luksmeta/luksmeta.8.adoc

luksmeta(8)
===========
:doctype: manpage

== NAME

luksmeta - Utility for storing metadata in a LUKSv1 header

== SYNOPSIS

*luksmeta test* -d DEVICE

*luksmeta nuke* -d DEVICE [-f]

*luksmeta init* -d DEVICE [-f] [-n]

*luksmeta show* -d DEVICE [-s SLOT]

*luksmeta save* -d DEVICE [-s SLOT]  -u UUID  < DATA

*luksmeta load* -d DEVICE  -s SLOT  [-u UUID] > DATA

*luksmeta wipe* -d DEVICE  -s SLOT  [-u UUID] [-f]

== OVERVIEW

The *luksmeta* utility enables an administrator to store metadata in the gap
between the end of the LUKSv1 header and the start of the encrypted data. This
is useful for storing data that is available before the volume is unlocked,
usually for use during the volume unlock process.

The metadata is stored in a series of UUID-typed slots, allowing multiple
blocks of metadata. Although the *luksmeta* slots are inspired by the LUKS
slots, they are functionally independent and share only a casual relationship.
Slots merely provide a hint that a given chunk of metadata is associated with
a specific LUKSv1 password (in a slot with the same number). However,
*luksmeta* itself is indifferent to the relationship between a LUKSv1 slot
and the correspondly numbered *luksmeta* slot, with one exception (detailed
below).

After a LUKSv1 volume is initialized using *cryptsetup*(8), it must also be
initialized for metadata storage by *luksmeta init*. Once this is complete,
the device is ready to store medata.

Data can be written to a slot using *luksmeta save* or read from a slot
using *luksmeta load*. You can also erase the data in an existing slot using
*luksmeta wipe* or query the slots using *luksmeta show*.

== UUID GENERATION

It is often presumed that saving metadata to a slot requires a specific UUID
or that there is an official registry of UUID types. This is incorrect.

UUID stands for Universally Unique IDentifier. UUIDs are a standardized,
widely-used data type used for identification without a central registry. For
the relevant standards, see ISO 9834-8:2005 and RFC 4122.

UUIDs are large enough that collision is practically impossible. So if your
application wants to store data in a *luksmeta* slot, just generate your own
UUID and use it consistently to refer to your type of data. If you have
multiple types of data, feel free to generate multiple UUIDs.

The easiest way to generate a UUID is to use *uuidgen*(1). However, any compliant
UUID generator will suffice.

== INITIALIZATION

Before reading or writing metadata, the LUKSv1 block device must be
initialized for metadata storage. Three commands help with this process:
*luksmeta test*, *luksmeta nuke* and *luksmeta init*.

The *luksmeta test* command simply checks an existing block device to see
if it is initialized for metadata storage. This command does not provide any
output, so be sure to check its return code (see below).

The *luksmeta nuke* command will zero (erase) the entire LUKSv1 header gap.
Since this operation is destructive, user confirmation will be required before
clearing the gap unless the *-f* option is supplied.

The *luksmeta init* command initializes the LUKSv1 block device for metadata
storage. This process will wipe out any data in the LUKSv1 header gap. For
this reason, this command will require user confirmation before any data is
written unless the *-f* option is supplied. Note that this command succeeds
without any modification if the device is already initialized. If you would
like to force the creation of clean initialization state, you can specify the
*-n* option to nuke the LUKSv1 header gap before initialization (but after
user confirmation).

== METADATA STATE

The *luksmeta show* command displays the current state of slots on the LUKSv1
block device. If no slot is specified, it prints a table consisting of the
slot number, the corresponding LUKSv1 slot state and the UUID of the data
stored in the *luksmeta* slot (or "empty" if no data is stored). If a slot is
specified, this command simply prints out the UUID of the data in the slot. If
the slot does not contain data, it prints nothing.

== MANAGING METADATA

Managing the metadata in the slots is performed with three commands:
*luksmeta save*, *luksmeta load* and *luksmeta wipe*. These commands write
metadata to a slot, read metadata from a slot and erase metadata in a slot,
respectively.

The *luksmeta save* command reads metadata on standard input and writes it to
the specified slot using the specified UUID. If no slot is specified,
*luksmeta* will search for the first slot number for which the LUKSv1 slot
is inactive and the *luksmeta* slot is empty. This represents the only
official correlation between LUKSv1 slots and *luksmeta* slots. In this case,
the metadata is written to the first applicable slot using the specified UUID
and the slot number is printed to standard output. In either case, this
command will never overwrite existing data. To replace data in a slot you will
need to execute *luksmeta wipe* before *luksmeta save*.

The *luksmeta load* command reads data from the specified slot and writes it
to standard output. If a UUID is specified, the command will verify that the
UUID associated with the metadata in the slot matches the specified UUID. This
type check helps to ensure that you always receive the type of data you are
expecting as output. If the UUIDs do not match, the command will fail.

The *luksmeta wipe* command erases the data from the given slot. If a UUID is
specified, the command will verify that the UUID associated with the metadata
in the slot matches the specified UUID. This type check helps to ensure that
you only erase the data you intended to erase. Because this is a destructive
operation, this command will require user confirmation before any data is
erased, unless the *-f* option is supplied. Note that this command succeeds
if you attempt to wipe a slot that is already empty.

== CAVEATS

The amount of storage in the LUKSv1 header gap is extremely limited. It also
varies based upon the configuration used by LUKSv1 at device initialization
time. In some LUKSv1 configurations, there is not even enough space for
all the metadata slots even at the smallest possible slot size.

During the design of this utility, we considered it likely that users would
want to reduce the number of usable slots in exchange for more storage space
in the slots used. In order to provide this flexibility, the amount of storage
available per-slot is dynamic. Put simply, slots are not a fixed size. This
means that it is possible (and even somewhat likely) to encounter an error
during *luksmeta save* indicating that there is insufficient space.

This error is not a programming bug. If you encounter this error it likely
means that either all space is being consumed by the already-written slots or
that the metadata you are attempting to write simply does not fit.

You can attempt to resolve this problem by calling *luksmeta wipe* on slots
that are no longer in use. This will release the storage space for use by
other slots. Note that *luksmeta* does not, however, currently perform
defragmentation since the number of usable blocks is rather limited. You can
attempt to manually get around this by extracting all slot data, wiping the
slots and reloading them in order. However, this operation is potentially
dangerous and should be undertaken with great care.

== OPTIONS

* *-d* _DEVICE_, *--device*=_DEVICE_ :
  The device on which to perform the operation.

* *-s* _SLOT_, *--slot*=_SLOT_ :
  The slot number on which to perform the operation.

* *-u* _UUID_, *--uuid*=_UUID_ :
  The UUID to associate with the operation.

* *-f*, *--force* :
  Forcibly suppress all user prompting.

== RETURN VALUES

This command uses the return values as defined by *sysexits.h*. The following
are general errors (and corresponding numeric exit codes) whose meaning is
shared by all *luksmeta* commands:

* *EX_OK*         (0): The operation was successful.
* *EX_USAGE*     (64): The program was called with invalid parameters.
* *EX_DATAERR*   (65): The specified UUID does not match the slot UUID.
* *EX_NOINPUT*   (66): An error occurred while reading from standard input.
* *EX_OSERR*     (71): An undefined operating system error occurred.
* *EX_OSFILE*    (72): The device is not initialized or is corrupted.
* *EX_CANTCREAT* (73): There is insufficient space in LUKSv1 header.
* *EX_IOERR*     (74): An IO error occurred when writing to the device.
* *EX_NOPERM*    (77): The user did not grant permission during confirmation.

Additionally, *luksmeta save* will return *EX_UNAVAILABLE* when you attempt
to save data into a slot that is already used. Likewise, *luksmeta load* will
return *EX_UNAVAILABLE* when you attempt to read from an empty slot.

== EXAMPLES

Destroy all data (including LUKSMeta data) in the LUKSv1 header gap and
initialize the gap for LUKSMeta storage:

    $ luksmeta init -n -f -d /dev/sdz

If already initialized, do nothing. Otherwise, destroy all non-LUKSMeta data
in the LUKSv1 header gap and initialize the gap for LUKSMeta storage:

    $ luksmeta init -f -d /dev/sdz

Write some data to a slot:

    $ UUID=*uuidgen*
    $ echo $UUID
    31c25e3b-b8e2-4eaa-a427-23aa882feef2
    $ echo "Hello, World" | luksmeta save -d /dev/sdz -s 0 -u $UUID

Read the data back:

    $ luksmeta load -d /dev/sdz -s 0 -u $UUID
    Hello, World

Wipe the data from the slot:

    $ luksmeta wipe -d /dev/sdz -s 0 -u $UUID

Erase all trace of LUKSMeta:

    $ luksmeta nuke -f -d /dev/sdz

== AUTHOR

Nathaniel McCallum <npmccallum@redhat.com>

== SEE ALSO

*cryptsetup*(8),
*uuidgen*(1)