Friday, January 27, 2012

Digging Tunnels: Connection Problem on Debian

If you follow the instructions on my first "Digging Tunnels" post, to setup ssh tunneling over SSL with stunnel, then you may find that you can't access your Debian box from another Debian box. I got the following error:
SSL_accept: 1408F10B: error:1408F10B:SSL routines:SSL3_GET_RECORD:wrong version number

The fix (found on is to make sure you have the following in lines in /etc/stunnel/stunnel.conf:
sslVersion = all
options = NO_SSLv2

Friday, January 20, 2012

Boot Floppy and ISO Images with GRUB2 on Debian

I've once described my setup for booting floppy disk images. That setup works rather nicely, but there's a better way of doing this on Debian:
  1. install grub-imageboot:
    aptitude install grub-imageboot
  2. put all image files you want to be able to boot in /boot/images
  3. run (as root):
and that's it!

grub-imageboot performs the same task as my original setup, in much the same way, but it also supports ISO images, is easier to install and much easier to uninstall.

A related tool, from the same authors (the fine Grml team) is grml-rescueboot, which lets you boot into a Grml rescue system from Grml ISO images placed in /boot/grml.

Thursday, January 12, 2012

Stuttering Video Playback Due To USB Device Reset

My kids don't usually complain when they're watching a movie on my PC, if video playback stutters. It seems that as long as playback resumes in a second or less, they don't mind the interruptions. I guess they've learned that if they do complain, chances are that they'll have to wait several long minutes, until their OCD inflicted dad "fixes" the problem.

I hate it when video playback stutters. So much so that I usually first download whatever video I want to watch and then watch it offline. Which is why I am a get-flash-videos fanboy.

But last time we were hit with such a problem, our lame ISP was not the culprit. This time, the video source was a backup ISO image of some DVD, stored on an external hard disk, connected to a Debian box (my old, headless laptop) over USB, accessed via SAMBA over a wireless connection, and played back on my WinXP laptop. The hiccups were rather noticeable, even by my lag-tolerant kids, to the point that they complained about it.

At first, it seemed that debugging this would be a daunting task - there are just so many factors involved. But I got lucky - I tried playing that same DVD image on my wife's laptop, over a wired network connection, and was hit by the same playback hiccups. This ruled out most of the suspects except for my old Debian laptop.

And yet, I had no idea what to look for now, especially because I can't just playback video on my old laptop - it's screen is busted... So I just browsed for any suspicious message in the output of dmesg - and found the following message, repeated several times:
"reset high speed USB device using ehci_hcd and address 3"

These messages seemed very relevant, and a quick search brought me to several blog posts, forum threads and bug reports.
I tried one the solutions/workarounds suggested on those links:
echo 128 > /sys/block/sdc/device/max_sectors
and it actually did the trick and made the hiccups go away.

The next challenge was to make this happen on system start-up. The Right Thing™ to do would have been to modify /etc/sysctl.conf, but there's always the chance that the relevant block device would be renamed, e.g. when another external disk drive is attached. So I ended up adding the following line line to /etc/init.d/
echo 128 > /sys/block/$(readlink -f /dev/data | sed -e 's/1//g' -e 's/\/dev\///g')/device/max_sectors
where /dev/data is a symbolic link that points to this block device, which is generated with an appropriate UDEV rule.