1 CMPT 471 Networking II DHCP © Janice Regan, 2006-2013.

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Presentation transcript:

1 CMPT 471 Networking II DHCP © Janice Regan,

2 DHCP software  A commonly used DHCP package used on linux networks is the ISC (Internet software consortium) DHCP package  To help understand how this package is used read the linux documentation for The DHCP relay agent, dhcrelay (posted) The DHCP client, dhclient The DHCP server, dhcpd

© Janice Regan, DHCP relay agents  Two types  One runs on IP routers (some dedicated routers include embedded DHCP relay agents for example the Cisco router in the lab)  One runs on general purpose hosts (may be operating as routers)  Relay agents can replace a DHCP server on some segments of a multisegment network installation  Each segment of a network running DHCP must contain either (or both)  At least 1 DHCP server  At least 1 DHCP relay agent (usually associated with the routers attached to the segment)

© Janice Regan, DHCP relay agents  Relay agents forward DHCP messages from clients on a network segment with no DHCP server to a DHCP server on another network segment  Remember that the original DHCPDISCOVER message is broadcast on the network segment containing the source host. It must reach either a DHCP server or a DHCP relay agent.  If it reaches a relay agent then it will be forwarded to a DHCP server on another network segment or to another relay agent.  To forward a DCHP message the relay agent needs  the information in the DHCP message  One or more IP address that all DHCP messages it receives are forwarded to. (>1 if more than 1 DHCP server)

© Janice Regan, DHCP Relay agent operation  It increments the hop count in the DHCP message  Inserts the IP address of the relay agent into the router IP address field if the router (gateway) IP address is  The router IP address will be set to when a DHCPDISCOVER message is created.  The router IP address will thus record the IP address of the first relay agent on the path to the DHCP server  Appends any relay agent options  The end option will be moved to follow any appended options  Sends the packet the DHCP server or next relay agent

© Janice Regan, DHCP servers and Relay Agents  The server will receive and process the request, adding the configuration information to the message  The server will unicast the response message (encapsulated in an IP datagram) to the address extracted from the router address field. The response does not passes through only one relay agent, the first relay agent encountered by the request.  When the packet reaches the host running the relay agent it will be sent to the relay agent

© Janice Regan, DHCP Relay Agent operation  Relay agent receives the message from the DHCP server  Relay agent checks the router address field to determine the network interface over which the message should be sent  The router address will indicate which interface the original message arrived through  Relay agent checks the broadcast flag to see if the packet should be broadcast or unicast the host that sent the request  Relay agent sends the message to the host that sent the request

© Janice Regan, Avoiding DHCP message collisions  If a network is shut down (like in a power failure) and all hosts come up at the same time the server may not be able to deal with all the resulting discover messages  Each client is required to delay its initial DHCP message by a random time between 0 and 10 seconds  The delay time between retransmissions is randomized by -1 to +1 seconds

© Janice Regan, DHCP configuration  Host or router running the DHCP server will have a configuration file at a default location of /etc/dhcpd.conf  Defines the operation of DHCP in the network  Specifies basic network architecture  Specifies lease times and other necessary information  For details of configuration refer to reference  The DHCP Handbook, Droms and Lemon

© Janice Regan, Subnet declarations  Defines scope: range of addresses in network # Subnet 1 subnet netmask { # subnet declarations } # Subnet 2 subnet netmask { # subnet declarations }

© Janice Regan, Subnet Address allocation  Define range of addresses available for allocation # subnet 1 subnet netmask { range # reserved for DHCP server # reserved for DNS server # reserved for router }

© Janice Regan, Router option  Define range of addresses available for allocation # subnet 1 subnet netmask { range # reserved for DHCP server # reserved for DNS server # reserved for router option routers }

© Janice Regan, DNS server option  Define range of addresses available for allocation # Subnet 1 subnet netmask { range # reserved for DHCP server # reserved for DNS server # reserved for router option domain-name-servers }

© Janice Regan, Subnet mask option  Define range of addresses available for allocation # Subnet 1 subnet netmask { range # reserved for DHCP server # reserved for DNS server # reserved for router option subnet-mask }

© Janice Regan, Specifying lease times # Subnet 1 subnet netmask { range #default lease 30 days, maximum 45 days default-lease-time maximum-lease-time minimum-lease-time 600 }

© Janice Regan, Lease times  Can be specified by client as an option in DHCP messages  If specified time is less the min-lease-time, min-lease-time will be used  If specified time is greater the max-lease- time, max-lease-time will be used

© Janice Regan, Static Allocation using DHCP  Can statically allocate addresses for particular hosts  Microsoft DHCP uses the term reservation rather than static allocation  Can use all static addresses  Can mix static and dynamic addresses  For example static addresses for fixed wired clients, dynamic addresses for mobile client

© Janice Regan, Specifying static leases # Subnet 1 Subnet netmask { range default-lease-time } host dhcpserver { fixed address hardware ethernet 00:20:78:10:c8:12 default-lease-time 1000 }

© Janice Regan, Host statements  Used for static ip addresses and enhanced security  Any request from a client that matches (same ethernet address) a host statement is considered to be from a “known” client.  Other requests are from “unknown” clients  The name in the host statement is used only to identify the host statement it is not the clients hostname  Can use host statements to make the dhcp server treat some addresses differently from other

© Janice Regan, Using pools  Can use host statements to make the dhcp server treat some addresses differently from other  Example: permit only known clients to obtain leases from the specified range of addresses. Provide a small range for any clients pool { deny unknown clients range } pool { allow unknown clients range }