Network: DHCP DORA process

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The DHCP (Dynamic Host Configuration Protocol) DORA process is a series of steps used by a DHCP client to obtain network configuration information from a DHCP server. “DORA” stands for Discover, Offer, Request, and Acknowledge. Here’s an explanation of each step:

  1. Discover (D):
    • In the Discover step, the DHCP client broadcasts a DHCP Discover message to locate available DHCP servers on the network.
    • The Discover message is sent as a broadcast packet with the destination IP address set to 255.255.255.255 and the destination MAC address set to ff:ff:ff:ff:ff:ff.
    • The Discover message includes the client’s hardware (MAC) address, identifying itself to potential DHCP servers.
    • The DHCP Discover message may also include optional parameters requested by the client, such as subnet mask, default gateway, DNS server, etc.
    • The client waits for DHCP Offer messages from available DHCP servers.
  2. Offer (O):
    • Upon receiving the DHCP Discover message, DHCP servers on the network respond with DHCP Offer messages.
    • Each DHCP server that receives the Discover message checks its available IP address pool and configuration settings to determine if it can fulfill the client’s request.
    • A DHCP Offer message includes an available IP address (leased from the server’s pool), subnet mask, lease duration, default gateway, DNS server, and any other configuration options requested by the client.
    • The DHCP Offer message is unicast to the client’s MAC address, as indicated in the Discover message.
    • If multiple DHCP servers respond with Offer messages, the client typically selects the first Offer it receives, although it may evaluate Offers based on other criteria such as lease duration or server preference.
  3. Request (R):
    • Upon receiving one or more DHCP Offer messages, the client selects an Offer and broadcasts a DHCP Request message to the DHCP servers.
    • The Request message confirms the selection of a specific DHCP server’s Offer and requests allocation of the offered IP address and associated configuration parameters.
    • If the client received multiple Offer messages, it may include the IP address of the chosen server in the Request message to ensure that the server knows it has been selected.
    • The Request message also serves as notification to other DHCP servers that their Offers were not accepted.
  4. Acknowledge (A):
    • After receiving the DHCP Request message, the DHCP server that made the Offer sends a DHCP Acknowledge (ACK) message to the client.
    • The Acknowledge message confirms the allocation of the requested IP address and provides the client with the lease duration and any other configuration parameters.
    • The Acknowledge message is unicast to the client’s MAC address.
    • Upon receiving the Acknowledge message, the client completes the configuration process, configures its network interface with the allocated IP address and other parameters, and begins using the network.

Overall, the DHCP DORA process allows DHCP clients to dynamically obtain network configuration information from DHCP servers, simplifying the process of network configuration and management in IP-based networks.

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