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  <refentryinfo>
  15    &apt-author.jgunthorpe;
  16    &apt-author.team;
  17    &apt-email;
  18    &apt-product;
  19    <!-- The last update date -->
  20    <date>2012-05-21T00:00:00Z</date>
  21  </refentryinfo>
  22
  23  <refmeta>
  24    <refentrytitle>apt-secure</refentrytitle>
  25    <manvolnum>8</manvolnum>
  26    <refmiscinfo class="manual">APT</refmiscinfo>
  27  </refmeta>
  28
  29 <!-- NOTE: This manpage has been written based on the
  30      Securing Debian Manual ("Debian Security
  31      Infrastructure" chapter) and on documentation
  32      available at the following sites:
  33      http://wiki.debian.net/?apt06
  34      http://www.syntaxpolice.org/apt-secure/
  35      http://www.enyo.de/fw/software/apt-secure/
  36 -->
  37 <!-- TODO: write a more verbose example of how it works with
  38      a sample similar to
  39      http://www.debian-administration.org/articles/174
  40      ?
  41 -->
  42
  43
  44  <!-- Man page title -->
  45  <refnamediv>
  46     <refname>apt-secure</refname>
  47     <refpurpose>Archive authentication support for APT</refpurpose>
  48  </refnamediv>
  49
  50  <refsect1><title>Description</title>
  51    <para>
  52    Starting with version 0.6, <command>apt</command> contains code
  53    that does signature checking of the Release file for all
  54    archives. This ensures that packages in the archive can't be
  55    modified by people who have no access to the Release file signing
  56    key.
  57    </para>
  58
  59    <para>
  60    If a package comes from a archive without a signature or with a
  61    signature that apt does not have a key for that package is
  62    considered untrusted and installing it will result in a big
  63    warning. <command>apt-get</command> will currently only warn
  64    for unsigned archives, future releases might force all sources
  65    to be verified before downloading packages from them.
  66    </para>
  67
  68    <para>
  69    The package frontends &apt-get;, &aptitude; and &synaptic; support this new
  70    authentication feature.
  71    </para>
  72 </refsect1>
  73
  74  <refsect1><title>Trusted archives</title>
  75
  76    <para>
  77    The chain of trust from an apt archive to the end user is made up of
  78    different steps. <command>apt-secure</command> is the last step in
  79    this chain, trusting an archive does not mean that the packages
  80    that you trust it do not contain malicious code but means that you
  81    trust the archive maintainer. It's the archive maintainer
  82    responsibility to ensure that the archive integrity is correct.
  83    </para>
  84
  85    <para>apt-secure does not review signatures at a
  86    package level. If you require tools to do this you should look at
  87    <command>debsig-verify</command> and
  88    <command>debsign</command> (provided in the debsig-verify and
  89    devscripts packages respectively).</para>
  90
  91    <para>
  92    The chain of trust in Debian starts when a maintainer uploads a new
  93    package or a new version of a package to the Debian archive. This
  94    upload in order to become effective needs to be signed by a key of
  95    a maintainer within the Debian maintainer's keyring (available in
  96    the debian-keyring package). Maintainer's keys are signed by
  97    other maintainers following pre-established procedures to
  98    ensure the identity of the key holder.
  99    </para>
 100
 101    <para>
 102    Once the uploaded package is verified and included in the archive,
 103    the maintainer signature is stripped off, an MD5 sum of the package
 104    is computed and put in the Packages file. The MD5 sum of all of the
 105    packages files are then computed and put into the Release file. The
 106    Release file is then signed by the archive key (which is created
 107    once a year) and distributed through the FTP server. This key is
 108    also on the Debian keyring.
 109    </para>
 110
 111    <para>
 112    Any end user can check the signature of the Release file, extract the MD5
 113    sum of a package from it and compare it with the MD5 sum of the
 114    package he downloaded. Prior to version 0.6 only the MD5 sum of the
 115    downloaded Debian package was checked. Now both the MD5 sum and the
 116    signature of the Release file are checked.
 117    </para>
 118
 119    <para>Notice that this is distinct from checking signatures on a
 120    per package basis. It is designed to prevent two possible attacks:
 121    </para>
 122
 123     <itemizedlist>
 124        <listitem><para><literal>Network "man in the middle"
 125        attacks</literal>. Without signature checking, a malicious
 126        agent can introduce himself in the package download process and
 127        provide malicious software either by controlling a network
 128        element (router, switch, etc.) or by redirecting traffic to a
 129        rogue server (through arp or DNS spoofing
 130        attacks).</para></listitem>
 131
 132        <listitem><para><literal>Mirror network compromise</literal>.
 133         Without signature checking, a malicious agent can compromise a
 134         mirror host and modify the files in it to propagate malicious
 135         software to all users downloading packages from that
 136         host.</para></listitem>
 137     </itemizedlist>
 138
 139    <para>However, it does not defend against a compromise of the
 140    Debian master server itself (which signs the packages) or against a
 141    compromise of the key used to sign the Release files. In any case,
 142    this mechanism can complement a per-package signature.</para>
 143 </refsect1>
 144
 145  <refsect1><title>User configuration</title>
 146    <para>
 147    <command>apt-key</command> is the program that manages the list
 148    of keys used by apt. It can be used to add or remove keys although
 149    an installation of this release will automatically provide the
 150    default Debian archive signing keys used in the Debian package
 151    repositories.
 152    </para>
 153    <para>
 154    In order to add a new key you need to first download it
 155    (you should make sure you are using a trusted communication channel
 156    when retrieving it), add it with <command>apt-key</command> and
 157    then run <command>apt-get update</command> so that apt can download
 158    and verify the <filename>InRelease</filename> or <filename>Release.gpg</filename>
 159    files from the archives you have configured.
 160    </para>
 161 </refsect1>
 162
 163 <refsect1><title>Archive configuration</title>
 164    <para>
 165    If you want to provide archive signatures in an archive under your
 166    maintenance you have to:
 167    </para>
 168
 169      <itemizedlist>
 170        <listitem><para><emphasis>Create a toplevel Release
 171        file</emphasis>,  if it does not exist already. You can do this
 172        by running <command>apt-ftparchive release</command>
 173        (provided in apt-utils).</para></listitem>
 174
 175       <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
 176       <command>gpg --clearsign -o InRelease Release</command> and
 177       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 178
 179       <listitem><para><emphasis>Publish the key fingerprint</emphasis>,
 180       that way your users will know what key they need to import in
 181       order to authenticate the files in the
 182       archive.</para></listitem>
 183
 184     </itemizedlist>
 185
 186     <para>Whenever the contents of the archive changes (new packages
 187     are added or removed) the archive maintainer has to follow the
 188     first two steps previously outlined.</para>
 189
 190 </refsect1>
 191
 192 <refsect1><title>See Also</title>
 193 <para>
 194 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 195 &debsign; &debsig-verify;, &gpg;
 196 </para>
 197
 198 <para>For more background information you might want to review the
 199 <ulink
 200 url="http://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
 201 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 202 (available also in the harden-doc package) and the
 203 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 204 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 205
 206 </refsect1>
 207
 208  &manbugs;
 209  &manauthor;
 210
 211 <refsect1><title>Manpage Authors</title>
 212
 213 <para>This man-page is based on the work of Javier Fernández-Sanguino
 214 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 215 </para>
 216
 217 </refsect1>
 218
 219
 220 </refentry>
 221