Download DHCP

DHCP
DHCP
• Dynamic Host Configuration Protocol (DHCP)
• From 1993
• An extension of BOOTP, very similar to DHCP
• Same port numbers as BOOTP
• DHCP is the preferred mechanism for dynamic assignment
of IP addresses
• DHCP can interoperate with BOOTP clients.
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DHCP
• Dynamic Host Configuration Protocol
• It is a method for assigning Internet Protocol (IP)
addresses permanently or to individual computers
in an organization’s network
• DHCP lets a network administrator supervise and
distribute IP addresses from a central point and
automatically sends a new IP address when a
computer is plugged into a different place in the
network
MOTIVATION FOR DHCP
• Static Vs Dynamic IP
• Configuration parameters for network hosts
 IP address
 Router
 Subnet Mask
 Others..
DYNAMIC HOST CONFIGURATION
PROTOCOL (DHCP)
• Four Key benefits to DHCP:
•
•
•
•
Centralized administration of IP configuration.
Dynamic host configuration.
Seamless IP host configuration.
Flexibility and scalability.
DISADVANTAGES OF DHCP
• When DHCP server is unavailable, client is
unable to access enterprises network
• Your machine name does not change when
you get a new IP address
• Uses UDP, an unreliable and insecure protocol.
• DNS cannot be used for DHCP configured hosts.
SECURITY PROBLEM
• DHCP is an unauthenticated protocol
When connecting to a network, the user is not
required to provide credentials in order to obtain
a lease
Malicious users with physical access to the DHCPenabled network can instigate a denial-of-service
attack on DHCP servers by requesting many
leases from the server, thereby depleting the
number of leases that are available to other
DHCP clients
LIMITATIONS
• Some machines on your network need to be at
fixed addresses, for example servers and routers
• You need to be able to assign a machine to run the
DHCP server continually as it must be available at
all times when clients need IP access
DHCP INTERACTION (SIMPLIFIED)
Argon
128.143.137.144
00:a0:24:71:e4:44
DHCP Server
DHCP Response:
IP address: 128.143.137.144
Default gateway: 128.143.137.1
Netmask: 255.255.0.0
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DHCP OPERATION
DHCP Client
00:a0:24:71:e4:44
DHCP Server
DHCPDISCOVER
• DCHP DISCOVER
Sent to 255.255.255.255
DHCP Server
DHCP Client
00:a0:24:71:e4:44
DCHP OFFER
DHCPOFFER
DHCP Server
DHCPOFFER
DHCP Server
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DHCP OPERATION
DHCP Client
00:a0:24:71:e4:44
DHCP Server
DHCPREQUEST
DCHP DISCOVER
DHCPACK
At this time, the DHCP
client can start to use the IP
address
DHCP Server
DHCP Client
00:a0:24:71:e4:44
DHCP Server
DHCPREQUEST
Renewing a Lease
(sent when 50% of lease
has expired)
If DHCP server sends
DHCPNACK, then
address is released.
DHCPACK
DHCP Server
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DHCP OPERATION
DHCP Client
00:a0:24:71:e4:44
DHCP Server
DHCPRELEASE
DCHP RELEASE
At this time, the DHCP
client has released the IP
address
DHCP Server
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BOOTP/DHCP MESSAGE FORMAT
OpCode (1 = Req, 2 =
Reply)
Hardware Type (1 =
Ethernet)
Number of Seconds
Hardware Address
Hop Count
Length
Unused (in BOOTP)
Flags (in DHCP)
Transaction ID
Client IP address
Your IP address
Server IP address
Gateway IP address
Client hardware address (16 bytes)
Server host name (64 bytes)
Boot file name (128 bytes)
Options
(There are >100 different options)
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DHCP MESSAGE TYPE
• Message type is sent as an
option.
Value
Message Type
1
DHCPDISCOVER
2
DHCPOFFER
3
DHCPREQUEST
4
DHCPDECLINE
5
DHCPACK
6
DHCPNAK
7
DHCPRELEASE
8
DHCPINFORM
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