DHCP DORA process

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DHCP service works by using the DORA (Discover, Offer, Request and Acknowledgment) process (you can trace on the whole process using a network monitor utility):

1.      DHCPDISCOVER

2.      DHCPOFFER 

3.      DHCPREQUEST

4.   DHCPACK 

DHCPNACK: The DHCPNACK or Negative Acknowledgment is a packet that the server sends if the IP address is not available in stand of DHCPACK (in use on other client for example) or the address is no longer valid.

In case of DHCPNACK the client must restart the lease process in order to get an IP address.

Figure 3-4 DORA process

Note: DHCP service uses port 67/UDP in the DHCP server, and 68/UDP at the DHCP clients. It's recommended to check that your firewall doesn't block these ports in order to able the DHCP server and clients to communicate.

Description of the communication steps

1.      The client broadcasts a DHCPDISCOVER.

2.      Each server may respond with a DHCPOFFER message.

3.      The client receives one or more DHCPOFFER messages from one or more servers and chooses one server from which to request configuration parameters.

The client broadcasts a DHCPREQUEST message.

4.      Those servers not selected by the DHCPREQUEST message use the message as notification that the client has declined that server's offer. The server selected in the DHCPREQUEST message commits the responds with a DHCPACK message containing the configuration parameters for the requesting client.

5.      The client receives the DHCPACK message with configuration parameters. At this point, the client is configured. If the client receives a DHCPNAK message, the client restarts the configuration process.

6.      The client may choose to relinquish its lease on a network address by sending a DHCPRELEASE message to the server (e.g. on shutdown).

7.      The server receives the DHCPRELEASE message and marks the lease as free.