1. Using DHCP (Dynamic Host Configuration Protocol ), Why ? DHCP protocol is used to assign IP addresses to hosts or workstations on the network. Usually a DHCP server on the network performs this function. Basically it "leases" out address for specific times to the various hosts. If a host does not use a given address for some period of time, that IP address can then be assigned to another machine by the DHCP server. When assignments are made or changed, the DHCP server must update the information in the DNS server.

2. DHCP Lease Stages 1.Lease Request - The client sends a broadcast requesting an IP address 2.Lease Offer - The server sends the above information and marks the offered address as unavailable. The message sent is a DHCPOFFER broadcast message. 3.Lease Acceptance - The first offer received by the client is accepted. The acceptance is sent from the client as a broadcast (DHCPREQUEST message) including the IP address of the DNS server that sent the accepted offer. Other DHCP servers retract their offers and mark the offered address as available and the accepted address as unavailable. 4.Server lease acknowledgement - The server sends a DHCPACK or a DHCPNACK if an unavailable address was requested.

3. DHCP Server

4. Client Reservation Client Reservation is used to be sure a computer gets the same IP address all the time. Therefore since DHCP IP address assignments use MAC addresses to control assignments, the following are required for client reservation: MAC (hardware) address IP address

5. DHCP Managed Configuration of TCP/IP Hosts

6. Outline DHCP purpose and goals Background and history of DHCP Case Study Operational details Using DHCP

7. Purpose of DHCP From RFC2131: The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host and a mechanism for allocation of network addresses to hosts.

8. DHCP functional goals A host without a valid IP address locates and communicates with a DHCP server A DHCP server passes configuration parameters, including an IP address, to the host The DHCP server may dynamically allocate addresses to hosts and reuse addresses

9. DHCP functional goals Hosts can detect when they require a new IP address Unavailability of DHCP server has minimal effect on operation of hosts

10. What does DHCP do? Provides protocol stack, application and other configuration parameters to hosts Eliminates need for individual, manual configuration for hosts Includes administrative controls for network administrators

11. What does DHCP do? Backward compatible packet format for BOOTP interoperation (RFC 1542) Can coexist with hosts that have pre- assigned IP addresses and hosts that do not participate in DHCP

12. Design Goals Eliminate manual configuration of hosts Prevent use of any IP address by more than one host Should not require a server on every subnet Allow for multiple servers

13. Design Goals Provide a mechanism, not a policy Provide same configuration - including IP address - to a host whenever possible

14. What can you do with DHCP Plug-and-play Move desktop PCs between offices Renumber Other restructuring - change subnet masks Mobile IP - laptops Moving equipment - cartable

15. What DHCP doesn’t do Support multiple addresses per interface Inform running host that parameters have changed Propagate new addresses to DNS Support inter-server communication Provide authenticated message delivery

16. What DHCP doesn’t do Configure routers and other network equipment Design network addressing plan Determine other configuration parameters Locate other servers

17. Outline DHCP purpose and goals Background and history of DHCP Case Study Operational details Using DHCP

18. What is DHCP and where does it come from? Internet Engineering Task Force (IETF) Dynamic Host Configuration Working Group (DHC WG) BOOTP

19. IETF standards Formal process for development, review and acceptance of TCP/IP protocol suite standards Initial specifications published as Internet Drafts (I-Ds) Accepted specifications published as Request for Comments (RFCs)

20. Protocol status DHCP has been accepted as a Draft Standard; the specifications are published in: –RFC 2131: Dynamic Host Configuration Protocol –RFC 2132: DHCP Options and BOOTP Vendor Extensions Several additional options are in development

21. Implementation status DHCP is an open standard, with freely available specifications Can be (and has been) implemented entirely from the specification Commercial implementations are widely available Non-commerical implementations are also available

22. DHCP Resources Compilation of DHCP-related WWW links and other information: http://www.dhcp.org DHCP FAQ (maintained by John Wobus) dhcp-v4@bucknell.edu mailing list (admin requests to listserv@bucknell.edu )

23. DHCP Resources IETF information can be retrieved from: http://www.ietf.cnri.reston.va.us I-Ds and RFCs can also be retrieved from: http://www.rfc-editor.org

24. Related work RARP/DRARP TFTP ICMP Router Discovery Mobile IP Wireless/cellular IP

25. Outline DHCP purpose and goals Background and history of DHCP Case Study Operational details Using DHCP

26. Generic Startup, Inc. – GSI GSI is a medium–sized startup with about 200 employees Internal TCP/IP network – “intranet” Network Architect is responsible for network design, planning and operation

27. Intranet architecture Intranet uses Ethernet throughout 5 internal Ethernet segments –4 segments for desktops –1 segment for servers Connected through single router

28. TCP/IP addressing Network architect has obtained Class C network address 201.155.7.0 for GSI Subnetted for segments –/27 subnet mask –8 possible subnets, 32 hosts per subnet

29. 201.157.7.32 Intranet addressing 201.157.7.64 201.157.7.128 201.157.7.96 201.157.7.192 Router

30. DHCP on the GSI intranet Network architect plans addressing scheme and locations of servers DHCP server attached to 201.157.7.192 subnet Desktop clients contact server at startup for IP address and configuration parameters

31. GSI uses DHCP to… Configure new computers Reconfigure relocated computers Accommodate laptops Renumber network

32. Planning for DHCP Preparation for DHCP requires careful planning IP addressing strategy –Consider current needs –Allow for growth Network architect configures rules for addressing strategy into DHCP server

33. Newly installed computer Newly installed computer locates DHCP server Server consults address scheme rules –Picks an address –Determines other configuration parameters “Plug-and-play”

34. Newly installed computer 201.157.7.96 201.157.7.198 Router New computer DHCP server

35. Relocated computer Computer retains address When restarted, computer checks with server to confirm address If address OK, computer retains old address If computer attached to different subnet, obtains new address

36. Relocated computer 201.157.7.64 201.157.7.96 201.157.7.198 Router 201.157.7.98 DHCP server

37. Using DHCP with legacy equipment DHCP server not required to make every address on a subnet available for allocation DHCP server not required to answer every incoming request Network architect can configure server to reserve (not allocate) addresses

38. Growth – new computers on a subnet So … GSI grows and hires new employees Each gets a new computer; new computers are allocated addresses from DHCP pool Suppose addresses in a subnet are all allocated?

39. DHCP and new computers DHCP server will hand out all available addresses Limited number of addresses can be shared (if all computers not on simultaneously) Eventually, network architect will have to allocate more addresses

40. Reusing addresses Server can reuse abandoned addresses –Address initially allocated for fixed time called a lease –Client can extend lease If lease expires, server can reallocate Reallocation only when necessary (e.g., LRU) is a good idea…

41. Growth – multiple IP networks on a subnet /27 subnet accommodates only 30 computers Suppose application development group grows to 40? Add second IP subnet to existing Ethernet segment

42. Multiple IP networks on a subnet 201.157.7.32 201.157.7.64 201.157.7.128 201.157.7.96 201.157.7.160 201.157.7.192 Router

43. Reconfiguring the server for multiple networks Server configuration file defines multiple subnets and address pools on one physical segment Server chooses address from pools for the segment Server checks DHCP client address against all subnets on the segment

44. Growth – changing subnet masks In some cases, subnet growth can be managed with a change to the subnet mask –201.157.7.128/27 and 201.157.7.160/27 can be combined into 201.157.7.128/26 –Network infrastructure must accommodate VLSMs Must change subnet masks on attached clients

45. Passing new subnet masks to clients At next reboot, DHCP client will contact server Server returns new subnet mask with acknowledgment Client records and uses new mask

46. Growth – renumbering Eventually, GIS network architect obtains second class C address: 202.5.77.0 Subnet numbers are reallocated among network segments Many computers now on “wrong” subnet

47. Renumbered GSI network 202.5.77.128 201.157.7.64 201.157.7.32 202.5.77.64 201.157.7.128 Router 201.157.7.98

48. Using DHCP for renumbering Set up plan for renumbering –New network architecture –Network addresses, server addresses –Timing of cutovers Force DHCP clients to contact server for notification about new address –Set short leases –Require all clients be rebooted

49. Using DHCP for renumbering Rebooting, although not elegant, probably most reliable Schedule subnet cutover for overnight or weekend, force reboot through “alternate protocol” (e.g.., e–mail to all users)

50. Outline DHCP purpose and goals Background and history of DHCP Case Study Operational details Using DHCP

51. Server manages client configurations Provide a variety of mechanisms for controlled configuration Can override default parameters from Host Requirements

52. Address allocation Static (BOOTP): client must be pre- configured into database Automatic: server can allocate new address to client Dynamic: server can allocate and reuse addresses

53. Leases Dynamic addresses are allocated for a period of time known as the lease Client is allowed to use the address until the lease expires

54. Leases Client MUST NOT use the address after the lease expires, even if there are active connections using the address Server MUST NOT reuse the address before the lease expires

55. Motivation for leases An IP internet may not always be completely operational; there may not always be connectivity between any two hosts, so: –Can’t use distributed (client-based) assignment of addresses –Can’t use “address defense” before server reuse of addresses

56. Motivation for leases Leases guarantee an agreement as to when an address may be safely reused even if the server can’t contact the client

57. Address reuse Server MAY choose to reuse an address by reassigning it to a different client after the lease has expired Server can check using ICMP echo to see if the address is still in use (but no response is not a definitive answer!)

58. Address reuse Allows address sharing –From old computers replaced by new ones –Among a pool of computers not always using TCP/IP –For transient hosts like laptops

59. Address allocation details Clients check on address validity at reboot time (renumbering) Clients can extend the lease on an address at startup time

60. Address allocation details Clients can extend the lease on an address as expiration time approaches (without closing and restarting existing connections) Clients with addresses that have been configured manually can use DHCP to obtain other configuration parameters

61. Four ways a client uses DHCP INIT - acquire an IP address and configuration information INIT-REBOOT - confirm validity of previously acquired address and configuration RENEWING - extend a lease from the original server REBINDING - extend a lease from any server

62. Obtaining an initial address Client broadcasts DISCOVER to locate servers Server chooses address and replies Client selects a server and sends REQUEST for address Server commits allocation and returns ACK

63. Rebooting client Client puts address in REQUEST and broadcasts Server checks validity and returns ACK with parameters If client address is invalid – e.g., client is attached to a new network – server replies with NAK and client restarts

64. Extending a lease Client puts requested lease extension in REQUEST and sends to server Server commits extension and returns ACK with parameters

65. DHCP options Options carry additional configuration information to client –DHCP message type –Subnet mask, default routers, DNS server –Many others … Carried as fields in DHCP message

66. Configuration with options Network architect configures server to select and return options and values Client can explicitly request specific options

67. Relay agents Using hardware and IP broadcast still limits DHCP message from client to single physical network Relay agent, on same subnet as client, forwards DHCP messages between clients and servers

68. Relay agents Relay agent and server exchange messages using unicast UDP –Servers can be located anywhere on intranet –Servers can be centrally located for ease of administration Very simple in function, implementation Usually, but not necessarily, located in routers

69. Outline DHCP purpose and goals Background and history of DHCP Case Study Operational details Using DHCP

70. Using multiple servers Clients must be implemented for multiple servers; e.g., receiving multiple OFFER messages Using multiple servers can provide increased reliability through redundancy

71. Using multiple servers All coordination must be managed by DHCP administrator –Distributed database –Off-line batch updates –Manually

72. Strategies for using multiple servers Split address pool for each subnet among servers Coordinate leases off-line Reallocate addresses when needed

73. Lease times and strategies Choice of lease times made by DHCP administrator Long lease times decrease traffic and server load, short lease times increase flexibility

74. Lease times and strategies Should choose lease time allow for server unavailability –Allows clients to use old addresses –For example, long enough to span weekends Can assign different leases to desktop computers, cartable systems and laptops

75. Changing other configuration parameters Other configuration parameters such as print servers may change Reconfigure DHCP server with new parameters At next reconfirmation, clients will get new addresses

76. Moving a client to a new location User may get moved to a new location on a different subnet User may arrange to move computer system without contacting network administrator DHCP will allocate address for new location

77. Moving a client to a new location What about old lease? –New server can notify network administrator about address allocation –Client can issue RELEASE before moving from old location Or, might be appropriate to leave old lease in place…

78. Replacing a system User may get new computer on desktop Network administrator wants to allocate same IP address to the new computer – but, new computer will have different hardware address Use client id as system identifier and transfer to new system

79. Limitations to DHCP Coordination among multiple servers DHCP interaction with DNS Security/authentication New options IPv6 Opportunities for enhancement

80. Coordination among multiple servers Becomes a distributed database problem Several strategies have been proposed “Failover protocol” now in development

81. Dynamic DNS When client is allocated a new address, DNS records need to be updated –A record: Name to IP address –PTR record: IP address to name DHCP to be extended to allow coordination between client and server –Which does updates? –Error conditions?

82. Security/Authentication Unauthorized – either intentional or accidental – server can cause denial of service problems Some sites may want to limit IP address allocation to authorized client

83. Security/Authentication Authentication based on shared secret key, an authentication ticket and a message digest Assures source of message is valid and message hasn’t been tampered with en route Schiller/Huitema/Droms/Arbaugh proposal in process

84. New options acceptance New options must have non–overlapping option codes Codes handed out by Internet Assigned Numbers Authority (IANA) New mechanism will approve each new option as a separate RFC (like TELNET)

85. IPv6 IP Version 6 (aka IPv6 or IPng) is a new internet protocol to replace IP Includes new features for host configuration: –Router advertisement –Autoconfiguration –Link-local addresses

86. IPv6 To accommodate sites that want centralized management of addresses, DHCP for IPv6 (DHCPv6) is being developed by the DHC WG.

87. Summary DHCP works today as a tool for automatic configuration of TCP/IP hosts It is an open Internet standard and interoperable client implementations are widely available Provides automation for routine configuration tasks, once network architect has configured network and addressing plan Ongoing work will extend DHCP with authentication, DHCP-DNS interaction and inter-server communication