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KaliLinux Common Service configuration tutorial DHCP Service Workflow

The DHCP service workflow is shown in figure 1.1.

The specific workflow is as follows:

(1) the DHCP client sends out DHCP Discover messages by broadcasting.

(2) all DHCP servers (DHCP Server-A, ServerB) can receive DHCP Discover messages sent by DHCP clients. All DHCP servers respond by sending a DHCP Offer message to the DHCP client. The "Your (Client) IP Address" field in the DHCP Offer message is the IP address that the DHCP server can provide to the DHCP client, and the DHCP server will put its own IP address in the "option" field so that the DHCP client can distinguish between different DHCP servers. The DHCP server will have a record of an assigned IP address after sending this message.

(3) the DHCP client can only process one of the DHCP Offer messages. The general rule is that DHCP Client handles the first DHCP Offer messages received. The DHCP client sends out a broadcast DHCP Request message, adding the IP address of the selected DHCP server and the required IP address in the options field.

(4) after receiving the DHCP Request message, the DHCP server determines whether the IP address in the option field is the same as its own address. If different, the DHCP server (such as Server-A) clears the corresponding IP address allocation record without doing any processing; if it is the same, the DHCP server (such as Server-B) responds to a DHCP ACK message to the DHCP client and adds the lease information of the IP address in the option field.

(5) after receiving the DHCP ACK message, the DHCP client checks whether the IP address assigned by the DHCP server can be used. If it can be used, the DHCP client successfully obtains the IP address and automatically starts the renewal process based on the IP address; if the DHCP client finds that the assigned IP address has been used, the DHCP client sends a DHCP Decline message to the DHCP server, informing the DHCP server to disable the IP address, and then the DHCP client starts a new address application process.

(6) after successfully obtaining the IP address, the DHCP client can release its IP address at any time by sending DHCP Release messages. When the DHCP server receives the DHCP Release message, it reclaims the corresponding IP address and reassigns it.

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