Transcript Dynamic Host Configuration Protocol

Dynamic Host Configuration
Protocol [DHCP]
-RFC 2131
CIS 856: TCP/IP and Upper Layer Protocols
Karthik Ravindra
Nov 11, 2008
Motivation
What’s
my IP
Where am
I?
DNS Server
File
Server
192.168.1.3
192.168.1.4
ee:ee:ee:ee:ee:01
ee:ee:ee:ee:ee:02
ee:ee:ee:ee:ee:03
ee:ee:ee:ee:ee:04
 Diskless Workstations 
Who has
my boot
file?
Where is
my boot
file?
Router
192.168.1.1
Boot Server
192.168.1.2
Internet
RARP
• Provides IP Address for the diskless computer
• However
– RARP is a user process but works over link layer
directly
• RARP server and system must be on same network
– Returns only IP address
• Booting and configuration parameters not returned
• Host needs ICMP to complete booting
• Can’t relay RARP requests to a central server.
BOOTP
BOOTP Operation
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Port 68/UDP
Is server
name
my
name?
NO
Optional
Do I
know the
host with
server
name?
YES
BOOTP Client (A)
I’ll discard
this PDU
NO
YES
BOOTP Server (B)
BOOTREQUEST
YES
Is client
IP
address
0.0.0.0?
Is server
name
local?
D S
YES
BOOTP
Is client
hardware
address
in our
DB?
Port 67/UDP
PDU
|
67 | 68 255.255.255.255
NO
S
S
I’ll unicast this PDU to the
host who has server name
ee:ee:ee:ee:ee:02
after updating
the gateway
0.0.0.0
interface address field
S
S
D
| ff:ff:ff:ff:ff:ff
S D
I’ll create
the
YES
192.168.1.2
| 255.255.255.255 67| 68
ee:ee:ee:ee:ee:02
| ee:ee:ee:ee:ee:05
BOOTREPLY PDU
and send to the client
S Source
D Destination
D
BOOTREPLY
NO
D
D
BOOTP
UDP
IP
BOOTP PDU
Ethernet
Identify the limitation ?
Is server
name
my
name?
NO
Optional
YES
Is server
name
local?
YES
YES
NO
I’ll discard
this PDU
YES
Is client
IP
address
0.0.0.0?
Is client
hardware
address
in our
DB?
Do I
know the
host with
server
name?
YES
NO
NO
NO
I’ll create the
BOOTREPLY PDU
and send to the client
I’ll unicast this PDU to the
host who has server name
after updating the gateway
interface address field
Identify the limitation ?
ee:ee:ee:ee:ee:01
ee:ee:ee:ee:ee:03
ee:ee:ee:ee:ee:02
DNS
Server
192.168.1.3
ee:ee:ee:ee:ee:04
File
Server
192.168.1.4
Router
New Roaming
Client
What is my network
configuration?
192.168.1.1
Boot Server
192.168.1.2
Internet
I don’t know, I’ve never
seen you here before.
Purpose of DHCP
– Automates the assignment of unique IP addresses,
subnet masks, default gateways, and other IP
parameters to individual computers and devices on
the network.
– Reduces the work necessary to administer a large IP
(Internet Protocol) 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.
DHCP Address Allocation
• Manual allocation
• Automatic allocation
• Dynamic allocation
DHCP as compared to BOOTP
•
•
•
•
Is an extension of BOOTP.
Allows diskless clients configure automatically.
Centralizes and manages the allocation of TCP/IP.
Automatically assigns IP config settings
Manually
configured IP
DHCP clients get
IP config settings
from DHCP server
Without DHCP Servers
• Administrative overhead
– Need to keep track of all IP assignments
– Need to manually enter IP settings
• Bad or duplicate IP addresses
– Mistakes will happen!
• Moving between subnets
– Need to manually change IP settings
DHCP Operation Phases
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
DHCP Operation
• DCHP
DISCOVER
DHCP Client
00:a0:24:71:e4:44
DHCP Server
DHCPREQUEST
DHCPACK
At this time, the
DHCP client can
start using the IP
address
DHCP Server
DHCP Client
00:a0:24:71:e4:44
• Renewing a Lease
DHCP Server
DHCPREQUEST
DHCPACK
DHCP Server
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
Initial Message Flow
Server A
Client
DHCPDISCOVER
DHCPOFFER
Server B
DHCPDISCOVER
DHCPOFFER
Client attempts to discover
available DHCP servers
Servers reply with offers
Client collects offers and
decides which offer to accept
DHCPREQUEST
Configuration complete
DHCPREQUEST
DHCPACK
Graceful shutdown
DHCPRELEASE
Client broadcasts request for
one of the received offers
Server acknowledges client’s
use of IP address
Client explicitly releases
use of IP address
Renewal Message Flow
Server A
Client
Server B
T1 elapses
DHCPREQUEST
Configuration complete
DHCPACK
Client unicasts request to
continue using IP address
Server acknowledges
request and updates lease
T1 elapses
DHCPREQUEST
Client unicasts request to
continue using IP address
T2 elapses
DHCPREQUEST
DHCPACK
DHCPREQUEST
Configuration complete
Client broadcasts request to
continue using IP address
Server acknowledges
request and updates lease
Client FSM (Simplified)
DHCPNAK/
Stop using IP address
INIT
DHCPNAK,
Lease expires/
Stop using IP
address
DHCPNAK/
Discard offer
RENEWING
T2/Broadcast DHCPREQUEST
REBINDING
DHCPACK/Set T1,T2
DHCPACK (in use)/
DHCPDECLINE
DHCPACK/Set T1,T2
-/DHCPDISCOVER
REQUESTING
Select offer/DHCPREQUEST
SELECTING
DHCPOFFER/
Process offer
T1/
Unicast
DHCPREQUEST
DHCPACK/Set T1,T2
BOUND
Relay Agents
DHCP Pros
• Reduces a great deal of manual configuration work.
• Mobility of devices between networks and to
automatically obtain valid configuration parameters for
the current network can be of great benefit.
• Because IP addresses are only allocated when clients
are actually active, it is possible to reduce the total
number of addresses in use in an organization.
DHCP Cons
• Potentially unauthorized clients
• Malicious client could exhaust address
• Malicious server (Rogue server)
– Supply incorrect configuration parameters
– Supply malicious configuration parameters
Summary
• BOOTP allows default router, subnet mask, DNS
• DHCP allows dynamic allocation
• DHCP is backward compatible with BOOTP
DHCP PDU Types
DHCP PDU
Use
DHCPDISCOVER Client broadcast to find its local servers
DHCPOFFER
Server response to client offering configuration
parameters
DHCPREQUEST
Client broadcast requesting offered parameters
DHCPDECLINE
Client to server notification that IP address is in use
DHCPACK
Server response to client confirming a request
DHCPNAK
Server response to client denying a request
DHCPRELEASE
Client notice to server on release of address
DHCPINFORM
Client request to server for additional configuration
parameters
Figure 17-5
DHCP transition diagram
Exchanging messages
Exchanging messages