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I'm trying to use cryptdisks_start to open a LUKS device that is defined in my /etc/crypttab. Unfortunately, the command fails with the following error message:

martin ~ # cryptdisks_start luks-01a2e5d8-9211-40ce-b160-d3f973d1a155
 * Starting crypto disk...
 * luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 (starting)..
 * luks-01a2e5d8-9211-40ce-b160-d3f973d1a155: the precheck for '/dev/disk/by-uuid/01a2e5d8-9211-40ce-b160-d3f973d1a155' failed:  - The device /dev/disk/by-uuid/01a2e5d8-9211-40ce-b160-d3f973d1a155 contains a filesystem type crypto_LUKS.
 * luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 (failed)...

A rather strange error, because of course that device has to contain a crypto_LUKS filesystem!

The relevant line from /etc/crypttab, as set up by GNOME Disks:

luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 UUID=01a2e5d8-9211-40ce-b160-d3f973d1a155 /etc/luks-keys/luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 nofail
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up vote 4 down vote accepted

It doesn't work because the /etc/crypttab line is missing the option keyword luks. Changing the line to this resolved the issue:

luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 UUID=01a2e5d8-9211-40ce-b160-d3f973d1a155 /etc/luks-keys/luks-01a2e5d8-9211-40ce-b160-d3f973d1a155 luks,nofail

This is due to the fact that cryptdisks_start uses the options to determine what kind of encryption is being used so that it will use the correct command to open the device. Without the luks option, cryptdisks_start will try to open the device as a plain dm-crypt device with cryptsetup create. Luckily a sanity check prevents this, although it causes a confusing error message.

Apparently the Disks tool of GNOME3 writes this incorrect line to /etc/crypttab when using the unlock icon and saving the passphrase.

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