VM Serial Console part 2

Wednesday, November 9, 2011

Fooling around a bit more with accessing a VM's serial console from a KVM hypervisor with

virsh console mymachine

I found one thing that doesn't carry over from the host to the VM is the terminal window size, so if you try to use something like vim through the console connection, it seems to assume a 80x25 or so window, and when vim exits your console is all screwed up.

It looks like a serial connection doesn't have an out-of-band way of passing that info the way telnet or ssh does, so you have set it manually. You can discover your settings on the host machine with

stty size

which should show something like:

60 142

on the VM, the same command probably shows

0 0

zero rows and columns, no wonder it's confused. Fix it by setting the VM to have the same rows and columns as the host with something like:

stty rows 60 columns 142

and you're in business.

Enabling VM serial console on stock Ubuntu 10.04 server

Wednesday, November 2, 2011

So I've been running Ubuntu 10.04 server virtual machines on a host running KVM as the hypervisor, and thought I should take a look at accessing the VM's console from the host, in case there's a problem with the networking on the VM.

The hosts's VM libvirt definition shows a serial port and console defined with

<serial type='pty'>
  <source path='/dev/pts/1'/>
  <target port='0'/>
  <alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/1'>
  <source path='/dev/pts/1'/>
  <target type='serial' port='0'/>
  <alias name='serial0'/>
</console>

and within the stock Ubuntu 10.04 server VM, dmesg | grep ttyS0 shows:

[    0.174722] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    0.175027] 00:05: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A

So the virtual hardware is all setup on both ends, but ps aux | grep ttyS0 doesn't show anything

We need to have a process listening to that port. To do that, create a file named /etc/init/ttyS0.conf with these contents:

# 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 38400 ttyS0 xterm-color

and then run

initctl start ttyS0

back in the host machine run virsh list to find the name or id number of your VM, and then

virsh console <your-vm-name-or-number>

to connect, hit return and you should see a login prompt.

Customizing cloned Ubuntu VMs

Sunday, October 30, 2011

I was playing with creating and cloning Ubuntu virtual machines the other day, and got to the point where I had a nicely setup reference image that I could just copy to fire up additional VMs that would be in a pretty usable state.

There are a few things within a cloned VM that you'd want to change if you were going to keep the new instance around, such as the hostname, SSH host keys, and disk UUIDs. I threw together a simple shell script to take care of these things automatically.

#!/bin/sh
#
# Updates for cloned Ubuntu VM
#

#
# Some initial settings cloned from the master
#
ROOT=/dev/vda1
SWAP=/dev/vdb1
LONG_HOSTNAME=ubuntu.local
SHORT_HOSTNAME=ubuntu

if [ -z $1 ]
then
    echo "Usage: $0 <new-hostname>"
    exit 1
fi

# 
# Update hostname
#
shorthost=`echo $1 | cut -d . -f 1`
echo $1 >/etc/hostname
hostname $1
sed -i -e "s/$LONG_HOSTNAME/$1/g" /etc/hosts
sed -i -e "s/$SHORT_HOSTNAME/$shorthost/g" /etc/hosts

#
# Generate new SSH host keys
#
rm /etc/ssh/ssh_host_*
dpkg-reconfigure openssh-server

#
# Change root partition UUID
#
OLD_UUID=`blkid -o value $ROOT | head -n 1`
NEW_UUID=`uuidgen`
tune2fs -U $NEW_UUID $ROOT
sed -i -e "s/$OLD_UUID/$NEW_UUID/g" /etc/fstab /boot/grub/grub.cfg

#
# Change swap partition UUID
#
OLD_UUID=`blkid -o value $SWAP | head -n 1`
NEW_UUID=`uuidgen`
swapoff $SWAP
mkswap -U $NEW_UUID $SWAP
swapon $SWAP
sed -i -e "s/$OLD_UUID/$NEW_UUID/g" /etc/fstab

#
# Remove udev lines forcing new MAC address to probably show up as eth1
#
sed -i -e "/PCI device/d"     /etc/udev/rules.d/70-persistent-net.rules
sed -i -e "/SUBSYSTEM==/d" /etc/udev/rules.d/70-persistent-net.rules

echo "UUID and hostname updated, udev nic lines removed,  be sure to reboot"

I'd then run it on the cloned machine with something like

update_clone.sh mynewmachine.foobar.com

This somewhat particular to my specific master VM, in that it's expecting one disk dedicated to root and one disk dedicated to swap, and the VM was created with ubuntu.local as the hostname. Hopefully though it'll give some ideas about what to look for and how to script those changes.