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438 lines
19 KiB
XML
438 lines
19 KiB
XML
<?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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SPDX-License-Identifier: LGPL-2.1+
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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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</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 are delimited by white space.</para>
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<para>Each line is in the form<programlisting><replaceable>name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>password</replaceable> <replaceable>options</replaceable></programlisting>
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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 an 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>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>discard</option></term>
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<listitem><para>Allow discard requests to be passed through the encrypted block
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device. This improves performance on SSD storage but has security implications.
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</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>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>_netdev</option></term>
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<listitem><para>Marks this cryptsetup device as requiring network. It will be
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started after the network is available, similarly to
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<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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units marked with <option>_netdev</option>. The service unit to set up this device
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will be ordered between <filename>remote-fs-pre.target</filename> and
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<filename>remote-cryptsetup.target</filename>, instead of
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<filename>cryptsetup-pre.target</filename> and
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<filename>cryptsetup.target</filename>.</para>
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<para>Hint: if this device is used for a mount point that is specified in
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<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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the <option>_netdev</option> option should also be used for the mount
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point. Otherwise, a dependency loop might be created where the mount point
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will be pulled in by <filename>local-fs.target</filename>, while the
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service to configure the network is usually only started <emphasis>after</emphasis>
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the local file system has been mounted.</para>
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</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 added to <filename>cryptsetup.target</filename>.
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This means that it will not be automatically unlocked on boot, unless something else pulls
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it in. In particular, if the device is used for a mount point, it'll be unlocked
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automatically during boot, unless the mount point itself is also disabled with
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<option>noauto</option>.</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>This device will not be a hard dependency of
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<filename>cryptsetup.target</filename>. It'll be still pulled in and started, but the system
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will not wait for the device to show up and be unlocked, and boot will not fail if this is
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unsuccessful. Note that other units that depend on the unlocked device may still fail. In
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particular, if the device is used for a mount point, the mount point itself is also needs to
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have <option>noauto</option> option, or the boot will fail if the device is not unlocked
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successfully.</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. This
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option is only relevant for plain devices.</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>skip=</option></term>
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<listitem><para>How many 512-byte sectors of the encrypted data to skip at the
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beginning. This is different from the <option>offset=</option> option with respect
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to the sector numbers used in initialization vector (IV) calculation. Using
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<option>offset=</option> will shift the IV calculation by the same negative
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amount. Hence, if <option>offset=<replaceable>n</replaceable></option> is given,
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sector <replaceable>n</replaceable> will get a sector number of 0 for the IV
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calculation. Using <option>skip=</option> causes sector
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<replaceable>n</replaceable> to also be the first sector of the mapped device, but
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with its number for IV generation being <replaceable>n</replaceable>.</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>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>sector-size=</option></term>
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<listitem><para>Specifies the sector size in bytes. 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>tcrypt-veracrypt</option></term>
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<listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of
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TrueCrypt that is mostly compatible, but uses different, stronger key
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derivation algorithms that cannot be detected without this flag.
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Enabling this option could substantially slow down unlocking, because
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VeraCrypt's key derivation takes much longer than TrueCrypt's. 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>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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<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 device to show up
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before giving up on the entry. The argument is a time in seconds or explicitly
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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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</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='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</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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