man/crypttab.xml

   1 <?xml version="1.0"?>
   2 <!--*-nxml-*-->
   3 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!--
   5   SPDX-License-Identifier: LGPL-2.1+
   6
   7   This is based on crypttab(5) from Fedora's initscripts package, which in
   8   turn is based on Debian's version.
   9
  10   The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
  11 -->
  12 <refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP'>
  13
  14   <refentryinfo>
  15     <title>crypttab</title>
  16     <productname>systemd</productname>
  17   </refentryinfo>
  18
  19   <refmeta>
  20     <refentrytitle>crypttab</refentrytitle>
  21     <manvolnum>5</manvolnum>
  22   </refmeta>
  23
  24   <refnamediv>
  25     <refname>crypttab</refname>
  26     <refpurpose>Configuration for encrypted block devices</refpurpose>
  27   </refnamediv>
  28
  29   <refsynopsisdiv>
  30     <para><filename>/etc/crypttab</filename></para>
  31   </refsynopsisdiv>
  32
  33   <refsect1>
  34     <title>Description</title>
  35
  36     <para>The <filename>/etc/crypttab</filename> file describes
  37     encrypted block devices that are set up during system boot.</para>
  38
  39     <para>Empty lines and lines starting with the <literal>#</literal>
  40     character are ignored. Each of the remaining lines describes one
  41     encrypted block device. Fields are delimited by white space.</para>
  42
  43     <para>Each line is in the form<programlisting><replaceable>name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>password</replaceable> <replaceable>options</replaceable></programlisting>
  44     The first two fields are mandatory, the remaining two are
  45     optional.</para>
  46
  47     <para>Setting up encrypted block devices using this file supports
  48     three encryption modes: LUKS, TrueCrypt and plain. See
  49     <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  50     for more information about each mode. When no mode is specified in
  51     the options field and the block device contains a LUKS signature,
  52     it is opened as a LUKS device; otherwise, it is assumed to be in
  53     raw dm-crypt (plain mode) format.</para>
  54
  55     <para>The first field contains the name of the resulting encrypted
  56     block device; the device is set up within
  57     <filename>/dev/mapper/</filename>.</para>
  58
  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>
  62
  63     <para>The third field specifies the encryption password. If the
  64     field is not present or the password is set to
  65     <literal>none</literal> or <literal>-</literal>, the password has
  66     to be manually entered during system boot. Otherwise, the field is
  67     interpreted as an absolute path to a file containing the encryption
  68     password. For swap encryption, <filename>/dev/urandom</filename>
  69     or the hardware device <filename>/dev/hw_random</filename> can be
  70     used as the password file; using <filename>/dev/random</filename>
  71     may prevent boot completion if the system does not have enough
  72     entropy to generate a truly random encryption key.</para>
  73
  74     <para>The fourth field, if present, is a comma-delimited list of
  75     options. The following options are recognized:</para>
  76
  77     <variablelist class='fstab-options'>
  78
  79       <varlistentry>
  80         <term><option>cipher=</option></term>
  81
  82         <listitem><para>Specifies the cipher to use. See
  83         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  84         for possible values and the default value of this option. A
  85         cipher with unpredictable IV values, such as
  86         <literal>aes-cbc-essiv:sha256</literal>, is
  87         recommended.</para></listitem>
  88       </varlistentry>
  89
  90       <varlistentry>
  91         <term><option>discard</option></term>
  92
  93         <listitem><para>Allow discard requests to be passed through the encrypted block
  94         device. This improves performance on SSD storage but has security implications.
  95         </para></listitem>
  96       </varlistentry>
  97
  98       <varlistentry>
  99         <term><option>hash=</option></term>
 100
 101         <listitem><para>Specifies the hash to use for password
 102         hashing. See
 103         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 104         for possible values and the default value of this
 105         option.</para></listitem>
 106       </varlistentry>
 107
 108       <varlistentry>
 109         <term><option>header=</option></term>
 110
 111         <listitem><para>Use a detached (separated) metadata device or
 112         file where the LUKS header is stored. This option is only
 113         relevant for LUKS devices. See
 114         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 115         for possible values and the default value of this
 116         option.</para></listitem>
 117       </varlistentry>
 118
 119       <varlistentry>
 120         <term><option>keyfile-offset=</option></term>
 121
 122         <listitem><para>Specifies the number of bytes to skip at the
 123         start of the key file. See
 124         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 125         for possible values and the default value of this
 126         option.</para></listitem>
 127       </varlistentry>
 128
 129       <varlistentry>
 130         <term><option>keyfile-size=</option></term>
 131
 132         <listitem><para>Specifies the maximum number of bytes to read
 133         from the key file. See
 134         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 135         for possible values and the default value of this option. This
 136         option is ignored in plain encryption mode, as the key file
 137         size is then given by the key size.</para></listitem>
 138       </varlistentry>
 139
 140       <varlistentry>
 141         <term><option>key-slot=</option></term>
 142
 143         <listitem><para>Specifies the key slot to compare the
 144         passphrase or key against. If the key slot does not match the
 145         given passphrase or key, but another would, the setup of the
 146         device will fail regardless. This option implies
 147         <option>luks</option>. See
 148         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 149         for possible values. The default is to try all key slots in
 150         sequential order.</para></listitem>
 151       </varlistentry>
 152
 153       <varlistentry>
 154         <term><option>luks</option></term>
 155
 156         <listitem><para>Force LUKS mode. When this mode is used, the
 157         following options are ignored since they are provided by the
 158         LUKS header on the device: <option>cipher=</option>,
 159         <option>hash=</option>,
 160         <option>size=</option>.</para></listitem>
 161       </varlistentry>
 162
 163       <varlistentry>
 164         <term><option>_netdev</option></term>
 165
 166         <listitem><para>Marks this cryptsetup device as requiring network. It will be
 167         started after the network is available, similarly to
 168         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 169         units marked with <option>_netdev</option>. The service unit to set up this device
 170         will be ordered between <filename>remote-fs-pre.target</filename> and
 171         <filename>remote-cryptsetup.target</filename>, instead of
 172         <filename>cryptsetup-pre.target</filename> and
 173         <filename>cryptsetup.target</filename>.</para>
 174
 175         <para>Hint: if this device is used for a mount point that is specified in
 176         <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 177         the <option>_netdev</option> option should also be used for the mount
 178         point. Otherwise, a dependency loop might be created where the mount point
 179         will be pulled in by <filename>local-fs.target</filename>, while the
 180         service to configure the network is usually only started <emphasis>after</emphasis>
 181         the local file system has been mounted.</para>
 182         </listitem>
 183       </varlistentry>
 184
 185       <varlistentry>
 186         <term><option>noauto</option></term>
 187
 188         <listitem><para>This device will not be added to <filename>cryptsetup.target</filename>.
 189         This means that it will not be automatically unlocked on boot, unless something else pulls
 190         it in. In particular, if the device is used for a mount point, it'll be unlocked
 191         automatically during boot, unless the mount point itself is also disabled with
 192         <option>noauto</option>.</para></listitem>
 193       </varlistentry>
 194
 195       <varlistentry>
 196         <term><option>nofail</option></term>
 197
 198         <listitem><para>This device will not be a hard dependency of
 199         <filename>cryptsetup.target</filename>. It'll be still pulled in and started, but the system
 200         will not wait for the device to show up and be unlocked, and boot will not fail if this is
 201         unsuccessful. Note that other units that depend on the unlocked device may still fail. In
 202         particular, if the device is used for a mount point, the mount point itself is also needs to
 203         have <option>noauto</option> option, or the boot will fail if the device is not unlocked
 204         successfully.</para></listitem>
 205       </varlistentry>
 206
 207       <varlistentry>
 208         <term><option>offset=</option></term>
 209
 210         <listitem><para>Start offset in the backend device, in 512-byte sectors. This
 211         option is only relevant for plain devices.</para></listitem>
 212       </varlistentry>
 213
 214       <varlistentry>
 215         <term><option>plain</option></term>
 216
 217         <listitem><para>Force plain encryption mode.</para></listitem>
 218       </varlistentry>
 219
 220       <varlistentry>
 221         <term><option>read-only</option></term><term><option>readonly</option></term>
 222
 223         <listitem><para>Set up the encrypted block device in read-only
 224         mode.</para></listitem>
 225       </varlistentry>
 226
 227       <varlistentry>
 228         <term><option>skip=</option></term>
 229
 230         <listitem><para>How many 512-byte sectors of the encrypted data to skip at the
 231         beginning. This is different from the <option>offset=</option> option with respect
 232         to the sector numbers used in initialization vector (IV) calculation. Using
 233         <option>offset=</option> will shift the IV calculation by the same negative
 234         amount. Hence, if <option>offset=<replaceable>n</replaceable></option> is given,
 235         sector <replaceable>n</replaceable> will get a sector number of 0 for the IV
 236         calculation. Using <option>skip=</option> causes sector
 237         <replaceable>n</replaceable> to also be the first sector of the mapped device, but
 238         with its number for IV generation being <replaceable>n</replaceable>.</para>
 239
 240         <para>This option is only relevant for plain devices.</para>
 241         </listitem>
 242       </varlistentry>
 243
 244       <varlistentry>
 245         <term><option>size=</option></term>
 246
 247         <listitem><para>Specifies the key size in bits. See
 248         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 249         for possible values and the default value of this
 250         option.</para></listitem>
 251       </varlistentry>
 252
 253       <varlistentry>
 254         <term><option>swap</option></term>
 255
 256         <listitem><para>The encrypted block device will be used as a
 257         swap device, and will be formatted accordingly after setting
 258         up the encrypted block device, with
 259         <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
 260         This option implies <option>plain</option>.</para>
 261
 262         <para>WARNING: Using the <option>swap</option> option will
 263         destroy the contents of the named partition during every boot,
 264         so make sure the underlying block device is specified
 265         correctly.</para></listitem>
 266       </varlistentry>
 267
 268       <varlistentry>
 269         <term><option>tcrypt</option></term>
 270
 271         <listitem><para>Use TrueCrypt encryption mode. When this mode
 272         is used, the following options are ignored since they are
 273         provided by the TrueCrypt header on the device or do not
 274         apply:
 275         <option>cipher=</option>,
 276         <option>hash=</option>,
 277         <option>keyfile-offset=</option>,
 278         <option>keyfile-size=</option>,
 279         <option>size=</option>.</para>
 280
 281         <para>When this mode is used, the passphrase is read from the
 282         key file given in the third field. Only the first line of this
 283         file is read, excluding the new line character.</para>
 284
 285         <para>Note that the TrueCrypt format uses both passphrase and
 286         key files to derive a password for the volume. Therefore, the
 287         passphrase and all key files need to be provided. Use
 288         <option>tcrypt-keyfile=</option> to provide the absolute path
 289         to all key files. When using an empty passphrase in
 290         combination with one or more key files, use
 291         <literal>/dev/null</literal> as the password file in the third
 292         field.</para></listitem>
 293       </varlistentry>
 294
 295       <varlistentry>
 296         <term><option>tcrypt-hidden</option></term>
 297
 298         <listitem><para>Use the hidden TrueCrypt volume. This option
 299         implies <option>tcrypt</option>.</para>
 300
 301         <para>This will map the hidden volume that is inside of the
 302         volume provided in the second field. Please note that there is
 303         no protection for the hidden volume if the outer volume is
 304         mounted instead. See
 305         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 306         for more information on this limitation.</para></listitem>
 307       </varlistentry>
 308
 309       <varlistentry>
 310         <term><option>tcrypt-keyfile=</option></term>
 311
 312         <listitem><para>Specifies the absolute path to a key file to
 313         use for a TrueCrypt volume. This implies
 314         <option>tcrypt</option> and can be used more than once to
 315         provide several key files.</para>
 316
 317         <para>See the entry for <option>tcrypt</option> on the
 318         behavior of the passphrase and key files when using TrueCrypt
 319         encryption mode.</para></listitem>
 320       </varlistentry>
 321
 322       <varlistentry>
 323         <term><option>tcrypt-system</option></term>
 324
 325         <listitem><para>Use TrueCrypt in system encryption mode. This
 326         option implies <option>tcrypt</option>.</para></listitem>
 327       </varlistentry>
 328
 329       <varlistentry>
 330         <term><option>tcrypt-veracrypt</option></term>
 331
 332         <listitem><para>Check for a VeraCrypt volume.  VeraCrypt is a fork of
 333         TrueCrypt that is mostly compatible, but uses different, stronger key
 334         derivation algorithms that cannot be detected without this flag.
 335         Enabling this option could substantially slow down unlocking, because
 336         VeraCrypt's key derivation takes much longer than TrueCrypt's.  This
 337         option implies <option>tcrypt</option>.</para></listitem>
 338       </varlistentry>
 339
 340       <varlistentry>
 341         <term><option>timeout=</option></term>
 342
 343         <listitem><para>Specifies the timeout for querying for a
 344         password. If no unit is specified, seconds is used. Supported
 345         units are s, ms, us, min, h, d. A timeout of 0 waits
 346         indefinitely (which is the default).</para></listitem>
 347       </varlistentry>
 348
 349       <varlistentry>
 350         <term><option>tmp</option></term>
 351
 352         <listitem><para>The encrypted block device will be prepared
 353         for using it as <filename>/tmp</filename>; it will be
 354         formatted using
 355         <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
 356         This option implies <option>plain</option>.</para>
 357
 358         <para>WARNING: Using the <option>tmp</option> option will
 359         destroy the contents of the named partition during every boot,
 360         so make sure the underlying block device is specified
 361         correctly.</para></listitem>
 362       </varlistentry>
 363
 364       <varlistentry>
 365         <term><option>tries=</option></term>
 366
 367         <listitem><para>Specifies the maximum number of times the user
 368         is queried for a password. The default is 3. If set to 0, the
 369         user is queried for a password indefinitely.</para></listitem>
 370       </varlistentry>
 371
 372       <varlistentry>
 373         <term><option>verify</option></term>
 374
 375         <listitem><para> If the encryption password is read from
 376         console, it has to be entered twice to prevent
 377         typos.</para></listitem>
 378       </varlistentry>
 379
 380       <varlistentry>
 381         <term><option>x-systemd.device-timeout=</option></term>
 382
 383         <listitem><para>Specifies how long systemd should wait for a device to show up
 384         before giving up on the entry. The argument is a time in seconds or explicitly
 385         specified units of
 386         <literal>s</literal>,
 387         <literal>min</literal>,
 388         <literal>h</literal>,
 389         <literal>ms</literal>.
 390         </para></listitem>
 391       </varlistentry>
 392
 393     </variablelist>
 394
 395     <para>At early boot and when the system manager configuration is
 396     reloaded, this file is translated into native systemd units by
 397     <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
 398   </refsect1>
 399
 400   <refsect1>
 401     <title>Example</title>
 402     <example>
 403       <title>/etc/crypttab example</title>
 404       <para>Set up four encrypted block devices. One using LUKS for
 405       normal storage, another one for usage as a swap device and two
 406       TrueCrypt volumes.</para>
 407
 408       <programlisting>luks       UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
 409 swap       /dev/sda7       /dev/urandom       swap
 410 truecrypt  /dev/sda2       /etc/container_password  tcrypt
 411 hidden     /mnt/tc_hidden  /dev/null    tcrypt-hidden,tcrypt-keyfile=/etc/keyfile</programlisting>
 412     </example>
 413   </refsect1>
 414
 415   <refsect1>
 416     <title>See Also</title>
 417     <para>
 418       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 419       <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 420       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 421       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 422       <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 423       <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 424       <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 425     </para>
 426   </refsect1>
 427
 428 </refentry>