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<title>Configuration management</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"></meta></head><body><div class="article"><div class="titlepage"><div><div><h2 class="title"><a id="id-1"></a>Configuration management</h2></div><div><h3 class="subtitle"><em>Protocol version 2.1</em></h3></div><div><div class="author"><h3 class="author"><span class="firstname">Wichert</span> <span class="surname">Akkerman</span></h3><div class="affiliation"><span class="orgname">The Debian Project<br></br></span><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:wakkerma@debian.org">wakkerma@debian.org</a>&gt;</code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Joey</span> <span class="surname">Hess</span></h3><div class="affiliation"><span class="orgname">The Debian Project<br></br></span><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:joeyh@debian.org">joeyh@debian.org</a>&gt;</code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="othername">The Debian Policy Mailing List</span></h3><div class="affiliation"><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:debian-policy@lists.debian.org">debian-policy@lists.debian.org</a>&gt;</code></p></div></div></div></div><div><p class="releaseinfo">Revision 7.1, Debian Policy 4.6.0.1, 2021-08-18</p></div><div><p class="copyright">Copyright © 1998, 1999, 2000 Wichert Akkerman, Joey Hess</p></div><div><div class="legalnotice"><a id="id-1.1.8"></a><p>
        These are the copyright dates of the original configuration
        management protocol document.  Since then, this document has been
        updated by many others.  No comprehensive collection of copyright
        notices for subsequent work exists.
      </p><p>
        Redistribution and use in source and binary forms, with or without
        modification, are permitted provided that the following conditions
        are met:
      </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
            Redistributions of source code must retain the above copyright
            notice, this list of conditions and the following disclaimer.
          </p></li><li class="listitem"><p>
            Redistributions in binary form must reproduce the above
            copyright notice, this list of conditions and the following
            disclaimer in the documentation and/or other materials
            provided with the distribution.
          </p></li><li class="listitem"><p>
            Neither the name of the Debian Project nor the names of its
            contributors may be used to endorse or promote products
            derived from this software without specific prior written
            permission.
          </p></li></ol></div><p>
        THIS MANUAL IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
        "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
        LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
        FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
        COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
        INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
        (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
        SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
        HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
        CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
        OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
        EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
      </p></div></div></div><hr></hr></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="#id-1.2">1. 
      Introduction
    </a></span></dt><dt><span class="sect1"><a href="#id-1.3">2. 
      Configuration Data
    </a></span></dt><dd><dl><dt><span class="sect2"><a href="#id-1.3.2">2.1. 
        The configuration space
      </a></span></dt></dl></dd><dt><span class="sect1"><a href="#id-1.4">3. 
      Templates
    </a></span></dt><dd><dl><dt><span class="sect2"><a href="#id-1.4.4">3.1. 
        Template information
      </a></span></dt></dl></dd><dt><span class="sect1"><a href="#id-1.5">4. 
      Configuration frontends
    </a></span></dt><dt><span class="sect1"><a href="#id-1.6">5. 
      Communication with the frontend
    </a></span></dt><dt><span class="sect1"><a href="#id-1.7">6. 
      Debian install-time configuration
    </a></span></dt></dl></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.2"></a>1. 
      Introduction
    </h2></div></div></div><p>
      Configuration management is quickly becoming a very important issue.
      Having programs which do cool stuff is great, but we need to store
      their configuration as well. We see more and more different
      configuration systems being introduced all the time, which is not very
      practical. This text introduces a general configuration management
      system which flexible enough to be used for all kinds of applications.
    </p></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.3"></a>2. 
      Configuration Data
    </h2></div></div></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a id="id-1.3.2"></a>2.1. 
        The configuration space
      </h3></div></div></div><p>
        All configuration information is stored in what I call the
        configuration space. This is a database with a special design
        which resembles the method we look at configuration information.
	This is done by defining a hierarchy of information. Each package
	receives its own space in the hierarchy.  Each package is free to
	use a flat space, or divide its space further into
	sub-hierarchies.  If multiple packages share a common purpose they
	may use a shared toplevel hierarchy, preferably with the same name
	as a shared (virtual) package name (for example, both
	<span class="application">mutt</span> and <span class="application">elm</span>
	can use <code class="literal">mail-reader</code>,
	<span class="application">strn</span> and <span class="application">nn</span>
	could use <code class="literal">news-reader</code>).  This
	shared tree can also be used as a default, ie a variable
	<code class="literal">news-reader/nntpserver</code> can be used by
	<span class="application">strn</span> if <code class="literal">strn/nntpserver</code>
	does not exist.
      </p><p>
        Each variable in the configuration space has some information
	associated with it. Most importantly, it has a value. It also may
	have a set of flags and a set of substitution data.
      </p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.4"></a>3. 
      Templates
    </h2></div></div></div><p>
      Each variable in the configuration space is associated with some
      meta-data. The minimum meta-data associated with a variable is:
      long and short description, type, and default value. The meta-data
      is essentially static; the protocol described below does not allow it
      to be changed.
    </p><p>
      The meta-data exists in a space with similar naming
      properties to the configuration space described above, and typically
      one variable in the configuration space will have associated with it
      metadata with the same name in the meta-data space. However, this need
      not be the case; many different variables can all be associated with
      the same meta-data. In effect the meta-data serves as a template
      for the configuration variable.
    </p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a id="id-1.4.4"></a>3.1. 
        Template information
      </h3></div></div></div><p>
	So, what do we need to store in a variable template? Of course we
	need a name to identify the template. Template names are made up of
	components separated by the character `/' (slash).
	Each component is limited to alphanumerics and `+' `-' `.' `_'
	(plus, minus, full stop, underscore).
      </p><p>
        A type is also needed so data can be verified. Here is a table
	of common types; implementations are free to make up more.
	</p><div class="table"><a id="id-1.4.4.3.1"></a><p class="title"><strong>Table 1. Available data types</strong></p><div class="table-contents"><table class="table" summary="Available data types" border="1"><colgroup><col></col><col></col></colgroup><thead><tr><th>Type</th><th>Description</th></tr></thead><tbody><tr><td>string</td><td>Holds any arbitrary string of data.</td></tr><tr><td>boolean</td><td>
         Holds "<code class="literal">true</code>" or "<code class="literal">false</code>".
      </td></tr><tr><td>select</td><td>
        Holds one of a finite number of possible values. These
        values must be specified in a field named
        <code class="literal">Choices:</code>. Separate the possible values
        with commas and spaces, like this:
        <code class="literal">
          Choices: yes, no, maybe
        </code>
      </td></tr><tr><td>multiselect</td><td>
        Just like the select data type, except the user can choose any
        number of items from the list. This means that the
        <code class="literal">Default:</code> field and the actual value of the
        question may be a comma and space delimited list of values,
        just like the <code class="literal">Choices:</code> field.
      </td></tr><tr><td>note</td><td>
        This template is a note that can be displayed to the user. As
        opposed to text, it is something important, that the user really
        should see. If it is not possible to display it, it might be
        saved to a log file or mailbox for them to see later.
      </td></tr><tr><td>text</td><td>
        This template is a scrap of text that can be displayed to
        the user. It's intended to be used for mostly cosmetic
        reasons, touching up around other questions that are asked
        at the same time. Unlike a note, it isn't treated as
        something the user should definitely see. Less complex frontends
        may refuse to ever display this type of element.
      </td></tr><tr><td>password</td><td>
        Holds a password. Use with caution. Be aware that the password
        the user enters will be written to a database. You
        should consider clearing that value out of the database as soon
        as is possible.
      </td></tr><tr><td>title</td><td>
        Holds a (short) string that can be displayed using the SETTITLE
        command. Only the value of the short description will be used.
      </td></tr></tbody></table></div></div><p><br class="table-break"></br>

</p><p>
	Of course a default value is useful as well, and
	finally we need a description of the variable. We actually use two
	descriptions: a short one (limited to 50 characters or so) and an
	extended one.
      </p><p>
        The extended description may be word-wrapped by the
	FrontEnd. To make separate paragraphs in it, use <code class="literal">.</code>
	on a line by itself to separate them. Text in the extended
	description that is prefaced by additional whitespace will not be
	wordwrapped. Both the description and extended
        description may have substitutions embedded in them. Ie,
	<code class="literal">${foo}</code>. These will be expanded when the
	descriptions are displayed.
      </p><p>
        This information is stored in a template file that consists of
	stanzas in a rfc-822 compliant format, separated by blank lines.
	Here is an example:
	</p><pre class="programlisting">
Template: hostname
Type: string
Default: debian
Description: unqualified hostname for this computer
 This is the name by which this computer will be known on the network. It
 has to be a unique name in your domain.

Template: domain
Type: string
Description: domain for this computer
  This is the domain your computer is a member of. Typically it is
  something like "mycompany.com" or "myuniversity.edu".</pre><p>
      </p><p>
	For localization, the description field (and also the choices
	field of a select or multiselect type question, and the
	default field of a string or password type question) can be
	supplemented with versions for other languages. These are
	named <span class="emphasis"><em>Description-ll</em></span>,
	<span class="emphasis"><em>Description-ll_LL</em></span>,
	<span class="emphasis"><em>Description-ll_LL.encoding</em></span> and so on.
      </p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.5"></a>4. 
      Configuration frontends
    </h2></div></div></div><p>
      Of course applications can use the database and meta-database directly.
      But there should be a simple system to interact with the user that is
      simple and modular enough to be used with systems ranging from
      shell-scripts to Fortran programs. To do this we define a general
      frontend that can be driven using the simplest and most common form of
      communication: stdin and stdout.
    </p><p>
      Using this simple form of communication gives us a great advantage: it
      becomes easy to change the frontend. That means the user can switch
      between a console, a graphical or even a web-interface at will.
    </p><p>
      Besides being able to switch between types of frontends there is
      another important aspect of a good user interface: user friendliness.
      We have to account for the fact that some users know more then others
      and change the information we show or ask from the user. We do this by
      giving everything a priority and giving the user control over what
      kind of questions he wants to see. Experts can request to see
      everything, while novices get the option of only seeing only important
      questions. Finally there is an option to simply skip all questions, so
      it becomes possible to do automatic configuration using default values
      or values that are downloaded into the database from a remote
      location. This makes it simple for example to install and manage
      clusters or lab rooms or do installs for dummies.
    </p></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.6"></a>5. 
      Communication with the frontend
    </h2></div></div></div><p>
      This communication between the frontend and the application should be
      as simple as possible. Since most IO implementations default to
      line-buffered IO, so we use a simple language where each command is
      exactly one line.
    </p><p>
      After sending each command to stdout, the client
      should read one line from stdin. This is the response to the command,
      and it will be in the form of a number followed by whitespace and an
      optional string of text. The number is the status code, while the
      text provides additional information.
      </p><div class="table"><a id="id-1.6.3.1"></a><p class="title"><strong>Table 2. Numeric status codes</strong></p><div class="table-contents"><table class="table" summary="Numeric status codes" border="1"><colgroup><col></col><col></col></colgroup><thead><tr><th>Range</th><th>Description</th></tr></thead><tbody><tr><td>0</td><td>success</td></tr><tr><td>1-9</td><td>reserved</td></tr><tr><td>10-19</td><td>invalid parameters</td></tr><tr><td>20-29</td><td>syntax errors</td></tr><tr><td>30-99</td><td>command-specific return codes</td></tr><tr><td>100-109</td><td>internal errors</td></tr><tr><td>110-255</td><td>reserved</td></tr></tbody></table></div></div><p><br class="table-break"></br>

</p><p>
      Here are the currently supported commands.
    </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><a id="command_version"></a>
      VERSION
      <em class="parameter"><code>number</code></em>
    </p><p>
      This exchanges with the frontend the protocol version number that is
      being used. The current version is 2.1. Versions in the 2.x series
      will be backwards-compatible. You may specify the protocol version
      number you are speaking. The frontend will return the version of the
      protocol it speaks. If the version you specify is too low, this
      command will return the numeric return code <code class="literal">30</code>.
    </p></li><li class="listitem"><p><a id="command_capb"></a>
      CAPB
      <em class="parameter"><code>capabilities</code></em>
    </p><p>
      This exchanges with the frontend a list of supported capabilities
      Capabilities both the frontend and your confmodule support may be
      used; the capabilities supported by the frontend are returned by
      this command.
      </p><div class="table"><a id="id-1.6.5.2.2.1"></a><p class="title"><strong>Table 3. Currently used capabilities</strong></p><div class="table-contents"><table class="table" summary="Currently used capabilities" border="1"><colgroup><col></col><col></col></colgroup><thead><tr><th>capability</th><th>description</th></tr></thead><tbody><tr><td>backup</td><td>
	        Backing up to a previous step is supported.
	      </td></tr><tr><td>escape</td><td>
	        The frontend expects commands sent to it to have
	        backslashes and newlines quoted as <code class="literal">\\</code>
	        and <code class="literal">\n</code> respectively and will in turn
	        quote backslashes and newlines in its replies.  See
	        <span class="citerefentry"><span class="refentrytitle">debconf-escape</span>(1)</span>.
	      </td></tr><tr><td>multiselect</td><td>
	        The multiselect data type is supported. You do not need to
	        check this capability if you depend on any modern version
	        of debconf.
	      </td></tr></tbody></table></div></div><p><br class="table-break"></br>
    </p></li><li class="listitem"><p><a id="command_settitle"></a>
      SETTITLE
      <em class="parameter"><code>template</code></em>
    </p><p>
      You can use this command to set a title in the frontend. This may
      appear in different ways, depending on the frontend being used, for
      example it might change the title of the frontend's window. If you
      don't specify anything, a title will automatically be generated.
    </p><p>
      Using a template has the advantage that titles are translatable and
      that they can be maintained in the same place as other text
      displayed to users.
    </p></li><li class="listitem"><p><a id="command_title"></a>
      TITLE
      <em class="parameter"><code>string</code></em>
    </p><p>
      Similar to SETTITLE, but takes a string instead of a template as
      parameter. Consequence is that the title will not be translatable,
      unless some other mechanism (like gettext) is used.
    </p></li><li class="listitem"><p><a id="command_stop"></a>
      STOP
    </p><p>
      This command tells the frontend you're done talking to it. Typically
      the frontend can detect the termination of your program and this
      command is not necessary.
    </p></li><li class="listitem"><p><a id="command_input"></a>
      INPUT
      <em class="parameter"><code>priority</code></em>
      <em class="parameter"><code>question</code></em>
    </p><p>
      This tells the frontend to display a question (or other type of
      item) to the user. <em class="parameter"><code>question</code></em> is the name of
      the item to display, all other information about the item is
      retrieved from the templates described previously.
      <em class="parameter"><code>priority</code></em> is how important it is that the user
      be prompted. The frontend need only ask this question if the
      priority is high enough. The question is not displayed until a go
      command is given. This allows us to ask multiple questions in a
      single screen. Once a question has been displayed to the user and
      the user has provided input, the frontend will set the
      <code class="literal">seen</code> flag.
      </p><div class="table"><a id="id-1.6.5.6.2.4"></a><p class="title"><strong>Table 4. Supported priorities</strong></p><div class="table-contents"><table class="table" summary="Supported priorities" border="1"><colgroup><col></col><col></col></colgroup><thead><tr><th>Priority</th><th>Description</th></tr></thead><tbody><tr><td>low</td><td>
	  Very trivial items that have defaults that
	  will work in the vast majority of cases.
	</td></tr><tr><td>medium</td><td>
          Normal items that have reasonable defaults.
	</td></tr><tr><td>high</td><td>
          Items that don't have a reasonable 
	  default.
	</td></tr><tr><td>critical</td><td>
	  Items that will probably break
	  the system without user intervention.
        </td></tr></tbody></table></div></div><p><br class="table-break"></br>

</p><p>
      Note that the frontend decides if the user is actually prompted or
      not. If the user has already answered a question, they are normally
      not asked it again even if input is called again. And if the user is
      ignoring low priority items, they will not see them. In either of
      these cases, this command returns the numeric return code
      <code class="literal">30</code>.
    </p></li><li class="listitem"><p><a id="command_beginblock"></a>
      BEGINBLOCK
    </p></li><li class="listitem"><p><a id="command_endblock"></a>
      ENDBLOCK
    </p><p>
      Some frontends are able to display a number of items to the user at
      once. To do this, they need to be given blocks of input commands,
      enclosed in the BEGINBLOCK and ENDBLOCK commands. Blocks can be
      nested and very advanced frontends may use this as a user interface
      hint.
    </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
        There is an implicit block around any set of INPUT commands that
        are not enclosed in an explicit block.
      </p></div></li><li class="listitem"><p><a id="command_go"></a>
      GO
    </p><p>
      Shows the current set of accumulated items to the user and lets them
      fill in values, etc. If the backup capability is supported and the
      user indicates they want to back up a step, this command returns the
      numeric return code <code class="literal">30</code>.
    </p></li><li class="listitem"><p><a id="command_clear"></a>
      CLEAR
    </p><p>
      Clears the accumulated set of INPUT commands without displaying them
      to the user.
    </p></li><li class="listitem"><p><a id="command_get"></a>
      GET <em class="parameter"><code>question</code></em>
    </p><p>
      Ask the frontend to tell you how the user answered a question. The
      value is returned to you.
    </p></li><li class="listitem"><p><a id="command_set"></a>
      SET
      <em class="parameter"><code>question</code></em>
      <em class="parameter"><code>value</code></em>
    </p><p>
      Set the answer of a question to a value.
    </p></li><li class="listitem"><p><a id="command_reset"></a>
      RESET <em class="parameter"><code>question</code></em>
    </p><p>
      Reset the question to its default value. This includes resetting
      flags to their defaults.
    </p></li><li class="listitem"><p><a id="command_subst"></a>
      SUBST
      <em class="parameter"><code>question</code></em>
      <em class="parameter"><code>key</code></em>
      <em class="parameter"><code>value</code></em>
    </p><p>
      Questions (and other items) can have substitutions embedded in their
      descriptions (and, currently in their choices fields). These
      substitutions look like "<code class="literal">${key}</code>". When the
      question is displayed, the substitutions are replaced with their
      values. This command can be used to set the value of a substitution.
    </p></li><li class="listitem"><p><a id="command_fget"></a>
      FGET
      <em class="parameter"><code>question</code></em>
      <em class="parameter"><code>flag</code></em>
    </p><p>
      Questions (and other items) can have flags associated with them. The
      flags have a value of "<code class="literal">true</code>" or
      "<code class="literal">false</code>". This command returns the value of a
      flag.
    </p></li><li class="listitem"><p><a id="command_fset"></a>
      FSET
      <em class="parameter"><code>question</code></em>
      <em class="parameter"><code>flag</code></em>
      <em class="parameter"><code>value</code></em>
    </p><p>
      This sets the state of a flag on a question. Valid states for the
      flag are "<code class="literal">true</code>" and "<code class="literal">false</code>".
    </p><p>
      One common flag is the "<code class="literal">seen</code>" flag. It is
      normally only set if a user already seen a question.  Typically,
      frontends only display questions to users if they have the seen flag
      set to "false". Sometimes you want the user to see a question again
      -- in these cases you can set the seen flag to false to force the
      frontend to redisplay it.
    </p><p>
      Note that as a special convenience behavior, frontends will
      redisplay already seen questions if the question was first seen by
      the user in the same confmodule run. This makes it easy for a
      confmodule to back up to previous questions without having to reset
      the seen flag.
    </p></li><li class="listitem"><p><a id="command_metaget"></a>
      METAGET
      <em class="parameter"><code>question</code></em>
      <em class="parameter"><code>field</code></em>
    </p><p>
      This returns the value of any field of a question (the description,
      for example).
    </p></li><li class="listitem"><p><a id="command_register"></a>
      REGISTER
      <em class="parameter"><code>template</code></em>
      <em class="parameter"><code>question</code></em>
    </p><p>
      This creates a new question that is bound to a template. By default
      each template has an associated question with the same
      name. However, any number of questions can really be associated with
      a template, and this lets you create more such questions.
    </p></li><li class="listitem"><p><a id="command_unregister"></a>
      UNREGISTER
      <em class="parameter"><code>question</code></em>
    </p><p>
      This removes a question from the database.
    </p></li><li class="listitem"><p><a id="command_purge"></a>
      PURGE
    </p><p>
      Call this in your postrm when your package is purged. It removes all
      templates and questions your package has generated.
    </p></li></ul></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id-1.7"></a>6. 
      Debian install-time configuration
    </h2></div></div></div><p>
      Debian has had an excellent packaging system for a long time now. There is
      one thing missing though: a system to handle the configuration of
      packages so we don't have to stop the installation every time a package
      needs some data from the user or wants to show some information.
    </p><p>
      We want to make a package which does not break older dpkg's, and we
      want to be able to get the configuration information before the package
      is unpacked. To do this we add two new files, config and templates, to
      the control.tar.gz of a .deb package. Since all installation-software
      (apt, dselect, dpkg) download the package before installing it, we can
      extract this before the package is unpacked.
    </p><p>
      The templates file lists the templates for variables that this package
      uses. This is done using the format as used in the example in the
      section on templates.
    </p><p>
      The config-file contains a new element, which I call the
      configmodule.  This is a program that will determine the
      configuration before the package is unpacked.  This means it is
      usually run <span class="emphasis"><em>before</em></span> the preinst, and before
      the package is unpacked!
      </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>Please see debconf-devel(7) for details.</p></div><p>
      This is done to make sure that we can
      use the desired configuration in the preinst if necessary.
    </p><p>
      How does the configmodule get its information?  The configmodule
      needs a way to retrieve information from the configuration space, ask
      the user for information if necessary, etc. But we don't want to
      implement a user interface for each package. To solve this we use a
      separate frontend as specified in the section on frontends.
    </p></div></div></body></html>

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Code Editor : debconf_specification.html