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<refentry xmlns:xi="http://www.w3.org/2001/XInclude">
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<title>Mandos Manual</title>
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<!-- NWalsh’s docbook scripts use this to generate the footer: -->
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<productname>Mandos</productname>
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<productnumber>&version;</productnumber>
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<date>&TIMESTAMP;</date>
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<firstname>Björn</firstname>
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<surname>Påhlsson</surname>
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<email>belorn@recompile.se</email>
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<firstname>Teddy</firstname>
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<surname>Hogeborn</surname>
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<email>teddy@recompile.se</email>
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<holder>Teddy Hogeborn</holder>
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<holder>Björn Påhlsson</holder>
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<xi:include href="legalnotice.xml"/>
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<refentrytitle>intro</refentrytitle>
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<manvolnum>8mandos</manvolnum>
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<refname>intro</refname>
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Introduction to the Mandos system
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<refsect1 id="description">
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<title>DESCRIPTION</title>
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This is the the Mandos system, which allows computers to have
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encrypted root file systems and at the same time be capable of
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remote and/or unattended reboots.
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The computers run a small client program in the initial RAM disk
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environment which will communicate with a server over a network.
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All network communication is encrypted using TLS. The clients
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are identified by the server using a TLS public key; each client
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has one unique to it. The server sends the clients an encrypted
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password. The encrypted password is decrypted by the clients
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using a separate OpenPGP key, and the password is then used to
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unlock the root file system, whereupon the computers can
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continue booting normally.
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<refsect1 id="introduction">
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<title>INTRODUCTION</title>
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<!-- This paragraph is a combination and paraphrase of two
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quotes from the 1995 movie “The Usual Suspects”. -->
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You know how it is. You’ve heard of it happening. The Man
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comes and takes away your servers, your friends’ servers, the
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servers of everybody in the same hosting facility. The servers
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of their neighbors, and their neighbors’ friends. The servers
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of people who owe them money. And like
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<emphasis>that</emphasis>, they’re gone. And you doubt you’ll
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That is why your servers have encrypted root file systems.
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However, there’s a downside. There’s no going around it:
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rebooting is a pain. Dragging out that rarely-used keyboard and
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screen and unraveling cables behind your servers to plug them in
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to type in that password is messy, especially if you have many
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servers. There are some people who do clever things like using
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serial line consoles and daisy-chain it to the next server, and
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keep all the servers connected in a ring with serial cables,
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which will work, if your servers are physically close enough.
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There are also other out-of-band management solutions, but with
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<emphasis>all</emphasis> these, you still have to be on hand and
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manually type in the password at boot time. Otherwise the
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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
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run over to the server to type in the password?
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Well, with Mandos, you (almost) can! The gain in convenience
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will only be offset by a small loss in security. The setup is
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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
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network. The password will <emphasis>not</emphasis> be stored
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in plaintext, but encrypted with OpenPGP. To decrypt this
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password, a key is needed. This key (the Mandos client key)
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will not be stored there, but back on the original server
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(henceforth known as the Mandos client) in the initial RAM disk
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image. Oh, and all network Mandos client/server communications
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will be encrypted, using TLS (SSL).
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So, at boot time, the Mandos client will ask for its encrypted
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data over the network, decrypt the data to get the password, use
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the password to decrypt the root file system, and the client can
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then continue booting.
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Now, of course the initial RAM disk image is not on the
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encrypted root file system, so anyone who had physical access
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could take the Mandos client computer offline and read the disk
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with their own tools to get the authentication keys used by a
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client. <emphasis>But</emphasis>, by then the Mandos server
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should notice that the original server has been offline for too
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long, and will no longer give out the encrypted key. The timing
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here is the only real weak point, and the method, frequency and
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timeout of the server’s checking can be adjusted to any desired
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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
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<emphasis>that</emphasis> server is encrypted.)
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<title>FREQUENTLY ASKED QUESTIONS</title>
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Couldn’t the security be defeated by…
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<refsect2 id="quick">
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<title>Grabbing the Mandos client key from the
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initrd <emphasis>really quickly</emphasis>?</title>
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This, as mentioned above, is the only real weak point. But if
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you set the timing values tight enough, this will be really
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difficult to do. An attacker would have to physically
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disassemble the client computer, extract the key from the
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initial RAM disk image, and then connect to a <emphasis>still
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online</emphasis> Mandos server to get the encrypted key, and do
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all this <emphasis>before</emphasis> the Mandos server timeout
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kicks in and the Mandos server refuses to give out the key to
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Now, as the typical procedure seems to be to barge in and turn
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off and grab <emphasis>all</emphasis> computers, to maybe look
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at them months later, this is not likely. If someone does that,
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the whole system <emphasis>will</emphasis> lock itself up
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completely, since Mandos servers are no longer running.
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For sophisticated attackers who <emphasis>could</emphasis> do
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the clever thing, <emphasis>and</emphasis> had physical access
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to the server for enough time, it would be simpler to get a key
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for an encrypted file system by using hardware memory scanners
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and reading it right off the memory bus.
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<refsect2 id="replay">
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<title>Replay attacks?</title>
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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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<title>Man-in-the-middle?</title>
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No. The server only gives out the passwords to clients which
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have <emphasis>in the TLS handshake</emphasis> proven that
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they do indeed hold the private key corresponding to that
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<refsect2 id="sniff">
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<title>How about sniffing the network traffic and decrypting it
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later by physically grabbing the Mandos client and using its
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We only use <acronym>PFS</acronym> (Perfect Forward Security)
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key exchange algorithms in TLS, which protects against this.
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<refsect2 id="physgrab">
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<title>Physically grabbing the Mandos server computer?</title>
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You could protect <emphasis>that</emphasis> computer the
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old-fashioned way, with a must-type-in-the-password-at-boot
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method. Or you could have two computers be the Mandos server
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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
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available servers. This means that if just one reboots then
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the other can bring it back up, but if both reboot at the same
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time they will stay down until someone types in the password
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<refsect2 id="fakecheck">
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<title>Faking checker results?</title>
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If the Mandos client does not have an SSH server, the default
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is for the Mandos server to use
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<quote><literal>fping</literal></quote>, the replies to which
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could be faked to eliminate the timeout. But this could
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easily be changed to any shell command, with any security
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measures you like. If the Mandos client
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<emphasis>has</emphasis> an SSH server, the default
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configuration (as generated by
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<command>mandos-keygen</command> with the
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<option>--password</option> option) is for the Mandos server
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to use an <command>ssh-keyscan</command> command with strict
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keychecking, which can not be faked. Alternatively, IPsec
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could be used for the ping packets, making them secure.
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<refsect1 id="security">
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<title>SECURITY</title>
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So, in summary: The only weakness in the Mandos system is from
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The power to come in and physically take your servers,
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<emphasis>and</emphasis>
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The cunning and patience to do it carefully, one at a time,
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and <emphasis>quickly</emphasis>, faking Mandos
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client/server responses for each one before the timeout.
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While there are some who may be threatened by people who have
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<emphasis>both</emphasis> these attributes, they do not,
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probably, constitute the majority.
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If you <emphasis>do</emphasis> face such opponents, you must
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figure that they could just as well open your servers and read
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the file system keys right off the memory by running wires to
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What Mandos is designed to protect against is
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<emphasis>not</emphasis> such determined, focused, and competent
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attacks, but against the early morning knock on your door and
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the sudden absence of all the servers in your server room.
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Which it does nicely.
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<refsect1 id="plugins">
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<title>PLUGINS</title>
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In the early designs, the
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<citerefentry><refentrytitle>mandos-client</refentrytitle
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><manvolnum>8mandos</manvolnum></citerefentry> 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. Other ways of retrieving a password could easily be
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envisoned, but this multiplicity of purpose was seen to be too
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complex to be a viable way to continue. Instead, the original
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program was separated into <citerefentry><refentrytitle
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>mandos-client</refentrytitle><manvolnum>8mandos</manvolnum
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></citerefentry> and <citerefentry><refentrytitle
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>password-prompt</refentrytitle><manvolnum>8mandos</manvolnum
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></citerefentry>, and a <citerefentry><refentrytitle
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>plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
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></citerefentry> exist to run them both in parallel, allowing
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the first successful plugin to provide the password. This
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opened up for any number of additional plugins to run, all
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competing to be the first to find a password and provide it to
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Four additional plugins are provided:
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<citerefentry><refentrytitle>plymouth</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>
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This prompts for a password when using <citerefentry>
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<refentrytitle>plymouth</refentrytitle><manvolnum
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>8</manvolnum></citerefentry>.
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<citerefentry><refentrytitle>usplash</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>
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This prompts for a password when using <citerefentry>
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<refentrytitle>usplash</refentrytitle><manvolnum
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>8</manvolnum></citerefentry>.
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<citerefentry><refentrytitle>splashy</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>
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This prompts for a password when using <citerefentry>
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<refentrytitle>splashy</refentrytitle><manvolnum
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>8</manvolnum></citerefentry>.
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<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>
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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
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More plugins can easily be written and added by the system
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administrator; see the section called "WRITING PLUGINS" in
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<citerefentry><refentrytitle>plugin-runner</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry> to learn the
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<refsect1 id="systemd">
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<title>SYSTEMD</title>
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More advanced startup systems like <citerefentry><refentrytitle
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>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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already have their own plugin-like mechanisms for allowing
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multiple agents to independently retrieve a password and deliver
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it to the subsystem requesting a password to unlock the root
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file system. On these systems, it would make no sense to run
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<citerefentry><refentrytitle>plugin-runner</refentrytitle
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><manvolnum>8mandos</manvolnum></citerefentry>, the plugins of
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which would largely duplicate the work of (and conflict with)
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the existing systems prompting for passwords.
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As for <citerefentry><refentrytitle>systemd</refentrytitle
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><manvolnum>1</manvolnum></citerefentry> in particular, it has
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url="https://systemd.io/PASSWORD_AGENTS/">Password
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Agents</ulink> system. Mandos uses this via its
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<citerefentry><refentrytitle>password-agent</refentrytitle
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><manvolnum>8mandos</manvolnum></citerefentry> program, which is
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run instead of <citerefentry><refentrytitle
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>plugin-runner</refentrytitle><manvolnum>8mandos</manvolnum
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></citerefentry> when <citerefentry><refentrytitle
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>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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is used during system startup.
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<xi:include href="bugs.xml"/>
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<refsect1 id="see_also">
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<title>SEE ALSO</title>
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<citerefentry><refentrytitle>mandos</refentrytitle>
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<manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos.conf</refentrytitle>
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<manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos-clients.conf</refentrytitle>
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<manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos-ctl</refentrytitle>
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<manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos-monitor</refentrytitle>
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<manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>plugin-runner</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>password-agent</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos-client</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>password-prompt</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>plymouth</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>usplash</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>splashy</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>askpass-fifo</refentrytitle>
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<manvolnum>8mandos</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>mandos-keygen</refentrytitle>
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<manvolnum>8</manvolnum></citerefentry>
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<ulink url="https://www.recompile.se/mandos">Mandos</ulink>
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The Mandos home page.
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