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

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 PC
whose MAC Address is used in this program (the 
PC and the Host computer must be connected over
LAN). 

This article is contributed by Kishlay Verma. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Reference :
https://en.wikipedia.org/wiki/Wake-on-LAN
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