3 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.5//EN"
4 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
6 SPDX-License-Identifier: LGPL-2.1-or-later
8 This is based on crypttab(5) from Fedora's initscripts package, which in
9 turn is based on Debian's version.
11 The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
13 <refentry id=
"crypttab" conditional='HAVE_LIBCRYPTSETUP' xmlns:
xi=
"http://www.w3.org/2001/XInclude">
16 <title>crypttab
</title>
17 <productname>systemd
</productname>
21 <refentrytitle>crypttab
</refentrytitle>
22 <manvolnum>5</manvolnum>
26 <refname>crypttab
</refname>
27 <refpurpose>Configuration for encrypted block devices
</refpurpose>
31 <para><filename>/etc/crypttab
</filename></para>
35 <title>Description
</title>
37 <para>The
<filename>/etc/crypttab
</filename> file describes
38 encrypted block devices that are set up during system boot.
</para>
40 <para>Empty lines and lines starting with the
<literal>#
</literal>
41 character are ignored. Each of the remaining lines describes one
42 encrypted block device. Fields are delimited by white space.
</para>
44 <para>Each line is in the form
<programlisting><replaceable>volume-name
</replaceable> <replaceable>encrypted-device
</replaceable> <replaceable>key-file
</replaceable> <replaceable>options
</replaceable></programlisting>
45 The first two fields are mandatory, the remaining two are
48 <para>Setting up encrypted block devices using this file supports
49 three encryption modes: LUKS, TrueCrypt and plain. See
50 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
51 for more information about each mode. When no mode is specified in
52 the options field and the block device contains a LUKS signature,
53 it is opened as a LUKS device; otherwise, it is assumed to be in
54 raw dm-crypt (plain mode) format.
</para>
56 <para>The first field contains the name of the resulting encrypted volume; its block device is set up
57 below
<filename>/dev/mapper/
</filename>.
</para>
59 <para>The second field contains a path to the underlying block
60 device or file, or a specification of a block device via
61 <literal>UUID=
</literal> followed by the UUID.
</para>
63 <para>The third field specifies an absolute path to a file with the encryption key. Optionally,
64 the path may be followed by
<literal>:
</literal> and an fstab device specification (e.g. starting with
65 <literal>LABEL=
</literal> or similar); in which case the path is taken relative to the device file system
66 root. If the field is not present or is
<literal>none
</literal> or
<literal>-
</literal>, a key file
67 named after the volume to unlock (i.e. the first column of the line), suffixed with
68 <filename>.key
</filename> is automatically loaded from the
<filename>/etc/cryptsetup-keys.d/
</filename>
69 and
<filename>/run/cryptsetup-keys.d/
</filename> directories, if present. Otherwise, the password has to
70 be manually entered during system boot. For swap encryption,
<filename>/dev/urandom
</filename> may be
71 used as key file.
</para>
73 <para>The fourth field, if present, is a comma-delimited list of
74 options. The following options are recognized:
</para>
76 <variablelist class='fstab-options'
>
79 <term><option>cipher=
</option></term>
81 <listitem><para>Specifies the cipher to use. See
<citerefentry
82 project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
83 for possible values and the default value of this option. A cipher with unpredictable IV values, such
84 as
<literal>aes-cbc-essiv:sha256
</literal>, is recommended. Embedded commas in the cipher
85 specification need to be escaped by preceding them with a backslash, see example below.
</para>
90 <term><option>discard
</option></term>
92 <listitem><para>Allow discard requests to be passed through the encrypted block
93 device. This improves performance on SSD storage but has security implications.
98 <term><option>hash=
</option></term>
100 <listitem><para>Specifies the hash to use for password
102 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
103 for possible values and the default value of this
104 option.
</para></listitem>
108 <term><option>header=
</option></term>
110 <listitem><para>Use a detached (separated) metadata device or
111 file where the LUKS header is stored. This option is only
112 relevant for LUKS devices. See
113 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
114 for possible values and the default value of this
117 <para>Optionally, the path may be followed by
<literal>:
</literal> and an fstab device specification
118 (e.g. starting with
<literal>UUID=
</literal> or similar); in which case, the path is relative to the
119 device file system root. The device gets mounted automatically for LUKS device activation duration only.
124 <term><option>keyfile-offset=
</option></term>
126 <listitem><para>Specifies the number of bytes to skip at the
127 start of the key file. See
128 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
129 for possible values and the default value of this
130 option.
</para></listitem>
134 <term><option>keyfile-size=
</option></term>
136 <listitem><para>Specifies the maximum number of bytes to read
137 from the key file. See
138 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
139 for possible values and the default value of this option. This
140 option is ignored in plain encryption mode, as the key file
141 size is then given by the key size.
</para></listitem>
145 <term><option>keyfile-erase
</option></term>
147 <listitem><para>If enabled, the specified key file is erased after the volume is activated or when
148 activation fails. This is in particular useful when the key file is only acquired transiently before
149 activation (e.g. via a file in
<filename>/run/
</filename>, generated by a service running before
150 activation), and shall be removed after use. Defaults to off.
</para></listitem>
154 <term><option>key-slot=
</option></term>
156 <listitem><para>Specifies the key slot to compare the
157 passphrase or key against. If the key slot does not match the
158 given passphrase or key, but another would, the setup of the
159 device will fail regardless. This option implies
160 <option>luks
</option>. See
161 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
162 for possible values. The default is to try all key slots in
163 sequential order.
</para></listitem>
167 <term><option>keyfile-timeout=
</option></term>
169 <listitem><para> Specifies the timeout for the device on
170 which the key file resides and falls back to a password if
171 it could not be mounted. See
172 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
173 for key files on external devices.
178 <term><option>luks
</option></term>
180 <listitem><para>Force LUKS mode. When this mode is used, the
181 following options are ignored since they are provided by the
182 LUKS header on the device:
<option>cipher=
</option>,
183 <option>hash=
</option>,
184 <option>size=
</option>.
</para></listitem>
188 <term><option>bitlk
</option></term>
190 <listitem><para>Decrypt Bitlocker drive. Encryption parameters
191 are deduced by cryptsetup from Bitlocker header.
</para></listitem>
195 <term><option>_netdev
</option></term>
197 <listitem><para>Marks this cryptsetup device as requiring network. It will be
198 started after the network is available, similarly to
199 <citerefentry><refentrytitle>systemd.mount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
200 units marked with
<option>_netdev
</option>. The service unit to set up this device
201 will be ordered between
<filename>remote-fs-pre.target
</filename> and
202 <filename>remote-cryptsetup.target
</filename>, instead of
203 <filename>cryptsetup-pre.target
</filename> and
204 <filename>cryptsetup.target
</filename>.
</para>
206 <para>Hint: if this device is used for a mount point that is specified in
207 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
208 the
<option>_netdev
</option> option should also be used for the mount
209 point. Otherwise, a dependency loop might be created where the mount point
210 will be pulled in by
<filename>local-fs.target
</filename>, while the
211 service to configure the network is usually only started
<emphasis>after
</emphasis>
212 the local file system has been mounted.
</para>
217 <term><option>noauto
</option></term>
219 <listitem><para>This device will not be added to
<filename>cryptsetup.target
</filename>.
220 This means that it will not be automatically unlocked on boot, unless something else pulls
221 it in. In particular, if the device is used for a mount point, it'll be unlocked
222 automatically during boot, unless the mount point itself is also disabled with
223 <option>noauto
</option>.
</para></listitem>
227 <term><option>nofail
</option></term>
229 <listitem><para>This device will not be a hard dependency of
230 <filename>cryptsetup.target
</filename>. It'll still be pulled in and started, but the system
231 will not wait for the device to show up and be unlocked, and boot will not fail if this is
232 unsuccessful. Note that other units that depend on the unlocked device may still fail. In
233 particular, if the device is used for a mount point, the mount point itself also needs to
234 have the
<option>nofail
</option> option, or the boot will fail if the device is not unlocked
235 successfully.
</para></listitem>
239 <term><option>offset=
</option></term>
241 <listitem><para>Start offset in the backend device, in
512-byte sectors. This
242 option is only relevant for plain devices.
</para></listitem>
246 <term><option>plain
</option></term>
248 <listitem><para>Force plain encryption mode.
</para></listitem>
252 <term><option>read-only
</option></term><term><option>readonly
</option></term>
254 <listitem><para>Set up the encrypted block device in read-only
255 mode.
</para></listitem>
259 <term><option>same-cpu-crypt
</option></term>
261 <listitem><para>Perform encryption using the same cpu that IO was submitted on. The default is to use
262 an unbound workqueue so that encryption work is automatically balanced between available CPUs.
</para>
264 <para>This requires kernel
4.0 or newer.
</para>
269 <term><option>submit-from-crypt-cpus
</option></term>
271 <listitem><para>Disable offloading writes to a separate thread after encryption. There are some
272 situations where offloading write requests from the encryption threads to a dedicated thread degrades
273 performance significantly. The default is to offload write requests to a dedicated thread because it
274 benefits the CFQ scheduler to have writes submitted using the same context.
</para>
276 <para>This requires kernel
4.0 or newer.
</para>
281 <term><option>skip=
</option></term>
283 <listitem><para>How many
512-byte sectors of the encrypted data to skip at the
284 beginning. This is different from the
<option>offset=
</option> option with respect
285 to the sector numbers used in initialization vector (IV) calculation. Using
286 <option>offset=
</option> will shift the IV calculation by the same negative
287 amount. Hence, if
<option>offset=
<replaceable>n
</replaceable></option> is given,
288 sector
<replaceable>n
</replaceable> will get a sector number of
0 for the IV
289 calculation. Using
<option>skip=
</option> causes sector
290 <replaceable>n
</replaceable> to also be the first sector of the mapped device, but
291 with its number for IV generation being
<replaceable>n
</replaceable>.
</para>
293 <para>This option is only relevant for plain devices.
</para>
298 <term><option>size=
</option></term>
300 <listitem><para>Specifies the key size in bits. See
301 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
302 for possible values and the default value of this
303 option.
</para></listitem>
307 <term><option>sector-size=
</option></term>
309 <listitem><para>Specifies the sector size in bytes. See
310 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
311 for possible values and the default value of this
312 option.
</para></listitem>
316 <term><option>swap
</option></term>
318 <listitem><para>The encrypted block device will be used as a
319 swap device, and will be formatted accordingly after setting
320 up the encrypted block device, with
321 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
322 This option implies
<option>plain
</option>.
</para>
324 <para>WARNING: Using the
<option>swap
</option> option will
325 destroy the contents of the named partition during every boot,
326 so make sure the underlying block device is specified
327 correctly.
</para></listitem>
331 <term><option>tcrypt
</option></term>
333 <listitem><para>Use TrueCrypt encryption mode. When this mode
334 is used, the following options are ignored since they are
335 provided by the TrueCrypt header on the device or do not
337 <option>cipher=
</option>,
338 <option>hash=
</option>,
339 <option>keyfile-offset=
</option>,
340 <option>keyfile-size=
</option>,
341 <option>size=
</option>.
</para>
343 <para>When this mode is used, the passphrase is read from the
344 key file given in the third field. Only the first line of this
345 file is read, excluding the new line character.
</para>
347 <para>Note that the TrueCrypt format uses both passphrase and
348 key files to derive a password for the volume. Therefore, the
349 passphrase and all key files need to be provided. Use
350 <option>tcrypt-keyfile=
</option> to provide the absolute path
351 to all key files. When using an empty passphrase in
352 combination with one or more key files, use
353 <literal>/dev/null
</literal> as the password file in the third
354 field.
</para></listitem>
358 <term><option>tcrypt-hidden
</option></term>
360 <listitem><para>Use the hidden TrueCrypt volume. This option
361 implies
<option>tcrypt
</option>.
</para>
363 <para>This will map the hidden volume that is inside of the
364 volume provided in the second field. Please note that there is
365 no protection for the hidden volume if the outer volume is
367 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
368 for more information on this limitation.
</para></listitem>
372 <term><option>tcrypt-keyfile=
</option></term>
374 <listitem><para>Specifies the absolute path to a key file to
375 use for a TrueCrypt volume. This implies
376 <option>tcrypt
</option> and can be used more than once to
377 provide several key files.
</para>
379 <para>See the entry for
<option>tcrypt
</option> on the
380 behavior of the passphrase and key files when using TrueCrypt
381 encryption mode.
</para></listitem>
385 <term><option>tcrypt-system
</option></term>
387 <listitem><para>Use TrueCrypt in system encryption mode. This
388 option implies
<option>tcrypt
</option>.
</para></listitem>
392 <term><option>tcrypt-veracrypt
</option></term>
394 <listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of
395 TrueCrypt that is mostly compatible, but uses different, stronger key
396 derivation algorithms that cannot be detected without this flag.
397 Enabling this option could substantially slow down unlocking, because
398 VeraCrypt's key derivation takes much longer than TrueCrypt's. This
399 option implies
<option>tcrypt
</option>.
</para></listitem>
403 <term><option>timeout=
</option></term>
405 <listitem><para>Specifies the timeout for querying for a
406 password. If no unit is specified, seconds is used. Supported
407 units are s, ms, us, min, h, d. A timeout of
0 waits
408 indefinitely (which is the default).
</para></listitem>
412 <term><option>tmp=
</option></term>
414 <listitem><para>The encrypted block device will be prepared for using it as
415 <filename>/tmp/
</filename>; it will be formatted using
<citerefentry
416 project='man-pages'
><refentrytitle>mkfs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>. Takes
417 a file system type as argument, such as
<literal>ext4
</literal>,
<literal>xfs
</literal> or
418 <literal>btrfs
</literal>. If no argument is specified defaults to
<literal>ext4
</literal>. This
419 option implies
<option>plain
</option>.
</para>
421 <para>WARNING: Using the
<option>tmp
</option> option will destroy the contents of the named partition
422 during every boot, so make sure the underlying block device is specified correctly.
</para></listitem>
426 <term><option>tries=
</option></term>
428 <listitem><para>Specifies the maximum number of times the user
429 is queried for a password. The default is
3. If set to
0, the
430 user is queried for a password indefinitely.
</para></listitem>
434 <term><option>verify
</option></term>
436 <listitem><para>If the encryption password is read from console, it has to be entered twice to
437 prevent typos.
</para></listitem>
441 <term><option>pkcs11-uri=
</option></term>
443 <listitem><para>Takes a
<ulink url=
"https://tools.ietf.org/html/rfc7512">RFC7512 PKCS#
11 URI
</ulink>
444 pointing to a private RSA key which is used to decrypt the key specified in the third column of the
445 line. This is useful for unlocking encrypted volumes through security tokens or smartcards. See below
446 for an example how to set up this mechanism for unlocking a LUKS volume with a YubiKey security
447 token. The specified URI can refer directly to a private RSA key stored on a token or alternatively
448 just to a slot or token, in which case a search for a suitable private RSA key will be performed. In
449 this case if multiple suitable objects are found the token is refused. The key configured in the
450 third column is passed as is to RSA decryption. The resulting decrypted key is then base64 encoded
451 before it is used to unlock the LUKS volume.
</para></listitem>
455 <term><option>try-empty-password=
</option></term>
457 <listitem><para>Takes a boolean argument. If enabled, right before asking the user for a password it
458 is first attempted to unlock the volume with an empty password. This is useful for systems that are
459 initialized with an encrypted volume with only an empty password set, which shall be replaced with a
460 suitable password during first boot, but after activation.
</para></listitem>
464 <term><option>x-systemd.device-timeout=
</option></term>
466 <listitem><para>Specifies how long systemd should wait for a device to show up
467 before giving up on the entry. The argument is a time in seconds or explicitly
469 <literal>s
</literal>,
470 <literal>min
</literal>,
471 <literal>h
</literal>,
472 <literal>ms
</literal>.
477 <term><option>x-initrd.attach
</option></term>
479 <listitem><para>Setup this encrypted block device in the initramfs, similarly to
480 <citerefentry><refentrytitle>systemd.mount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
481 units marked with
<option>x-initrd.mount
</option>.
</para>
483 <para>Although it's not necessary to mark the mount entry for the root file system with
484 <option>x-initrd.mount
</option>,
<option>x-initrd.attach
</option> is still recommended with
485 the encrypted block device containing the root file system as otherwise systemd will
486 attempt to detach the device during the regular system shutdown while it's still in
487 use. With this option the device will still be detached but later after the root file
488 system is unmounted.
</para>
490 <para>All other encrypted block devices that contain file systems mounted in the initramfs
491 should use this option.
</para>
497 <para>At early boot and when the system manager configuration is
498 reloaded, this file is translated into native systemd units by
499 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
503 <title>Examples
</title>
505 <title>/etc/crypttab example
</title>
506 <para>Set up four encrypted block devices. One using LUKS for normal storage, another one for usage as
507 a swap device and two TrueCrypt volumes. For the fourth device, the option string is interpreted as two
508 options
<literal>cipher=xchacha12,aes-adiantum-plain64
</literal>,
509 <literal>keyfile-timeout=
10s
</literal>.
</para>
511 <programlisting>luks UUID=
2505567a-
9e27-
4efe-a4d5-
15ad146c258b
512 swap /dev/sda7 /dev/urandom swap
513 truecrypt /dev/sda2 /etc/container_password tcrypt
514 hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
515 external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=
10s,cipher=xchacha12\,aes-adiantum-plain64
520 <title>Yubikey-based Volume Unlocking Example
</title>
522 <para>The PKCS#
11 logic allows hooking up any compatible security token that is capable of storing RSA
523 decryption keys. Here's an example how to set up a Yubikey security token for this purpose, using
524 <citerefentry project='debian'
><refentrytitle>ykmap
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
525 from the yubikey-manager project:
</para>
527 <programlisting><xi:include href=
"yubikey-crypttab.sh" parse=
"text" /></programlisting>
529 <para>A few notes on the above:
</para>
532 <listitem><para>We use RSA (and not ECC), since Yubikeys support PKCS#
11 Decrypt() only for RSA keys
</para></listitem>
533 <listitem><para>We use RSA2048, which is the longest key size current Yubikeys support
</para></listitem>
534 <listitem><para>LUKS key size must be shorter than
2048bit due to RSA padding, hence we use
128 bytes
</para></listitem>
535 <listitem><para>We use Yubikey key slot
9d, since that's apparently the keyslot to use for decryption purposes,
536 <ulink url=
"https://developers.yubico.com/PIV/Introduction/Certificate_slots.html">see
537 documentation
</ulink>.
</para></listitem>
544 <title>See Also
</title>
546 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
547 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
548 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
549 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
550 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
551 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
552 <citerefentry project='man-pages'
><refentrytitle>mke2fs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>