DHCP

1
DHCP

• and how it works

2
Overview

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

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• Defined by RFC 1541 (superseded by RFC 2131)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)

4
Basics

• Runs over UDP
• Utilizing ports
• 67 connections to server
• 68 connections to client
• Extension of BOOTP (protocol used for simple
  interaction)
• Uses clientserver model

5
Case

• Got a network
• Want to add new clients
• DHCP-Server doesnt work
• How do clients get their
• IP Address -or-
• network configuration parameters (Subnet Mask,
  Default Gateway, DNS and WINS)
• Need replacement!!

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

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

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

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

10
Case - in practice

• Server sends
• DHCPACK
• Lease is finalized
• Client starts using IP
• DHCPNAK
• Client resumes from DHCPDISCOVER point
• If client doesnt want IP - DHCPDECLINE is sent
• DHCPRELEASE gives IP back into pool

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

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

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

14
Case - Theory

• Lease duration
• Client holds IP when not connected
• Clients retire
• Servers/Databases should have constant IPs
• Analyze the NW
• Often changing clients?
• Servers/databases?
• Enough IP Addresses available?

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!!!!

16
Case - in practice

• Servers
• WEB-Server 192.168.1.1
• Access Point 192.168.1.2
• Lecturers via Ethernet
• Range from .100 to .102
• Students via WLAN
• Range from .200 to .230

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

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

19
Conclusion

• Thank you for your attention
• any questions??

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