Tag: linux

Wake-on-LAN (WoL) in Linux is disabled by default. How to enable it without ethtool.

Accomplishing the simple often takes a day or two.

According to countless tutorials on the internet, in order for a working WoL, you need to fulfill the following preconditions (where point 4 below is rarely mentioned [that’s why this article exists]):

  1. An ethernet hardware/card supporting WoL
  2. Enabling WoL in the BIOS
  3. Enabling the WoL feature on the ethernet card (usually using the user space tool ethtool)
  4. Enabling the WoL feature in the Linux kernel (and this was the major pitfall for me)

Support of WoL can be checked for a given ethernet card in the following way:

ethtool  | grep Wake-on

According to my own experiments on my local hardware, WoL seems to always be enabled by default:

Supports Wake-on: pumbg
Wake-on: g

Because it already prints Wake-on: g, signalling that WoL is enabled (for Magic Packets), one is tempted to not run ethtool -s <card_label> wol g to achieve the same.

However, this ethtool command does at least one other thing, being equivalent to the following:

echo enabled > /sys/class/net/<card_label>/device/power/wakeup

And the main pitfall is: After a reboot, cat /sys/class/net/<card_label>/device/power/wakeup will return disabled.

That is why attempts to make WoL work without ethtool may fail. The feature will stay disabled in the Linux kernel, which will completely shut off the ethernet card in most powersave and power-off states. (I verified this observing the LEDs of a connected ethernet switch).

Conclusion

On modern Linux systems, given supporting hardware, it seems to be enough to write the string enabled to /sys/class/net/<card_label>/device/power/wakeup to make WoL work. ethtool doesn’t seem to be required any more for that, since Ethernet cards seem to already be configured properly by default. The Ethernet card will stay powered in most power-save and power-off states, so that it can receive Wake-on-LAN packets.

Resources

https://www.kernel.org/doc/html/v4.14/driver-api/pm/devices.html
https://www.kernel.org/doc/Documentation/power/states.txt

File permissions for successful SSH login via authorized_keys

Note: This post is 9 years old. Some information may no longer be correct or even relevant. Please, keep this in mind while reading.

If you want to ssh into your server without being repeatedly prompted for the password you can copy your public ssh key into a file called authorized_keys  in the .ssh subdirectory of the home directory of the remove server account. However, this works only if the permissions for this file are set correctly.

First, if you have not done so already, generate the public key for your local user:

ssh-keygen

This will create a file ~/.ssh/id_rsa.pub

Append the only line in this file into the file ~/.ssh/authorized_keys  of the remote user account. Create the directory and file if it does not exist.

Now try to ssh into your remote account. If ssh is still asking for the remote user’s password, check the permissions of the following files and directories:

  • The permissions of the home directory of the remote user must be 755
  • The permissions of the remote .ssh directory must be 700
  • The permissions of the remote authorized_keys file must be 600

… of course all of those must be owned by the remote user, and not by root.

Now, you should be able to ssh into the remote account without being asked for the password!