path: root/man/crypttab.xml

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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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  This file is part of systemd.

  Copyright 2012 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
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  along with systemd; If not, see <http://www.gnu.org/licenses/>.

  This is based on crypttab(5) from Fedora's initscripts package, which in
  turn is based on Debian's version.

  The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.

-->
<refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP'>

        <refentryinfo>
                <title>crypttab</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Documentation</contrib>
                                <firstname>Miloslav</firstname>
                                <surname>Trmac</surname>
                                <email>mitr@redhat.com</email>
                        </author>
                        <author>
                                <contrib>Documentation</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>crypttab</refentrytitle>
                <manvolnum>5</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>crypttab</refname>
                <refpurpose>Configuration for encrypted block devices</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <para><filename>/etc/crypttab</filename></para>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para>The <filename>/etc/crypttab</filename> file
                describes encrypted block devices that are set up
                during system boot.</para>

                <para>Empty lines and lines starting with the <literal>#</literal>
                character are ignored.  Each of the remaining lines
                describes one encrypted block device, fields on the
                line are delimited by white space.  The first two
                fields are mandatory, the remaining two are
                optional.</para>

                <para>Setting up encrypted block devices using this file
                supports three encryption modes: LUKS, TrueCrypt and plain.
                See <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                for more information about each mode. When no mode is specified
                in the options field and the block device contains a LUKS
                signature, it is opened as a LUKS device; otherwise, it is
                assumed to be in raw dm-crypt (plain mode) format.</para>

                <para>The first field contains the name of the
                resulting encrypted block device; the device is set up
                within <filename>/dev/mapper/</filename>.</para>

                <para>The second field contains a path to the
                underlying block device or file, or a specification of a block
                device via <literal>UUID=</literal> followed by the
                UUID.</para>

                <para>The third field specifies the encryption
                password.  If the field is not present or the password
                is set to <literal>none</literal> or <literal>-</literal>,
                the password has to be manually entered during system boot.
                Otherwise, the field is interpreted as a absolute path to
                a file containing the encryption password. For swap encryption,
                <filename>/dev/urandom</filename> or the hardware
                device <filename>/dev/hw_random</filename> can be used
                as the password file; using
                <filename>/dev/random</filename> may prevent boot
                completion if the system does not have enough entropy
                to generate a truly random encryption key.</para>

                <para>The fourth field, if present, is a
                comma-delimited list of options.  The following
                options are recognized:</para>

                <variablelist class='crypttab-options'>

                        <varlistentry>
                                <term><varname>allow-discards</varname></term>

                                <listitem><para>Allow discard requests to be
                                passed through the encrypted block device. This
                                improves performance on SSD storage but has
                                security implications.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>cipher=</varname></term>

                                <listitem><para>Specifies the cipher to use. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for possible values and the default value of
                                this option. A cipher with unpredictable IV
                                values, such as <literal>aes-cbc-essiv:sha256</literal>,
                                is recommended.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>hash=</varname></term>

                                <listitem><para>Specifies the hash to use for
                                password hashing. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for possible values and the default value of
                                this option.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>keyfile-offset=</varname></term>

                                <listitem><para>Specifies the number of bytes to
                                skip at the start of the key file. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for possible values and the default value of
                                this option.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>keyfile-size=</varname></term>

                                <listitem><para>Specifies the maximum number
                                of bytes to read from the key file. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for possible values and the default value of
                                this option. This option is ignored in plain
                                encryption mode, as the key file size is then
                                given by the key size.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>luks</varname></term>

                                <listitem><para>Force LUKS mode. When this mode
                                is used, the following options are ignored since
                                they are provided by the LUKS header on the
                                device: <varname>cipher=</varname>,
                                <varname>hash=</varname>,
                                <varname>size=</varname>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>noauto</varname></term>

                                <listitem><para>This device will not be
                                automatically unlocked on boot.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>nofail</varname></term>

                                <listitem><para>The system will not wait for the
                                device to show up and be unlocked at boot, and
                                not fail the boot if it does not show up.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>plain</varname></term>

                                <listitem><para>Force plain encryption mode.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>read-only</varname></term><term><varname>readonly</varname></term>

                                <listitem><para>Set up the encrypted block
                                device in read-only mode.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>size=</varname></term>

                                <listitem><para>Specifies the key size
                                in bits. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for possible values and the default value of
                                this option.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>swap</varname></term>

                                <listitem><para>The encrypted block device will
                                be used as a swap device, and will be formatted
                                accordingly after setting up the encrypted
                                block device, with
                                <citerefentry><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
                                This option implies <varname>plain</varname>.</para>

                                <para>WARNING: Using the <varname>swap</varname>
                                option will destroy the contents of the named
                                partition during every boot, so make sure the
                                underlying block device is specified correctly.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tcrypt</varname></term>

                                <listitem><para>Use TrueCrypt encryption mode.
                                When this mode is used, the following options are
                                ignored since they are provided by the TrueCrypt
                                header on the device or do not apply:
                                <varname>cipher=</varname>,
                                <varname>hash=</varname>,
                                <varname>keyfile-offset=</varname>,
                                <varname>keyfile-size=</varname>,
                                <varname>size=</varname>.</para>

                                <para>When this mode is used, the passphrase is
                                read from the key file given in the third field.
                                Only the first line of this file is read,
                                excluding the new line character.</para>

                                <para>Note that the TrueCrypt format uses both
                                passphrase and key files to derive a password
                                for the volume. Therefore, the passphrase and
                                all key files need to be provided. Use
                                <varname>tcrypt-keyfile=</varname> to provide
                                the absolute path to all key files. When using
                                an empty passphrase in combination with one or
                                more key files, use <literal>/dev/null</literal>
                                as the password file in the third field.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tcrypt-hidden</varname></term>

                                <listitem><para>Use the hidden TrueCrypt volume.
                                This implies <varname>tcrypt</varname>.</para>

                                <para>This will map the hidden volume that is
                                inside of the volume provided in the second
                                field. Please note that there is no protection
                                for the hidden volume if the outer volume is
                                mounted instead. See
                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for more information on this limitation.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tcrypt-keyfile=</varname></term>

                                <listitem><para>Specifies the absolute path to a
                                key file to use for a TrueCrypt volume. This
                                implies <varname>tcrypt</varname> and can be
                                used more than once to provide several key
                                files.</para>

                                <para>See the entry for <varname>tcrypt</varname>
                                on the behavior of the passphrase and key files
                                when using TrueCrypt encryption mode.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tcrypt-system</varname></term>

                                <listitem><para>Use TrueCrypt in system
                                encryption mode. This implies
                                <varname>tcrypt</varname>.</para>

                                <para>Please note that when using this mode, the
                                whole device needs to be given in the second
                                field instead of the partition. For example: if
                                <literal>/dev/sda2</literal> is the system
                                encrypted TrueCrypt patition, <literal>/dev/sda</literal>
                                has to be given.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>timeout=</varname></term>

                                <listitem><para>Specifies the timeout for
                                querying for a password. If no unit is
                                specified, seconds is used. Supported units are
                                s, ms, us, min, h, d. A timeout of 0 waits
                                indefinitely (which is the default).</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tmp</varname></term>

                                <listitem><para>The encrypted block device will
                                be prepared for using it as <filename>/tmp</filename>;
                                it will be formatted using
                                <citerefentry><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
                                This option implies <varname>plain</varname>.</para>

                                <para>WARNING: Using the <varname>tmp</varname>
                                option will destroy the contents of the named
                                partition during every boot, so make sure the
                                underlying block device is specified correctly.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>tries=</varname></term>

                                <listitem><para>Specifies the maximum number of
                                times the user is queried for a password.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>verify</varname></term>

                                <listitem><para> If the encryption password is
                                read from console, it has to be entered twice to
                                prevent typos.</para></listitem>
                        </varlistentry>

                </variablelist>

                <para>At early boot and when the system manager
                configuration is reloaded, this file is translated into
                native systemd units
                by <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
        </refsect1>

        <refsect1>
                <title>Example</title>
                <example>
                        <title>/etc/crypttab example</title>
                        <para>Set up four encrypted block devices. One using
                        LUKS for normal storage, another one for usage as a swap
                        device and two TrueCrypt volumes.</para>

                        <programlisting>luks       UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
swap       /dev/sda7       /dev/urandom             swap
truecrypt  /dev/sda2       /etc/container_password  tcrypt
hidden     /mnt/tc_hidden  /null                    tcrypt-hidden,tcrypt-keyfile=/etc/keyfile</programlisting>
                </example>
        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>