man/crypttab.xml

   1 <?xml version="1.0"?>
   2 <!--*-nxml-*-->
   3 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   4   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This is based on crypttab(5) from Fedora's initscripts package, which in
   9   turn is based on Debian's version.
  10
  11   The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
  12 -->
  13 <refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP'>
  14
  15   <refentryinfo>
  16     <title>crypttab</title>
  17     <productname>systemd</productname>
  18   </refentryinfo>
  19
  20   <refmeta>
  21     <refentrytitle>crypttab</refentrytitle>
  22     <manvolnum>5</manvolnum>
  23   </refmeta>
  24
  25   <refnamediv>
  26     <refname>crypttab</refname>
  27     <refpurpose>Configuration for encrypted block devices</refpurpose>
  28   </refnamediv>
  29
  30   <refsynopsisdiv>
  31     <para><filename>/etc/crypttab</filename></para>
  32   </refsynopsisdiv>
  33
  34   <refsect1>
  35     <title>Description</title>
  36
  37     <para>The <filename>/etc/crypttab</filename> file describes
  38     encrypted block devices that are set up during system boot.</para>
  39
  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>
  43
  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
  46     optional.</para>
  47
  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>
  55
  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>
  59
  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>
  63
  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>
  74
  75     <para>The fourth field, if present, is a comma-delimited list of
  76     options. The following options are recognized:</para>
  77
  78     <variablelist class='fstab-options'>
  79
  80       <varlistentry>
  81         <term><option>cipher=</option></term>
  82
  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>
  89       </varlistentry>
  90
  91       <varlistentry>
  92         <term><option>discard</option></term>
  93
  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.
  96         </para></listitem>
  97       </varlistentry>
  98
  99       <varlistentry>
 100         <term><option>hash=</option></term>
 101
 102         <listitem><para>Specifies the hash to use for password
 103         hashing. See
 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>
 107       </varlistentry>
 108
 109       <varlistentry>
 110         <term><option>header=</option></term>
 111
 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>
 118       </varlistentry>
 119
 120       <varlistentry>
 121         <term><option>keyfile-offset=</option></term>
 122
 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>
 128       </varlistentry>
 129
 130       <varlistentry>
 131         <term><option>keyfile-size=</option></term>
 132
 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>
 139       </varlistentry>
 140
 141       <varlistentry>
 142         <term><option>key-slot=</option></term>
 143
 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>
 152       </varlistentry>
 153
 154       <varlistentry>
 155         <term><option>luks</option></term>
 156
 157         <listitem><para>Force LUKS mode. When this mode is used, the
 158         following options are ignored since they are provided by the
 159         LUKS header on the device: <option>cipher=</option>,
 160         <option>hash=</option>,
 161         <option>size=</option>.</para></listitem>
 162       </varlistentry>
 163
 164       <varlistentry>
 165         <term><option>_netdev</option></term>
 166
 167         <listitem><para>Marks this cryptsetup device as requiring network. It will be
 168         started after the network is available, similarly to
 169         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 170         units marked with <option>_netdev</option>. The service unit to set up this device
 171         will be ordered between <filename>remote-fs-pre.target</filename> and
 172         <filename>remote-cryptsetup.target</filename>, instead of
 173         <filename>cryptsetup-pre.target</filename> and
 174         <filename>cryptsetup.target</filename>.</para>
 175
 176         <para>Hint: if this device is used for a mount point that is specified in
 177         <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 178         the <option>_netdev</option> option should also be used for the mount
 179         point. Otherwise, a dependency loop might be created where the mount point
 180         will be pulled in by <filename>local-fs.target</filename>, while the
 181         service to configure the network is usually only started <emphasis>after</emphasis>
 182         the local file system has been mounted.</para>
 183         </listitem>
 184       </varlistentry>
 185
 186       <varlistentry>
 187         <term><option>noauto</option></term>
 188
 189         <listitem><para>This device will not be added to <filename>cryptsetup.target</filename>.
 190         This means that it will not be automatically unlocked on boot, unless something else pulls
 191         it in. In particular, if the device is used for a mount point, it'll be unlocked
 192         automatically during boot, unless the mount point itself is also disabled with
 193         <option>noauto</option>.</para></listitem>
 194       </varlistentry>
 195
 196       <varlistentry>
 197         <term><option>nofail</option></term>
 198
 199         <listitem><para>This device will not be a hard dependency of
 200         <filename>cryptsetup.target</filename>. It'll be still pulled in and started, but the system
 201         will not wait for the device to show up and be unlocked, and boot will not fail if this is
 202         unsuccessful. Note that other units that depend on the unlocked device may still fail. In
 203         particular, if the device is used for a mount point, the mount point itself is also needs to
 204         have <option>noauto</option> option, or the boot will fail if the device is not unlocked
 205         successfully.</para></listitem>
 206       </varlistentry>
 207
 208       <varlistentry>
 209         <term><option>offset=</option></term>
 210
 211         <listitem><para>Start offset in the backend device, in 512-byte sectors. This
 212         option is only relevant for plain devices.</para></listitem>
 213       </varlistentry>
 214
 215       <varlistentry>
 216         <term><option>plain</option></term>
 217
 218         <listitem><para>Force plain encryption mode.</para></listitem>
 219       </varlistentry>
 220
 221       <varlistentry>
 222         <term><option>read-only</option></term><term><option>readonly</option></term>
 223
 224         <listitem><para>Set up the encrypted block device in read-only
 225         mode.</para></listitem>
 226       </varlistentry>
 227
 228       <varlistentry>
 229         <term><option>same-cpu-crypt</option></term>
 230
 231         <listitem><para>Perform encryption using the same cpu that IO was submitted on. The default is to use
 232         an unbound workqueue so that encryption work is automatically balanced between available CPUs.</para>
 233         <para>This requires kernel 4.0 or newer.</para>
 234         </listitem>
 235       </varlistentry>
 236
 237       <varlistentry>
 238         <term><option>submit-from-crypt-cpus</option></term>
 239
 240         <listitem><para>Disable offloading writes to a separate thread after encryption. There are some
 241         situations where offloading write bios from the encryption threads to a single thread degrades
 242         performance significantly. The default is to offload write bios to the same thread because it benefits
 243         CFQ to have writes submitted using the same context.</para>
 244         <para>This requires kernel 4.0 or newer.</para>
 245         </listitem>
 246       </varlistentry>
 247
 248       <varlistentry>
 249         <term><option>skip=</option></term>
 250
 251         <listitem><para>How many 512-byte sectors of the encrypted data to skip at the
 252         beginning. This is different from the <option>offset=</option> option with respect
 253         to the sector numbers used in initialization vector (IV) calculation. Using
 254         <option>offset=</option> will shift the IV calculation by the same negative
 255         amount. Hence, if <option>offset=<replaceable>n</replaceable></option> is given,
 256         sector <replaceable>n</replaceable> will get a sector number of 0 for the IV
 257         calculation. Using <option>skip=</option> causes sector
 258         <replaceable>n</replaceable> to also be the first sector of the mapped device, but
 259         with its number for IV generation being <replaceable>n</replaceable>.</para>
 260
 261         <para>This option is only relevant for plain devices.</para>
 262         </listitem>
 263       </varlistentry>
 264
 265       <varlistentry>
 266         <term><option>size=</option></term>
 267
 268         <listitem><para>Specifies the key size in bits. See
 269         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 270         for possible values and the default value of this
 271         option.</para></listitem>
 272       </varlistentry>
 273
 274       <varlistentry>
 275         <term><option>sector-size=</option></term>
 276
 277         <listitem><para>Specifies the sector size in bytes. See
 278         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 279         for possible values and the default value of this
 280         option.</para></listitem>
 281       </varlistentry>
 282
 283       <varlistentry>
 284         <term><option>swap</option></term>
 285
 286         <listitem><para>The encrypted block device will be used as a
 287         swap device, and will be formatted accordingly after setting
 288         up the encrypted block device, with
 289         <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
 290         This option implies <option>plain</option>.</para>
 291
 292         <para>WARNING: Using the <option>swap</option> option will
 293         destroy the contents of the named partition during every boot,
 294         so make sure the underlying block device is specified
 295         correctly.</para></listitem>
 296       </varlistentry>
 297
 298       <varlistentry>
 299         <term><option>tcrypt</option></term>
 300
 301         <listitem><para>Use TrueCrypt encryption mode. When this mode
 302         is used, the following options are ignored since they are
 303         provided by the TrueCrypt header on the device or do not
 304         apply:
 305         <option>cipher=</option>,
 306         <option>hash=</option>,
 307         <option>keyfile-offset=</option>,
 308         <option>keyfile-size=</option>,
 309         <option>size=</option>.</para>
 310
 311         <para>When this mode is used, the passphrase is read from the
 312         key file given in the third field. Only the first line of this
 313         file is read, excluding the new line character.</para>
 314
 315         <para>Note that the TrueCrypt format uses both passphrase and
 316         key files to derive a password for the volume. Therefore, the
 317         passphrase and all key files need to be provided. Use
 318         <option>tcrypt-keyfile=</option> to provide the absolute path
 319         to all key files. When using an empty passphrase in
 320         combination with one or more key files, use
 321         <literal>/dev/null</literal> as the password file in the third
 322         field.</para></listitem>
 323       </varlistentry>
 324
 325       <varlistentry>
 326         <term><option>tcrypt-hidden</option></term>
 327
 328         <listitem><para>Use the hidden TrueCrypt volume. This option
 329         implies <option>tcrypt</option>.</para>
 330
 331         <para>This will map the hidden volume that is inside of the
 332         volume provided in the second field. Please note that there is
 333         no protection for the hidden volume if the outer volume is
 334         mounted instead. See
 335         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 336         for more information on this limitation.</para></listitem>
 337       </varlistentry>
 338
 339       <varlistentry>
 340         <term><option>tcrypt-keyfile=</option></term>
 341
 342         <listitem><para>Specifies the absolute path to a key file to
 343         use for a TrueCrypt volume. This implies
 344         <option>tcrypt</option> and can be used more than once to
 345         provide several key files.</para>
 346
 347         <para>See the entry for <option>tcrypt</option> on the
 348         behavior of the passphrase and key files when using TrueCrypt
 349         encryption mode.</para></listitem>
 350       </varlistentry>
 351
 352       <varlistentry>
 353         <term><option>tcrypt-system</option></term>
 354
 355         <listitem><para>Use TrueCrypt in system encryption mode. This
 356         option implies <option>tcrypt</option>.</para></listitem>
 357       </varlistentry>
 358
 359       <varlistentry>
 360         <term><option>tcrypt-veracrypt</option></term>
 361
 362         <listitem><para>Check for a VeraCrypt volume.  VeraCrypt is a fork of
 363         TrueCrypt that is mostly compatible, but uses different, stronger key
 364         derivation algorithms that cannot be detected without this flag.
 365         Enabling this option could substantially slow down unlocking, because
 366         VeraCrypt's key derivation takes much longer than TrueCrypt's.  This
 367         option implies <option>tcrypt</option>.</para></listitem>
 368       </varlistentry>
 369
 370       <varlistentry>
 371         <term><option>timeout=</option></term>
 372
 373         <listitem><para>Specifies the timeout for querying for a
 374         password. If no unit is specified, seconds is used. Supported
 375         units are s, ms, us, min, h, d. A timeout of 0 waits
 376         indefinitely (which is the default).</para></listitem>
 377       </varlistentry>
 378
 379       <varlistentry>
 380         <term><option>tmp</option></term>
 381
 382         <listitem><para>The encrypted block device will be prepared
 383         for using it as <filename>/tmp</filename>; it will be
 384         formatted using
 385         <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
 386         This option implies <option>plain</option>.</para>
 387
 388         <para>WARNING: Using the <option>tmp</option> option will
 389         destroy the contents of the named partition during every boot,
 390         so make sure the underlying block device is specified
 391         correctly.</para></listitem>
 392       </varlistentry>
 393
 394       <varlistentry>
 395         <term><option>tries=</option></term>
 396
 397         <listitem><para>Specifies the maximum number of times the user
 398         is queried for a password. The default is 3. If set to 0, the
 399         user is queried for a password indefinitely.</para></listitem>
 400       </varlistentry>
 401
 402       <varlistentry>
 403         <term><option>verify</option></term>
 404
 405         <listitem><para> If the encryption password is read from
 406         console, it has to be entered twice to prevent
 407         typos.</para></listitem>
 408       </varlistentry>
 409
 410       <varlistentry>
 411         <term><option>x-systemd.device-timeout=</option></term>
 412
 413         <listitem><para>Specifies how long systemd should wait for a device to show up
 414         before giving up on the entry. The argument is a time in seconds or explicitly
 415         specified units of
 416         <literal>s</literal>,
 417         <literal>min</literal>,
 418         <literal>h</literal>,
 419         <literal>ms</literal>.
 420         </para></listitem>
 421       </varlistentry>
 422
 423     </variablelist>
 424
 425     <para>At early boot and when the system manager configuration is
 426     reloaded, this file is translated into native systemd units by
 427     <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
 428   </refsect1>
 429
 430   <refsect1>
 431     <title>Example</title>
 432     <example>
 433       <title>/etc/crypttab example</title>
 434       <para>Set up four encrypted block devices. One using LUKS for
 435       normal storage, another one for usage as a swap device and two
 436       TrueCrypt volumes.</para>
 437
 438       <programlisting>luks       UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
 439 swap       /dev/sda7       /dev/urandom       swap
 440 truecrypt  /dev/sda2       /etc/container_password  tcrypt
 441 hidden     /mnt/tc_hidden  /dev/null    tcrypt-hidden,tcrypt-keyfile=/etc/keyfile</programlisting>
 442     </example>
 443   </refsect1>
 444
 445   <refsect1>
 446     <title>See Also</title>
 447     <para>
 448       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 449       <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 450       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 451       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 452       <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 453       <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 454       <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 455     </para>
 456   </refsect1>
 457
 458 </refentry>