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These are useful for plain devices as they don't have any metadata by themselves. Instead of using an unreliable hardcoded device name in crypttab you can then put static metadata at the start of the partition for a stable UUID or label. https://bugs.freedesktop.org/show_bug.cgi?id=87717 https://bugs.debian.org/751707 https://launchpad.net/bugs/953875
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417 lines
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<?xml version="1.0"?>
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<!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd 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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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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This is based on crypttab(5) from Fedora's initscripts package, which in
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turn is based on Debian's version.
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The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
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-->
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<refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP'>
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<refentryinfo>
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<title>crypttab</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Documentation</contrib>
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<firstname>Miloslav</firstname>
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<surname>Trmac</surname>
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<email>mitr@redhat.com</email>
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</author>
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<author>
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<contrib>Documentation</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>crypttab</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>crypttab</refname>
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<refpurpose>Configuration for encrypted block devices</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/crypttab</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>The <filename>/etc/crypttab</filename> file describes
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encrypted block devices that are set up during system boot.</para>
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<para>Empty lines and lines starting with the <literal>#</literal>
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character are ignored. Each of the remaining lines describes one
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encrypted block device, fields on the line are delimited by white
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space. The first two fields are mandatory, the remaining two are
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optional.</para>
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<para>Setting up encrypted block devices using this file supports
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three encryption modes: LUKS, TrueCrypt and plain. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for more information about each mode. When no mode is specified in
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the options field and the block device contains a LUKS signature,
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it is opened as a LUKS device; otherwise, it is assumed to be in
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raw dm-crypt (plain mode) format.</para>
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<para>The first field contains the name of the resulting encrypted
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block device; the device is set up within
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<filename>/dev/mapper/</filename>.</para>
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<para>The second field contains a path to the underlying block
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device or file, or a specification of a block device via
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<literal>UUID=</literal> followed by the UUID.</para>
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<para>The third field specifies the encryption password. If the
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field is not present or the password is set to
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<literal>none</literal> or <literal>-</literal>, the password has
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to be manually entered during system boot. Otherwise, the field is
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interpreted as a absolute path to a file containing the encryption
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password. For swap encryption, <filename>/dev/urandom</filename>
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or the hardware device <filename>/dev/hw_random</filename> can be
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used as the password file; using <filename>/dev/random</filename>
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may prevent boot completion if the system does not have enough
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entropy to generate a truly random encryption key.</para>
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<para>The fourth field, if present, is a comma-delimited list of
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options. The following options are recognized:</para>
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<variablelist class='fstab-options'>
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<varlistentry>
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<term><option>discard</option></term>
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<listitem><para>Allow discard requests to be passed through
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the encrypted block device. This improves performance on SSD
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storage but has security implications.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>cipher=</option></term>
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<listitem><para>Specifies the cipher to use. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this option. A
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cipher with unpredictable IV values, such as
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<literal>aes-cbc-essiv:sha256</literal>, is
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recommended.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>hash=</option></term>
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<listitem><para>Specifies the hash to use for password
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hashing. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this
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option.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>header=</option></term>
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<listitem><para>Use a detached (separated) metadata device or
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file where the LUKS header is stored. This option is only
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relevant for LUKS devices. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this
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option.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>offset=</option></term>
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<listitem><para>Start offset in the backend device, in 512-byte sectors.
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This option is only relevant for plain devices.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>skip=</option></term>
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<listitem><para>How many 512-byte sectors of the encrypted data to skip
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at the beginning. This is different from the <option>--offset</option>
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option with respect to the sector numbers used in initialization vector
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(IV) calculation. Using <option>--offset</option> will shift the IV
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calculation by the same negative amount. Hence, if <option>--offset n</option>,
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sector n will get a sector number of 0 for the IV calculation.
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Using <option>--skip</option> causes sector n to also be the first
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sector of the mapped device, but with its number for IV generation is n.</para>
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<para>This option is only relevant for plain devices.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>keyfile-offset=</option></term>
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<listitem><para>Specifies the number of bytes to skip at the
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start of the key file. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this
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option.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>keyfile-size=</option></term>
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<listitem><para>Specifies the maximum number of bytes to read
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from the key file. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this option. This
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option is ignored in plain encryption mode, as the key file
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size is then given by the key size.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>key-slot=</option></term>
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<listitem><para>Specifies the key slot to compare the
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passphrase or key against. If the key slot does not match the
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given passphrase or key, but another would, the setup of the
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device will fail regardless. This option implies
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<option>luks</option>. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values. The default is to try all key slots in
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sequential order.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>luks</option></term>
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<listitem><para>Force LUKS mode. When this mode is used, the
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following options are ignored since they are provided by the
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LUKS header on the device: <option>cipher=</option>,
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<option>hash=</option>,
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<option>size=</option>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>noauto</option></term>
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<listitem><para>This device will not be automatically unlocked
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on boot.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>nofail</option></term>
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<listitem><para>The system will not wait for the device to
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show up and be unlocked at boot, and not fail the boot if it
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does not show up.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>plain</option></term>
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<listitem><para>Force plain encryption mode.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>read-only</option></term><term><option>readonly</option></term>
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<listitem><para>Set up the encrypted block device in read-only
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mode.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>size=</option></term>
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<listitem><para>Specifies the key size in bits. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for possible values and the default value of this
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option.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>swap</option></term>
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<listitem><para>The encrypted block device will be used as a
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swap device, and will be formatted accordingly after setting
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up the encrypted block device, with
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<citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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This option implies <option>plain</option>.</para>
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<para>WARNING: Using the <option>swap</option> option will
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destroy the contents of the named partition during every boot,
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so make sure the underlying block device is specified
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correctly.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tcrypt</option></term>
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<listitem><para>Use TrueCrypt encryption mode. When this mode
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is used, the following options are ignored since they are
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provided by the TrueCrypt header on the device or do not
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apply:
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<option>cipher=</option>,
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<option>hash=</option>,
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<option>keyfile-offset=</option>,
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<option>keyfile-size=</option>,
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<option>size=</option>.</para>
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<para>When this mode is used, the passphrase is read from the
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key file given in the third field. Only the first line of this
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file is read, excluding the new line character.</para>
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<para>Note that the TrueCrypt format uses both passphrase and
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key files to derive a password for the volume. Therefore, the
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passphrase and all key files need to be provided. Use
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<option>tcrypt-keyfile=</option> to provide the absolute path
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to all key files. When using an empty passphrase in
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combination with one or more key files, use
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<literal>/dev/null</literal> as the password file in the third
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field.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tcrypt-hidden</option></term>
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<listitem><para>Use the hidden TrueCrypt volume. This option
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implies <option>tcrypt</option>.</para>
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<para>This will map the hidden volume that is inside of the
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volume provided in the second field. Please note that there is
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no protection for the hidden volume if the outer volume is
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mounted instead. See
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for more information on this limitation.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tcrypt-keyfile=</option></term>
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<listitem><para>Specifies the absolute path to a key file to
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use for a TrueCrypt volume. This implies
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<option>tcrypt</option> and can be used more than once to
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provide several key files.</para>
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<para>See the entry for <option>tcrypt</option> on the
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behavior of the passphrase and key files when using TrueCrypt
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encryption mode.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tcrypt-system</option></term>
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<listitem><para>Use TrueCrypt in system encryption mode. This
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option implies <option>tcrypt</option>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>timeout=</option></term>
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<listitem><para>Specifies the timeout for querying for a
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password. If no unit is specified, seconds is used. Supported
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units are s, ms, us, min, h, d. A timeout of 0 waits
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indefinitely (which is the default).</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>x-systemd.device-timeout=</option></term>
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<listitem><para>Specifies how long systemd should wait for a
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device to show up before giving up on the entry. The argument
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is a time in seconds or explicitly specified units of
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<literal>s</literal>,
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<literal>min</literal>,
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<literal>h</literal>,
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<literal>ms</literal>.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tmp</option></term>
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<listitem><para>The encrypted block device will be prepared
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for using it as <filename>/tmp</filename>; it will be
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formatted using
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<citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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This option implies <option>plain</option>.</para>
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<para>WARNING: Using the <option>tmp</option> option will
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destroy the contents of the named partition during every boot,
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so make sure the underlying block device is specified
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correctly.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>tries=</option></term>
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<listitem><para>Specifies the maximum number of times the user
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is queried for a password. The default is 3. If set to 0, the
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user is queried for a password indefinitely.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>verify</option></term>
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<listitem><para> If the encryption password is read from
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console, it has to be entered twice to prevent
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typos.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>At early boot and when the system manager configuration is
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reloaded, this file is translated into native systemd units by
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<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
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</refsect1>
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<refsect1>
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<title>Example</title>
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<example>
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<title>/etc/crypttab example</title>
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<para>Set up four encrypted block devices. One using LUKS for
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normal storage, another one for usage as a swap device and two
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TrueCrypt volumes.</para>
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<programlisting>luks UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
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swap /dev/sda7 /dev/urandom swap
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truecrypt /dev/sda2 /etc/container_password tcrypt
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hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile</programlisting>
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</example>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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