systemd/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

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  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>.

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<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 project='die-net'><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='fstab-options'>

      <varlistentry>
        <term><option>discard</option></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><option>cipher=</option></term>

        <listitem><para>Specifies the cipher to use. See
        <citerefentry project='die-net'><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><option>hash=</option></term>

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

      <varlistentry>
        <term><option>header=</option></term>

        <listitem><para>Use a detached (separated) metadata device or
        file where the LUKS header is stored. This option is only
        relevant for LUKS devices. See
        <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for possible values and the default value of this
        option.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>offset=</option></term>

        <listitem><para>Start offset in the backend device, in 512-byte sectors.
        This option is only relevant for plain devices.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>skip=</option></term>

        <listitem><para>How many 512-byte sectors of the encrypted data to skip
        at the beginning. This is different from the <option>--offset</option>
        option with respect to the sector numbers used in initialization vector
        (IV) calculation. Using <option>--offset</option> will shift the IV
        calculation by the same negative amount.  Hence, if <option>--offset n</option>,
        sector n will  get a sector number of 0 for the IV calculation.
        Using <option>--skip</option> causes sector n to also be the first
        sector of the mapped device, but with its number for IV generation is n.</para>

        <para>This option is only relevant for plain devices.</para>
        </listitem>
      </varlistentry>

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

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

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

        <listitem><para>Specifies the maximum number of bytes to read
        from the key file. See
        <citerefentry project='die-net'><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><option>key-slot=</option></term>

        <listitem><para>Specifies the key slot to compare the
        passphrase or key against. If the key slot does not match the
        given passphrase or key, but another would, the setup of the
        device will fail regardless. This option implies
        <option>luks</option>. See
        <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for possible values. The default is to try all key slots in
        sequential order.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>luks</option></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: <option>cipher=</option>,
        <option>hash=</option>,
        <option>size=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>noauto</option></term>

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

      <varlistentry>
        <term><option>nofail</option></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><option>plain</option></term>

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

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

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

      <varlistentry>
        <term><option>size=</option></term>

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

      <varlistentry>
        <term><option>swap</option></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 project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        This option implies <option>plain</option>.</para>

        <para>WARNING: Using the <option>swap</option> 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><option>tcrypt</option></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:
        <option>cipher=</option>,
        <option>hash=</option>,
        <option>keyfile-offset=</option>,
        <option>keyfile-size=</option>,
        <option>size=</option>.</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
        <option>tcrypt-keyfile=</option> 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><option>tcrypt-hidden</option></term>

        <listitem><para>Use the hidden TrueCrypt volume. This option
        implies <option>tcrypt</option>.</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 project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for more information on this limitation.</para></listitem>
      </varlistentry>

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

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

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

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

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

      <varlistentry>
        <term><option>timeout=</option></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><option>x-systemd.device-timeout=</option></term>

        <listitem><para>Specifies how long systemd should wait for a
        device to show up before giving up on the entry. The argument
        is a time in seconds or explicitly specified units of
        <literal>s</literal>,
        <literal>min</literal>,
        <literal>h</literal>,
        <literal>ms</literal>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tmp</option></term>

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

        <para>WARNING: Using the <option>tmp</option> 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><option>tries=</option></term>

        <listitem><para>Specifies the maximum number of times the user
        is queried for a password. The default is 3. If set to 0, the
        user is queried for a password indefinitely.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>verify</option></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  /dev/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 project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>