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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY TIMESTAMP "2019-08-04">
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<!ENTITY % common SYSTEM "common.ent">
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%common;
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]>
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<refentry xmlns:xi="http://www.w3.org/2001/XInclude">
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   <refentryinfo>
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    <title>Mandos Manual</title>
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    <!-- NWalsh’s docbook scripts use this to generate the footer: -->
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    <productname>Mandos</productname>
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    <productnumber>&version;</productnumber>
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    <date>&TIMESTAMP;</date>
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    <authorgroup>
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      <author>
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	<firstname>Björn</firstname>
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	<surname>Påhlsson</surname>
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	<address>
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	  <email>belorn@recompile.se</email>
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	</address>
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      </author>
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      <author>
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	<firstname>Teddy</firstname>
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	<surname>Hogeborn</surname>
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	<address>
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	  <email>teddy@recompile.se</email>
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	</address>
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      </author>
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    </authorgroup>
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    <copyright>
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      <year>2011</year>
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      <year>2012</year>
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      <year>2013</year>
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      <year>2014</year>
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      <year>2015</year>
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      <year>2016</year>
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      <year>2017</year>
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      <year>2018</year>
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      <year>2019</year>
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      <holder>Teddy Hogeborn</holder>
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      <holder>Björn Påhlsson</holder>
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    </copyright>
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    <xi:include href="legalnotice.xml"/>
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  </refentryinfo>
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  <refmeta>
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    <refentrytitle>intro</refentrytitle>
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    <manvolnum>8mandos</manvolnum>
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  </refmeta>
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  <refnamediv>
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    <refname>intro</refname>
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    <refpurpose>
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      Introduction to the Mandos system
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    </refpurpose>
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  </refnamediv>
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  <refsect1 id="description">
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    <title>DESCRIPTION</title>
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    <para>
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      This is the the Mandos system, which allows computers to have
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      encrypted root file systems and at the same time be capable of
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      remote and/or unattended reboots.
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    </para>
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    <para>
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      The computers run a small client program in the initial RAM disk
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      environment which will communicate with a server over a network.
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      All network communication is encrypted using TLS.  The clients
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      are identified by the server using a TLS public key; each client
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      has one unique to it.  The server sends the clients an encrypted
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      password.  The encrypted password is decrypted by the clients
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      using a separate OpenPGP key, and the password is then used to
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      unlock the root file system, whereupon the computers can
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      continue booting normally.
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    </para>
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  </refsect1>
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  <refsect1 id="introduction">
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    <title>INTRODUCTION</title>
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    <para>
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      <!-- This paragraph is a combination and paraphrase of two
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           quotes from the 1995 movie “The Usual Suspects”. -->
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      You know how it is.  You’ve heard of it happening.  The Man
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      comes and takes away your servers, your friends’ servers, the
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      servers of everybody in the same hosting facility. The servers
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      of their neighbors, and their neighbors’ friends.  The servers
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      of people who owe them money.  And like
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      <emphasis>that</emphasis>, they’re gone.  And you doubt you’ll
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      ever see them again.
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    </para>
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    <para>
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      That is why your servers have encrypted root file systems.
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      However, there’s a downside.  There’s no going around it:
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      rebooting is a pain.  Dragging out that rarely-used keyboard and
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      screen and unraveling cables behind your servers to plug them in
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      to type in that password is messy, especially if you have many
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      servers.  There are some people who do clever things like using
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      serial line consoles and daisy-chain it to the next server, and
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      keep all the servers connected in a ring with serial cables,
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      which will work, if your servers are physically close enough.
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      There are also other out-of-band management solutions, but with
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      <emphasis>all</emphasis> these, you still have to be on hand and
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      manually type in the password at boot time.  Otherwise the
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      server just sits there, waiting for a password.
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    </para>
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    <para>
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      Wouldn’t it be great if you could have the security of encrypted
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      root file systems and still have servers that could boot up
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      automatically if there was a short power outage while you were
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      asleep?  That you could reboot at will, without having someone
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      run over to the server to type in the password?
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    </para>
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    <para>
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      Well, with Mandos, you (almost) can!  The gain in convenience
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      will only be offset by a small loss in security.  The setup is
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      as follows:
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    </para>
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    <para>
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      The server will still have its encrypted root file system.  The
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      password to this file system will be stored on another computer
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      (henceforth known as the Mandos server) on the same local
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      network.  The password will <emphasis>not</emphasis> be stored
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      in plaintext, but encrypted with OpenPGP.  To decrypt this
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      password, a key is needed.  This key (the Mandos client key)
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      will not be stored there, but back on the original server
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      (henceforth known as the Mandos client) in the initial RAM disk
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      image.  Oh, and all network Mandos client/server communications
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      will be encrypted, using TLS (SSL).
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    </para>
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    <para>
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      So, at boot time, the Mandos client will ask for its encrypted
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      data over the network, decrypt the data to get the password, use
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      the password to decrypt the root file system, and the client can
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      then continue booting.
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    </para>
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    <para>
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      Now, of course the initial RAM disk image is not on the
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      encrypted root file system, so anyone who had physical access
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      could take the Mandos client computer offline and read the disk
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      with their own tools to get the authentication keys used by a
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      client.  <emphasis>But</emphasis>, by then the Mandos server
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      should notice that the original server has been offline for too
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      long, and will no longer give out the encrypted key.  The timing
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      here is the only real weak point, and the method, frequency and
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      timeout of the server’s checking can be adjusted to any desired
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      level of paranoia.
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    </para>
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    <para>
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      (The encrypted keys on the Mandos server is on its normal file
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      system, so those are safe, provided the root file system of
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      <emphasis>that</emphasis> server is encrypted.)
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    </para>
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  </refsect1>
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  <refsect1 id="faq">
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    <title>FREQUENTLY ASKED QUESTIONS</title>
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    <para>
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      Couldn’t the security be defeated by…
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    </para>
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    <refsect2 id="quick">
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      <title>Grabbing the Mandos client key from the
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      initrd <emphasis>really quickly</emphasis>?</title>
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    <para>
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      This, as mentioned above, is the only real weak point.  But if
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      you set the timing values tight enough, this will be really
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      difficult to do.  An attacker would have to physically
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      disassemble the client computer, extract the key from the
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      initial RAM disk image, and then connect to a <emphasis>still
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      online</emphasis> Mandos server to get the encrypted key, and do
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      all this <emphasis>before</emphasis> the Mandos server timeout
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      kicks in and the Mandos server refuses to give out the key to
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      anyone.
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    </para>
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    <para>
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      Now, as the typical procedure seems to be to barge in and turn
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      off and grab <emphasis>all</emphasis> computers, to maybe look
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      at them months later, this is not likely.  If someone does that,
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      the whole system <emphasis>will</emphasis> lock itself up
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      completely, since Mandos servers are no longer running.
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    </para>
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    <para>
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      For sophisticated attackers who <emphasis>could</emphasis> do
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      the clever thing, <emphasis>and</emphasis> had physical access
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      to the server for enough time, it would be simpler to get a key
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      for an encrypted file system by using hardware memory scanners
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      and reading it right off the memory bus.
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    </para>
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    </refsect2>
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    <refsect2 id="replay">
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      <title>Replay attacks?</title>
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      <para>
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	Nope, the network stuff is all done over TLS, which provides
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	protection against that.
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      </para>
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    </refsect2>
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    <refsect2 id="mitm">
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      <title>Man-in-the-middle?</title>
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      <para>
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	No.  The server only gives out the passwords to clients which
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	have <emphasis>in the TLS handshake</emphasis> proven that
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	they do indeed hold the private key corresponding to that
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	client.
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      </para>
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    </refsect2>
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    <refsect2 id="sniff">
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      <title>How about sniffing the network traffic and decrypting it
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      later by physically grabbing the Mandos client and using its
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      key?</title>
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      <para>
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	We only use <acronym>PFS</acronym> (Perfect Forward Security)
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	key exchange algorithms in TLS, which protects against this.
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      </para>
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    </refsect2>
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    <refsect2 id="physgrab">
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      <title>Physically grabbing the Mandos server computer?</title>
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      <para>
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	You could protect <emphasis>that</emphasis> computer the
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	old-fashioned way, with a must-type-in-the-password-at-boot
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	method.  Or you could have two computers be the Mandos server
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	for each other.
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      </para>
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      <para>
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	Multiple Mandos servers can coexist on a network without any
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	trouble.  They do not clash, and clients will try all
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	available servers.  This means that if just one reboots then
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	the other can bring it back up, but if both reboot at the same
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	time they will stay down until someone types in the password
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	on one of them.
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      </para>
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    </refsect2>
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    <refsect2 id="fakecheck">
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      <title>Faking checker results?</title>
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      <para>
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	If the Mandos client does not have an SSH server, the default
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	is for the Mandos server to use
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	<quote><literal>fping</literal></quote>, the replies to which
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	could be faked to eliminate the timeout.  But this could
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	easily be changed to any shell command, with any security
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	measures you like.  If the Mandos client
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	<emphasis>has</emphasis> an SSH server, the default
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	configuration (as generated by
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	<command>mandos-keygen</command> with the
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	<option>--password</option> option) is for the Mandos server
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	to use an <command>ssh-keyscan</command> command with strict
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	keychecking, which can not be faked.  Alternatively, IPsec
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	could be used for the ping packets, making them secure.
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      </para>
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    </refsect2>
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  </refsect1>
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  <refsect1 id="security">
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    <title>SECURITY</title>
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    <para>
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      So, in summary:  The only weakness in the Mandos system is from
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      people who have:
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    </para>
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    <orderedlist>
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      <listitem>
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	<para>
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	  The power to come in and physically take your servers,
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	  <emphasis>and</emphasis>
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	</para>
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      </listitem>
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      <listitem>
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	<para>
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	  The cunning and patience to do it carefully, one at a time,
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	  and <emphasis>quickly</emphasis>, faking Mandos
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	  client/server responses for each one before the timeout.
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	</para>
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      </listitem>
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    </orderedlist>
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    <para>
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      While there are some who may be threatened by people who have
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      <emphasis>both</emphasis> these attributes, they do not,
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      probably, constitute the majority.
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    </para>
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    <para>
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      If you <emphasis>do</emphasis> face such opponents, you must
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      figure that they could just as well open your servers and read
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      the file system keys right off the memory by running wires to
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      the memory bus.
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    </para>
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    <para>
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      What Mandos is designed to protect against is
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      <emphasis>not</emphasis> such determined, focused, and competent
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      attacks, but against the early morning knock on your door and
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      the sudden absence of all the servers in your server room.
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      Which it does nicely.
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    </para>
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  </refsect1>
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  <refsect1 id="plugins">
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    <title>PLUGINS</title>
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    <para>
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      In the early designs, the
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      <citerefentry><refentrytitle>mandos-client</refentrytitle
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      ><manvolnum>8mandos</manvolnum></citerefentry> program (which
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      retrieves a password from the Mandos server) also prompted for a
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      password on the terminal, in case a Mandos server could not be
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      found.  Other ways of retrieving a password could easily be
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      envisoned, but this multiplicity of purpose was seen to be too
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      complex to be a viable way to continue.  Instead, the original
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      program was separated into <citerefentry><refentrytitle
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      >mandos-client</refentrytitle><manvolnum>8mandos</manvolnum
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      ></citerefentry> and <citerefentry><refentrytitle
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      >password-prompt</refentrytitle><manvolnum>8mandos</manvolnum
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      ></citerefentry>, and a <citerefentry><refentrytitle
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      >plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
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      ></citerefentry> exist to run them both in parallel, allowing
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      the first successful plugin to provide the password.  This
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      opened up for any number of additional plugins to run, all
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      competing to be the first to find a password and provide it to
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      the plugin runner.
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    </para>
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    <para>
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      Four additional plugins are provided:
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    </para>
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    <variablelist>
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      <varlistentry>
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	<term>
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	  <citerefentry><refentrytitle>plymouth</refentrytitle>
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	  <manvolnum>8mandos</manvolnum></citerefentry>
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	</term>
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	<listitem>
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	  <para>
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	    This prompts for a password when using <citerefentry>
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	    <refentrytitle>plymouth</refentrytitle><manvolnum
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	    >8</manvolnum></citerefentry>.
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	  </para>
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	</listitem>
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      </varlistentry>
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      <varlistentry>
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	<term>
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	  <citerefentry><refentrytitle>usplash</refentrytitle>
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	  <manvolnum>8mandos</manvolnum></citerefentry>
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	</term>
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	<listitem>
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	  <para>
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	    This prompts for a password when using <citerefentry>
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	    <refentrytitle>usplash</refentrytitle><manvolnum
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	    >8</manvolnum></citerefentry>.
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	  </para>
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	</listitem>
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      </varlistentry>
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      <varlistentry>
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	<term>
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	  <citerefentry><refentrytitle>splashy</refentrytitle>
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	  <manvolnum>8mandos</manvolnum></citerefentry>
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	</term>
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	<listitem>
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	  <para>
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	    This prompts for a password when using <citerefentry>
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	    <refentrytitle>splashy</refentrytitle><manvolnum
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	    >8</manvolnum></citerefentry>.
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	  </para>
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	</listitem>
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      </varlistentry>
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      <varlistentry>
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	<term>
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	  <citerefentry><refentrytitle>askpass-fifo</refentrytitle>
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	  <manvolnum>8mandos</manvolnum></citerefentry>
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	</term>
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	<listitem>
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	  <para>
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	    To provide compatibility with the "askpass" program from
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	    cryptsetup, this plugin listens to the same FIFO as
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	    askpass would do.
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	  </para>
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	</listitem>
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      </varlistentry>
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    </variablelist>
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    <para>
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      More plugins can easily be written and added by the system
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      administrator; see the section called "WRITING PLUGINS" in
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      <citerefentry><refentrytitle>plugin-runner</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry> to learn the
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      plugin requirements.
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    </para>
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  </refsect1>
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  <refsect1 id="systemd">
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    <title>SYSTEMD</title>
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    <para>
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      More advanced startup systems like <citerefentry><refentrytitle
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      >systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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      already have their own plugin-like mechanisms for allowing
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      multiple agents to independently retrieve a password and deliver
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      it to the subsystem requesting a password to unlock the root
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      file system.  On these systems, it would make no sense to run
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      <citerefentry><refentrytitle>plugin-runner</refentrytitle
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      ><manvolnum>8mandos</manvolnum></citerefentry>, the plugins of
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      which would largely duplicate the work of (and conflict with)
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      the existing systems prompting for passwords.
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    </para>
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    <para>
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      As for <citerefentry><refentrytitle>systemd</refentrytitle
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      ><manvolnum>1</manvolnum></citerefentry> in particular, it has
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      its own <ulink
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      url="https://www.freedesktop.org/wiki/Software/systemd/PasswordAgents/"
407
      >Password Agents</ulink> system.  Mandos uses this via its
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      <citerefentry><refentrytitle>password-agent</refentrytitle
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      ><manvolnum>8mandos</manvolnum></citerefentry> program, which
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      is run instead of <citerefentry><refentrytitle
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      >plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
412
      ></citerefentry> when <citerefentry><refentrytitle
413
      >systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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      is used during system startup.
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    </para>
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  </refsect1>
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  <refsect1 id="bugs">
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    <title>BUGS</title>
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    <xi:include href="bugs.xml"/>
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  </refsect1>
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  <refsect1 id="see_also">
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    <title>SEE ALSO</title>
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    <para>
425
      <citerefentry><refentrytitle>mandos</refentrytitle>
426
      <manvolnum>8</manvolnum></citerefentry>,
427
      <citerefentry><refentrytitle>mandos.conf</refentrytitle>
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      <manvolnum>5</manvolnum></citerefentry>,
429
      <citerefentry><refentrytitle>mandos-clients.conf</refentrytitle>
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      <manvolnum>5</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>mandos-ctl</refentrytitle>
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      <manvolnum>8</manvolnum></citerefentry>,
433
      <citerefentry><refentrytitle>mandos-monitor</refentrytitle>
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      <manvolnum>8</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>plugin-runner</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>password-agent</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>mandos-client</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>password-prompt</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>plymouth</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>usplash</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>splashy</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>askpass-fifo</refentrytitle>
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      <manvolnum>8mandos</manvolnum></citerefentry>,
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      <citerefentry><refentrytitle>mandos-keygen</refentrytitle>
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      <manvolnum>8</manvolnum></citerefentry>
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    </para>
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    <variablelist>
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      <varlistentry>
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	<term>
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	  <ulink url="https://www.recompile.se/mandos">Mandos</ulink>
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	</term>
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	<listitem>
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	  <para>
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	    The Mandos home page.
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	  </para>
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	</listitem>
464
      </varlistentry>
465
    </variablelist>
466
  </refsect1>
467
</refentry>
468
<!-- Local Variables: -->
469
<!-- time-stamp-start: "<!ENTITY TIMESTAMP [\"']" -->
470
<!-- time-stamp-end: "[\"']>" -->
471
<!-- time-stamp-format: "%:y-%02m-%02d" -->
472
<!-- End: -->