4
- Have your cake and eat it too!
6
You know how it is. You’ve heard of it happening. The Man comes
7
and takes away your servers, your friends’ servers, the servers of
8
everybody in the same hosting facility. The servers of their
9
neighbors, and their neighbors’ friends. The servers of people who
10
owe them money. And like *that*, they’re gone. And you doubt
11
you’ll ever see them again.
13
That is why your servers have encrypted root file systems. However,
14
there’s a downside. There’s no going around it: rebooting is a
15
pain. Dragging out that rarely-used keyboard and screen and
16
unraveling cables behind your servers to plug them in to type in
17
that password is messy, especially if you have many servers. There
18
are some people who do clever things like using serial line consoles
19
and daisy-chain it to the next server, and keep all the servers
20
connected in a ring with serial cables, which will work, if your
21
servers are physically close enough. There are also other
22
out-of-band management solutions, but with *all* these, you still
23
have to be on hand and manually type in the password at boot time.
24
Otherwise the server just sits there, waiting for a password.
26
Wouldn’t it be great if you could have the security of encrypted
27
root file systems and still have servers that could boot up
28
automatically if there was a short power outage while you were
29
asleep? That you could reboot at will, without having someone run
30
over to the server to type in the password?
32
Well, with Mandos, you (almost) can! The gain in convenience will
33
only be offset by a small loss in security. The setup is as
36
The server will still have its encrypted root file system. The
37
password to this file system will be stored on another computer
38
(henceforth known as the Mandos server) on the same local network.
39
The password will *not* be stored in plaintext, but encrypted with
40
OpenPGP. To decrypt this password, a key is needed. This key (the
41
Mandos client key) will not be stored there, but back on the
42
original server (henceforth known as the Mandos client) in the
43
initial RAM disk image. Oh, and all network Mandos client/server
44
communications will be encrypted, using TLS (SSL).
46
So, at boot time, the Mandos client will ask for its encrypted data
47
over the network, decrypt it to get the password, use it to decrypt
48
the root file, and continue booting.
50
Now, of course the initial RAM disk image is not on the encrypted
51
root file system, so anyone who had physical access could take the
52
Mandos client computer offline and read the disk with their own
53
tools to get the authentication keys used by a client. *But*, by
54
then the Mandos server should notice that the original server has
55
been offline for too long, and will no longer give out the encrypted
56
key. The timing here is the only real weak point, and the method,
57
frequency and timeout of the server’s checking can be adjusted to
58
any desired level of paranoia
60
(The encrypted keys on the Mandos server is on its normal file
61
system, so those are safe, provided the root file system of *that*
64
* FAQ - couldn’t the security be defeated by...
66
** Grabbing the Mandos client key from the initrd *really quickly*?
67
This, as mentioned above, is the only real weak point. But if you
68
set the timing values tight enough, this will be really difficult
69
to do. An attacker would have to physically disassemble the client
70
computer, extract the key from the initial RAM disk image, and then
71
connect to a *still online* Mandos server to get the encrypted key,
72
and do all this *before* the Mandos server timeout kicks in and the
73
Mandos server refuses to give out the key to anyone.
75
Now, as the typical procedure seems to be to barge in and turn off
76
and grab *all* computers, to maybe look at them months later, this
77
is not likely. If someone does that, the whole system *will* lock
78
itself up completely, since Mandos servers are no longer running.
80
For sophisticated attackers who *could* do the clever thing, *and*
81
had physical access to the server for enough time, it would be
82
simpler to get a key for an encrypted file system by using hardware
83
memory scanners and reading it right off the memory bus.
86
Nope, the network stuff is all done over TLS, which provides
87
protection against that.
90
No. The server only gives out the passwords to clients which have
91
*in the TLS handshake* proven that they do indeed hold the OpenPGP
92
private key corresponding to that client.
94
** Physically grabbing the Mandos server computer?
95
You could protect *that* computer the old-fashioned way, with a
96
must-type-in-the-password-at-boot method. Or you could have two
97
computers be the Mandos server for each other.
99
Multiple Mandos servers can coexist on a network without any
100
trouble. They do not clash, and clients will try all available
101
servers. This means that if just one reboots then the other can
102
bring it back up, but if both reboots at the same time they will
103
stay down until someone types in the password on one of them.
105
** Faking ping replies?
106
The default for the server is to use "fping", the replies to which
107
could be faked to eliminate the timeout. But this could easily be
108
changed to any shell command, with any security measures you like.
109
It could, for instance, be changed to an SSH command with strict
110
keychecking, which could not be faked. Or IPsec could be used for
111
the ping packets, making them secure.
114
So, in summary: The only weakness in the Mandos system is from
116
1. The power to come in and physically take your servers, *and*
117
2. The cunning and patience to do it carefully, one at a time, and
118
*quickly*, faking Mandos client/server responses for each one
121
While there are some who may be threatened by people who have *both*
122
these attributes, they do not, probably, constitute the majority.
124
If you *do* face such opponents, you must figure that they could
125
just as well open your servers and read the file system keys right
126
off the memory by running wires to the memory bus.
128
What Mandos is designed to protect against is *not* such determined,
129
focused, and competent attacks, but against the early morning knock
130
on your door and the sudden absence of all the servers in your
131
server room. Which it does nicely.
134
In the early designs, the mandos-client(8mandos) program (which
135
retrieves a password from the Mandos server) also prompted for a
136
password on the terminal, in case a Mandos server could not be
137
found. This duality of purpose was seen to be too complex to be a
138
viable way to continue. Instead, the programs are now separated
139
into mandos-client(8mandos) and password-prompt(8mandos), and a
140
plugin-runner(8mandos) exist to run them both in parallel, allowing
141
the first plugin to succeed to provide the password. This opened up
142
for any number of additional plugins to run, all competing to be the
143
first to find a password and provide it to the plugin runner.
145
Three additional plugins are provided:
147
This prompts for a password when using usplash(8).
149
This prompts for a password when using splashy(8).
150
* askpass-fifo(8mandos)
151
To provide compatibility with the "askpass" program from
152
cryptsetup, this plugin listens to the same FIFO as askpass would
155
(None of these take any options or reads any files.)
157
More plugins could easily be written and added by the system
158
administrator; see the section called "WRITING PLUGINS" in
159
plugin-runner(8mandos) to learn the plugin requirements.
163
Copyright © 2008 Teddy Hogeborn
168
This program is free software: you can redistribute it and/or
169
modify it under the terms of the GNU General Public License as
170
published by the Free Software Foundation, either version 3 of the
171
License, or (at your option) any later version.
173
This program is distributed in the hope that it will be useful, but
174
WITHOUT ANY WARRANTY; without even the implied warranty of
175
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
176
General Public License for more details.
178
You should have received a copy of the GNU General Public License
179
along with this program. If not, see
180
<http://www.gnu.org/licenses/>.
1
Please see: http://www.recompile.se/mandos/man/intro.8mandos
3
This information previously in this file has been moved to the
4
intro(8mandos) manual page. Go to the above URL, or run this command:
8
In short, this is the Mandos system; it allows computers to have
9
encrypted root file systems and at the same time be capable of remote
10
and/or unattended reboots.
added
removed
README
