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Technical Documents

The Dedicated Host shell

When you take out a dedicated host with Bytemark, we provide a login to our Dedicated Host Shell system. This interface provides two functions essential to managing a dedicated host remotely: serial console access and the ability to turn your machine off and on (power cycle).

To log on, you should be in receipt of an ssh key for your dedicated host called (e.g.) joebloggs.key. You need to supply this key to your ssh client instead of a password. The name of the key file identifies your unique dedicated host name on our network, and determines the address to which you’ll need to connect in order to get to your shell:

If you are connecting from a Linux host, the command to use is:

ssh -i joebloggs.dh.key dhshell@joebloggs.dhadmin.bytemark.co.uk

Note ssh insists that the permissions on the keyfile are 0700. If your key has different permissions you’ll receive a prompt for a passphrase, even if one has not been set.

With PuTTY on Windows use the .ppk key eg joebloggs.dh.ppk

The dhshell interface

Once you’ve connected you will see the dedicated host shell interface, allowing you to immediately see the output of the serial console of your machine and call up the other functions.

The display should look something like this:

Note the two status lines at the bottom remove two lines from your terminal size – if you run editors or other full-screen applications and want to avoid the odd bit of corruption, you may want to expand your terminal by two lines to compensate.

The serial line will usually show a login prompt unless you have altered your /etc/inittab to remove it. We don’t recommend this, as the serial login is your primary means of fixing a broken network or firewall. You will also see kernel messages printed here; this is where to look for messages indicating disc failure or excessive network activity.

If you don’t see a login prompt straight away you may need to press return a couple of times.

If the server is really wedged, you can use the kernel system request facility to cause a reboot. To send a system request over the serial line, you send a break (press ‘Ctrl+p’ then press ‘b’), followed quickly by one of the Sysrq letters: common ones are S for synchronize discs, U for unmount discs and B to force a reboot. Others are documented in the kernel Documentation/sysrq.txt file.

To exit the serial console press ‘Ctrl-p x’, as indicated by the quick help displayed at the bottom of the window.

dhshell keybindings

The dhshell process by default only shows you the serial console of your machine. There are other functions available which you can access with the following keys:

Ctrl-p [ Scroll backwards
Ctrl-p b Send a break
Ctrl-p f Enter fullscreen mode with no status bars
Ctrl-p F Leave fullscreen mode
Ctrl-p h Read the help
Ctrl-p r Reboot your server
Ctrl-p w Show the watchdog editor
Ctrl-p x Exit

As the shell is running within GNU Screen you can use most of the standard GNU screen keybindings will work – just note that we’ve changed the prefix from Ctrl-a to Ctrl-p, to avoid confusing when running nested screens.

PowerCycling Your Machine

If your machine is really stuck and you can do nothing else with it then we’d suggest powercycling it. This is not clean and is essentially the same as pulling the plug on your server with no warning.

To powercycle your machine press ‘Ctrl-p r’ and confirm your decision. The machine should then reboot, and after a few seconds you’ll be able to see the serial console output our boot loader messages. The sequence ‘Ctrl-p s’ (long powercycle) is similar except this allows the power to be disconnected for up to 60 seconds; it’s useful with some motherboards that seem to have sufficient capacitance to prevent the simple powercycle from working.

Booting setup

You might want to read more about our servers’ booting setup if you need to rescue or reinstall your host from scratch.

Using the watchdog

You can also set up watchdog checks in exactly the same way as for virtual machines.

To open the watchdog interface press ‘Ctrl-p w’. This will open up a new window which allows you to enter watchdog commands, once you’ve finished you can close it by entering ‘exit’.

Take a look at the documentation for the syntax of the rules you can add, and a brief overview of other commands you may run.

Previous Addresses

You might previously have connected to a different address to access your dedicated host. These were the two we used to advise people to use:

 dhshell.rsh.bytemark.co.uk
 dhshell.man.bytemark.co.uk

These addresses are specific to the two data centers we host machines within and may stop working in future as we expand the dedicated host shell facilities.

We recommend that you no long use either of these addresses to avoid future changes.

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