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>keyfile-timeout=</option></term>
 156
 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.
 162         </para></listitem>
 163       </varlistentry>
 164
 165       <varlistentry>
 166         <term><option>luks</option></term>
 167
 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>
 173       </varlistentry>
 174
 175       <varlistentry>
 176         <term><option>_netdev</option></term>
 177
 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>
 186
 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>
 194         </listitem>
 195       </varlistentry>
 196
 197       <varlistentry>
 198         <term><option>noauto</option></term>
 199
 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>
 205       </varlistentry>
 206
 207       <varlistentry>
 208         <term><option>nofail</option></term>
 209
 210         <listitem><para>This device will not be a hard dependency of
 211         <filename>cryptsetup.target</filename>. It'll still be 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 also needs to
 215         have the <option>nofail</option> option, or the boot will fail if the device is not unlocked
 216         successfully.</para></listitem>
 217       </varlistentry>
 218
 219       <varlistentry>
 220         <term><option>offset=</option></term>
 221
 222         <listitem><para>Start offset in the backend device, in 512-byte sectors. This
 223         option is only relevant for plain devices.</para></listitem>
 224       </varlistentry>
 225
 226       <varlistentry>
 227         <term><option>plain</option></term>
 228
 229         <listitem><para>Force plain encryption mode.</para></listitem>
 230       </varlistentry>
 231
 232       <varlistentry>
 233         <term><option>read-only</option></term><term><option>readonly</option></term>
 234
 235         <listitem><para>Set up the encrypted block device in read-only
 236         mode.</para></listitem>
 237       </varlistentry>
 238
 239       <varlistentry>
 240         <term><option>same-cpu-crypt</option></term>
 241
 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>
 245         </listitem>
 246       </varlistentry>
 247
 248       <varlistentry>
 249         <term><option>submit-from-crypt-cpus</option></term>
 250
 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>
 256         </listitem>
 257       </varlistentry>
 258
 259       <varlistentry>
 260         <term><option>skip=</option></term>
 261
 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>
 271
 272         <para>This option is only relevant for plain devices.</para>
 273         </listitem>
 274       </varlistentry>
 275
 276       <varlistentry>
 277         <term><option>size=</option></term>
 278
 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>
 283       </varlistentry>
 284
 285       <varlistentry>
 286         <term><option>sector-size=</option></term>
 287
 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>
 292       </varlistentry>
 293
 294       <varlistentry>
 295         <term><option>swap</option></term>
 296
 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>
 302
 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>
 307       </varlistentry>
 308
 309       <varlistentry>
 310         <term><option>tcrypt</option></term>
 311
 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
 315         apply:
 316         <option>cipher=</option>,
 317         <option>hash=</option>,
 318         <option>keyfile-offset=</option>,
 319         <option>keyfile-size=</option>,
 320         <option>size=</option>.</para>
 321
 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>
 325
 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>
 334       </varlistentry>
 335
 336       <varlistentry>
 337         <term><option>tcrypt-hidden</option></term>
 338
 339         <listitem><para>Use the hidden TrueCrypt volume. This option
 340         implies <option>tcrypt</option>.</para>
 341
 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
 345         mounted instead. See
 346         <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 347         for more information on this limitation.</para></listitem>
 348       </varlistentry>
 349
 350       <varlistentry>
 351         <term><option>tcrypt-keyfile=</option></term>
 352
 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>
 357
 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>
 361       </varlistentry>
 362
 363       <varlistentry>
 364         <term><option>tcrypt-system</option></term>
 365
 366         <listitem><para>Use TrueCrypt in system encryption mode. This
 367         option implies <option>tcrypt</option>.</para></listitem>
 368       </varlistentry>
 369
 370       <varlistentry>
 371         <term><option>tcrypt-veracrypt</option></term>
 372
 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>
 379       </varlistentry>
 380
 381       <varlistentry>
 382         <term><option>timeout=</option></term>
 383
 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>
 388       </varlistentry>
 389
 390       <varlistentry>
 391         <term><option>tmp</option></term>
 392
 393         <listitem><para>The encrypted block device will be prepared
 394         for using it as <filename>/tmp</filename>; it will be
 395         formatted using
 396         <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
 397         This option implies <option>plain</option>.</para>
 398
 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>
 403       </varlistentry>
 404
 405       <varlistentry>
 406         <term><option>tries=</option></term>
 407
 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>
 411       </varlistentry>
 412
 413       <varlistentry>
 414         <term><option>verify</option></term>
 415
 416         <listitem><para> If the encryption password is read from
 417         console, it has to be entered twice to prevent
 418         typos.</para></listitem>
 419       </varlistentry>
 420
 421       <varlistentry>
 422         <term><option>x-systemd.device-timeout=</option></term>
 423
 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
 426         specified units of
 427         <literal>s</literal>,
 428         <literal>min</literal>,
 429         <literal>h</literal>,
 430         <literal>ms</literal>.
 431         </para></listitem>
 432       </varlistentry>
 433
 434     </variablelist>
 435
 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>
 439   </refsect1>
 440
 441   <refsect1>
 442     <title>Example</title>
 443     <example>
 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>
 448
 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>
 454     </example>
 455   </refsect1>
 456
 457   <refsect1>
 458     <title>See Also</title>
 459     <para>
 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>
 467     </para>
 468   </refsect1>
 469
 470 </refentry>