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- Have your cake and eat it too!
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You know how it is. You’ve heard of it happening. The Man comes
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and takes away your servers, your friends’ servers, the servers of
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everybody in the same hosting facility. The servers of their
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neighbors, and their neighbors’ friends. The servers of people who
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owe them money. And like *that*, they’re gone. And you doubt
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you’ll ever see them again.
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That is why your servers have encrypted root file systems. However,
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there’s a downside. There’s no going around it: rebooting is a
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pain. Dragging out that rarely-used keyboard and screen and
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unraveling cables behind your servers to plug them in to type in
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that password is messy, especially if you have many servers. There
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are some people who do clever things like using serial line consoles
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and daisy-chain it to the next server, and keep all the servers
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connected in a ring with serial cables, which will work, if your
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servers are physically close enough. There are also other
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out-of-band management solutions, but with *all* these, you still
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have to be on hand and manually type in the password at boot time.
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Otherwise the server just sits there, waiting for a password.
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Wouldn’t it be great if you could have the security of encrypted
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root file systems and still have servers that could boot up
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automatically if there was a short power outage while you were
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asleep? That you could reboot at will, without having someone run
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over to the server to type in the password?
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Well, with Mandos, you (almost) can! The gain in convenience will
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only be offset by a small loss in security. The setup is as
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The server will still have its encrypted root file system. The
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password to this file system will be stored on another computer
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(henceforth known as the Mandos server) on the same local network.
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The password will *not* be stored in plaintext, but encrypted with
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OpenPGP. To decrypt this password, a key is needed. This key (the
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Mandos client key) will not be stored there, but back on the
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original server (henceforth known as the Mandos client) in the
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initial RAM disk image. Oh, and all network Mandos client/server
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communications will be encrypted, using TLS (SSL).
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So, at boot time, the Mandos client will ask for its encrypted data
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over the network, decrypt it to get the password, use it to decrypt
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the root file, and continue booting.
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Now, of course the initial RAM disk image is not on the encrypted
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root file system, so anyone who had physical access could take the
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Mandos client computer offline and read the disk with their own
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tools to get the authentication keys used by a client. *But*, by
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then the Mandos server should notice that the original server has
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been offline for too long, and will no longer give out the encrypted
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key. The timing here is the only real weak point, and the method,
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frequency and timeout of the server’s checking can be adjusted to
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any desired level of paranoia
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(The encrypted keys on the Mandos server is on its normal file
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system, so those are safe, provided the root file system of *that*
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* FAQ - couldn’t the security be defeated by...
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** Grabbing the Mandos client key from the initrd *really quickly*?
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This, as mentioned above, is the only real weak point. But if you
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set the timing values tight enough, this will be really difficult
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to do. An attacker would have to physically disassemble the client
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computer, extract the key from the initial RAM disk image, and then
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connect to a *still online* Mandos server to get the encrypted key,
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and do all this *before* the Mandos server timeout kicks in and the
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Mandos server refuses to give out the key to anyone.
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Now, as the typical procedure seems to be to barge in and turn off
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and grab *all* computers, to maybe look at them months later, this
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is not likely. If someone does that, the whole system *will* lock
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itself up completely, since Mandos servers are no longer running.
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For sophisticated attackers who *could* do the clever thing, *and*
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had physical access to the server for enough time, it would be
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simpler to get a key for an encrypted file system by using hardware
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memory scanners and reading it right off the memory bus.
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Nope, the network stuff is all done over TLS, which provides
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protection against that.
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No. The server only gives out the passwords to clients which have
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*in the TLS handshake* proven that they do indeed hold the OpenPGP
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private key corresponding to that client.
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** Physically grabbing the Mandos server computer?
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You could protect *that* computer the old-fashioned way, with a
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must-type-in-the-password-at-boot method. Or you could have two
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computers be the Mandos server for each other.
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Multiple Mandos servers can coexist on a network without any
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trouble. They do not clash, and clients will try all available
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servers. This means that if just one reboots then the other can
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bring it back up, but if both reboots at the same time they will
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stay down until someone types in the password on one of them.
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** Faking ping replies?
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The default for the server is to use "fping", the replies to which
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could be faked to eliminate the timeout. But this could easily be
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changed to any shell command, with any security measures you like.
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It could, for instance, be changed to an SSH command with strict
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keychecking, which could not be faked. Or IPsec could be used for
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the ping packets, making them secure.
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So, in summary: The only weakness in the Mandos system is from
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1. The power to come in and physically take your servers, *and*
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2. The cunning and patience to do it carefully, one at a time, and
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*quickly*, faking Mandos client/server responses for each one
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While there are some who may be threatened by people who have *both*
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these attributes, they do not, probably, constitute the majority.
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If you *do* face such opponents, you must figure that they could
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just as well open your servers and read the file system keys right
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off the memory by running wires to the memory bus.
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What Mandos is designed to protect against is *not* such determined,
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focused, and competent attacks, but against the early morning knock
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on your door and the sudden absence of all the servers in your
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server room. Which it does nicely.
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In the early designs, the mandos-client(8mandos) program (which
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retrieves a password from the Mandos server) also prompted for a
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password on the terminal, in case a Mandos server could not be
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found. This duality of purpose was seen to be too complex to be a
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viable way to continue. Instead, the programs are now separated
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into mandos-client(8mandos) and password-prompt(8mandos), and a
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plugin-runner(8mandos) exist to run them both in parallel, allowing
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the first plugin to succeed to provide the password. This opened up
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for any number of additional plugins to run, all competing to be the
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first to find a password and provide it to the plugin runner.
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Three additional plugins are provided:
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This prompts for a password when using usplash(8).
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This prompts for a password when using splashy(8).
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* askpass-fifo(8mandos)
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To provide compatibility with the "askpass" program from
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cryptsetup, this plugin listens to the same FIFO as askpass would
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(None of these take any options or reads any files.)
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More plugins could easily be written and added by the system
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administrator; see the section called "WRITING PLUGINS" in
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plugin-runner(8mandos) to learn the plugin requirements.
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Copyright © 2008,2009 Teddy Hogeborn
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Copyright © 2008,2009 Björn Påhlsson
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This program is free software: you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see
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<http://www.gnu.org/licenses/>.
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