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+
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'
>
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>name
</replaceable> <replaceable>encrypted-device
</replaceable> <replaceable>password
</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
57 block device; the device is set up within
58 <filename>/dev/mapper/
</filename>.
</para>
60 <para>The second field contains a path to the underlying block
61 device or file, or a specification of a block device via
62 <literal>UUID=
</literal> followed by the UUID.
</para>
64 <para>The third field specifies the encryption password. If the
65 field is not present or the password is set to
66 <literal>none
</literal> or
<literal>-
</literal>, the password has
67 to be manually entered during system boot. Otherwise, the field is
68 interpreted as an absolute path to a file containing the encryption
69 password. For swap encryption,
<filename>/dev/urandom
</filename>
70 or the hardware device
<filename>/dev/hw_random
</filename> can be
71 used as the password file; using
<filename>/dev/random
</filename>
72 may prevent boot completion if the system does not have enough
73 entropy to generate a truly random encryption key.
</para>
75 <para>The fourth field, if present, is a comma-delimited list of
76 options. The following options are recognized:
</para>
78 <variablelist class='fstab-options'
>
81 <term><option>cipher=
</option></term>
83 <listitem><para>Specifies the cipher to use. See
84 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
85 for possible values and the default value of this option. A
86 cipher with unpredictable IV values, such as
87 <literal>aes-cbc-essiv:sha256
</literal>, is
88 recommended.
</para></listitem>
92 <term><option>discard
</option></term>
94 <listitem><para>Allow discard requests to be passed through the encrypted block
95 device. This improves performance on SSD storage but has security implications.
100 <term><option>hash=
</option></term>
102 <listitem><para>Specifies the hash to use for password
104 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
105 for possible values and the default value of this
106 option.
</para></listitem>
110 <term><option>header=
</option></term>
112 <listitem><para>Use a detached (separated) metadata device or
113 file where the LUKS header is stored. This option is only
114 relevant for LUKS devices. See
115 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
116 for possible values and the default value of this
117 option.
</para></listitem>
121 <term><option>keyfile-offset=
</option></term>
123 <listitem><para>Specifies the number of bytes to skip at the
124 start of the key file. See
125 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
126 for possible values and the default value of this
127 option.
</para></listitem>
131 <term><option>keyfile-size=
</option></term>
133 <listitem><para>Specifies the maximum number of bytes to read
134 from the key file. See
135 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
136 for possible values and the default value of this option. This
137 option is ignored in plain encryption mode, as the key file
138 size is then given by the key size.
</para></listitem>
142 <term><option>key-slot=
</option></term>
144 <listitem><para>Specifies the key slot to compare the
145 passphrase or key against. If the key slot does not match the
146 given passphrase or key, but another would, the setup of the
147 device will fail regardless. This option implies
148 <option>luks
</option>. See
149 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
150 for possible values. The default is to try all key slots in
151 sequential order.
</para></listitem>
155 <term><option>keyfile-timeout=
</option></term>
157 <listitem><para> Specifies the timeout for the device on
158 which the key file resides and falls back to a password if
159 it could not be mounted. See
160 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
161 for key files on external devices.
166 <term><option>luks
</option></term>
168 <listitem><para>Force LUKS mode. When this mode is used, the
169 following options are ignored since they are provided by the
170 LUKS header on the device:
<option>cipher=
</option>,
171 <option>hash=
</option>,
172 <option>size=
</option>.
</para></listitem>
176 <term><option>_netdev
</option></term>
178 <listitem><para>Marks this cryptsetup device as requiring network. It will be
179 started after the network is available, similarly to
180 <citerefentry><refentrytitle>systemd.mount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
181 units marked with
<option>_netdev
</option>. The service unit to set up this device
182 will be ordered between
<filename>remote-fs-pre.target
</filename> and
183 <filename>remote-cryptsetup.target
</filename>, instead of
184 <filename>cryptsetup-pre.target
</filename> and
185 <filename>cryptsetup.target
</filename>.
</para>
187 <para>Hint: if this device is used for a mount point that is specified in
188 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
189 the
<option>_netdev
</option> option should also be used for the mount
190 point. Otherwise, a dependency loop might be created where the mount point
191 will be pulled in by
<filename>local-fs.target
</filename>, while the
192 service to configure the network is usually only started
<emphasis>after
</emphasis>
193 the local file system has been mounted.
</para>
198 <term><option>noauto
</option></term>
200 <listitem><para>This device will not be added to
<filename>cryptsetup.target
</filename>.
201 This means that it will not be automatically unlocked on boot, unless something else pulls
202 it in. In particular, if the device is used for a mount point, it'll be unlocked
203 automatically during boot, unless the mount point itself is also disabled with
204 <option>noauto
</option>.
</para></listitem>
208 <term><option>nofail
</option></term>
210 <listitem><para>This device will not be a hard dependency of
211 <filename>cryptsetup.target
</filename>. It'll be still pulled in and started, but the system
212 will not wait for the device to show up and be unlocked, and boot will not fail if this is
213 unsuccessful. Note that other units that depend on the unlocked device may still fail. In
214 particular, if the device is used for a mount point, the mount point itself is also needs to
215 have
<option>noauto
</option> option, or the boot will fail if the device is not unlocked
216 successfully.
</para></listitem>
220 <term><option>offset=
</option></term>
222 <listitem><para>Start offset in the backend device, in
512-byte sectors. This
223 option is only relevant for plain devices.
</para></listitem>
227 <term><option>plain
</option></term>
229 <listitem><para>Force plain encryption mode.
</para></listitem>
233 <term><option>read-only
</option></term><term><option>readonly
</option></term>
235 <listitem><para>Set up the encrypted block device in read-only
236 mode.
</para></listitem>
240 <term><option>same-cpu-crypt
</option></term>
242 <listitem><para>Perform encryption using the same cpu that IO was submitted on. The default is to use
243 an unbound workqueue so that encryption work is automatically balanced between available CPUs.
</para>
244 <para>This requires kernel
4.0 or newer.
</para>
249 <term><option>submit-from-crypt-cpus
</option></term>
251 <listitem><para>Disable offloading writes to a separate thread after encryption. There are some
252 situations where offloading write bios from the encryption threads to a single thread degrades
253 performance significantly. The default is to offload write bios to the same thread because it benefits
254 CFQ to have writes submitted using the same context.
</para>
255 <para>This requires kernel
4.0 or newer.
</para>
260 <term><option>skip=
</option></term>
262 <listitem><para>How many
512-byte sectors of the encrypted data to skip at the
263 beginning. This is different from the
<option>offset=
</option> option with respect
264 to the sector numbers used in initialization vector (IV) calculation. Using
265 <option>offset=
</option> will shift the IV calculation by the same negative
266 amount. Hence, if
<option>offset=
<replaceable>n
</replaceable></option> is given,
267 sector
<replaceable>n
</replaceable> will get a sector number of
0 for the IV
268 calculation. Using
<option>skip=
</option> causes sector
269 <replaceable>n
</replaceable> to also be the first sector of the mapped device, but
270 with its number for IV generation being
<replaceable>n
</replaceable>.
</para>
272 <para>This option is only relevant for plain devices.
</para>
277 <term><option>size=
</option></term>
279 <listitem><para>Specifies the key size in bits. See
280 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
281 for possible values and the default value of this
282 option.
</para></listitem>
286 <term><option>sector-size=
</option></term>
288 <listitem><para>Specifies the sector size in bytes. See
289 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
290 for possible values and the default value of this
291 option.
</para></listitem>
295 <term><option>swap
</option></term>
297 <listitem><para>The encrypted block device will be used as a
298 swap device, and will be formatted accordingly after setting
299 up the encrypted block device, with
300 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
301 This option implies
<option>plain
</option>.
</para>
303 <para>WARNING: Using the
<option>swap
</option> option will
304 destroy the contents of the named partition during every boot,
305 so make sure the underlying block device is specified
306 correctly.
</para></listitem>
310 <term><option>tcrypt
</option></term>
312 <listitem><para>Use TrueCrypt encryption mode. When this mode
313 is used, the following options are ignored since they are
314 provided by the TrueCrypt header on the device or do not
316 <option>cipher=
</option>,
317 <option>hash=
</option>,
318 <option>keyfile-offset=
</option>,
319 <option>keyfile-size=
</option>,
320 <option>size=
</option>.
</para>
322 <para>When this mode is used, the passphrase is read from the
323 key file given in the third field. Only the first line of this
324 file is read, excluding the new line character.
</para>
326 <para>Note that the TrueCrypt format uses both passphrase and
327 key files to derive a password for the volume. Therefore, the
328 passphrase and all key files need to be provided. Use
329 <option>tcrypt-keyfile=
</option> to provide the absolute path
330 to all key files. When using an empty passphrase in
331 combination with one or more key files, use
332 <literal>/dev/null
</literal> as the password file in the third
333 field.
</para></listitem>
337 <term><option>tcrypt-hidden
</option></term>
339 <listitem><para>Use the hidden TrueCrypt volume. This option
340 implies
<option>tcrypt
</option>.
</para>
342 <para>This will map the hidden volume that is inside of the
343 volume provided in the second field. Please note that there is
344 no protection for the hidden volume if the outer volume is
346 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
347 for more information on this limitation.
</para></listitem>
351 <term><option>tcrypt-keyfile=
</option></term>
353 <listitem><para>Specifies the absolute path to a key file to
354 use for a TrueCrypt volume. This implies
355 <option>tcrypt
</option> and can be used more than once to
356 provide several key files.
</para>
358 <para>See the entry for
<option>tcrypt
</option> on the
359 behavior of the passphrase and key files when using TrueCrypt
360 encryption mode.
</para></listitem>
364 <term><option>tcrypt-system
</option></term>
366 <listitem><para>Use TrueCrypt in system encryption mode. This
367 option implies
<option>tcrypt
</option>.
</para></listitem>
371 <term><option>tcrypt-veracrypt
</option></term>
373 <listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of
374 TrueCrypt that is mostly compatible, but uses different, stronger key
375 derivation algorithms that cannot be detected without this flag.
376 Enabling this option could substantially slow down unlocking, because
377 VeraCrypt's key derivation takes much longer than TrueCrypt's. This
378 option implies
<option>tcrypt
</option>.
</para></listitem>
382 <term><option>timeout=
</option></term>
384 <listitem><para>Specifies the timeout for querying for a
385 password. If no unit is specified, seconds is used. Supported
386 units are s, ms, us, min, h, d. A timeout of
0 waits
387 indefinitely (which is the default).
</para></listitem>
391 <term><option>tmp
</option></term>
393 <listitem><para>The encrypted block device will be prepared
394 for using it as
<filename>/tmp
</filename>; it will be
396 <citerefentry project='man-pages'
><refentrytitle>mke2fs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
397 This option implies
<option>plain
</option>.
</para>
399 <para>WARNING: Using the
<option>tmp
</option> option will
400 destroy the contents of the named partition during every boot,
401 so make sure the underlying block device is specified
402 correctly.
</para></listitem>
406 <term><option>tries=
</option></term>
408 <listitem><para>Specifies the maximum number of times the user
409 is queried for a password. The default is
3. If set to
0, the
410 user is queried for a password indefinitely.
</para></listitem>
414 <term><option>verify
</option></term>
416 <listitem><para> If the encryption password is read from
417 console, it has to be entered twice to prevent
418 typos.
</para></listitem>
422 <term><option>x-systemd.device-timeout=
</option></term>
424 <listitem><para>Specifies how long systemd should wait for a device to show up
425 before giving up on the entry. The argument is a time in seconds or explicitly
427 <literal>s
</literal>,
428 <literal>min
</literal>,
429 <literal>h
</literal>,
430 <literal>ms
</literal>.
436 <para>At early boot and when the system manager configuration is
437 reloaded, this file is translated into native systemd units by
438 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
442 <title>Example
</title>
444 <title>/etc/crypttab example
</title>
445 <para>Set up four encrypted block devices. One using LUKS for
446 normal storage, another one for usage as a swap device and two
447 TrueCrypt volumes.
</para>
449 <programlisting>luks UUID=
2505567a-
9e27-
4efe-a4d5-
15ad146c258b
450 swap /dev/sda7 /dev/urandom swap
451 truecrypt /dev/sda2 /etc/container_password tcrypt
452 hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
453 external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=
10s
</programlisting>
458 <title>See Also
</title>
460 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
461 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
462 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
463 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
464 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
465 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
466 <citerefentry project='man-pages'
><refentrytitle>mke2fs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>