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<?xml version="1.0"?>
<!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
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
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 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 fstab device specification
(e.g. starting with <literal>LABEL=</literal> or similar); in which case the path is taken relative to
the device 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>
<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 fstab 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>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 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>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 alphabenumeric 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 below) the socket source name is picked in
identical fashion, except that the literal string <literal>/cryptsetup-pkcs11/</literal> is used. 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 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 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
<citerefentry project='debian'><refentrytitle>ykmap</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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 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>
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