1
<?xml version="1.0" encoding="UTF-8"?>
2
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4
<!ENTITY TIMESTAMP "2019-03-30">
5
<!ENTITY % common SYSTEM "common.ent">
9
<refentry xmlns:xi="http://www.w3.org/2001/XInclude">
11
<title>Mandos Manual</title>
12
<!-- NWalsh’s docbook scripts use this to generate the footer: -->
13
<productname>Mandos</productname>
14
<productnumber>&version;</productnumber>
15
<date>&TIMESTAMP;</date>
18
<firstname>Björn</firstname>
19
<surname>Påhlsson</surname>
21
<email>belorn@recompile.se</email>
25
<firstname>Teddy</firstname>
26
<surname>Hogeborn</surname>
28
<email>teddy@recompile.se</email>
42
<holder>Teddy Hogeborn</holder>
43
<holder>Björn Påhlsson</holder>
45
<xi:include href="legalnotice.xml"/>
49
<refentrytitle>intro</refentrytitle>
50
<manvolnum>8mandos</manvolnum>
54
<refname>intro</refname>
56
Introduction to the Mandos system
60
<refsect1 id="description">
61
<title>DESCRIPTION</title>
63
This is the the Mandos system, which allows computers to have
64
encrypted root file systems and at the same time be capable of
65
remote and/or unattended reboots.
68
The computers run a small client program in the initial RAM disk
69
environment which will communicate with a server over a network.
70
All network communication is encrypted using TLS. The clients
71
are identified by the server using a TLS public key; each client
72
has one unique to it. The server sends the clients an encrypted
73
password. The encrypted password is decrypted by the clients
74
using a separate OpenPGP key, and the password is then used to
75
unlock the root file system, whereupon the computers can
76
continue booting normally.
80
<refsect1 id="introduction">
81
<title>INTRODUCTION</title>
83
<!-- This paragraph is a combination and paraphrase of two
84
quotes from the 1995 movie “The Usual Suspects”. -->
85
You know how it is. You’ve heard of it happening. The Man
86
comes and takes away your servers, your friends’ servers, the
87
servers of everybody in the same hosting facility. The servers
88
of their neighbors, and their neighbors’ friends. The servers
89
of people who owe them money. And like
90
<emphasis>that</emphasis>, they’re gone. And you doubt you’ll
94
That is why your servers have encrypted root file systems.
95
However, there’s a downside. There’s no going around it:
96
rebooting is a pain. Dragging out that rarely-used keyboard and
97
screen and unraveling cables behind your servers to plug them in
98
to type in that password is messy, especially if you have many
99
servers. There are some people who do clever things like using
100
serial line consoles and daisy-chain it to the next server, and
101
keep all the servers connected in a ring with serial cables,
102
which will work, if your servers are physically close enough.
103
There are also other out-of-band management solutions, but with
104
<emphasis>all</emphasis> these, you still have to be on hand and
105
manually type in the password at boot time. Otherwise the
106
server just sits there, waiting for a password.
109
Wouldn’t it be great if you could have the security of encrypted
110
root file systems and still have servers that could boot up
111
automatically if there was a short power outage while you were
112
asleep? That you could reboot at will, without having someone
113
run over to the server to type in the password?
116
Well, with Mandos, you (almost) can! The gain in convenience
117
will only be offset by a small loss in security. The setup is
121
The server will still have its encrypted root file system. The
122
password to this file system will be stored on another computer
123
(henceforth known as the Mandos server) on the same local
124
network. The password will <emphasis>not</emphasis> be stored
125
in plaintext, but encrypted with OpenPGP. To decrypt this
126
password, a key is needed. This key (the Mandos client key)
127
will not be stored there, but back on the original server
128
(henceforth known as the Mandos client) in the initial RAM disk
129
image. Oh, and all network Mandos client/server communications
130
will be encrypted, using TLS (SSL).
133
So, at boot time, the Mandos client will ask for its encrypted
134
data over the network, decrypt it to get the password, use it to
135
decrypt the root file, and continue booting.
138
Now, of course the initial RAM disk image is not on the
139
encrypted root file system, so anyone who had physical access
140
could take the Mandos client computer offline and read the disk
141
with their own tools to get the authentication keys used by a
142
client. <emphasis>But</emphasis>, by then the Mandos server
143
should notice that the original server has been offline for too
144
long, and will no longer give out the encrypted key. The timing
145
here is the only real weak point, and the method, frequency and
146
timeout of the server’s checking can be adjusted to any desired
150
(The encrypted keys on the Mandos server is on its normal file
151
system, so those are safe, provided the root file system of
152
<emphasis>that</emphasis> server is encrypted.)
157
<title>FREQUENTLY ASKED QUESTIONS</title>
159
Couldn’t the security be defeated by…
161
<refsect2 id="quick">
162
<title>Grabbing the Mandos client key from the
163
initrd <emphasis>really quickly</emphasis>?</title>
165
This, as mentioned above, is the only real weak point. But if
166
you set the timing values tight enough, this will be really
167
difficult to do. An attacker would have to physically
168
disassemble the client computer, extract the key from the
169
initial RAM disk image, and then connect to a <emphasis>still
170
online</emphasis> Mandos server to get the encrypted key, and do
171
all this <emphasis>before</emphasis> the Mandos server timeout
172
kicks in and the Mandos server refuses to give out the key to
176
Now, as the typical procedure seems to be to barge in and turn
177
off and grab <emphasis>all</emphasis> computers, to maybe look
178
at them months later, this is not likely. If someone does that,
179
the whole system <emphasis>will</emphasis> lock itself up
180
completely, since Mandos servers are no longer running.
183
For sophisticated attackers who <emphasis>could</emphasis> do
184
the clever thing, <emphasis>and</emphasis> had physical access
185
to the server for enough time, it would be simpler to get a key
186
for an encrypted file system by using hardware memory scanners
187
and reading it right off the memory bus.
191
<refsect2 id="replay">
192
<title>Replay attacks?</title>
194
Nope, the network stuff is all done over TLS, which provides
195
protection against that.
200
<title>Man-in-the-middle?</title>
202
No. The server only gives out the passwords to clients which
203
have <emphasis>in the TLS handshake</emphasis> proven that
204
they do indeed hold the private key corresponding to that
209
<refsect2 id="sniff">
210
<title>How about sniffing the network traffic and decrypting it
211
later by physically grabbing the Mandos client and using its
214
We only use <acronym>PFS</acronym> (Perfect Forward Security)
215
key exchange algorithms in TLS, which protects against this.
219
<refsect2 id="physgrab">
220
<title>Physically grabbing the Mandos server computer?</title>
222
You could protect <emphasis>that</emphasis> computer the
223
old-fashioned way, with a must-type-in-the-password-at-boot
224
method. Or you could have two computers be the Mandos server
228
Multiple Mandos servers can coexist on a network without any
229
trouble. They do not clash, and clients will try all
230
available servers. This means that if just one reboots then
231
the other can bring it back up, but if both reboot at the same
232
time they will stay down until someone types in the password
237
<refsect2 id="fakecheck">
238
<title>Faking checker results?</title>
240
If the Mandos client does not have an SSH server, the default
241
is for the Mandos server to use
242
<quote><literal>fping</literal></quote>, the replies to which
243
could be faked to eliminate the timeout. But this could
244
easily be changed to any shell command, with any security
245
measures you like. If the Mandos client
246
<emphasis>has</emphasis> an SSH server, the default
247
configuration (as generated by
248
<command>mandos-keygen</command> with the
249
<option>--password</option> option) is for the Mandos server
250
to use an <command>ssh-keyscan</command> command with strict
251
keychecking, which can not be faked. Alternatively, IPsec
252
could be used for the ping packets, making them secure.
257
<refsect1 id="security">
258
<title>SECURITY</title>
260
So, in summary: The only weakness in the Mandos system is from
266
The power to come in and physically take your servers,
267
<emphasis>and</emphasis>
272
The cunning and patience to do it carefully, one at a time,
273
and <emphasis>quickly</emphasis>, faking Mandos
274
client/server responses for each one before the timeout.
279
While there are some who may be threatened by people who have
280
<emphasis>both</emphasis> these attributes, they do not,
281
probably, constitute the majority.
284
If you <emphasis>do</emphasis> face such opponents, you must
285
figure that they could just as well open your servers and read
286
the file system keys right off the memory by running wires to
290
What Mandos is designed to protect against is
291
<emphasis>not</emphasis> such determined, focused, and competent
292
attacks, but against the early morning knock on your door and
293
the sudden absence of all the servers in your server room.
294
Which it does nicely.
298
<refsect1 id="plugins">
299
<title>PLUGINS</title>
301
In the early designs, the
302
<citerefentry><refentrytitle>mandos-client</refentrytitle
303
><manvolnum>8mandos</manvolnum></citerefentry> program (which
304
retrieves a password from the Mandos server) also prompted for a
305
password on the terminal, in case a Mandos server could not be
306
found. Other ways of retrieving a password could easily be
307
envisoned, but this multiplicity of purpose was seen to be too
308
complex to be a viable way to continue. Instead, the original
309
program was separated into <citerefentry><refentrytitle
310
>mandos-client</refentrytitle><manvolnum>8mandos</manvolnum
311
></citerefentry> and <citerefentry><refentrytitle
312
>password-prompt</refentrytitle><manvolnum>8mandos</manvolnum
313
></citerefentry>, and a <citerefentry><refentrytitle
314
>plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
315
></citerefentry> exist to run them both in parallel, allowing
316
the first successful plugin to provide the password. This
317
opened up for any number of additional plugins to run, all
318
competing to be the first to find a password and provide it to
322
Four additional plugins are provided:
327
<citerefentry><refentrytitle>plymouth</refentrytitle>
328
<manvolnum>8mandos</manvolnum></citerefentry>
332
This prompts for a password when using <citerefentry>
333
<refentrytitle>plymouth</refentrytitle><manvolnum
334
>8</manvolnum></citerefentry>.
340
<citerefentry><refentrytitle>usplash</refentrytitle>
341
<manvolnum>8mandos</manvolnum></citerefentry>
345
This prompts for a password when using <citerefentry>
346
<refentrytitle>usplash</refentrytitle><manvolnum
347
>8</manvolnum></citerefentry>.
353
<citerefentry><refentrytitle>splashy</refentrytitle>
354
<manvolnum>8mandos</manvolnum></citerefentry>
358
This prompts for a password when using <citerefentry>
359
<refentrytitle>splashy</refentrytitle><manvolnum
360
>8</manvolnum></citerefentry>.
366
<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
367
<manvolnum>8mandos</manvolnum></citerefentry>
371
To provide compatibility with the "askpass" program from
372
cryptsetup, this plugin listens to the same FIFO as
379
More plugins can easily be written and added by the system
380
administrator; see the section called "WRITING PLUGINS" in
381
<citerefentry><refentrytitle>plugin-runner</refentrytitle>
382
<manvolnum>8mandos</manvolnum></citerefentry> to learn the
389
<xi:include href="bugs.xml"/>
392
<refsect1 id="see_also">
393
<title>SEE ALSO</title>
395
<citerefentry><refentrytitle>mandos</refentrytitle>
396
<manvolnum>8</manvolnum></citerefentry>,
397
<citerefentry><refentrytitle>mandos.conf</refentrytitle>
398
<manvolnum>5</manvolnum></citerefentry>,
399
<citerefentry><refentrytitle>mandos-clients.conf</refentrytitle>
400
<manvolnum>5</manvolnum></citerefentry>,
401
<citerefentry><refentrytitle>mandos-ctl</refentrytitle>
402
<manvolnum>8</manvolnum></citerefentry>,
403
<citerefentry><refentrytitle>mandos-monitor</refentrytitle>
404
<manvolnum>8</manvolnum></citerefentry>,
405
<citerefentry><refentrytitle>plugin-runner</refentrytitle>
406
<manvolnum>8mandos</manvolnum></citerefentry>,
407
<citerefentry><refentrytitle>mandos-client</refentrytitle>
408
<manvolnum>8mandos</manvolnum></citerefentry>,
409
<citerefentry><refentrytitle>password-prompt</refentrytitle>
410
<manvolnum>8mandos</manvolnum></citerefentry>,
411
<citerefentry><refentrytitle>plymouth</refentrytitle>
412
<manvolnum>8mandos</manvolnum></citerefentry>,
413
<citerefentry><refentrytitle>usplash</refentrytitle>
414
<manvolnum>8mandos</manvolnum></citerefentry>,
415
<citerefentry><refentrytitle>splashy</refentrytitle>
416
<manvolnum>8mandos</manvolnum></citerefentry>,
417
<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
418
<manvolnum>8mandos</manvolnum></citerefentry>,
419
<citerefentry><refentrytitle>mandos-keygen</refentrytitle>
420
<manvolnum>8</manvolnum></citerefentry>
425
<ulink url="https://www.recompile.se/mandos">Mandos</ulink>
429
The Mandos home page.
436
<!-- Local Variables: -->
437
<!-- time-stamp-start: "<!ENTITY TIMESTAMP [\"']" -->
438
<!-- time-stamp-end: "[\"']>" -->
439
<!-- time-stamp-format: "%:y-%02m-%02d" -->
added
removed
intro.xml
