1 DHCP by steffen diehl

DHCP

…and how it works

2 DHCP Overview

Overview

• Introduction• Basics• Case - We play DHCP Server

• Problems • Tasks• Solutions• Apply what you've learned

• Conclusion

3 DHCP in general

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• Dynamic Host Configuration Protocol• Standard protocol• Defined by RFC 1541 (superseded by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 DHCP techiques

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

5 DHCP problem

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!!

6 DHCP server tasks I

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 DHCP parts of a DHCP Server

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 DHCP client conditions

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 DHCP steps in becoming part of the network I

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 DHCP steps in becoming part of the network II

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

11 DHCP types of assigning addresses I

Case - Theory

• Static/Manual allocation • Server allocates IP chosen by the Admin• Server configuration includes

• IP-Address -and-• MAC-Addressfor every client

• DHCP only used to convey assigned address to client

12 DHCP types of assigning addresses II

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 clientsend

13 DHCP lease duration

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 DHCP problems

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?

15 DHCP server tasks II

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 DHCP find a solution!!!

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 DHCP negative aspects

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 DHCP positive aspects

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 DHCP the end

Conclusion

Thank you for your attention

…any questions??

20 DHCP

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