DHCP Dynamic Host Configuration Protocol

1
DHCP Dynamic Host Configuration Protocol

• By
• Curtis Wright III

2
DHCP Definition

• Dynamic Host Configuration Protocol(DHCP)
  provides a framework for passing configuration
  information to hosts on a TCP/IP network. DHCP is
  based on the Bootstrap Protocol(BOOTP) adding the
  capability of automatic allocation of reusable
  network addresses and additional configuration
  options. DHCP captures the behavior of BOOTP
  relay agents. DHCP participants can interoperate
  with BOOTP participants.

3
DHCP Origins

• DHCP is the result of the Dynamic Host
  Configuration (DHC) working group of the IETF
  addressing the issue they defined on their first
  meeting in April 1989 in Cocoa Beach, FL of the
  automated allocation of a network address and
  transmission of other parameters such as the
  subnet mast and a default router.

4
DHCP RFC Sequence

• RFC 1531 which was published in October of 1993
  followed by 1541 same and month followed by RFC
  2131 in 1997.
• Ralph Droms was the original author of DHCP and
  it currently involved on its ongoing evolution.

5
Predecessors to DHCP

1. Reverse Address Resolution Protocol (RARP)
2. Bootstrap Protocol (BOOTP)

6
RARP

• Provides a mechanism for a host to determine its
  IP address when it is only aware of its MAC
  address.
• RARP can determine and configure a diskless
  workstation with an IP address.
• No method for determining and delivering other
  configuration data.
• RARP server can service only a single subnet
  because of its complete reliance on Ethernet
  broadcasts.
• RFC 903

7
BOOTP

• Developed as answer to RARPs limitations.
• BOOTP allows hosts to be dynamically configured
  to use the TCP/IP protocol suite.
• BOOTP delivers configuration information
  automatically instead of each TCP/IP host on a
  network being manually configured.
• RFC 951

8
BOOTP

• BOOTP is a client/server process where the BOOTP
  client, during the boot phase, requests
  configuration information from a BOOTP server.
• After receiving a request from a BOOTP client,
  the server looks up the clients MAC address in
  its BOOTP configuration database and sends a
  reply containing IP configuration information.
• Client receives the reply and configures its
  TCP/IP stack.
• BOOTP client will also load a boot file if the
  BOOTP server supplies a path using the fully
  qualified filename.
• The magic cookie, a mechanism for a BOOTP server
  to supply vendor-specific operating system(OS)
  options to a BOOTP client. Options include DNS
  servers, WINS or NetBIOS name server, time
  servers, etc.

9
BOOTP

• Even with these improvements BOOTP had serious
  drawbacks such as
• 1. BOOTP database was a static text file.
• File had to be maintained by hand as changes
  were made to the network.
• 2. Unable to dynamically allocate and
  distribute IP addresses.
• Dynamic IP addressing was critical when IP
  address space was limited.

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BOOTP Conversation

• Two types of BOOTP messages
• 1. BOOTREQUEST
• 2. BOOTREPLY

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BOOTREQUEST/BOOTREPLY

• Have the sane packet structure.
• BOOTREQUEST includes information that allows the
  BOOTP server to determine what configuration data
  it must supply the requesting host.
• BOOTREPLY includes the information that the
  client requested of the server.

12
BOOTREQUEST

• Contains following information
• 1. Sources MAC address
• 2.Destinations MAC address
• 3. Destinations IP address
• 4.Sources IP address
• 5. Destination server hostname
• 6. Boot filename
• 7. Vendor-specific data

13
BOOTP/DHCP similarities

• DHCP has many of the same characteristics as
  BOOTP such as
• 1.Client/server operating model
• 2.Packet structure
• 3.UDP port numbers
• a. messages from a client to a server are
  sent to the server port (67).
• b. Messages from a server to a client are
  sent to the client port (68).

14
BOOTP/DHCP Differences

• There are two primary differences between DHCP
  and BOOTP
• DHCP defines a mechanism through which clients
  can be assigned a network address for a finite
  lease, allowing for serial reassignment of
  network addresses to different clients.
• DHCP provides the mechanism for a client to
  acquire all of the IP configuration parameters
  that it needs in order to operate.

15
DHCP Message format
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DHCP Message Field Desciptions
Fields Octets Descriptions
op 1 op code/message type
htype 1 Hardware address type
hlen 1 hardware address length
hops 1 Client sets to zero, optionally used by relay agents when booting via relay agent
xid 4 Transaction ID
secs 2 Filled in by client, seconds elapsed since client began address acquisition or renewal process.
flags 2 flags
ciaddr 4 Client IP address
yiaddr 4 your (client) IP address
siaddr 4 IP address of next server to use in bootstrap returned in DHCPOFFER, DHCPACK by server.
giaddr 4 Relay agent IP address
chaddr 16 Client hardware address
sname 64 Optional server host name, null terminated string.
file 128 Boot file name
options var Optional parameters field.
17
DHCP Allocation Mechanisms

• DHCP supports three mechanisms for IP allocation
• 1.Automatic a permanent IP address is
  assigned to a client.
• 2.Dynamic an IP address is assigned for a
  limited amount of time.
• 3.Manual a clients IP address is assigned
  by network administrator, and DHCP conveys that
  address to the client.
• A network could use one or more of these
  mechanisms depending on the network
  administrators policies.

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Dynamic Allocation

• 1. Allows automatic reuse of an address that is
  no longer needed by the client to which it is
  assigned.
• 2. Useful for assigning an address to a client
  that will be connected to the network temporarily
  or for sharing a limited pool of IP addresses
  among a group of clients that do not need
  permanent IP addresses.
• 3. Good for assigning an IP address to a new
  client being permanently connected to a network
  having a limited number of IP addresses that it
  is necessary to reclaim them when old clients are
  retired.

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DHCP server and client messages
From client From server
DHCPDISCOVER DHCPOFFER
DHCPREQUEST DHCPACK
DHCPDECLINE DHCPNACK
DHCPRELEASE
DHCPINFORM
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DHCP messages

• DHCPDISCOVER- Client broadcast to locate
  available servers.
• DHCPOFFER- Server to client in response to
  DHCPDISCOVER with offer of configuration
  parameters.
• DHCPREQUEST Client message to servers either
  (a) requesting offered parameters from one server
  and implicitly declining offers from all others,
  (b) confirming correctness of previously
  allocated address after, e.g., system reboot, or
  (c) extending the lease on a particular network
  address
• DHCPACK Server to client with configuration
  parameters, including committed network address.

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DHCP messages (contd)

• DHCPNACK Server to client indicating client
  notion of network address is incorrect or
  clients lease as expired.
• DHCPDECLINE Client to server indicating network
  address is already in use.
• DHCPRELEASE Client to server relinquishing
  network address and cancelling remaining lease
• DHCPINFORM Client to server, asking only for
  local configuration parameters client already
  has externally configured network address.

22
DHCP client and server messages when allocating a
new network address
Server (Not Selected)
Client
Server(selected)
Begins initialization
DHCPDISCOVER
DHCPDISCOVER
Determines configuration
Determines Configuration
DHCPOFFER
Collects Replies Selects configuration
DHCPREQUEST
DHCPREQUEST
Commits configuration
DHCPACK
Initialization complete
Graceful shutdown
DHCPRELEASE
Discards lease
23
DHCP client and server messages for previously
allocated network address
Server
Server
Client
Begins Initialization
Locates Configuration
Locates Configuration
DHCPACK
DHCPACK
Initialization complete
Subsequent DHCPACKs ignored
24
Constructing and sending DHCP messages

• DHCP clients and servers both construct DHCP
  messages by filling in fields in the fixed format
  section of the message and appending tagged data
  items in the variable length option area.
• Options area includes first a four-octet magic
  cookie, followed by the options which comes from
  BOOTP.
• The last option must ALWAYS be the end option.

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Constructing and sending DHCP messages (contd)

• DHCP messages broadcast by a client prior to that
  client obtaining its IP address must have the
  source address field in the IP header set to 0.
• DHCP clients are responsible for all message
  retransmission. The client MUST adopt a
  retransmission strategy that incorporates a
  randomized exponential backoff algorithm to
  determine the delay between retransmissions.
• DHCP servers and BOOTP relay agents attempt to
  deliver DHCPOFFER, DHCPACK, and DHCPNACK messages
  directly to the client using unicast delivery.

26
DHCP server administrative controls

• A DHCP server has a block of network addresses
  from with it can satisfy requests for new
  addresses.
• Each server maintains a database of allocated
  addresses and leases in local permanent storage.
• DHCP server are not required to respond to every
  DHCPDISCOVER and DHCPREQUEST message they
  receive.
• DHCP server needs to use some unique identifier
  to associate a client with its lease.
• DHCP clients are free to use any strategy in
  selecting a DHCP server among those from which
  the client receives a DHCPOFFER message.

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DHCP Server Behavior

• While not required for correct operation of DHCP,
  the sever SHOULD NOT use the selected network
  address before the client responds to the
  servers DHCPOFFER message.
• Procedure for server choosing an expiration time
  for a lease
• If the client has not requested a specific lease
  in the DHCPDISCOVER message and the client
  already has an assigned network address, the
  server returns the lease expiration time
  previously assigned to that address(note that the
  client must explicitly request a specific lease
  to extend the expiration time on a previously
  assigned address),
• ELSEIF
• the client has not requested a specific
  lease in the DHCPDISCOVER message and the client
  does not have an assigned network address, the
  server assigns a locally configured default lease
  time, ELSE
• If the client has requested a specific lease in
  the DHCPDISCOVER message(regardless of whether
  the client has an assigned network address) , the
  server may choose either to return the requested
  lease( if the lease is acceptable to local
  policy) or select another lease.

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DHCP server behavior (contd)

• The sever must return to the client
• The clients network address
• The expiration time for the clients lease
• Parameters requested by the client based on
  specified rules. 4. The server MUST supply as
  many of the requested parameters as possible and
  MUST omit any parameters it cannot provide.
• 5.Any parameters from the existing binding that
  differ from the Host Requirements Document
  defaults,
• 6.Any parameters specific to this client
• 7.Any parameter specific to this clients class
  the parameters must be identified by an exact
  match between the clients vendor class
  identifiers and the clients classes identified
  in the server
• 8.Parameters with non-default values on the
  clients subnet.

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State transition diagram for client
INIT- REBOOT
INIT
DHCPNAK/ Restart
Send DHCPREQUEST
DHCPACK(not accept)/Send DHCPDECLINE
Send DHCPDISCOVER
REBOOTING
DHCPNAK/ Discard offer
SELECTING
DHCPOFFER/ Discard
DHCPOFFER/Collect Replies
REQUESTING
Select offer/send DHCPREQUEST
REBINDING
DHCPAK/ Halt Network
DHCPACK/Record lease, set Timers T1, T2
DHCPACK/Record lease, set timers T1, T2
DHCPACK/Record lease, Set timers T1, T2
DHCPACK/Record lease,set timers T1, T2
RENEWING
BOUND
T1 expires Send DHCPREQUEST to Leasing server
DHCPACK/Record lease, set timers T1, T2
DHCPOFFER,DHCPACK,DHCPNAK Discard
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Some DHCP drawbacks

• A malicious user could make trouble by putting up
  an unofficial DHCP server.
• The "broadcast flag" DHCP includes a way in
  which client implementations unable to receive a
  packet with a specific IP address can ask the
  server or relay agent to use the broadcast IP
  address in the replies (a "flag" set by the
  client in the requests
• An easy mistake to make in setting up a DHCP
  server is to fail to set all the necessary global
  parameters.
• Long leases can be disadvantageous in cases where
  you need to change a configuration parameter or
  withdraw an address from use.

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DHCP security

• DHCP which is built on UDP and IP and is used to
  make maintenance of remote and/or diskless hosts
  easier in its current form is quite insecure.

32
IPv6(IPng) or IP Next Generation

• It has been foreseen due to the rapid growth of
  the Internet and its initial design that there
  will come a point where there will not be any
  available addresses to connect new hosts.
• IPv6 is one of several solutions that has been
  proposed to deal with this issue.
• IPv6 has a stateless autoconfiguration
  capability.
• Stateless autoconfiguration does not require
  manual host configuration, minimal router
  configuration, and no additional servers. A host
  is able generate its own addresses using a
  combination of locally available information and
  information advertised by routers.

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Future of DHCP

• 1. DHCP for IPv6 is the future for DHCP .
• 2. DHCPv6 complements IPv6 by providing a
• stateful autoconfiguration option to facilitate
  the automatic configuration of DHCP clients(RFC
  3315).
• In stateful autoconfiguration, hosts obtain
  interface addresses and/or configuration
  information and parameters from a server. The
  server maintains a database of the addresses that
  have been to which hosts.
• 3.DHCP clients can get configuration options such
  as DNS server addresses.
• 4. DHCPv6 uses a combination of unicast and
  multicast messages instead of broadcast message
  for the majority of its conversation.

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References

• 1.www.dhcp.org
• 2.DHCP for Windows 2000
• 3.The DHCP handbook

35
Exam Questions

• Provide timeline diagram of messages exchanged
  between DHCP client and servers when allocating a
  new network address.
• List the predecessors to DHCP with their
  advantages and disadvantages and explain why DCHP
  is better.

36
Exam Question Answers

• Server Client
  Server
• (not selected)
  
  (selected)
• V
  V
  V
• 
  
  
• 
  Begins Initialization
  
• 
  
  
• ----------------------
  ---------/ \ ----------------------------------
  ------
• / DHCPDISCOVER
  DHCPDISCOVER \
• 
  
  
• Determines Configuration
  Determines
  Configuration
• 
  
  
• \-----------------------
  --------------------------
  --------/
• DHCPOFFER \
  / DHCPOFFER
• 
  \ /
  
• 
  Collects Replies
  
• 
  \
  
• 
  Selects Configuration
  
• 
  
  

37
Exam Question Answers(Contd)

• Predecessors to DHCP are
• 1.RARP(Reverse Address Resolution Protocol)
• Benefits
• a. Provides a mechanism for a host to
  determine its IP address when it is only aware of
  its MAC address
• b. RARP can determine and configure a
  diskless workstation with an IP address.
• Disadvantages
• a. No method for determining and
  delivering other configuration data.
• b. RARP server can service only a single
  subnet because of its complete reliance on
  Ethernet broadcasts

38
Exam Question Answers(Contd)

• BOOTP(Bootstrap Protocol)
• Benefits
• a. BOOTP allows hosts to be dynamically
  configured to use the TCP/IP protocol suite
• b. BOOTP delivers configuration information
  automatically instead of each TCP/IP host on a
  network being manually configured
• Disadvantages
• a. BOOTP database was a static text file
  which was maintained by hand as network changes
  were made.
• b. Unable to dynamically allocate and
  distribute IP addresses.

39
Exam Question Answers(Contd)

• DHCP is better because
• a. DHCP defines a mechanism through which
  clients can be assigned a network address for a
  finite lease, allowing for serial reassignment of
  network addresses to different clients.
• b. DHCP provides the mechanism for a client to
  acquire all of the IP configuration parameters
  that it needs in order to operate.