1CRYPTTAB(5) crypttab CRYPTTAB(5)
2
6 crypttab - Configuration for encrypted block devices
7
9 /etc/crypttab
10
12 The /etc/crypttab file describes encrypted block devices that are set
13 up during system boot.
14 Empty lines and lines starting with the "#" character are ignored. Each
16 of the remaining lines describes one encrypted block device. Fields are
17 delimited by white space.
18 Each line is in the form
20 volume-name encrypted-device key-file options
22 The first two fields are mandatory, the remaining two are optional.
24 Setting up encrypted block devices using this file supports three
26 encryption modes: LUKS, TrueCrypt and plain. See cryptsetup(8) for more
27 information about each mode. When no mode is specified in the options
28 field and the block device contains a LUKS signature, it is opened as a
29 LUKS device; otherwise, it is assumed to be in raw dm-crypt (plain
30 mode) format.
31 The first field contains the name of the resulting encrypted volume;
33 its block device is set up below /dev/mapper/.
34 The second field contains a path to the underlying block device or
36 file, or a specification of a block device via "UUID=" followed by the
37 UUID.
38 The third field specifies an absolute path to a file to read the
40 encryption key from. Optionally, the path may be followed by ":" and an
41 fstab device specification (e.g. starting with "LABEL=" or similar); in
42 which case, the path is relative to the device file system root. If the
43 field is not present or set to "none" or "-", a key file named after
44 the volume to unlock (i.e. the first column of the line), suffixed with
45 .key is automatically loaded from the /etc/cryptsetup-keys.d/ and
46 /run/cryptsetup-keys.d/ directories, if present. Otherwise, the
47 password has to be manually entered during system boot. For swap
48 encryption, /dev/urandom may be used as key file.
49 The fourth field, if present, is a comma-delimited list of options. The
51 following options are recognized:
52 cipher=
54 Specifies the cipher to use. See cryptsetup(8) for possible values
55 and the default value of this option. A cipher with unpredictable
56 IV values, such as "aes-cbc-essiv:sha256", is recommended.
57 discard
59 Allow discard requests to be passed through the encrypted block
60 device. This improves performance on SSD storage but has security
61 implications.
62 hash=
64 Specifies the hash to use for password hashing. See cryptsetup(8)
65 for possible values and the default value of this option.
66 header=
68 Use a detached (separated) metadata device or file where the LUKS
69 header is stored. This option is only relevant for LUKS devices.
70 See cryptsetup(8) for possible values and the default value of this
71 option.
72 keyfile-offset=
74 Specifies the number of bytes to skip at the start of the key file.
75 See cryptsetup(8) for possible values and the default value of this
76 option.
77 keyfile-size=
79 Specifies the maximum number of bytes to read from the key file.
80 See cryptsetup(8) for possible values and the default value of this
81 option. This option is ignored in plain encryption mode, as the key
82 file size is then given by the key size.
83 keyfile-erase
85 If enabled, the specified key file is erased after the volume is
86 activated or when activation fails. This is in particular useful
87 when the key file is only acquired transiently before activation
88 (e.g. via a file in /run/, generated by a service running before
89 activation), and shall be removed after use. Defaults to off.
90 key-slot=
92 Specifies the key slot to compare the passphrase or key against. If
93 the key slot does not match the given passphrase or key, but
94 another would, the setup of the device will fail regardless. This
95 option implies luks. See cryptsetup(8) for possible values. The
96 default is to try all key slots in sequential order.
97 keyfile-timeout=
99 Specifies the timeout for the device on which the key file resides
100 and falls back to a password if it could not be mounted. See
101 systemd-cryptsetup-generator(8) for key files on external devices.
102 luks
104 Force LUKS mode. When this mode is used, the following options are
105 ignored since they are provided by the LUKS header on the device:
106 cipher=, hash=, size=.
107 bitlk
109 Decrypt Bitlocker drive. Encryption parameters are deduced by
110 cryptsetup from Bitlocker header.
111 _netdev
113 Marks this cryptsetup device as requiring network. It will be
114 started after the network is available, similarly to
115 systemd.mount(5) units marked with _netdev. The service unit to set
116 up this device will be ordered between remote-fs-pre.target and
117 remote-cryptsetup.target, instead of cryptsetup-pre.target and
118 cryptsetup.target.
119 Hint: if this device is used for a mount point that is specified in
121 fstab(5), the _netdev option should also be used for the mount
122 point. Otherwise, a dependency loop might be created where the
123 mount point will be pulled in by local-fs.target, while the service
124 to configure the network is usually only started after the local
125 file system has been mounted.
126 noauto
128 This device will not be added to cryptsetup.target. This means that
129 it will not be automatically unlocked on boot, unless something
130 else pulls it in. In particular, if the device is used for a mount
131 point, it'll be unlocked automatically during boot, unless the
132 mount point itself is also disabled with noauto.
133 nofail
135 This device will not be a hard dependency of cryptsetup.target.
136 It'll still be pulled in and started, but the system will not wait
137 for the device to show up and be unlocked, and boot will not fail
138 if this is unsuccessful. Note that other units that depend on the
139 unlocked device may still fail. In particular, if the device is
140 used for a mount point, the mount point itself also needs to have
141 the nofail option, or the boot will fail if the device is not
142 unlocked successfully.
143 offset=
145 Start offset in the backend device, in 512-byte sectors. This
146 option is only relevant for plain devices.
147 plain
149 Force plain encryption mode.
150 read-only, readonly
152 Set up the encrypted block device in read-only mode.
153 same-cpu-crypt
155 Perform encryption using the same cpu that IO was submitted on. The
156 default is to use an unbound workqueue so that encryption work is
157 automatically balanced between available CPUs.
158 This requires kernel 4.0 or newer.
160 submit-from-crypt-cpus
162 Disable offloading writes to a separate thread after encryption.
163 There are some situations where offloading write requests from the
164 encryption threads to a dedicated thread degrades performance
165 significantly. The default is to offload write requests to a
166 dedicated thread because it benefits the CFQ scheduler to have
167 writes submitted using the same context.
168 This requires kernel 4.0 or newer.
170 no-read-workqueue
172 Bypass dm-crypt internal workqueue and process read requests
173 synchronously. The default is to queue these requests and process
174 them asynchronously.
175 This requires kernel 5.9 or newer.
177 no-write-workqueue
179 Bypass dm-crypt internal workqueue and process write requests
180 synchronously. The default is to queue these requests and process
181 them asynchronously.
182 This requires kernel 5.9 or newer.
184 skip=
186 How many 512-byte sectors of the encrypted data to skip at the
187 beginning. This is different from the offset= option with respect
188 to the sector numbers used in initialization vector (IV)
189 calculation. Using offset= will shift the IV calculation by the
190 same negative amount. Hence, if offset=n is given, sector n will
191 get a sector number of 0 for the IV calculation. Using skip= causes
192 sector n to also be the first sector of the mapped device, but with
193 its number for IV generation being n.
194 This option is only relevant for plain devices.
196 size=
198 Specifies the key size in bits. See cryptsetup(8) for possible
199 values and the default value of this option.
200 sector-size=
202 Specifies the sector size in bytes. See cryptsetup(8) for possible
203 values and the default value of this option.
204 swap
206 The encrypted block device will be used as a swap device, and will
207 be formatted accordingly after setting up the encrypted block
208 device, with mkswap(8). This option implies plain.
209 WARNING: Using the swap option will destroy the contents of the
211 named partition during every boot, so make sure the underlying
212 block device is specified correctly.
213 tcrypt
215 Use TrueCrypt encryption mode. When this mode is used, the
216 following options are ignored since they are provided by the
217 TrueCrypt header on the device or do not apply: cipher=, hash=,
218 keyfile-offset=, keyfile-size=, size=.
219 When this mode is used, the passphrase is read from the key file
221 given in the third field. Only the first line of this file is read,
222 excluding the new line character.
223 Note that the TrueCrypt format uses both passphrase and key files
225 to derive a password for the volume. Therefore, the passphrase and
226 all key files need to be provided. Use tcrypt-keyfile= to provide
227 the absolute path to all key files. When using an empty passphrase
228 in combination with one or more key files, use "/dev/null" as the
229 password file in the third field.
230 tcrypt-hidden
232 Use the hidden TrueCrypt volume. This option implies tcrypt.
233 This will map the hidden volume that is inside of the volume
235 provided in the second field. Please note that there is no
236 protection for the hidden volume if the outer volume is mounted
237 instead. See cryptsetup(8) for more information on this limitation.
238 tcrypt-keyfile=
240 Specifies the absolute path to a key file to use for a TrueCrypt
241 volume. This implies tcrypt and can be used more than once to
242 provide several key files.
243 See the entry for tcrypt on the behavior of the passphrase and key
245 files when using TrueCrypt encryption mode.
246 tcrypt-system
248 Use TrueCrypt in system encryption mode. This option implies
249 tcrypt.
250 tcrypt-veracrypt
252 Check for a VeraCrypt volume. VeraCrypt is a fork of TrueCrypt that
253 is mostly compatible, but uses different, stronger key derivation
254 algorithms that cannot be detected without this flag. Enabling this
255 option could substantially slow down unlocking, because VeraCrypt's
256 key derivation takes much longer than TrueCrypt's. This option
257 implies tcrypt.
258 timeout=
260 Specifies the timeout for querying for a password. If no unit is
261 specified, seconds is used. Supported units are s, ms, us, min, h,
262 d. A timeout of 0 waits indefinitely (which is the default).
263 tmp=
265 The encrypted block device will be prepared for using it as /tmp/;
266 it will be formatted using mkfs(8). Takes a file system type as
267 argument, such as "ext4", "xfs" or "btrfs". If no argument is
268 specified defaults to "ext4". This option implies plain.
269 WARNING: Using the tmp option will destroy the contents of the
271 named partition during every boot, so make sure the underlying
272 block device is specified correctly.
273 tries=
275 Specifies the maximum number of times the user is queried for a
276 password. The default is 3. If set to 0, the user is queried for a
277 password indefinitely.
278 verify
280 If the encryption password is read from console, it has to be
281 entered twice to prevent typos.
282 pkcs11-uri=
284 Takes a RFC7512 PKCS#11 URI[1] pointing to a private RSA key which
285 is used to decrypt the key specified in the third column of the
286 line. This is useful for unlocking encrypted volumes through
287 security tokens or smartcards. See below for an example how to set
288 up this mechanism for unlocking a LUKS volume with a YubiKey
289 security token. The specified URI can refer directly to a private
290 RSA key stored on a token or alternatively just to a slot or token,
291 in which case a search for a suitable private RSA key will be
292 performed. In this case if multiple suitable objects are found the
293 token is refused. The key configured in the third column is passed
294 as is to RSA decryption. The resulting decrypted key is then base64
295 encoded before it is used to unlock the LUKS volume.
296 try-empty-password=
298 Takes a boolean argument. If enabled, right before asking the user
299 for a password it is first attempted to unlock the volume with an
300 empty password. This is useful for systems that are initialized
301 with an encrypted volume with only an empty password set, which
302 shall be replaced with a suitable password during first boot, but
303 after activation.
304 x-systemd.device-timeout=
306 Specifies how long systemd should wait for a device to show up
307 before giving up on the entry. The argument is a time in seconds or
308 explicitly specified units of "s", "min", "h", "ms".
309 x-initrd.attach
311 Setup this encrypted block device in the initramfs, similarly to
312 systemd.mount(5) units marked with x-initrd.mount.
313 Although it's not necessary to mark the mount entry for the root
315 file system with x-initrd.mount, x-initrd.attach is still
316 recommended with the encrypted block device containing the root
317 file system as otherwise systemd will attempt to detach the device
318 during the regular system shutdown while it's still in use. With
319 this option the device will still be detached but later after the
320 root file system is unmounted.
321 All other encrypted block devices that contain file systems mounted
323 in the initramfs should use this option.
324 At early boot and when the system manager configuration is reloaded,
326 this file is translated into native systemd units by systemd-
327 cryptsetup-generator(8).
328
330 Example 1. /etc/crypttab example
331 Set up four encrypted block devices. One using LUKS for normal storage,
333 another one for usage as a swap device and two TrueCrypt volumes.
334 luks UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
336 swap /dev/sda7 /dev/urandom swap
337 truecrypt /dev/sda2 /etc/container_password tcrypt
338 hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
339 external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=10s
340 Example 2. Yubikey-based Volume Unlocking Example
342 The PKCS#11 logic allows hooking up any compatible security token that
344 is capable of storing RSA decryption keys. Here's an example how to set
345 up a Yubikey security token for this purpose, using ykmap(1) from the
346 yubikey-manager project:
347 # Make sure no one can read the files we generate but us
349 umask 077
350 # Destroy any old key on the Yubikey (careful!)
352 ykman piv reset
353 # Generate a new private/public key pair on the device, store the public key in 'pubkey.pem'.
355 ykman piv generate-key -a RSA2048 9d pubkey.pem
356 # Create a self-signed certificate from this public key, and store it on the
358 # device. The "subject" should be an arbitrary string to identify the token in
359 # the p11tool output below.
360 ykman piv generate-certificate --subject "Knobelei" 9d pubkey.pem
361 # Check if the newly create key on the Yubikey shows up as token in PKCS#11. Have a look at the output, and
363 # copy the resulting token URI to the clipboard.
364 p11tool --list-tokens
365 # Generate a (secret) random key to use as LUKS decryption key.
367 dd if=/dev/urandom of=plaintext.bin bs=128 count=1
368 # Encode the secret key also as base64 text (with all whitespace removed)
370 base64 < plaintext.bin | tr -d '\n\r\t ' > plaintext.base64
371 # Encrypt this newly generated (binary) LUKS decryption key using the public key whose private key is on the
373 # Yubikey, store the result in /etc/cryptsetup-keys.d/mytest.key, where we'll look for it during boot.
374 mkdir -p /etc/cryptsetup-keys.d
375 sudo openssl rsautl -encrypt -pubin -inkey pubkey.pem -in plaintext.bin -out /etc/cryptsetup-keys.d/mytest.key
376 # Configure the LUKS decryption key on the LUKS device. We use very low pbkdf settings since the key already
378 # has quite a high quality (it comes directly from /dev/urandom after all), and thus we don't need to do much
379 # key derivation. Replace /dev/sdXn by the partition to use (e.g. sda1)
380 sudo cryptsetup luksAddKey /dev/sdXn plaintext.base64 --pbkdf=pbkdf2 --pbkdf-force-iterations=1000
381 # Now securely delete the plain text LUKS key, we don't need it anymore, and since it contains secret key
383 # material it should be removed from disk thoroughly.
384 shred -u plaintext.bin plaintext.base64
385 # We don't need the public key anymore either, let's remove it too. Since this one is not security
387 # sensitive we just do a regular "rm" here.
388 rm pubkey.pem
389 # Test: Let's run systemd-cryptsetup to test if this all worked. The option string should contain the full
391 # PKCS#11 URI we have in the clipboard; it tells the tool how to decipher the encrypted LUKS key. Note that
392 # systemd-cryptsetup automatically searches for the encrypted key in /etc/cryptsetup-keys.d/, hence we do
393 # not need to specify the key file path explicitly here.
394 sudo systemd-cryptsetup attach mytest /dev/sdXn - 'pkcs11-uri=pkcs11:...'
395 # If that worked, let's now add the same line persistently to /etc/crypttab, for the future.
397 sudo bash -c 'echo "mytest /dev/sdXn - \'pkcs11-uri=pkcs11:...\'" >> /etc/crypttab'
398 A few notes on the above:
400 · We use RSA (and not ECC), since Yubikeys support PKCS#11 Decrypt()
402 only for RSA keys
403 · We use RSA2048, which is the longest key size current Yubikeys
405 support
406 · LUKS key size must be shorter than 2048bit due to RSA padding,
408 hence we use 128 bytes
409 · We use Yubikey key slot 9d, since that's apparently the keyslot to
411 use for decryption purposes, see documentation[2].
412
414 systemd(1), systemd-cryptsetup@.service(8), systemd-cryptsetup-
415 generator(8), fstab(5), cryptsetup(8), mkswap(8), mke2fs(8)
416
418 1. RFC7512 PKCS#11 URI
419 https://tools.ietf.org/html/rfc7512
420 2. see documentation
422 https://developers.yubico.com/PIV/Introduction/Certificate_slots.html
423systemd 246 CRYPTTAB(5)