP addresses to hosts is static 7 DHCP Interaction (simplified) Argon 128.143.137.144 00:a0:24:71:e4:44 DHCP Server DHCP Response: IP address: 128.143.137.144 Default gateway: 128.143.137.1 Netmask: 255.255.0.0 8 BOOTP/DHCP Message Format OpCode Hardware Type Number of Seconds Hardware Address Hop Count Length Unused (in BOOTP) Flags (in DHCP) Transaction ID Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server host name (64 bytes) Boot file name (128 bytes) Options (There are >100 different options) 9 BOOTP/DHCP OpCode: 1 (Request), 2(Reply) Note: DHCP message type is sent in an option Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to boot Client IP address, Your IP address, server IP address, Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank 10 Lesson: Adding and Authorizing the DHCP Server Service Why Use DHCP? What Is Automatic Private IP Addressing? How DHCP Allocates IP Addresses How the DHCP Lease Generation Process Works How the DHCP Lease Renewal Process Works How a DHCP Server Service Is Authorized Practice: Adding and Authorizing a DHCP Server Service Why Use DHCP? DHCP reduces the complexity and amount of administrative work by using automatic TCP/IP configuration Manual TCP/IP Configuration Automatic TCP/IP Configuration IP addresses are entered manually IP addresses are supplied automatically IP address could be entered incorrectly Correct configuration information is ensured Communication and network issues can result Client configuration is updated automatically Frequent computer moves increase administrative effort A common source of network problems is eliminated What Is Automatic Private IP Addressing? APIPA automatically self-configures addresses when there is no DHCP server available Advantages Disadvantages Serves as a DHCP server failover mechanism for small networks Forces assignment of addresses typically not used Automatically assigns an IP address in a specific range Conceals possible connectivity problems Does not work outside 169.254.x.x subnet Is not routable How DHCP Allocates IP Addresses DHCP Client2: IP configuration from DHCP server Non-DHCP Client: Static IP configuration Lease Renewal Lease Generation DHCP Server DHCP Client1: IP configuration from DHCP server DHCP Database IP Address1: Leased to DHCP Client1 IP Address2: Leased to DHCP Client2 IP Address3: Available to be leased How the DHCP Lease Generation Process Works DHCP Server2 DHCP Server1 DHCP Client 1 DHCP client broadcasts a DHCPDISCOVER packet 2 DHCP servers broadcast a DHCPOFFER packet 3 DHCP client broadcasts a DHCPREQUEST packet 4 DHCP Server1 broadcasts a DHCPACK packet How the DHCP Lease Renewal Process Works DHCP Server2 DHCP Server1 DHCP Client 50% 100% of lease 87.5% 50% of of oflease lease lease duration duration has has expired expired DHCPfails Client sends DHCPREQUEST packet client packet If 1 the client tosends renewaait’s itsDHCPREQUEST lease, lease, after after 50% 87.5% of of thethe lease lease has duration then expired, has expired, the DHCP then lease thegeneration DHCP leaseprocess renewalstarts process overwill Server1 sends a DHCPACK begin again with againa after DHCP87.5% client ofbroadcasting the lease duration apacket DHCPDISCOVER has expired 2 DHCP How a DHCP Server Service Is Authorized Domain Controller If DHCP Server1 findswith its IP DHCP Server1 checks the addresscontroller on the list, service domain to the obtain a list starts and supports DHCP clients of authorized DHCP servers DHCP Server1 Authorized Active Directory Services DHCP requests DHCP Server2 DHCP Client Unauthorized DHCPServer2 Server2 does checks If DHCP notwith findthe its IP DHCP client receives IP address domain controller obtain a list of address on the list,tothe service does from authorized DHCP Server1 authorized DHCPDHCP servers not start and support clients Does not service DHCP requests DHCP 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 leased DHCP Server LAN A LAN B Scope A Scope B Scope Properties Network ID Lease duration Scope name Subnet mask Network IP address range Exclusion range What Is Network Monitor? Network Monitor: 2 3 Captures a sample of network traffic Uses filters to select specific packets Decodes the packets 1 4 Compiles network statistics What Is a DHCP Reservation? A reservation is a specific IP address, within a scope, that is permanently reserved for lease to a specific DHCP client Workstation 1 Subnet A File and Print Server Subnet B DHCP Server IP Address1: Leased to Workstation 1 IP Address2: Leased to Workstation 2 IP Address3: Reserved for File and Print Server Workstation 2 What Are DHCP Options? DHCP options are configuration parameters that a DHCP server assigns to clients DHCP Client DHCP Client IP Configuration Data Client’s IP address Client’s subnet mask DHCP options DHCP Server How DHCP-Server, Scope, and Reserved-Client Options Are Applied DHCP Server File and Print Server Windows 98 Scope A Scope B Router Windows XP Windows XP DHCP option applied at the reserved-client server scope levellevel How DHCP Class-Level Options Are Applied DHCP Server File and Print Server Windows 98 Router Router Scope A Scope B Windows XP Windows XP DHCP option applied at the class level Lesson: Configuring a DHCP Relay Agent What Is a DHCP Relay Agent? How a DHCP Relay Agent Works How a DHCP Relay Agent Uses Hop Count How a DHCP Relay Agent Uses Boot Threshold Practice: Configuring a DHCP Relay Agent What Is a DHCP Relay Agent? A DHCP relay agent is a computer or router that listens for DHCP/BOOTP broadcasts from DHCP clients and then relays those messages DHCP Relay Agent DHCP Server Unicast Broadcast Broadcast Subnet A Subnet B Routers (Non–RFC 1542 Compliant) Client Client Client Client How a DHCP Relay Agent Works DHCP Relay Agent Client1 1 2 3 4 5 6 7 8 Client2 DHCP Server Router Non-RFC 1542 Compliant (Non–RFC Compliant) Client3 Client1 broadcasts a DHCPDISCOVER packet Relay agent forwards the DHCPDISCOVER message to the DHCP server Server sends a DHCPOFFER message to the DHCP relay agent Relay agent broadcasts the DHCPOFFER packet Client1 broadcasts a DHCPREQUEST packet Relay agent forwards the DHCPREQUEST message to the DHCP server Server sends a DHCPACK message to the DHCP relay agent Relay agent broadcasts the DHCPACK packet How a DHCP Relay Agent Uses Hop Count The hop count threshold is the number of routers through which the packet can be transmitted before it is discarded DHCP Relay Agent 2 Hop Count = 2 DHCP Relay Agent 1 DHCP Server How a DHCP Relay Agent Uses Boot Threshold The boot threshold is the time the DHCP relay agent will wait for a DHCP server response before forwarding the request DHCP Server 2 Boot Threshold = 10 seconds DHCP Relay Agent Local DHCP Server DHCP Server 3