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  1. …and how it works DHCP by steffen diehl

  2. Overview • Introduction • Basics • Case - We play DHCP Server • Problems • Tasks • Solutions • Apply what you've learned • Conclusion Overview

  3. Basics • Dynamic Host Configuration Protocol • Standard protocol • Defined by RFC 1541 (superseded by RFC 2131) • Created by the Host Configuration Working Group of the Internet Engineering Task Force (IETF) in general

  4. Basics • 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 techiques

  5. Case • 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!! problem

  6. 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

  7. 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

  8. 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

  9. 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

  10. Case - in practice • Server sends • DHCPACK • Lease is finalized • Client starts using IP • DHCPNAK • 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

  11. 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

  12. 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 end

  13. 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

  14. 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? problems

  15. 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

  16. 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!!! back

  17. Conclusion • 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

  18. Conclusion • 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

  19. Conclusion Thank you for your attention …any questions?? the end

  20. Refrences • RFC 2131, Dynamic Host Configuration Protocol • RFC 2322, Management of IP numbers by peg-dhcp • www.ietf.org, Internet Engineering Task Force • www.microsoft.com, Microsoft Knowledge Base Article - 169289