doc/apt-secure.8.xml

   1 <?xml version="1.0" encoding="utf-8" standalone="no"?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
   4
   5 <!ENTITY % aptent SYSTEM "apt.ent">
   6 %aptent;
   7
   8 <!ENTITY % aptverbatiment SYSTEM "apt-verbatim.ent">
   9 %aptverbatiment;
  10
  11 ]>
  12
  13 <refentry>
  14  &apt-docinfo;
  15
  16  <refmeta>
  17    <refentrytitle>apt-secure</refentrytitle>
  18    <manvolnum>8</manvolnum>
  19    <refmiscinfo class="manual">APT</refmiscinfo>
  20  </refmeta>
  21
  22 <!-- NOTE: This manpage has been written based on the
  23      Securing Debian Manual ("Debian Security
  24      Infrastructure" chapter) and on documentation
  25      available at the following sites:
  26      http://wiki.debian.net/?apt06
  27      http://www.syntaxpolice.org/apt-secure/
  28      http://www.enyo.de/fw/software/apt-secure/
  29 -->
  30 <!-- TODO: write a more verbose example of how it works with
  31      a sample similar to
  32      http://www.debian-administration.org/articles/174
  33      ?
  34 -->
  35
  36
  37  <!-- Man page title -->
  38  <refnamediv>
  39     <refname>apt-secure</refname>
  40     <refpurpose>Archive authentication support for APT</refpurpose>
  41  </refnamediv>
  42
  43  <refsect1><title>Description</title>
  44    <para>
  45    Starting with version 0.6, <command>apt</command> contains code
  46    that does signature checking of the Release file for all
  47    archives. This ensures that packages in the archive can't be
  48    modified by people who have no access to the Release file signing
  49    key.
  50    </para>
  51
  52    <para>
  53    If a package comes from a archive without a signature or with a
  54    signature that apt does not have a key for that package is
  55    considered untrusted and installing it will result in a big
  56    warning. <command>apt-get</command> will currently only warn
  57    for unsigned archives, future releases might force all sources
  58    to be verified before downloading packages from them.
  59    </para>
  60
  61    <para>
  62    The package frontends &apt-get;, &aptitude; and &synaptic; support this new
  63    authentication feature.
  64    </para>
  65 </refsect1>
  66
  67  <refsect1><title>Trusted archives</title>
  68
  69    <para>
  70    The chain of trust from an apt archive to the end user is made up of
  71    different steps. <command>apt-secure</command> is the last step in
  72    this chain, trusting an archive does not mean that the packages
  73    that you trust it do not contain malicious code but means that you
  74    trust the archive maintainer. It's the archive maintainer
  75    responsibility to ensure that the archive integrity is correct.
  76    </para>
  77
  78    <para>apt-secure does not review signatures at a
  79    package level. If you require tools to do this you should look at
  80    <command>debsig-verify</command> and
  81    <command>debsign</command> (provided in the debsig-verify and
  82    devscripts packages respectively).</para>
  83
  84    <para>
  85    The chain of trust in Debian starts when a maintainer uploads a new
  86    package or a new version of a package to the Debian archive. This
  87    upload in order to become effective needs to be signed by a key of
  88    a maintainer within the Debian maintainer's keyring (available in
  89    the debian-keyring package). Maintainer's keys are signed by
  90    other maintainers following pre-established procedures to
  91    ensure the identity of the key holder.
  92    </para>
  93
  94    <para>
  95    Once the uploaded package is verified and included in the archive,
  96    the maintainer signature is stripped off, an MD5 sum of the package
  97    is computed and put in the Packages file. The MD5 sum of all of the
  98    packages files are then computed and put into the Release file. The
  99    Release file is then signed by the archive key (which is created
 100    once a year) and distributed through the FTP server. This key is
 101    also on the Debian keyring.
 102    </para>
 103
 104    <para>
 105    Any end user can check the signature of the Release file, extract the MD5
 106    sum of a package from it and compare it with the MD5 sum of the
 107    package he downloaded. Prior to version 0.6 only the MD5 sum of the
 108    downloaded Debian package was checked. Now both the MD5 sum and the
 109    signature of the Release file are checked.
 110    </para>
 111
 112    <para>Notice that this is distinct from checking signatures on a
 113    per package basis. It is designed to prevent two possible attacks:
 114    </para>
 115
 116     <itemizedlist>
 117        <listitem><para><literal>Network "man in the middle"
 118        attacks</literal>. Without signature checking, a malicious
 119        agent can introduce himself in the package download process and
 120        provide malicious software either by controlling a network
 121        element (router, switch, etc.) or by redirecting traffic to a
 122        rogue server (through arp or DNS spoofing
 123        attacks).</para></listitem>
 124
 125        <listitem><para><literal>Mirror network compromise</literal>.
 126         Without signature checking, a malicious agent can compromise a
 127         mirror host and modify the files in it to propagate malicious
 128         software to all users downloading packages from that
 129         host.</para></listitem>
 130     </itemizedlist>
 131
 132    <para>However, it does not defend against a compromise of the
 133    Debian master server itself (which signs the packages) or against a
 134    compromise of the key used to sign the Release files. In any case,
 135    this mechanism can complement a per-package signature.</para>
 136 </refsect1>
 137
 138  <refsect1><title>User configuration</title>
 139    <para>
 140    <command>apt-key</command> is the program that manages the list
 141    of keys used by apt. It can be used to add or remove keys although
 142    an installation of this release will automatically provide the
 143    default Debian archive signing keys used in the Debian package
 144    repositories.
 145    </para>
 146    <para>
 147    In order to add a new key you need to first download it
 148    (you should make sure you are using a trusted communication channel
 149    when retrieving it), add it with <command>apt-key</command> and
 150    then run <command>apt-get update</command> so that apt can download
 151    and verify the <filename>Release.gpg</filename> files from the archives you
 152    have configured.
 153    </para>
 154 </refsect1>
 155
 156 <refsect1><title>Archive configuration</title>
 157    <para>
 158    If you want to provide archive signatures in an archive under your
 159    maintenance you have to:
 160    </para>
 161
 162      <itemizedlist>
 163        <listitem><para><emphasis>Create a toplevel Release
 164        file</emphasis>,  if it does not exist already. You can do this
 165        by running <command>apt-ftparchive release</command>
 166        (provided in apt-utils).</para></listitem>
 167
 168       <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
 169       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 170
 171       <listitem><para><emphasis>Publish the key fingerprint</emphasis>,
 172       that way your users will know what key they need to import in
 173       order to authenticate the files in the
 174       archive.</para></listitem>
 175
 176     </itemizedlist>
 177
 178     <para>Whenever the contents of the archive changes (new packages
 179     are added or removed) the archive maintainer has to follow the
 180     first two steps previously outlined.</para>
 181
 182 </refsect1>
 183
 184 <refsect1><title>See Also</title>
 185 <para>
 186 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 187 &debsign; &debsig-verify;, &gpg;
 188 </para>
 189
 190 <para>For more background information you might want to review the
 191 <ulink
 192 url="http://www.debian.org/doc/manuals/securing-debian-howto/ch7.en.html">Debian
 193 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 194 (available also in the harden-doc package) and the
 195 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 196 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 197
 198 </refsect1>
 199
 200  &manbugs;
 201  &manauthor;
 202
 203 <refsect1><title>Manpage Authors</title>
 204
 205 <para>This man-page is based on the work of Javier Fernández-Sanguino
 206 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 207 </para>
 208
 209 </refsect1>
 210
 211
 212 </refentry>
 213