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+<?xml version="1.0" encoding="utf-8" standalone="no"?>
+<!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>2016-08-06T00:00:00Z</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 repositories. This ensures
+ that data like packages in the archive can't be modified by people who
+ have no access to the Release file signing key. Starting with version 1.1
+ <command>APT</command> requires repositories to provide recent authentication
+ information for unimpeded usage of the repository. Since version 1.5 changes
+ in the information contained in the Release file about the repository need to be
+ confirmed before APT continues to apply updates from this repository.
+ </para>
+
+ <para>
+ Note: All APT-based package management front-ends like &apt-get;, &aptitude;
+ and &synaptic; support this authentication feature, so this manpage uses
+ <literal>APT</literal> to refer to them all for simplicity only.
+ </para>
+</refsect1>
+
+ <refsect1><title>Unsigned Repositories</title>
+ <para>
+ If an archive has an unsigned Release file or no Release file at all
+ current APT versions will refuse to download data from them by default
+ in <command>update</command> operations and even if forced to download
+ front-ends like &apt-get; will require explicit confirmation if an
+ installation request includes a package from such an unauthenticated
+ archive.
+ </para>
+
+ <para>
+ You can force all APT clients to raise only warnings by setting the
+ configuration option <option>Acquire::AllowInsecureRepositories</option> to
+ <literal>true</literal>. Individual repositories can also be allowed to be insecure
+ via the &sources-list; option <literal>allow-insecure=yes</literal>.
+ Note that insecure repositories are strongly discouraged and all options
+ to force apt to continue supporting them will eventually be removed.
+ Users also have the <option>Trusted</option> option available to disable
+ even the warnings, but be sure to understand the implications as detailed in
+ &sources-list;.
+ </para>
+
+ <para>
+ A repository which previously was authenticated but would loose this state in
+ an <command>update</command> operation raises an error in all APT clients
+ irrespective of the option to allow or forbid usage of insecure repositories.
+ The error can be overcome by additionally setting
+ <option>Acquire::AllowDowngradeToInsecureRepositories</option>
+ to <literal>true</literal> or for Individual repositories with the &sources-list;
+ option <literal>allow-downgrade-to-insecure=yes</literal>.
+ </para>
+</refsect1>
+
+ <refsect1><title>Signed Repositories</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 (e.g.) 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 one of the Debian package maintainer keyrings (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. Similar procedures exist in all
+ Debian-based distributions.
+ </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
+ 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>Information changes</title>
+ <para>
+ A Release file contains beside the checksums for the files in the repository
+ also general information about the repository like the origin, codename or
+ version number of the release.
+ </para><para>
+ This information is shown in various places so a repository owner should always
+ ensure correctness. Further more user configuration like &apt-preferences;
+ can depend and make use of this information. Since version 1.5 the user must
+ therefore explicitly confirm changes to signal that the user is sufficiently
+ prepared e.g. for the new major release of the distribution shipped in the
+ repository (as e.g. indicated by the codename).
+ </para>
+</refsect1>
+
+<refsect1><title>User Configuration</title>
+ <para>
+ <command>apt-key</command> is the program that manages the list of keys used
+ by APT to trust repositories. It can be used to add or remove keys as well
+ as list the trusted keys. Limiting which key(s) are able to sign which archive
+ is possible via the <option>Signed-By</option> in &sources-list;.
+ </para><para>
+ Note that a default installation already contains all keys to securely
+ acquire packages from the default repositories, so fiddling with
+ <command>apt-key</command> is only needed if third-party repositories are
+ added.
+ </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>Repository 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>, so that your users
+ will know what key they need to import in order to authenticate the files
+ in the archive. It is best to ship your key in its own keyring package
+ like &keyring-distro; does with &keyring-package; to be able to
+ distribute updates and key transitions automatically later.
+ </para></listitem>
+
+ <listitem><para>
+ <emphasis>Provide instructions on how to add your archive and key</emphasis>.
+ If your users can't acquire your key securely the chain of trust described above is broken.
+ How you can help users add your key depends on your archive and target audience ranging
+ from having your keyring package included in another archive users already have configured
+ (like the default repositories of their distribution) to leveraging the web of trust.
+ </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="https://www.debian.org/doc/manuals/securing-debian-howto/ch7">Debian
+Security Infrastructure</ulink> chapter of the Securing Debian Manual
+(also available 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>