Workaround for routing WOL (Wake on LAN) packets with Linux

January 16, 2010

If you want to send a WOL packet to a PC within your subnet it is really easy. Just install a program like wakeonlan (apt-get install wakeonlan) and type something like:

wakeonlan 01:02:03:04:05:06

But how to you send a WOL packet to an other subnet? Basically you use a UDP packet and send it to the broadcast address of the other network. e.g. with wakeonlan it looks like this

wakeonlan -i 01:02:03:04:05:06

But you need support for this from your router, as normally they don’t allow sending to the broadcast address from other networks. Professional routers/layer3 switches have support for this (you just need to enable it), but you’ve a Linux router at home? (e.g. one with Openwrt or Debian/Ubuntu)

The simplest way to get it working is to enter following on the router (rerun it at every boot):

arp -s FF:FF:FF:FF:FF:FF

This tells the router that the given IP has a MAC address which is used for broadcasts. Now you only need to send the packet to this new “broadcast” address instead of the real one. So your wakeup call looks like this:

wakeonlan -i 01:02:03:04:05:06

ps: you should only enable something like this on a trusted network and the IP address you use should be not used by any other device.

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