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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.
</para>
<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>
As a temporary exception &apt-get; (not &apt;!) raises warnings only if it
encounters unauthenticated archives to give a slightly longer grace period
on this backward compatibility effecting change. This exception will be removed
in future releases and you can opt-out of this grace period by setting the
configuration option <option>Binary::apt-get::Acquire::AllowInsecureRepositories</option>
to <literal>false</literal> or <option>--no-allow-insecure-repositories</option>
on the command line.
</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>
<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>Trusted 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>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>Archive 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>