Magic packet

The magic packet is a frame that is most often sent as a broadcast and that contains anywhere within its payload 6 bytes of all 255 (FF FF FF FF FF FF in hexadecimal), followed by sixteen repetitions of the target computer's 48-bit MAC address, for a total of 102 bytes.

Since the magic packet is only scanned for the string above, and not actually parsed by a full protocol stack, it could be sent as payload of any network- and transport-layer protocol, although it is typically sent as a UDP datagram to port 0 (reserved port number),^[6] 7 (Echo Protocol) or 9 (Discard Protocol),^[7] or directly over Ethernet using EtherType 0x0842.^[8] A connection-oriented transport-layer protocol like TCP is less suited for this task as it requires establishing an active connection before sending user data.

A standard magic packet has the following basic limitations:

• Requires destination computer MAC address (also may require a SecureOn password)
• Does not provide a delivery confirmation
• May not work outside of the local network
• Requires hardware support for Wake-on-LAN in the destination computer
• Most 802.11 wireless interfaces do not maintain a link in low-power states and cannot receive a magic packet

The Wake-on-LAN implementation is designed to be simple and to be quickly processed by the circuitry present on the network interface card using minimal power. Because Wake-on-LAN operates below the IP protocol layer, IP addresses and DNS names are meaningless and so the MAC address is required.

Subnet directed broadcasts

A principal limitation of standard broadcast Wake-on-LAN is that broadcast packets are generally not routed. This prevents the technique being used in larger networks or over the Internet. Subnet-directed broadcasts (SDBs)^[9][10] may be used to overcome this limitation. SDB may require changes to the intermediate router configuration. SDBs are treated like unicast network packets until processed by the final (local) router. This router then broadcasts the packet using a layer-2 broadcast. This technique allows a broadcast to be initiated on a remote network but requires all intervening routers to forward the SDB.^[11][12] When preparing a network to forward SDB packets, care must be taken to filter packets so that only desired (e.g. WoL) SDB packets are permitted – otherwise the network may become a participant in DDoS attacks such as the Smurf attack.^[13]

Troubleshooting magic packets

Wake-on-LAN can be a difficult technology to implement because it requires appropriate BIOS/UEFI, network card and, sometimes, operating system and router support to function reliably. In some cases, hardware may wake from one low-power state but not from others. This means that due to hardware issues the computer may be wakeable from its soft off state (S5) but doesn't wake from sleep or hibernation or vice versa.

Starting with Windows Vista, the operating system logs all wake sources in the System event log. The Event Viewer and the powercfg.exe /lastwake command can retrieve them.^[14]

Unauthorized access

Magic packets are sent via the data link or OSI-2 layer, which can be used or abused by anyone on the same LAN, unless the L2 LAN equipment is capable of (and configured for) filtering such traffic to match site-wide security requirements.

Firewalls may be used to prevent clients among the public WAN from accessing the broadcast addresses of inside LAN segments, or routers may be configured to ignore subnet-directed broadcasts (see above).

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, by increasing the search space by 48 bits (6 bytes), up to 2⁹⁶ combinations if the MAC address is entirely unknown. However any network eavesdropping will expose the cleartext password. Still, only a few NIC and router manufacturers support such security features.

Abuse of the Wake-on-LAN feature only allows computers to be switched on; it does not in itself bypass password and other forms of security, and is unable to power off the machine once on. However, many client computers attempt booting from a PXE server when powered up by WoL. Therefore, a combination of DHCP and PXE servers on the network can sometimes be used to start a computer with an attacker's boot image, bypassing any security of the installed operating system and granting access to unprotected, local disks over the network.

Interactions with network access control

The use of Wake-on-LAN technology on enterprise networks can sometimes conflict with network access control solutions such as 802.1X or MAC-based authentication, which may prevent magic packet delivery if a machine's WoL hardware has not been designed to maintain a live authentication session while in a sleep state.^[15] Configuration of these two features in tandem often requires tuning of timing parameters and thorough testing.

Data privacy

Some PCs include technology built into the chipset to improve security for Wake-on-LAN. For example, Intel AMT (a component of Intel vPro technology), includes Transport Layer Security (TLS), an industry-standard protocol that strengthens encryption.^[16]

AMT uses TLS encryption to secure an out-of-band communication tunnel to an AMT-based PC for remote management commands such as Wake-on-LAN. AMT secures the communication tunnel with Advanced Encryption Standard (AES) 128-bit encryption and RSA keys with modulus lengths of 2,048 bits.^[17][18] 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.^[16]

IT shops using Wake-on-LAN 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.^[16] The Intel implementation also works for wireless networks.^[16]

Hardware implementations

Older 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 Wake-on-LAN 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 Wake-on-LAN cable.^[19]

Wake-on-LAN usually needs to be enabled in the Power Management section of a PC motherboard's BIOS/UEFI setup utility, although on some systems, such as Apple computers, it is enabled by default. On older systems the BIOS/UEFI setting may be referred to as WoL; on newer systems supporting PCI version 2.2, it may be referred to as PME (Power Management Events, which include WoL). It may also be necessary to configure the computer to reserve standby power for the network card when the system is shut down.

In addition, in order to get Wake-on-LAN to work, enabling this feature on the network interface card or on-board silicon is sometimes required. Details of how to do this depend upon the operating system and the device driver.

Laptops powered by the Intel Centrino Processor Technology or newer^[20] (with explicit BIOS/UEFI support) allow waking up the machine using Wake on Wireless LAN (WoWLAN).

In most modern PCs, ACPI is notified of the "waking up" and takes control of the power-up. In ACPI, OSPM must record the "wake source" or the device that is causing the power-up – the device being the "Soft" power switch, the NIC (via Wake-on-LAN), the cover being opened, a temperature change, etc.^[14]

The 3-pin WoL interface on the motherboard consist of pin-1 +5V DC (red), pin-2 Ground (black), pin-3 Wake signal (green or yellow).^[21] By supplying the pin-3 wake signal with +5V DC the computer will be triggered to power up provided WoL is enabled in the BIOS/UEFI configuration.

Creating and sending the magic packet

Sending a magic packet requires knowledge of the target computer's MAC address. Software to send WoL magic packets is available for all modern platforms, including Windows, Macintosh and Linux, plus many smartphones. Examples include: Wake On LAN GUI, LAN Helper, Magic Packet Utility, NetWaker for Windows, Nirsoft WakeMeOnLAN, WakeOnLANx, EMCO WOL, Aquila Tech Wake on LAN, ManageEngine WOL utility, FusionFenix and SolarWinds WOL Tool.^[22] There are also web sites that allow a Magic Packet to be sent online without charge. Example source code for a developer to add Wake-on-LAN to a program is readily available in many computer languages. The following example is in Python 3:

import socket

def wol(lunaMacAddress: bytes):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)

    magic = b'\xff' * 6 + lunaMacAddress * 16
    s.sendto(magic, ('<broadcast>', 7))

if __name__ == '__main__':
    # pass to wol the mac address of the Ethernet port of the appliance to wakeup
    wol(b'\x00\x15\xB2\xAA\x5B\x00')

Ensuring the magic packet travels from source to destination

If the sender is on the same subnet (local network, aka LAN) as the computer to be awakened there are generally no issues. When sending over the Internet, and in particular where a NAT (Network Address Translator) router, as typically deployed in most homes, is involved, special settings often need to be set. For example, in the router, the computer to be controlled needs to have a dedicated IP address assigned (aka a DHCP reservation). Also, since the controlled computer will be "sleeping" except for some electricity on to part of its LAN card, typically it will not be registered at the router as having an active IP lease.

Further, the WoL protocol operates on a "deeper level" in the multi-layer networking architecture. To ensure the magic packet gets from source to destination while the destination is sleeping, the ARP Binding (also known as IP & MAC binding) must typically be set in a NAT router. This allows the router to forward the magic packet to the sleeping computer's MAC adapter at a networking layer below typical IP usage. In the NAT router, ARP binding requires just a dedicated IP number and the MAC address of the destination computer. There are some security implications associated with ARP binding (see ARP spoofing); however, as long as none of the computers connected to the LAN are compromised, an attacker must use a computer that is connected directly to the target LAN (plugged into the LAN via cable, or by breaking through the Wi‑Fi connection security to gain access to the LAN).

Most home routers are able to send magic packets to LAN; for example, routers with the DD-WRT, Tomato or PfSense firmware have a built-in Wake-on-LAN client. OpenWrt supports both Linux implementations for WoL etherwake and WoLs.

Responding to the magic packet

Most WoL hardware functionally is typically blocked by default and needs to be enabled in using the system BIOS/UEFI. Further configuration from the OS is required in some cases, for example via the Device Manager network card properties on Windows operating systems.

Unattended operation

For a machine which is normally unattended, precautions need to be taken to make the Wake-on-LAN function as reliable as possible. For a machine procured to work in this way, Wake-on-LAN functionality is an important part of the purchase procedure.

Some machines do not support Wake-on-LAN 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.

Waking up without operator presence

If a machine that is not designed to support Wake-on-LAN is left powered down after power failure, it may be possible to set the BIOS/UEFI to start it up automatically on restoration of power, so that it is never left in an unresponsive state. A typical BIOS/UEFI 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/UEFI 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. Therefore, the use of a reliable server-class machine with RAID drives, redundant power supplies, etc., will help to maximize availability. Additionally, a device which can switch the machine off and on again, controlled perhaps by a remote 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.