3 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
5 SPDX-License-Identifier: LGPL-2.1+
7 This is based on crypttab(5) from Fedora's initscripts package, which in
8 turn is based on Debian's version.
10 The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
12 <refentry id=
"crypttab" conditional='HAVE_LIBCRYPTSETUP'
>
15 <title>crypttab
</title>
16 <productname>systemd
</productname>
20 <contrib>Documentation
</contrib>
21 <firstname>Miloslav
</firstname>
22 <surname>Trmac
</surname>
23 <email>mitr@redhat.com
</email>
26 <contrib>Documentation
</contrib>
27 <firstname>Lennart
</firstname>
28 <surname>Poettering
</surname>
29 <email>lennart@poettering.net
</email>
35 <refentrytitle>crypttab
</refentrytitle>
36 <manvolnum>5</manvolnum>
40 <refname>crypttab
</refname>
41 <refpurpose>Configuration for encrypted block devices
</refpurpose>
45 <para><filename>/etc/crypttab
</filename></para>
49 <title>Description
</title>
51 <para>The
<filename>/etc/crypttab
</filename> file describes
52 encrypted block devices that are set up during system boot.
</para>
54 <para>Empty lines and lines starting with the
<literal>#
</literal>
55 character are ignored. Each of the remaining lines describes one
56 encrypted block device. Fields are delimited by white space.
</para>
58 <para>Each line is in the form
<programlisting><replaceable>name
</replaceable> <replaceable>encrypted-device
</replaceable> <replaceable>password
</replaceable> <replaceable>options
</replaceable></programlisting>
59 The first two fields are mandatory, the remaining two are
62 <para>Setting up encrypted block devices using this file supports
63 three encryption modes: LUKS, TrueCrypt and plain. See
64 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
65 for more information about each mode. When no mode is specified in
66 the options field and the block device contains a LUKS signature,
67 it is opened as a LUKS device; otherwise, it is assumed to be in
68 raw dm-crypt (plain mode) format.
</para>
70 <para>The first field contains the name of the resulting encrypted
71 block device; the device is set up within
72 <filename>/dev/mapper/
</filename>.
</para>
74 <para>The second field contains a path to the underlying block
75 device or file, or a specification of a block device via
76 <literal>UUID=
</literal> followed by the UUID.
</para>
78 <para>The third field specifies the encryption password. If the
79 field is not present or the password is set to
80 <literal>none
</literal> or
<literal>-
</literal>, the password has
81 to be manually entered during system boot. Otherwise, the field is
82 interpreted as an absolute path to a file containing the encryption
83 password. For swap encryption,
<filename>/dev/urandom
</filename>
84 or the hardware device
<filename>/dev/hw_random
</filename> can be
85 used as the password file; using
<filename>/dev/random
</filename>
86 may prevent boot completion if the system does not have enough
87 entropy to generate a truly random encryption key.
</para>
89 <para>The fourth field, if present, is a comma-delimited list of
90 options. The following options are recognized:
</para>
92 <variablelist class='fstab-options'
>
95 <term><option>cipher=
</option></term>
97 <listitem><para>Specifies the cipher to use. See
98 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
99 for possible values and the default value of this option. A
100 cipher with unpredictable IV values, such as
101 <literal>aes-cbc-essiv:sha256
</literal>, is
102 recommended.
</para></listitem>
106 <term><option>discard
</option></term>
108 <listitem><para>Allow discard requests to be passed through the encrypted block
109 device. This improves performance on SSD storage but has security implications.
114 <term><option>hash=
</option></term>
116 <listitem><para>Specifies the hash to use for password
118 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
119 for possible values and the default value of this
120 option.
</para></listitem>
124 <term><option>header=
</option></term>
126 <listitem><para>Use a detached (separated) metadata device or
127 file where the LUKS header is stored. This option is only
128 relevant for LUKS devices. See
129 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
130 for possible values and the default value of this
131 option.
</para></listitem>
135 <term><option>keyfile-offset=
</option></term>
137 <listitem><para>Specifies the number of bytes to skip at the
138 start of the key file. See
139 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
140 for possible values and the default value of this
141 option.
</para></listitem>
145 <term><option>keyfile-size=
</option></term>
147 <listitem><para>Specifies the maximum number of bytes to read
148 from the key file. See
149 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
150 for possible values and the default value of this option. This
151 option is ignored in plain encryption mode, as the key file
152 size is then given by the key size.
</para></listitem>
156 <term><option>key-slot=
</option></term>
158 <listitem><para>Specifies the key slot to compare the
159 passphrase or key against. If the key slot does not match the
160 given passphrase or key, but another would, the setup of the
161 device will fail regardless. This option implies
162 <option>luks
</option>. See
163 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
164 for possible values. The default is to try all key slots in
165 sequential order.
</para></listitem>
169 <term><option>luks
</option></term>
171 <listitem><para>Force LUKS mode. When this mode is used, the
172 following options are ignored since they are provided by the
173 LUKS header on the device:
<option>cipher=
</option>,
174 <option>hash=
</option>,
175 <option>size=
</option>.
</para></listitem>
179 <term><option>_netdev
</option></term>
181 <listitem><para>Marks this cryptsetup device as requiring network. It will be
182 started after the network is available, similarly to
183 <citerefentry><refentrytitle>systemd.mount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
184 units marked with
<option>_netdev
</option>. The service unit to set up this device
185 will be ordered between
<filename>remote-fs-pre.target
</filename> and
186 <filename>remote-cryptsetup.target
</filename>, instead of
187 <filename>cryptsetup-pre.target
</filename> and
188 <filename>cryptsetup.target
</filename>.
</para>
190 <para>Hint: if this device is used for a mount point that is specified in
191 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
192 the
<option>_netdev
</option> option should also be used for the mount
193 point. Otherwise, a dependency loop might be created where the mount point
194 will be pulled in by
<filename>local-fs.target
</filename>, while the
195 service to configure the network is usually only started
<emphasis>after
</emphasis>
196 the local file system has been mounted.
</para>
201 <term><option>noauto
</option></term>
203 <listitem><para>This device will not be added to
<filename>cryptsetup.target
</filename>.
204 This means that it will not be automatically unlocked on boot, unless something else pulls
205 it in. In particular, if the device is used for a mount point, it'll be unlocked
206 automatically during boot, unless the mount point itself is also disabled with
207 <option>noauto
</option>.
</para></listitem>
211 <term><option>nofail
</option></term>
213 <listitem><para>This device will not be a hard dependency of
214 <filename>cryptsetup.target
</filename>. It'll be still pulled in and started, but the system
215 will not wait for the device to show up and be unlocked, and boot will not fail if this is
216 unsuccessful. Note that other units that depend on the unlocked device may still fail. In
217 particular, if the device is used for a mount point, the mount point itself is also needs to
218 have
<option>noauto
</option> option, or the boot will fail if the device is not unlocked
219 successfully.
</para></listitem>
223 <term><option>offset=
</option></term>
225 <listitem><para>Start offset in the backend device, in
512-byte sectors. This
226 option is only relevant for plain devices.
</para></listitem>
230 <term><option>plain
</option></term>
232 <listitem><para>Force plain encryption mode.
</para></listitem>
236 <term><option>read-only
</option></term><term><option>readonly
</option></term>
238 <listitem><para>Set up the encrypted block device in read-only
239 mode.
</para></listitem>
243 <term><option>skip=
</option></term>
245 <listitem><para>How many
512-byte sectors of the encrypted data to skip at the
246 beginning. This is different from the
<option>offset=
</option> option with respect
247 to the sector numbers used in initialization vector (IV) calculation. Using
248 <option>offset=
</option> will shift the IV calculation by the same negative
249 amount. Hence, if
<option>offset=
<replaceable>n
</replaceable></option> is given,
250 sector
<replaceable>n
</replaceable> will get a sector number of
0 for the IV
251 calculation. Using
<option>skip=
</option> causes sector
252 <replaceable>n
</replaceable> to also be the first sector of the mapped device, but
253 with its number for IV generation being
<replaceable>n
</replaceable>.
</para>
255 <para>This option is only relevant for plain devices.
</para>
260 <term><option>size=
</option></term>
262 <listitem><para>Specifies the key size in bits. See
263 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
264 for possible values and the default value of this
265 option.
</para></listitem>
269 <term><option>swap
</option></term>
271 <listitem><para>The encrypted block device will be used as a
272 swap device, and will be formatted accordingly after setting
273 up the encrypted block device, with
274 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
275 This option implies
<option>plain
</option>.
</para>
277 <para>WARNING: Using the
<option>swap
</option> option will
278 destroy the contents of the named partition during every boot,
279 so make sure the underlying block device is specified
280 correctly.
</para></listitem>
284 <term><option>tcrypt
</option></term>
286 <listitem><para>Use TrueCrypt encryption mode. When this mode
287 is used, the following options are ignored since they are
288 provided by the TrueCrypt header on the device or do not
290 <option>cipher=
</option>,
291 <option>hash=
</option>,
292 <option>keyfile-offset=
</option>,
293 <option>keyfile-size=
</option>,
294 <option>size=
</option>.
</para>
296 <para>When this mode is used, the passphrase is read from the
297 key file given in the third field. Only the first line of this
298 file is read, excluding the new line character.
</para>
300 <para>Note that the TrueCrypt format uses both passphrase and
301 key files to derive a password for the volume. Therefore, the
302 passphrase and all key files need to be provided. Use
303 <option>tcrypt-keyfile=
</option> to provide the absolute path
304 to all key files. When using an empty passphrase in
305 combination with one or more key files, use
306 <literal>/dev/null
</literal> as the password file in the third
307 field.
</para></listitem>
311 <term><option>tcrypt-hidden
</option></term>
313 <listitem><para>Use the hidden TrueCrypt volume. This option
314 implies
<option>tcrypt
</option>.
</para>
316 <para>This will map the hidden volume that is inside of the
317 volume provided in the second field. Please note that there is
318 no protection for the hidden volume if the outer volume is
320 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
321 for more information on this limitation.
</para></listitem>
325 <term><option>tcrypt-keyfile=
</option></term>
327 <listitem><para>Specifies the absolute path to a key file to
328 use for a TrueCrypt volume. This implies
329 <option>tcrypt
</option> and can be used more than once to
330 provide several key files.
</para>
332 <para>See the entry for
<option>tcrypt
</option> on the
333 behavior of the passphrase and key files when using TrueCrypt
334 encryption mode.
</para></listitem>
338 <term><option>tcrypt-system
</option></term>
340 <listitem><para>Use TrueCrypt in system encryption mode. This
341 option implies
<option>tcrypt
</option>.
</para></listitem>
345 <term><option>tcrypt-veracrypt
</option></term>
347 <listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of
348 TrueCrypt that is mostly compatible, but uses different, stronger key
349 derivation algorithms that cannot be detected without this flag.
350 Enabling this option could substantially slow down unlocking, because
351 VeraCrypt's key derivation takes much longer than TrueCrypt's. This
352 option implies
<option>tcrypt
</option>.
</para></listitem>
356 <term><option>timeout=
</option></term>
358 <listitem><para>Specifies the timeout for querying for a
359 password. If no unit is specified, seconds is used. Supported
360 units are s, ms, us, min, h, d. A timeout of
0 waits
361 indefinitely (which is the default).
</para></listitem>
365 <term><option>tmp
</option></term>
367 <listitem><para>The encrypted block device will be prepared
368 for using it as
<filename>/tmp
</filename>; it will be
370 <citerefentry project='man-pages'
><refentrytitle>mke2fs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
371 This option implies
<option>plain
</option>.
</para>
373 <para>WARNING: Using the
<option>tmp
</option> option will
374 destroy the contents of the named partition during every boot,
375 so make sure the underlying block device is specified
376 correctly.
</para></listitem>
380 <term><option>tries=
</option></term>
382 <listitem><para>Specifies the maximum number of times the user
383 is queried for a password. The default is
3. If set to
0, the
384 user is queried for a password indefinitely.
</para></listitem>
388 <term><option>verify
</option></term>
390 <listitem><para> If the encryption password is read from
391 console, it has to be entered twice to prevent
392 typos.
</para></listitem>
396 <term><option>x-systemd.device-timeout=
</option></term>
398 <listitem><para>Specifies how long systemd should wait for a device to show up
399 before giving up on the entry. The argument is a time in seconds or explicitly
401 <literal>s
</literal>,
402 <literal>min
</literal>,
403 <literal>h
</literal>,
404 <literal>ms
</literal>.
410 <para>At early boot and when the system manager configuration is
411 reloaded, this file is translated into native systemd units by
412 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
416 <title>Example
</title>
418 <title>/etc/crypttab example
</title>
419 <para>Set up four encrypted block devices. One using LUKS for
420 normal storage, another one for usage as a swap device and two
421 TrueCrypt volumes.
</para>
423 <programlisting>luks UUID=
2505567a-
9e27-
4efe-a4d5-
15ad146c258b
424 swap /dev/sda7 /dev/urandom swap
425 truecrypt /dev/sda2 /etc/container_password tcrypt
426 hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
</programlisting>
431 <title>See Also
</title>
433 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
434 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
435 <citerefentry><refentrytitle>systemd-cryptsetup-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
436 <citerefentry project='man-pages'
><refentrytitle>fstab
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
437 <citerefentry project='die-net'
><refentrytitle>cryptsetup
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
438 <citerefentry project='man-pages'
><refentrytitle>mkswap
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
439 <citerefentry project='man-pages'
><refentrytitle>mke2fs
</refentrytitle><manvolnum>8</manvolnum></citerefentry>