DHCP Message Types

Today I am going to talk about the DHCP message types and the meaning of all the requests sent by server and client in DHCP process. The messages are shown below

Messages Types

• DHCPDISCOVER 
• DHCPOFFER 
• DHCPREQUEST 
• DHCPDECLINE 
• DHCPACK 
• DHCPNAK 
• DHCPRELEASE 
• DHCPINFORM 

Fig 1.1- Sample DHCP request

Fig1.2 - Sample DHCP Response

DHCPDISCOVER: Broadcast by a client to find available DHCP servers. 

DHCPOFFER: Response from a server to a DHCPDISCOVER and offering IP address and other parameters. 

DHCPREQUEST: Message from a client to servers that does one of the following: 

• Requests the parameters offered by one of the servers and declines all other offers. 
• Verifies a previously allocated address after a system or network change (a reboot for example). 
• Requests the extension of a lease on a particular address. 

DHCPACK: Acknowledgement from server to client with parameters, including IP address. 

DHCPNACK: Negative acknowledgement from server to client, indicating that the client's lease has expired or that a requested IP address is incorrect. 

DHCPDECLINE: Message from client to server indicating that the offered address is already in use. 

DHCPRELEASE: Message from client to server canceling remainder of a lease and relinquishing network address. 

DHCPINFORM: Message from a client that already has an IP address (manually configured for example), requesting further configuration parameters from the DHCP server. 

Client-Server Interactions

• The client broadcasts a DHCPDISCOVER message on its local physical subnet. 
• The DHCPDISCOVER message may include some options such as network address suggestion or lease duration. 
• Each server may respond with a DHCPOFFER message that includes an available network address (your IP address) and other configuration options. 
• The servers record the address as offered to the client to prevent the same address being offered to other clients in the event of further DHCPDISCOVER messages being received before the first client has completed its configuration. 
• If the client receives one or more DHCPOFFER messages from one or more servers. 
• The client chooses one based on the configuration parameters offered and broadcasts a DHCPREQUEST message that includes the server identifier option to indicate which message it has selected and the requested IP address option, taken from your IP address in the selected offer. 
• In the event that no offers are received, if the client has knowledge of a previous network address, the client may reuse that address if its lease is still valid, until the lease expires.