1LUKSMETA(8) LUKSMETA(8)
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6 luksmeta - Utility for storing metadata in a LUKSv1 header
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9 luksmeta test -d DEVICE
10 luksmeta nuke -d DEVICE [-f]
12 luksmeta init -d DEVICE [-f] [-n]
14 luksmeta show -d DEVICE [-s SLOT]
16 luksmeta save -d DEVICE [-s SLOT] -u UUID < DATA
18 luksmeta load -d DEVICE -s SLOT [-u UUID] > DATA
20 luksmeta wipe -d DEVICE -s SLOT [-u UUID] [-f]
22
24 The luksmeta utility enables an administrator to store metadata in the
25 gap between the end of the LUKSv1 header and the start of the encrypted
26 data. This is useful for storing data that is available before the
27 volume is unlocked, usually for use during the volume unlock process.
28 The metadata is stored in a series of UUID-typed slots, allowing
30 multiple blocks of metadata. Although the luksmeta slots are inspired
31 by the LUKS slots, they are functionally independent and share only a
32 casual relationship. Slots merely provide a hint that a given chunk of
33 metadata is associated with a specific LUKSv1 password (in a slot with
34 the same number). However, luksmeta itself is indifferent to the
35 relationship between a LUKSv1 slot and the correspondly numbered
36 luksmeta slot, with one exception (detailed below).
37 After a LUKSv1 volume is initialized using cryptsetup(8), it must also
39 be initialized for metadata storage by luksmeta init. Once this is
40 complete, the device is ready to store medata.
41 Data can be written to a slot using luksmeta save or read from a slot
43 using luksmeta load. You can also erase the data in an existing slot
44 using luksmeta wipe or query the slots using luksmeta show.
45
47 It is often presumed that saving metadata to a slot requires a specific
48 UUID or that there is an official registry of UUID types. This is
49 incorrect.
50 UUID stands for Universally Unique IDentifier. UUIDs are a
52 standardized, widely-used data type used for identification without a
53 central registry. For the relevant standards, see ISO 9834-8:2005 and
54 RFC 4122.
55 UUIDs are large enough that collision is practically impossible. So if
57 your application wants to store data in a luksmeta slot, just generate
58 your own UUID and use it consistently to refer to your type of data. If
59 you have multiple types of data, feel free to generate multiple UUIDs.
60 The easiest way to generate a UUID is to use uuidgen(1). However, any
62 compliant UUID generator will suffice.
63
65 Before reading or writing metadata, the LUKSv1 block device must be
66 initialized for metadata storage. Three commands help with this
67 process: luksmeta test, luksmeta nuke and luksmeta init.
68 The luksmeta test command simply checks an existing block device to see
70 if it is initialized for metadata storage. This command does not
71 provide any output, so be sure to check its return code (see below).
72 The luksmeta nuke command will zero (erase) the entire LUKSv1 header
74 gap. Since this operation is destructive, user confirmation will be
75 required before clearing the gap unless the -f option is supplied.
76 The luksmeta init command initializes the LUKSv1 block device for
78 metadata storage. This process will wipe out any data in the LUKSv1
79 header gap. For this reason, this command will require user
80 confirmation before any data is written unless the -f option is
81 supplied. Note that this command succeeds without any modification if
82 the device is already initialized. If you would like to force the
83 creation of clean initialization state, you can specify the -n option
84 to nuke the LUKSv1 header gap before initialization (but after user
85 confirmation).
86
88 The luksmeta show command displays the current state of slots on the
89 LUKSv1 block device. If no slot is specified, it prints a table
90 consisting of the slot number, the corresponding LUKSv1 slot state and
91 the UUID of the data stored in the luksmeta slot (or "empty" if no data
92 is stored). If a slot is specified, this command simply prints out the
93 UUID of the data in the slot. If the slot does not contain data, it
94 prints nothing.
95
97 Managing the metadata in the slots is performed with three commands:
98 luksmeta save, luksmeta load and luksmeta wipe. These commands write
99 metadata to a slot, read metadata from a slot and erase metadata in a
100 slot, respectively.
101 The luksmeta save command reads metadata on standard input and writes
103 it to the specified slot using the specified UUID. If no slot is
104 specified, luksmeta will search for the first slot number for which the
105 LUKSv1 slot is inactive and the luksmeta slot is empty. This represents
106 the only official correlation between LUKSv1 slots and luksmeta slots.
107 In this case, the metadata is written to the first applicable slot
108 using the specified UUID and the slot number is printed to standard
109 output. In either case, this command will never overwrite existing
110 data. To replace data in a slot you will need to execute luksmeta wipe
111 before luksmeta save.
112 The luksmeta load command reads data from the specified slot and writes
114 it to standard output. If a UUID is specified, the command will verify
115 that the UUID associated with the metadata in the slot matches the
116 specified UUID. This type check helps to ensure that you always receive
117 the type of data you are expecting as output. If the UUIDs do not
118 match, the command will fail.
119 The luksmeta wipe command erases the data from the given slot. If a
121 UUID is specified, the command will verify that the UUID associated
122 with the metadata in the slot matches the specified UUID. This type
123 check helps to ensure that you only erase the data you intended to
124 erase. Because this is a destructive operation, this command will
125 require user confirmation before any data is erased, unless the -f
126 option is supplied. Note that this command succeeds if you attempt to
127 wipe a slot that is already empty.
128
130 The amount of storage in the LUKSv1 header gap is extremely limited. It
131 also varies based upon the configuration used by LUKSv1 at device
132 initialization time. In some LUKSv1 configurations, there is not even
133 enough space for all the metadata slots even at the smallest possible
134 slot size.
135 During the design of this utility, we considered it likely that users
137 would want to reduce the number of usable slots in exchange for more
138 storage space in the slots used. In order to provide this flexibility,
139 the amount of storage available per-slot is dynamic. Put simply, slots
140 are not a fixed size. This means that it is possible (and even somewhat
141 likely) to encounter an error during luksmeta save indicating that
142 there is insufficient space.
143 This error is not a programming bug. If you encounter this error it
145 likely means that either all space is being consumed by the
146 already-written slots or that the metadata you are attempting to write
147 simply does not fit.
148 You can attempt to resolve this problem by calling luksmeta wipe on
150 slots that are no longer in use. This will release the storage space
151 for use by other slots. Note that luksmeta does not, however, currently
152 perform defragmentation since the number of usable blocks is rather
153 limited. You can attempt to manually get around this by extracting all
154 slot data, wiping the slots and reloading them in order. However, this
155 operation is potentially dangerous and should be undertaken with great
156 care.
157
159 • -d DEVICE, --device=DEVICE : The device on which to perform the
160 operation.
161 • -s SLOT, --slot=SLOT : The slot number on which to perform the
163 operation.
164 • -u UUID, --uuid=UUID : The UUID to associate with the operation.
166 • -f, --force : Forcibly suppress all user prompting.
168
170 This command uses the return values as defined by sysexit.h. The
171 following are general errors whose meaning is shared by all luksmeta
172 commands:
173 • EX_OK : The operation was successful.
175 • EX_OSERR : An undefined operating system error occurred.
177 • EX_USAGE : The program was called with invalid parameters.
179 • EX_IOERR : An IO error occurred when writing to the device.
181 • EX_OSFILE : The device is not initialized or is corrupted.
183 • EX_NOPERM : The user did not grant permission during confirmation.
185 • EX_NOINPUT : An error occurred while reading from standard input.
187 • EX_DATAERR : The specified UUID does not match the slot UUID.
189 • EX_CANTCREAT : There is insufficient space in LUKSv1 header.
191 Additionally, luksmeta save will return EX_UNAVAILABLE when you attempt
193 to save data into a slot that is already used. Likewise, luksmeta load
194 will return EX_UNAVAILABLE when you attempt to read from an empty slot.
195
197 Destroy all data (including LUKSMeta data) in the LUKSv1 header gap and
198 initialize the gap for LUKSMeta storage:
199 $ luksmeta init -n -f -d /dev/sdz
201 If already initialized, do nothing. Otherwise, destroy all non-LUKSMeta
203 data in the LUKSv1 header gap and initialize the gap for LUKSMeta
204 storage:
205 $ luksmeta init -f -d /dev/sdz
207 Write some data to a slot:
209 $ UUID=*uuidgen*
211 $ echo $UUID
212 31c25e3b-b8e2-4eaa-a427-23aa882feef2
213 $ echo "Hello, World" | luksmeta save -d /dev/sdz -s 0 -u $UUID
214 Read the data back:
216 $ luksmeta load -d /dev/sdz -s 0 -u $UUID
218 Hello, World
219 Wipe the data from the slot:
221 $ luksmeta wipe -d /dev/sdz -s 0 -u $UUID
223 Erase all trace of LUKSMeta:
225 $ luksmeta nuke -f -d /dev/sdz
227
229 Nathaniel McCallum <npmccallum@redhat.com>
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232 cryptsetup(8), uuidgen(1)
233 07/21/2022 LUKSMETA(8)