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Xen: Running a Sid DomU with PyGrub on a Squeeze Dom0 //at 03:07 //by abe

from the one-and-a-half-generation-away dept.

I’m running one Debian Sid and one Jessie (Testing) Xen guest domain on a Debian Squeeze (Oldstable) Xen 4.0 running host server.

Recently I had to reboot one these virtual machines after more than a year of uptime. But the new 3.14 kernel from Debian Experimental didn’t boot. Neither did 3.13 from Debian Unstable. Nor did any other kernel image newer then the 3.5-trunk (from Debian Experimental back than) work.

Everytime pygrub bailed out with this error message:

Error: (2, 'Invalid kernel', 'xc_dom_find_loader: no loader found\n')

(Yes, the parentheses and the “\n” were part of the error message.)

After some searching on the web I found hints that this message may be caused by an unsupported compression type in the kernel image.

And indeed, if I unpack the “vmlinuz” with the extract-vmlinux tool which is part of Linux’ source code (but not yet part of any binary package in Debian), and use the extract file in grub’s menu.lst (which is then read by pygrub) instead, the DomU boots Linux kernel 3.14 again, even on a Squeeze-running Dom0.


PDiffs are still useful //at 03:26 //by abe

from the not-that-bad dept.

… probably just not as default.

I do agree with Richi and with Michael that disabling PDiffs by default gives the big majority of Debian Testing or Unstable users a speedier package list update.

I’m though not sure, if disabling PDiffs by default would

  • also have an performance impact on our mirrors — it surely would have a traffic impact on the mirrors;
  • really bring a benefit for Debian Stable users as Debian Stable changes seldomly and hence there are not that many PDiffs to download and apply — at least I can’t remember being annoyed by PDiffs anywhere else than on Debian Testing and Debian Unstable. Even the repositories with security updates don’t change that often.

Additionally I want to remind you that PDiffs per se are nothing bad and should be continued to be supported:

  • Because there are still areas, even in “civilized” countries, where only small bandwidth is available and where using PDiffs reduces the download time a lot. Yes, also in Germany. BTDT. Only until recently there was no Fibre, no DSL, very bad UTMS reception and otherwise just EDGE at my parents’ home. (LTE was available far too expensive until recently.) And I was very happy about not having to download 30 MB or such just for seeing if there are updates at all, because 25 kB/s was the fastest download rate I could get (peaks, not average).
  • Because it seems to be in fashion with big ISP near-to-monoplists, especially in Germany, to cut off your nice bandwidth if you transfer too many Megabytes. Keyword “Drosselkom”. If you happen to be a customer of such a shitty ISP, you may be happy to reduce your traffic amount by using PDiffs instead of downloading the full package list every time.

So yes, disabling PDiffs by default is probably ok, but the feature must be kept available for those who haven’t 100 MBit/s fibre connection into their homes or are sitting just one hop away from the next Debian mirror (like me at work :-).

Oh, and btw., for the very same reasons I’m also a big fan of debdelta which is approximately the same as PDiffs, just not for package lists but for binary packages. Using debdelta I was able to speed up my download rates over EDGE to up to virtual 100 kB/s, i.e. by factor four (depending on the packages). Just imagine a LibreOffice minor update at 15 kB/s average download rate. ;-)

And all these experiences were not made with a high-performance CPU but with the approximately 5 year old Intel Atom processor of my ASUS EeePC 900A. So I used PDiffs and debdelta even despite having a slight performance penalty on applying the diffs and deltas.


Showing packages newer than in archive with aptitude //at 22:14 //by abe

from the handy-aptitude-TUI-filters dept.

I happens quite often that I install a manually built, newer version of some package on a machine. Occassionally I forget to remove it or to downgrade it to the version in the APT repo.

$ apt-show-versions | fgrep newer

easily finds those packages.

But usually when doing such a check, I want this list of packages in my aptitude TUI to have a look at the other versions of that package and to take actions. And I don’t want to manually search for each of the package manually.

This can be done with the following “one-liner”:

# aptitude -o "Aptitude::Pkg-Display-Limit=( `apt-show-versions | fgrep newer | awk -F '[ :]' '{printf "~n ^"$1"$ | "}' | sed -e 's/| *$//'` )"

It uses apt-show-version’s output, searches for the right packages, takes the first column and transforms it into an aptitude search pattern matching all packages whose name is exactly one of the listed packages.

But this solution is quite ugly and slow. So I wondered if this is also doable with pure aptitude search patterns which likely would also be faster.

And after some playing around I found the following working aptitude search term:

~i ?any-version(!~O.) !~U !~o

This matches all packages which which are installed and which have a version which has no origin, i.e. no associated APT repository. Since this also matches all hold packages as well as all packages not available in any archive, I use !~U !~o to exclude those packages from that list again.

Since nobody can remember that nor wants to type that everytime needed, I added the following alias to my setup:

alias aptitude-newer-than-in-archive='aptitude -o "Aptitude::Pkg-Display-Limit=~i ?any-version(!~O.) !~U !~o"'

Only caveat so far:

It seems to also match packages from APT repos which haven’t set an “Origin”. This should not happen with any Debian or Ubuntu APT repository, but seems to happen occasionally with privately run APT repositories.

And using ~A instead of ~O, i.e. ~i ?any-version(!~A.), does not work for this case either, despite it matches installed packages of which versions not in any available archive exist. But unfortunately aptitude seems to remember in some way if a package was in some archive in the past, so this only shows packages installed with dpkg -i, but not packages removed from e.g. unstable but with older versions still being available in stable.

Next Debian Meetup in Zurich //at 20:16 //by abe

from the beat-the-drum dept.

The first Debian meetup in Zurich last month was quite a success and I look forward to further Debian meetups in Zurich — every first Tuesday of the month.

The next meetup will be

on Tuesday, 2013-Dec-03 starting at 18:30 CET
at St. Gallerhof, Konradstrasse 2, 8005 Zurich

Please note that this is a different location than last month.

Everybody who is interested in Debian is welcome to join us. Registering is not necessary.

There was also some interest in Debian meetups in other Swiss cities, namely Bern and somewhere around Lac Leman. In case you want a Debian meetup elsewhere in Switzerland, too, or if you’re interested in any Debian meetup in Switzerland, feel free to join the Swiss Debian Community Mailing List and help organising other Swiss Debian meetups.


Debian-Stammtisch in Zürich //at 12:49 //by abe

Aus der Let's-do-it Abteilung

Wir (Michael Stapelberg und ich) wollen in Zürich einen regelmässigen Debian-Stammtisch, eine Debian-Lokalgruppe etablieren.

Das erste Treffen findet statt:

  • am Dienstag, den 5. November 2013, ab 19 Uhr (CET)
  • im Gloria, Josefstrasse 59, 8005 Zürich.

Jeder, der sich für Debian interessiert, ist eingeladen. Eine Anmeldung ist nicht notwendig.

Als regelmässigen Termin visieren wir den ersten Dienstag im Monat an.

Diesen Termin haben wir gewählt, weil er unseren Recherchen nach keine Terminkonflikte mit den Lokalgruppen der LUGS in Zürich und Winterthur, der FSFE in Zürich, dem CCC ZH, den Tuxeros oder dem Webtuesday gibt. In Kauf genommen haben wir Kollisionen mit wöchentlichen Treffs in der Hackerspaces Ruum42 in St. Gallen und Reaktor23 in Waldshut-Tiengen.

Es kam aber dennoch bereits die erste Meldung bzgl. potentieller Terminkonflikte rein. Wir diskutieren gerne noch über den Termin, sowohl am Stammtisch selbst, aber auch auf der Community-Mailingliste. Bevorzugte Sprache auf der Mailingliste ist Englisch, Deutsch ist aber auch in Ordnung.

Potentielle Terminänderungen werden primär auf der Community-Mailingliste und im Debian-Wiki auf der Lokalgruppen-Seite bekanntgegeben.

Die Termine wollen wir aber auch über die LUGS-Termine-Liste, die es auch als iCal oder E-Mail-Reminder gibt, publizieren. Danach sollten die Termine auch auf Freie Termine erscheinen.

Hui, ist das lange her, daß ich hier was auf Deutsch geschrieben habe. :-)


Searching in Screen’s copy mode //at 23:43 //by abe

from the It-would-be-neat-if-that-would-work.-Oh,-it-does-work! dept.

I’m using GNU Screen daily for definitely more than a decade and I became maintainer of Debian’s screen package nearly exactly two years ago. Nevertheless it still happens occassionally that I discover features yet unknown to me. Recently I had one of these moments again:

I looked for a specific line in the long output of a command which has run inside a Screen session. For that I entered Screen’s copy mode with Ctrl-A [ and scrolled around with arrow keys and page-up and -down keys.

But didn’t find it. I thought, it would be cool if I can search for the string I’m looking for. Intuïtively I typed /, the search string and pressed enter. And it worked! It jumped to the next occurrence of that string.

Of course I immediately had to check if tmux has such a feature, too. And it indeed has, but it seems to be a less sophisticated implementation:

Feature Key-binding in GNU Screen Key-binding in Tmux
Switch into copy/scroll mode
(needed for the remainder)
Ctrl-A [ Ctrl-B [
Search for string once, forward / + string + Enter Ctrl-S + string + Enter
Search for string once, backward ? + string + Enter Ctrl-R + string + Enter
Search for string again, forward / Enter Ctrl-S Enter
Search for string again, backward ? Enter Ctrl-R Enter
Incremental search for string, forward Ctrl-S + string -
Incremental search for string, backward Ctrl-R + string -
(Incremental) search for next occurrence, forward Ctrl-S again -
(Incremental) search for next occurrence, backward Ctrl-R again -

Being able to do incremental search like with GNU Emacs gave me yet another reason for continuing to use Screen and not to switch Tmux. ;-)


How to make wget honour Content-Disposition headers //at 16:12 //by abe

from the DWIM dept.

Download links often point to CGI scripts which actually generate (or just fetch, i.e. proxy) the actual file to be downloaded, e.g. URLs like

Most of such CGI scripts send the real file name in the Content-Disposition header as specified in the MIME Specification.

All browsers I know (well, at least those I use regularily :-) handle that perfectly and propose the file name sent in the Content-Disposition header as file name for saving the downloaded name which is usually exactly what I want.

All browsers do that, …, just not my favourite commandline download tool GNU Wget … Downloading the above URL with wget would look like this with default settings:

$ wget ''
--2013-10-02 16:04:16--
Resolving (, 2606:2800:220:6d:26bf:1447:1097:aa7
Connecting to (|2606:2800:220:6d:26bf:1447:1097:aa7|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 2020 (2.0K) [text/plain]
Saving to: `download.cgi?file=foobar.txt'

100%[============================================>] 2,020       --.-K/s   in 0s

2013-10-02 16:04:24 (12.5 MB/s) - `download.cgi?file=foobar.txt' saved [2020/2020]


But luckily Wget can do that, it’s just not enabled by default — because it’s an experimental and possibly buggy feature, at least according to the man page. Well, works for me! :-)

You can easily enabled it by default for either your user or the whole system by placing the following line in your ~/.wgetrc or /etc/wgetrc:

content-disposition = on

Given the CGI script sends an appropriate Content-Disposition header, the above output now looks like this:

$ wget ''
--2013-10-02 16:04:16--
Resolving (, 2606:2800:220:6d:26bf:1447:1097:aa7
Connecting to (|2606:2800:220:6d:26bf:1447:1097:aa7|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 2020 (2.0K) [text/plain]
Saving to: `foobar.txt'

100%[============================================>] 2,020       --.-K/s   in 0s

2013-10-02 16:04:24 (12.5 MB/s) - `foobar.txt' saved [2020/2020]

Now Wget does what I mean!

You can also set this as flag on the commandline, but typing wget --content-disposition … everytime is surely not what I want. ;-)

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Hackergotchi of Axel Beckert


This is the blog or weblog of Axel Stefan Beckert (aka abe or XTaran) who thought, he would never start blogging... (He also once thought, that there is no reason to switch to this new ugly Netscape thing because Mosaïc works fine. That was about 1996.) Well, times change...

He was born 1975 at Villingen-Schwenningen, made his Abitur at Schwäbisch Hall, studied Computer Science with minor Biology at University of Saarland at Saarbrücken (Germany) and now lives in Zürich (Switzerland), working at the Network Security Group (NSG) of the Central IT Services (Informatikdienste) at ETH Zurich.

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