doc/apt-secure.8.xml

   1 <?xml version="1.0" encoding="utf-8" standalone="no"?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
   4 <!ENTITY % aptent SYSTEM "apt.ent"> %aptent;
   5 <!ENTITY % aptverbatiment SYSTEM "apt-verbatim.ent"> %aptverbatiment;
   6 <!ENTITY % aptvendor SYSTEM "apt-vendor.ent"> %aptvendor;
   7 ]>
   8
   9 <refentry>
  10  <refentryinfo>
  11    &apt-author.jgunthorpe;
  12    &apt-author.team;
  13    &apt-email;
  14    &apt-product;
  15    <!-- The last update date -->
  16    <date>2016-08-06T00:00:00Z</date>
  17  </refentryinfo>
  18
  19  <refmeta>
  20    <refentrytitle>apt-secure</refentrytitle>
  21    <manvolnum>8</manvolnum>
  22    <refmiscinfo class="manual">APT</refmiscinfo>
  23  </refmeta>
  24
  25 <!-- NOTE: This manpage has been written based on the
  26      Securing Debian Manual ("Debian Security
  27      Infrastructure" chapter) and on documentation
  28      available at the following sites:
  29      http://wiki.debian.net/?apt06
  30      http://www.syntaxpolice.org/apt-secure/
  31      http://www.enyo.de/fw/software/apt-secure/
  32 -->
  33 <!-- TODO: write a more verbose example of how it works with
  34      a sample similar to
  35      http://www.debian-administration.org/articles/174
  36      ?
  37 -->
  38
  39
  40  <!-- Man page title -->
  41  <refnamediv>
  42     <refname>apt-secure</refname>
  43     <refpurpose>Archive authentication support for APT</refpurpose>
  44  </refnamediv>
  45
  46  <refsect1><title>Description</title>
  47    <para>
  48    Starting with version 0.6, <command>APT</command> contains code that does
  49    signature checking of the Release file for all repositories. This ensures
  50    that data like packages in the archive can't be modified by people who
  51    have no access to the Release file signing key. Starting with version 1.1
  52    <command>APT</command> requires repositories to provide recent authentication
  53    information for unimpeded usage of the repository.
  54    </para>
  55
  56    <para>
  57    If an archive has an unsigned Release file or no Release file at all
  58    current APT versions will refuse to download data from them by default
  59    in <command>update</command> operations and even if forced to download
  60    front-ends like &apt-get; will require explicit confirmation if an
  61    installation request includes a package from such an unauthenticated
  62    archive.
  63    </para>
  64
  65    <para>
  66    As a temporary exception &apt-get; (not &apt;!) raises warnings only if it
  67    encounters unauthenticated archives to give a slightly longer grace period
  68    on this backward compatibility effecting change. This exception will be removed
  69    in future releases and you can opt-out of this grace period by setting the
  70    configuration option <option>Binary::apt-get::Acquire::AllowInsecureRepositories</option>
  71    to <literal>false</literal> or <option>--no-allow-insecure-repositories</option>
  72    on the command line.
  73    </para>
  74
  75    <para>
  76    You can force all APT clients to raise only warnings by setting the
  77    configuration option <option>Acquire::AllowInsecureRepositories</option> to
  78    <literal>true</literal>. Individual repositories can also be allowed to be insecure
  79    via the &sources-list; option <literal>allow-insecure=yes</literal>.
  80    Note that insecure repositories are strongly discouraged and all options
  81    to force apt to continue supporting them will eventually be removed.
  82    Users also have the <option>Trusted</option> option available to disable
  83    even the warnings, but be sure to understand the implications as detailed in
  84    &sources-list;.
  85    </para>
  86
  87    <para>
  88    A repository which previously was authenticated but would loose this state in
  89    an <command>update</command> operation raises an error in all APT clients
  90    irrespective of the option to allow or forbid usage of insecure repositories.
  91    The error can be overcome by additionally setting
  92    <option>Acquire::AllowDowngradeToInsecureRepositories</option>
  93    to <literal>true</literal> or for Individual repositories with the &sources-list;
  94    option <literal>allow-downgrade-to-insecure=yes</literal>.
  95    </para>
  96
  97    <para>
  98    Note: All APT-based package management front-ends like &apt-get;, &aptitude;
  99    and &synaptic; support this authentication feature, so this manpage uses
 100    <literal>APT</literal> to refer to them all for simplicity only.
 101    </para>
 102 </refsect1>
 103
 104  <refsect1><title>Trusted Repositories</title>
 105
 106    <para>
 107    The chain of trust from an APT archive to the end user is made up of
 108    several steps. <command>apt-secure</command> is the last step in
 109    this chain; trusting an archive does not mean that you trust its
 110    packages not to contain malicious code, but means that you
 111    trust the archive maintainer. It's the archive maintainer's
 112    responsibility to ensure that the archive's integrity is preserved.
 113    </para>
 114
 115    <para>apt-secure does not review signatures at a
 116    package level. If you require tools to do this you should look at
 117    <command>debsig-verify</command> and
 118    <command>debsign</command> (provided in the debsig-verify and
 119    devscripts packages respectively).</para>
 120
 121    <para>
 122    The chain of trust in Debian starts (e.g.) when a maintainer uploads a new
 123    package or a new version of a package to the Debian archive. In
 124    order to become effective, this upload needs to be signed by a key
 125    contained in one of the Debian package maintainer keyrings (available in
 126    the debian-keyring package). Maintainers' keys are signed by
 127    other maintainers following pre-established procedures to
 128    ensure the identity of the key holder. Similar procedures exist in all
 129    Debian-based distributions.
 130    </para>
 131
 132    <para>
 133    Once the uploaded package is verified and included in the archive,
 134    the maintainer signature is stripped off, and checksums of the package
 135    are computed and put in the Packages file. The checksums of all of the
 136    Packages files are then computed and put into the Release file. The
 137    Release file is then signed by the archive key for this &keyring-distro; release,
 138    and distributed alongside the packages and the Packages files on
 139    &keyring-distro; mirrors. The keys are in the &keyring-distro; archive keyring
 140    available in the &keyring-package; package.
 141    </para>
 142
 143    <para>
 144    End users can check the signature of the Release file, extract a checksum
 145    of a package from it and compare it with the checksum of the package
 146    they downloaded by hand - or rely on APT doing this automatically.
 147    </para>
 148
 149    <para>Notice that this is distinct from checking signatures on a
 150    per package basis. It is designed to prevent two possible attacks:
 151    </para>
 152
 153     <itemizedlist>
 154        <listitem><para><literal>Network "man in the middle"
 155        attacks</literal>. Without signature checking, malicious
 156        agents can introduce themselves into the package download process and
 157        provide malicious software either by controlling a network
 158        element (router, switch, etc.) or by redirecting traffic to a
 159        rogue server (through ARP or DNS spoofing
 160        attacks).</para></listitem>
 161
 162        <listitem><para><literal>Mirror network compromise</literal>.
 163         Without signature checking, a malicious agent can compromise a
 164         mirror host and modify the files in it to propagate malicious
 165         software to all users downloading packages from that
 166         host.</para></listitem>
 167     </itemizedlist>
 168
 169    <para>However, it does not defend against a compromise of the
 170    master server itself (which signs the packages) or against a
 171    compromise of the key used to sign the Release files. In any case,
 172    this mechanism can complement a per-package signature.</para>
 173 </refsect1>
 174
 175  <refsect1><title>User Configuration</title>
 176    <para>
 177    <command>apt-key</command> is the program that manages the list of keys used
 178    by APT to trust repositories. It can be used to add or remove keys as well
 179    as list the trusted keys. Limiting which key(s) are able to sign which archive
 180    is possible via the <option>Signed-By</option> in &sources-list;.
 181    </para><para>
 182    Note that a default installation already contains all keys to securely
 183    acquire packages from the default repositories, so fiddling with
 184    <command>apt-key</command> is only needed if third-party repositories are
 185    added.
 186    </para><para>
 187    In order to add a new key you need to first download it
 188    (you should make sure you are using a trusted communication channel
 189    when retrieving it), add it with <command>apt-key</command> and
 190    then run <command>apt-get update</command> so that apt can download
 191    and verify the <filename>InRelease</filename> or <filename>Release.gpg</filename>
 192    files from the archives you have configured.
 193    </para>
 194 </refsect1>
 195
 196 <refsect1><title>Archive Configuration</title>
 197    <para>
 198    If you want to provide archive signatures in an archive under your
 199    maintenance you have to:
 200    </para>
 201
 202      <itemizedlist>
 203        <listitem><para><emphasis>Create a toplevel Release
 204        file</emphasis>,  if it does not exist already. You can do this
 205        by running <command>apt-ftparchive release</command>
 206        (provided in apt-utils).</para></listitem>
 207
 208       <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
 209       <command>gpg --clearsign -o InRelease Release</command> and
 210       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 211
 212       <listitem><para>
 213       <emphasis>Publish the key fingerprint</emphasis>, so that your users
 214       will know what key they need to import in order to authenticate the files
 215       in the archive. It is best to ship your key in its own keyring package
 216       like &keyring-distro; does with &keyring-package; to be able to
 217       distribute updates and key transitions automatically later.
 218       </para></listitem>
 219
 220       <listitem><para>
 221       <emphasis>Provide instructions on how to add your archive and key</emphasis>.
 222       If your users can't acquire your key securely the chain of trust described above is broken.
 223       How you can help users add your key depends on your archive and target audience ranging
 224       from having your keyring package included in another archive users already have configured
 225       (like the default repositories of their distribution) to leveraging the web of trust.
 226       </para></listitem>
 227
 228     </itemizedlist>
 229
 230     <para>Whenever the contents of the archive change (new packages
 231     are added or removed) the archive maintainer has to follow the
 232     first two steps outlined above.</para>
 233
 234 </refsect1>
 235
 236 <refsect1><title>See Also</title>
 237 <para>
 238 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 239 &debsign;, &debsig-verify;, &gpg;
 240 </para>
 241
 242 <para>For more background information you might want to review the
 243 <ulink
 244 url="https://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
 245 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 246 (also available in the harden-doc package) and the
 247 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 248 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 249
 250 </refsect1>
 251
 252  &manbugs;
 253  &manauthor;
 254
 255 <refsect1><title>Manpage Authors</title>
 256
 257 <para>This man-page is based on the work of Javier Fernández-Sanguino
 258 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 259 </para>
 260
 261 </refsect1>
 262
 263
 264 </refentry>