DHCPRelease

DHCPRELEASE is a DHCP message used by a client to release a previously allocated IP address back to the DHCP server. Defined for IPv4 in RFC 2131, the message type is DHCPDISCOVER, OFFER, REQUEST, ACK, NAK, and RELEASE, with RELEASE being the seventh message type. The primary purpose of DHCPRELEASE is to inform the server that the client no longer needs the lease, allowing the server to return the address to the pool for reallocation.

In a DHCPRELEASE, the client typically sends a BOOTREQUEST (op=1) with the ciaddr field set to the IP address being released. The server identifier option (54) may be included to indicate the specific server the release targets, and the client identifier option (82) can help identify the client. The yiaddr field is not used in a release. Unlike other DHCP messages, a RELEASE may not require a formal acknowledgment from the server; some implementations do not send a DHCPACK in response, though a server may provide a brief acknowledgment or simply act on the release and free the lease.

Transmission generally uses UDP from client port 68 to server port 67. If the client cannot determine the server address, the release may be broadcast. Releasing a lease helps conserve address space on the DHCP server and prevents stale bindings when a device leaves the network or shuts down.

Common implementations include client commands such as dhclient -r on Unix-like systems and ipconfig /release on Windows.

---