apt/doc/apt-secure.8.xml

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 <refentryinfo>
   &apt-author.jgunthorpe;
   &apt-author.team;
   &apt-email;
   &apt-product;
   <!-- The last update date -->
   <date>2016-03-18T00:00:00Z</date>
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 <refmeta>
   <refentrytitle>apt-secure</refentrytitle>
   <manvolnum>8</manvolnum>
   <refmiscinfo class="manual">APT</refmiscinfo>
 </refmeta>

<!-- NOTE: This manpage has been written based on the
     Securing Debian Manual ("Debian Security
     Infrastructure" chapter) and on documentation
     available at the following sites:
     http://wiki.debian.net/?apt06
     http://www.syntaxpolice.org/apt-secure/
     http://www.enyo.de/fw/software/apt-secure/
-->
<!-- TODO: write a more verbose example of how it works with 
     a sample similar to 
     http://www.debian-administration.org/articles/174
     ?
--> 

 
 <!-- Man page title -->
 <refnamediv>
    <refname>apt-secure</refname>
    <refpurpose>Archive authentication support for APT</refpurpose>
 </refnamediv>

 <refsect1><title>Description</title>
   <para>
   Starting with version 0.6, <command>APT</command> contains code that does
   signature checking of the Release file for all repositories. This ensures
   that data like packages in the archive can't be modified by people who
   have no access to the Release file signing key. Starting with version 1.1
   <command>APT</command> requires repositories to provide recent authentication
   information for unimpeded usage of the repository.
   </para>

   <para>
   If an archive has an unsigned Release file or no Release file at all
   current APT versions will refuse to download data from them by default
   in <command>update</command> operations and even if forced to download
   front-ends like &apt-get; will require explicit confirmation if an
   installation request includes a package from such an unauthenticated
   archive.
   </para>

   <para>
   As a temporary exception &apt-get; (not &apt;!) raises warnings only if it
   encounters unauthenticated archives to give a slightly longer grace period
   on this backward compatibility effecting change. This exception will be removed
   in future releases and you can opt-out of this grace period by setting the
   configuration option <option>Binary::apt-get::Acquire::AllowInsecureRepositories</option>
   to <literal>false</literal> or <option>--no-allow-insecure-repositories</option>
   on the command line.
   </para>

   <para>
   You can force all APT clients to raise only warnings by setting the
   configuration option <option>Acquire::AllowInsecureRepositories</option> to
   <literal>true</literal>. Note that this option will eventually be removed.
   Users also have the <option>Trusted</option> option available to disable
   even the warnings, but be sure to understand the implications as detailed in
   &sources-list;.
   </para>

   <para>
   A repository which previously was authentication but would loose this state in
   an <command>update</command> operation raises an error in all APT clients
   irrespective of the option to allow or forbid usage of insecure repositories.
   The error can be overcome by additionally setting
   <option>Acquire::AllowDowngradeToInsecureRepositories</option>
   to <literal>true</literal>.
   </para>

   <para>
   Note: All APT-based package management front-ends like &apt-get;, &aptitude;
   and &synaptic; support this authentication feature, so this manpage uses
   <literal>APT</literal> to refer to them all for simplicity only.
   </para>
</refsect1>

 <refsect1><title>Trusted Repositories</title>

   <para>
   The chain of trust from an APT archive to the end user is made up of
   several steps. <command>apt-secure</command> is the last step in
   this chain; trusting an archive does not mean that you trust its
   packages not to contain malicious code, but means that you
   trust the archive maintainer. It's the archive maintainer's
   responsibility to ensure that the archive's integrity is preserved.
   </para>

   <para>apt-secure does not review signatures at a
   package level. If you require tools to do this you should look at
   <command>debsig-verify</command> and
   <command>debsign</command> (provided in the debsig-verify and
   devscripts packages respectively).</para>

   <para>
   The chain of trust in Debian starts (e.g.) when a maintainer uploads a new
   package or a new version of a package to the Debian archive. In
   order to become effective, this upload needs to be signed by a key
   contained in one of the Debian package maintainer keyrings (available in
   the debian-keyring package). Maintainers' keys are signed by
   other maintainers following pre-established procedures to
   ensure the identity of the key holder. Similar procedures exist in all
   Debian-based distributions.
   </para>

   <para>
   Once the uploaded package is verified and included in the archive,
   the maintainer signature is stripped off, and checksums of the package
   are computed and put in the Packages file. The checksums of all of the
   Packages files are then computed and put into the Release file. The
   Release file is then signed by the archive key for this &keyring-distro; release,
   and distributed alongside the packages and the Packages files on
   &keyring-distro; mirrors. The keys are in the &keyring-distro; archive keyring
   available in the &keyring-package; package.
   </para>

   <para>
   End users can check the signature of the Release file, extract a checksum
   of a package from it and compare it with the checksum of the package
   they downloaded by hand - or rely on APT doing this automatically.
   </para>

   <para>Notice that this is distinct from checking signatures on a
   per package basis. It is designed to prevent two possible attacks:
   </para>

    <itemizedlist>
       <listitem><para><literal>Network "man in the middle"
       attacks</literal>. Without signature checking, malicious
       agents can introduce themselves into the package download process and
       provide malicious software either by controlling a network
       element (router, switch, etc.) or by redirecting traffic to a
       rogue server (through ARP or DNS spoofing
       attacks).</para></listitem>
 
       <listitem><para><literal>Mirror network compromise</literal>.
        Without signature checking, a malicious agent can compromise a
        mirror host and modify the files in it to propagate malicious
        software to all users downloading packages from that
        host.</para></listitem>
    </itemizedlist>

   <para>However, it does not defend against a compromise of the
   master server itself (which signs the packages) or against a
   compromise of the key used to sign the Release files. In any case,
   this mechanism can complement a per-package signature.</para>
</refsect1>

 <refsect1><title>User Configuration</title>
   <para>
   <command>apt-key</command> is the program that manages the list of keys used
   by APT to trust repositories. It can be used to add or remove keys as well
   as list the trusted keys. Limiting which key(s) are able to sign which archive
   is possible via the <option>Signed-By</option> in &sources-list;.
   </para><para>
   Note that a default installation already contains all keys to securely
   acquire packages from the default repositories, so fiddling with
   <command>apt-key</command> is only needed if third-party repositories are
   added.
   </para><para>
   In order to add a new key you need to first download it
   (you should make sure you are using a trusted communication channel
   when retrieving it), add it with <command>apt-key</command> and
   then run <command>apt-get update</command> so that apt can download
   and verify the <filename>InRelease</filename> or <filename>Release.gpg</filename>
   files from the archives you have configured.
   </para>
</refsect1>

<refsect1><title>Archive Configuration</title>
   <para>
   If you want to provide archive signatures in an archive under your
   maintenance you have to:
   </para>

     <itemizedlist>
       <listitem><para><emphasis>Create a toplevel Release
       file</emphasis>,  if it does not exist already. You can do this
       by running <command>apt-ftparchive release</command> 
       (provided in apt-utils).</para></listitem>
   
      <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
      <command>gpg --clearsign -o InRelease Release</command> and
      <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>

      <listitem><para>
      <emphasis>Publish the key fingerprint</emphasis>, so that your users
      will know what key they need to import in order to authenticate the files
      in the archive. It is best to ship your key in its own keyring package
      like &keyring-distro; does with &keyring-package; to be able to
      distribute updates and key transitions automatically later.
      </para></listitem>

      <listitem><para>
      <emphasis>Provide instructions on how to add your archive and key</emphasis>.
      If your users can't acquire your key securely the chain of trust described above is broken.
      How you can help users add your key depends on your archive and target audience ranging
      from having your keyring package included in another archive users already have configured
      (like the default repositories of their distribution) to leveraging the web of trust.
      </para></listitem>

    </itemizedlist>

    <para>Whenever the contents of the archive change (new packages
    are added or removed) the archive maintainer has to follow the
    first two steps outlined above.</para>

</refsect1>

<refsect1><title>See Also</title> 
<para> 
&apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
&debsign;, &debsig-verify;, &gpg;
</para>

<para>For more background information you might want to review the
<ulink
url="https://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
Security Infrastructure</ulink> chapter of the Securing Debian Manual
(also available in the harden-doc package) and the
<ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
>Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>

</refsect1>

 &manbugs;
 &manauthor;

<refsect1><title>Manpage Authors</title> 

<para>This man-page is based on the work of Javier Fernández-Sanguino
Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
</para>

</refsect1>
 

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