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DHCP Managed Configuration of TCPIP Hosts

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Title: DHCP Managed Configuration of TCPIP Hosts


1
DHCP Managed Configuration of TCP/IP Hosts
  • Richard Perlman
  • perl_at_lucent.com

2
Outline
  • DHCP purpose and goals
  • Background and history of DHCP
  • Case Study
  • Operational details
  • Using DHCP

3
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.

4
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

5
DHCP functional goals
  • Hosts can detect when they require a new IP
    address
  • Unavailability of DHCP server has minimal effect
    on operation of hosts

6
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

7
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

8
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

9
Design Goals
  • Provide a mechanism, not a policy
  • Provide same configuration - including IP
    address - to a host whenever possible

10
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

11
What DHCP doesnt 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

12
What DHCP doesnt do
  • Configure routers and other network equipment
  • Design network addressing plan
  • Determine other configuration parameters
  • Locate other servers

13
Outline
  • DHCP purpose and goals
  • Background and history of DHCP
  • Case Study
  • Operational details
  • Using DHCP

14
What is DHCP and where does it come from?
  • Internet Engineering Task Force (IETF)
  • Dynamic Host Configuration Working Group (DHC WG)
  • BOOTP

15
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)

16
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

17
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

18
DHCP Resources
  • Compilation of DHCP-related WWW links and other
    information
  • http//www.dhcp.org
  • DHCP FAQ (maintained by John Wobus)
  • dhcp-v4_at_bucknell.edu mailing list (admin requests
    to listserv_at_bucknell.edu)

19
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

20
Related work
  • RARP/DRARP
  • TFTP
  • ICMP
  • Router Discovery
  • Mobile IP
  • Wireless/cellular IP

21
Outline
  • DHCP purpose and goals
  • Background and history of DHCP
  • Case Study
  • Operational details
  • Using DHCP

22
Generic Startup, Inc. GSI
  • GSI is a mediumsized startup with about 200
    employees
  • Internal TCP/IP network intranet
  • Network Architect is responsible for network
    design, planning and operation

23
Intranet architecture
  • Intranet uses Ethernet throughout
  • 5 internal Ethernet segments
  • 4 segments for desktops
  • 1 segment for servers
  • Connected through single router

24
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

25
Intranet addressing
201.157.7.64
201.157.7.192
Router
201.157.7.128
201.157.7.96
201.157.7.32
26
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

27
GSI uses DHCP to
  • Configure new computers
  • Reconfigure relocated computers
  • Accommodate laptops
  • Renumber network

28
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

29
Newly installed computer
  • Newly installed computer locates DHCP server
  • Server consults address scheme rules
  • Picks an address
  • Determines other configuration parameters
  • Plug-and-play

30
Newly installed computer
201.157.7.198
Router
DHCP server
201.157.7.96
New computer
31
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

32
Relocated computer
201.157.7.98
201.157.7.64
201.157.7.198
Router
DHCP server
201.157.7.96
33
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

34
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?

35
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

36
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

37
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

38
Multiple IP networks on a subnet
201.157.7.64
201.157.7.192
Router
201.157.7.128
201.157.7.96 201.157.7.160
201.157.7.32
39
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

40
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

41
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

42
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

43
Renumbered GSI network
201.157.7.64
201.157.7.128
Router
201.157.7.32
202.5.77.64
202.5.77.128
201.157.7.98
44
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

45
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..,
    email to all users)

46
Outline
  • DHCP purpose and goals
  • Background and history of DHCP
  • Case Study
  • Operational details
  • Using DHCP

47
Server manages client configurations
  • Provide a variety of mechanisms for controlled
    configuration
  • Can override default parameters from Host
    Requirements

48
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

49
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

50
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

51
Motivation for leases
  • An IP internet may not always be completely
    operational there may not always be connectivity
    between any two hosts, so
  • Cant use distributed (client-based) assignment
    of addresses
  • Cant use address defense before server reuse
    of addresses

52
Motivation for leases
  • Leases guarantee an agreement as to when an
    address may be safely reused even if the server
    cant contact the client

53
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!)

54
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

55
Address allocation details
  • Clients check on address validity at reboot time
    (renumbering)
  • Clients can extend the lease on an address at
    startup time

56
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

57
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

58
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

59
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

60
Extending a lease
  • Client puts requested lease extension in REQUEST
    and sends to server
  • Server commits extension and returns ACK with
    parameters

61
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

62
Configuration with options
  • Network architect configures server to select and
    return options and values
  • Client can explicitly request specific options

63
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

64
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

65
Outline
  • DHCP purpose and goals
  • Background and history of DHCP
  • Case Study
  • Operational details
  • Using DHCP

66
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

67
Using multiple servers
  • All coordination must be managed by DHCP
    administrator
  • Distributed database
  • Off-line batch updates
  • Manually

68
Strategies for using multiple servers
  • Split address pool for each subnet among servers
  • Coordinate leases off-line
  • Reallocate addresses when needed

69
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

70
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

71
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

72
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

73
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

74
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

75
Limitations to DHCP
Opportunities for enhancement
  • Coordination among multiple servers
  • DHCP interaction with DNS
  • Security/authentication
  • New options
  • IPv6

76
Coordination among multiple servers
  • Becomes a distributed database problem
  • Several strategies have been proposed
  • Failover protocol now in development

77
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?

78
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

79
Security/Authentication
  • Authentication based on shared secret key, an
    authentication ticket and a message digest
  • Assures source of message is valid and message
    hasnt been tampered with en route
  • Schiller/Huitema/Droms/Arbaugh proposal in process

80
New options acceptance
  • New options must have nonoverlapping option
    codes
  • Codes handed out by Internet Assigned Numbers
    Authority (IANA)
  • New mechanism will approve each new option as a
    separate RFC (like TELNET)

81
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

82
IPv6
  • To accommodate sites that want centralized
    management of addresses, DHCP for IPv6 (DHCPv6)
    is being developed by the DHC WG.

83
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

84
Summary
  • 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
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