Wake-on-LAN – How to Get It Working

networkingwake-on-lan

I have a PC behind a router with a dynamic IP address that I want to configure for wake-on-LAN.

How do I send the magic packet to the PC? Do I configure my router to forward UDP port 6 to the PC? How would I send the packet from another PC on the network? How about a PC outside the network?

Is this even possible?

Update: here is a pertinent configuration screen from my router. Any hope?

http://files.quickmediasolutions.com/router.png

Best Answer

Static ARP is completely unnecessary

You've got the first part right. To get the WOL packet into the network, forward the packet coming from a specific port to the broadcast address of the local network (255.255.255.255) or whatever subnet range you want WOL access to be enabled on.

The Destination MAC address of the packet should be set to ethernet broadcast or FF:FF:FF:FF:FF:FF. The ethernet type of the packet should be 0x0842 (Wake On Lan).

So, where does the MAC of the computer being woken up go?

In the magic packet itself. WOL packets were only really intended to be sent across a local network. They're blind to any protocol above the link layer. To get around this limitation, they broadcast (ethernet broadcast not IP broadcast) to all the computers on the network and each computer reads the magic packet to see if they're the one being called on.

The contents of the magic packet contain 16 copies of the MAC address of the computer being woken up.

The frame structure is as follows:

DA -> SA -> Type -> Magic Packet

Where:

DA = FF:FF:FF:FF:FF:FF
SA = [whatever the source MAC is]
Type = 0x0842
Magic = [The actual destination MAC repeated 16x]

If you want to test that the packets coming across are in the right format, use the following filter in Wireshark:

ether dst FF:FF:FF:FF:FF:FF and ether proto 0x0842

Basically, the WOL application needs to be capable of creating a packet that spoofs the Ethernet Destination address. There are tools online that can do such a thing but I'm not familiar with them.

Note: The reason I know so much about this is because I'm the author of the WOL parser for SharpPcap (pcap wrapper in C#). If there is sufficient demand, I could extend my console application to include packet sending (it currently only sniffs) and make it available as an OSS project.

Update: @Evan Anderson made a good point that I forgot to mention. Broadcasting incoming packets on a LAN is generally a bad idea. This solution will work but it's only a hack to circumvent the limitations of the Wake On Lan protocol.

The technique I've outlined will work for any computer on the LAN the way WOL was designed but could potentially open your network up to be used for as an attack (Smurf/Fraggle/Papasmurf) amplifier if someone were to send a specially crafted packet to the WOL port.

Evan Anderson's approach is technically more secure but is limited to unicast.

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