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In Do '..-backports' repositories automatically take precedence over the standard repos? in sources.list, the answer says that backports are never automatically used to update a standard package in the newer versions of Ubuntu.

How is this actually implemented? I've read something about pinning, but there is no /etc/apt/preferences file on my machine, and yet it shows the behavior explained in the answer.

Where is the special status of the backports repository actually configured, and how does apt know which packages have already been updated from there?

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If you open with a text editor the file

/var/lib/apt/lists/*_ubuntu_dists_oneiric-backports_Release

you can see near the beginning the following lines

NotAutomatic: yes
ButAutomaticUpgrades: yes

They are responsible for the behavior described for the newer Ubuntu releases.
Modifying the line has no effect, probably because the APT system has that information already stored in some sort of cache. Also, the flag would be reset to its previous content on the next update.

This behavior is similar to that of Debian, as you can see on the page Debian Backport >> Instructions.

Also see the Ubuntu Wiki page about Backports: UbuntuBackports.

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