/etc/apt/preferences is a file in /etc/apt, the AptDirectory.
This page has been written by ZugSchlus, who not even remotely grasps the concept of pinning. So, please take the words "probably", "needs to be verified" and similiar wordings literally, and document your findings (may they be "this page is right" or "this page is wrong", optionally "this page is wrong because") here.
Description of Package Selection Process
TODO: This needs to be verified
Ignore packages that don't meet version criteria
Ignore packages lower versioned than current, unless their priority is > 1000
Install highest priority remaining package
In case of priority tie, take pinned package.
This step should probably be skipped if the pin doesn't match anything
In case of no pinned package, take highest version.
Examples of /etc/apt/preferences file
Package: * Pin: release o=Debian,a=testing Pin-Priority: 900
Package: * Pin: release o=Debian,a=unstable Pin-Priority: 300
Package: * Pin: release o=Debian Pin-Priority: -1
Missing: Documentation what this preferences file does.
ZugSchlus tries to explain:
All packages from a distribution called testing are pinned to 900
All packages from a distribution called unstable are pinned to 300
All other packages from Debian are pinned at -1 and thus never installed.
Problem: This pin behaves differently depending on which target release is set in other parts of apt configuration. Hence, this example cannot really be documented without adding more information. A non-pinned package being part of the target release has default priority 990, while other non-pinned packages have a default priority of 500.
Objective: On an unstable system, pull dpatch from experimental.
A possible (and not completely correct) solution:
Have both unstable and experimental in sources.list
In Absence of a preferences file, experimental will be automatically be pinned to priority 1. Question: Is this hardcoded, or configured somewhere on the target system, or on the Mirror?
Pin the wanted packages to a value x with 100<x<500:
Package: dpatch Pin: release o=Debian,a=experimental Pin-Priority: 450
A value > 500 will always select a package from experimental, even preventing a higher numbered unstable package from being installed, and selecting "no package at all" ahead of all available packages if the distribution not containing that package is pinned higher.
Unfortunately, the Package field understands neither wildcards nor regular expressions. You need one pin stanza per package.
Pinning the package to 450 will probably not automatically update to the experimental package, but it will probably track the package in experimental as long as it stays higher-versioend than the one in unstable.
apt-cache policy package gives information about the selection process. Unfortunately, it is not widely known what the output means. The following is a try to interpret:
$ apt-cache policy exim4-daemon-light exim4-daemon-light Installed: 4.50-1 Candidate: 4.50-1 Package Pin: (not found) Version Table: 4.50-4 555 500 http://mirror sid/main Packages *** 4.50-1 555 100 /var/lib/dpkg/status 4.44-2 555 500 http://mirror sarge/main Packages
The priority of each version/location is the number at the left of it. In this case, 500, 100, and 500. It is unclear what the number at the right of the version number means. Some people believe that it is just the last period that was specified for this package in /etc/apt/preferences for that package, while others said it is the actual pin priority being placed on the package.
TODO: This section was written after taking a lot of more or less educated guesses. Would somebody with real knowledge of apt please verify?
ZugSchlus, who tried to add some explanations and more examples
with help from Adeodato Simó and Paul Hampson on firstname.lastname@example.org