Feb 02 2014

Migrating from KVM to LXC

Category: LinuxTuxevara @ 18:58

After I had to replace the mainboard of my HTPC, on which also two other virtual machines were running on KVM, the kvm_amd module crashed on every boot on the replacement hardware. Though KVM still worked, I don’t like to see any modules crashing on boot. I began asking myself whether I really need full KVM virtualization or if some kind of container based virtualization would do a good or even better job for me.

After reading into the pros and cons of different solutions, I concluded that LinuX Containers (LXC) should probably first choice for me. As the HTPC is running on Ubuntu 12.04, which also has Apparmor profiles that enhance the security of LXC’s weak security concept, I actually decided to continue with LXC.

I don’t want to explain how to install LXC, because this is already covered by many other sites. The only thing that I found which is not covered well enough, is the conversion of machines from KVM (or similar) to LXC. In my case the KVM guests where using RAW disk images, so I am exclusively focusing on converting such below.

Step 1 – Prepare the rootfs folder

First, the new target folder for the root file system of the LXC guest must be created.

mkdir -p /var/lib/lxc//rootfs

Step 2 – Mount the RAW image

Then the RAW disk image must be mounted to access the files. In my case the disk contained only one partition. Modify the mount command may be necessary.

kpartx -a
mount /dev/mapper/loop0p1 /mnt

Step 3 – Copy files to rootfs

Now that the content of the RAW image can be accessed, the files can be copied into the new rootfs folder created in step 1. I have been using the below command for years, to create more or less exact copies:

cd /mnt
find . -xdev | cpio -pmv /var/lib/lxc//rootfs

Step 4 – Modify the configuration

Now comes the trickiest part: The configuration of the new machine must be modified, otherwise it is unlikely that it will boot successfully. Most required changes can be extracted from the template files in /usr/lib/lxc/templates/, which are typically used for the creation of new machines. Below are the modification for Debian machines, which worked perfectly well for Debian Lenny (shame in me) and Squeeze.


cat < $rootfs/etc/inittab
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
# Normally not reached, but fallthrough in case of emergency.
1:2345:respawn:/sbin/getty 38400 console
c1:12345:respawn:/sbin/getty 38400 tty1 linux
c2:12345:respawn:/sbin/getty 38400 tty2 linux
c3:12345:respawn:/sbin/getty 38400 tty3 linux
c4:12345:respawn:/sbin/getty 38400 tty4 linux

mkdir -p $rootfs/selinux
echo 0 > $rootfs/selinux/enforce

mknod $rootfs/dev/tty1 c 4 1
mknod $rootfs/dev/tty2 c 4 2
mknod $rootfs/dev/tty3 c 4 3
mknod $rootfs/dev/tty4 c 4 4

# reconfigure some services

locale="$LANG $(echo $LANG | cut -d. -f2)"
chroot $rootfs echo "locales locales/default_environment_locale select $LANG" | chroot $rootfs sh -c "LANG=C debconf-set-selections"
chroot $rootfs echo "locales locales/default_environment_locale seen true" | chroot $rootfs sh -c "LANG=C debconf-set-selections"
chroot $rootfs echo "locales locales/locales_to_be_generated seen true" | chroot $rootfs sh -c "LANG=C debconf-set-selections"
chroot $rootfs sed -i -e "0,/^[# ]*$locale *$/ s/^[# ]*$locale *$/$locale/" /etc/locale.gen
chroot $rootfs sh -c "LANG=C dpkg-reconfigure locales -f noninteractive"

# remove pointless services in a container
chroot $rootfs /usr/sbin/update-rc.d -f checkroot.sh remove # S
chroot $rootfs /usr/sbin/update-rc.d checkroot.sh stop 09 S .

chroot $rootfs /usr/sbin/update-rc.d -f umountfs remove # 0 6
chroot $rootfs /usr/sbin/update-rc.d umountfs start 09 0 6 .

chroot $rootfs /usr/sbin/update-rc.d -f umountroot remove # 0 6
chroot $rootfs /usr/sbin/update-rc.d umountroot start 10 0 6 .

# The following initscripts don't provide an empty start or stop block.
# To prevent them being enabled on upgrades, we leave a start link on
# runlevel 3.
chroot $rootfs /usr/sbin/update-rc.d -f hwclock.sh remove # S 0 6
chroot $rootfs /usr/sbin/update-rc.d hwclock.sh start 10 3 .

chroot $rootfs /usr/sbin/update-rc.d -f hwclockfirst.sh remove # S
chroot $rootfs /usr/sbin/update-rc.d hwclockfirst start 08 3 .

chroot $rootfs /usr/sbin/update-rc.d -f module-init-tools remove # S
chroot $rootfs /usr/sbin/update-rc.d module-init-tools start 10 3 .

rm $rootfs/etc/udev/rules.d/70-persistent-net.rules

Step 5 – Create LXC config

Finally we have to create a LXC configuration file for the new machine. Lazy as I am, I have copied an existing config file into /var/lib/lxc// and modified the paths and network configuration accordingly.

After that the machine can be started with

lxc-start -n

Keep in mind that you won’t be able to detach from that console again. But to debug boot problems it is essential to not launch the machine in background mode (-d).

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Oct 31 2013

Setting IMAP INTERNALDATE to header date

Category: Computer,LinuxTuxevara @ 11:02

While setting up a self refilling test mail server, I came across the problem that I need other IMAP INTERNALDATEs (aka arrival date) than the create/modify time of the email file.
As my email generator script creates random dates for the email headers that are between 10 years back and today, it would make perfect sense to also use them as the arrival date of the message.

Five minutes later the following little script was finished, which I think could be pretty useful for anyone who has to update the arrival date in his IMAP server that uses Maildir format or similar. This could for instance become quite handy after an email migration where the IMAP INTERNALDATE could not be retained.


for FILE in `find $1 -type f`
    DATE=`grep "Date" $FILE | cut -d ":" -f 2- | sed -e 's/^ *//g' -e 's/ *$//g'`
    if [ -n "$DATE" ]
        echo "Setting modified time of \"$FILE\" to \"$DATE\"."
        touch -c $FILE --date="$DATE"
        echo "No date found in \"$FILE\"."

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Mar 28 2012

Heise Open: “LiMux: Billiger und robuster als Windows”

Category: Linux,PolitikTuxevara @ 20:27

Wie Heise Open heute in seinem Artikel schreibt, hat Oberbürgermeister Ude in München, festgestellt, dass es nach derzeitigem Verlauf des Projekts darauf hinaus läuft, dass die Kosten vom Linux-Einsatz dauerhaft geringer sind als Windows und auch noch deutlich robustere Systeme hervorbringt.

Selbst mich als Linux-Anhänger überrascht diese Aussage eines Amtsträgers doch sehr. Hoffen wir mal, dass es so weiter geht mit LiMux und es nicht im gleichen Debakel endet wir im Auswärtigen Amt.

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