How to set default APT source

Question

Running cat /etc/apt/apt.conf gives:

APT::Default-Release "stable";

I thought doing that would disallow the command apt-get install wajig to work, if a newer version was available anywhere but Stable.

How do I set up APT so that such a version would require me to specify the repository name in order to install it (e.g. apt-get --target-release testing wajig)?

[update] I wasn't aware that my question wasn't so clear. I want this to be an archive-wide setting (i.e. it should apply to each package in the Stable archive), not to some specific package.

Answer 1

Put this in your "/etc/apt/preferences":

Package: *
Pin: release a=stable
Pin-Priority: 900

Package: *
Pin: release o=Debian
Pin-Priority: -10

This is from man apt_preferences where P means Pin-Priority:

500 < P <=990
   causes a version to be installed unless there is a version available belonging to the target release or the installed version is more recent

P < 0
   prevents the version from being installed

See this Debian wiki page for something gentler than the manpage.

Answer 2

If there is a wajig package with a positive pin priority in any of your sources, apt-get install wajig will install it. Default-Release works like setting a high priority to that particular release. If I understand correctly, you'd like apt-get install wajig to work if squeeze has the latest version and not to work otherwise; I don't think that's possible.