systemd-stable/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 passphrase(s).</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 an encrypted key is stored on disk/removable media, acquired via
      <constant>AF_UNIX</constant>, or stored in the LUKS2 JSON token metadata header. The encrypted key is
      then decrypted by the PKCS#11 token with an RSA key stored on it, and then used to unlock the encrypted
      volume. Use the <option>pkcs11-uri=</option> option described below to use this mechanism.</para></listitem>

      <listitem><para>Similar, the key may be acquired via a FIDO2 compatible hardware security token (which
      must implement the "hmac-secret" extension). In this case a (during enrollment) randomly generated key
      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>Similar, 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>
        </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></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>

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

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

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

        <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>

      <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 RSA 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 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 RSA key stored on a token or alternatively
        just to a slot or token, in which case a search for a suitable private RSA key will be performed.  In
        this case if multiple suitable objects are found the token is refused. The encrypted key configured
        in the third column of the line is passed as is (i.e. in binary form, unprocessed) to RSA
        decryption. The resulting decrypted key 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></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></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></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 the LUKS2
        JSON token header contains a different value. It should normally not be necessary to override
        this.</para></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></listitem>
      </varlistentry>

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

        <listitem><para>Takes a comma 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></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></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>x-initrd.attach</option></term>

        <listitem><para>Setup this encrypted block device in the initramfs, 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 initramfs
        should use this option.</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><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> <literal>/cryptsetup/</literal> <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. Example (for a volume <literal>myvol</literal>):</para>

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

    <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 it to determine which key to send, allowing a single listening socket to serve keys for a
    multitude of volumes. If the PKCS#11 logic is used (see above) the socket source name is picked in
    identical fashion, except that the literal string <literal>/cryptsetup-pkcs11/</literal> is used (similar
    for FIDO2: <literal>/cryptsetup-fido2/</literal> and TPM2: <literal>/cryptsetup-tpm2/</literal>). This is
    done so that services providing key material know that not a secret key is requested but 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
      decryption 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,
        <ulink url="https://developers.yubico.com/PIV/Introduction/Certificate_slots.html">see
        documentation</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>
      <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>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</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>