By | 13 Aug 2008

Preparing to go asleep

To make Linux not shut down the network interface upon halt, edit the file /etc/init.d/halt, find the line with the halt command and remove the parameter -i if there. This parameter does ifdown on all networking interfaces. As the manpage for halt states, this is unneccessary for newer kernels – also it disables WOL.

The second step is to make sure, your card supports WOL. To find out, issue the sudo ethtool eth0 command. You should get something like this:

Settings for eth0:
    Supported ports: [ TP MII ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Advertised auto-negotiation: Yes
    Speed: 10Mb/s
    Duplex: Half
    Port: MII
    PHYAD: 1
    Transceiver: internal
    Auto-negotiation: on
    Supports Wake-on: g
    Wake-on: g
    Current message level: 0x00000007 (7)
    Link detected: no

The important lines are the Supports Wake-on and Wake-on ones. The “g” means it is enabled for MagicPacketâ„¢. If not, you should manually call the command:

ethtool -s eth0 wol g

This should enable WOL for the card. If this works, you have to issue this command after every bootup as the state will be back to disabled then. You might want to create a startup script.

Waking remote PCs

The simple way is to use wakeonlan which only supports MagicPacketâ„¢.

For a PC in a Class C network, use a call like this:

wakeonlan -i de:ad:be:ef:ca:fe

This would send the packet to the 192.168.1.x subnet and the PC with the specified MAC address should wake up.

Note: Some nVidia chipsets require the MAC address to be specified in reverse order. In the example this would be fe:ca:ef:be:ad:de.

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