1.. SPDX-License-Identifier: BSD-3-Clause 2 3========================================= 4Netlink protocol specifications (in YAML) 5========================================= 6 7Netlink protocol specifications are complete, machine readable descriptions of 8Netlink protocols written in YAML. The goal of the specifications is to allow 9separating Netlink parsing from user space logic and minimize the amount of 10hand written Netlink code for each new family, command, attribute. 11Netlink specs should be complete and not depend on any other spec 12or C header file, making it easy to use in languages which can't include 13kernel headers directly. 14 15Internally kernel uses the YAML specs to generate: 16 17 - the C uAPI header 18 - documentation of the protocol as a ReST file - see :ref:`Documentation/networking/netlink_spec/index.rst <specs>` 19 - policy tables for input attribute validation 20 - operation tables 21 22YAML specifications can be found under ``Documentation/netlink/specs/`` 23 24This document describes details of the schema. 25See :doc:`intro-specs` for a practical starting guide. 26 27All specs must be licensed under 28``((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)`` 29to allow for easy adoption in user space code. 30 31Compatibility levels 32==================== 33 34There are four schema levels for Netlink specs, from the simplest used 35by new families to the most complex covering all the quirks of the old ones. 36Each next level inherits the attributes of the previous level, meaning that 37user capable of parsing more complex ``genetlink`` schemas is also compatible 38with simpler ones. The levels are: 39 40 - ``genetlink`` - most streamlined, should be used by all new families 41 - ``genetlink-c`` - superset of ``genetlink`` with extra attributes allowing 42 customization of define and enum type and value names; this schema should 43 be equivalent to ``genetlink`` for all implementations which don't interact 44 directly with C uAPI headers 45 - ``genetlink-legacy`` - Generic Netlink catch all schema supporting quirks of 46 all old genetlink families, strange attribute formats, binary structures etc. 47 - ``netlink-raw`` - catch all schema supporting pre-Generic Netlink protocols 48 such as ``NETLINK_ROUTE`` 49 50The definition of the schemas (in ``jsonschema``) can be found 51under ``Documentation/netlink/``. 52 53Schema structure 54================ 55 56YAML schema has the following conceptual sections: 57 58 - globals 59 - definitions 60 - attributes 61 - operations 62 - multicast groups 63 64Most properties in the schema accept (or in fact require) a ``doc`` 65sub-property documenting the defined object. 66 67The following sections describe the properties of the most modern ``genetlink`` 68schema. See the documentation of :doc:`genetlink-c <c-code-gen>` 69for information on how C names are derived from name properties. 70 71See also :ref:`Documentation/core-api/netlink.rst <kernel_netlink>` for 72information on the Netlink specification properties that are only relevant to 73the kernel space and not part of the user space API. 74 75genetlink 76========= 77 78Globals 79------- 80 81Attributes listed directly at the root level of the spec file. 82 83name 84~~~~ 85 86Name of the family. Name identifies the family in a unique way, since 87the Family IDs are allocated dynamically. 88 89protocol 90~~~~~~~~ 91 92The schema level, default is ``genetlink``, which is the only value 93allowed for new ``genetlink`` families. 94 95definitions 96----------- 97 98Array of type and constant definitions. 99 100name 101~~~~ 102 103Name of the type / constant. 104 105type 106~~~~ 107 108One of the following types: 109 110 - const - a single, standalone constant 111 - enum - defines an integer enumeration, with values for each entry 112 incrementing by 1, (e.g. 0, 1, 2, 3) 113 - flags - defines an integer enumeration, with values for each entry 114 occupying a bit, starting from bit 0, (e.g. 1, 2, 4, 8) 115 116value 117~~~~~ 118 119The value for the ``const``. 120 121value-start 122~~~~~~~~~~~ 123 124The first value for ``enum`` and ``flags``, allows overriding the default 125start value of ``0`` (for ``enum``) and starting bit (for ``flags``). 126For ``flags`` ``value-start`` selects the starting bit, not the shifted value. 127 128Sparse enumerations are not supported. 129 130entries 131~~~~~~~ 132 133Array of names of the entries for ``enum`` and ``flags``. 134 135header 136~~~~~~ 137 138For C-compatible languages, header which already defines this value. 139In case the definition is shared by multiple families (e.g. ``IFNAMSIZ``) 140code generators for C-compatible languages may prefer to add an appropriate 141include instead of rendering a new definition. 142 143attribute-sets 144-------------- 145 146This property contains information about netlink attributes of the family. 147All families have at least one attribute set, most have multiple. 148``attribute-sets`` is an array, with each entry describing a single set. 149 150Note that the spec is "flattened" and is not meant to visually resemble 151the format of the netlink messages (unlike certain ad-hoc documentation 152formats seen in kernel comments). In the spec subordinate attribute sets 153are not defined inline as a nest, but defined in a separate attribute set 154referred to with a ``nested-attributes`` property of the container. 155 156Spec may also contain fractional sets - sets which contain a ``subset-of`` 157property. Such sets describe a section of a full set, allowing narrowing down 158which attributes are allowed in a nest or refining the validation criteria. 159Fractional sets can only be used in nests. They are not rendered to the uAPI 160in any fashion. 161 162name 163~~~~ 164 165Uniquely identifies the attribute set, operations and nested attributes 166refer to the sets by the ``name``. 167 168subset-of 169~~~~~~~~~ 170 171Re-defines a portion of another set (a fractional set). 172Allows narrowing down fields and changing validation criteria 173or even types of attributes depending on the nest in which they 174are contained. The ``value`` of each attribute in the fractional 175set is implicitly the same as in the main set. 176 177attributes 178~~~~~~~~~~ 179 180List of attributes in the set. 181 182.. _attribute_properties: 183 184Attribute properties 185-------------------- 186 187name 188~~~~ 189 190Identifies the attribute, unique within the set. 191 192type 193~~~~ 194 195Netlink attribute type, see :ref:`attr_types`. 196 197.. _assign_val: 198 199value 200~~~~~ 201 202Numerical attribute ID, used in serialized Netlink messages. 203The ``value`` property can be skipped, in which case the attribute ID 204will be the value of the previous attribute plus one (recursively) 205and ``1`` for the first attribute in the attribute set. 206 207Attributes (and operations) use ``1`` as the default value for the first 208entry (unlike enums in definitions which start from ``0``) because 209entry ``0`` is almost always reserved as undefined. Spec can explicitly 210set value to ``0`` if needed. 211 212Note that the ``value`` of an attribute is defined only in its main set 213(not in subsets). 214 215enum 216~~~~ 217 218For integer types specifies that values in the attribute belong 219to an ``enum`` or ``flags`` from the ``definitions`` section. 220 221enum-as-flags 222~~~~~~~~~~~~~ 223 224Treat ``enum`` as ``flags`` regardless of its type in ``definitions``. 225When both ``enum`` and ``flags`` forms are needed ``definitions`` should 226contain an ``enum`` and attributes which need the ``flags`` form should 227use this attribute. 228 229nested-attributes 230~~~~~~~~~~~~~~~~~ 231 232Identifies the attribute space for attributes nested within given attribute. 233Only valid for complex attributes which may have sub-attributes. 234 235multi-attr (arrays) 236~~~~~~~~~~~~~~~~~~~ 237 238Boolean property signifying that the attribute may be present multiple times. 239Allowing an attribute to repeat is the recommended way of implementing arrays 240(no extra nesting). 241 242byte-order 243~~~~~~~~~~ 244 245For integer types specifies attribute byte order - ``little-endian`` 246or ``big-endian``. 247 248checks 249~~~~~~ 250 251Input validation constraints used by the kernel. User space should query 252the policy of the running kernel using Generic Netlink introspection, 253rather than depend on what is specified in the spec file. 254 255The validation policy in the kernel is formed by combining the type 256definition (``type`` and ``nested-attributes``) and the ``checks``. 257 258sub-type 259~~~~~~~~ 260 261Legacy families have special ways of expressing arrays. ``sub-type`` can be 262used to define the type of array members in case array members are not 263fully defined as attributes (in a bona fide attribute space). For instance 264a C array of u32 values can be specified with ``type: binary`` and 265``sub-type: u32``. Binary types and legacy array formats are described in 266more detail in :doc:`genetlink-legacy`. 267 268display-hint 269~~~~~~~~~~~~ 270 271Optional format indicator that is intended only for choosing the right 272formatting mechanism when displaying values of this type. Currently supported 273hints are ``hex``, ``mac``, ``fddi``, ``ipv4``, ``ipv6`` and ``uuid``. 274 275operations 276---------- 277 278This section describes messages passed between the kernel and the user space. 279There are three types of entries in this section - operations, notifications 280and events. 281 282Operations describe the most common request - response communication. User 283sends a request and kernel replies. Each operation may contain any combination 284of the two modes familiar to netlink users - ``do`` and ``dump``. 285``do`` and ``dump`` in turn contain a combination of ``request`` and 286``response`` properties. If no explicit message with attributes is passed 287in a given direction (e.g. a ``dump`` which does not accept filter, or a ``do`` 288of a SET operation to which the kernel responds with just the netlink error 289code) ``request`` or ``response`` section can be skipped. 290``request`` and ``response`` sections list the attributes allowed in a message. 291The list contains only the names of attributes from a set referred 292to by the ``attribute-set`` property. 293 294Notifications and events both refer to the asynchronous messages sent by 295the kernel to members of a multicast group. The difference between the 296two is that a notification shares its contents with a GET operation 297(the name of the GET operation is specified in the ``notify`` property). 298This arrangement is commonly used for notifications about 299objects where the notification carries the full object definition. 300 301Events are more focused and carry only a subset of information rather than full 302object state (a made up example would be a link state change event with just 303the interface name and the new link state). Events contain the ``event`` 304property. Events are considered less idiomatic for netlink and notifications 305should be preferred. 306 307list 308~~~~ 309 310The only property of ``operations`` for ``genetlink``, holds the list of 311operations, notifications etc. 312 313Operation properties 314-------------------- 315 316name 317~~~~ 318 319Identifies the operation. 320 321value 322~~~~~ 323 324Numerical message ID, used in serialized Netlink messages. 325The same enumeration rules are applied as to 326:ref:`attribute values<assign_val>`. 327 328attribute-set 329~~~~~~~~~~~~~ 330 331Specifies the attribute set contained within the message. 332 333do 334~~~ 335 336Specification for the ``doit`` request. Should contain ``request``, ``reply`` 337or both of these properties, each holding a :ref:`attr_list`. 338 339dump 340~~~~ 341 342Specification for the ``dumpit`` request. Should contain ``request``, ``reply`` 343or both of these properties, each holding a :ref:`attr_list`. 344 345notify 346~~~~~~ 347 348Designates the message as a notification. Contains the name of the operation 349(possibly the same as the operation holding this property) which shares 350the contents with the notification (``do``). 351 352event 353~~~~~ 354 355Specification of attributes in the event, holds a :ref:`attr_list`. 356``event`` property is mutually exclusive with ``notify``. 357 358mcgrp 359~~~~~ 360 361Used with ``event`` and ``notify``, specifies which multicast group 362message belongs to. 363 364.. _attr_list: 365 366Message attribute list 367---------------------- 368 369``request``, ``reply`` and ``event`` properties have a single ``attributes`` 370property which holds the list of attribute names. 371 372Messages can also define ``pre`` and ``post`` properties which will be rendered 373as ``pre_doit`` and ``post_doit`` calls in the kernel (these properties should 374be ignored by user space). 375 376mcast-groups 377------------ 378 379This section lists the multicast groups of the family. 380 381list 382~~~~ 383 384The only property of ``mcast-groups`` for ``genetlink``, holds the list 385of groups. 386 387Multicast group properties 388-------------------------- 389 390name 391~~~~ 392 393Uniquely identifies the multicast group in the family. Similarly to 394Family ID, Multicast Group ID needs to be resolved at runtime, based 395on the name. 396 397.. _attr_types: 398 399Attribute types 400=============== 401 402This section describes the attribute types supported by the ``genetlink`` 403compatibility level. Refer to documentation of different levels for additional 404attribute types. 405 406Common integer types 407-------------------- 408 409``sint`` and ``uint`` represent signed and unsigned 64 bit integers. 410If the value can fit on 32 bits only 32 bits are carried in netlink 411messages, otherwise full 64 bits are carried. Note that the payload 412is only aligned to 4B, so the full 64 bit value may be unaligned! 413 414Common integer types should be preferred over fix-width types in majority 415of cases. 416 417Fix-width integer types 418----------------------- 419 420Fixed-width integer types include: 421``u8``, ``u16``, ``u32``, ``u64``, ``s8``, ``s16``, ``s32``, ``s64``. 422 423Note that types smaller than 32 bit should be avoided as using them 424does not save any memory in Netlink messages (due to alignment). 425See :ref:`pad_type` for padding of 64 bit attributes. 426 427The payload of the attribute is the integer in host order unless ``byte-order`` 428specifies otherwise. 429 43064 bit values are usually aligned by the kernel but it is recommended 431that the user space is able to deal with unaligned values. 432 433.. _pad_type: 434 435pad 436--- 437 438Special attribute type used for padding attributes which require alignment 439bigger than standard 4B alignment required by netlink (e.g. 64 bit integers). 440There can only be a single attribute of the ``pad`` type in any attribute set 441and it should be automatically used for padding when needed. 442 443flag 444---- 445 446Attribute with no payload, its presence is the entire information. 447 448binary 449------ 450 451Raw binary data attribute, the contents are opaque to generic code. 452 453string 454------ 455 456Character string. Unless ``checks`` has ``unterminated-ok`` set to ``true`` 457the string is required to be null terminated. 458``max-len`` in ``checks`` indicates the longest possible string, 459if not present the length of the string is unbounded. 460 461Note that ``max-len`` does not count the terminating character. 462 463nest 464---- 465 466Attribute containing other (nested) attributes. 467``nested-attributes`` specifies which attribute set is used inside. 468