The title says it all. If you don’t know what a Magic Packet is then read here. Below is the code of a shell script (I call magic_shutdown) that uses tcpdump to listen for Magic Packets. When it receives on then the shell script verifies its content to make sure that this packet was meant for this computer. Note this script doesn’t require that your network interface card support Wake-on-LAN. It has no special hardware dependencies.

All-in-all, this script lets you create create a shortcut which will act as remote power button for your computer. Press it to turn the computer ON. Press it again to turn the computer OFF.

Get magic_shutdown‘s code
License: GNU Public License version 3.

[code lang=”bash”]#!/bin/bash
#Author:AppleGrew
#License:GPL version 3

listenPort=9
interface="eth0"

#Forking to daemonize…
if [[ "$2" != "forked" ]]
then
echo "Forking $0…"
"$0" "$1" forked &
echo "Forked."
exit 0
fi

#Creating pid file
ppid=$$
echo $ppid >"$1"

echo "Started"
mac=`ifconfig "$interface"|head -n1|sed -e ‘s/.*HWaddr \([0-9:a-fA-F]*\)/\1/g’ -e ‘s/://g’`
pckt_expect=`echo "ff ff ff ff ff ff $mac $mac $mac $mac $mac $mac $mac $mac"|sed ‘s/ //g’|tr ‘A-Z’ ‘a-z’`
while `true`
do
pckt_data=`tcpdump -i "$interface" -x -c 1 udp port ${listenPort}`
if [[ $? != 0 ]]
then
echo "tcpdump returned error."
exit 1
fi

pckt_data=`echo "$pckt_data" | \
grep ‘0x[0-9]*:’| \
tr ‘A-Z’ ‘a-z’| \
sed ‘s/[ \t]//g’| \
sed ‘s/0x[0-9]*:\([0-9a-f]*\)/\1/g’| \
tr -d ‘\n\r’| \
cut -c 57-`
if [[ "$pckt_data" == "$pckt_expect" ]]
then
echo "Matched! Received Magic packet shutting down…"
shutdown -P now #Not recommended
#For Gnome #Doesn’t work dunno why?
#dbus-send –session –dest=org.gnome.PowerManager \
# –type=method_call –print-reply –reply-timeout=2000 \
# /org/gnome/PowerManager org.gnome.PowerManager.Shutdown
#For KDE 3.5
#dcop `dcop|grep power-manager` power-manager shutdown
exit 0
fi
done

echo "EXITED"
exit 0[/code]

Below is the startup script that must be used to launch magic_shutdown script.

Get launch_magic_shutdown‘s code
License: GNU Public License version 3.

[code lang=”bash”]#!/bin/bash
#Author:AppleGrew
#License:GPL version 3

SCRIPT="/opt/magic_shutdown"
PID_FILE="/var/run/magic_shutdown.pid"
case "$1" in
start)
test -f "$PID_FILE" && echo "Already Running…" && exit 1
"$SCRIPT" "$PID_FILE"
echo "Started"
;;
stop)
pid=`cat "$PID_FILE"`

tcpPid=`pgrep -P $pid tcpdump`
kill -9 $pid
kill -2 $tcpPid
if [ -f "$PID_FILE" ] && ! ps -p $pid >/dev/null
then
rm -f "$PID_FILE"
else
echo "Failed to delete pid file. Maybe its already deleted."
fi
echo "Stopped"
;;
esac[/code]

Installations:-
Assuming that you have downloaded the above two codes into your home directory. Now run the following commands.

sudo cp launch_magic_shutdown /etc/init.d
sudo chmod a+x /etc/init.d/launch_magic_shutdown
cd /etc/rc2.d
sudo ln -s ../init.d/launch_magic_shutdown S99launch_magic_shutdown

sudo cp magic_shutdown /opt/magic_shutdown
sudo chmod a+x /opt/magic_shutdown

Hope this helps. Report in the comments section if you encounter any problem.

---

Update: Stuart’s Script
Stuart (see comment section) posted his version of magic_shutdown script. If the above one doesn’t work then maybe you should give this one a try.
[code lang=”bash”]#!/bin/sh
#Author: Stuart
#Original Author:AppleGrew
#License:GPL version 3

#Forking to daemonize…
if [[ "$2" != "forked" ]]
then
echo "Forking $0…"
"$0" "$1" forked &
echo "Forked."
exit 0
fi

#Creating pid file
ppid=$$
echo $ppid >"$1"

echo "Started"
interface=`route -n | grep "^0.0.0.0" | awk -F " " ‘{print $8}’`
mac=`ifconfig "$interface"|head -n1|sed -e ‘s/.*HWaddr \([0-9:a-fA-F]*\)/\1/g’ -e ‘s/://g’`
pckt_expect=`echo "$mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac $mac"|sed ‘s/ //g’|tr ‘A-Z’ ‘a-z’`
while `true`
do
pckt_data=`tcpdump -i "$interface" -s 0 -x -c 1 \( \(ether dst "$mac" and not ip and not arp and not rarp\) or \(udp port 9\) \)`
if [[ $? != 0 ]]
then
echo "tcpdump returned error."
exit 1
fi
pckt_data=`echo "$pckt_data" | \
grep ‘0x[0-9]*:’| \
tr ‘A-Z’ ‘a-z’| \
sed ‘s/[ \t]//g’| \
sed ‘s/0x[0-9]*:\([0-9a-f]*\)/\1/g’| \
tr -d ‘\n\r’ | \
awk -F "ffffffffffff" ‘{print $2}’`
if [[ "$pckt_data" == "$pckt_expect" ]]
then
echo "Matched! Received Magic packet shutting down…"
rm -f $1
/sbin/poweroff
exit 0
fi
done

echo "EXITED"
exit 0[/code]