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  1. Introduction
  2. OSI Layers
  3. Addresses
  4. SNMP
  5. Network Tools
  6. Routing
  7. ARP
  8. WINS
  9. DNS
  10. NIS
  11. DHCP
  12. Name Resolution
  13. RAS
  14. Connections
  15. Test Pointers
  16. Items to remember
  17. Terms
  18. Credits

DHCP

DHCP RFCs are 1533, 1534, 1541, and 1542. Sent from DHCP server:

  • IP address
  • Netmask
  • Default Gateway address
  • DNS server addresse(s)
  • NetBIOS Name server (NBNS) address(es). (On Windows servers)
  • Lease period in hours
  • IP address of DHCP server.

Lease Stages:

  1. Lease Request - The client sends a broadcast requesting an IP address
  2. Lease Offer - The server sends the above information and marks the offered address as unavailable. The message sent is a DHCPOFFER broadcast message.
  3. Lease Acceptance - The first offer received by the client is accepted. The acceptance is sent from the client as a broadcast (DHCPREQUEST message) including the IP address of the DNS server that sent the accepted offer. Other DHCP servers retract their offers and mark the offered address as available and the accepted address as unavailable.
  4. Server lease acknowledgement - The server sends a DHCPACK or a DHCPNACK if an unavailable address was requested.

DHCP discover message - The initial broadcast sent by the client to obtain a DHCP lease. It contains the client MAC address and computer name. This is a broadcast using 255.255.255.255 as the destination address and 0.0.0.0 as the source address. The request is sent, then the client waits one second for an offer. The request is repeated at 9, 13, and 16 second intervals with additional 0 to 1000 milliseconds of randomness. The attempt is repeated every 5 minutes thereafter. The client uses port 67 and the server uses port 68.

DHCP Lease Renewal

After 50% of the lease time has passed, the client will attempt to renew the lease with the original DHCP server that it obtained the lease from using a DHCPREQUEST message. Any time the client boots and the lease is 50% or more passed, the client will attempt to renew the lease. At 87.5% of the lease completion, the client will attempt to contact any DHCP server for a new lease. If the lease expires, the client will send a request as in the initial boot when the client had no IP address. If this fails, the client TCP/IP stack will cease functioning.

DHCP File information storage:

  • WFW - \windows\DHCP.BIN (encrypted)
  • WinNT - HKey_Local_Machine\System\CurrentControlSet\Services\Adapter\Parameter\Tcpip

DHCP Scope and Subnets

One DHCP scope is required for each subnet.

DHCP Relay Agents

May be placed in two places:

  • Routers
  • Subnets that don't have a DHCP server to forward DHCP requests.

Client Reservation

Client Reservation is used to be sure a computer gets the same IP address all the time. Therefore since DHCP IP address assignments use MAC addresses to control assignments, the following are required for client reservation:

  • MAC (hardware) address
  • IP address

Exclusion Range

Exclusion range is used to reserve a bank of IP addresses so computers with static IP addresses, such as servers may use the assigned addresses in this range. These addresses are not assigned by the DHCP server.

DHCP and WINS

To use WINS the DHCP server must specify:

  • WINS server IP address.
  • NetBIOS resolution mode (B, P, N, or H - node).

DHCP backup interval

Configured in the registry at:

\hkey\local_machine\system\currentcontrolset\services\dhcp\server\parameters