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>2015-10-14T00:00:00Z</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.
  52    </para>
  53
  54    <para>
  55    If an archive doesn't have a signed Release file or no Release file at all
  56    current APT versions will raise a warning in <command>update</command>
  57    operations and frontends like <command>apt-get</command> will require
  58    explicit confirmation if an installation request includes a package from
  59    such an unauthenticated archive.
  60    </para>
  61
  62    <para>
  63    In the future APT will refuse to work with unauthenticated repositories by
  64    default until support for them is removed entirely. Users have the option to
  65    opt-in to this behavior already by setting the configuration option
  66    <option>Acquire::AllowInsecureRepositories</option> to <literal>false</literal>.
  67    </para>
  68
  69    <para>
  70    Note: All APT-based package management frontends like &apt-get;, &aptitude;
  71    and &synaptic; support this authentication feature, so this manpage uses
  72    <literal>APT</literal> to refer to them all for simplicity only.
  73    </para>
  74 </refsect1>
  75
  76  <refsect1><title>Trusted repositories</title>
  77
  78    <para>
  79    The chain of trust from an APT archive to the end user is made up of
  80    several steps. <command>apt-secure</command> is the last step in
  81    this chain; trusting an archive does not mean that you trust its
  82    packages not to contain malicious code, but means that you
  83    trust the archive maintainer. It's the archive maintainer's
  84    responsibility to ensure that the archive's integrity is preserved.
  85    </para>
  86
  87    <para>apt-secure does not review signatures at a
  88    package level. If you require tools to do this you should look at
  89    <command>debsig-verify</command> and
  90    <command>debsign</command> (provided in the debsig-verify and
  91    devscripts packages respectively).</para>
  92
  93    <para>
  94    The chain of trust in Debian e.g. starts when a maintainer uploads a new
  95    package or a new version of a package to the Debian archive. In
  96    order to become effective, this upload needs to be signed by a key
  97    contained in one of the Debian package maintainers keyrings (available in
  98    the debian-keyring package). Maintainers' keys are signed by
  99    other maintainers following pre-established procedures to
 100    ensure the identity of the key holder. Similar procedures exist in all
 101    Debian-based distributions.
 102    </para>
 103
 104    <para>
 105    Once the uploaded package is verified and included in the archive,
 106    the maintainer signature is stripped off, and checksums of the package
 107    are computed and put in the Packages file. The checksums of all of the
 108    Packages files are then computed and put into the Release file. The
 109    Release file is then signed by the archive key for this &keyring-distro; release,
 110    and distributed alongside the packages and the Packages files on
 111    &keyring-distro; mirrors. The keys are in the &keyring-distro; archive keyring
 112    available in the &keyring-package; package.
 113    </para>
 114
 115    <para>
 116    End users can check the signature of the Release file, extract a checksum
 117    of a package from it and compare it with the checksum of the package
 118    they downloaded by hand - or rely on APT doing this automatically.
 119    </para>
 120
 121    <para>Notice that this is distinct from checking signatures on a
 122    per package basis. It is designed to prevent two possible attacks:
 123    </para>
 124
 125     <itemizedlist>
 126        <listitem><para><literal>Network "man in the middle"
 127        attacks</literal>. Without signature checking, malicious
 128        agents can introduce themselves into the package download process and
 129        provide malicious software either by controlling a network
 130        element (router, switch, etc.) or by redirecting traffic to a
 131        rogue server (through ARP or DNS spoofing
 132        attacks).</para></listitem>
 133
 134        <listitem><para><literal>Mirror network compromise</literal>.
 135         Without signature checking, a malicious agent can compromise a
 136         mirror host and modify the files in it to propagate malicious
 137         software to all users downloading packages from that
 138         host.</para></listitem>
 139     </itemizedlist>
 140
 141    <para>However, it does not defend against a compromise of the
 142    master server itself (which signs the packages) or against a
 143    compromise of the key used to sign the Release files. In any case,
 144    this mechanism can complement a per-package signature.</para>
 145 </refsect1>
 146
 147  <refsect1><title>User configuration</title>
 148    <para>
 149    <command>apt-key</command> is the program that manages the list of keys used
 150    by APT to trust repositories. It can be used to add or remove keys as well
 151    as list the trusted keys. Limiting which key(s) are able to sign which archive
 152    is possible via the <option>Signed-By</option> in &sources-list;.
 153    </para><para>
 154    Note that a default installation already contains all keys to securely
 155    acquire packages from the default repositories, so fiddling with
 156    <command>apt-key</command> is only needed if third-party repositories are
 157    added.
 158    </para><para>
 159    In order to add a new key you need to first download it
 160    (you should make sure you are using a trusted communication channel
 161    when retrieving it), add it with <command>apt-key</command> and
 162    then run <command>apt-get update</command> so that apt can download
 163    and verify the <filename>InRelease</filename> or <filename>Release.gpg</filename>
 164    files from the archives you have configured.
 165    </para>
 166 </refsect1>
 167
 168 <refsect1><title>Archive configuration</title>
 169    <para>
 170    If you want to provide archive signatures in an archive under your
 171    maintenance you have to:
 172    </para>
 173
 174      <itemizedlist>
 175        <listitem><para><emphasis>Create a toplevel Release
 176        file</emphasis>,  if it does not exist already. You can do this
 177        by running <command>apt-ftparchive release</command>
 178        (provided in apt-utils).</para></listitem>
 179
 180       <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
 181       <command>gpg --clearsign -o InRelease Release</command> and
 182       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 183
 184       <listitem><para>
 185       <emphasis>Publish the key fingerprint</emphasis>, that way your users
 186       will know what key they need to import in order to authenticate the files
 187       in the archive. It is best to ship your key in its own keyring package
 188       like &keyring-distro; does with &keyring-package; to be able to
 189       distribute updates and key transitions automatically later.
 190       </para></listitem>
 191
 192       <listitem><para>
 193       <emphasis>Provide instructions on how to add your archive and key</emphasis>.
 194       If your users can't acquire your key securely the chain of trust described above is broken.
 195       How you can help users add your key depends on your archive and target audience ranging
 196       from having your keyring package included in another archive users already have configured
 197       (like the default repositories of their distribution) to leverage the web of trust.
 198       </para></listitem>
 199
 200     </itemizedlist>
 201
 202     <para>Whenever the contents of the archive change (new packages
 203     are added or removed) the archive maintainer has to follow the
 204     first two steps outlined above.</para>
 205
 206 </refsect1>
 207
 208 <refsect1><title>See Also</title>
 209 <para>
 210 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 211 &debsign;, &debsig-verify;, &gpg;
 212 </para>
 213
 214 <para>For more background information you might want to review the
 215 <ulink
 216 url="https://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
 217 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 218 (available also in the harden-doc package) and the
 219 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 220 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 221
 222 </refsect1>
 223
 224  &manbugs;
 225  &manauthor;
 226
 227 <refsect1><title>Manpage Authors</title>
 228
 229 <para>This man-page is based on the work of Javier Fernández-Sanguino
 230 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 231 </para>
 232
 233 </refsect1>
 234
 235
 236 </refentry>
 237