Enable WakeonLan for your Linux server

Wakeonlan (wol) enables you to switch ON remote servers without physically accessing it. Wakeonlan sends magic packets to wake-on-LAN enabled ethernet adapters and motherboards to switch on remote computers.

By mistake, when you shutdown a system instead of rebooting, you can use Wakeonlan to power on the server remotely. Also, If you have a server that don’t need to be up and running 24×7, you can turn off and turn on the server remotely anytime you want.

This article gives a brief overview of Wake-On-LAN and instructions to set up Wakeonlan feature.

Overview of Wake-On-LAN

  • You can use Wakeonlan when a machine is connected to LAN, and you know the MAC address of that machine.
  • Your NIC should support wakeonlan feature, and it should be enabled before the
    shut down. In most cases, by default wakeonlan is enabled on the NIC.
  • You need to send the magic packet from another machine which is connected to the same network ( LAN ). You need root access to send magic packet. wakeonlan package should be installed on the machine.
  • When the system crashes because of power failure, for the first time you cannot switch on your machine using this facility. But after the first first boot you can use wakeonlan to turn it on, if the server gets shutdown for some reason.
  • WakeonLan is also referred as wol.

Check whether wol is supported on the NIC

Execute the following ethtool command in the server which you want to switch ON from a remote place.

# ethtool eth0
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: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pumbg  [ Note: check whether flag g is present ]
        Wake-on: g [ Note: g mean enabled. d means disabled ]
        Current message level: 0x00000001 (1)
        Link detected: yes

If  Supports Wake-on is g, then the support for wol feature is enabled on the NIC card.

Enabling wol option on the Ethernet Card

By default the Wake-on will be set to g in most of the machines. If not, use ethtool to set the g flag to the wol option of the NIC card as shown below.

# ethtool -s eth0 wol g

Note: You should execute ethtool as root, else you may get following error message.

$ /sbin/ethtool eth0
Settings for eth0:
Cannot get device settings: Operation not permitted
Cannot get wake-on-lan settings: Operation not permitted
        Current message level: 0x000000ff (255)
Cannot get link status: Operation not permitted

Install wakeonlan package on a different machine

Install the wakeonlan package in the machine from where you need to send the magic packet to switch on your server.

# apt-get install wakeonlan

Note down the MAC address of the remote server

Note down the MAC address of the server that you wish to switch on remotely.

# ifconfig
eth0     Link encap:Ethernet  HWaddr 00:16:k5:64:A9:68  [ Mac address ]
          inet addr:192.168.6.56  Bcast:192.168.6.255  Mask:255.255.255.0
          inet6 addr: fe80::216:17ff:fe6b:289/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3179855 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2170162 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3832534893 (3.5 GB)  TX bytes:390304845 (372.2 MB)
          Interrupt:17

Finally, Switch ON the machine remotely without physical access

When the server is not up, execute the following command from another machine which is connected to the same LAN. Once the magic packet is sent, the remote system will start to boot.

# wakeonlan 00:16:k5:64:A9:68

Rebooting your system using the SysRq key

If you are working on kernel development, or device drivers, or running a code that could cause kernel panic, SysRq key will be very valuable. The magic SysRq key is a key combination in the Linux kernel which allows the user to perform various low level commands regardless of the system’s state.

It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem. The key combination consists ofAlt+SysRq+commandkey. In many systems the SysRq key is the printscreen key.

First, you need to enable the SysRq key, as shown below.

echo "1" > /proc/sys/kernel/sysrq

List of SysRq Command Keys

Following are the command keys available for Alt+SysRq+commandkey.

  • ‘k’ – Kills all the process running on the current virtual console.
  • ‘s’ – This will attempt to sync all the mounted file system.
  • ‘b’ – Immediately reboot the system, without unmounting partitions or syncing.
  • ‘e’ – Sends SIGTERM to all process except init.
  • ‘m’ – Output current memory information to the console.
  • ‘i’ – Send the SIGKILL signal to all processes except init
  • ‘r’ – Switch the keyboard from raw mode (the mode used by programs such as X11), to XLATE mode.
  • ‘s’ – sync all mounted file system.
  • ‘t’ – Output a list of current tasks and their information to the console.
  • ‘u’ – Remount all mounted filesystems in readonly mode.
  • ‘o’ – Shutdown the system immediately.
  • ‘p’ – Print the current registers and flags to the console.
  • ’0-9′ – Sets the console log level, controlling which kernel messages will be printed to your console.
  • ‘f’ – Will call oom_kill to kill process which takes more memory.
  • ‘h’ – Used to display the help. But any other keys than the above listed will print help.

We can also do this by echoing the keys to the /proc/sysrq-trigger file. For example, to re-boot a system you can perform the following.

echo "b" > /proc/sysrq-trigger

Perform a Safe reboot of Linux using Magic SysRq Key

To perform a safe reboot of a Linux computer which hangs up, do the following. This will avoid the fsck during the next re-booting. i.e Press Alt+SysRq+letter highlighted below.

  • unRaw (take control of keyboard back from X11,
  • tErminate (send SIGTERM to all processes, allowing them to terminate gracefully),
  • kIll (send SIGILL to all processes, forcing them to terminate immediately),
  • Sync (flush data to disk),
  • Unmount (remount all filesystems read-only),
  • reBoot.

Using the ifconfig command

Ifconfig command is used to configure network interfaces. ifconfig stands for interface configurator. Ifconfig is widely used to initialize the network interface and to enable or disable the interfaces.

In this article, let us review 7 common usages of ifconfig command.

1. View Network Settings of an Ethernet Adapter

Ifconfig, when invoked with no arguments will display all the details of currently active interfaces. If you give the interface name as an argument, the details of that specific interface will be displayed.

# ifconfig eth0

eth0   Link encap:Ethernet  HWaddr 00:2D:32:3E:39:3B
inet addr:192.168.2.2  Bcast:192.168.2.255  Mask:255.255.255.0
inet6 addr: fe80::21d:92ff:fede:499b/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:977839669 errors:0 dropped:1990 overruns:0 frame:0
TX packets:1116825094 errors:8 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2694625909 (2.5 GiB)  TX bytes:4106931617 (3.8 GiB)
Interrupt:185 Base address:0xdc00

2. Display Details of All interfaces Including Disabled Interfaces

# ifconfig -a

3. Disable an Interface

# ifconfig eth0 down

4. Enable an Interface

# ifconfig eth0 up

5. Assign ip-address to an Interface

Assign 192.168.2.2 as the IP address for the interface eth0.

# ifconfig eth0 192.168.2.2


Change Subnet mask of the interface eth0.

# ifconfig eth0 netmask 255.255.255.0


Change Broadcast address of the interface eth0.

# ifconfig eth0 broadcast 192.168.2.255


Assign ip-address, netmask and broadcast at the same time to interface eht0.

# ifconfig eth0 192.168.2.2 netmask 255.255.255.0 broadcast 192.168.2.255

6. Change MTU

This will change the Maximum transmission unit (MTU) to XX. MTU is the maximum number of octets the interface is able to handle in one transaction. For Ethernet the Maximum transmission unit by default is 1500.

 

 

# ifconfig eth0 mtu XX

7. Promiscuous mode

By default when a network card receives a packet, it checks whether the packet belongs to itself. If not, the interface card normally drops the packet. But in promiscuous mode, the card doesn’t drop the packet. Instead, it will accept all the packets which flows through the network card.


Superuser privilege is required to set an interface in promiscuous mode. Most network monitor tools use the promiscuous mode to capture the packets and to analyze the network traffic.


Following will put the interface in promiscuous mode.

# ifconfig eth0 promisc


Following will put the interface in normal mode.

# ifconfig eth0 -promisc

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