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 password.

       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 secure.

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

        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 do.

       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 © 2011-2017 Teddy Hogeborn, Björn Påhlsson

       This manual page 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.

       This manual page is distributed in the hope that it will be useful, but
       WITHOUT ANY WARRANTY; without even the implied warranty of
       Public License for more details.

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

        1. Mandos

Mandos 1.7.15                      2017-02-23                     INTRO(8mandos)