Giuseppe Parrello


How to send a WOL message to run a script

In this page I describe how to send a "Wake-On-Lan" message through the network in order to run a script on a router (Asus RT-AC56U) - we use some Entware packages, so please refer to this page on how to install Entware on a router.
Let's start with assumption that using a Wake-On-Lan message is certainly not recommended because anyone can send this message, just know the recipient's IP address, the MAC address of the network device to be turned on and the destination port. And to make matters worse, the Wake-On-Lan protocol does not provide a password. So we will use this procedure only to send non-significant messages, for example to run a script that allows you to enable a secondary functionality.
To send the Wake-On-Lan message we can use any program that supports Wake-On-Lan (refer to this page to use my application for Android operating system), instead to receive the above Wake-On-Lan message on the router we will use an Entware package called "socat".

How to install "socat" package on router

In order to install the "socat" daemon with the Entware packages, we need to execute the following command :
opkg install socat coreutils-od
The "socat" package contains only the "/opt/bin/socat" daemon, which we will analyze later. While the "coreutils-od" package contains "/opt/bin/od" command used by the script we will analyze later.

How to setup the program to send the Wake-On-Lan message

The program that sends the Wake-On-Lan message must be set using the following parameters:

How to setup a script used by the "socat" daemon

As we will see later, the "socat" daemon will pass the received Wake-On-Lan message to a script and this script will be able to read the Wake-On-Lan message and perform certain actions. Here below we see an example of a script, we name it "" (remember to properly set the execution permissions of the above script using the command "chmod + x" otherwise the "socat" daemon will not be able to execute it):


echo "PID: $$"
echo "$MESSAGE"

hexvar=$(echo $MESSAGE | /opt/bin/od -t x1)
echo $hexvar

# execute action 1
if echo $hexvar | grep -q -i "ff ff ff ff ff ff 11 22 33 44 55 66"; then
    # WOL message is received, we run a command
    echo "1 String contains hex is true."
    echo "1 String contains hex is not true."

# execute action 2
if echo $hexvar | grep -q -i "ff ff ff ff ff ff 11 77 88 99 AA BB"; then
    # WOL message is received, we run a command
    echo "2 String contains hex is true."
    echo "2 String contains hex is not true."

As is clear from the above-mentioned script, we perform the following operations:

The "echo" instructions included in the script are only used for debugging issues, they can be deleted once the procedure is understood.
Obviously we can insert more "if" instructions with different MAC addresses, be careful to use a space character between a hexadecimal number and the next one. Practically, in the program that must send the Wake-On-Lan message, we can add more Wake-On-Lan entries, but the only thing that will change will be the value of the dummy MAC address; the values of the IP address of the router and the port will always remain the same.

How to run the "socat" daemon to receive the Wake-On-Lan message

Once we have set the program that should send the Wake-On-Lan message and once we have set the script that will receive the Wake-On-Lan message, we can run the "socat" daemon. A typical instruction we can perform is as follows:
/opt/bin/socat -u -T1 UDP-LISTEN:25000,fork system:/opt/etc/
In the above-mentioned instruction we have the following parameters (for more information on the "socat" daemon refer to this page on Internet):

The above-mentioned instruction, if executed in the router's terminal, will be executed in debug mode as "root" user. For security reasons, I recommend running the "socat" daemon using the "nobody" user (obviously the script will run as "nobody" user, as well as all the commands executed within the script). To do this, you need to use the "daemonize" command, which can be installed via an Entware package with the "opkg install daemonize" instruction.
Therefore the "socat" daemon can be executed with the following instruction:
daemonize -u nobody /opt/bin/socat -u -T1 UDP-LISTEN:25000,fork system:/opt/etc/