DHCP
Dynamic Host Configuration Protocol
Agenda
• Introduction
• BOOTP Packet Format
• DHCP
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Introduction – RARP is limited
• RARP only returns IP address
• Does not cross routers
• Difficult to program
 Need for mask
 Need for default gateway
 Need for DNS server
 Need for ...
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BOOTP Packet Format
IP datagram
UDP datagram
IP
Header
20 bytes
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UDP
Header
BOOTP request/reply
8 bytes
300 bytes
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BOOTP Header
Opcode
(1=request, 2=reply)
Hardware type
1=Ethernet
Hardware
Address length
Hop count
Transaction ID
Number of seconds
(Unused)
300 bytes
Client IP address
Your IP address
Server IP address
Gateway IP address
Client hardware address
Server hostname
Boot filename
Vendor specific information
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UDP Port numbers
• Client uses port 68
• Server uses port 67
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DHCP – Dynamic host configuration protocol
• Extension of BOOTP
• Provides TCP/IP configuration parameters
• RFC 2131
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DHCP – DHCP Discover
The Client is looking for available DHCP servers
1
Client
DHCP SERVER
DHCP
DISCOVER
Broadcast
Source IP Address = 0.0.0.0
Destination IP Address = 255.255.255.255
UDP port 67
Client IP Address = 0.0.0.0
Hardware Address = 08007. . .
...
...
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DHCP – DHCP Offer
The servers response to the clients DHCP OFFER
Client
2
DHCP
OFFER
Broadcast
DHCP SERVER
Destination IP Address = 255.255.255.255
UDP Port 68
Your IP Address = 131.107.7.29
Hardware Address = 08007. . .
Subnet mask
Default gateway
Lease time
Renewal time
DNS server
WINS server
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DHCP – DHCP REQUEST
Intends to accept the parameters
3
Client
DHCP
REQUEST
Broadcast
DHCP SERVER
Source IP Address = 0.0.0.0
Destination IP Address = 255.255.255.255
UDP port 67
Client IP Address = 131.107.7.29
Server IP Address = 131.107.7.1
Hardware Address = 08007. . .
Subnet mask
Default gateway
Lease time
Renewal time
DNS server
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T
Leuven
–
Information
department
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WINS
server
10
DHCP – DHCP ACK
The servers confirms the DHCP REQUEST
+ updates his database
4
Client
4
DHCP
ACK
Broadcast
DHCP Table
131.107.7.7 08004. . .
131.107.7.29 08007. . .
DHCP SERVER
UDP Port 68
IP Address = 131.107.7.29
Hardware Address = 08007. . .
Subnet mask
Default gateway
Lease time
Renewal time
DNS server
WINS server
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DHCP – Gratuitous ARP
Client checks for unique IP address
5
Client
ARP
REQUEST
Broadcast
IP Address = 131.107.7.29
Hardware Address = 08007. . .
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DHCP – Renewing the Lease
After T1 (= 0,5 x LT) the client will renew the lease
By issuing a DHCP REQUEST
Client
DHCP
REQUEST
DHCP SERVER
Source IP Address = 131.107.7.29
Destination IP Address = 255.255.255.255
UDP port 67
Client IP Address = 131.107.7.29
Server IP Address = 131.107.7.1
Hardware Address = 08007. . .
Subnet mask
Default gateway
Lease time
Renewal time
DNS server
WINS server
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DHCP – Renewing the Lease after REBOOT
After REBOOT the client will renew the lease
By issuing a DHCP REQUEST
Client
DHCP
REQUEST
Broadcast
DHCP SERVER
Source IP Address = 0.0.0.0
Destination IP Address = 255.255.255.255
UDP port 67
Client IP Address = 131.107.7.29
Server IP Address = 131.107.7.1
Hardware Address = 08007. . .
Subnet mask
Default gateway
Lease time
Renewal time
DNS server
WINS server
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DHCP on a routed network

Broadcast does not cross routers !
IP helper address = 10.1.1.1
DHCP
DISCOVER
Broadcast
1
A
BOOTP/DHCP
RELAY AGENT
DHCP
DISCOVER
unicast
10.2.2.1
Gateway IP address = 10.2.2.1
Client
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B
DHCP Server
10.1.1.1
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DHCP on a routed network

Broadcast does not cross routers !
IP helper address = 10.1.1.1
DHCP
OFFER
Broadcast
BOOTP/DHCP
RELAY AGENT
10.2.2.1
DHCP
OFFER
unicast
3
4
A
Destination IP Address = 10.2.2.1
UDP Port 67
Your IP Address = 10.2.2.7
Server IP Address = 10.1.1.1
Gateway IP Address = 10.2.2.1
Hardware Address = 08007. . .
DHCP Server
Subnet mask
10.1.1.1
Default gateway
Lease time
...
2003-2004 - Information management
B
Client
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DHCP Commands
• WIN NT/2K
– ipconfig /release
– ipconfig /renew
• WIN95
– winipcfg
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DHCP

DHCPNACK


DHCPDECLINE


The server.to.client communication, refusing
the request for configuration parameter.
The client.to.server communication, indicating
that the network address is already in use. (arp)
DHCPRELEASE

The client.to.server communication,
relinquishing network address and canceling
remaining lease.
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