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

Apt is configured by several resources, including:

Contents

  1. apt_preferences (APT pinning)
    1. Force installation of a newer package
    2. Prevent/selective installation from third-party a repository
  2. apt.conf
  3. Command line options

apt_preferences (APT pinning)

ManPage: 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.

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

$ apt-cache policy claws-mail
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 that case 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 DebianStable repository with a priority of 500
 500 https://deb.debian.org/debian stable/main amd64 Packages
     o=Debian,n=stable,l=Debian,c=main,b=amd64
     origin deb.debian.org

 # The repository for Debian 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 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 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

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

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

Force installation of a newer package

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

apt install -t stretch-backports claws-mail

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

To allow upgrades of this package from stretch-backports, 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 newer version. Running an ?AptUpgrade will automatically pick up newer versions from stable-backports.

Prevent/selective installation from third-party a repository

To prevent installation of newer packages from a third-party repository (don't enable DebianUnstable repositories on Stable, see 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.

apt.conf

See 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:

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

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

Command line options

PackageManagementTools such as apt, aptitude, apt-get... allow you to set specific configuration options for a single run. See their respective ?ManPages.

CategoryPackageManagement | CategorySoftware | CategorySystemAdministration