[dm-crypt] UUID question

Milan Broz gmazyland at gmail.com
Tue Dec 18 09:21:48 CET 2012


On 12/18/2012 01:36 AM, Arno Wagner wrote:
> On Mon, Dec 17, 2012 at 04:10:50PM -0800, David Li wrote:
>> Hi, I wonder if the dm-crypt partition UUID (shown in blkid -p <dev>) can
>> be used to uniquely associate it with the set of keys the partition will
>> need. Are there any cases that the UUID would change during the partition's
>> lifetime?
> 
> The UUID is actually a filesystem attribute, not a partition 
> attribute. That said, for purpose of an UUID, LUKS is regarded as a 
> filesystem, which is IMO the correct way to view it, but not a 
> perfect one. So, yes, the UUID will change if you do a luksFormat 
> (aptly named if LUKS is regarded as a filesystem), but it will not 
> change otherwise. As a luksFormat invalidates all keys, that should 
> do for your purpose. 

Well, it is more complicated. blkid recognizes UUID from metadata
on disk. There are several groups of metadata and there are priorities
(raid devices have priority to filesystem for example).
UUID is generic attribute, even MD devices, LVM PVs etc have UUID.

LUKS is basically handled like MD (raid) device.

Anyway, question was if UUID can change during lifetime - no.
(reformat is not part of lifetime, you will lose data)
To be precise, you can change UUID but it must be explicit user action
(see man cryptsetup).

And it is preferred way to reference LUKS device by its UUID (if the
physical disk is moved likde sdb->sdc, it still works).

An example:

# blkid /dev/sdb
/dev/sdb: UUID="bb0c71ca-24c0-4a73-b7ff-ebdbcf152040" TYPE="crypto_LUKS"

# blkid -U bb0c71ca-24c0-4a73-b7ff-ebdbcf152040
/dev/sdb


And cryptsetup itself (in recent versions) recognizes UUID as device parameter:

#cryptsetup luksOpen UUID=bb0c71ca-24c0-4a73-b7ff-ebdbcf152040 test
Enter passphrase for /dev/disk/by-uuid/bb0c71ca-24c0-4a73-b7ff-ebdbcf152040: 

And you should be able to use UUID in /etc/crypttab as well.

Milan


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