Program to remotely Power On a PC over the internet using the Wake-on-LAN protocol.

• Difficulty Level : Hard
• Last Updated : 18 Oct, 2021

Wake-on-LAN (WoL) is an Ethernet or token ring computer networking standard that allows a computer to be turned on or awakened by a network message. 

• The message is usually sent to the target computer by a program executed on a device connected to the same local area network, such as a smartphone.
• It is also possible to initiate the message from another network by using subnet-directed broadcasts or a WOL gateway service.
• Equivalent terms include wake on WAN, remote wake-up, power on by LAN, power up by LAN, resume by LAN, resume on LAN and wake up on LAN.

Principle of operation 

• Wake-on-LAN (“WOL”) is implemented using a specially designed packet called a magic packet, which is sent to all computers in a network, among them the computer to be awakened.
• The magic packet contains the MAC address of the destination computer, an identifying number built into each network interface card (“NIC”) or other ethernet devices in a computer, that enables it to be uniquely recognized and addressed on a network.
• Powered-down or turned-off computers capable of Wake-on-LAN will contain network devices able to “listen” to incoming packets in low-power mode while the system is powered down.
• If a magic packet is received that is directed to the device’s MAC address, the NIC signals the computer’s power supply or motherboard to initiate system wake-up, much in the same way as pressing the power button would do.
• The magic packet is sent on the data link layer (layer 2 in the OSI model) and when sent, is broadcast to all attached devices on a given network, using the network broadcast address; the IP-address (layer 3 in the OSI model) is not used.

In order for Wake-on-LAN to work, parts of the network interface need to stay on. This consumes a small amount of standby power, much less than normal operating power. Disabling wake-on-LAN when not needed can therefore vary slightly reduce power consumption on computers that are switched off but still plugged into a power socket.

Magic Packet Structure 
The magic packet is a broadcast frame containing 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 may be sent as any network- and transport-layer protocol, although it is typically sent as a UDP datagram to port 0, 7, or 9, or directly over Ethernet as EtherType 0x0842.

A standard magic packet has the following basic limitations:  

1. Requires destination computer MAC address (also may require a SecureOn password).
2. Do not provide a delivery confirmation.
3. May not work outside of the local network.
4. Requires hardware support of Wake-On-LAN on the destination computer.
5. 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 very simple and to be quickly processed by the circuitry present on the network interface card with minimal power requirement. Because Wake-on-LAN operates below the IP protocol layer the MAC address is required and makes IP addresses and DNS names meaningless.

• C++

// C program to remotely Power On a PC over the

// internet using the Wake-on-LAN protocol.

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

#include <string.h>

#include <sys/types.h>

int main()

{

    int i;

    unsigned char toSend[102],mac[6];

    struct sockaddr_in udpClient, udpServer;

    int broadcast = 1 ;

    // UDP Socket creation

    int udpSocket = socket(AF_INET, SOCK_DGRAM, 0);

    // Manipulating the Socket

    if (setsockopt(udpSocket, SOL_SOCKET, SO_BROADCAST,

                  &broadcast, sizeof broadcast) == -1)

    {

        perror("setsockopt (SO_BROADCAST)");

        exit(EXIT_FAILURE);

    }

    udpClient.sin_family = AF_INET;

    udpClient.sin_addr.s_addr = INADDR_ANY;

    udpClient.sin_port = 0;

    //Binding the socket

    bind(udpSocket, (struct sockaddr*)&udpClient, sizeof(udpClient));

    for (i=0; i<6; i++)

        toSend[i] = 0xFF;

    // Let the MAC Address be ab:cd:ef:gh:ij:kl

    mac[0] = 0xab;  // 1st octet of the MAC Address

    mac[1] = 0xcd;  // 2nd octet of the MAC Address

    mac[2] = 0xef;  // 3rd octet of the MAC Address

    mac[3] = 0xgh;  // 4th octet of the MAC Address

    mac[4] = 0xij;  // 5th octet of the MAC Address

    mac[5] = 0xkl;  // 6th octet of the MAC Address

    for (i=1; i<=16; i++)

        memcpy(&toSend[i*6], &mac, 6*sizeof(unsigned char));

    udpServer.sin_family = AF_INET;

    // Broadcast address

    udpServer.sin_addr.s_addr = inet_addr("10.89.255.255");

    udpServer.sin_port = htons(9);

    sendto(udpSocket, &toSend, sizeof(unsigned char) * 102, 0,

             (struct sockaddr*)&udpServer, sizeof(udpServer));

    return 0;

}

Output: 

This program will power on the switched-off PCwhose MAC Address is used in this program (thePC and the Host computer must be connected overLAN).