if file is inaccessible for _apt, disable privilege drop in acquire

[apt.git] / doc / apt-secure.8.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % aptent SYSTEM "apt.ent"> %aptent;
<!ENTITY % aptverbatiment SYSTEM "apt-verbatim.ent"> %aptverbatiment;
<!ENTITY % aptvendor SYSTEM "apt-vendor.ent"> %aptvendor;
]>

<refentry>
 <refentryinfo>
   &apt-author.jgunthorpe;
   &apt-author.team;
   &apt-email;
   &apt-product;
   <!-- The last update date -->
   <date>2012-06-09T00:00:00Z</date>
 </refentryinfo>

 <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
   archives. This ensures that packages in the archive can't be
   modified by people who have no access to the Release file signing
   key.
   </para>

   <para>
   If a package comes from a archive without a signature, or with a
   signature that apt does not have a key for, that package is
   considered untrusted, and installing it will result in a big
   warning. <command>apt-get</command> will currently only warn
   for unsigned archives; future releases might force all sources
   to be verified before downloading packages from them.
   </para>

   <para>
   The package frontends &apt-get;, &aptitude; and &synaptic; support this new
   authentication feature.
   </para>
</refsect1>

 <refsect1><title>Trusted archives</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 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 the Debian Maintainers keyring (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.
   </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
   Debian 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. It can be used to add or remove keys, although
   an installation of this release will automatically contain the
   default Debian archive signing keys used in the Debian package
   repositories.
   </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>,
      that way your users will know what key they need to import in
      order to authenticate the files in the
      archive.</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="http://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
Security Infrastructure</ulink> chapter of the Securing Debian Manual
(available also 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>
 

</refentry>