Apple patents DHCP hack for Wake-on-WAN
Apple has received another patent relating to waking sleeping devices on a network, this time for waking devices on a remote LAN:
In a sleep state, a system can typically receive packets of lower-layer protocols, such as Ethernet, but cannot respond to ARP (Address Resolution Protocol) requests. Unfortunately, being able to respond to ARP packets is often necessary for receiving packets of upper-layer protocols, such as IP (Internet Protocol). Hence, the wake-up packet is generally sent using a lower-layer protocol, such as Ethernet, instead of using an upper-layer protocol, such as IP. Unfortunately, such lower-layer protocols can only be used for sending packets within a LAN.
— Description, U.S. Patent 7,447,927, November 4, 2008
Apple’s solution cleverly exploits the existing DHCP protocol to allow it to work on existing devices, much like Bonjour cleverly exploits DNS:
Specifically, one embodiment of the present invention exploits the following properties of DHCP: (a) most routers in a network are DHCP relay agents, (b) DHCP packets can carry a user-defined payload, and (c) DHCP allows a remote system to specify a hardware address which the relay agent uses to send a DHCP packet to the target system.
— Description, U.S. Patent 7,447,927, November 4, 2008
A previous patent relating to waking sleeping devices on a network involved sleep proxies.