DHCP principles in Linux

DHCP Principle

DHCP Lease Four Steps

A: Client makes IP request

When a DHCP client starts, it will automatically configure its IP address to 0.0.0.0. Since normal communication cannot be performed using 0.0.0.0, the client must obtain a legal address through the DHCP server. Since the client does not know the IP address of the DHCP server, it uses the address of 0.0.0.0 as the source address, UDP68 port as the source port, 255.255.255.255 as the destination address, and UDP67 port as the destination port to broadcast the requested IP address information. . The broadcast information contains the MAC address and computer name of the DHCP client so that the DHCP server can determine which client sent the request.

B: Server response

When the DHCP server receives the information from the client requesting an IP address, it searches its own IP address pool to see if there is a legal IP address to provide to the client. If so, the DHCP server marks the IP address and adds it to the DHCPOFFER message. Then the DHCP server broadcasts a DHCPOFFER message that includes the following information: the MAC address of the DHCP client; the legal IP address provided by the DHCP server; Subnet mask; default gateway (routing); lease period; IP address of the DHCP server. Because the DHCP client does not yet have an IP address, the DHCP server uses its own IP address as the source address, UDP67 port as the source port, 255.255.255.255 as the destination address, and UDP68 port as the destination port to broadcast the DHCPOFFER message.

C: Client selects IP

The DHCP client selects an IP address from the first DHCPOFFER message received, and the DHCP server that issued the IP address reserves the address so that the address cannot be provided to another DHCP client. The third phase of the DHCP lease occurs after the client receives the DHCPOFFER from the first DHCP server and selects an IP address. The client broadcasts a DHCPREQUEST message to all DHCP servers to indicate that it accepts the offer. The DHCPREQUEST message includes the service identifier (IP address) of the server that provided the IP configuration for this client. The DHCP server looks at the Server Identifier field to determine if it itself has been selected to provide IP addresses to the specified clients. If those DHCPOFFERs are rejected, the DHCP server unoffers and reserves their IP addresses for the next IP lease request.

During the client's IP selection process, although the client has selected an IP address, it has not yet configured an IP address. There may be several DHCP servers in a network, so the client still uses the address of 0.0.0.0 as Source address, use UDP68 port as the source port, use 255.255.255.255 as the destination address, use UDP67 port as the destination port to broadcast DHCPREQUEST information.

D: Server confirms lease

After receiving the DHCPREQUEST message, the DHCP server broadcasts a successful confirmation to the client in the form of a DHCPACK message. This message contains the valid lease of the IP address and other possible configuration information. Although the server confirmed the client's lease request, the client has not received the server's DHCPACK message, so the server still uses its own IP address as the source address, UDP67 port as the source port, and 255.255.255.255 as the destination address, using UDP68 port serves as the destination port to broadcast DHCPACK information. When the client receives the DHCPACK message, it configures an IP address and completes TCP/IP initialization.

Server confirms lease: DHCPNACK (DHCPNAK)

If DHCPREQUEST is unsuccessful, for example, the client attempts to lease the previous IP address but the IP address is no longer available, or the IP is invalid because the client moved to another subnet, the DHCP server will broadcast a negative confirmation message DHCPNACK. When the client receives an unsuccessful acknowledgment, it restarts the DHCP lease process.

DHCP lease renewal

The DHCP client will send a DHCPREQUEST message packet directly to the DHCP server that provides it with an IP address when 50% of the lease period has expired. If the client receives the DHCPACK message packet responded by the server, the client updates its configuration based on the new lease period and other updated TCP/IP parameters provided in the packet, and the IP lease update is completed. If no reply is received from the server, the client continues to use the existing IP address because 50% of the current lease period remains.

If there is no update when 50% of the lease period has passed, the DHCP client will contact the DHCP server that provided it with the IP address again when 75% of the lease period has passed. If it is still unsuccessful, when the lease reaches 100%, the DHCP client must give up the IP address and apply again. If no DHCP server is available at this time, the DHCP client will use a random address in 169.254.0.0/16 and try again every 5 minutes.

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