DHCP
…and how it works
1
DHCP
by steffen diehl
Overview
• Introduction
• Basics
• Case - We play DHCP Server
•
•
•
•
Problems
Tasks
Solutions
Apply what you've learned
• Conclusion
2
DHCP
Overview
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• Defined by RFC 1541 (superseded by RFC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)
3
DHCP
in general
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
4
DHCP
techiques
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!!
5
DHCP
problem
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
6
DHCP
server tasks I
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
7
DHCP
parts of a DHCP Server
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
8
DHCP
client conditions
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
9
DHCP
steps in becoming part of the network I
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
10
DHCP
steps in becoming part of the network II
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
11
DHCP
types of assigning addresses I
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
12
DHCP
types of assigning addresses II
end
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
13
DHCP
lease duration
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?
14
DHCP
problems
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!!!!
15
DHCP
server tasks II
Case - in practice
• Servers:
• WEB-Server
• Access Point
192.168.1.1
192.168.1.2
• Lecturers via Ethernet:
• Range from .100 to .102
• Students via WLAN:
• Range from .200 to .230
16
DHCP
find a solution!!!
back
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
17
DHCP
negative 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
18
DHCP
positive aspects
Conclusion
Thank you for your attention
…any questions??
19
DHCP
the end
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
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DHCP