RARP

1
RARP

• Reverse Address Resolution Protocol (RARP) is
  closely related to the Address Resolution
  Protocol (ARP) used on Transmission Control
  Protocol/Internet Protocol (TCP/IP) networks, but
  with a difference.
• With RARP, a computer transmits a message
  containing its data-link layer hardware address
  to a server and receives a message containing an
  Internet Protocol (IP) address for it to use.
• ARP resolves IP addresses into hardware
  addresses.
• An RARP server uses the same message format as
  ARP and is a simple mechanism for assigning IP
  addresses over the network.
• RARP assigns a workstation an IP address only it
  cannot assign a subnet mask or other TCP/IP
  configuration parameters.

2
BOOTP

• The Bootstrap Protocol (BOOTP) is based on
  Request for Comments (RFC) 951, with extensions
  in RFC 1533 and RFC 1542.
• BOOTP can supply clients with subnet masks,
  router addresses, and other TCP/IP configuration
  parameters in addition to IP addresses.

3
DHCP Design Goals

• Dynamic Host Configuration Protocol (DHCP) client
  computers and DHCP clients should require no
  manual configuration.
• DHCP should not require a server on each subnet.
• DHCP clients must be able to handle multiple
  responses to a request for configuration
  parameters.
• DHCP must coexist with statically configured,
  nonparticipating hosts and with existing network
  protocol implementations.

4
DHCP Design Goals (Cont.)

• DHCP must be able to provide service to existing
  BOOTP clients.
• DHCP must ensure that only one DHCP client at a
  time will use any specific network address.
• DHCP clients must retain their configurations
  across a DHCP client reboot or a DHCP server
  reboot.
• DHCP must support fixed or permanent allocation
  of configuration parameters to specific clients.

5
How a DHCP Client Interacts with a DHCP Server
6
DHCP Address Allocation Methods

• Manual allocation. The assignment of a single
  predetermined IP address to a specific DHCP
  client, identified by its data-link layer
  hardware address
• Automatic allocation. The permanent assignment of
  IP addresses from a scope
• Dynamic allocation. The assignment of IP
  addresses from a scope on a leased basis

7
DHCP Components

• DHCP consists of the following three parts
• A service that responds to DHCP requests from
  clients and maintains records of IP address
  assignments
• A client that generates requests for TCP/IP
  parameters and configures the TCP/IP client
• The protocol that the clients and server use to
  communicate

8
The DHCP Message Format
9
DHCP Message Fields

• Op (1 byte). Specifies whether the message
  originated at a client or a server
• Htype (1 byte). Specifies the type of hardware
  address in the Chaddr field
• Hlen (1 byte). Specifies the length of the
  hardware address in the Chaddr field, in bytes
• Hops (1 byte). Specifies the number of routers in
  the path between the client and the server
• Xid (4 bytes). Contains a transaction identifier
  used to associate requests and replies
• Secs (2 bytes). Specifies the elapsed time (in
  seconds) since the beginning of an address
  allocation or lease renewal process

10
DHCP Message Fields (Cont.)

• Flags (2 bytes). Indicates whether DHCP servers
  and relay agents should use broadcast
  transmissions instead of unicast transmissions to
  communicate with a client
• Ciaddr (4 bytes). Contains the client computers
  IP address when it is in the bound, renewal, or
  rebinding state
• Yiaddr (4 bytes). Contains the IP address being
  offered to a client by a server
• Siaddr (4 bytes). Specifies the IP address of the
  next server in a bootstrap sequence used only
  when the DHCP server supplies an executable boot
  file to a diskless workstation
• Giaddr (4 bytes). Contains the IP address of a
  DHCP relay agent located on a different network,
  when necessary

11
DHCP Message Fields (Cont.)

• Chaddr (16 bytes). Contains the hardware address
  of the client system, using the type and length
  specified in the Htype and Hlen fields
• Sname (64 bytes). Contains either the host name
  of the DHCP server or overflow data from the
  Options field
• File (128 bytes). Contains the name and path to
  an executable boot file for diskless workstations
  
• Options (variable). Contains a series of DHCP
  options, which specify the configuration
  parameters for the client computer

12
DHCP Message Type Option Values

• 1DHCPDISCOVER. Used by clients to request
  configuration parameters from a DHCP server
• 2DHCPOFFER. Used by servers to offer IP
  addresses to requesting clients
• 3DHCPREQUEST. Used by clients to accept or renew
  an IP address assignment
• 4DHCPDECLINE. Used by clients to reject an
  offered IP address
• 5DHCPACK. Used by servers to acknowledge a
  clients acceptance of an offered IP address
• 6DHCPNAK. Used by servers to reject a clients
  acceptance of an offered IP address
• 7DHCPRELEASE. Used by clients to terminate an IP
  address lease
• 8DHCPINFORM. Used by clients to obtain
  additional TCP/IP configuration parameters from a
  server

13
Other DHCP Options

• Host-specific IP layer parameters. Configure
  various IP settings on the client computer, such
  as the default Time to Live settings
• Interface-specific IP layer parameters. Configure
  settings that are particular to the network
  interface over which the DHCP message arrives
• Link layer parameters. Configure settings
  specific to the link (or data-link) layer
  protocol running on the client computer
• TCP parameters. Configure TCP-specific settings
  on the client, such as the TCP Time to Live value
  
• Application and service parameters. Configure
  settings for specific application layer processes
  
• DHCP extensions. Configure settings that control
  the behavior of the DHCP client itself, such as
  the length of the DHCP address lease and the
  renewal and rebinding time values

14
DHCP Client/Server Communications
15
DHCP Address Assignment

• 1. The client uses the DHCPDISCOVER message type
  to generate a series of DHCP packets, which it
  transmits as broadcasts.
• 2. When a DHCP server receives a DHCPDISCOVER
  message from a client, it uses the DHCPOFFER
  message type to generate a response containing an
  IP address and other DHCP options.
• 3. After a specified period of time, the client
  stops broadcasting and accepts one of the offered
  IP addresses by transmitting a DHCPREQUEST
  message.
• 4. On receipt of the DHCPREQUEST message, the
  server whose offer the client is accepting
  commits the offered IP address and other settings
  to its database.
• 5. To conclude its part of the transaction, the
  server sends a DHCPACK message to the client.
• 6. After receiving a DHCPACK message from the
  server, the client performs a final test by
  generating a series of ARP broadcasts to ensure
  that no other system on the network is using the
  assigned IP address.

16
DHCP Lease Renewal
17
The DHCP Lease Renewal Process

• 1. When a client reaches the renewal time value,
  or T1 value, of its lease, it enters the renewing
  state and begins transmitting DHCPREQUEST
  messages to the leasing server as unicasts.
• 2. If the server is available to receive the
  message, it responds with a DHCPACK message,
  which renews the lease, or a DHCPNAK message,
  which terminates the lease.
• 3. If the server does not respond to the
  DHCPREQUEST unicast, the client continues
  transmitting until it reaches the rebinding time
  value, or T2 value.
• 4. The client enters the rebinding state and
  begins transmitting DHCPREQUEST messages as
  broadcasts to any DHCP server.
• A server can respond with either a DHCPACK or
  DHCPNAK message.
• If the lease time expires with no response from
  any DHCP server, the clients IP address is
  released and all its TCP/IP communication ceases.

18
Planning a DHCP Implementation

• When planning a DHCP implementation, you must
  consider many factors, including the following
• The number of clients you intend to support
• The configuration of your internetwork
• The performance capabilities of your servers

19
The Windows 2003 DHCP Server Service Installation
Process

• The overall process of installing Microsoft
  Windows 2003 DHCP Server and preparing it for use
  consists of the following steps
• 1. Install the Windows 2003 DHCP Server service
  software.
• 2. Authorize the server in the Active Directory
  service.
• 3. Create one or more scopes on the server.
• 4. Configure DHCP options.
• 5. Activate the scope or scopes.

20
Installing the Windows 2003 DHCP Server Service

• 1. Click Start, point to Settings, and then
  select Control Panel.
• 2. Double-click the Add/Remove Programs icon in
  Control Panel.
• 3. Click Add/Remove Windows Components.
• 4. In the Components list, select Networking
  Services, and then click Details.
• 5. In the Subcomponents Of Networking Services
  list, select the check box next to the Dynamic
  Host Configuration Protocol (DHCP) entry, and
  then click OK.
• 6. Click Next to install the DHCP Server
  service.
• 7. Click Finish to complete the installation.
• 8. In the System Settings Change message box,
  click Yes to restart the computer.

21
Authorizing a DHCP Server

• 1. Log on to the network, using an account that
  is a member of the Enterprise Admins group.
• 2. Click Start, point to Programs, and then
  select DHCP from the Administrative Tools program
  group.
• 3. In the scope pane, select the server you
  installed on the local computer, and then select
  Authorize from the Action menu.
• 4. Press the F5 key to refresh the server
  display, and then click the DHCP header at the
  root of the scope tree.
• The Status of the server has changed from Not
  Authorized to Running.

22
Scope Creation Restrictions

• You can create only one scope per subnet on a
  single DHCP server.
• Reservations must be included in a scope.
• Statically configured IP addresses must not be
  included in a scope.

23
Creating a Scope

• 1. Click Start, point to Programs, and then
  select DHCP from the Administrative Tools program
  group.
• 2. Select the entry for your DHCP server in the
  scope pane, and then select New Scope from the
  Action menu.
• 3. Click Next to bypass the Welcome page.
• 4. In the Name and Description boxes, type a
  name and (optionally) a descriptive string for
  the scope you are creating, and then click Next.
• 5. In the Start IP Address and End IP Address
  fields, type the beginning and ending IP
  addresses of the address range you want the DHCP
  server to assign.
• 6. Specify the subnet mask for the addresses
  that the server will assign, either by indicating
  the number of bits in the network identifier in
  the Length selector or by typing the mask in the
  Subnet Mask box, using dotted decimal notation
  then click Next.

24
Creating a Scope (Cont.)

• 7. (Optional) Type a range of addresses that you
  want to exclude from the range you specified in
  the IP Address Range page in the Start IP Address
  and End IP Address box, and then click Add to add
  the range to the Excluded Address Range List.
  Click Next.
• 8. Specify the time interval (in days, hours, or
  minutes, or all three) for the IP address leases
  the server will assign for this scope, and then
  click Next.
• 9. Click No, I Will Configure These Options
  Later, and then click Next.
• 10. Click Finish to create the scope.
• The scope now appears in the DHCP console.

25
Configuring DHCP Options

• 1. Click Start, point to Programs, and then
  select DHCP from the Administrative Tools group.
• 2. Select the Server Options or Scope Options
  heading under a particular scope, and then select
  Configure Options from the Action menu.
• 3. Highlight the 003 Router entry.
• 4. Select the check box next to the 003 Router
  entry.
• 5. In the Server Name box, type the name of the
  router that you want the DHCP clients to use as
  their default gateway.
• 6. Click Resolve to resolve the name and add the
  address to the IP Address box.

26
Configuring DHCP Options (Cont.)

• 7. Click Add to add the IP address to the list.
• 8. Repeat steps 57 to add additional router
  addresses.
• 9. Select the check box next to the 003 DNS
  Servers entry in the Available Options list.
• 10. Repeat steps 57 to specify the names or IP
  addresses of the DNS servers that you want your
  DHCP clients to use.
• 11. Scroll down in the Available Options list and
  select the check box next to the 044 WINS/NBNS
  Servers option.
• 12. Repeat steps 57 to specify the names of IP
  addresses of the Windows Internet Name Service
  (WINS) server that you want your DHCP clients to
  use.
• 13. Click OK to apply the options you have
  configured.

27
Creating a Reservation

• 1. Click Start, point to Programs, and then
  select DHCP from the Administrative Tools program
  group.
• 2. In the scope pane, expand the scope in which
  you want to create the reservation.
• 3. Select the Reservations heading, and then
  select New Reservation from the Action menu.
• 4. Type a name for the reservation in the
  Reservation Name box.
• 5. Type the IP address you want to assign in the
  IP Address box.
• 6. In the MAC Address box, type the data-link
  layer hardware address of the computer to which
  you want to assign the IP address.
• 7. Click Add to create the reservation.
• 8. Repeat steps 47 to create additional
  reservations for the scope, or click Close to
  close the New Reservation dialog box.
• The reservations you created now appear in the
  detail pane of the DHCP console.

28
The Server Statistics Dialog Box
29
Configuring the Windows 2003 DHCP Relay Agent

• 1. Open the Routing And Remote Access console.
• 2. Expand the icon for the server you want to
  configure, expand the IP Routing icon, and then
  click the DHCP Relay Agent icon.
• 3. Select Properties from the Action menu to
  display the DHCP Relay Agent Properties dialog
  box.
• 4. Type the IP address of the DHCP server you
  want the relay agent to use in the Server Address
  box, and then click Add.
• 5. Repeat step 4 to add additional DHCP servers
  to the list, and then click OK.

30
The DNS Tab of a DHCP Server's Properties Dialog
Box
31
Preventing DHCP Problems

• Use the 80/20 design rule for balancing scope
  distribution of addresses where multiple DHCP
  servers are used to service the same scope.
• Use server-side conflict detection on DHCP
  servers only when it is needed.
• Create reservations on all DHCP servers that can
  potentially service the reserved client.
• DHCP is disk-intensive purchase hardware with
  optimal disk performance characteristics.
• Keep audit logging enabled for use in
  troubleshooting.

32
Troubleshooting DHCP Clients

• Most DHCP-related problems start as a failed IP
  configuration attempt at a client, so it is good
  practice to start there.
• If a DHCP-related problem does not originate at
  the client, check the system event log and DHCP
  server audit logs for clues.

33
Troubleshooting DHCP Servers

• When a DHCP server fails to provide leases to its
  clients, clients often discover the failure in
  one of the following ways
• The client might be configured to use an IP
  address not provided by the server.
• The server sends a negative response to the
  client, and the client displays an error message
  indicating that a DHCP server could not be found.
  
• The server leases an address to the client but
  the client appears to have other network
  configurationbased problems.