fix two typos in apt.conf & apt-cache manpage

[apt.git] / doc / apt.conf.5.xml

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<refentry>

 <refentryinfo>
   &apt-author.jgunthorpe;
   &apt-author.team;
   <author>
     &apt-name.dburrows;
     <contrib>Initial documentation of Debug::*.</contrib>
     <email>dburrows@debian.org</email>
   </author>
   &apt-email;
   &apt-product;
   <!-- The last update date -->
   <date>2015-12-14T00:00:00Z</date>
 </refentryinfo>
 
 <refmeta>
   <refentrytitle>apt.conf</refentrytitle>
   <manvolnum>5</manvolnum>
   <refmiscinfo class="manual">APT</refmiscinfo>
 </refmeta>
 
 <!-- Man page title -->
 <refnamediv>
    <refname>apt.conf</refname>
    <refpurpose>Configuration file for APT</refpurpose>
 </refnamediv>

 <refsect1><title>Description</title>
   <para><filename>/etc/apt/apt.conf</filename> is the main configuration
   file shared by all the tools in the APT suite of tools, though it is by
   no means the only place options can be set. The suite also shares a common
   command line parser to provide a uniform environment.</para>

   <orderedlist>
      <para>When an APT tool starts up it will read the configuration files
      in the following order:</para>
      <listitem><para>the file specified by the <envar>APT_CONFIG</envar>
         environment variable (if any)</para></listitem>
      <listitem><para>all files in <literal>Dir::Etc::Parts</literal> in
         alphanumeric ascending order which have either no or "<literal>conf</literal>"
         as filename extension and which only contain alphanumeric,
         hyphen (-), underscore (_) and period (.) characters.
         Otherwise APT will print a notice that it has ignored a file, unless that
         file matches a pattern in the <literal>Dir::Ignore-Files-Silently</literal>
         configuration list - in which case it will be silently ignored.</para></listitem>
      <listitem><para>the main configuration file specified by
         <literal>Dir::Etc::main</literal></para></listitem>
      <listitem><para>all options set in the binary specific configuration
            subtree are moved into the root of the tree.</para></listitem>
      <listitem><para>the command line options are applied to override the
         configuration directives or to load even more configuration files.</para></listitem>
   </orderedlist>
   </refsect1>
   <refsect1><title>Syntax</title>
   <para>The configuration file is organized in a tree with options organized into
   functional groups. Option specification is given with a double colon
   notation; for instance <literal>APT::Get::Assume-Yes</literal> is an option within 
   the APT tool group, for the Get tool. Options do not inherit from their 
   parent groups.</para> 

   <para>Syntactically the configuration language is modeled after what the ISC tools
   such as bind and dhcp use. Lines starting with
   <literal>//</literal> are treated as comments (ignored), as well as all text
   between <literal>/*</literal> and <literal>*/</literal>, just like C/C++ comments.
   Each line is of the form
   <literal>APT::Get::Assume-Yes "true";</literal>.
   The quotation marks and trailing semicolon are required.
   The value must be on one line, and there is no kind of string concatenation.
   Values must not include backslashes or extra quotation marks.
   Option names are made up of alphanumeric characters and the characters "/-:._+".
   A new scope can be opened with curly braces, like this:</para>

<informalexample><programlisting>   
APT {
  Get {
    Assume-Yes "true";
    Fix-Broken "true";
  };
};
</programlisting></informalexample>

   <para>with newlines placed to make it more readable. Lists can be created by 
   opening a scope and including a single string enclosed in quotes followed by a
   semicolon. Multiple entries can be included, separated by a semicolon.</para>

<informalexample><programlisting>   
DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt";};
</programlisting></informalexample>

   <para>In general the sample configuration file &configureindex;
   is a good guide for how it should look.</para>

   <para>Case is not significant in names of configuration items, so in the
   previous example you could use <literal>dpkg::pre-install-pkgs</literal>.</para>

   <para>Names for the configuration items are optional if a list is defined as can be seen in
   the <literal>DPkg::Pre-Install-Pkgs</literal> example above. If you don't specify a name a
   new entry will simply add a new option to the list. If you specify a name you can override
   the option in the same way as any other option by reassigning a new value to the option.</para>

   <para>Two special commands are defined: <literal>#include</literal> (which is
   deprecated and not supported by alternative implementations) and
   <literal>#clear</literal>. <literal>#include</literal> will include the
   given file, unless the filename ends in a slash, in which case the whole
   directory is included.
   <literal>#clear</literal> is used to erase a part of the configuration tree. The
   specified element and all its descendants are erased.
   (Note that these lines also need to end with a semicolon.)</para>

   <para>
   The <literal>#clear</literal> command is the only way to delete a list or
   a complete scope. Reopening a scope (or using the syntax described below
   with an appended <literal>::</literal>) will <emphasis>not</emphasis>
   override previously written entries. Options can only be overridden by
   addressing a new value to them - lists and scopes can't be overridden,
   only cleared.
   </para>

   <para>All of the APT tools take an -o option which allows an arbitrary configuration 
   directive to be specified on the command line. The syntax is a full option
   name (<literal>APT::Get::Assume-Yes</literal> for instance) followed by an equals
   sign then the new value of the option. To append a new element to a list, add a
   trailing <literal>::</literal> to the name of the list.
   (As you might suspect, the scope syntax can't be used on the command line.)</para>

   <para>
   Note that appending items to a list using <literal>::</literal> only works
   for one item per line, and that you should not use it in combination with
   the scope syntax (which adds <literal>::</literal> implicitly). Using both
   syntaxes together will trigger a bug which some users unfortunately depend
   on: an option with the unusual name "<literal>::</literal>" which acts
   like every other option with a name. This introduces many problems; for
   one thing, users who write multiple lines in this
   <emphasis>wrong</emphasis> syntax in the hope of appending to a list will
   achieve the opposite, as only the last assignment for this option
   "<literal>::</literal>" will be used. Future versions of APT will raise
   errors and stop working if they encounter this misuse, so please correct
   such statements now while APT doesn't explicitly complain about them.
   </para>
 </refsect1>

 <refsect1><title>The APT Group</title>
   <para>This group of options controls general APT behavior as well as holding the
   options for all of the tools.</para>

   <variablelist>
     <varlistentry><term><option>Architecture</option></term>
     <listitem><para>System Architecture; sets the architecture to use when fetching files and
     parsing package lists. The internal default is the architecture apt was 
     compiled for.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Architectures</option></term>
     <listitem><para>
     All Architectures the system supports. For instance, CPUs implementing
     the <literal>amd64</literal> (also called <literal>x86-64</literal>)
     instruction set are also able to execute binaries compiled for the
     <literal>i386</literal> (<literal>x86</literal>) instruction set. This
     list is used when fetching files and parsing package lists. The
     initial default is always the system's native architecture
     (<literal>APT::Architecture</literal>), and foreign architectures are
     added to the default list when they are registered via
     <command>dpkg --add-architecture</command>.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Build-Profiles</option></term>
     <listitem><para>
     List of all build profiles enabled for build-dependency resolution,
     without the "<literal>profile.</literal>" namespace prefix.
     By default this list is empty. The <envar>DEB_BUILD_PROFILES</envar>
     as used by &dpkg-buildpackage; overrides the list notation.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Default-Release</option></term>
     <listitem><para>Default release to install packages from if more than one
     version is available. Contains release name, codename or release version. Examples: 'stable', 'testing',
     'unstable', '&debian-stable-codename;', '&debian-testing-codename;', '4.0', '5.0*'. See also &apt-preferences;.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Ignore-Hold</option></term>
     <listitem><para>Ignore held packages; this global option causes the problem resolver to
     ignore held packages in its decision making.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Clean-Installed</option></term>
     <listitem><para>Defaults to on. When turned on the autoclean feature will remove any packages
     which can no longer be downloaded from the cache. If turned off then
     packages that are locally installed are also excluded from cleaning - but
     note that APT provides no direct means to reinstall them.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Immediate-Configure</option></term>
     <listitem><para>
     Defaults to on, which will cause APT to install essential and important
     packages as soon as possible in an install/upgrade operation, in order
     to limit the effect of a failing &dpkg; call. If this option is
     disabled, APT treats an important package in the same way as an extra
     package: between the unpacking of the package A and its configuration
     there can be many other unpack or configuration calls for other
     unrelated packages B, C etc. If these cause the &dpkg; call to fail
     (e.g. because package B's maintainer scripts generate an error), this
     results in a system state in which package A is unpacked but
     unconfigured - so any package depending on A is now no longer
     guaranteed to work, as its dependency on A is no longer satisfied.
     </para><para>
     The immediate configuration marker is also applied in the potentially
     problematic case of circular dependencies, since a dependency with the
     immediate flag is equivalent to a Pre-Dependency. In theory this allows
     APT to recognise a situation in which it is unable to perform immediate
     configuration, abort, and suggest to the user that the option should be
     temporarily deactivated in order to allow the operation to proceed.
     Note the use of the word "theory" here; in the real world this problem
     has rarely been encountered, in non-stable distribution versions, and
     was caused by wrong dependencies of the package in question or by a
     system in an already broken state; so you should not blindly disable
     this option, as the scenario mentioned above is not the only problem it
     can help to prevent in the first place.
     </para><para>
     Before a big operation like <literal>dist-upgrade</literal> is run
     with this option disabled you should try to explicitly
     <literal>install</literal> the package APT is unable to configure
     immediately; but please make sure you also report your problem to your
     distribution and to the APT team with the buglink below, so they can
     work on improving or correcting the upgrade process.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Force-LoopBreak</option></term>
     <listitem><para>
     Never enable this option unless you <emphasis>really</emphasis> know
     what you are doing. It permits APT to temporarily remove an essential
     package to break a Conflicts/Conflicts or Conflicts/Pre-Depends loop
     between two essential packages. <emphasis>Such a loop should never exist
     and is a grave bug</emphasis>. This option will work if the essential
     packages are not <command>tar</command>, <command>gzip</command>,
     <command>libc</command>, <command>dpkg</command>, <command>dash</command>
     or anything that those packages depend on.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Cache-Start</option></term><term><option>Cache-Grow</option></term><term><option>Cache-Limit</option></term>
     <listitem><para>APT uses since version 0.7.26 a resizable memory mapped cache file to store the available
     information. <literal>Cache-Start</literal> acts as a hint of the size the cache will grow to,
     and is therefore the amount of memory APT will request at startup. The default value is
     20971520 bytes (~20 MB). Note that this amount of space needs to be available for APT;
     otherwise it will likely fail ungracefully, so for memory restricted devices this value should
     be lowered while on systems with a lot of configured sources it should be increased.
     <literal>Cache-Grow</literal> defines in bytes with the default of 1048576 (~1 MB) how much
     the cache size will be increased in the event the space defined by <literal>Cache-Start</literal>
     is not enough. This value will be applied again and again until either the cache is big
     enough to store all information or the size of the cache reaches the <literal>Cache-Limit</literal>.
     The default of <literal>Cache-Limit</literal> is 0 which stands for no limit.
     If <literal>Cache-Grow</literal> is set to 0 the automatic growth of the cache is disabled.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Build-Essential</option></term>
     <listitem><para>Defines which packages are considered essential build dependencies.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Get</option></term>
     <listitem><para>The Get subsection controls the &apt-get; tool; please see its
     documentation for more information about the options here.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Cache</option></term>
     <listitem><para>The Cache subsection controls the &apt-cache; tool; please see its
     documentation for more information about the options here.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>CDROM</option></term>
     <listitem><para>The CDROM subsection controls the &apt-cdrom; tool; please see its
     documentation for more information about the options here.</para></listitem>
     </varlistentry>
   </variablelist>
 </refsect1>

 <refsect1><title>The Acquire Group</title>
   <para>The <literal>Acquire</literal> group of options controls the
   download of packages as well as the various "acquire methods" responsible
   for the download itself (see also &sources-list;).</para>

   <variablelist>
     <varlistentry><term><option>Check-Valid-Until</option></term>
     <listitem><para>
     Security related option defaulting to true, as giving a Release file's
     validation an expiration date prevents replay attacks over a long
     timescale, and can also for example help users to identify mirrors
     that are no longer updated - but the feature depends on the
     correctness of the clock on the user system. Archive maintainers are
     encouraged to create Release files with the
     <literal>Valid-Until</literal> header, but if they don't or a
     stricter value is desired the <literal>Max-ValidTime</literal>
     option below can be used.
     The <option>Check-Valid-Until</option> option of &sources-list; entries should be
     preferred to disable the check selectively instead of using this global override.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Max-ValidTime</option></term>
         <listitem><para>Maximum time (in seconds) after its creation (as indicated
         by the <literal>Date</literal> header) that the <filename>Release</filename>
         file should be considered valid.
         If the Release file itself includes a <literal>Valid-Until</literal> header
         the earlier date of the two is used as the expiration date.
         The default value is <literal>0</literal> which stands for "valid forever".
         Archive specific settings can be made by appending the label of the archive
         to the option name. Preferably, the same can be achieved for specific
         &sources-list; entries by using the <option>Valid-Until-Max</option> option there.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Min-ValidTime</option></term>
         <listitem><para>Minimum time (in seconds) after its creation (as indicated
         by the <literal>Date</literal> header) that the <filename>Release</filename>
         file should be considered valid.
         Use this if you need to use a seldom updated (local) mirror of a more
         frequently updated archive with a <literal>Valid-Until</literal> header
         instead of completely disabling the expiration date checking.
         Archive specific settings can and should be used by appending the label of
         the archive to the option name. Preferably, the same can be achieved for specific
         &sources-list; entries by using the <option>Valid-Until-Min</option> option there.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>PDiffs</option></term>
         <listitem><para>Try to download deltas called <literal>PDiffs</literal> for
         indexes (like <filename>Packages</filename> files) instead of
         downloading whole ones. True by default. Preferably, this can be set
         for specific &sources-list; entries or index files by using the
         <option>PDiffs</option> option there.</para>
         <para>Two sub-options to limit the use of PDiffs are also available:
         <literal>FileLimit</literal> can be used to specify a maximum number of
         PDiff files should be downloaded to update a file. <literal>SizeLimit</literal>
         on the other hand is the maximum percentage of the size of all patches
         compared to the size of the targeted file. If one of these limits is
         exceeded the complete file is downloaded instead of the patches.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>By-Hash</option></term>
         <listitem><para>Try to download indexes via an URI constructed from a
         hashsum of the expected file rather than downloaded via a well-known
         stable filename. True by default, but automatically disabled if the
         source indicates no support for it. Usage can be forced with the special
         value "force". Preferably, this can be set for specific &sources-list; entries
         or index files by using the <option>By-Hash</option> option there.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Queue-Mode</option></term>
     <listitem><para>Queuing mode; <literal>Queue-Mode</literal> can be one of <literal>host</literal> or 
     <literal>access</literal> which determines how  APT parallelizes outgoing 
     connections. <literal>host</literal> means that one connection per target host 
     will be opened, <literal>access</literal> means that one connection per URI type 
     will be opened.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Retries</option></term>
     <listitem><para>Number of retries to perform. If this is non-zero APT will retry failed 
     files the given number of times.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Source-Symlinks</option></term>
     <listitem><para>Use symlinks for source archives. If set to true then source archives will
     be symlinked when possible instead of copying. True is the default.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>http</option></term>
     <listitem><para><literal>http::Proxy</literal> sets the default proxy to use for HTTP
     URIs. It is in the standard form of <literal>http://[[user][:pass]@]host[:port]/</literal>.
     Per host proxies can also be specified by using the form
     <literal>http::Proxy::&lt;host&gt;</literal> with the special keyword <literal>DIRECT</literal> 
         meaning to use no proxies. If no one of the above settings is specified,
         <envar>http_proxy</envar> environment variable
         will be used.</para>

     <para>Three settings are provided for cache control with HTTP/1.1 compliant
     proxy caches.
     <literal>No-Cache</literal> tells the proxy not to use its cached
     response under any circumstances.
     <literal>Max-Age</literal> sets the allowed maximum age (in seconds) of
     an index file in the cache of the proxy.
     <literal>No-Store</literal> specifies that the proxy should not store
     the requested archive files in its cache, which can be used to prevent
     the proxy from polluting its cache with (big) .deb files.</para>

     <para>The option <literal>timeout</literal> sets the timeout timer used by the method;
     this value applies to the connection as well as the data timeout.</para>

     <para>The setting <literal>Acquire::http::Pipeline-Depth</literal> can be used to
     enable HTTP pipelining (RFC 2616 section 8.1.2.2) which can be beneficial e.g. on
     high-latency connections. It specifies how many requests are sent in a pipeline.
     APT tries to detect and workaround misbehaving webservers and proxies at runtime, but
     if you know that yours does not conform to the HTTP/1.1 specification pipelining can
     be disabled by setting the value to 0. It is enabled by default with the value 10.</para>

     <para><literal>Acquire::http::AllowRedirect</literal> controls whether APT will follow
     redirects, which is enabled by default.</para>

     <para>The used bandwidth can be limited with
     <literal>Acquire::http::Dl-Limit</literal> which accepts integer
     values in kilobytes per second. The default value is 0 which
     deactivates the limit and tries to use all available bandwidth.
     Note that this option implicitly disables downloading from
     multiple servers at the same time.</para>

     <para><literal>Acquire::http::User-Agent</literal> can be used to set a different
     User-Agent for the http download method as some proxies allow access for clients
     only if the client uses a known identifier.</para>

     <para><literal>Acquire::http::Proxy-Auto-Detect</literal> can be used to
     specify an external command to discover the http proxy to use. Apt expects
     the command to output the proxy on stdout in the style
     <literal>http://proxy:port/</literal>. This will override the
     generic <literal>Acquire::http::Proxy</literal> but not any specific
     host proxy configuration set via 
     <literal>Acquire::http::Proxy::$HOST</literal>.

     See the &squid-deb-proxy-client; package for an example implementation that
     uses avahi. This option takes precedence over the legacy option name 
     <literal>ProxyAutoDetect</literal>.
     </para>

     </listitem>
     </varlistentry>

     <varlistentry><term><option>https</option></term>
         <listitem><para>
         The <literal>Cache-control</literal>, <literal>Timeout</literal>,
         <literal>AllowRedirect</literal>, <literal>Dl-Limit</literal> and
         <literal>proxy</literal> options work for HTTPS URIs in the same way
         as for the <literal>http</literal> method, and default to the same
         values if they are not explicitly set. The
         <literal>Pipeline-Depth</literal> option is not yet supported.
         </para>

         <para><literal>CaInfo</literal> suboption specifies place of file that
         holds info about trusted certificates.
         <literal>&lt;host&gt;::CaInfo</literal> is the corresponding per-host option.
         <literal>Verify-Peer</literal> boolean suboption determines whether or not the
         server's host certificate should be verified against trusted certificates.
         <literal>&lt;host&gt;::Verify-Peer</literal> is the corresponding per-host option.
         <literal>Verify-Host</literal> boolean suboption determines whether or not the
         server's hostname should be verified.
         <literal>&lt;host&gt;::Verify-Host</literal> is the corresponding per-host option.
         <literal>SslCert</literal> determines what certificate to use for client
         authentication. <literal>&lt;host&gt;::SslCert</literal> is the corresponding per-host option.
         <literal>SslKey</literal> determines what private key to use for client
         authentication. <literal>&lt;host&gt;::SslKey</literal> is the corresponding per-host option.
         <literal>SslForceVersion</literal> overrides default SSL version to use.
         It can contain either of the strings '<literal>TLSv1</literal>' or
         '<literal>SSLv3</literal>'.
         <literal>&lt;host&gt;::SslForceVersion</literal> is the corresponding per-host option.
         </para></listitem></varlistentry>

     <varlistentry><term><option>ftp</option></term>
     <listitem><para>
     <literal>ftp::Proxy</literal> sets the default proxy to use for FTP URIs.
     It is in the standard form of <literal>ftp://[[user][:pass]@]host[:port]/</literal>.
     Per host proxies can also be specified by using the form
     <literal>ftp::Proxy::&lt;host&gt;</literal> with the special keyword <literal>DIRECT</literal> 
         meaning to use no proxies. If no one of the above settings is specified,
         <envar>ftp_proxy</envar> environment variable
         will be used. To use an FTP
     proxy you will have to set the <literal>ftp::ProxyLogin</literal> script in the 
     configuration file. This entry specifies the commands to send to tell 
     the proxy server what to connect to. Please see 
     &configureindex; for an example of 
     how to do this. The substitution variables representing the corresponding
     URI component are <literal>$(PROXY_USER)</literal>,
     <literal>$(PROXY_PASS)</literal>, <literal>$(SITE_USER)</literal>,
     <literal>$(SITE_PASS)</literal>, <literal>$(SITE)</literal> and
     <literal>$(SITE_PORT)</literal>.</para>

     <para>The option <literal>timeout</literal> sets the timeout timer used by the method;
     this value applies to the connection as well as the data timeout.</para>

     <para>Several settings are provided to control passive mode. Generally it is 
     safe to leave passive mode on; it works in nearly every environment. 
     However, some situations require that passive mode be disabled and port 
     mode FTP used instead. This can be done globally or for connections that 
     go through a proxy or for a specific host (see the sample config file 
     for examples).</para>

     <para>It is possible to proxy FTP over HTTP by setting the <envar>ftp_proxy</envar>
     environment variable to an HTTP URL - see the discussion of the http method
     above for syntax. You cannot set this in the configuration file and it is
     not recommended to use FTP over HTTP due to its low efficiency.</para>

     <para>The setting <literal>ForceExtended</literal> controls the use of RFC2428 
     <literal>EPSV</literal> and <literal>EPRT</literal> commands. The default is false, which means
     these commands are only used if the control connection is IPv6. Setting this
     to true forces their use even on IPv4 connections. Note that most FTP servers
     do not support RFC2428.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>cdrom</option></term>
     <listitem><para>
     For URIs using the <literal>cdrom</literal> method, the only configurable
     option is the mount point, <literal>cdrom::Mount</literal>, which must be
     the mount point for the CD-ROM (or DVD, or whatever) drive as specified in
     <filename>/etc/fstab</filename>. It is possible to provide alternate mount
     and unmount commands if your mount point cannot be listed in the fstab.
     The syntax is to put <literallayout>/cdrom/::Mount "foo";</literallayout> within
     the <literal>cdrom</literal> block. It is important to have the trailing slash.
     Unmount commands can be specified using UMount.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>gpgv</option></term>
     <listitem><para>
     For GPGV URIs the only configurable option is <literal>gpgv::Options</literal>,
     which passes additional parameters to gpgv.
     </para></listitem>
     </varlistentry>

     <varlistentry><term><option>CompressionTypes</option></term>
     <listitem><para>List of compression types which are understood by the acquire methods.
     Files like <filename>Packages</filename> can be available in various compression formats.
     By default the acquire methods can decompress <command>bzip2</command>, <command>lzma</command>
     and <command>gzip</command> compressed files; with this setting more formats can be added
     on the fly or the used method can be changed. The syntax for this is:
     <synopsis>Acquire::CompressionTypes::<replaceable>FileExtension</replaceable> "<replaceable>Methodname</replaceable>";</synopsis>
     </para><para>Also, the <literal>Order</literal> subgroup can be used to define in which order
     the acquire system will try to download the compressed files. The acquire system will try the first
     and proceed with the next compression type in this list on error, so to prefer one over the other type
     simply add the preferred type first - default types not already added will be implicitly appended
     to the end of the list, so e.g. <synopsis>Acquire::CompressionTypes::Order:: "gz";</synopsis> can
     be used to prefer <command>gzip</command> compressed files over <command>bzip2</command> and <command>lzma</command>.
     If <command>lzma</command> should be preferred over <command>gzip</command> and <command>bzip2</command> the
     configure setting should look like this: <synopsis>Acquire::CompressionTypes::Order { "lzma"; "gz"; };</synopsis>
     It is not needed to add <literal>bz2</literal> to the list explicitly as it will be added automatically.</para>
     <para>Note that the
     <literal>Dir::Bin::<replaceable>Methodname</replaceable></literal>
     will be checked at run time. If this option has been set, the
     method will only be used if this file exists; e.g. for the
     <literal>bzip2</literal> method (the inbuilt) setting is:
     <literallayout>Dir::Bin::bzip2 "/bin/bzip2";</literallayout>
     Note also that list entries specified on the command line will be added at the end of the list
     specified in the configuration files, but before the default entries. To prefer a type in this case
     over the ones specified in the configuration files you can set the option direct - not in list style.
     This will not override the defined list; it will only prefix the list with this type.</para>
     <para>The special type <literal>uncompressed</literal> can be used to give uncompressed files a
     preference, but note that most archives don't provide uncompressed files so this is mostly only
     useable for local mirrors.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>GzipIndexes</option></term>
         <listitem><para>
         When downloading <literal>gzip</literal> compressed indexes (Packages, Sources, or
         Translations), keep them gzip compressed locally instead of unpacking
         them. This saves quite a lot of disk space at the expense of more CPU
         requirements when building the local package caches. False by default.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Languages</option></term>
     <listitem><para>The Languages subsection controls which <filename>Translation</filename> files are downloaded
     and in which order APT tries to display the description-translations. APT will try to display the first
     available description in the language which is listed first. Languages can be defined with their
     short or long language codes. Note that not all archives provide <filename>Translation</filename>
     files for every language - the long language codes are especially rare.</para>
     <para>The default list includes "environment" and "en". "<literal>environment</literal>" has a special meaning here:
     it will be replaced at runtime with the language codes extracted from the <literal>LC_MESSAGES</literal> environment variable.
     It will also ensure that these codes are not included twice in the list. If <literal>LC_MESSAGES</literal>
     is set to "C" only the <filename>Translation-en</filename> file (if available) will be used.
     To force APT to use no Translation file use the setting <literal>Acquire::Languages=none</literal>. "<literal>none</literal>"
     is another special meaning code which will stop the search for a suitable <filename>Translation</filename> file.
     This tells APT to download these translations too, without actually
     using them unless the environment specifies the languages. So the
     following example configuration will result in the order "en, de" in an
     English locale or "de, en" in a German one. Note that "fr" is
     downloaded, but not used unless APT is used in a French locale (where
     the order would be "fr, de, en").
     <programlisting>Acquire::Languages { "environment"; "de"; "en"; "none"; "fr"; };</programlisting></para>
     <para>Note: To prevent problems resulting from APT being executed in different environments
     (e.g. by different users or by other programs) all Translation files which are found in
     <filename>/var/lib/apt/lists/</filename> will be added to the end of the list
     (after an implicit "<literal>none</literal>").</para>
     </listitem>
     </varlistentry>

     <varlistentry><term><option>ForceIPv4</option></term>
         <listitem><para>
         When downloading, force to use only the IPv4 protocol.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>ForceIPv6</option></term>
         <listitem><para>
         When downloading, force to use only the IPv6 protocol.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>MaxReleaseFileSize</option></term>
         <listitem><para>
           The maximum file size of Release/Release.gpg/InRelease files.
           The default is 10MB.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>EnableSrvRecords</option></term>
         <listitem><para>
           This option controls if apt will use the DNS SRV server record
           as specified in RFC 2782 to select an alternative server to
           connect to.
           The default is "true".
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>AllowInsecureRepositories</option></term>
         <listitem><para>
           Allow the update operation to load data files from
           a repository without a trusted signature. If enabled this
           option no data files will be loaded and the update
           operation fails with a error for this source. The default
           is false for backward compatibility. This will be changed
           in the future.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>AllowDowngradeToInsecureRepositories</option></term>
         <listitem><para>
           Allow that a repository that was previously gpg signed to become
           unsigned durign a update operation. When there is no valid signature
           of a previously trusted repository apt will refuse the update. This
           option can be used to override this protection. You almost certainly
           never want to enable this. The default is false.

           Note that apt will still consider packages from this source
           untrusted and warn about them if you try to install
           them.
         </para></listitem>
     </varlistentry>

     <varlistentry><term><option>Changelogs::URI</option> scope</term>
        <listitem><para>
          Acquiring changelogs can only be done if an URI is known from where to get them.
          Preferable the Release file indicates this in a 'Changelogs' field. If this isn't
          available the Label/Origin field of the Release file is used to check if a
          <literal>Acquire::Changelogs::URI::Label::<replaceable>LABEL</replaceable></literal> or
          <literal>Acquire::Changelogs::URI::Origin::<replaceable>ORIGIN</replaceable></literal> option
          exists and if so this value is taken. The value in the Release file can be overridden
          with <literal>Acquire::Changelogs::URI::Override::Label::<replaceable>LABEL</replaceable></literal>
          or <literal>Acquire::Changelogs::URI::Override::Origin::<replaceable>ORIGIN</replaceable></literal>.

          The value should be a normal URI to a text file, except that package specific data is
          replaced with the placeholder <literal>@CHANGEPATH@</literal>. The
          value for it is: 1. if the package is from a component (e.g. <literal>main</literal>)
          this is the first part otherwise it is omitted, 2. the first letter of source package name,
          except if the source package name starts with '<literal>lib</literal>' in which case it will
          be the first four letters. 3. The complete source package name. 4. the complete name again and
          5. the source version.
          The first (if present), second, third and fourth part are separated by a slash ('<literal>/</literal>')
          and between the fourth and fifth part is an underscore ('<literal>_</literal>').

          The special value '<literal>no</literal>' is available for this option indicating that
          this source can't be used to acquire changelog files from. Another source will be tried
          if available in this case.
        </para></listitem>
    </varlistentry>

   </variablelist>
 </refsect1>

 <refsect1><title>Binary specific configuration</title>
    <para>Especially with the introduction of the <command>apt</command> binary
       it can be useful to set certain options only for a specific binary as
       even options which look like they would effect only a certain binary like
       <option>APT::Get::Show-Versions</option> effect
       <command>apt-get</command> as well as <command>apt</command>.
    </para>
    <para>Setting an option for a specific binary only can be achieved by
       setting the option inside the
       <option>Binary::<replaceable>specific-binary</replaceable></option>
       scope. Setting the option <option>APT::Get::Show-Versions</option> for
       the <command>apt</command> only can e.g. by done by setting
       <option>Binary::apt::APT::Get::Show-Versions</option> instead.</para>
    <para>Note that as seen in the DESCRIPTION section further above you can't
       set binary-specific options on the commandline itself nor in
       configuration files loaded via the commandline.</para>
 </refsect1>

 <refsect1><title>Directories</title>

   <para>The <literal>Dir::State</literal> section has directories that pertain to local 
   state information. <literal>lists</literal> is the directory to place downloaded 
   package lists in and <literal>status</literal> is the name of the &dpkg; status file.
   <literal>preferences</literal> is the name of the APT <filename>preferences</filename> file.
   <literal>Dir::State</literal> contains the default directory to prefix on all
   sub-items if they do not start with <filename>/</filename> or <filename>./</filename>.</para>

   <para><literal>Dir::Cache</literal> contains locations pertaining to local cache 
   information, such as the two package caches <literal>srcpkgcache</literal> and 
   <literal>pkgcache</literal> as well as the location to place downloaded archives, 
   <literal>Dir::Cache::archives</literal>. Generation of caches can be turned off
   by setting <literal>pkgcache</literal> or <literal>srcpkgcache</literal> to
   <literal>""</literal>.  This will slow down startup but save disk space. It
   is probably preferable to turn off the pkgcache rather than the srcpkgcache.
   Like <literal>Dir::State</literal> the default directory is contained in
   <literal>Dir::Cache</literal></para>

   <para><literal>Dir::Etc</literal> contains the location of configuration files, 
   <literal>sourcelist</literal> gives the location of the sourcelist and 
   <literal>main</literal> is the default configuration file (setting has no effect,
   unless it is done from the config file specified by 
   <envar>APT_CONFIG</envar>).</para>

   <para>The <literal>Dir::Parts</literal> setting reads in all the config fragments in 
   lexical order from the directory specified. After this is done then the
   main config file is loaded.</para>

   <para>Binary programs are pointed to by <literal>Dir::Bin</literal>. <literal>Dir::Bin::Methods</literal> 
   specifies the location of the method handlers and <literal>gzip</literal>, 
   <literal>bzip2</literal>, <literal>lzma</literal>,
   <literal>dpkg</literal>, <literal>apt-get</literal> <literal>dpkg-source</literal> 
   <literal>dpkg-buildpackage</literal> and <literal>apt-cache</literal> specify the location
   of the respective programs.</para>

   <para>
     The configuration item <literal>RootDir</literal> has a special
     meaning.  If set, all paths will be
     relative to <literal>RootDir</literal>, <emphasis>even paths that
     are specified absolutely</emphasis>.  So, for instance, if
     <literal>RootDir</literal> is set to
     <filename>/tmp/staging</filename> and
     <literal>Dir::State::status</literal> is set to
     <filename>/var/lib/dpkg/status</filename>, then the status file
     will be looked up in
     <filename>/tmp/staging/var/lib/dpkg/status</filename>.
     If you want to prefix only relative paths, set <literal>Dir</literal> instead.
   </para>

   <para>
      The <literal>Ignore-Files-Silently</literal> list can be used to specify
      which files APT should silently ignore while parsing the files in the
      fragment directories. Per default a file which end with <literal>.disabled</literal>,
      <literal>~</literal>, <literal>.bak</literal> or <literal>.dpkg-[a-z]+</literal>
      is silently ignored. As seen in the last default value these patterns can use regular
      expression syntax.
   </para>
 </refsect1>
 
 <refsect1><title>APT in DSelect</title>
   <para>   
   When APT is used as a &dselect; method several configuration directives
   control the default behavior. These are in the <literal>DSelect</literal> section.</para>
   
   <variablelist>
     <varlistentry><term><option>Clean</option></term>
     <listitem><para>Cache Clean mode; this value may be one of
     <literal>always</literal>, <literal>prompt</literal>,
     <literal>auto</literal>, <literal>pre-auto</literal> and
     <literal>never</literal>.
     <literal>always</literal> and <literal>prompt</literal> will remove
     all packages from the cache after upgrading, <literal>prompt</literal>
     (the default) does so conditionally.
     <literal>auto</literal> removes only those packages which are no longer
     downloadable (replaced with a new version for instance).
     <literal>pre-auto</literal> performs this action before downloading
     new packages.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>options</option></term>
     <listitem><para>The contents of this variable are passed to &apt-get; as command line
     options when it is run for the install phase.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Updateoptions</option></term>
     <listitem><para>The contents of this variable are passed to &apt-get; as command line
     options when it is run for the update phase.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>PromptAfterUpdate</option></term>
     <listitem><para>If true the [U]pdate operation in &dselect; will always prompt to continue. 
     The default is to prompt only on error.</para></listitem>
     </varlistentry>
   </variablelist>
 </refsect1>
 
 <refsect1><title>How APT calls &dpkg;</title>
   <para>Several configuration directives control how APT invokes &dpkg;. These are 
   in the <literal>DPkg</literal> section.</para>

   <variablelist>
     <varlistentry><term><option>options</option></term>
     <listitem><para>This is a list of options to pass to &dpkg;. The options must be specified
     using the list notation and each list item is passed as a single argument
     to &dpkg;.</para></listitem>
     </varlistentry>
     
     <varlistentry><term><option>Pre-Invoke</option></term><term><option>Post-Invoke</option></term>
     <listitem><para>This is a list of shell commands to run before/after invoking &dpkg;. 
     Like <literal>options</literal> this must be specified in list notation. The 
     commands are invoked in order using <filename>/bin/sh</filename>; should any 
     fail APT will abort.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Pre-Install-Pkgs</option></term>
     <listitem><para>This is a list of shell commands to run before invoking &dpkg;. Like
     <literal>options</literal> this must be specified in list notation. The commands
     are invoked in order using <filename>/bin/sh</filename>; should any fail APT 
     will abort. APT will pass the filenames of all .deb files it is going to
     install to the commands, one per line on the requested file descriptor, defaulting
     to standard input.</para>

     <para>Version 2 of this protocol sends more information through the requested
     file descriptor: a line with the text <literal>VERSION 2</literal>,
     the APT configuration space, and a list of package actions with filename
     and version information.</para>

     <para>Each configuration directive line has the form
     <literal>key=value</literal>.  Special characters (equal signs, newlines,
     nonprintable characters, quotation marks, and percent signs in
     <literal>key</literal> and newlines, nonprintable characters, and percent
     signs in <literal>value</literal>) are %-encoded. Lists are represented
     by multiple <literal>key::=value</literal> lines with the same key. The
     configuration section ends with a blank line.</para>

     <para>Package action lines consist of five fields in Version 2: package
     name (without architecture qualification even if foreign), old version,
     direction of version change (&lt; for upgrades, &gt; for downgrades, = for
     no change), new version, action. The version fields are "-" for no version
     at all (for example when installing a package for the first time; no
     version is treated as earlier than any real version, so that is an
     upgrade, indicated as <literal>- &lt; 1.23.4</literal>). The action field
     is "**CONFIGURE**" if the package is being configured, "**REMOVE**" if it
     is being removed, or the filename of a .deb file if it is being
     unpacked.</para>

     <para>In Version 3 after each version field follows the architecture
     of this version, which is "-" if there is no version, and a field showing
     the MultiArch type "same", "foreign", "allowed" or "none". Note that "none"
     is an incorrect typename which is just kept to remain compatible, it
     should be read as "no" and users are encouraged to support both.</para>

     <para>The version of the protocol to be used for the command
     <literal><replaceable>cmd</replaceable></literal> can be chosen by setting
     <literal>DPkg::Tools::options::<replaceable>cmd</replaceable>::Version</literal>
     accordingly, the default being version 1. If APT isn't supporting the requested
     version it will send the information in the highest version it has support for instead.
     </para>

     <para>The file descriptor to be used to send the information can be requested with
     <literal>DPkg::Tools::options::<replaceable>cmd</replaceable>::InfoFD</literal>
     which defaults to <literal>0</literal> for standard input and is available since
     version 0.9.11. Support for the option can be detected by looking for the environment
     variable <envar>APT_HOOK_INFO_FD</envar> which contains the number of the used
     file descriptor as a confirmation.</para>
     </listitem>
     </varlistentry>

     <varlistentry><term><option>Run-Directory</option></term>
     <listitem><para>APT chdirs to this directory before invoking &dpkg;, the default is 
     <filename>/</filename>.</para></listitem>
     </varlistentry>

     <varlistentry><term><option>Build-options</option></term>
     <listitem><para>These options are passed to &dpkg-buildpackage; when compiling packages;
     the default is to disable signing and produce all binaries.</para></listitem>
     </varlistentry>
   </variablelist>

   <refsect2><title>dpkg trigger usage (and related options)</title>
     <para>APT can call &dpkg; in such a way as to let it make aggressive use of triggers over
     multiple calls of &dpkg;. Without further options &dpkg; will use triggers once each time it runs.
     Activating these options can therefore decrease the time needed to perform the
     install or upgrade. Note that it is intended to activate these options per default in the
     future, but as it drastically changes the way APT calls &dpkg; it needs a lot more testing.
     <emphasis>These options are therefore currently experimental and should not be used in
     production environments.</emphasis> It also breaks progress reporting such that all front-ends will
     currently stay around half (or more) of the time in the 100% state while it actually configures
     all packages.</para>
     <para>Note that it is not guaranteed that APT will support these options or that these options will
     not cause (big) trouble in the future. If you have understand the current risks and problems with
     these options, but are brave enough to help testing them, create a new configuration file and test a
     combination of options. Please report any bugs, problems and improvements you encounter and make sure
     to note which options you have used in your reports. Asking &dpkg; for help could also be useful for
     debugging proposes, see e.g. <command>dpkg --audit</command>. A defensive option combination would be
<literallayout>DPkg::NoTriggers "true";
PackageManager::Configure "smart";
DPkg::ConfigurePending "true";
DPkg::TriggersPending "true";</literallayout></para>

     <variablelist>
       <varlistentry><term><option>DPkg::NoTriggers</option></term>
       <listitem><para>Add the no triggers flag to all &dpkg; calls (except the ConfigurePending call).
       See &dpkg; if you are interested in what this actually means. In short: &dpkg; will not run the
       triggers when this flag is present unless it is explicitly called to do so in an extra call.
       Note that this option exists (undocumented) also in older APT versions with a slightly different
       meaning: Previously these option only append --no-triggers to the configure calls to &dpkg; -
       now APT will also add this flag to the unpack and remove calls.</para></listitem>
       </varlistentry>
       <varlistentry><term><option>PackageManager::Configure</option></term>
       <listitem><para>Valid values are "<literal>all</literal>",
       "<literal>smart</literal>" and "<literal>no</literal>".
       The default value is "<literal>all</literal>", which causes APT to
       configure all packages. The "<literal>smart</literal>" way is to
       configure only packages which need to be configured before another
       package can be unpacked (Pre-Depends), and let the rest be configured
       by &dpkg; with a call generated by the ConfigurePending option (see
       below). On the other hand, "<literal>no</literal>" will not configure
       anything, and totally relies on &dpkg; for configuration (which at the
       moment will fail if a Pre-Depends is encountered). Setting this option
       to any value other than <literal>all</literal> will implicitly also
       activate the next option by default, as otherwise the system could end
       in an unconfigured and potentially unbootable state.</para></listitem>
       </varlistentry>
       <varlistentry><term><option>DPkg::ConfigurePending</option></term>
       <listitem><para>If this option is set APT will call <command>dpkg --configure --pending</command>
       to let &dpkg; handle all required configurations and triggers. This option is activated automatically
       per default if the previous option is not set to <literal>all</literal>, but deactivating it could be useful
       if you want to run APT multiple times in a row - e.g. in an installer. In these sceneries you could
       deactivate this option in all but the last run.</para></listitem>
       </varlistentry>
       <varlistentry><term><option>DPkg::TriggersPending</option></term>
       <listitem><para>Useful for the <literal>smart</literal> configuration as a package which has pending
       triggers is not considered as <literal>installed</literal>, and &dpkg; treats them as <literal>unpacked</literal>
       currently which is a showstopper for Pre-Dependencies (see debbugs #526774). Note that this will
       process all triggers, not only the triggers needed to configure this package.</para></listitem>
       </varlistentry>
       <varlistentry><term><option>OrderList::Score::Immediate</option></term>
       <listitem><para>Essential packages (and their dependencies) should be configured immediately
       after unpacking. It is a good idea to do this quite early in the upgrade process as these
       configure calls also currently require <literal>DPkg::TriggersPending</literal> which
       will run quite a few triggers (which may not be needed). Essentials get per default a high score
       but the immediate flag is relatively low (a package which has a Pre-Depends is rated higher).
       These option and the others in the same group can be used to change the scoring. The following
       example shows the settings with their default values.
       <literallayout>OrderList::Score {
        Delete 500;
        Essential 200;
        Immediate 10;
        PreDepends 50;
};</literallayout>
       </para></listitem>
       </varlistentry>
     </variablelist>
   </refsect2>
 </refsect1>

 <refsect1>
   <title>Periodic and Archives options</title>
   <para><literal>APT::Periodic</literal> and <literal>APT::Archives</literal>
   groups of options configure behavior of apt periodic updates, which is
   done by the <literal>/etc/cron.daily/apt</literal> script. See the top of
   this script for the brief documentation of these options.
   </para>
 </refsect1>

 <refsect1>
   <title>Debug options</title>
   <para>
     Enabling options in the <literal>Debug::</literal> section will
     cause debugging information to be sent to the standard error
     stream of the program utilizing the <literal>apt</literal>
     libraries, or enable special program modes that are primarily
     useful for debugging the behavior of <literal>apt</literal>.
     Most of these options are not interesting to a normal user, but a
     few may be:

     <itemizedlist>
       <listitem>
         <para>
           <literal>Debug::pkgProblemResolver</literal> enables output
           about the decisions made by
           <literal>dist-upgrade, upgrade, install, remove, purge</literal>.
         </para>
       </listitem>

       <listitem>
         <para>
           <literal>Debug::NoLocking</literal> disables all file
           locking.  This can be used to run some operations (for
           instance, <literal>apt-get -s install</literal>) as a
           non-root user.
         </para>
       </listitem>

       <listitem>
         <para>
           <literal>Debug::pkgDPkgPM</literal> prints out the actual
           command line each time that <literal>apt</literal> invokes
           &dpkg;.
         </para>
       </listitem>

       <listitem>
         <para>
           <literal>Debug::IdentCdrom</literal> disables the inclusion
           of statfs data in CD-ROM IDs.  <!-- TODO: provide a
           motivating example, except I haven't a clue why you'd want
           to do this. -->
         </para>
       </listitem>
     </itemizedlist>
   </para>

   <para>
     A full list of debugging options to apt follows.
   </para>

   <variablelist>
     <varlistentry>
       <term><option>Debug::Acquire::cdrom</option></term>

       <listitem>
         <para>
           Print information related to accessing
           <literal>cdrom://</literal> sources.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::Acquire::ftp</option></term>

       <listitem>
         <para>
           Print information related to downloading packages using
           FTP.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::Acquire::http</option></term>

       <listitem>
         <para>
           Print information related to downloading packages using
           HTTP.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::Acquire::https</option></term>

       <listitem>
         <para>
           Print information related to downloading packages using
           HTTPS.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::Acquire::gpgv</option></term>

       <listitem>
         <para>
           Print information related to verifying cryptographic
           signatures using <literal>gpg</literal>.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::aptcdrom</option></term>

       <listitem>
         <para>
           Output information about the process of accessing
           collections of packages stored on CD-ROMs.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::BuildDeps</option></term>
       <listitem>
         <para>
           Describes the process of resolving build-dependencies in
           &apt-get;.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::Hashes</option></term>
       <listitem>
         <para>
           Output each cryptographic hash that is generated by the
           <literal>apt</literal> libraries.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::IdentCDROM</option></term>
       <listitem>
         <para>
           Do not include information from <literal>statfs</literal>,
           namely the number of used and free blocks on the CD-ROM
           filesystem, when generating an ID for a CD-ROM.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::NoLocking</option></term>
       <listitem>
         <para>
           Disable all file locking.  For instance, this will allow
           two instances of <quote><literal>apt-get
           update</literal></quote> to run at the same time.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAcquire</option></term>

       <listitem>
         <para>
           Log when items are added to or removed from the global
           download queue.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAcquire::Auth</option></term>
       <listitem>
         <para>
           Output status messages and errors related to verifying
           checksums and cryptographic signatures of downloaded files.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAcquire::Diffs</option></term>
       <listitem>
         <para>
           Output information about downloading and applying package
           index list diffs, and errors relating to package index list
           diffs.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAcquire::RRed</option></term>

       <listitem>
         <para>
           Output information related to patching apt package lists
           when downloading index diffs instead of full indices.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAcquire::Worker</option></term>

       <listitem>
         <para>
           Log all interactions with the sub-processes that actually
           perform downloads.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgAutoRemove</option></term>

       <listitem>
         <para>
           Log events related to the automatically-installed status of
           packages and to the removal of unused packages.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgDepCache::AutoInstall</option></term>
       <listitem>
         <para>
           Generate debug messages describing which packages are being
           automatically installed to resolve dependencies.  This
           corresponds to the initial auto-install pass performed in,
           e.g., <literal>apt-get install</literal>, and not to the
           full <literal>apt</literal> dependency resolver; see
           <literal>Debug::pkgProblemResolver</literal> for that.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgDepCache::Marker</option></term>
       <listitem>
        <para>
           Generate debug messages describing which packages are marked
          as keep/install/remove while the ProblemResolver does his work.
          Each addition or deletion may trigger additional actions;
          they are shown indented two additional spaces under the original entry.
          The format for each line is <literal>MarkKeep</literal>,
          <literal>MarkDelete</literal> or <literal>MarkInstall</literal> followed by
          <literal>package-name &lt;a.b.c -&gt; d.e.f | x.y.z&gt; (section)</literal>
          where <literal>a.b.c</literal> is the current version of the package,
          <literal>d.e.f</literal> is the version considered for installation and
          <literal>x.y.z</literal> is a newer version, but not considered for installation
          (because of a low pin score). The later two can be omitted if there is none or if
          it is the same as the installed version.
          <literal>section</literal> is the name of the section the package appears in.
        </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgDPkgPM</option></term>
       <listitem>
         <para>
           When invoking &dpkg;, output the precise command line with
           which it is being invoked, with arguments separated by a
           single space character.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgDPkgProgressReporting</option></term>
       <listitem>
         <para>
           Output all the data received from &dpkg; on the status file
           descriptor and any errors encountered while parsing it.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgOrderList</option></term>

       <listitem>
         <para>
           Generate a trace of the algorithm that decides the order in
           which <literal>apt</literal> should pass packages to
           &dpkg;.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgPackageManager</option></term>

       <listitem>
         <para>
           Output status messages tracing the steps performed when
           invoking &dpkg;.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgPolicy</option></term>

       <listitem>
         <para>
           Output the priority of each package list on startup.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgProblemResolver</option></term>

       <listitem>
         <para>
           Trace the execution of the dependency resolver (this
           applies only to what happens when a complex dependency
           problem is encountered).
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::pkgProblemResolver::ShowScores</option></term>
       <listitem>
        <para>
          Display a list of all installed packages with their calculated score
          used by the pkgProblemResolver. The description of the package
          is the same as described in <literal>Debug::pkgDepCache::Marker</literal>
        </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::sourceList</option></term>

       <listitem>
         <para>
           Print information about the vendors read from
           <filename>/etc/apt/vendors.list</filename>.
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>Debug::RunScripts</option></term>
       <listitem>
        <para>
          Display the external commands that are called by apt hooks.
          This includes e.g. the config options
          <literal>DPkg::{Pre,Post}-Invoke</literal> or
          <literal>APT::Update::{Pre,Post}-Invoke</literal>.
        </para>
       </listitem>
     </varlistentry>

<!-- 2009/07/11 Currently used nowhere. The corresponding code
is commented.
     <varlistentry>
       <term><literal>Debug::Vendor</literal></term>

       <listitem>
         <para>
           Print information about each vendor.
         </para>
       </listitem>
     </varlistentry>
-->

   </variablelist>
 </refsect1>
 
 <refsect1><title>Examples</title>
   <para>&configureindex; is a 
   configuration file showing example values for all possible 
   options.</para>
 </refsect1>
 
 <refsect1><title>Files</title>
   <variablelist>
      &file-aptconf;
   </variablelist>
 </refsect1>
 
 <refsect1><title>See Also</title>
   <para>&apt-cache;, &apt-config;<!-- ? reading apt.conf -->, &apt-preferences;.</para>
 </refsect1>

 &manbugs;
 
</refentry>