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<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"" [
<!ENTITY % aptent SYSTEM "apt.ent">
<refmiscinfo class="manual">APT</refmiscinfo>
<!-- NOTE: This manpage has been written based on the
Securing Debian Manual ("Debian Security
Infrastructure" chapter) and on documentation
available at the following sites:
<!-- TODO: write a more verbose example of how it works with
a sample similar to
<!-- Man page title -->
<refpurpose>Archive authentication support for APT</refpurpose>
Starting with version 0.6, <command>apt</command> contains code
that does signature checking of the Release file for all
archives. This ensures that packages in the archive can't be
modified by people who have no access to the Release file signing
If a package comes from a archive without a signature or with a
signature that apt does not have a key for that package is
considered untrusted and installing it will result in a big
warning. <command>apt-get</command> will currently only warn
for unsigned archives, future releases might force all sources
to be verified before downloading packages from them.
The package frontends &apt-get;, &aptitude; and &synaptic; support this new
authentication feature.
<refsect1><title>Trusted archives</title>
The chain of trust from an apt archive to the end user is made up of
different steps. <command>apt-secure</command> is the last step in
this chain, trusting an archive does not mean that the packages
that you trust it do not contain malicious code but means that you
trust the archive maintainer. It's the archive maintainer
responsibility to ensure that the archive integrity is correct.
<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>
The chain of trust in Debian starts when a maintainer uploads a new
package or a new version of a package to the Debian archive. This
upload in order to become effective needs to be signed by a key of
a maintainer within the Debian maintainer's keyring (available in
the debian-keyring package). Maintainer's keys are signed by
other maintainers following pre-established procedures to
ensure the identity of the key holder.
Once the uploaded package is verified and included in the archive,
the maintainer signature is stripped off, an MD5 sum of the package
is computed and put in the Packages file. The MD5 sum 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 (which is created
once a year) and distributed through the FTP server. This key is
also on the Debian keyring.
Any end user can check the signature of the Release file, extract the MD5
sum of a package from it and compare it with the MD5 sum of the
package he downloaded. Prior to version 0.6 only the MD5 sum of the
downloaded Debian package was checked. Now both the MD5 sum and the
signature of the Release file are checked.
<para>Notice that this is distinct from checking signatures on a
per package basis. It is designed to prevent two possible attacks:
<listitem><para><literal>Network "man in the middle"
attacks</literal>. Without signature checking, a malicious
agent can introduce himself in 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
<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
<para>However, it does not defend against a compromise of the
Debian 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><title>User configuration</title>
<command>apt-key</command> is the program that manages the list
of keys used by apt. It can be used to add or remove keys although
an installation of this release will automatically provide the
default Debian archive signing keys used in the Debian package
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>Release.gpg</filename> files from the archives you
have configured.
<refsect1><title>Archive configuration</title>
If you want to provide archive signatures in an archive under your
maintenance you have to:
<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 -abs -o Release.gpg Release</command>.</para></listitem>
<listitem><para><emphasis>Publish the key fingerprint</emphasis>,
that way your users will know what key they need to import in
order to authenticate the files in the
<para>Whenever the contents of the archive changes (new packages
are added or removed) the archive maintainer has to follow the
first two steps previously outlined.</para>
<refsect1><title>See Also</title>
&apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
&debsign; &debsig-verify;, &gpg;
<para>For more background information you might want to review the
Security Infrastructure</ulink> chapter of the Securing Debian Manual
(available also in the harden-doc package) and the
<ulink url=""
>Strong Distribution HOWTO</ulink> by V. Alex Brennen. </para>
<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.