DHCPNAK

DHCPNAK stands for DHCP Negative Acknowledgement. It is a DHCP message, type 6, sent by a DHCP server to indicate that a client's request for an IP address or configuration cannot be granted. It is used in the DHCP exchange after a client has sent a DHCPREQUEST, typically when the requested IP address is unavailable, not valid for the client's subnet, or no longer matches the server's records (for example if the lease has expired or the client identifier does not match the server's data). A DHCPNAK signals that the current parameters cannot be used and the client must start the process over.

In normal operation, DHCP clients proceed through a four-step process: DISCOVER, OFFER, REQUEST, ACK. If the REQUEST cannot be satisfied, the server responds with a NAK instead of an ACK. The DHCPNAK usually includes the server identifier (option 54) to identify the responding server, but it does not convey a lease; the client must start a new exchange to obtain a new lease.

On receiving a DHCPNAK, the client should abandon its current IP configuration, return to the INIT state, and send a new DHCPDISCOVER to obtain a valid lease on a suitable IP address. DHCPNAKs help enforce consistency between the server's address pool and the client's requested parameters and can occur in situations such as network reconfiguration, subnet changes, or stale state on the client.