4.3.2 DHCPREQUEST message

A DHCPREQUEST message may come from a client responding to a DHCPOFFER message from a server, from a client verifying a previously allocated IP address or from a client extending the lease on a network address. If the DHCPREQUEST message contains a 'server identifier' option, the message is in response to a DHCPOFFER message. Otherwise, the message is a request to verify or extend an existing lease. If the client uses a 'client identifier' in a DHCPREQUEST message, it MUST use that same 'client identifier' in all subsequent messages. If the client included a list of requested parameters in a DHCPDISCOVER message, it MUST include that list in all subsequent messages.

Any configuration parameters in the DHCPACK message SHOULD NOT conflict with those in the earlier DHCPOFFER message to which the client is responding. The client SHOULD use the parameters in the DHCPACK message for configuration.

Clients send DHCPREQUEST messages as follows:

• DHCPREQUEST generated during SELECTING state:

  Client inserts the address of the selected server in 'server identifier', 'ciaddr' MUST be zero, 'requested IP address' MUST be filled in with the yiaddr value from the chosen DHCPOFFER.

  Note that the client may choose to collect several DHCPOFFER messages and select the "best" offer. The client indicates its selection by identifying the offering server in the DHCPREQUEST message. If the client receives no acceptable offers, the client may choose to try another DHCPDISCOVER message. Therefore, the servers may not receive a specific DHCPREQUEST from which they can decide whether or not the client has accepted the offer. Because the servers have not committed any network address assignments on the basis of a DHCPOFFER, servers are free to reuse offered network addresses in response to subsequent requests. As an implementation detail, servers SHOULD NOT reuse offered addresses and may use an implementation-specific timeout mechanism to decide when to reuse an offered address.

• DHCPREQUEST generated during INIT-REBOOT state:

  'server identifier' MUST NOT be filled in, 'requested IP address' option MUST be filled in with client's notion of its previously assigned address. 'ciaddr' MUST be zero. The client is seeking to verify a previously allocated, cached configuration. Server SHOULD send a DHCPNAK message to the client if the 'requested IP address' is incorrect, or is on the wrong network.

  Determining whether a client in the INIT-REBOOT state is on the correct network is done by examining the contents of 'giaddr', the 'requested IP address' option, and a database lookup. If the DHCP server detects that the client is on the wrong net (i.e., the result of applying the local subnet mask or remote subnet mask (if 'giaddr' is not zero) to 'requested IP address' option value doesn't match reality), then the server SHOULD send a DHCPNAK message to the client.

  If the network is correct, then the DHCP server should check if the client's notion of its IP address is correct. If not, then the server SHOULD send a DHCPNAK message to the client. If the DHCP server has no record of this client, then it MUST remain silent, and MAY output a warning to the network administrator. This behavior is necessary for peaceful coexistence of non- communicating DHCP servers on the same wire.

  If 'giaddr' is 0x0 in the DHCPREQUEST message, the client is on the same subnet as the server. The server MUST broadcast the DHCPNAK message to the 0xffffffff broadcast address because the client may not have a correct network address or subnet mask, and the client may not be answering ARP requests.

  If 'giaddr' is set in the DHCPREQUEST message, the client is on a different subnet. The server MUST set the broadcast bit in the DHCPNAK, so that the relay agent will broadcast the DHCPNAK to the client, because the client may not have a correct network address or subnet mask, and the client may not be answering ARP requests.

• DHCPREQUEST generated during RENEWING state:

  'server identifier' MUST NOT be filled in, 'requested IP address' option MUST NOT be filled in, 'ciaddr' MUST be filled in with client's IP address. In this situation, the client is completely configured, and is trying to extend its lease. This message will be unicast, so no relay agents will be involved in its transmission. Because 'giaddr' is therefore not filled in, the DHCP server will trust the value in 'ciaddr', and use it when replying to the client.

  A client MAY choose to renew or extend its lease prior to T1. The server may choose not to extend the lease (as a policy decision by the network administrator), but should return a DHCPACK message regardless.

• DHCPREQUEST generated during REBINDING state:

  'server identifier' MUST NOT be filled in, 'requested IP address' option MUST NOT be filled in, 'ciaddr' MUST be filled in with client's IP address. In this situation, the client is completely configured, and is trying to extend its lease. This message MUST be broadcast to the 0xffffffff IP broadcast address. The DHCP server SHOULD check 'ciaddr' for correctness before replying to the DHCPREQUEST.

  The DHCPREQUEST from a REBINDING client is intended to accommodate sites that have multiple DHCP servers and a mechanism for maintaining consistency among leases managed by multiple servers. A DHCP server MAY extend a client's lease only if it has local administrative authority to do so.