Oct 312012

As a Linux enthusiast and Gentoo user I was always looking for the perfect boot experience. While I managed to boot my kernel with EFI and grub 2 (as described in my wiki), I still had some troubles with OpenRC playing nice with my LVM-only setup initialized by dracut. Tonight I finally figured out the missing configuration pieces to shut up all warnings on system init.

Initial situation
All my Linux partitions are stored in a single LVM volume group, to stay as flexible as possible:

merkur ~ # lsblk
└─sda5 8:5 0 49.5G 0 part
├─vg_merkur-slash (dm-0) 253:0 0 2.5G 0 lvm /
├─vg_merkur-boot (dm-1) 253:1 0 200M 0 lvm /boot
├─vg_merkur-tmp (dm-2) 253:2 0 6G 0 lvm /tmp
├─vg_merkur-swap (dm-3) 253:3 0 4G 0 lvm [SWAP]
├─vg_merkur-var (dm-4) 253:4 0 4G 0 lvm /var
├─vg_merkur-usr (dm-5) 253:5 0 12.8G 0 lvm /usr
└─vg_merkur-opt (dm-6) 253:6 0 8G 0 lvm /opt

My boot toolset currently consists of grub-2.00-r1, kernel-3.6.4, dracut-024, lvm-2.02.95-r4 and openrc-0.11.2

Kernel Configuration
Before compiling the kernel, make sure to include all the required configurations. For this setup, the most important ones are:


Dracut Configuration
Before installing dracut, the desired modules have to be configured in /etc/make.conf. :

DRACUT_MODULES="caps lvm mdraid syslog"

For this setup at least the “lvm” module is mandatory. Further dracut was built with the “device-mapper” USE flag enabled.

Altough some Linux developers (especially from Red Hat/Fedora) advice against a separate /usr partition because of many boot time dependencies on this system path, I didn’t bother much to change my years old setup. Since version 014, dracut includes a module to fill this gap (/usr/lib/dracut/modules.d/98usrmount/mount-usr.sh). It simply mounts the /usr partition right after the root file system early in the boot process. Therefore we have to make sure that the dracut modules “usrmount” and “lvm” are included in the initramfs, which was possible without any manual modification of /etc/dracut.conf, when generating the boot image with:

dracut -H

Kernel Command Line Configuration
Dracut runtime parameters are given on the kernel command line in the Grub configuration. To automatically enable the LVM Volume Group and spawning a debug shell in case the boot should fail, I added the following parameters in grub:

root=/dev/vg_merkur/slash rd.lvm.vg=vg_merkur rd.shell

LVM Configuration
Since dracut is now responsible to enable our volume group, the corresponding init script has to be disabled:

rc-update del lvm boot

Fsck and Fstab
When booting the system now, the /etc/init.d/fsck script will complain that it cannot check the file systems which are already mounted. Fortunately, the init script allows us to define that fsck should be only run when specific “fs_passno” values are set. I therefore this value to “1” for the file systems which are mounted by dracut and to “2” for all the file systems which should be checked by OpenRC. Take care, when specifying a value of “0”, the file system will be never checked for consistency:

# [fs] [mountpoint] [type] [opts] [dump/pass]
/dev/vg_merkur/boot /boot ext2 noatime,nosuid,nodev 0 2
/dev/vg_merkur/slash / ext4 noatime,discard 0 1
/dev/vg_merkur/usr /usr ext4 noatime,discard,nodev 0 1
/dev/vg_merkur/var /var ext4 noatime,discard,nosuid,nodev 0 2
/dev/vg_merkur/opt /opt ext4 noatime,discard,nosuid,nodev 0 2
/dev/vg_merkur/tmp /tmp ext4 noatime,discard,nosuid,nodev 0 2
/dev/vg_merkur/swap none swap sw 0 0

In /etc/conf.d/fsck we then can define, that the fsck init script should only care about file systems with a “fs_passno” larger than “1”:


That’s it… If you have some questions or hints, please leave a comment.

Oct 182009

Since I was using the nice FineGradePermissions feature of the trac 0.11 release, and Debian was only providing trac-0.10.3 in Etch, I had a custom trac installation running on my Etch server. For migrating to Lenny you would normally think that it’s enough to just copy your project directory to the new installation. Unfortunately this results in a nasty error message:

DatabaseError: file is encrypted or is not a database

Hmn, so let’s check the trac migration guide which advises you to first export the sqlite database with sqlite3 in a plain SQL file. Not much luck here either, the result is an empty database:

# sqlite3 trac.db .dump

The reason is the trac installation in Etch was using the python-sqlite-1.0.1 back-end which uses the SQLite 2 format while in Lenny there is python-pysqlite2-2.4.1 which only knows about SQLite 3.

The conversion from SQLite 2 to 3 can be done by first exporting the database with the sqlite tool and then re-importing it with sqlite3:

# sqlite trac.db .dump | sqlite3 trac3.db

More infos about this can be found at the trac upgrade notes from 0.8.x to 0.9.

Finally your trac installation should work again as usual.

Oct 162008

Xen is nowadays a widespread approach to run virtual hosts. Thanks to its flexible configuration it’s not only possible to run virtual Linux hosts but nearly any operating system that is compatible to the Xen hypervisor machine architecture will possibly run as guest host. The Xen paravirtualization Continue reading »

Oct 032007

Recently I wanted to set up a testing server for the different virtualization techniques for Linux. For this I have an Asus P5LD2 mainboard with an Intel dual core Pentium D 3,2 GHz which supports the Virtual Machine Extensions (VMX). Thanks to this I can compile Xen with the ‘hvm’ USE-flag and run fully virtualized guest operating systems on my Xen supervisor. This means I could run nearly every i386 compatible operating system (even Windows 😉 ) in my Xen environment. Without such hardware every guest operating system has to have a Xen enabled kernel.

Another approach with the same result is the open source project QEMU. Its abstraction level is higher than with Xen and it can even emulate different target architectures from your current x86 host. So far x86_64, ARM, SPARC, PowerPC, MIPS and M68k target systems are supported. Its guest operating system does not need any single change to run on QEMU. This makes it very comfortable to test new live CDs or operating system images. But it is not so trivial to setup QEMU and Xen together on a Gentoo machine.

How to setup QEMU on 32bit Gentoo in Xen dom0?

If you compile Xen on a 32bit host you have to add ‘-mno-tls-direct-seg-refs’ to your CFLAGS. That is because the glibc TLS library is implemented in a way that will conflict with how Xen uses segment registers. For compiling the non-patched QEMU 0.9.0 you have to use a gcc version 3.x. The nowadays default gcc 4.x is not yet supported. After several compile failures I finally found to setup QEMU the following way:

1. For compiling gcc-3.x remove the ‘-mno-tls-direct-seg-refs’ from /etc/make.conf and set the ‘nossp’ and ‘nopie’ USE-flags. Otherwise gcc or later qemu will not compile.

2. Switch to gcc-3.x before compiling qemu-softmmu, qemu-user and qemu. In my case it’s: gcc-config i686-pc-linux-gnu-3.3.6

3. Check your CFLAGS again because the optimization flags for gcc 4.x are not always backwards compatible to gcc-3.x. In my case the make.conf looks like this:

# gcc-3.x
CFLAGS="-march=pentium4 -O2 -pipe -fomit-frame-pointer -mno-tls-direct-seg-refs"

# gcc-4.x for compiling gcc-3.3
#CFLAGS="-march=prescott -O2 -pipe -fomit-frame-pointer"

# gcc-4.x
#CFLAGS="-march=prescott -O2 -pipe -fomit-frame-pointer -mno-tls-direct-seg-refs"

4. Now you can compile QEMU. Do not forget to switch back to your original CFLAGS and gcc-4.x after successfully emerging QEMU. I recommend to you to also build the QEMU kernel accelerator module kqemu which has to be compiled with the same compiler as the kernel itself.

Now Xen and QEMU are able to run whatever operating system image you give them. Have fun with playing around…

Additional links:

Jul 242007

I think everybody that runs an own Linux server with the SSH daemon listening on port 22 is sooner or later annoyed by the amount of password attacks done by bots somewhere out in the Internet. What can you do against it?

Blocking via iptables ‘recent’ module
How you can do this on a Gentoo system is described in the Gentoo Wiki here. Because it blocks the connection attempt only due to the number of tries within a certain time it is a very basic solution and needs quite a lot of testing to examine the good parameters for the ‘hitcount’ and ‘seconds’ arguments. You do not unintentionally want to block yourself when you only try to open several connections within a short time period. So not really the thing I recommend here.

Log parsing with sshguard
sshguard uses another approach. It parses the SSH log messages and searches for login failures. For example when you try to connect with a non-existent user sshguard catches it and creates an iptables deny rule. But also sshguard has a small design mistake. It wants you to create a sshguard chain in iptables and redirect all the traffic to the chain assuming that your default INPUT policy is ACCEPT. When it wants to block a host it runs iptables -A sshguard -s host-to-block -j DROP. In case you have your policy set to DROP you cannot configure the iptables to accept the allowed SSH traffic because else the blocking rules will not work anymore. I made a small patch to change the blocking command to insert the rule in first place of the chain. After you applied the patch you have to make sure that you setup your iptables the following way:

iptables -N sshguard
iptables -A sshguard -j ACCEPT
iptables -A INPUT -p tcp --dport ssh -j sshguard

Further you have to edit your system logger configuration file. Please read the documentation.

For all the lazy people I even made a Ebuild that also adds a second patch where you can disable the IPv6 ability of sshguard. You can find it here.

Jul 092007

YES, YES, YES! Finally I made it.

Since a while I am playing with the migration functionality of a Xen domU between two Xen servers. It allows to move a virtual machine without reboot between two physical machines. But my problem was: It did not hold what it promised.

Trying all possible versions of Xen and the xen-sources in the Gentoo Portage I was able to migrate a domU but not when it was configured as SMP machine. After I found that the kernel has to include the CONFIG_HOTPLUG_CPU option, I was able to migrate the domU at least one time from one host to the other. When repeating this procedure the domU crashed with a kernel oops. By finally trying the latest Xen 3.1 version from the unofficial mescalito portage overlay with the xen-sources-2.6.18, repeated migration of a domU between the Xen servers does work now. Prerequisite is that both dom0 and domU run the 2.6.18 kernel. I am trying to run this in production now and will see how reliable it is.

I just remember that I did also remove the memory restriction from the dom0. The dom0_mem parameter in the grub configuration is optional and when omitting it the dom0 can use the unused memory for itself.

Jul 072007

For my home server I wanted to act a Xen domU as CUPS printing server. Therefore I needed access to my parallel printer connected to the Xen server. How are I/O-ports handled in Xen?

The I/O-addresses of the devices which can be found with cat /proc/ioports can be redirected from the Xen dom0. You have to enter the address range of the device you want to use in a domU in their configuration file (/etc/xen/domain-config). For my parallel port it would be:

ioports = [ '0278-027a' ]

Further I make sure that my dom0 does not access the device by removing parallel port support in the kernel configuration. When you boot then a domU kernel with parallel port and printer support it can access the device natively. Here a cut out from dmesg:

parport0: PC-style at 0x278 [PCSPP,TRISTATE]
parport0: Printer, Hewlett-Packard HP LaserJet 6P
lp0: using parport0 (polling).

In CUPS the printer is now accessible under /dev/lp0.

Unfortunately the documentation about this feature of Xen is a little bit rare. Another interesting thing is the redirection of PCI devices to a domU. Unfortunately I did not test this at my setup yet. I hope to come back to this topic later. For your creativity here some more examples from the net:

  • dulug.de – serielle Schnittstelle mit XEN
  • wiki.xensource.com – Assign hardware to DomU with PCI Backend as module
  • PCI Delegation in Xen
  • Jul 132006

    Die kürzlich gefundene Kernel-Schwachstelle CVE-2006-2451 der Kernel >2.6.13 und < und <, wodurch ein entfernter Angreifer root-Privilegien erhalten kann, wurde am prominenten Beispiel des Debian Entwicklungsservers Gluck demonstriert. Dank der schnellen Reaktion der Administratoren wurde nur /bin/ping komprimitiert. Sonst wurden noch keine veränderten Daten/Packete oder Programme entdeckt. Das zeigt wieder einmal, dass man als Administrator gut dran tut seine Kernelversionen aktuell zu halten und die Maschinen genau zu überwachen.

    Quelle: debian.org: Debian Server restored after Compromise

    Update – 7. August 2006: Für alle, welche noch einen der betroffenen Kernel auf einem produktiven System einsetzen, der Standardkernel (2.6.8) von Debian Sarge ist übrigens nicht betroffen, gibt es ein Workaround: Mitigating against recent GNU/Linux kernel bugs

    Jun 282006

    Vielleicht ist es schon jemandem aufgefallen. Ganz unten auf der Seite ist eine Statuszeile von Spam Karma 2 einem Programm, welches dieses WordPress-Blog von ungewollten Einträgen schützt. Seit ich dieses SK2 Plugin vor genau einer Woche installiert habe, sind jetzt sage und schreibe bereits 124 Spam Einträge gefiltert worden. Nein, ich habe definitiv keine Lust, dass jemand auf meiner Seite mit zwielichtigen Sachen wirbt. Die geposteten Links müssen aber nicht immer nur auf Werbeangebote führen. Im schlimmeren Fall haben sie auf ihrer Zielseite Code eingebaut, um durch eine Browserschwachstelle ungewollte Programme zu installieren. Ich möchte nicht unerwähnt lassen, dass der Internet Explorer immer noch Angriffsziel Nummer eins ist und daher nicht als bevorzugter Browser gewählt werden sollte. Diese Spameinträge werden dann schliesslich auch von solchen ungewollten Programmen gemacht, sogenannten Spam Bots. Das macht die Filterung relativ einfach. Beispielsweise wird auf die Zeit geschaut, welche verstreicht nach dem Aufrufen der Seite bis zum Abschicken des Kommentars. Nun kommen wir aber zum eigentlichen Problem, nämlich der Ursache dieser Bots. Und schon sind wir wieder einmal bei Windows angelangt. Leider ist dieses Betriebssystem für die meisten Benutzer so einfach zu bedienen, dass sie keine weiteren Ansprüche stellen sich tiefer mit der Materie auseinander zu setzen, was bei Windows leider eindeutig nötig wäre… hier haben wir jetzt den Mist. Merci

    Jun 242006

    Vor einiger Zeit habe ich berichtet, wie man ohne Windows Media Player die neusten .wmv Codecs installiert (Apple OS X und Windows Media Files). Wenn man aber versucht die Windows Media Components auf einem Intel Mac einzurichten, meldet der Installer, dass diese nicht unter dem x86 OS X laufen. Eine kompatible Version ist zwar angekündigt, aber noch nicht erhältlich. Genau hier kommt jetzt Rosetta ins Spiel. Dabei handelt es sich um einen im Apple OS X integrierten Binary Translator, welcher ermöglicht, Programme welche für die PowerPC Architektur kompiliert wurden, auch auf Intel Macs laufen zu lassen. In unserem Fall wenden wir ihn auf den Installer an, welcher das “Flip4Mac WMV.mpkg” installiert. d.h. Rechtsklick auf /Applications/Utilities/Installer. Dann auf “Get Info” und dort “Open with Rosetta”. Danach können wir die Windows Media Components wie gewohnt installieren. Nun muss man natürlich auch noch den QuickTime Player unter Rosetta laufen lassen und kann dann sämtliche Windows Media Files auch auf seinem Intel Mac geniessen.

    Quelle: OSNews.org: How to Get Your Intel Mac to Play WMV Files

    Update – 7. August 2006: Mittlerweile ist die Version 2.1 der Windows Media Components for Quicktime herausgekommen, welche auch nativ unter Intel Macs funktioniert: Flip4Mac WMV 2.1