Posts Tagged ‘Linux’

Wireless USB keyboards and delayed keystrokes

April 29, 2012 Leave a comment

Everytime I was using my wireless USB keyboard with my laptop while the power cable was not connected, the keyboard behaviour was lousy, and keystrokes tend to be delayed by several seconds. The kernel logs said something like Read more…

Tell Xorg to re-grab the keyboard

December 29, 2011 Leave a comment

OK, I was doing some debugging with Xorg, and thought I had to use the Magic SysRq key to kill it. But when I had pressed Alt-SysRq-R to give the keyboard control from Xorg back to the kernel, it turned out that I not longer needed to do another SysRq because my Xorg magically worked again… 😉 Unfortunately now, everytime I pressed Alt-F4 to close a window, I found myself on tty4… rather poor. So I needed some way to tell Xorg to grab the keyboard again, and there it is: Just open an xterm and execute

sudo kbd_mode -s
Categories: fix, howto Tags: , , , ,

X screen shots from the console

March 24, 2011 2 comments

I had to debug a machine that was behind a DSL-2000 connection, and had about 20 KB/s of upstream. For that, I needed to see what was going on on the X screen, but due to the low bandwidth (and a screen resolution of 1920×1080), VNC was about as fast as 2 frames per minute.

But I found a comparable replacement: The ImageMagick suite has a program called import that allows you to dump the contents of the X screen to a image file. So I took a few screen shots from the console via DISPLAY=:0 import -window root foo.png and then copied the files to my machine.

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Categories: howto Tags: , , , , ,

Change partition type without reformatting

January 2, 2011 Leave a comment

Note to myself: it is possible to change the partition type of a already formatted (and used) partition. For example, if you have already formatted the partition with NTFS, but accidentally had created it with partition type 0x83 (Linux), so Windows can’t read it, since it expects 0x07 (HPFS/NTFS). On Linux, you can use sfdisk for that purpose:

# Be root
dd if=/dev/sdb of=sdb-bootsector count=1  # backup boot sector
sfdisk -d /dev/sdb | sed -e 's/Id=83/Id=07/' > /tmp/sdb.txt   
sfdisk /dev/sdb < /tmp/sdb.txt

(fill in the right values for your case)

Of course, good old fdisk works also, use the t command.


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SSH key authentication with encrypted home directories

October 9, 2010 5 comments

Yesterday, I ran into an interesting problem: I tried to set up SSH public key authentication between two of my machines, c3po and r2d2, so I could log in from rohieb@r2d2 to rohieb@c3po without a passphrase. However, everytime I tried to login to c3po, I was prompted to enter the passwort for rohieb@c3po, and the debug output mentioned something that thekey could not be verified. More astonishing, when I established a second SSH connection while the first was still running, I was not prompted for a password, and debug output said that key authentication had been sucessful. I googled a bit, and after a while got to this comment on Launchpad, mentioning problems when the user on the remote machine had its home directory encrypted through ecryptfs – which was the case for me. Of course, since ecryptfs only encrypts the user’s home after he has been authenticated, the SSH daemon cannot read his ~/.ssh/authorized_keys at the first time, and falls back to password authentication.

The Launchpad comment proposes to first unmount the ecryptfs filesystem, then store ~/.ssh/authorized_keys unencrypted, and then mount the encrypted home again (note that no program should be running that could try to access your home directory):

 $ ecryptfs-umount-private
 $ cd $HOME
 $ chmod 700 .
 $ mkdir -m 700 .ssh
 $ chmod 500 .
 $ echo $YOUR_REAL_PUBLIC_KEY > .ssh/authorized_keys
 $ ecryptfs-mount-private

This works indeed, but has the drawback that key authentication only works for the first login, because ecryptfs hides the unencrypted files when it mounts the encrypted directory on login; and you had to synchronize the encrypted and the unencrypted version of authorized_keys everytime you add a new key. To circumvent that, I simply moved the file to /etc/ssh/authorized_keys/rohieb (with the file only readable and writable by me, and /etc/ssh/authorized_keys writeable for all users) and adjusting /etc/ssh/sshd_config appropriately:

$ sudo vi /etc/ssh/sshd_config  # or use gedit instead of vi, or whatever...
[... some lines ...]
AuthorizedKeysFile /etc/ssh/authorized_keys/%u
[... some more lines ...]
$ sudo /etc/init.d/ssh restart


There is a better approach instead, which doesn’t need the SSHd config to be edited at all:

  • login to the user on the remote machine
  • create /home/.ecryptfs/$USER/.ssh and put your authorized_hosts there
  • symlink your encrypted version there:
    $ ln -s /home/.ecryptfs/$USER/.ssh/authorized_hosts ~/.ssh/authorized_hosts
  • symlink your unencrypted version there (as above, make sure no process wants to write to your home directory in the meantime):
    $ ecryptf-umount-private
    $ mkdir ~/.ssh
    $ ln -s /home/.ecryptfs/$USER/.ssh/authorized_hosts ~/.ssh/authorized_hosts
    $ ecryptfs-mount-private

The paths are for Ubuntu 9.10 (Karmic Koala) and later. On other systems, you might want to replace /home/.ecryptfs with /var/lib/ecryptfs.

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ZSNES on AMD64 Ubuntu

October 6, 2010 11 comments

[ Update, 2013-10: This post post is not up to date anymore. On newer Debians (since 7.0/wheezy) and Ubuntus (at least since 12.04, Precise Pangolin), you should be able to install zsnes out of the box: sudo apt-get install zsnes:i386. For details see the MultiArch documentation for Debian and Ubuntu. ]

Before I had bought my current hardware, I was working on a 32-bit-based system, and I really appreciated ZSNES as an SNES emulator. But unfortunately, my new hardware was an AMD64 system, and there is currently no ZSNES package for 64-bit Ubuntu or Debian 😦 So I decided to google a bit about the issue, but it took me until now (a year later) to get ZSNES finally working on my machine. The problem is, if you build ZSNES on a 64-bit machine, all the application does is segfault at start, and if you try to compile for 32-bit systems, you get errors about missing 32-bit libs (in particular, configure does not find a suitable libsdl). Instead, if you just take the binary which was compiled on a 32-bit system, and install ia32-libs everything seems to work—at least I was able to play a few levels of Super Mario World succesfully 🙂

So here was my idea: take the 32-bit package from the Ubuntu repository, and just change the Architecture control field, and by this fool dpkg 😛 And as it turned out, this idea worked great. You can get the Debian package here if you want, it should work for Ubuntu Karmic and Lucid, as well as for Debian testing (but I only tested it on Lucid, so there is no warranty here—but I’m happy to hear if it works :)):

For the curious people reading here, here is what I actually did:

  1. wget
  2. ar x zsnes_1.510-2.2ubuntu3_i386.deb
  3. tar xzf data.tar.gz
  4. Edited usr/share/applications/zsnes.desktop and added -ad sdl to the Exec: field, otherwise it would just segfault on the first run:
    Exec=zsnes -ad sdl
  5. Edited usr/share/doc/zsnes/changelog.Debian.gz and added a new changelog entry for the version
  6. tar xzf control.tar.gz
  7. Edited the control file, changed the Version: and Architecture: field to amd64, added the ia32-libs dependency, and set myself as maintainer:
    Package: zsnes
    Version: 1.510-2.2ubuntu3~ppa1
    Architecture: amd64
    Maintainer: Roland Hieber <>
    Installed-Size: 4160
    Depends: ia32-libs, libao2 (>= 0.8.8), libc6 (>= 2.4), libgcc1 (>= 1:4.1.1), libgl1-mesa-glx | libgl1, libpng12-0 (>= 1.2.13-4), libsdl1.2debian (>= 1.2.10-1), libstdc++6 (>= 4.1.1), zlib1g (>= 1:
  8. Changed the md5sums file for the right values for usr/share/applications/zsnes.desktop and usr/share/doc/zsnes/changelog.Debian.gz (I used the md5sum command and copy-pasted it)
  9. tar czf control.tar.gz control md5sums postrm postinst
  10. tar czf data.tar.gz usr/
  11. ar r zsnes_1.510-2.2ubuntu3~ppa1_amd64.deb debian-binary control.tar.gz data.tar.gz

I’m afraid that I can’t put the package to PPA, Launchpad only accepts source packages for uploads, and builds the binary packages itself, both for i386 and AMD64. This approach can not be used here, since we needed the i386 binary for AMD64.

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Standby with Lenovo Thinkpad SL510 on Ubuntu Lucid

July 30, 2010 Leave a comment

…note to myself: Remove a potentially mounted SD card before suspending your SL510, otherwise the kernel gets stuck…

Update: Turned out the SD card was a bit buggy, the driver mostly got a timeout when trying to speak with it.