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Wolfgang Karall's blog

Checking if you're running the currently installed Debian kernel

Note: this doesn't work across ABI changes, i.e. when 3.2.0-4-amd64 becomes 3.2.0-5-amd64

Find the package version for the most recently installed kernel:

dpkg -l linux-image-$(uname -r) | awk '/^ii/{ print $3 }'

Get the same information from the running kernel:

uname --kernel-version  | awk '{ print $NF }'

Compare.

Caveats when switching Xen domUs to pygrub

When switching some old Xen guests (domUs) to pygrub, some things to remember:

  • using the bootloader config setting, you must remove the config settings kernel, extra, ramdisk and root, otherwise the domU will not boot.
  • if you happen to use multiple disk devices, make sure the one with the kernel/initrd is listed first, otherwise you'll get the error
    Error: Boot loader didn't return any data

Finding Debian packages installed from backports.debian.org

Using backports is a nice way to get newer versions of software while continuing to use Debian stable. If you ever want to list the packages installed from the backports repository, aptitude can help:

aptitude search '~S ~i ~O"Debian Backports"'

You get the search term "Debian Backports" (the so-called origin) for the backports packages from the output of

apt-cache policy

For a list of valid origins depending on your APT sources, we can filter this by using grep and sort:

apt-cache policy | grep -o o=[^,]* | sort -u

So of course this also works to find out which packages got installed from other repositories.

Forcing gzip compression when building Debian packages

If you're building packages on a recent Debian-based distribution and are getting

dpkg-deb: file `blah.deb' contains ununderstood data member data.tar.xz     , giving up

when trying to install the package on an old system with dpkg version < 1.15.6, you can force the usage of the gzip compression algorithm by changing the dh_builddeb in debian/rules to:

dh_builddeb -- -Z gzip

If you do not have an old-style debian/rules files where all the dh_* calls are listed but only one generic stanza like

%:
        dh $@

then you can add

override_dh_builddeb:
        dh_builddeb -- -Zgzip

below that to set the option for dh_builddeb.

Putting fail2ban to good use: whitelisting SSH logins

I don't like fail2ban all that much (neither do I like denyhosts or other similar software, I'm really egalitarian there), because it is based on the idea of using blacklists. Which don't work. If you don't believe me and rather believe more famous people, read http://www.codinghorror.com/blog/2007/12/blacklists-dont-work.html

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