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Thread: Wake-on-Lan and PC Power Use.

  1. #1
    Flower Child stytagm's Avatar
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    Wake-on-Lan and PC Power Use.

    Hi guys, I'm just wondering weather you can set up a workstation to wake on Lan triggered from a VPN?

    Eh? I hear you ask. At work we tend to leave our computers on 24/7 incase we need to log in from home to do some work, either routine or because a problem's come up (we have to support our client's sites 24/7).

    But of course, most of the time they're just sucking down electricity for no good reason, so I was wondering about weather you can trigger a computer to wake on lan, having logged in via a vpn?

    Also how much electricity/money might you save? Our Boss is very much driven by the bottom line, so If I can prove he'll save a worthwhile amount of money, he'll enforce a policy.

    Once upon a time, someone touted around a figure of 30pence / 24 hours as the cost of a running computer, but that was when we all used 200w psu's I think, so does anyone know how much they use today?

    Also how much power does a computer on standby use? What kind of state does a mobo need to be left in so that wake-on-lan will work? Does anyone have any experience of this?

    Cheers - Andrew.
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    Ah, Mrs. Peel! mike_w's Avatar
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    For wake-on-LAN to work, the motherboard just needs power i.e. it just needs to have the power cord plugged in. At least, that's what it has needed in my experience (along with the MAC address).
    "Well, there was your Uncle Tiberius who died wrapped in cabbage leaves but we assumed that was a freak accident."

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    Seething Cauldron of Hatred TheAnimus's Avatar
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    Indeed the system needs power, but can be switched off, the PSU however is still on, and often kicking out about 2.5Amps at 5v.

    In practice enless you've got a load of USB junk plugged in a PC will use about 5-10W when its switched off, but left on (ATX only this).

    WOL (wake on LAN) requires that you send a specail packet to its MAC address, not its IP address (enless you have that stored in you 'arp cache') the result of this is you can't reliably send them via an internetwork, that is to say you must be on the same physical network, with no active routing going on (you don't need to go via the gateway to talk to the computer).

    Harvesting all the MAC addresses would be trival, you could even do it via a broadcast packet.

    When your logged in via a VPN you'll have to let the computer know which interface you intend to send the WOL packet out on, this is not that straightfoward, and would probably be easyest to send it out on each interface instead!

    which OS are you using for the VPN?
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