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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY TIMESTAMP "2019-02-09">
<!ENTITY % common SYSTEM "common.ent">
%common;
]>
<refentry xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>Mandos Manual</title>
<!-- NWalsh’s docbook scripts use this to generate the footer: -->
<productname>Mandos</productname>
<productnumber>&version;</productnumber>
<date>&TIMESTAMP;</date>
<authorgroup>
<author>
<firstname>Björn</firstname>
<surname>Påhlsson</surname>
<address>
<email>belorn@recompile.se</email>
</address>
</author>
<author>
<firstname>Teddy</firstname>
<surname>Hogeborn</surname>
<address>
<email>teddy@recompile.se</email>
</address>
</author>
</authorgroup>
<copyright>
<year>2011</year>
<year>2012</year>
<year>2013</year>
<year>2014</year>
<year>2015</year>
<year>2016</year>
<year>2017</year>
<year>2018</year>
<holder>Teddy Hogeborn</holder>
<holder>Björn Påhlsson</holder>
</copyright>
<xi:include href="legalnotice.xml"/>
</refentryinfo>
<refmeta>
<refentrytitle>intro</refentrytitle>
<manvolnum>8mandos</manvolnum>
</refmeta>
<refnamediv>
<refname>intro</refname>
<refpurpose>
Introduction to the Mandos system
</refpurpose>
</refnamediv>
<refsect1 id="description">
<title>DESCRIPTION</title>
<para>
This is the the Mandos system, which allows computers to have
encrypted root file systems and at the same time be capable of
remote and/or unattended reboots.
</para>
<para>
The computers run a small client program in the initial RAM disk
environment which will communicate with a server over a network.
All network communication is encrypted using TLS. The clients
are identified by the server using a TLS public key; each client
has one unique to it. The server sends the clients an encrypted
password. The encrypted password is decrypted by the clients
using a separate OpenPGP key, and the password is then used to
unlock the root file system, whereupon the computers can
continue booting normally.
</para>
</refsect1>
<refsect1 id="introduction">
<title>INTRODUCTION</title>
<para>
<!-- This paragraph is a combination and paraphrase of two
quotes from the 1995 movie “The Usual Suspects”. -->
You know how it is. You’ve heard of it happening. The Man
comes and takes away your servers, your friends’ servers, the
servers of everybody in the same hosting facility. The servers
of their neighbors, and their neighbors’ friends. The servers
of people who owe them money. And like
<emphasis>that</emphasis>, they’re gone. And you doubt you’ll
ever see them again.
</para>
<para>
That is why your servers have encrypted root file systems.
However, there’s a downside. There’s no going around it:
rebooting is a pain. Dragging out that rarely-used keyboard and
screen and unraveling cables behind your servers to plug them in
to type in that password is messy, especially if you have many
servers. There are some people who do clever things like using
serial line consoles and daisy-chain it to the next server, and
keep all the servers connected in a ring with serial cables,
which will work, if your servers are physically close enough.
There are also other out-of-band management solutions, but with
<emphasis>all</emphasis> these, you still have to be on hand and
manually type in the password at boot time. Otherwise the
server just sits there, waiting for a password.
</para>
<para>
Wouldn’t it be great if you could have the security of encrypted
root file systems and still have servers that could boot up
automatically if there was a short power outage while you were
asleep? That you could reboot at will, without having someone
run over to the server to type in the password?
</para>
<para>
Well, with Mandos, you (almost) can! The gain in convenience
will only be offset by a small loss in security. The setup is
as follows:
</para>
<para>
The server will still have its encrypted root file system. The
password to this file system will be stored on another computer
(henceforth known as the Mandos server) on the same local
network. The password will <emphasis>not</emphasis> be stored
in plaintext, but encrypted with OpenPGP. To decrypt this
password, a key is needed. This key (the Mandos client key)
will not be stored there, but back on the original server
(henceforth known as the Mandos client) in the initial RAM disk
image. Oh, and all network Mandos client/server communications
will be encrypted, using TLS (SSL).
</para>
<para>
So, at boot time, the Mandos client will ask for its encrypted
data over the network, decrypt it to get the password, use it to
decrypt the root file, and continue booting.
</para>
<para>
Now, of course the initial RAM disk image is not on the
encrypted root file system, so anyone who had physical access
could take the Mandos client computer offline and read the disk
with their own tools to get the authentication keys used by a
client. <emphasis>But</emphasis>, by then the Mandos server
should notice that the original server has been offline for too
long, and will no longer give out the encrypted key. The timing
here is the only real weak point, and the method, frequency and
timeout of the server’s checking can be adjusted to any desired
level of paranoia
</para>
<para>
(The encrypted keys on the Mandos server is on its normal file
system, so those are safe, provided the root file system of
<emphasis>that</emphasis> server is encrypted.)
</para>
</refsect1>
<refsect1 id="faq">
<title>FREQUENTLY ASKED QUESTIONS</title>
<para>
Couldn’t the security be defeated by…
</para>
<refsect2 id="quick">
<title>Grabbing the Mandos client key from the
initrd <emphasis>really quickly</emphasis>?</title>
<para>
This, as mentioned above, is the only real weak point. But if
you set the timing values tight enough, this will be really
difficult to do. An attacker would have to physically
disassemble the client computer, extract the key from the
initial RAM disk image, and then connect to a <emphasis>still
online</emphasis> Mandos server to get the encrypted key, and do
all this <emphasis>before</emphasis> the Mandos server timeout
kicks in and the Mandos server refuses to give out the key to
anyone.
</para>
<para>
Now, as the typical procedure seems to be to barge in and turn
off and grab <emphasis>all</emphasis> computers, to maybe look
at them months later, this is not likely. If someone does that,
the whole system <emphasis>will</emphasis> lock itself up
completely, since Mandos servers are no longer running.
</para>
<para>
For sophisticated attackers who <emphasis>could</emphasis> do
the clever thing, <emphasis>and</emphasis> had physical access
to the server for enough time, it would be simpler to get a key
for an encrypted file system by using hardware memory scanners
and reading it right off the memory bus.
</para>
</refsect2>
<refsect2 id="replay">
<title>Replay attacks?</title>
<para>
Nope, the network stuff is all done over TLS, which provides
protection against that.
</para>
</refsect2>
<refsect2 id="mitm">
<title>Man-in-the-middle?</title>
<para>
No. The server only gives out the passwords to clients which
have <emphasis>in the TLS handshake</emphasis> proven that
they do indeed hold the private key corresponding to that
client.
</para>
</refsect2>
<refsect2 id="sniff">
<title>How about sniffing the network traffic and decrypting it
later by physically grabbing the Mandos client and using its
key?</title>
<para>
We only use <acronym>PFS</acronym> (Perfect Forward Security)
key exchange algorithms in TLS, which protects against this.
</para>
</refsect2>
<refsect2 id="physgrab">
<title>Physically grabbing the Mandos server computer?</title>
<para>
You could protect <emphasis>that</emphasis> computer the
old-fashioned way, with a must-type-in-the-password-at-boot
method. Or you could have two computers be the Mandos server
for each other.
</para>
<para>
Multiple Mandos servers can coexist on a network without any
trouble. They do not clash, and clients will try all
available servers. This means that if just one reboots then
the other can bring it back up, but if both reboot at the same
time they will stay down until someone types in the password
on one of them.
</para>
</refsect2>
<refsect2 id="fakecheck">
<title>Faking checker results?</title>
<para>
If the Mandos client does not have an SSH server, the default
is for the Mandos server to use
<quote><literal>fping</literal></quote>, the replies to which
could be faked to eliminate the timeout. But this could
easily be changed to any shell command, with any security
measures you like. If the Mandos client
<emphasis>has</emphasis> an SSH server, the default
configuration (as generated by
<command>mandos-keygen</command> with the
<option>--password</option> option) is for the Mandos server
to use an <command>ssh-keyscan</command> command with strict
keychecking, which can not be faked. Alternatively, IPsec
could be used for the ping packets, making them secure.
</para>
</refsect2>
</refsect1>
<refsect1 id="security">
<title>SECURITY</title>
<para>
So, in summary: The only weakness in the Mandos system is from
people who have:
</para>
<orderedlist>
<listitem>
<para>
The power to come in and physically take your servers,
<emphasis>and</emphasis>
</para>
</listitem>
<listitem>
<para>
The cunning and patience to do it carefully, one at a time,
and <emphasis>quickly</emphasis>, faking Mandos
client/server responses for each one before the timeout.
</para>
</listitem>
</orderedlist>
<para>
While there are some who may be threatened by people who have
<emphasis>both</emphasis> these attributes, they do not,
probably, constitute the majority.
</para>
<para>
If you <emphasis>do</emphasis> face such opponents, you must
figure that they could just as well open your servers and read
the file system keys right off the memory by running wires to
the memory bus.
</para>
<para>
What Mandos is designed to protect against is
<emphasis>not</emphasis> such determined, focused, and competent
attacks, but against the early morning knock on your door and
the sudden absence of all the servers in your server room.
Which it does nicely.
</para>
</refsect1>
<refsect1 id="plugins">
<title>PLUGINS</title>
<para>
In the early designs, the
<citerefentry><refentrytitle>mandos-client</refentrytitle
><manvolnum>8mandos</manvolnum></citerefentry> program (which
retrieves a password from the Mandos server) also prompted for a
password on the terminal, in case a Mandos server could not be
found. Other ways of retrieving a password could easily be
envisoned, but this multiplicity of purpose was seen to be too
complex to be a viable way to continue. Instead, the original
program was separated into <citerefentry><refentrytitle
>mandos-client</refentrytitle><manvolnum>8mandos</manvolnum
></citerefentry> and <citerefentry><refentrytitle
>password-prompt</refentrytitle><manvolnum>8mandos</manvolnum
></citerefentry>, and a <citerefentry><refentrytitle
>plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
></citerefentry> exist to run them both in parallel, allowing
the first successful plugin to provide the password. This
opened up for any number of additional plugins to run, all
competing to be the first to find a password and provide it to
the plugin runner.
</para>
<para>
Four additional plugins are provided:
</para>
<variablelist>
<varlistentry>
<term>
<citerefentry><refentrytitle>plymouth</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>
</term>
<listitem>
<para>
This prompts for a password when using <citerefentry>
<refentrytitle>plymouth</refentrytitle><manvolnum
>8</manvolnum></citerefentry>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<citerefentry><refentrytitle>usplash</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>
</term>
<listitem>
<para>
This prompts for a password when using <citerefentry>
<refentrytitle>usplash</refentrytitle><manvolnum
>8</manvolnum></citerefentry>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<citerefentry><refentrytitle>splashy</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>
</term>
<listitem>
<para>
This prompts for a password when using <citerefentry>
<refentrytitle>splashy</refentrytitle><manvolnum
>8</manvolnum></citerefentry>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>
</term>
<listitem>
<para>
To provide compatibility with the "askpass" program from
cryptsetup, this plugin listens to the same FIFO as
askpass would do.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
More plugins can easily be written and added by the system
administrator; see the section called "WRITING PLUGINS" in
<citerefentry><refentrytitle>plugin-runner</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry> to learn the
plugin requirements.
</para>
</refsect1>
<refsect1 id="bugs">
<title>BUGS</title>
<xi:include href="bugs.xml"/>
</refsect1>
<refsect1 id="see_also">
<title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>mandos</refentrytitle>
<manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos.conf</refentrytitle>
<manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos-clients.conf</refentrytitle>
<manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos-ctl</refentrytitle>
<manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos-monitor</refentrytitle>
<manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>plugin-runner</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos-client</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>password-prompt</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>plymouth</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>usplash</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>splashy</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
<manvolnum>8mandos</manvolnum></citerefentry>,
<citerefentry><refentrytitle>mandos-keygen</refentrytitle>
<manvolnum>8</manvolnum></citerefentry>
</para>
<variablelist>
<varlistentry>
<term>
<ulink url="https://www.recompile.se/mandos">Mandos</ulink>
</term>
<listitem>
<para>
The Mandos home page.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
</refentry>
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