How to wake-on-lan via internet with python code?

Question

My home network diagram is simple as below:

internet       --> Optical modem --> router      --> pc1 ,pc2
111.111.111.111--> 192.168.1.1   -->192.168.31.1 --> 192.168.31.144,192.168.31.173

The ISP provide a static ip ,supposing it's 111.111.111.111,optical modem's ip address is 192.168.1.1,router's ip address is 192.168.31.1, ip address for pc1 is 192.168.31.144 , ip address for pc2 is 192.168.31.173,the pc1's mac address is xx.xx.xx.xx.

The openwrt is running on my router,adding some port forwarding rules in the router and open firewall:

forwarding rule
name        protocl       outer port    inner IP address   inner port 
wakeonwan   UDP           9             192.168.31.144      9
ssh         TCP and UDP   10000         192.168.31.1        22

And bind the ip address and mac in openwrt:

ip address               mac               interface
192.168.31.144           xx.xx.xx.xx       ??

Which interface to choose from the below options?

Login my router remotely:

ssh [email protected] -p 10000

Issue wol command from the router:

/usr/bin/wol -i 192.168.31.255 xx.xx.xx.xx

The pc1 can be waked on from the router!
Turn off pc1,wakeup it from pc2 with the following python code wakeonlan.py:

from wakeonlan import send_magic_packet
send_magic_packet('xx.xx.xx.xx')

Executing the command python3 wakeonlan.py in pc2 can wakeup my pc1 successfully.

Executing the command python3 wakeonwan.py remotely(for example--in my company's pc) can't wakeup my pc1.

cat wakeonwan.py
mac = "xx.xx.xx.xx"
target_ip = "111.111.111.111"
from wakeonlan import send_magic_packet
send_magic_packet(mac, ip_address=target_ip,port=9)

It encounter no error,why pc1 can't wakeup with wakeonwan.py?
Let's do it with a lib--paramiko.

pip install paramiko

Eidt wakeonwan-paramiko.py:

import paramiko
from contextlib import contextmanager
host = '111.111.111.111'
username = 'root'
password = 'password'
ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy()) 
ssh.connect(host, username=username, password=password,port=10000)
stdin, stdout, stderr = ssh.exec_command("/usr/bin/wol -i 192.168.31.255 xx:xx:xx:xx")
ssh.close()

python3 wakeonwan-paramiko.py can wakeup my home pc from external network,i feel it is more simple to edit wakeonwan.py to wakeup pc remotely?
How to fix it?

Answer 1

The tricky part in sending Wake-on-LAN packets in from beyond the local network is in getting the router send the packet out to the local network, so that the completely passive PC1 can receive it.

The magic packet is addressed to the router's external IP address, which makes it an unicast packet. The forwarding rule in the router specifies the target PC's own IP address as the inner IP address. So, per the standard procedure for forwarding unicast traffic, the router will first make an ARP request to find out the MAC address of the target PC... but gets no answer, because the target PC is powered off. A network interface that is waiting for wake-on-LAN packets is in a low-power receive-only mode: it will not typically be able to answer to ARP requests.

As the ARP request fails, the router might send back an ICMP error packet indicating "host unreachable"... but the Python wakeonlan module does not seem to have the facilities to process such an error response, even assuming the ICMP message does not get filtered out by paranoid firewalls on its way back.

The fact that the MAC address is embedded into the magic packet payload does not help, because the router does neither know nor care about that. For the router, the magic packet is just another TCP or UDP packet that will be processed just as usual.

You would have to configure the forwarding rule send the magic packet into the broadcast address of the internal network, in order to make it a broadcast. That way, the router will not need to make an ARP query before passing the packet into the inner network. So, if the netmask of your internal network is 255.255.255.0, then you should use 192.168.31.255 as the inner IP address in the forwarding rule.

But... as @A.B mentioned in the comments, Linux-based routers don't generally like transforming unicasts into broadcasts, so this approach might not work.

An alternative solution would be to add a static ARP table entry for the target PC into the router's ARP table, so it would not need to make the ARP query, but could just proceed into forwarding the packet to the inner network.

By quick source code inspection, the wakeonlan python module seems to use SOCK_DGRAM, which means it uses UDP. So you should be able to change the forwarding rule from "TCP and UDP" to "UDP only".