| <?xml version="1.0"?> |
| <!--*-nxml-*--> |
| <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
| "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
| <!-- |
| SPDX-License-Identifier: LGPL-2.1-or-later |
| |
| This is based on crypttab(5) from Fedora's initscripts package, which in |
| turn is based on Debian's version. |
| |
| The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>. |
| --> |
| <refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP' xmlns:xi="http://www.w3.org/2001/XInclude"> |
| |
| <refentryinfo> |
| <title>crypttab</title> |
| <productname>systemd</productname> |
| </refentryinfo> |
| |
| <refmeta> |
| <refentrytitle>crypttab</refentrytitle> |
| <manvolnum>5</manvolnum> |
| </refmeta> |
| |
| <refnamediv> |
| <refname>crypttab</refname> |
| <refpurpose>Configuration for encrypted block devices</refpurpose> |
| </refnamediv> |
| |
| <refsynopsisdiv> |
| <para><filename>/etc/crypttab</filename></para> |
| </refsynopsisdiv> |
| |
| <refsect1> |
| <title>Description</title> |
| |
| <para>The <filename>/etc/crypttab</filename> file describes |
| encrypted block devices that are set up during system boot.</para> |
| |
| <para>Empty lines and lines starting with the <literal>#</literal> |
| character are ignored. Each of the remaining lines describes one |
| encrypted block device. Fields are delimited by white space.</para> |
| |
| <para>Each line is in the form<programlisting><replaceable>volume-name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>key-file</replaceable> <replaceable>options</replaceable></programlisting> |
| The first two fields are mandatory, the remaining two are |
| optional.</para> |
| |
| <para>Setting up encrypted block devices using this file supports |
| three encryption modes: LUKS, TrueCrypt and plain. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for more information about each mode. When no mode is specified in |
| the options field and the block device contains a LUKS signature, |
| it is opened as a LUKS device; otherwise, it is assumed to be in |
| raw dm-crypt (plain mode) format.</para> |
| |
| <para>The first field contains the name of the resulting encrypted volume; its block device is set up |
| below <filename>/dev/mapper/</filename>.</para> |
| |
| <para>The second field contains a path to the underlying block |
| device or file, or a specification of a block device via |
| <literal>UUID=</literal> followed by the UUID.</para> |
| |
| <para>The third field specifies an absolute path to a file with the encryption key. Optionally, |
| the path may be followed by <literal>:</literal> and an fstab device specification (e.g. starting with |
| <literal>LABEL=</literal> or similar); in which case the path is taken relative to the device file system |
| root. If the field is not present or is <literal>none</literal> or <literal>-</literal>, a key file |
| named after the volume to unlock (i.e. the first column of the line), suffixed with |
| <filename>.key</filename> is automatically loaded from the <filename>/etc/cryptsetup-keys.d/</filename> |
| and <filename>/run/cryptsetup-keys.d/</filename> directories, if present. Otherwise, the password has to |
| be manually entered during system boot. For swap encryption, <filename>/dev/urandom</filename> may be |
| used as key file.</para> |
| |
| <para>The fourth field, if present, is a comma-delimited list of |
| options. The following options are recognized:</para> |
| |
| <variablelist class='fstab-options'> |
| |
| <varlistentry> |
| <term><option>cipher=</option></term> |
| |
| <listitem><para>Specifies the cipher to use. See <citerefentry |
| project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this option. A cipher with unpredictable IV values, such |
| as <literal>aes-cbc-essiv:sha256</literal>, is recommended. Embedded commas in the cipher |
| specification need to be escaped by preceding them with a backslash, see example below.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>discard</option></term> |
| |
| <listitem><para>Allow discard requests to be passed through the encrypted block |
| device. This improves performance on SSD storage but has security implications. |
| </para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>hash=</option></term> |
| |
| <listitem><para>Specifies the hash to use for password |
| hashing. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this |
| option.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>header=</option></term> |
| |
| <listitem><para>Use a detached (separated) metadata device or |
| file where the LUKS header is stored. This option is only |
| relevant for LUKS devices. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this |
| option.</para> |
| |
| <para>Optionally, the path may be followed by <literal>:</literal> and an fstab device specification |
| (e.g. starting with <literal>UUID=</literal> or similar); in which case, the path is relative to the |
| device file system root. The device gets mounted automatically for LUKS device activation duration only. |
| </para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>keyfile-offset=</option></term> |
| |
| <listitem><para>Specifies the number of bytes to skip at the |
| start of the key file. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this |
| option.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>keyfile-size=</option></term> |
| |
| <listitem><para>Specifies the maximum number of bytes to read |
| from the key file. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this option. This |
| option is ignored in plain encryption mode, as the key file |
| size is then given by the key size.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>keyfile-erase</option></term> |
| |
| <listitem><para>If enabled, the specified key file is erased after the volume is activated or when |
| activation fails. This is in particular useful when the key file is only acquired transiently before |
| activation (e.g. via a file in <filename>/run/</filename>, generated by a service running before |
| activation), and shall be removed after use. Defaults to off.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>key-slot=</option></term> |
| |
| <listitem><para>Specifies the key slot to compare the |
| passphrase or key against. If the key slot does not match the |
| given passphrase or key, but another would, the setup of the |
| device will fail regardless. This option implies |
| <option>luks</option>. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values. The default is to try all key slots in |
| sequential order.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>keyfile-timeout=</option></term> |
| |
| <listitem><para> Specifies the timeout for the device on |
| which the key file resides and falls back to a password if |
| it could not be mounted. See |
| <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for key files on external devices. |
| </para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>luks</option></term> |
| |
| <listitem><para>Force LUKS mode. When this mode is used, the |
| following options are ignored since they are provided by the |
| LUKS header on the device: <option>cipher=</option>, |
| <option>hash=</option>, |
| <option>size=</option>.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>bitlk</option></term> |
| |
| <listitem><para>Decrypt Bitlocker drive. Encryption parameters |
| are deduced by cryptsetup from Bitlocker header.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>_netdev</option></term> |
| |
| <listitem><para>Marks this cryptsetup device as requiring network. It will be |
| started after the network is available, similarly to |
| <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
| units marked with <option>_netdev</option>. The service unit to set up this device |
| will be ordered between <filename>remote-fs-pre.target</filename> and |
| <filename>remote-cryptsetup.target</filename>, instead of |
| <filename>cryptsetup-pre.target</filename> and |
| <filename>cryptsetup.target</filename>.</para> |
| |
| <para>Hint: if this device is used for a mount point that is specified in |
| <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
| the <option>_netdev</option> option should also be used for the mount |
| point. Otherwise, a dependency loop might be created where the mount point |
| will be pulled in by <filename>local-fs.target</filename>, while the |
| service to configure the network is usually only started <emphasis>after</emphasis> |
| the local file system has been mounted.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>noauto</option></term> |
| |
| <listitem><para>This device will not be added to <filename>cryptsetup.target</filename>. |
| This means that it will not be automatically unlocked on boot, unless something else pulls |
| it in. In particular, if the device is used for a mount point, it'll be unlocked |
| automatically during boot, unless the mount point itself is also disabled with |
| <option>noauto</option>.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>nofail</option></term> |
| |
| <listitem><para>This device will not be a hard dependency of |
| <filename>cryptsetup.target</filename>. It'll still be pulled in and started, but the system |
| will not wait for the device to show up and be unlocked, and boot will not fail if this is |
| unsuccessful. Note that other units that depend on the unlocked device may still fail. In |
| particular, if the device is used for a mount point, the mount point itself also needs to |
| have the <option>nofail</option> option, or the boot will fail if the device is not unlocked |
| successfully.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>offset=</option></term> |
| |
| <listitem><para>Start offset in the backend device, in 512-byte sectors. This |
| option is only relevant for plain devices.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>plain</option></term> |
| |
| <listitem><para>Force plain encryption mode.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>read-only</option></term><term><option>readonly</option></term> |
| |
| <listitem><para>Set up the encrypted block device in read-only |
| mode.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>same-cpu-crypt</option></term> |
| |
| <listitem><para>Perform encryption using the same cpu that IO was submitted on. The default is to use |
| an unbound workqueue so that encryption work is automatically balanced between available CPUs.</para> |
| |
| <para>This requires kernel 4.0 or newer.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>submit-from-crypt-cpus</option></term> |
| |
| <listitem><para>Disable offloading writes to a separate thread after encryption. There are some |
| situations where offloading write requests from the encryption threads to a dedicated thread degrades |
| performance significantly. The default is to offload write requests to a dedicated thread because it |
| benefits the CFQ scheduler to have writes submitted using the same context.</para> |
| |
| <para>This requires kernel 4.0 or newer.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>skip=</option></term> |
| |
| <listitem><para>How many 512-byte sectors of the encrypted data to skip at the |
| beginning. This is different from the <option>offset=</option> option with respect |
| to the sector numbers used in initialization vector (IV) calculation. Using |
| <option>offset=</option> will shift the IV calculation by the same negative |
| amount. Hence, if <option>offset=<replaceable>n</replaceable></option> is given, |
| sector <replaceable>n</replaceable> will get a sector number of 0 for the IV |
| calculation. Using <option>skip=</option> causes sector |
| <replaceable>n</replaceable> to also be the first sector of the mapped device, but |
| with its number for IV generation being <replaceable>n</replaceable>.</para> |
| |
| <para>This option is only relevant for plain devices.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>size=</option></term> |
| |
| <listitem><para>Specifies the key size in bits. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this |
| option.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>sector-size=</option></term> |
| |
| <listitem><para>Specifies the sector size in bytes. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for possible values and the default value of this |
| option.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>swap</option></term> |
| |
| <listitem><para>The encrypted block device will be used as a |
| swap device, and will be formatted accordingly after setting |
| up the encrypted block device, with |
| <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>. |
| This option implies <option>plain</option>.</para> |
| |
| <para>WARNING: Using the <option>swap</option> option will |
| destroy the contents of the named partition during every boot, |
| so make sure the underlying block device is specified |
| correctly.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tcrypt</option></term> |
| |
| <listitem><para>Use TrueCrypt encryption mode. When this mode |
| is used, the following options are ignored since they are |
| provided by the TrueCrypt header on the device or do not |
| apply: |
| <option>cipher=</option>, |
| <option>hash=</option>, |
| <option>keyfile-offset=</option>, |
| <option>keyfile-size=</option>, |
| <option>size=</option>.</para> |
| |
| <para>When this mode is used, the passphrase is read from the |
| key file given in the third field. Only the first line of this |
| file is read, excluding the new line character.</para> |
| |
| <para>Note that the TrueCrypt format uses both passphrase and |
| key files to derive a password for the volume. Therefore, the |
| passphrase and all key files need to be provided. Use |
| <option>tcrypt-keyfile=</option> to provide the absolute path |
| to all key files. When using an empty passphrase in |
| combination with one or more key files, use |
| <literal>/dev/null</literal> as the password file in the third |
| field.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tcrypt-hidden</option></term> |
| |
| <listitem><para>Use the hidden TrueCrypt volume. This option |
| implies <option>tcrypt</option>.</para> |
| |
| <para>This will map the hidden volume that is inside of the |
| volume provided in the second field. Please note that there is |
| no protection for the hidden volume if the outer volume is |
| mounted instead. See |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| for more information on this limitation.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tcrypt-keyfile=</option></term> |
| |
| <listitem><para>Specifies the absolute path to a key file to |
| use for a TrueCrypt volume. This implies |
| <option>tcrypt</option> and can be used more than once to |
| provide several key files.</para> |
| |
| <para>See the entry for <option>tcrypt</option> on the |
| behavior of the passphrase and key files when using TrueCrypt |
| encryption mode.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tcrypt-system</option></term> |
| |
| <listitem><para>Use TrueCrypt in system encryption mode. This |
| option implies <option>tcrypt</option>.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tcrypt-veracrypt</option></term> |
| |
| <listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of |
| TrueCrypt that is mostly compatible, but uses different, stronger key |
| derivation algorithms that cannot be detected without this flag. |
| Enabling this option could substantially slow down unlocking, because |
| VeraCrypt's key derivation takes much longer than TrueCrypt's. This |
| option implies <option>tcrypt</option>.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>timeout=</option></term> |
| |
| <listitem><para>Specifies the timeout for querying for a |
| password. If no unit is specified, seconds is used. Supported |
| units are s, ms, us, min, h, d. A timeout of 0 waits |
| indefinitely (which is the default).</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tmp=</option></term> |
| |
| <listitem><para>The encrypted block device will be prepared for using it as |
| <filename>/tmp/</filename>; it will be formatted using <citerefentry |
| project='man-pages'><refentrytitle>mkfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>. Takes |
| a file system type as argument, such as <literal>ext4</literal>, <literal>xfs</literal> or |
| <literal>btrfs</literal>. If no argument is specified defaults to <literal>ext4</literal>. This |
| option implies <option>plain</option>.</para> |
| |
| <para>WARNING: Using the <option>tmp</option> option will destroy the contents of the named partition |
| during every boot, so make sure the underlying block device is specified correctly.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>tries=</option></term> |
| |
| <listitem><para>Specifies the maximum number of times the user |
| is queried for a password. The default is 3. If set to 0, the |
| user is queried for a password indefinitely.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>verify</option></term> |
| |
| <listitem><para>If the encryption password is read from console, it has to be entered twice to |
| prevent typos.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>pkcs11-uri=</option></term> |
| |
| <listitem><para>Takes a <ulink url="https://tools.ietf.org/html/rfc7512">RFC7512 PKCS#11 URI</ulink> |
| pointing to a private RSA key which is used to decrypt the key specified in the third column of the |
| line. This is useful for unlocking encrypted volumes through security tokens or smartcards. See below |
| for an example how to set up this mechanism for unlocking a LUKS volume with a YubiKey security |
| token. The specified URI can refer directly to a private RSA key stored on a token or alternatively |
| just to a slot or token, in which case a search for a suitable private RSA key will be performed. In |
| this case if multiple suitable objects are found the token is refused. The key configured in the |
| third column is passed as is to RSA decryption. The resulting decrypted key is then base64 encoded |
| before it is used to unlock the LUKS volume.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>try-empty-password=</option></term> |
| |
| <listitem><para>Takes a boolean argument. If enabled, right before asking the user for a password it |
| is first attempted to unlock the volume with an empty password. This is useful for systems that are |
| initialized with an encrypted volume with only an empty password set, which shall be replaced with a |
| suitable password during first boot, but after activation.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>x-systemd.device-timeout=</option></term> |
| |
| <listitem><para>Specifies how long systemd should wait for a device to show up |
| before giving up on the entry. The argument is a time in seconds or explicitly |
| specified units of |
| <literal>s</literal>, |
| <literal>min</literal>, |
| <literal>h</literal>, |
| <literal>ms</literal>. |
| </para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><option>x-initrd.attach</option></term> |
| |
| <listitem><para>Setup this encrypted block device in the initramfs, similarly to |
| <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
| units marked with <option>x-initrd.mount</option>.</para> |
| |
| <para>Although it's not necessary to mark the mount entry for the root file system with |
| <option>x-initrd.mount</option>, <option>x-initrd.attach</option> is still recommended with |
| the encrypted block device containing the root file system as otherwise systemd will |
| attempt to detach the device during the regular system shutdown while it's still in |
| use. With this option the device will still be detached but later after the root file |
| system is unmounted.</para> |
| |
| <para>All other encrypted block devices that contain file systems mounted in the initramfs |
| should use this option.</para> |
| </listitem> |
| </varlistentry> |
| |
| </variablelist> |
| |
| <para>At early boot and when the system manager configuration is |
| reloaded, this file is translated into native systemd units by |
| <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para> |
| </refsect1> |
| |
| <refsect1> |
| <title>Examples</title> |
| <example> |
| <title>/etc/crypttab example</title> |
| <para>Set up four encrypted block devices. One using LUKS for normal storage, another one for usage as |
| a swap device and two TrueCrypt volumes. For the fourth device, the option string is interpreted as two |
| options <literal>cipher=xchacha12,aes-adiantum-plain64</literal>, |
| <literal>keyfile-timeout=10s</literal>.</para> |
| |
| <programlisting>luks UUID=2505567a-9e27-4efe-a4d5-15ad146c258b |
| swap /dev/sda7 /dev/urandom swap |
| truecrypt /dev/sda2 /etc/container_password tcrypt |
| hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile |
| external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=10s,cipher=xchacha12\,aes-adiantum-plain64 |
| </programlisting> |
| </example> |
| |
| <example> |
| <title>Yubikey-based Volume Unlocking Example</title> |
| |
| <para>The PKCS#11 logic allows hooking up any compatible security token that is capable of storing RSA |
| decryption keys. Here's an example how to set up a Yubikey security token for this purpose, using |
| <citerefentry project='debian'><refentrytitle>ykmap</refentrytitle><manvolnum>1</manvolnum></citerefentry> |
| from the yubikey-manager project:</para> |
| |
| <programlisting><xi:include href="yubikey-crypttab.sh" parse="text" /></programlisting> |
| |
| <para>A few notes on the above:</para> |
| |
| <itemizedlist> |
| <listitem><para>We use RSA (and not ECC), since Yubikeys support PKCS#11 Decrypt() only for RSA keys</para></listitem> |
| <listitem><para>We use RSA2048, which is the longest key size current Yubikeys support</para></listitem> |
| <listitem><para>LUKS key size must be shorter than 2048bit due to RSA padding, hence we use 128 bytes</para></listitem> |
| <listitem><para>We use Yubikey key slot 9d, since that's apparently the keyslot to use for decryption purposes, |
| <ulink url="https://developers.yubico.com/PIV/Introduction/Certificate_slots.html">see |
| documentation</ulink>.</para></listitem> |
| </itemizedlist> |
| |
| </example> |
| </refsect1> |
| |
| <refsect1> |
| <title>See Also</title> |
| <para> |
| <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
| <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
| <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
| <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
| <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
| <citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
| <citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
| </para> |
| </refsect1> |
| |
| </refentry> |