INTRO(8mandos)                   Mandos Manual                  INTRO(8mandos)

       intro - Introduction to the Mandos system

       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.

       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 an OpenPGP 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 the same OpenPGP
       key, and the password is then used to unlock the root file system,
       whereupon the computers can continue booting normally.

       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 that, they’re gone. And you doubt you’ll ever see them again.

       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 all
       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

       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?

       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:

       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 not 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).

       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.

       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.  But, 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

       (The encrypted keys on the Mandos server is on its normal file system,
       so those are safe, provided the root file system of that server is

       Couldn’t the security be defeated by...

   Grabbing the Mandos client key from the initrd really quickly?
       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
       still online Mandos server to get the encrypted key, and do all this
       before the Mandos server timeout kicks in and the Mandos server refuses
       to give out the key to anyone.

       Now, as the typical procedure seems to be to barge in and turn off and
       grab all computers, to maybe look at them months later, this is not
       likely. If someone does that, the whole system will lock itself up
       completely, since Mandos servers are no longer running.

       For sophisticated attackers who could do the clever thing, and 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.

   Replay attacks?
       Nope, the network stuff is all done over TLS, which provides protection
       against that.

       No. The server only gives out the passwords to clients which have in
       the TLS handshake proven that they do indeed hold the OpenPGP private
       key corresponding to that client.

   How about sniffing the network traffic and decrypting it later by
       physically grabbing the Mandos client and using its key?
       We only use PFS (Perfect Forward Security) key exchange algorithms in
       TLS, which protects against this.

   Physically grabbing the Mandos server computer?
       You could protect that 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.

       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.

   Faking checker results?
       If the Mandos client does not have an SSH server, the default is for
       the Mandos server to use “fping”, 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 has
       an SSH server, the default configuration (as generated by mandos-keygen
       with the --password option) is for the Mandos server to use an
       ssh-keyscan command with strict keychecking, which can not be faked.
       Alternatively, IPsec could be used for the ping packets, making them

       So, in summary: The only weakness in the Mandos system is from people
       who have:

        1. The power to come in and physically take your servers, and

        2. The cunning and patience to do it carefully, one at a time, and
           quickly, faking Mandos client/server responses for each one before
           the timeout.

       While there are some who may be threatened by people who have both
       these attributes, they do not, probably, constitute the majority.

       If you do 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.

       What Mandos is designed to protect against is not 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.

       In the early designs, the mandos-client(8mandos) 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 mandos-
       client(8mandos) and password-prompt(8mandos), and a plugin-
       runner(8mandos) 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.

       Four additional plugins are provided:

           This prompts for a password when using plymouth(8).

           This prompts for a password when using usplash(8).

           This prompts for a password when using splashy(8).

           To provide compatibility with the "askpass" program from
           cryptsetup, this plugin listens to the same FIFO as askpass would

       More plugins can easily be written and added by the system
       administrator; see the section called "WRITING PLUGINS" in plugin-
       runner(8mandos) to learn the plugin requirements.

       Please report bugs to the Mandos development mailing list:
       <> (subscription required). Note that this list
       is public. The developers can be reached privately at
       <> (OpenPGP key fingerprint 153A 37F1 0BBA 0435 987F
       2C4A 7223 2973 CA34 C2C4 for encrypted mail).

       mandos(8), mandos.conf(5), mandos-clients.conf(5), mandos-ctl(8),
       mandos-monitor(8), plugin-runner(8mandos), mandos-client(8mandos),
       password-prompt(8mandos), plymouth(8mandos), usplash(8mandos),
       splashy(8mandos), askpass-fifo(8mandos), mandos-keygen(8)

           The Mandos home page.

       Copyright © 2011-2018 Teddy Hogeborn, Björn Påhlsson

       This manual page is part of Mandos.

       Mandos is free software: you can redistribute it and/or modify it under
       the terms of the GNU General Public License as published by the Free
       Software Foundation, either version 3 of the License, or (at your
       option) any later version.

       Mandos is distributed in the hope that it will be useful, but WITHOUT
       ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
       FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
       for more details.

       You should have received a copy of the GNU General Public License along
       with Mandos. If not, see

        1. Mandos

Mandos 1.7.19                     2018-02-08                    INTRO(8mandos)