1 .. SPDX-License-Identifier: BSD-3-Clause 1 .. SPDX-License-Identifier: BSD-3-Clause
2 2
3 ========================================= 3 =========================================
4 Netlink protocol specifications (in YAML) 4 Netlink protocol specifications (in YAML)
5 ========================================= 5 =========================================
6 6
7 Netlink protocol specifications are complete, 7 Netlink protocol specifications are complete, machine readable descriptions of
8 Netlink protocols written in YAML. The goal of 8 Netlink protocols written in YAML. The goal of the specifications is to allow
9 separating Netlink parsing from user space log 9 separating Netlink parsing from user space logic and minimize the amount of
10 hand written Netlink code for each new family, 10 hand written Netlink code for each new family, command, attribute.
11 Netlink specs should be complete and not depen 11 Netlink specs should be complete and not depend on any other spec
12 or C header file, making it easy to use in lan 12 or C header file, making it easy to use in languages which can't include
13 kernel headers directly. 13 kernel headers directly.
14 14
15 Internally kernel uses the YAML specs to gener 15 Internally kernel uses the YAML specs to generate:
16 16
17 - the C uAPI header 17 - the C uAPI header
18 - documentation of the protocol as a ReST fil !! 18 - documentation of the protocol as a ReST file
19 - policy tables for input attribute validatio 19 - policy tables for input attribute validation
20 - operation tables 20 - operation tables
21 21
22 YAML specifications can be found under ``Docum 22 YAML specifications can be found under ``Documentation/netlink/specs/``
23 23
24 This document describes details of the schema. 24 This document describes details of the schema.
25 See :doc:`intro-specs` for a practical startin 25 See :doc:`intro-specs` for a practical starting guide.
26 26
27 All specs must be licensed under 27 All specs must be licensed under
28 ``((GPL-2.0 WITH Linux-syscall-note) OR BSD-3- 28 ``((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)``
29 to allow for easy adoption in user space code. 29 to allow for easy adoption in user space code.
30 30
31 Compatibility levels 31 Compatibility levels
32 ==================== 32 ====================
33 33
34 There are four schema levels for Netlink specs 34 There are four schema levels for Netlink specs, from the simplest used
35 by new families to the most complex covering a 35 by new families to the most complex covering all the quirks of the old ones.
36 Each next level inherits the attributes of the 36 Each next level inherits the attributes of the previous level, meaning that
37 user capable of parsing more complex ``genetli 37 user capable of parsing more complex ``genetlink`` schemas is also compatible
38 with simpler ones. The levels are: 38 with simpler ones. The levels are:
39 39
40 - ``genetlink`` - most streamlined, should be 40 - ``genetlink`` - most streamlined, should be used by all new families
41 - ``genetlink-c`` - superset of ``genetlink`` 41 - ``genetlink-c`` - superset of ``genetlink`` with extra attributes allowing
42 customization of define and enum type and v 42 customization of define and enum type and value names; this schema should
43 be equivalent to ``genetlink`` for all impl 43 be equivalent to ``genetlink`` for all implementations which don't interact
44 directly with C uAPI headers 44 directly with C uAPI headers
45 - ``genetlink-legacy`` - Generic Netlink catc 45 - ``genetlink-legacy`` - Generic Netlink catch all schema supporting quirks of
46 all old genetlink families, strange attribu 46 all old genetlink families, strange attribute formats, binary structures etc.
47 - ``netlink-raw`` - catch all schema supporti 47 - ``netlink-raw`` - catch all schema supporting pre-Generic Netlink protocols
48 such as ``NETLINK_ROUTE`` 48 such as ``NETLINK_ROUTE``
49 49
50 The definition of the schemas (in ``jsonschema 50 The definition of the schemas (in ``jsonschema``) can be found
51 under ``Documentation/netlink/``. 51 under ``Documentation/netlink/``.
52 52
53 Schema structure 53 Schema structure
54 ================ 54 ================
55 55
56 YAML schema has the following conceptual secti 56 YAML schema has the following conceptual sections:
57 57
58 - globals 58 - globals
59 - definitions 59 - definitions
60 - attributes 60 - attributes
61 - operations 61 - operations
62 - multicast groups 62 - multicast groups
63 63
64 Most properties in the schema accept (or in fa 64 Most properties in the schema accept (or in fact require) a ``doc``
65 sub-property documenting the defined object. 65 sub-property documenting the defined object.
66 66
67 The following sections describe the properties 67 The following sections describe the properties of the most modern ``genetlink``
68 schema. See the documentation of :doc:`genetli 68 schema. See the documentation of :doc:`genetlink-c <c-code-gen>`
69 for information on how C names are derived fro 69 for information on how C names are derived from name properties.
70 70
71 See also :ref:`Documentation/core-api/netlink. 71 See also :ref:`Documentation/core-api/netlink.rst <kernel_netlink>` for
72 information on the Netlink specification prope 72 information on the Netlink specification properties that are only relevant to
73 the kernel space and not part of the user spac 73 the kernel space and not part of the user space API.
74 74
75 genetlink 75 genetlink
76 ========= 76 =========
77 77
78 Globals 78 Globals
79 ------- 79 -------
80 80
81 Attributes listed directly at the root level o 81 Attributes listed directly at the root level of the spec file.
82 82
83 name 83 name
84 ~~~~ 84 ~~~~
85 85
86 Name of the family. Name identifies the family 86 Name of the family. Name identifies the family in a unique way, since
87 the Family IDs are allocated dynamically. 87 the Family IDs are allocated dynamically.
88 88
>> 89 version
>> 90 ~~~~~~~
>> 91
>> 92 Generic Netlink family version, default is 1.
>> 93
89 protocol 94 protocol
90 ~~~~~~~~ 95 ~~~~~~~~
91 96
92 The schema level, default is ``genetlink``, wh 97 The schema level, default is ``genetlink``, which is the only value
93 allowed for new ``genetlink`` families. 98 allowed for new ``genetlink`` families.
94 99
95 definitions 100 definitions
96 ----------- 101 -----------
97 102
98 Array of type and constant definitions. 103 Array of type and constant definitions.
99 104
100 name 105 name
101 ~~~~ 106 ~~~~
102 107
103 Name of the type / constant. 108 Name of the type / constant.
104 109
105 type 110 type
106 ~~~~ 111 ~~~~
107 112
108 One of the following types: 113 One of the following types:
109 114
110 - const - a single, standalone constant 115 - const - a single, standalone constant
111 - enum - defines an integer enumeration, with 116 - enum - defines an integer enumeration, with values for each entry
112 incrementing by 1, (e.g. 0, 1, 2, 3) 117 incrementing by 1, (e.g. 0, 1, 2, 3)
113 - flags - defines an integer enumeration, wit 118 - flags - defines an integer enumeration, with values for each entry
114 occupying a bit, starting from bit 0, (e.g. 119 occupying a bit, starting from bit 0, (e.g. 1, 2, 4, 8)
115 120
116 value 121 value
117 ~~~~~ 122 ~~~~~
118 123
119 The value for the ``const``. 124 The value for the ``const``.
120 125
121 value-start 126 value-start
122 ~~~~~~~~~~~ 127 ~~~~~~~~~~~
123 128
124 The first value for ``enum`` and ``flags``, al 129 The first value for ``enum`` and ``flags``, allows overriding the default
125 start value of ``0`` (for ``enum``) and starti 130 start value of ``0`` (for ``enum``) and starting bit (for ``flags``).
126 For ``flags`` ``value-start`` selects the star 131 For ``flags`` ``value-start`` selects the starting bit, not the shifted value.
127 132
128 Sparse enumerations are not supported. 133 Sparse enumerations are not supported.
129 134
130 entries 135 entries
131 ~~~~~~~ 136 ~~~~~~~
132 137
133 Array of names of the entries for ``enum`` and 138 Array of names of the entries for ``enum`` and ``flags``.
134 139
135 header 140 header
136 ~~~~~~ 141 ~~~~~~
137 142
138 For C-compatible languages, header which alrea 143 For C-compatible languages, header which already defines this value.
139 In case the definition is shared by multiple f 144 In case the definition is shared by multiple families (e.g. ``IFNAMSIZ``)
140 code generators for C-compatible languages may 145 code generators for C-compatible languages may prefer to add an appropriate
141 include instead of rendering a new definition. 146 include instead of rendering a new definition.
142 147
143 attribute-sets 148 attribute-sets
144 -------------- 149 --------------
145 150
146 This property contains information about netli 151 This property contains information about netlink attributes of the family.
147 All families have at least one attribute set, 152 All families have at least one attribute set, most have multiple.
148 ``attribute-sets`` is an array, with each entr 153 ``attribute-sets`` is an array, with each entry describing a single set.
149 154
150 Note that the spec is "flattened" and is not m 155 Note that the spec is "flattened" and is not meant to visually resemble
151 the format of the netlink messages (unlike cer 156 the format of the netlink messages (unlike certain ad-hoc documentation
152 formats seen in kernel comments). In the spec 157 formats seen in kernel comments). In the spec subordinate attribute sets
153 are not defined inline as a nest, but defined 158 are not defined inline as a nest, but defined in a separate attribute set
154 referred to with a ``nested-attributes`` prope 159 referred to with a ``nested-attributes`` property of the container.
155 160
156 Spec may also contain fractional sets - sets w 161 Spec may also contain fractional sets - sets which contain a ``subset-of``
157 property. Such sets describe a section of a fu 162 property. Such sets describe a section of a full set, allowing narrowing down
158 which attributes are allowed in a nest or refi 163 which attributes are allowed in a nest or refining the validation criteria.
159 Fractional sets can only be used in nests. The 164 Fractional sets can only be used in nests. They are not rendered to the uAPI
160 in any fashion. 165 in any fashion.
161 166
162 name 167 name
163 ~~~~ 168 ~~~~
164 169
165 Uniquely identifies the attribute set, operati 170 Uniquely identifies the attribute set, operations and nested attributes
166 refer to the sets by the ``name``. 171 refer to the sets by the ``name``.
167 172
168 subset-of 173 subset-of
169 ~~~~~~~~~ 174 ~~~~~~~~~
170 175
171 Re-defines a portion of another set (a fractio 176 Re-defines a portion of another set (a fractional set).
172 Allows narrowing down fields and changing vali 177 Allows narrowing down fields and changing validation criteria
173 or even types of attributes depending on the n 178 or even types of attributes depending on the nest in which they
174 are contained. The ``value`` of each attribute 179 are contained. The ``value`` of each attribute in the fractional
175 set is implicitly the same as in the main set. 180 set is implicitly the same as in the main set.
176 181
177 attributes 182 attributes
178 ~~~~~~~~~~ 183 ~~~~~~~~~~
179 184
180 List of attributes in the set. 185 List of attributes in the set.
181 186
182 .. _attribute_properties: 187 .. _attribute_properties:
183 188
184 Attribute properties 189 Attribute properties
185 -------------------- 190 --------------------
186 191
187 name 192 name
188 ~~~~ 193 ~~~~
189 194
190 Identifies the attribute, unique within the se 195 Identifies the attribute, unique within the set.
191 196
192 type 197 type
193 ~~~~ 198 ~~~~
194 199
195 Netlink attribute type, see :ref:`attr_types`. 200 Netlink attribute type, see :ref:`attr_types`.
196 201
197 .. _assign_val: 202 .. _assign_val:
198 203
199 value 204 value
200 ~~~~~ 205 ~~~~~
201 206
202 Numerical attribute ID, used in serialized Net 207 Numerical attribute ID, used in serialized Netlink messages.
203 The ``value`` property can be skipped, in whic 208 The ``value`` property can be skipped, in which case the attribute ID
204 will be the value of the previous attribute pl 209 will be the value of the previous attribute plus one (recursively)
205 and ``1`` for the first attribute in the attri 210 and ``1`` for the first attribute in the attribute set.
206 211
207 Attributes (and operations) use ``1`` as the d 212 Attributes (and operations) use ``1`` as the default value for the first
208 entry (unlike enums in definitions which start 213 entry (unlike enums in definitions which start from ``0``) because
209 entry ``0`` is almost always reserved as undef 214 entry ``0`` is almost always reserved as undefined. Spec can explicitly
210 set value to ``0`` if needed. 215 set value to ``0`` if needed.
211 216
212 Note that the ``value`` of an attribute is def 217 Note that the ``value`` of an attribute is defined only in its main set
213 (not in subsets). 218 (not in subsets).
214 219
215 enum 220 enum
216 ~~~~ 221 ~~~~
217 222
218 For integer types specifies that values in the 223 For integer types specifies that values in the attribute belong
219 to an ``enum`` or ``flags`` from the ``definit 224 to an ``enum`` or ``flags`` from the ``definitions`` section.
220 225
221 enum-as-flags 226 enum-as-flags
222 ~~~~~~~~~~~~~ 227 ~~~~~~~~~~~~~
223 228
224 Treat ``enum`` as ``flags`` regardless of its 229 Treat ``enum`` as ``flags`` regardless of its type in ``definitions``.
225 When both ``enum`` and ``flags`` forms are nee 230 When both ``enum`` and ``flags`` forms are needed ``definitions`` should
226 contain an ``enum`` and attributes which need 231 contain an ``enum`` and attributes which need the ``flags`` form should
227 use this attribute. 232 use this attribute.
228 233
229 nested-attributes 234 nested-attributes
230 ~~~~~~~~~~~~~~~~~ 235 ~~~~~~~~~~~~~~~~~
231 236
232 Identifies the attribute space for attributes 237 Identifies the attribute space for attributes nested within given attribute.
233 Only valid for complex attributes which may ha 238 Only valid for complex attributes which may have sub-attributes.
234 239
235 multi-attr (arrays) 240 multi-attr (arrays)
236 ~~~~~~~~~~~~~~~~~~~ 241 ~~~~~~~~~~~~~~~~~~~
237 242
238 Boolean property signifying that the attribute 243 Boolean property signifying that the attribute may be present multiple times.
239 Allowing an attribute to repeat is the recomme 244 Allowing an attribute to repeat is the recommended way of implementing arrays
240 (no extra nesting). 245 (no extra nesting).
241 246
242 byte-order 247 byte-order
243 ~~~~~~~~~~ 248 ~~~~~~~~~~
244 249
245 For integer types specifies attribute byte ord 250 For integer types specifies attribute byte order - ``little-endian``
246 or ``big-endian``. 251 or ``big-endian``.
247 252
248 checks 253 checks
249 ~~~~~~ 254 ~~~~~~
250 255
251 Input validation constraints used by the kerne 256 Input validation constraints used by the kernel. User space should query
252 the policy of the running kernel using Generic 257 the policy of the running kernel using Generic Netlink introspection,
253 rather than depend on what is specified in the 258 rather than depend on what is specified in the spec file.
254 259
255 The validation policy in the kernel is formed 260 The validation policy in the kernel is formed by combining the type
256 definition (``type`` and ``nested-attributes`` 261 definition (``type`` and ``nested-attributes``) and the ``checks``.
257 262
258 sub-type 263 sub-type
259 ~~~~~~~~ 264 ~~~~~~~~
260 265
261 Legacy families have special ways of expressin 266 Legacy families have special ways of expressing arrays. ``sub-type`` can be
262 used to define the type of array members in ca 267 used to define the type of array members in case array members are not
263 fully defined as attributes (in a bona fide at 268 fully defined as attributes (in a bona fide attribute space). For instance
264 a C array of u32 values can be specified with 269 a C array of u32 values can be specified with ``type: binary`` and
265 ``sub-type: u32``. Binary types and legacy arr 270 ``sub-type: u32``. Binary types and legacy array formats are described in
266 more detail in :doc:`genetlink-legacy`. 271 more detail in :doc:`genetlink-legacy`.
267 272
268 display-hint 273 display-hint
269 ~~~~~~~~~~~~ 274 ~~~~~~~~~~~~
270 275
271 Optional format indicator that is intended onl 276 Optional format indicator that is intended only for choosing the right
272 formatting mechanism when displaying values of 277 formatting mechanism when displaying values of this type. Currently supported
273 hints are ``hex``, ``mac``, ``fddi``, ``ipv4`` 278 hints are ``hex``, ``mac``, ``fddi``, ``ipv4``, ``ipv6`` and ``uuid``.
274 279
275 operations 280 operations
276 ---------- 281 ----------
277 282
278 This section describes messages passed between 283 This section describes messages passed between the kernel and the user space.
279 There are three types of entries in this secti 284 There are three types of entries in this section - operations, notifications
280 and events. 285 and events.
281 286
282 Operations describe the most common request - 287 Operations describe the most common request - response communication. User
283 sends a request and kernel replies. Each opera 288 sends a request and kernel replies. Each operation may contain any combination
284 of the two modes familiar to netlink users - ` 289 of the two modes familiar to netlink users - ``do`` and ``dump``.
285 ``do`` and ``dump`` in turn contain a combinat 290 ``do`` and ``dump`` in turn contain a combination of ``request`` and
286 ``response`` properties. If no explicit messag 291 ``response`` properties. If no explicit message with attributes is passed
287 in a given direction (e.g. a ``dump`` which do 292 in a given direction (e.g. a ``dump`` which does not accept filter, or a ``do``
288 of a SET operation to which the kernel respond 293 of a SET operation to which the kernel responds with just the netlink error
289 code) ``request`` or ``response`` section can 294 code) ``request`` or ``response`` section can be skipped.
290 ``request`` and ``response`` sections list the 295 ``request`` and ``response`` sections list the attributes allowed in a message.
291 The list contains only the names of attributes 296 The list contains only the names of attributes from a set referred
292 to by the ``attribute-set`` property. 297 to by the ``attribute-set`` property.
293 298
294 Notifications and events both refer to the asy 299 Notifications and events both refer to the asynchronous messages sent by
295 the kernel to members of a multicast group. Th 300 the kernel to members of a multicast group. The difference between the
296 two is that a notification shares its contents 301 two is that a notification shares its contents with a GET operation
297 (the name of the GET operation is specified in 302 (the name of the GET operation is specified in the ``notify`` property).
298 This arrangement is commonly used for notifica 303 This arrangement is commonly used for notifications about
299 objects where the notification carries the ful 304 objects where the notification carries the full object definition.
300 305
301 Events are more focused and carry only a subse 306 Events are more focused and carry only a subset of information rather than full
302 object state (a made up example would be a lin 307 object state (a made up example would be a link state change event with just
303 the interface name and the new link state). Ev 308 the interface name and the new link state). Events contain the ``event``
304 property. Events are considered less idiomatic 309 property. Events are considered less idiomatic for netlink and notifications
305 should be preferred. 310 should be preferred.
306 311
307 list 312 list
308 ~~~~ 313 ~~~~
309 314
310 The only property of ``operations`` for ``gene 315 The only property of ``operations`` for ``genetlink``, holds the list of
311 operations, notifications etc. 316 operations, notifications etc.
312 317
313 Operation properties 318 Operation properties
314 -------------------- 319 --------------------
315 320
316 name 321 name
317 ~~~~ 322 ~~~~
318 323
319 Identifies the operation. 324 Identifies the operation.
320 325
321 value 326 value
322 ~~~~~ 327 ~~~~~
323 328
324 Numerical message ID, used in serialized Netli 329 Numerical message ID, used in serialized Netlink messages.
325 The same enumeration rules are applied as to 330 The same enumeration rules are applied as to
326 :ref:`attribute values<assign_val>`. 331 :ref:`attribute values<assign_val>`.
327 332
328 attribute-set 333 attribute-set
329 ~~~~~~~~~~~~~ 334 ~~~~~~~~~~~~~
330 335
331 Specifies the attribute set contained within t 336 Specifies the attribute set contained within the message.
332 337
333 do 338 do
334 ~~~ 339 ~~~
335 340
336 Specification for the ``doit`` request. Should 341 Specification for the ``doit`` request. Should contain ``request``, ``reply``
337 or both of these properties, each holding a :r 342 or both of these properties, each holding a :ref:`attr_list`.
338 343
339 dump 344 dump
340 ~~~~ 345 ~~~~
341 346
342 Specification for the ``dumpit`` request. Shou 347 Specification for the ``dumpit`` request. Should contain ``request``, ``reply``
343 or both of these properties, each holding a :r 348 or both of these properties, each holding a :ref:`attr_list`.
344 349
345 notify 350 notify
346 ~~~~~~ 351 ~~~~~~
347 352
348 Designates the message as a notification. Cont 353 Designates the message as a notification. Contains the name of the operation
349 (possibly the same as the operation holding th 354 (possibly the same as the operation holding this property) which shares
350 the contents with the notification (``do``). 355 the contents with the notification (``do``).
351 356
352 event 357 event
353 ~~~~~ 358 ~~~~~
354 359
355 Specification of attributes in the event, hold 360 Specification of attributes in the event, holds a :ref:`attr_list`.
356 ``event`` property is mutually exclusive with 361 ``event`` property is mutually exclusive with ``notify``.
357 362
358 mcgrp 363 mcgrp
359 ~~~~~ 364 ~~~~~
360 365
361 Used with ``event`` and ``notify``, specifies 366 Used with ``event`` and ``notify``, specifies which multicast group
362 message belongs to. 367 message belongs to.
363 368
364 .. _attr_list: 369 .. _attr_list:
365 370
366 Message attribute list 371 Message attribute list
367 ---------------------- 372 ----------------------
368 373
369 ``request``, ``reply`` and ``event`` propertie 374 ``request``, ``reply`` and ``event`` properties have a single ``attributes``
370 property which holds the list of attribute nam 375 property which holds the list of attribute names.
371 376
372 Messages can also define ``pre`` and ``post`` 377 Messages can also define ``pre`` and ``post`` properties which will be rendered
373 as ``pre_doit`` and ``post_doit`` calls in the 378 as ``pre_doit`` and ``post_doit`` calls in the kernel (these properties should
374 be ignored by user space). 379 be ignored by user space).
375 380
376 mcast-groups 381 mcast-groups
377 ------------ 382 ------------
378 383
379 This section lists the multicast groups of the 384 This section lists the multicast groups of the family.
380 385
381 list 386 list
382 ~~~~ 387 ~~~~
383 388
384 The only property of ``mcast-groups`` for ``ge 389 The only property of ``mcast-groups`` for ``genetlink``, holds the list
385 of groups. 390 of groups.
386 391
387 Multicast group properties 392 Multicast group properties
388 -------------------------- 393 --------------------------
389 394
390 name 395 name
391 ~~~~ 396 ~~~~
392 397
393 Uniquely identifies the multicast group in the 398 Uniquely identifies the multicast group in the family. Similarly to
394 Family ID, Multicast Group ID needs to be reso 399 Family ID, Multicast Group ID needs to be resolved at runtime, based
395 on the name. 400 on the name.
396 401
397 .. _attr_types: 402 .. _attr_types:
398 403
399 Attribute types 404 Attribute types
400 =============== 405 ===============
401 406
402 This section describes the attribute types sup 407 This section describes the attribute types supported by the ``genetlink``
403 compatibility level. Refer to documentation of 408 compatibility level. Refer to documentation of different levels for additional
404 attribute types. 409 attribute types.
405 410
406 Common integer types !! 411 Scalar integer types
407 -------------------- 412 --------------------
408 413
409 ``sint`` and ``uint`` represent signed and uns !! 414 Fixed-width integer types:
410 If the value can fit on 32 bits only 32 bits a <<
411 messages, otherwise full 64 bits are carried. <<
412 is only aligned to 4B, so the full 64 bit valu <<
413 <<
414 Common integer types should be preferred over <<
415 of cases. <<
416 <<
417 Fix-width integer types <<
418 ----------------------- <<
419 <<
420 Fixed-width integer types include: <<
421 ``u8``, ``u16``, ``u32``, ``u64``, ``s8``, ``s 415 ``u8``, ``u16``, ``u32``, ``u64``, ``s8``, ``s16``, ``s32``, ``s64``.
422 416
423 Note that types smaller than 32 bit should be 417 Note that types smaller than 32 bit should be avoided as using them
424 does not save any memory in Netlink messages ( 418 does not save any memory in Netlink messages (due to alignment).
425 See :ref:`pad_type` for padding of 64 bit attr 419 See :ref:`pad_type` for padding of 64 bit attributes.
426 420
427 The payload of the attribute is the integer in 421 The payload of the attribute is the integer in host order unless ``byte-order``
428 specifies otherwise. 422 specifies otherwise.
429 <<
430 64 bit values are usually aligned by the kerne <<
431 that the user space is able to deal with unali <<
432 423
433 .. _pad_type: 424 .. _pad_type:
434 425
435 pad 426 pad
436 --- 427 ---
437 428
438 Special attribute type used for padding attrib 429 Special attribute type used for padding attributes which require alignment
439 bigger than standard 4B alignment required by 430 bigger than standard 4B alignment required by netlink (e.g. 64 bit integers).
440 There can only be a single attribute of the `` 431 There can only be a single attribute of the ``pad`` type in any attribute set
441 and it should be automatically used for paddin 432 and it should be automatically used for padding when needed.
442 433
443 flag 434 flag
444 ---- 435 ----
445 436
446 Attribute with no payload, its presence is the 437 Attribute with no payload, its presence is the entire information.
447 438
448 binary 439 binary
449 ------ 440 ------
450 441
451 Raw binary data attribute, the contents are op 442 Raw binary data attribute, the contents are opaque to generic code.
452 443
453 string 444 string
454 ------ 445 ------
455 446
456 Character string. Unless ``checks`` has ``unte 447 Character string. Unless ``checks`` has ``unterminated-ok`` set to ``true``
457 the string is required to be null terminated. 448 the string is required to be null terminated.
458 ``max-len`` in ``checks`` indicates the longes 449 ``max-len`` in ``checks`` indicates the longest possible string,
459 if not present the length of the string is unb 450 if not present the length of the string is unbounded.
460 451
461 Note that ``max-len`` does not count the termi 452 Note that ``max-len`` does not count the terminating character.
462 453
463 nest 454 nest
464 ---- 455 ----
465 456
466 Attribute containing other (nested) attributes 457 Attribute containing other (nested) attributes.
467 ``nested-attributes`` specifies which attribut 458 ``nested-attributes`` specifies which attribute set is used inside.
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