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In the example above, the package that would be installed ({{{Candidate}}}) would be the older, {{{3.14}}} version from {{{stretch/main}}}. {{{stretch-backports/main}}} has a newer version {{{3.17}}}, but a lower priority ({{{100}}} vs {{{500}}} for stretch) In the example above, the package that would be installed ({{{Candidate}}}) would be the older, {{{3.14}}} version from `stretch/main`. {{{stretch-backports/main}}} has a newer version {{{3.17}}}, but a lower priority ({{{100}}} vs {{{500}}} for stretch)
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Now installing the {{{claws-mail}}} package will install the version from {{ stretch-backports }}. Running an AptUpgrade will automatically pick up newer versions from {{{stable-backports}}}. Running {{{apt-cache policy}}} again you would see: Now installing the {{{claws-mail}}} package will install the version from ` stretch-backports`. Running an AptUpgrade will automatically pick up newer versions from {{{stable-backports}}}. Running {{{apt-cache policy}}} again you would see:
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 * John H. Robinson's [[https://web.archive.org/web/20021010033255/http://jaqque.sbih.org/kplug/apt-pinning.html|Apt-Pinning for beginners]] (2002)

WIP: refactoring of AptConf, AptPreferences and other apt configuration pages

Apt is configured by several resources, including:

  • ?SourcesLists - lists of software repositories (sources)

  • ?AptKeys - keys for secure authentication of packages

  • apt_preferences and apt.conf described below

  • Runtime options/command-line flags of PackageManagementTools

apt_preferences (APT pinning)

man 5 apt_preferences

When multiple Apt repositories are enabled, a package can exist in several of them. To know which one should be installed, Apt assigns priorities to packages. The default is 500.

  • If the packages have the same priority, the package with a higher version number (most recent) wins.
  • If packages have different priorities, the one with the higher priority wins.

Pinning allows changing priorities for only some packages/repositories, so that you can:

  • Prefer a DebianBackports package over a DebianStable one: by default Debian backports repositories have a lower priority than stable (100). They won't be installed or upgraded unless explicitly configured to (or the package only exists in backports).

  • Only allow some packages from a third-party repository, and ignore the other even if more recent: you may want to add experimental/unstable/third-party repositories with extra/more recent software, but only allow some of these packages to be installed.

  • Force a package downgrade (not recommended)

<!> With a few exceptions (DebianBackports) it is not recommended to mix repositories/releases unless they were specially prepared . See DontBreakDebian.. Don't enable DebianUnstable repositories on DebianStable. When pinning, you must ensure compatibility of packages by yourself since Debian does not guarantee it.

To view the priority of a specific package, use apt-cache policy mypackage:

$ apt-cache policy claws-mail
  Installed : (none)
  Candidate : 3.14.1-3+b1
 Version table :
     3.17.1-1~bpo9+1 100
        100 https://deb.debian.org/debian stretch-backports/main amd64 Packages
     3.14.1-3+b1 500
        500 https://deb.debian.org/debian stretch/main amd64 Package

In the example above, the package that would be installed (Candidate) would be the older, 3.14 version from stretch/main. stretch-backports/main has a newer version 3.17, but a lower priority (100 vs 500 for stretch)

To view the global priority for each Apt source (repository):

$ apt-cache policy 
Package files: 
 # The default https://wiki.debian.org/DebianStable repository with a priority of 500
 500 https://deb.debian.org/debian stable/main amd64 Packages
     origin deb.debian.org

 # The repository for Debian https://wiki.debian.org/PointReleases (security and grave bug fixes ~every 2 months)
 500 https://deb.debian.org/debian stable-updates/main amd64 Packages
     release o=Debian,a=oldstable-updates,n=stable-updates,l=Debian,c=main,b=amd64
     origin deb.debian.org

 # The https://wiki.debian.org/DebianSecurity repository with short response time for security fixes
 500 http://security.debian.org stable/updates/main amd64 Packages
     release v=9,o=Debian,a=oldstable,n=stable,l=Debian-Security,c=main,b=amd64
     origin security.debian.org

 # The https://wiki.debian.org/DebianBackports repository, comes with a default priority of 100
 100 https://deb.debian.org/debian stable-backports/main amd64 Packages
     release o=Debian Backports,a=stable-backports,n=stable-backports,l=Debian Backports,c=main,b=amd64
     origin deb.debian.org

 # The priority of locally installed packages
 100 /var/lib/dpkg/status 
     release a=now

Force installation of a package from a repository

To tell Apt to install a package from stretch-backports, even if the package has a low priority:

apt install -t stretch-backports claws-mail

Note that the package will not be automatically upgraded when running an ?AptUpgrade.

Always prefer packages from a repository

To always prefer packages from stretch-backports (and hence allow ?AptUpgrades), set a higher priority) for this package coming from the stretch-backports release. Edit the file /etc/apt/preferences.d/99debian-backports (create it):

Package: claws-mail
Pin: release a=stretch-backports
Pin-Priority: 900

Now installing the claws-mail package will install the version from  stretch-backports. Running an ?AptUpgrade will automatically pick up newer versions from stable-backports. Running apt-cache policy again you would see:

Pinned packages:
     claws-mail -> 3.17.1-1~bpo9+1 with priority 900

Prevent/selective installation from third-party a repository

To prevent installation of newer packages from a third-party repository (DontBreakDebian), even if it has equal priority, edit the file /etc/apt/preferences.d/99my-custom-repository:

# Never prefer packages from the my-custom-repo repository
Package: *
Pin: origin o=my-custom-repo
Pin-Priority: 1

# Allow upgrading only my-specific-software from my-custom-repo
Package: my-specific-software
Pin: origin o=my-custom-repo
Pin-Priority: 500

File naming in /etc/apt/preferences.d/ is free but the last in alphabetical order takes precedence.

The * after Package:is not a wildcard, but a special case that means "everything". Wildcards are NOT supported. However, traling wildcards are accepted in versions (2.6* will match both 2.6 and 2.6.18)

Other pinning notes

In addition origin, you can pin packages based on other variables. apt-cache policy shows other variables that can be used as the Pin: key:

   1 https://deb.debian.org/debian stretch-backports/non-free i386 Packages
     release o=Debian Backports,a=stretch-backports,n=stretch-backports,l=Debian Backports,c=non-free,b=i386
     origin deb.debian.org
  • release: the DebianRelease full name

  • archive: archive (base directory in the repository)

  • c,component: main/contrib/non-free

  • origin: domain name of the repository (ToDo verify)

  • l,label: ToDo

  • b,architecture: processor architecture

These variables are provided by Release files in Debian repositories.

See also:


man 5 apt.conf

Apt accepts configuration files (without extension) in /etc/apt/apt.conf.d/. These are processed by Apt in numeric/alphabetical order. /etc/apt/apt.conf is also valid but deprecated.

These files can contain various directives:

  • Dpkg::Pre-Install-Pkgs {"mycommand";};: executes mycommand before package installation/unpacking by Dpkg.

  • Dpkg::Pre-Invoke {"mycommand";};: executes mycommand before apt calls dpkg

  • Dpkg::Post-Invoke {"mycommand";};: executes mycommand after apt calls dpkg

  • Pre-Install-Pkgs: ??? ToDo

  • APT::Default-Release "testing";: sets the default Debian release used by Apt (ToDo use case?)

  • APT::Cache-Limit 10000000;: ??? ToDo

  • Apt::Get::Purge;: ??? ToDo

  • Acquire::http::Proxy "http://proxy:8080";: sets the proxy for HTTP downloads

  • Acquire::https::Proxy "https://proxy:8443";: sets the proxy for HTTPS downloads

  • Acquire::http::Timeout "2";: sets the timeout for HTTP downloads

  • Acquire::https::Timeout "2";: sets the timeout for HTTPS downloads

  • Acquire::ftp::Timeout "2";: sets the timeout for FTP downloads

If you really have to use FTP, this sets the FTP proxy:

   Proxy "ftp://proxy:2121/";
      "USER $(SITE_USER)@$(SITE)";
      "PASS $(SITE_PASS)";

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