Ubuntu, downgrading a package
Situation You would like to install a package but the dependency of this package require an older version of another package (we call it libABC). The current version of libABC is 6.8.1.blub but we would like libABC 6.4.1.blub.
Solution Use the following command to show the different versions, that are available:
$ apt-cache showpkg libABC Versions: 6.8.1.blub ([...]) Description Language: File: [...] MD5: f61a69e6c5dea15bee1ea40fb16e0a5c 6.4.1.blub ([...]) Description Language: File: [...] MD5: 85e8417bf7ce534d4649cd38557780b1
You should see the lower version there. If not, PANIC!
Next we tell the system to install that package, but we will specify the lower version number using the equals (=):
apt-get install =libABC-6.4.1.blub
Now we need to hold back (under gentoo it called "mask") the new package version. Currently i've found the solution with "forbid-version":
$ aptitude forbid-version libABC-6.8.1.blub
A better way could be "holding an package back", some information could be found in the Ubuntu Help - Pinning HowTo.