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Wake on LAN (WOL, sometimes WoL) is an Ethernet computer networking standard that allows a computer to be turned on or woken up remotely by a network message sent usually by a simple program executed on another computer on the network.
[edit] Technical details[edit] System requirementsWake on LAN (WoL) support is implemented on the motherboard of a computer and the network interface, and as such, is not dependent on the operating system running on the hardware, although the operating system can sometimes control the WoL behaviour. If the network interface is a plug-in card rather than being integrated into the motherboard, the card may need to be connected to the motherboard by a cable. Motherboards with an embedded Ethernet controller which supports WoL do not need a cable. [edit] Intel/AMD based PCOlder motherboards must have a WAKEUP-LINK header onboard connected to the network card via a special 3-pin cable; however, systems supporting the PCI 2.2 standard and with a PCI 2.2 compliant network adapter card do not usually require a WoL cable as the required standby power is relayed through the PCI bus. PCI version 2.2 supports PME (Power Management Events). PCI cards send and receive PME signals via the PCI socket directly, without the need for a WOL cable.[1] Wake on LAN must be enabled in the Power Management section of a PC motherboard's BIOS setup utility. It may also be necessary to configure the computer to reserve power for the network card when the system is shut down. In addition, in order to get WoL to work it is sometimes required to enable this feature on the interface card. Details of how to do this depend upon the operating system and the device driver. Laptops powered by the Intel 3945 chipset or newer (with explicit BIOS support) allow waking up the machine using wireless network connection (802.11 protocol). This is called Wake on Wireless LAN (WoWLAN).[2] [edit] Mac hardware (OS X)Modern Mac hardware features integrated WoL functionality, controlled via the OS X System Preferences Energy Saver panel, in the Options tab. Marking the "Wake for Ethernet network administrator access" checkbox enables WoL. Apple's Apple Remote Desktop client management system can be used to send WoL packets, but there are also freeware and shareware Mac OS X applications available. [edit] How it worksWake-on-LAN is platform-independent, so any application on any platform that sends Magic Packets can wake up computers running on any platform. It is not restricted to LAN (Local area network) traffic. The computer to be woken is shut down (Sleeping, Hibernating or Soft Off, i.e. ACPI state G1 or G2), with power reserved for the network card, but not disconnected from its power source. The network card listens for a specific packet containing its MAC address, called the "Magic Packet," broadcast on the broadcast address for that particular subnet (or an entire LAN, though this requires special hardware or configuration). The magic packet is sent on the data link or OSI-2 layer and broadcast to all NICs within the network of the broadcast address; the IP-address (OSI-3 layer) is not used. When the listening computer receives this packet, the network card checks the packet for the correct information. If the Magic Packet is valid, the network card takes the computer out of hibernation or standby, or starts it up. In order for Wake on LAN to work, parts of the network interface need to stay on. This consumes a standby power, small compared to the computer's normal operating power. If Wake on LAN is not needed, disabling it may reduce power consumption while the computer is switched off but still plugged in.[3] [edit] Magic PacketThe Magic Packet is a broadcast frame containing anywhere within its payload 6 bytes of ones (resulting in hexadecimal FF FF FF FF FF FF) followed by sixteen repetitions of the target computer's MAC address. Since the Magic Packet is only scanned for the string above, and not actually parsed by a full protocol stack, it may be sent as a broadcast packet of any network- and transport-layer protocol. It is typically sent as a UDP datagram to port 0, 7 or 9, or, in former times, as an IPX packet. [edit] Sending the Magic PacketAll software and services mentioned in this section are some of the many available, and are not particularly recommended above any other. Many web sites allow a magic packet to be sent to a specified MAC address without charge[4][5]. Simple dedicated programs and scripts have been written to send magic packets to a specified Ethernet adapter, either locally or over the Internet. Many can be downloaded free of charge. Free WoL programs for Microsoft Windows GUI, Windows command line, ASP, _vbscript_, scripting COM, and Pocket PC are available here amongst other places; for Apple Macintosh here; and executable programs, C source code and Perl and Python scripts for Linux, Unix, NetWare, Win32, OS/2, Mac OS X, and BeOS 5 here. It is also possible, of course, for any script or program which has access to the target LAN, locally or over the Internet, to include code to wake up a machine, perhaps to make available a resource supplied by a machine which may be switched off at the time. Source code for a developer to add WoL to a program is readily available in many computer languages[6]. As a concrete example (which is not intended to endorse or recommend Microsoft, Depicus, or OpenOffice.org software over any other), a networked machine running Microsoft Windows and with a copy of the Depicus command-line program wolcmd.exe or equivalent can access resources on a machine called Server connected to the LAN and with MAC address 01:23:45:67:89:AB by invoking a Windows script file called, say, WakeRun.CMD either directly by the user or from within a program:
wolcmd 0123456789AB 255.255.255.255 255.255.255.255 7 This will wake up machine Server, then open a file on it called \Letters\Contract.odt in a local word processor. [edit] SecurityMagic packets are sent via the data link or OSI-2 layer, which is not secure and can be used or abused by anyone on the same LAN. Firewalls may prevent clients within the public WAN from accessing the broadcast address of the private LAN. Certain NICs support a security feature called "SecureOn". It allows users to store within the NIC a hexadecimal password of 6 bytes. Clients have to append this password to the magic packet. The NIC wakes the system only if the MAC address and password are correct. This security measure significantly decreases the risk of successful brute force attacks: Still, only a few NIC and router manufacturers seem to support such security features. Abuse of the WoL feature only allows computers to be switched on; it does not in itself bypass password and other forms of security. [edit] TLS Encryption for WOLSome PCs include technology built into the chipset to improve security for WOL. For example, Intel AMT (a component of Intel vPro technology), includes Transport Layer Security (TLS), an industry-standard protocol that strengthens encryption.[7] AMT uses TLS encryption to secure an out-of-band communication tunnel to an AMT-based PC for remote management commands such as WOL. AMT secures the communication tunnel Advanced Encryption Standard (AES) 128-bit encryption and RSA keys with modulus lengths of 2048 bits.[8][9] Because the encrypted communication is out-of-band, the PC’s hardware and firmware receive the magic packet before network traffic reaches the software stack for the operating system (OS). Since the encrypted communication occurs “below” the OS level, it is less vulnerable to attacks by viruses, worms, and other threats that typically target the OS level.[10] IT shops using WOL through the Intel AMT implementation can wake an AMT PC over network environments that require TLS-based security, such as IEEE 802.1x, Cisco Self Defending Network (SDN), and Microsoft Network Access Protection (NAP) environments.[10] The Intel implementation also works for wireless networks.[10] [edit] Other machine states and LAN wakeup signalsIn the early days of Wake On LAN the situation was relatively simple: a machine was connected to power but switched off, and it was arranged that a special packet be sent to switch the machine on. Since then many options have been added and standards agreed. A machine can be in 7 power states from S0 (fully on) through S5 (powered down but plugged in) and disconnected from power, with names such as "sleep", "standby", and "hibernate". In some reduced-power modes the system state is stored in RAM and the machine can "wake up" very quickly; in others the state is saved to disc and the motherboard powered down, taking at least several seconds to awake. The machine can be woken from a reduced-power state by a variety of signals. In a particular example, the Gigabyte 8KNXP motherboard with built-in Intel PRO/1000 CT network adapter, there are 3 motherboard BIOS settings and 6 network adapter settings which affect wakeup. The problem is often to prevent the machine waking up immediately after going to a reduced power state. Clearly the machine's BIOS must be set to allow WOL. To allow wakeup from powered-down state S5, wakeup on PME is also required. The Intel adapter allows "Wake on Directed Packet", "Wake on Magic Packet", "Wake on Magic Packet from power off state", and "Wake on Link"[11]. Wake on Directed Packet is particularly useful as the machine will automatically come out of standby or hibernation when it is referenced, without the user or application needing to explicitly send a magic packet. Unfortunately in many networks waking on directed packet (any packet with the adapter's MAC address or IP address) or on link is likely to cause wakeup immediately after going to a low-power state. Details for any particular motherboard and network adapter are to be found in the relevant manuals; there is no general method. Knowledge of signals on the network may also be needed to prevent spurious wakening. [edit] Unattended operationFor a machine which is normally unattended precautions need to be taken to make the WoL function as reliable as possible. For a machine procured to work in this way, WoL functionality is an important part of the purchase procedure. Some machines do not support WoL after they have been disconnected from power, e.g., when power is restored after a power failure. Use of an uninterruptible power supply (UPS) will give protection against a short period without power, although the battery will discharge during a prolonged power cut. If a machine is not designed to support WoL if left powered down after power failure, it may be possible to set the BIOS to start it up automatically on restoration of power, so that it is never left in an unresponsive state. A typical BIOS setting is "AC BACK function" which may be "ON", "OFF", or "Memory". "ON" is the correct setting in this case; "memory", which restores the machine to the state it was in when power was lost, may leave a machine which was hibernating in an unwakeable state. Other problems can affect the ability to start or control the machine remotely: hardware failure of the machine or network, failure of the BIOS settings battery (the machine will halt when started before the network connection is made, displaying an error message and requiring a keypress), loss of control of the machine due to software problems (machine hang, termination of remote control or networking software, etc.), and virus infection or hard disk corruption. Use of a reliable server-class machine with RAID drives, redundant power supplies, etc., will help to maximize availability. A device which can switch the machine off and on again, controlled perhaps by a telephone signal, can force a reboot which will clear problems due to misbehaving software. For a machine not in constant use, energy can be conserved by putting the machine into low-power RAM standby after a short timeout period. If a connection delay of a minute or two is acceptable, the machine can timeout into hibernation, powered off with its state saved to disk. [edit] Wake on InternetSee also: Sleep Proxy Service
The computer being woken does not know whether the wakeup signal comes from another machine on the same network or from anywhere else. If the magic packet can be made to reach a computer, it can originate anywhere, e.g., from the Internet. This can be achieved by a Virtual Private Network (VPN), which makes the remote computer appear to be a member of the Local Area Network (LAN). In the absence of a VPN, a computer connected to a router can be woken if a magic packet sent over the Internet is routed to it. This requires any firewall to be set up to allow entry of the WOL signal to a specified port. The port can be forwarded to the computer to be woken up; or some routers permit the packet to be broadcast to the entire LAN. [edit] See also[edit] References
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