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Dynamic Host Configuration Protocol

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DHCP provides configuration parameters(rfc 1122) to internet hosts(TCP/IP) ... that key value is (IP-subnet-number, hardware-address) unless he client supplies ... – PowerPoint PPT presentation

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Title: Dynamic Host Configuration Protocol


1
Dynamic Host Configuration Protocol
  • Rishabh Parekh

2
Introduction
  • DHCP provides configuration parameters(rfc 1122)
    to internet hosts(TCP/IP)
  • It has the capability for automatic allocation of
    reusable network addresses
  • It is based on bootstrap protocol BOOTP(RFC 951)
  • DHCP participants can interoperate with BOOTP
    participants and BOOTP relay agents
  • It is based on client-server model where
    designated DHCP server hosts allocate network
    addresses and deliver configuration parameters to
    dynamically configured hosts
  • It is not designed to be used for configuring
    routers

3
Design Goals
  • Clients should not require any manual
    configuration
  • System administrators should be able to enforce
    local address allocation and configuration
    policies
  • DHCP should not require a server on each subnet
  • It must co-exist with statically configured
    clients and non-participating hosts
  • DHCP must interoperate with BOOTP relay agents
    and provide services to BOOTP clients
  • DHCP must guarantee to assign a unique network
    address which has not been allocated
  • Retain DHCP client configuration across DHCP
    client and server reboot

4
Protocol
  • Format of DHCP message
  • 0 1 2
    3
  • 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
    3 4 5 6 7 8 9 0 1
  • -----------------------
    ---------
  • op (1) htype (1) hlen (1)
    hops (1)
  • --------------------------------------------
    ----------------
  • xid (4)
  • ---------------------------------------------
    -----------------
  • secs (2)
    flags (2)
  • ---------------------------------------------
    -----------------
  • ciaddr (4)
  • ----------------------------------------------
    -----------------
  • yiaddr (4)
  • ----------------------------------------------
    -----------------
  • siaddr (4)
  • ----------------------------------------------
    -----------------
  • giaddr (4)
  • ----------------------------------------------
    -----------------


5
Parameter Repository
  • Persistent storage of network parameters for
    network clients
  • DHCP stores a key-value entry for each client
  • Key is a unique identifier that identifies a
    client within a subnet.
  • E.x. IP-subnet-number, hardware-address
  • IP-subnet-number, hostname
  • Value contains configuration parameters for the
    client
  • Protocol specifies that key value is
    (IP-subnet-number, hardware-address) unless he
    client supplies the client-identifier option.

6
Allocation of Network Addresses
  • Mechanisms of Address allocation
  • Automatic - permanent IP address allocated to the
    client
  • Manual - clients IP address is assigned by the
    network administrator. DHCP is just used to
    convey this address to the client.
  • Dynamic - DHCP assigns an IP address to a client
    for a limited amount of time (called lease) or
    until the client explicitly relinquishes the
    address.
  • Dynamic Allocation
  • it is useful for assigning an address to client
    that will be connected to network only
    temporarily
  • sharing a limited pool of IP addresses among a
    group of clients(like ISPs)
  • a client requests an address and the server
    guarantess that the addresses will not be
    reallocated for the period of the lease
  • The client may
  • extend its lease with subsequent requests
  • release the address back to the server when the
    lease expires
  • ask for permanent address by taking an infinite
    lease

7
Client-Server Interaction
  • DHCP uses UDP
  • DHCP messages from client to server are sent to
    the DHCP server port (67)
  • DHCP messages from server to client are sent to
    the DHCP client port (68)
  • DHCP Message Types
  • Message Use
  • ------- ---
  • DHCPDISCOVER - Client broadcast to locate
    available servers.
  • DHCPOFFER - Server to client in response
    to DHCPDISCOVER with
  • offer of configuration
    parameters.
  • DHCPREQUEST - Client message to servers
    either (a) requesting
  • offered parameters from one
    server and implicitly
  • declining offers from all
    others, (b) confirming
  • correctness of previously
    allocated address after,
  • e.g., system reboot, or (c)
    extending the lease on a
  • particular network address.

8
Allocating Network Address
  • Server Client Server
  • (not selected)
    (selected)
  • v v v

  • Begins initialization

  • _____________/\____________
  • /DHCPDISCOVER DHCPDISCOVER
    \

  • Determines
    Determines
  • configuration
    configuration

  • \ ____________/
  • \________ /DHCPOFFER
  • DHCPOFFER\ /
  • \
  • Collects replies
  • \
  • Selects configuration

9
Reusing allocated addresses
  • Server Client Server
  • v v
    v

  • Begins
  • initialization

  • /\
  • _________ __/ \__________
  • /DHCPREQU EST
    DHCPREQUEST\
  • /
    \

  • Locates
    Locates
  • configuration
    configuration

  • \
    /
  • \
    ___________/
  • \ / DHCPACK
  • \ _______ /

10
Conclusion
  • Topics not covered
  • When should clients use DHCP?
  • Specification of the DHCP protocol
  • Constructing and sending DHCP messages
  • DHCP server behavior
  • DHCP client behavior
  • Topics not understood
  • How can a client really accept a IP packet
    (containing DHCP configuration parameters) before
    its TCP/IP software is configured.
  • Related Work
  • BOOTP - transport mechanism for collection of
    configuration information
  • Network Information Protocol - distributed
    mechanism for dynamic IP address assignment
  • Resource Location Protocol - provides for
    location of higher resources
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