systemd/man/crypttab.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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<!--
  SPDX-License-Identifier: LGPL-2.1-or-later

  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' xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>crypttab</title>
    <productname>systemd</productname>
  </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 are delimited by white space.</para>

    <para>Each line is in the form<programlisting><replaceable>volume-name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>key-file</replaceable> <replaceable>options</replaceable></programlisting>
    The first two fields are mandatory, the remaining two are
    optional.</para>

    <para>Setting up encrypted block devices using this file supports four encryption modes: LUKS, TrueCrypt,
    BitLocker 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 four fields of <filename>/etc/crypttab</filename> are defined as follows:</para>

    <orderedlist>

      <listitem><para>The first field contains the name of the resulting volume with decrypted data; its
      block device is set up below <filename>/dev/mapper/</filename>.</para></listitem>

      <listitem><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></listitem>

      <listitem><para>The third field specifies an absolute path to a file with the encryption
      key. Optionally, the path may be followed by <literal>:</literal> and an
      <filename>/etc/fstab</filename> style device specification (e.g. starting with
      <literal>LABEL=</literal> or similar); in which case the path is taken relative to the specified
      device's file system root. If the field is not present or is <literal>none</literal> or
      <literal>-</literal>, a key file named after the volume to unlock (i.e. the first column of the line),
      suffixed with <filename>.key</filename> is automatically loaded from the
      <filename>/etc/cryptsetup-keys.d/</filename> and <filename>/run/cryptsetup-keys.d/</filename>
      directories, if present. Otherwise, the password has to be manually entered during system boot. For
      swap encryption, <filename>/dev/urandom</filename> may be used as key file, resulting in a randomized
      key.</para>

      <para>If the specified key file path refers to an <constant>AF_UNIX</constant> stream socket in the
      file system, the key is acquired by connecting to the socket and reading it from the connection. This
      allows the implementation of a service to provide key information dynamically, at the moment when it is
      needed. For details see below.</para></listitem>

      <listitem><para>The fourth field, if present, is a comma-delimited list of options. The supported
      options are listed below.</para></listitem>
    </orderedlist>
  </refsect1>

  <refsect1>
    <title>Key Acquisition</title>

    <para>Six different mechanisms for acquiring the decryption key or passphrase unlocking the encrypted
    volume are supported. Specifically:</para>

    <orderedlist>

      <listitem><para>Most prominently, the user may be queried interactively during volume activation
      (i.e. typically at boot), asking them to type in the necessary passphrases.</para></listitem>

      <listitem><para>The (unencrypted) key may be read from a file on disk, possibly on removable media. The third field
      of each line encodes the location, for details see above.</para></listitem>

      <listitem><para>The (unencrypted) key may be requested from another service, by specifying an
      <constant>AF_UNIX</constant> file system socket in place of a key file in the third field. For details
      see above and below.</para></listitem>

      <listitem><para>The key may be acquired via a PKCS#11 compatible hardware security token or
      smartcard. In this case a saved key used in unlock process is stored on disk/removable media, acquired via
      <constant>AF_UNIX</constant>, or stored in the LUKS2 JSON token metadata header. For RSA, the saved key
      is an encrypted volume key. The encrypted volume key is then decrypted by the PKCS#11 token with an RSA
      private key stored on it, and used to unlock the encrypted volume. For elliptic-curve (EC) cryptography,
      the saved key is the public key generated in enrollment process. The public key is then used to derive
      a shared secret with a private key stored in the PKCS#11 token. The derived shared secret is then used
      to unlock the volume. Use the <option>pkcs11-uri=</option> option described below to use this mechanism.
      </para></listitem>

      <listitem><para>Similarly, the key may be acquired via a FIDO2 compatible hardware security token
      (which must implement the "hmac-secret" extension). In this case a key generated randomly during
      enrollment is stored on disk/removable media, acquired via <constant>AF_UNIX</constant>, or stored in
      the LUKS2 JSON token metadata header. The random key is hashed via a keyed hash function (HMAC) on the
      FIDO2 token, using a secret key stored on the token that never leaves it. The resulting hash value is
      then used as key to unlock the encrypted volume. Use the <option>fido2-device=</option> option
      described below to use this mechanism.</para></listitem>

      <listitem><para>Similarly, the key may be acquired via a TPM2 security chip. In this case a (during
      enrollment) randomly generated key — encrypted by an asymmetric key derived from the TPM2 chip's seed
      key — is stored on disk/removable media, acquired via <constant>AF_UNIX</constant>, or stored in the
      LUKS2 JSON token metadata header. Use the <option>tpm2-device=</option> option described below to use
      this mechanism.</para></listitem>
    </orderedlist>

    <para>For the latter five mechanisms the source for the key material used for unlocking the volume is
    primarily configured in the third field of each <filename>/etc/crypttab</filename> line, but may also
    configured in <filename>/etc/cryptsetup-keys.d/</filename> and
    <filename>/run/cryptsetup-keys.d/</filename> (see above) or in the LUKS2 JSON token header (in case of
    the latter three). Use the
    <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    tool to enroll PKCS#11, FIDO2 and TPM2 devices in LUKS2 volumes.</para>
  </refsect1>

  <refsect1>
    <title>Supported Options</title>

    <para>The following options may be used in the fourth field of each line:</para>

    <variablelist class='fstab-options'>

      <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. Embedded commas in the cipher
        specification need to be escaped by preceding them with a backslash, see example below.</para>

        <xi:include href="version-info.xml" xpointer="v186"/>
        </listitem>
      </varlistentry>

      <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>

        <xi:include href="version-info.xml" xpointer="v207"/></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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

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

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

        <para>Optionally, the path may be followed by <literal>:</literal> and an
        <filename>/etc/fstab</filename> device specification (e.g. starting with <literal>UUID=</literal> or
        similar); in which case, the path is relative to the device file system root. The device gets mounted
        automatically for LUKS device activation duration only.</para>

        <xi:include href="version-info.xml" xpointer="v219"/></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>

        <xi:include href="version-info.xml" xpointer="v187"/></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>

        <xi:include href="version-info.xml" xpointer="v188"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>keyfile-erase</option></term>

        <listitem><para>If enabled, the specified key file is erased after the volume is activated or when
        activation fails. This is in particular useful when the key file is only acquired transiently before
        activation (e.g. via a file in <filename>/run/</filename>, generated by a service running before
        activation), and shall be removed after use. Defaults to off.</para>

        <xi:include href="version-info.xml" xpointer="v246"/></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>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

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

        <listitem><para> Specifies the timeout for the device on
        which the key file resides or the device used as the key file,
        and falls back to a password if it could not be accessed. See
        <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for key files on external devices.
        </para>

        <xi:include href="version-info.xml" xpointer="v243"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>link-volume-key=</option></term>

        <listitem><para>Specifies the kernel keyring and key description
        (see <citerefentry project='man-pages'><refentrytitle>keyrings</refentrytitle><manvolnum>7</manvolnum></citerefentry>)
        where LUKS2 volume key gets linked during device activation. The kernel keyring
        description and key description must be separated by <literal>::</literal>.</para>

        <para>The kernel keyring part can be a string description or a predefined
        kernel keyring prefixed with <literal>@</literal> (e.g.: to use <literal>@s</literal> session or
        <literal>@u</literal> user keyring directly). The type prefix text in the kernel keyring description
        is not required. The specified kernel keyring must already exist at the time of device activation.</para>

        <para>The key part is a string description optionally prefixed by a <literal>%key_type:</literal>.
        If no type is specified, the <literal>user</literal> type key is linked by default. See
        <citerefentry project='man-pages'><refentrytitle>keyctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        for more information on key descriptions (KEY IDENTIFIERS section).</para>

        <para>Note that the linked volume key is not cleaned up automatically when the device is detached.</para>

        <xi:include href="version-info.xml" xpointer="v256"/></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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>bitlk</option></term>

        <listitem><para>Decrypt BitLocker drive. Encryption parameters
        are deduced by cryptsetup from BitLocker header.</para>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>_netdev</option></term>

        <listitem><para>Marks this cryptsetup device as requiring network. It will be
        started after the network is available, similarly to
        <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        units marked with <option>_netdev</option>. The service unit to set up this device
        will be ordered between <filename>remote-fs-pre.target</filename> and
        <filename>remote-cryptsetup.target</filename>, instead of
        <filename>cryptsetup-pre.target</filename> and
        <filename>cryptsetup.target</filename>.</para>

        <para>Hint: if this device is used for a mount point that is specified in
        <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        the <option>_netdev</option> option should also be used for the mount
        point. Otherwise, a dependency loop might be created where the mount point
        will be pulled in by <filename>local-fs.target</filename>, while the
        service to configure the network is usually only started <emphasis>after</emphasis>
        the local file system has been mounted.</para>

        <xi:include href="version-info.xml" xpointer="v235"/>
        </listitem>
      </varlistentry>

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

        <listitem><para>This device will not be added to <filename>cryptsetup.target</filename>.
        This means that it will not be automatically unlocked on boot, unless something else pulls
        it in. In particular, if the device is used for a mount point, it'll be unlocked
        automatically during boot, unless the mount point itself is also disabled with
        <option>noauto</option>.</para>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

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

        <listitem><para>This device will not be a hard dependency of
        <filename>cryptsetup.target</filename>. It'll still be pulled in and started, but the system
        will not wait for the device to show up and be unlocked, and boot will not fail if this is
        unsuccessful. Note that other units that depend on the unlocked device may still fail. In
        particular, if the device is used for a mount point, the mount point itself also needs to
        have the <option>nofail</option> option, or the boot will fail if the device is not unlocked
        successfully. If a keyfile and/or a <option>header</option> are specified, the dependencies on
        their respective directories will also not be fatal, so that umounting said directories will
        not cause the generated cryptset unit to be deactivated.</para>

        <xi:include href="version-info.xml" xpointer="v186"/></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>

        <xi:include href="version-info.xml" xpointer="v220"/></listitem>
      </varlistentry>

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

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

        <xi:include href="version-info.xml" xpointer="v186"/></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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>same-cpu-crypt</option></term>

        <listitem><para>Perform encryption using the same CPU that IO was submitted on. The default is to use
        an unbound workqueue so that encryption work is automatically balanced between available CPUs.</para>

        <para>This requires kernel 4.0 or newer.</para>

        <xi:include href="version-info.xml" xpointer="v242"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>submit-from-crypt-cpus</option></term>

        <listitem><para>Disable offloading writes to a separate thread after encryption. There are some
        situations where offloading write requests from the encryption threads to a dedicated thread degrades
        performance significantly. The default is to offload write requests to a dedicated thread because it
        benefits the CFQ scheduler to have writes submitted using the same context.</para>

        <para>This requires kernel 4.0 or newer.</para>

        <xi:include href="version-info.xml" xpointer="v242"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>no-read-workqueue</option></term>

        <listitem><para>Bypass dm-crypt internal workqueue and process read requests synchronously. The
        default is to queue these requests and process them asynchronously.</para>

        <para>This requires kernel 5.9 or newer.</para>

        <xi:include href="version-info.xml" xpointer="v248"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><option>no-write-workqueue</option></term>

        <listitem><para>Bypass dm-crypt internal workqueue and process write requests synchronously. The
        default is to queue these requests and process them asynchronously.</para>

        <para>This requires kernel 5.9 or newer.</para>

        <xi:include href="version-info.xml" xpointer="v248"/>
        </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=<replaceable>n</replaceable></option> is given,
        sector <replaceable>n</replaceable> will get a sector number of 0 for the IV
        calculation. Using <option>skip=</option> causes sector
        <replaceable>n</replaceable> to also be the first sector of the mapped device, but
        with its number for IV generation being <replaceable>n</replaceable>.</para>

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

        <xi:include href="version-info.xml" xpointer="v220"/>
        </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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

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

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

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <warning>
          <para>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>
        </warning>

        <xi:include href="version-info.xml" xpointer="v186"/></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>

        <xi:include href="version-info.xml" xpointer="v206"/></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>

        <xi:include href="version-info.xml" xpointer="v206"/></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>

        <xi:include href="version-info.xml" xpointer="v206"/></listitem>
      </varlistentry>

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

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

        <xi:include href="version-info.xml" xpointer="v206"/></listitem>
      </varlistentry>

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

        <listitem><para>Check for a VeraCrypt volume.  VeraCrypt is a fork of
        TrueCrypt that is mostly compatible, but uses different, stronger key
        derivation algorithms that cannot be detected without this flag.
        Enabling this option could substantially slow down unlocking, because
        VeraCrypt's key derivation takes much longer than TrueCrypt's.  This
        option implies <option>tcrypt</option>.</para>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>veracrypt-pim=</option></term>

        <listitem><para>Specifies a custom Personal Iteration Multiplier (PIM)
        value, which can range from 0..2147468 for standard veracrypt volumes
        and 0..65535 for veracrypt system volumes. A value of 0 will imply the
        VeraCrypt default.

        This option is only effective when <option>tcrypt-veracrypt</option> is
        set.</para>

        <para>Note that VeraCrypt enforces a minimal allowed PIM value depending on the
        password strength and the hash algorithm used for key derivation, however
        <option>veracrypt-pim=</option> is not checked against these bounds.
        See
        <ulink url="https://www.veracrypt.fr/en/Personal%20Iterations%20Multiplier%20%28PIM%29.html">Veracrypt Personal Iterations Multiplier</ulink>
        documentation for more information.</para>

        <xi:include href="version-info.xml" xpointer="v254"/>
        </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>

        <xi:include href="version-info.xml" xpointer="v186"/></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>mkfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>. Takes
        a file system type as argument, such as <literal>ext4</literal>, <literal>xfs</literal> or
        <literal>btrfs</literal>. If no argument is specified defaults to <literal>ext4</literal>. This
        option implies <option>plain</option>.</para>

        <warning>
          <para>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>
        </warning>

        <xi:include href="version-info.xml" xpointer="v186"/></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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>headless=</option></term>

        <listitem><para>Takes a boolean argument, defaults to false. If true, never query interactively
        for the password/PIN. Useful for headless systems.</para>

        <xi:include href="version-info.xml" xpointer="v249"/></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>

        <xi:include href="version-info.xml" xpointer="v186"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>password-echo=yes|no|masked</option></term>

        <listitem><para>Controls whether to echo passwords or security token PINs
        that are read from console. Takes a boolean or the special string <literal>masked</literal>.
        The default is <option>password-echo=masked</option>.</para>

        <para>If enabled, the typed characters are echoed literally. If disabled,
        the typed characters are not echoed in any form, the user will not get
        feedback on their input. If set to <literal>masked</literal>, an asterisk
        (<literal>*</literal>) is echoed for each character typed. Regardless of
        which mode is chosen, if the user hits the tabulator key (<literal>↹</literal>)
        at any time, or the backspace key (<literal>⌫</literal>) before any other
        data has been entered, then echo is turned off.</para>

        <xi:include href="version-info.xml" xpointer="v249"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>pkcs11-uri=</option></term>

        <listitem><para>Takes either the special value <literal>auto</literal> or an <ulink
        url="https://tools.ietf.org/html/rfc7512">RFC7512 PKCS#11 URI</ulink> pointing to a private key
        which is used to decrypt the encrypted key specified in the third column of the line. This is useful
        for unlocking encrypted volumes through PKCS#11 compatible security tokens or smartcards. See below
        for an example how to set up this mechanism for unlocking a LUKS2 volume with a YubiKey security
        token.</para>

        <para>If specified as <literal>auto</literal> the volume must be of type LUKS2 and must carry PKCS#11
        security token metadata in its LUKS2 JSON token section. In this mode the URI and the encrypted key
        are automatically read from the LUKS2 JSON token header. Use
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        as a simple tool for enrolling PKCS#11 security tokens or smartcards in a way compatible with
        <literal>auto</literal>. In this mode the third column of the line should remain empty (that is,
        specified as <literal>-</literal>).</para>

        <para>The specified URI can refer directly to a private key stored on a token or alternatively
        just to a slot or token, in which case a search for a suitable private key will be performed.  In
        this case if multiple suitable objects are found the token is refused. The keyfile configured
        in the third column of the line is used as is (i.e. in binary form, unprocessed). The resulting
        decrypted key (for RSA) or derived shared secret (for ECC) is then Base64 encoded before it is used
        to unlock the LUKS volume.</para>

        <para>Use <command>systemd-cryptenroll --pkcs11-token-uri=list</command> to list all suitable PKCS#11
        security tokens currently plugged in, along with their URIs.</para>

        <para>Note that many newer security tokens that may be used as PKCS#11 security token typically also
        implement the newer and simpler FIDO2 standard. Consider using <option>fido2-device=</option>
        (described below) to enroll it via FIDO2 instead. Note that a security token enrolled via PKCS#11
        cannot be used to unlock the volume via FIDO2, unless also enrolled via FIDO2, and vice
        versa.</para>

        <xi:include href="version-info.xml" xpointer="v245"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>fido2-device=</option></term>

        <listitem><para>Takes either the special value <literal>auto</literal> or the path to a
        <literal>hidraw</literal> device node (e.g. <filename>/dev/hidraw1</filename>) referring to a FIDO2
        security token that implements the <literal>hmac-secret</literal> extension (most current hardware
        security tokens do). See below for an example how to set up this mechanism for unlocking an encrypted
        volume with a FIDO2 security token.</para>

        <para>If specified as <literal>auto</literal> the FIDO2 token device is automatically discovered, as
        it is plugged in.</para>

        <para>FIDO2 volume unlocking requires a client ID hash (CID) to be configured via
        <option>fido2-cid=</option> (see below) and a key to pass to the security token's HMAC functionality
        (configured in the line's third column) to operate. If not configured and the volume is of type
        LUKS2, the CID and the key are read from LUKS2 JSON token metadata instead. Use
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        as simple tool for enrolling FIDO2 security tokens, compatible with this automatic mode, which is
        only available for LUKS2 volumes.</para>

        <para>Use <command>systemd-cryptenroll --fido2-device=list</command> to list all suitable FIDO2
        security tokens currently plugged in, along with their device nodes.</para>

        <para>This option implements the following mechanism: the configured key is hashed via they HMAC
        keyed hash function the FIDO2 device implements, keyed by a secret key embedded on the device. The
        resulting hash value is Base64 encoded and used to unlock the LUKS2 volume. As it should not be
        possible to extract the secret from the hardware token, it should not be possible to retrieve the
        hashed key given the configured key — without possessing the hardware token.</para>

        <para>Note that many security tokens that implement FIDO2 also implement PKCS#11, suitable for
        unlocking volumes via the <option>pkcs11-uri=</option> option described above. Typically the newer,
        simpler FIDO2 standard is preferable.</para>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>fido2-cid=</option></term>

        <listitem><para>Takes a Base64 encoded FIDO2 client ID to use for the FIDO2 unlock operation. If
        specified, but <option>fido2-device=</option> is not, <option>fido2-device=auto</option> is
        implied. If <option>fido2-device=</option> is used but <option>fido2-cid=</option> is not, the volume
        must be of LUKS2 type, and the CID is read from the LUKS2 JSON token header. Use
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        for enrolling a FIDO2 token in the LUKS2 header compatible with this automatic
        mode.</para>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>fido2-rp=</option></term>

        <listitem><para>Takes a string, configuring the FIDO2 Relying Party (rp) for the FIDO2 unlock
        operation. If not specified <literal>io.systemd.cryptsetup</literal> is used, except if the LUKS2
        JSON token header contains a different value. It should normally not be necessary to override
        this.</para>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-device=</option></term>

        <listitem><para>Takes either the special value <literal>auto</literal> or the path to a device node
        (e.g. <filename>/dev/tpmrm0</filename>) referring to a TPM2 security chip. See below for an example
        how to set up this mechanism for unlocking an encrypted volume with a TPM2 chip.</para>

        <para>Use <option>tpm2-pcrs=</option> (see below) to configure the set of TPM2 PCRs to bind the
        volume unlocking to. Use
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        as simple tool for enrolling TPM2 security chips in LUKS2 volumes.</para>

        <para>If specified as <literal>auto</literal> the TPM2 device is automatically discovered. Use
        <command>systemd-cryptenroll --tpm2-device=list</command> to list all suitable TPM2 devices currently
        available, along with their device nodes.</para>

        <para>This option implements the following mechanism: when enrolling a TPM2 device via
        <command>systemd-cryptenroll</command> on a LUKS2 volume, a randomized key unlocking the volume is
        generated on the host and loaded into the TPM2 chip where it is encrypted with an asymmetric
        "primary" key pair derived from the TPM2's internal "seed" key. Neither the seed key nor the primary
        key are permitted to ever leave the TPM2 chip — however, the now encrypted randomized key may. It is
        saved in the LUKS2 volume JSON token header. When unlocking the encrypted volume, the primary key
        pair is generated on the TPM2 chip again (which works as long as the chip's seed key is correctly
        maintained by the TPM2 chip), which is then used to decrypt (on the TPM2 chip) the encrypted key from
        the LUKS2 volume JSON token header saved there during enrollment. The resulting decrypted key is then
        used to unlock the volume. When the randomized key is encrypted the current values of the selected
        PCRs (see below) are included in the operation, so that different PCR state results in different
        encrypted keys and the decrypted key can only be recovered if the same PCR state is
        reproduced.</para>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-pcrs=</option></term>

        <listitem><para>Takes a <literal>+</literal> separated list of numeric TPM2 PCR (i.e. "Platform
        Configuration Register") indexes to bind the TPM2 volume unlocking to. This option is only useful
        when TPM2 enrollment metadata is not available in the LUKS2 JSON token header already, the way
        <command>systemd-cryptenroll</command> writes it there. If not used (and no metadata in the LUKS2
        JSON token header defines it), defaults to a list of a single entry: PCR 7. Assign an empty string to
        encode a policy that binds the key to no PCRs, making the key accessible to local programs regardless
        of the current PCR state.</para>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-pin=</option></term>

        <listitem><para>Takes a boolean argument, defaults to <literal>false</literal>. Controls whether
        TPM2 volume unlocking is bound to a PIN in addition to PCRs. Similarly, this option is only useful
        when TPM2 enrollment metadata is not available.</para>

        <xi:include href="version-info.xml" xpointer="v251"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-signature=</option></term>

        <listitem><para>Takes an absolute path to a TPM2 PCR JSON signature file, as produced by the
        <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        tool. This permits locking LUKS2 volumes to any PCR values for which a valid signature matching a
        public key specified at key enrollment time can be provided. See
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        for details on enrolling TPM2 PCR public keys. If this option is not specified but it is attempted to
        unlock a LUKS2 volume with a signed TPM2 PCR enrollment a suitable signature file
        <filename>tpm2-pcr-signature.json</filename> is searched for in <filename>/etc/systemd/</filename>,
        <filename>/run/systemd/</filename>, <filename>/usr/lib/systemd/</filename> (in this
        order).</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-pcrlock=</option></term>

        <listitem><para>Takes an absolute path to a TPM2 pcrlock policy file, as produced by the
        <citerefentry><refentrytitle>systemd-pcrlock</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        tool. This permits locking LUKS2 volumes to a local policy of allowed PCR values with
        variants. See
        <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        for details on enrolling TPM2 pcrlock policies. If this option is not specified but it is attempted
        to unlock a LUKS2 volume with a TPM2 pcrlock enrollment a suitable signature file
        <filename>pcrlock.json</filename> is searched for in <filename>/run/systemd/</filename> and
        <filename>/var/lib/systemd/</filename> (in this order).</para>

        <xi:include href="version-info.xml" xpointer="v255"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-measure-pcr=</option></term>

        <listitem><para>Controls whether to measure the volume key of the encrypted volume to a TPM2 PCR. If
        set to "no" (which is the default) no PCR extension is done. If set to "yes" the volume key is
        measured into PCR 15. If set to a decimal integer in the range 0…23 the volume key is measured into
        the specified PCR. The volume key is measured along with the activated volume name and its UUID. This
        functionality is particularly useful for the encrypted volume backing the root file system, as it
        then allows later TPM objects to be securely bound to the root file system and hence the specific
        installation.</para>

        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>tpm2-measure-bank=</option></term>

        <listitem><para>Selects one or more TPM2 PCR banks to measure the volume key into, as configured with
        <option>tpm2-measure-pcr=</option> above. Multiple banks may be specified, separated by a colon
        character. If not specified automatically determines available and used banks. Expects a message
        digest name (e.g. <literal>sha1</literal>, <literal>sha256</literal>, …) as argument, to identify the
        bank.</para>

        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>token-timeout=</option></term>

        <listitem><para>Specifies how long to wait at most for configured security devices (i.e. FIDO2,
        PKCS#11, TPM2) to show up. Takes a time value in seconds (but other time units may be specified too,
        see <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for supported formats). Defaults to 30s. Once the specified timeout elapsed authentication via
        password is attempted. Note that this timeout applies to waiting for the security device to show up —
        it does not apply to the PIN prompt for the device (should one be needed) or similar. Pass 0 to turn
        off the time-out and wait forever.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>try-empty-password=</option></term>

        <listitem><para>Takes a boolean argument. If enabled, right before asking the user for a password it
        is first attempted to unlock the volume with an empty password. This is useful for systems that are
        initialized with an encrypted volume with only an empty password set, which shall be replaced with a
        suitable password during first boot, but after activation.</para>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

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

        <listitem><para>Specifies how long systemd should wait for a block 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>

        <xi:include href="version-info.xml" xpointer="v216"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>x-initrd.attach</option></term>

        <listitem><para>Setup this encrypted block device in the initrd, similarly to
        <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        units marked with <option>x-initrd.mount</option>.</para>

        <para>Although it's not necessary to mark the mount entry for the root file system with
        <option>x-initrd.mount</option>, <option>x-initrd.attach</option> is still recommended with
        the encrypted block device containing the root file system as otherwise systemd will
        attempt to detach the device during the regular system shutdown while it's still in
        use. With this option the device will still be detached but later after the root file
        system is unmounted.</para>

        <para>All other encrypted block devices that contain file systems mounted in the initrd should use
        this option.</para>

        <xi:include href="version-info.xml" xpointer="v245"/>
        </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><constant>AF_UNIX</constant> Key Files</title>

    <para>If the key file path (as specified in the third column of <filename>/etc/crypttab</filename>
    entries, see above) refers to an <constant>AF_UNIX</constant> stream socket in the file system, the key
    is acquired by connecting to the socket and reading the key from the connection. The connection is made
    from an <constant>AF_UNIX</constant> socket name in the abstract namespace, see <citerefentry
    project='man-pages'><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
    details. The source socket name is chosen according the following format:</para>

    <programlisting><constant>NUL</constant> <replaceable>RANDOM</replaceable> /cryptsetup/ <replaceable>VOLUME</replaceable></programlisting>

    <para>In other words: a <constant>NUL</constant> byte (as required for abstract namespace sockets),
    followed by a random string (consisting of alphanumeric characters only), followed by the literal
    string <literal>/cryptsetup/</literal>, followed by the name of the volume to acquire they key
    for. For example, for the volume <literal>myvol</literal>:</para>

    <programlisting>\0d7067f78d9827418/cryptsetup/myvol</programlisting>

    <para>Services listening on the <constant>AF_UNIX</constant> stream socket may query the source socket
    name with <citerefentry
    project='man-pages'><refentrytitle>getpeername</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
    and use this to determine which key to send, allowing a single listening socket to serve keys for
    multiple volumes. If the PKCS#11 logic is used (see above), the socket source name is picked in similar
    fashion, except that the literal string <literal>/cryptsetup-pkcs11/</literal> is used. And similarly for
    FIDO2 (<literal>/cryptsetup-fido2/</literal>) and TPM2 (<literal>/cryptsetup-tpm2/</literal>). A different
    path component is used so that services providing key material know that the secret key was not requested
    directly, but instead an encrypted key that will be decrypted via the PKCS#11/FIDO2/TPM2 logic to acquire
    the final secret key.</para>
  </refsect1>

  <refsect1>
    <title>Examples</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. For the fourth device, the option string is interpreted as two
      options <literal>cipher=xchacha12,aes-adiantum-plain64</literal>,
      <literal>keyfile-timeout=10s</literal>.</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
external   /dev/sda3       keyfile:LABEL=keydev keyfile-timeout=10s,cipher=xchacha12\,aes-adiantum-plain64
</programlisting>
    </example>

    <example>
      <title>Yubikey-based PKCS#11 Volume Unlocking Example</title>

      <para>The PKCS#11 logic allows hooking up any compatible security token that is capable of storing RSA
      or EC cryptographic keys for unlocking an encrypted volume. Here's an example how to set up a Yubikey
      security token for this purpose on a LUKS2 volume, using <citerefentry
      project='debian'><refentrytitle>ykmap</refentrytitle><manvolnum>1</manvolnum></citerefentry> from the
      yubikey-manager project to initialize the token and
      <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      to add it in the LUKS2 volume:</para>

      <programlisting><xi:include href="yubikey-crypttab.sh" parse="text" /></programlisting>

      <para>A few notes on the above:</para>

      <itemizedlist>
        <listitem><para>We use RSA2048, which is the longest key size current Yubikeys support</para></listitem>
        <listitem><para>We use Yubikey key slot 9d, since that's apparently the keyslot to use for decryption purposes,
        see
        <ulink url="https://developers.yubico.com/PIV/Introduction/Certificate_slots.html">Yubico PIV certificate slots</ulink>.
        </para></listitem>
      </itemizedlist>
    </example>

    <example>
      <title>FIDO2 Volume Unlocking Example</title>

      <para>The FIDO2 logic allows using any compatible FIDO2 security token that implements the
      <literal>hmac-secret</literal> extension for unlocking an encrypted volume. Here's an example how to
      set up a FIDO2 security token for this purpose for a LUKS2 volume, using
      <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>:</para>

      <programlisting><xi:include href="fido2-crypttab.sh" parse="text" /></programlisting>
    </example>

    <example>
      <title>TPM2 Volume Unlocking Example</title>

      <para>The TPM2 logic allows using any TPM2 chip supported by the Linux kernel for unlocking an
      encrypted volume. Here's an example how to set up a TPM2 chip for this purpose for a LUKS2 volume,
      using
      <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>:</para>

      <programlisting><xi:include href="tpm2-crypttab.sh" parse="text" /></programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>