doc/acquire-additional-files.txt

   1 # Acquire additional files in 'update' operations
   2
   3 The download and verification of data from multiple sources in different
   4 compression formats, with partial downloads and patches is an involved
   5 process which is hard to implement correctly and securely.
   6
   7 APT frontends share the code and binaries to make this happen in libapt
   8 with the Acquire system, supported by helpers shipped in the apt package
   9 itself and additional transports in individual packages like
  10 apt-transport-https.
  11
  12 For its own operation libapt needs or can make use of Packages, Sources
  13 and Translation-* files, which it will acquire by default, but
  14 a repository might contain more data files (e.g.  Contents) a frontend
  15 might want to use and would therefore need to be downloaded as well
  16 (e.g. apt-file).
  17
  18 This file describes the configuration scheme such a frontend can use to
  19 instruct the Acquire system to download those additional files.
  20
  21 Note that you can't download files from other sources. It must be files
  22 in the same repository and below the Release file. The Release file must
  23 also contain hashes for the file itself as well as for the compressed
  24 file if wanted, otherwise a download isn't even tried!
  25
  26
  27 # The Configuration Stanza
  28
  29 The Acquire system uses the same configuration settings to implement the
  30 files it downloads by default. These settings are the default, but if
  31 they would be written in a configuration file the configuration
  32 instructing the Acquire system to download the Packages files would look
  33 like this (see also apt.conf(5) manpage for configuration file syntax):
  34
  35         APT::Acquire::Targets::deb::Packages {
  36                 URI "$(COMPONENT)/binary-$(ARCHITECTURE)/Packages";
  37                 ShortDescription "Packages";
  38                 Description "$(SITE) $(RELEASE)/$(COMPONENT) $(ARCHITECTURE) Packages";
  39
  40                 flatURI "Packages";
  41                 flatDescription "$(SITE) $(RELEASE) Packages";
  42
  43                 Optional "false";
  44         };
  45
  46 All files which should be downloaded (nicknamed 'Targets') are mentioned
  47 below the APT::Acquire::Targets scope. 'deb' is here the type of the
  48 sources.list entry the file should be acquired for. The only other
  49 supported value is hence 'deb-src'. Beware: You can't specify multiple
  50 types here and you can't download the same URI for multiple types!
  51
  52 After the type you can pick any valid and unique string which preferable
  53 refers to the file it downloads (In the example we picked 'Packages').
  54 This string is never shown or used.
  55
  56 All targets have three main properties you can define:
  57 * URI: The identifier of the file to be downloaded as used in the
  58   Release file.  It is also the relative location of the file from the
  59   Release file.  You can neither download from a different server
  60   entirely (absolute URI) nor should you try to access directories above
  61   the Release file (e.g. "../../").
  62 * ShortDescription: Very short string intended to be displayed to the
  63   user e.g.  while reporting progress. apt will e.g. use this string in
  64   the last line to indicate progress of e.g. the download of a specific
  65   item.
  66 * Description: A preferable human understandable and readable identifier
  67   of which file is acquired exactly. Mainly used for progress reporting
  68   and error messages. apt will e.g. use this string in the Get/Hit/Err
  69   progress lines.
  70
  71 Additional optional properties:
  72 * flat{URI,Description}: APT supports two types of repositories:
  73   dists-style repositories which are the default and by far the most
  74   common which are named after the fact that the files are in an
  75   elaborated directory structure.  In contrast a flat-style repositories
  76   lumps all files together in one directory.  Support for these flat
  77   repositories exists mainly for legacy purposes only.  It is therefore
  78   recommend to not set these values.
  79 * Optional: The default value is 'true' and should be kept at this
  80   value.  If enabled the acquire system will skip the download if the
  81   file isn't mentioned in the Release file. Otherwise this is treated as
  82   a hard error and the update process fails.
  83
  84
  85 Note that the acquire system will automatically choose to download
  86 a compressed file if it is available and uncompress it for you, just as
  87 it will also use pdiff patching if provided by the repository and
  88 enabled by the user. You only have to ensure that the Release file
  89 contains the information about the compressed files/pdiffs to make this
  90 happen. NO properties have to be set to enable this.
  91
  92 # More examples
  93
  94 The stanzas for Translation-* files as well as for Sources files would
  95 look like this:
  96
  97 APT::Acquire::Targets {
  98         deb::Translations {
  99                 URI "$(COMPONENT)/i18n/Translation-$(LANGUAGE)";
 100                 ShortDescription "Translation-$(LANGUAGE)";
 101                 Description "$(SITE) $(RELEASE)/$(COMPONENT) Translation-$(LANGUAGE)";
 102
 103                 flatURI "$(LANGUAGE)";
 104                 flatDescription "$(SITE) $(RELEASE) Translation-$(LANGUAGE)";
 105         };
 106
 107         deb-src::Sources {
 108                 URI "$(COMPONENT)/source/Sources";
 109                 ShortDescription "Sources";
 110                 Description "$(SITE) $(RELEASE)/$(COMPONENT) Sources";
 111
 112                 flatURI "Sources";
 113                 flatDescription "$(SITE) $(RELEASE) Sources";
 114
 115                 Optional "false";
 116         };
 117 };
 118
 119 # Substitution variables
 120
 121 As seen in the examples, properties can contain placeholders filled in
 122 by the acquire system. The following variables are known; note that
 123 unknown variables have no default value nor are they touched: They are
 124 printed literally.
 125
 126 * $(SITE): An identifier of the site we access, e.g.
 127   "http://example.org/".
 128 * $(RELEASE): This is usually an archive- or codename, e.g. "stable" or
 129   "stretch".  Note that flat-style repositories do not have a archive-
 130   or codename per-se, so the value might very well be just "/" or so.
 131 * $(COMPONENT): as given in the sources.list, e.g. "main", "non-free" or
 132   "universe".  Note that flat-style repositories again do not really
 133   have a meaningful value here.
 134 * $(LANGUAGE): Values are all entries (expect "none") of configuration
 135   option Acquire::Languages, e.g. "en", "de" or "de_AT".
 136
 137 These values are defined both for 'deb' as well as 'deb-src' targets.
 138 'deb' targets additional have the variable:
 139
 140 * $(ARCHITECTURE): Values are all entries of configuration option
 141   APT::Architectures (potentially modified by sources.list options),
 142   e.g. "amd64", "i386" or "armel".