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+

  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>name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>password</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
    three encryption modes: LUKS, TrueCrypt and plain. See
    <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    for more information about each mode. When no mode is specified in
    the options field and the block device contains a LUKS signature,
    it is opened as a LUKS device; otherwise, it is assumed to be in
    raw dm-crypt (plain mode) format.</para>

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

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

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

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

    <variablelist class='fstab-options'>

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

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

      <varlistentry>
        <term><option>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></listitem>
      </varlistentry>

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

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

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

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

      <varlistentry>
        <term><option>key-slot=</option></term>

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

      <varlistentry>
        <term><option>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>_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 bios from the encryption threads to a single thread degrades
        performance significantly. The default is to offload write bios to the same thread because it benefits
        CFQ to have writes submitted using the same context.</para>
        <para>This requires kernel 4.0 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>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        This option implies <option>plain</option>.</para>

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

      <varlistentry>
        <term><option>tries=</option></term>

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

      <varlistentry>
        <term><option>verify</option></term>

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

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

        <listitem><para>Takes a <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 key specified in the third column of the
        line. This is useful for unlocking encrypted volumes through security tokens or smartcards. See below
        for an example how to set up this mechanism for unlocking a LUKS volume with a YubiKey security
        token. 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 key configured in the
        third column is passed as is to RSA decryption. The resulting decrypted key is then base64 encoded
        before it is used to unlock the LUKS volume.</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>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.</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</programlisting>
    </example>

    <example>
      <title>Yubikey-based Volume Unlocking Example</title>

      <para>The PKCS#11 logic allows hooking up any compatible security token that is capable of storing RSA
      decryption keys. Here's an example how to set up a Yubikey security token for this purpose, using
      <command>ykman</command> from the yubikey-manager project:</para>

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

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

<itemizedlist>
  <listitem><para>We use RSA (and not ECC), since Yubikeys support PKCS#11 Decrypt() only for RSA keys</para></listitem>
  <listitem><para>We use RSA2048, which is the longest key size current Yubikeys support</para></listitem>
  <listitem><para>LUKS key size must be shorter than 2048bit due to RSA padding, hence we use 128 bytes</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>
  </refsect1>

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