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