And how it works
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DHCP - PowerPoint PPT Presentation

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…and how it works. DHCP. Overview. Introduction Basics Case - We play DHCP Server Problems Tasks Solutions Apply what you've learned Conclusion. Basics. D ynamic H ost C onfiguration P rotocol Standard protocol Defined by RFC 1541 (superseded by RFC 2131) Created by the

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And how it works l.jpg

…and how it works


by steffen diehl

Overview l.jpg

  • Introduction

  • Basics

  • Case - We play DHCP Server

    • Problems

    • Tasks

    • Solutions

    • Apply what you've learned

  • Conclusion


Basics l.jpg

  • Dynamic Host Configuration Protocol

  • Standard protocol

  • Defined by RFC 1541 (superseded by RFC 2131)

  • Created by the

    Host Configuration Working Group

    of the

    Internet Engineering Task Force (IETF)

in general

Basics4 l.jpg

  • Runs over UDP

  • Utilizing ports:

    • 67 – connections to server

    • 68 – connections to client

  • Extension of BOOTP (protocol used for simple interaction)

  • Uses client–server model


Slide5 l.jpg

  • Got a network

  • Want to add new clients

  • DHCP-Server doesn’t work

  • How do clients get their:

    • IP Address -or-

    • network configuration parameters (Subnet Mask, Default Gateway, DNS and WINS)

  • Need replacement!!


Case theory l.jpg
Case - Theory

1st implementation goal of a DHCP Server

  • support of dynamic allocation of Network Addresses

    • chooses and allocates an IP address

    • gives network configuration parameters to the client

server tasks I

Case theory7 l.jpg
Case - Theory

  • Need 2 databases

    • Address Pool is the database which holds IP addresses and other network configuration parameters

    • Binding database keeps mapping between an Ethernet address and Address Pool entries

parts of a DHCP Server

Case theory8 l.jpg
Case - Theory

  • All interactions are initiated by a client

  • Server only replies

  • “obtain an IP address automatically” in NW options

  • configuring the host to the network is done by a simple handshake

client conditions

Case in practice l.jpg
Case – in practice

  • Client broadcasts DHCPDISCOVER

  • One or more servers return DHCPOFFER with available Address and NW Information

  • Clients chooses one offer that it likes best

  • broadcasts DHCPREQUEST to identify chosen Server/lease

  • DHCPREQUEST also to renew lease

steps in becoming part of the network I

Case in practice10 l.jpg
Case - in practice

  • Server sends


      • Lease is finalized

      • Client starts using IP


      • Client resumes from DHCPDISCOVER point

  • If client doesn’t want IP - DHCPDECLINE is sent

  • DHCPRELEASE gives IP back into pool

steps in becoming part of the network II

Case theory11 l.jpg
Case - Theory

  • Static/Manual allocation

    • Server allocates IP chosen by the Admin

    • Server configuration includes

      • IP-Address -and-

      • MAC-Address

        for every client

    • DHCP only used to convey assigned address to client

types of assigning addresses I

Case theory12 l.jpg
Case - Theory

  • Automatic allocation

    • IP address is permanently associated with a MAC address

    • till administrator intervenes the infinite lease

  • Dynamic allocation

    • Like Automatic allocation

    • Except:

      • server tracks leases

      • give IP addresses whose lease has expired to other DHCP clients

types of assigning addresses II


Case theory13 l.jpg
Case - Theory

  • Client is responsible to renew/release IP

  • Lease timestamps:

    • Total lease duration

    • T1 (0.5 * duration_of_lease)

      • client enters the RENEWING state

      • contacts the server that originally issued network address

    • T2 (0.875 * duration_of_lease)

      • client enters the REBINDING state

      • attempts to contact any server

lease duration

Case theory14 l.jpg
Case - Theory

  • Lease duration

    • Client holds IP when not connected

    • Clients retire

    • Servers/Databases should have constant IP´s

  • Analyze the NW

    • Often changing clients?

    • Servers/databases?

    • Enough IP Addresses available?


Case theory15 l.jpg
Case - Theory

By using the fitting Methods…

implementation goals 2-4 of a DHCP Server…

  • server should be able to recycle the IP address when the lease period is expired

  • A network administrator should not configure each client/user interaction of each client should not be required.

  • support of static allocation and an infinite lease

    …can also be realized!!!!

server tasks II

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Case - in practice

  • Servers:

    • WEB-Server

    • Access Point

  • Lecturers via Ethernet:

    • Range from .100 to .102

  • Students via WLAN:

    • Range from .200 to .230

find a solution!!!


Conclusion l.jpg

  • Insecurity of UDP

  • Broadcast of messages

  • Risk of wrong configuration causing

    • High traffic

    • Waste of IP Addresses

    • A lot more work than expected

  • Detailed and precise analysis before implementation needed

negative aspects

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  • works well if you have to manage a lot of mobile users:

    • people with laptops working in and out of the office

    • hosts coming and going with a great amount of frequency

  • perfect when Network parameters have changed

    • only one point you have to work at

  • sharing a limited pool of IP addresses

  • Mixture of allocation types can be used

positive aspects

Conclusion19 l.jpg

Thank you for your attention

…any questions??

the end

Refrences l.jpg

  • RFC 2131, Dynamic Host Configuration Protocol

  • RFC 2322, Management of IP numbers by peg-dhcp

  •, Internet Engineering Task Force

  •, Microsoft Knowledge Base Article - 169289