Workflow of the DHCP server

The use of DHCP servers is reflected in many networks. As an administrator, We need to master its construction, configuration, management, and maintenance. Before that, let's take a look at the DHCP service workflow.

The DHCP Service works like this:

1. Discovery stage,That is, the DHCP client is looking for a DHCP server. The DHCP Client sends DHCP discover discovery information to find the DHCP server in broadcast mode because the IP address of the DHCP server is unknown to the client. That is, it sends specific broadcast information to the IP address 255.255.255.255. Every host installed with TCP/IP protocol on the network will receive this broadcast information, but only the DHCP server will respond 1 ).

2. Provision stage,That is, the IP address provided by the DHCP server. The DHCP server that receives the DHCP discover discovery information in the network will respond, it selects a DHCP client from the IP address that has not been rented, send a DHCP offer containing the IP address for rent and other settings to the DHCP client. 2 ).

3. Select a stage,That is, the DHCP Client selects the IP address provided by a DHCP server. If Multiple DHCP servers provide information to DHCP offer, the DHCP client accepts only the information provided by the first DHCP offer, then, it answers a DHCP request information in broadcast mode, which contains the content that requests IP addresses from the selected DHCP server. The answer should be broadcast in order to notify all DHCP servers that they will select the IP address provided by a DHCP server 3 ).

 

4. confirmation stage,That is, the DHCP server confirms the stage of the provided IP address. When the DHCP server receives the DHCP request from the DHCP client, it sends a DHCP ack confirmation message containing the IP address provided by the DHCP Client and other settings to the DHCP client, the IP address provided by the DHCP client. Then, the DHCP client binds its TCP/IP protocol to the NIC. In addition, all other DHCP servers except the selected server of the DHCP client will reclaim the previously provided IP address 4 ).

5. Log On again,In the future, each time a DHCP client logs on to the network again, it does not need to send the DHCP discover discovery information, but directly sends the DHCP request information containing the previous IP address. When the DHCP server receives this information, it will attempt to allow the DHCP client to continue using the original IP address and answer a DHCP ack confirmation message. If the IP address cannot be assigned to the original DHCP client, then the DHCP server returns a DHCP nack denial message to the DHCP client. When the original DHCP client receives the DHCP nack denied information, it must resend the DHCP discover information to request a new IP address.

6. Update the lease,The IP address rented by the DHCP server to the DHCP Client generally has a lease term. After the lease term expires, the DHCP server will reclaim the IP address. If the DHCP Client wants to extend its IP lease, it must update its IP lease. When a DHCP Client is started and half of the IP Lease Term expires, the DHCP Client automatically sends an update message to the DHCP server.

For ease of understanding, we compare the DHCP client to a guest in a restaurant, and the DHCP server can have multiple waiters in a restaurant). The IP address is better than the food the customer needs. The whole process can be described as follows: When a guest walks into a restaurant, he asks, "Is there a waiter ?" DHCP discover), multiple waiters also replied: "Yes, I have chicken wings" "Yes, I have a hamburger" DHCP offer ). The guest said, "Okay, I want a hamburger." DHCP request, this guest is rigid and always chooses the food he heard for the first time.) The waiter with the hamburger responded: "Come" DHCP ack), and bring the food to the guests for them to enjoy and bind the NIC and IP address ). When the next time you come, you can directly ask the waiter to order a DHCP request for his favorite hamburger.) If there is a hamburger, the waiter will confirm it again and pick up DHCP ack ), if you have sold out, the waiter will tell the guest: "Sorry, you have sold out" DHCP nack ). Of course, the waiter will clean up the table every other time, unless the guests specifically state that the food will continue to eat, the waiter will take the leftovers away.