KVM Virtualization – Accessing pty Login Prompt in VM

kvm-virtualizationlibvirt

I have created a VM with virt-install, using as install media a Ubuntu 14.04.01 LTS server ISO. During the installation phase I was connected to the VM via the graphical console.

Now I have the VM running: I can ssh to it, and it is otherwise working fine. I can follow the boot process, with:

virsh console my-vm

But I am unable to see the login prompt. How can I access the login prompts with are normally allocated via the PTYs?

Best Answer

It is possible to access a KVM Guest directly using the Serial Console interface, in which case setting up bridged networking, SSH, and similar is not necessary. Access via the Serial Console provides an alternate way of accessing your servers to compliment or replace the default VNC access.

This is done using the virsh utility which is a shell interface for libvirt.

  1. Check whether a console device had been defined:

virsh ttyconsole my_vm If the output is shown(e.g. /dev/pts/41), it indicates the Guest has a console device already.

Otherwise, define one with virsh edit. Here is an example to be added inside <device></device>. Refer to libvirt XML format for details.

<console type='pty'>

  1. Configure a Serial Console in the Guest First, we need to configure a serial console in the guest, in order that it will accept a connection:

This is the configuration method for (guest) Ubuntu 9.10 (Karmic) and later versions.

sudo editor /etc/init/ttyS0.conf

Add the configuration:

# ttyS0 - getty
#
# This service maintains a getty on ttyS0 from the point the system is
# started until it is shut down again.

start on stopped rc RUNLEVEL=[2345]
stop on runlevel [!2345]

respawn
exec /sbin/getty -L 115200 ttyS0 xterm
  1. Run the following command to initiate the Serial Console (or restart the VM):

sudo start ttyS0

Here we are using "xterm" as the terminal type, which works well if we are connecting using a gnome-terminal (or xterm) instance. It is likely preferable to set this to match the terminal type which will be used to connect. This can be found using this command in the host/client terminal you will be using to connect:

echo $TERM

For example, if you are running screen it is "screen", for byobu "screen-bce", etc.

You can also set the variable temporarily when you are already connected to the console:

export TERM=screen

  1. Connect using virsh

First we connect to the (local) qemu session:

virsh -c qemu:///session (this is optional, and if you prefer not to use a virsh console, you can simply prepend commands with virsh in bash. I.e. virsh start my_vm, virsh console my_vm etc)

Now in the virsh shell, start the virtual machine:

start my_vm

When it has started connect to the console:

console my_vm

Press Return to get the login prompt, and login as usual (note that username input works even if the login promt is not shown).

  1. You're in!

  2. Misc notes

Use Ctrl + ] to exit the console.

If the console acts strangely or displays incorrectly, check that the TERM variable is set correctly (see above).

If the terminal seems to cut off the top of the output, change it to have 80x24 of viewing area, e.g. in byobu this means you will need to account for the statusbar and have the window itself at 80x26

The set of instructions has been shamelessly copied with minor adjustments from https://help.ubuntu.com/community/KVM/Access

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