DHCP

DHCP
Dynamic Host Configuration Protocol
CIS 856: TCP/IP and Upper Layer Protocols
Presented by Kyle Getz
October 20, 2005

Motivation for DHCP
 Configuration parameters for network hosts
 IP address
 Router
 Subnet Mask
 Many more…
 Before DHCP
 Manual assignment
 RARP
 BOOTP

DHCP Features
 Protocol for providing configuration
parameters to hosts over network
 Dynamic allocation of IP addresses
 Minimal human intervention

Sample Network
Router
Internet
DHCP
Server
UDP Port
68
UDP Port
68
UDP Port
68
UDP Port
67
DHCP
Clients

Preliminaries
 (DHCP) Message = DHCP-PDU (A-PDU)
 Client = DHCP Client
 Server = DHCP Server
 Well-known port numbers
 DHCP Server: UDP port 67
 DHCP Client: UDP port 68
 No ephemeral ports
 Broadcast and unicast used for PDU’s in both
directions
 “Broadcast”: link and IP addresses are broadcast
 “Unicast”: link and IP addresses are unicast

Initial Message Flow
Server A Client Server B
Client attempts to discover
available DHCP serversDHCPDISCOVE
R
DHCPDISCOVE
R
Servers reply with offers
DHCPOFFE
R
DHCPOFFE
R
Client collects offers and
decides which offer to accept
Client broadcasts request for
one of the received offersDHCPREQUES
T
DHCPREQUES
T
Server acknowledges client’s
use of IP addressDHCPAC
K
Configuration complete
Client explicitly releases
use of IP addressDHCPRELEA
SE
Graceful shutdown

DHCP Message Types
DHCP
Message
Use
DHCPDISCOVER Client broadcast to locate available servers
DHCPOFFER Server to client response offering configuration parameters
DHCPREQUEST Client broadcast requesting offered parameters
DHCPDECLINE Client to server notification that IP address is in use
DHCPACK Server to client response confirming a request
DHCPNAK Server to client response denying a request
DHCPRELEASE Client to server request to relinquish IP address
DHCPINFORM Client to server request for configuration parameters

Lease Renewal Times (Client)
 T1 < T2 < Lease time
 T1 default value = 1/2 of lease time
 T2 default value = 7/8 of lease time
 Communicated via DHCPOFFER, DHCPACK
 Client actions when times elapse
 T1: client must renew address with the DHCP server
 T2: client must renew address with any DHCP server
 Lease time: client must stop using IP address

Renewal Message Flow
Server A Client Server B
Client unicasts request to
continue using IP addressDHCPREQUES
T
Server acknowledges
request and updates leaseDHCPACK
Client broadcasts request to
T
DHCPREQUES
T
Server acknowledges request
and updates leaseDHCPAC
K
T1 elapses
T1 elapses
Client unicasts request to
T
T2 elapses

Client FSM (Simplified)
INIT
SELECTIN
G
-/DHCPDISCOV
ER
DHCPOFFER/
Process offer
REQUESTIN
G
Select
offer/DHCPREQUEST
BOUN
D
DHCPACK/Set T1,T2
DHCPACK/Set T1,T2
DHCPACK/Set T1,T2
RENEWIN
G
T1/ Unicast
DHCPREQUES
T
REBINDIN
G
T2/Broadcast
DHCPREQUEST
DHCPNAK/
Stop using IP
addressDHCPNAK,
Lease expires/
Stop using IP
address
DHCPACK (in
use)/
DHCPDECLINE
DHCPNAK/
Discard offer

Retransmissions
 Client responsible for all retransmissions
 Retransmission strategy
 Exponential backoff
 Randomized
 Recommendations
 Base delay doubled for each retransmission
 Random number picked from [-1,+1]
 Maximum base delay: 64 seconds

Server Storage
 Permanent storage
 Pool of available IP addresses
 Local configuration parameters
 Mapping between clients and leases
 Flexibility concerning storage update
 When DHCPOFFER sent
 When DHCPACK sent

Server Logic (Simplified)
Event Action Taken
DHCPDISCOVER
If current lease for client exists, send DHCPOFFER
Else, if IP address available, send DHCPOFFER
Else, do nothing
DHCPREQUEST If IP address available, send DHCPACK
Else, send DHCPNAK
DHCPDECLINE Mark IP address unavailable, notify network administrator
DHCPRELEASE Mark IP address available, delete lease
DHCPINFORM Send DHCPACK with configuration parameters
Lease expiration Mark IP address available, delete lease

DHCP PDU Format
32 Bits
Operation Code Hardware Type Hardware Length Hop Count
Transaction ID
Seconds Elapsed B Must Be Zero (MBZ)
Client IP address
Your IP address
Server IP address
Relay agent IP address
Client hardware address
(16 bytes)
Server host name
(64 bytes)
Boot file name
(128 bytes)
Options
(up to 312 bytes)
Magic Cookie

DHCP Options
255 End of options
Code Length Data
1 byte 1 byte Length bytes
0 Padding
1 4 255 255 255 0
Subnet Mask:
99 130 83 99Magic Cookie:
Option format:
One-byte options:
4 bytes

Another Sample Network
Router Internet
DHCP
Server
DHCP
Clients
Relay Agent
within

Relay Agents
 Remove restriction of having DHCP server on
every network
 Listen for DHCP messages and transmit them
to appropriate machine
 Client to server relay
 Broadcast from client → Unicast to server(s)
 Server to client relay
 Broadcast from server → Broadcast to client
 Unicast from server → Unicast to client

Advanced Topics
 Lease times
 Dynamic DNS
 Reliability
 Security

Lease Times
 Anywhere from 15 minutes – 1 year
 Common lease times & rationales
 15 minutes: Maximum number of addresses free
 3 days: Microsoft default
 4 months: Students can keep lease over summer
 Tradeoff

Dynamic DNS
 If IP address changes due to DHCP, DNS
entry is wrong
 Client or server can update DNS
 Option 81: Client FQDN
81 Length Flags rcode1 rcode2 Name…
1 byte 1 byte “Length” bytes

Reliability
 Two synchronized DHCP servers on the same
network: Primary, Secondary
 Permanent storage constantly communicated
 Failure: Secondary server takes over
Secondar
y Server
DHCP
Clients
Primary
Server

Security
 Potentially unauthorized clients
 Malicious client could exhaust address pool
 Malicious server (Rogue server)
 Supply incorrect configuration parameters
 Supply malicious configuration parameters

DHCP

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