--- /dev/null
+.. SPDX-License-Identifier: BSD-3-Clause
+
+=======================
+Introduction to Netlink
+=======================
+
+Netlink is often described as an ioctl() replacement.
+It aims to replace fixed-format C structures as supplied
+to ioctl() with a format which allows an easy way to add
+or extended the arguments.
+
+To achieve this Netlink uses a minimal fixed-format metadata header
+followed by multiple attributes in the TLV (type, length, value) format.
+
+Unfortunately the protocol has evolved over the years, in an organic
+and undocumented fashion, making it hard to coherently explain.
+To make the most practical sense this document starts by describing
+netlink as it is used today and dives into more "historical" uses
+in later sections.
+
+Opening a socket
+================
+
+Netlink communication happens over sockets, a socket needs to be
+opened first:
+
+.. code-block:: c
+
+ fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+
+The use of sockets allows for a natural way of exchanging information
+in both directions (to and from the kernel). The operations are still
+performed synchronously when applications send() the request but
+a separate recv() system call is needed to read the reply.
+
+A very simplified flow of a Netlink "call" will therefore look
+something like:
+
+.. code-block:: c
+
+ fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+
+ /* format the request */
+ send(fd, &request, sizeof(request));
+ n = recv(fd, &response, RSP_BUFFER_SIZE);
+ /* interpret the response */
+
+Netlink also provides natural support for "dumping", i.e. communicating
+to user space all objects of a certain type (e.g. dumping all network
+interfaces).
+
+.. code-block:: c
+
+ fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+
+ /* format the dump request */
+ send(fd, &request, sizeof(request));
+ while (1) {
+ n = recv(fd, &buffer, RSP_BUFFER_SIZE);
+ /* one recv() call can read multiple messages, hence the loop below */
+ for (nl_msg in buffer) {
+ if (nl_msg.nlmsg_type == NLMSG_DONE)
+ goto dump_finished;
+ /* process the object */
+ }
+ }
+ dump_finished:
+
+The first two arguments of the socket() call require little explanation -
+it is opening a Netlink socket, with all headers provided by the user
+(hence NETLINK, RAW). The last argument is the protocol within Netlink.
+This field used to identify the subsystem with which the socket will
+communicate.
+
+Classic vs Generic Netlink
+--------------------------
+
+Initial implementation of Netlink depended on a static allocation
+of IDs to subsystems and provided little supporting infrastructure.
+Let us refer to those protocols collectively as **Classic Netlink**.
+The list of them is defined on top of the ``include/uapi/linux/netlink.h``
+file, they include among others - general networking (NETLINK_ROUTE),
+iSCSI (NETLINK_ISCSI), and audit (NETLINK_AUDIT).
+
+**Generic Netlink** (introduced in 2005) allows for dynamic registration of
+subsystems (and subsystem ID allocation), introspection and simplifies
+implementing the kernel side of the interface.
+
+The following section describes how to use Generic Netlink, as the
+number of subsystems using Generic Netlink outnumbers the older
+protocols by an order of magnitude. There are also no plans for adding
+more Classic Netlink protocols to the kernel.
+Basic information on how communicating with core networking parts of
+the Linux kernel (or another of the 20 subsystems using Classic
+Netlink) differs from Generic Netlink is provided later in this document.
+
+Generic Netlink
+===============
+
+In addition to the Netlink fixed metadata header each Netlink protocol
+defines its own fixed metadata header. (Similarly to how network
+headers stack - Ethernet > IP > TCP we have Netlink > Generic N. > Family.)
+
+A Netlink message always starts with struct nlmsghdr, which is followed
+by a protocol-specific header. In case of Generic Netlink the protocol
+header is struct genlmsghdr.
+
+The practical meaning of the fields in case of Generic Netlink is as follows:
+
+.. code-block:: c
+
+ struct nlmsghdr {
+ __u32 nlmsg_len; /* Length of message including headers */
+ __u16 nlmsg_type; /* Generic Netlink Family (subsystem) ID */
+ __u16 nlmsg_flags; /* Flags - request or dump */
+ __u32 nlmsg_seq; /* Sequence number */
+ __u32 nlmsg_pid; /* Port ID, set to 0 */
+ };
+ struct genlmsghdr {
+ __u8 cmd; /* Command, as defined by the Family */
+ __u8 version; /* Irrelevant, set to 1 */
+ __u16 reserved; /* Reserved, set to 0 */
+ };
+ /* TLV attributes follow... */
+
+In Classic Netlink :c:member:`nlmsghdr.nlmsg_type` used to identify
+which operation within the subsystem the message was referring to
+(e.g. get information about a netdev). Generic Netlink needs to mux
+multiple subsystems in a single protocol so it uses this field to
+identify the subsystem, and :c:member:`genlmsghdr.cmd` identifies
+the operation instead. (See :ref:`res_fam` for
+information on how to find the Family ID of the subsystem of interest.)
+Note that the first 16 values (0 - 15) of this field are reserved for
+control messages both in Classic Netlink and Generic Netlink.
+See :ref:`nl_msg_type` for more details.
+
+There are 3 usual types of message exchanges on a Netlink socket:
+
+ - performing a single action (``do``);
+ - dumping information (``dump``);
+ - getting asynchronous notifications (``multicast``).
+
+Classic Netlink is very flexible and presumably allows other types
+of exchanges to happen, but in practice those are the three that get
+used.
+
+Asynchronous notifications are sent by the kernel and received by
+the user sockets which subscribed to them. ``do`` and ``dump`` requests
+are initiated by the user. :c:member:`nlmsghdr.nlmsg_flags` should
+be set as follows:
+
+ - for ``do``: ``NLM_F_REQUEST | NLM_F_ACK``
+ - for ``dump``: ``NLM_F_REQUEST | NLM_F_ACK | NLM_F_DUMP``
+
+:c:member:`nlmsghdr.nlmsg_seq` should be a set to a monotonically
+increasing value. The value gets echoed back in responses and doesn't
+matter in practice, but setting it to an increasing value for each
+message sent is considered good hygiene. The purpose of the field is
+matching responses to requests. Asynchronous notifications will have
+:c:member:`nlmsghdr.nlmsg_seq` of ``0``.
+
+:c:member:`nlmsghdr.nlmsg_pid` is the Netlink equivalent of an address.
+This field can be set to ``0`` when talking to the kernel.
+See :ref:`nlmsg_pid` for the (uncommon) uses of the field.
+
+The expected use for :c:member:`genlmsghdr.version` was to allow
+versioning of the APIs provided by the subsystems. No subsystem to
+date made significant use of this field, so setting it to ``1`` seems
+like a safe bet.
+
+.. _nl_msg_type:
+
+Netlink message types
+---------------------
+
+As previously mentioned :c:member:`nlmsghdr.nlmsg_type` carries
+protocol specific values but the first 16 identifiers are reserved
+(first subsystem specific message type should be equal to
+``NLMSG_MIN_TYPE`` which is ``0x10``).
+
+There are only 4 Netlink control messages defined:
+
+ - ``NLMSG_NOOP`` - ignore the message, not used in practice;
+ - ``NLMSG_ERROR`` - carries the return code of an operation;
+ - ``NLMSG_DONE`` - marks the end of a dump;
+ - ``NLMSG_OVERRUN`` - socket buffer has overflown, not used to date.
+
+``NLMSG_ERROR`` and ``NLMSG_DONE`` are of practical importance.
+They carry return codes for operations. Note that unless
+the ``NLM_F_ACK`` flag is set on the request Netlink will not respond
+with ``NLMSG_ERROR`` if there is no error. To avoid having to special-case
+this quirk it is recommended to always set ``NLM_F_ACK``.
+
+The format of ``NLMSG_ERROR`` is described by struct nlmsgerr::
+
+ ----------------------------------------------
+ | struct nlmsghdr - response header |
+ ----------------------------------------------
+ | int error |
+ ----------------------------------------------
+ | struct nlmsghdr - original request header |
+ ----------------------------------------------
+ | ** optionally (1) payload of the request |
+ ----------------------------------------------
+ | ** optionally (2) extended ACK |
+ ----------------------------------------------
+
+There are two instances of struct nlmsghdr here, first of the response
+and second of the request. ``NLMSG_ERROR`` carries the information about
+the request which led to the error. This could be useful when trying
+to match requests to responses or re-parse the request to dump it into
+logs.
+
+The payload of the request is not echoed in messages reporting success
+(``error == 0``) or if ``NETLINK_CAP_ACK`` setsockopt() was set.
+The latter is common
+and perhaps recommended as having to read a copy of every request back
+from the kernel is rather wasteful. The absence of request payload
+is indicated by ``NLM_F_CAPPED`` in :c:member:`nlmsghdr.nlmsg_flags`.
+
+The second optional element of ``NLMSG_ERROR`` are the extended ACK
+attributes. See :ref:`ext_ack` for more details. The presence
+of extended ACK is indicated by ``NLM_F_ACK_TLVS`` in
+:c:member:`nlmsghdr.nlmsg_flags`.
+
+``NLMSG_DONE`` is simpler, the request is never echoed but the extended
+ACK attributes may be present::
+
+ ----------------------------------------------
+ | struct nlmsghdr - response header |
+ ----------------------------------------------
+ | int error |
+ ----------------------------------------------
+ | ** optionally extended ACK |
+ ----------------------------------------------
+
+.. _res_fam:
+
+Resolving the Family ID
+-----------------------
+
+This section explains how to find the Family ID of a subsystem.
+It also serves as an example of Generic Netlink communication.
+
+Generic Netlink is itself a subsystem exposed via the Generic Netlink API.
+To avoid a circular dependency Generic Netlink has a statically allocated
+Family ID (``GENL_ID_CTRL`` which is equal to ``NLMSG_MIN_TYPE``).
+The Generic Netlink family implements a command used to find out information
+about other families (``CTRL_CMD_GETFAMILY``).
+
+To get information about the Generic Netlink family named for example
+``"test1"`` we need to send a message on the previously opened Generic Netlink
+socket. The message should target the Generic Netlink Family (1), be a
+``do`` (2) call to ``CTRL_CMD_GETFAMILY`` (3). A ``dump`` version of this
+call would make the kernel respond with information about *all* the families
+it knows about. Last but not least the name of the family in question has
+to be specified (4) as an attribute with the appropriate type::
+
+ struct nlmsghdr:
+ __u32 nlmsg_len: 32
+ __u16 nlmsg_type: GENL_ID_CTRL // (1)
+ __u16 nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK // (2)
+ __u32 nlmsg_seq: 1
+ __u32 nlmsg_pid: 0
+
+ struct genlmsghdr:
+ __u8 cmd: CTRL_CMD_GETFAMILY // (3)
+ __u8 version: 2 /* or 1, doesn't matter */
+ __u16 reserved: 0
+
+ struct nlattr: // (4)
+ __u16 nla_len: 10
+ __u16 nla_type: CTRL_ATTR_FAMILY_NAME
+ char data: test1\0
+
+ (padding:)
+ char data: \0\0
+
+The length fields in Netlink (:c:member:`nlmsghdr.nlmsg_len`
+and :c:member:`nlattr.nla_len`) always *include* the header.
+Attribute headers in netlink must be aligned to 4 bytes from the start
+of the message, hence the extra ``\0\0`` after ``CTRL_ATTR_FAMILY_NAME``.
+The attribute lengths *exclude* the padding.
+
+If the family is found kernel will reply with two messages, the response
+with all the information about the family::
+
+ /* Message #1 - reply */
+ struct nlmsghdr:
+ __u32 nlmsg_len: 136
+ __u16 nlmsg_type: GENL_ID_CTRL
+ __u16 nlmsg_flags: 0
+ __u32 nlmsg_seq: 1 /* echoed from our request */
+ __u32 nlmsg_pid: 5831 /* The PID of our user space process */
+
+ struct genlmsghdr:
+ __u8 cmd: CTRL_CMD_GETFAMILY
+ __u8 version: 2
+ __u16 reserved: 0
+
+ struct nlattr:
+ __u16 nla_len: 10
+ __u16 nla_type: CTRL_ATTR_FAMILY_NAME
+ char data: test1\0
+
+ (padding:)
+ data: \0\0
+
+ struct nlattr:
+ __u16 nla_len: 6
+ __u16 nla_type: CTRL_ATTR_FAMILY_ID
+ __u16: 123 /* The Family ID we are after */
+
+ (padding:)
+ char data: \0\0
+
+ struct nlattr:
+ __u16 nla_len: 9
+ __u16 nla_type: CTRL_ATTR_FAMILY_VERSION
+ __u16: 1
+
+ /* ... etc, more attributes will follow. */
+
+And the error code (success) since ``NLM_F_ACK`` had been set on the request::
+
+ /* Message #2 - the ACK */
+ struct nlmsghdr:
+ __u32 nlmsg_len: 36
+ __u16 nlmsg_type: NLMSG_ERROR
+ __u16 nlmsg_flags: NLM_F_CAPPED /* There won't be a payload */
+ __u32 nlmsg_seq: 1 /* echoed from our request */
+ __u32 nlmsg_pid: 5831 /* The PID of our user space process */
+
+ int error: 0
+
+ struct nlmsghdr: /* Copy of the request header as we sent it */
+ __u32 nlmsg_len: 32
+ __u16 nlmsg_type: GENL_ID_CTRL
+ __u16 nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK
+ __u32 nlmsg_seq: 1
+ __u32 nlmsg_pid: 0
+
+The order of attributes (struct nlattr) is not guaranteed so the user
+has to walk the attributes and parse them.
+
+Note that Generic Netlink sockets are not associated or bound to a single
+family. A socket can be used to exchange messages with many different
+families, selecting the recipient family on message-by-message basis using
+the :c:member:`nlmsghdr.nlmsg_type` field.
+
+.. _ext_ack:
+
+Extended ACK
+------------
+
+Extended ACK controls reporting of additional error/warning TLVs
+in ``NLMSG_ERROR`` and ``NLMSG_DONE`` messages. To maintain backward
+compatibility this feature has to be explicitly enabled by setting
+the ``NETLINK_EXT_ACK`` setsockopt() to ``1``.
+
+Types of extended ack attributes are defined in enum nlmsgerr_attrs.
+The two most commonly used attributes are ``NLMSGERR_ATTR_MSG``
+and ``NLMSGERR_ATTR_OFFS``.
+
+``NLMSGERR_ATTR_MSG`` carries a message in English describing
+the encountered problem. These messages are far more detailed
+than what can be expressed thru standard UNIX error codes.
+
+``NLMSGERR_ATTR_OFFS`` points to the attribute which caused the problem.
+
+Extended ACKs can be reported on errors as well as in case of success.
+The latter should be treated as a warning.
+
+Extended ACKs greatly improve the usability of Netlink and should
+always be enabled, appropriately parsed and reported to the user.
+
+Advanced topics
+===============
+
+Dump consistency
+----------------
+
+Some of the data structures kernel uses for storing objects make
+it hard to provide an atomic snapshot of all the objects in a dump
+(without impacting the fast-paths updating them).
+
+Kernel may set the ``NLM_F_DUMP_INTR`` flag on any message in a dump
+(including the ``NLMSG_DONE`` message) if the dump was interrupted and
+may be inconsistent (e.g. missing objects). User space should retry
+the dump if it sees the flag set.
+
+Introspection
+-------------
+
+The basic introspection abilities are enabled by access to the Family
+object as reported in :ref:`res_fam`. User can query information about
+the Generic Netlink family, including which operations are supported
+by the kernel and what attributes the kernel understands.
+Family information includes the highest ID of an attribute kernel can parse,
+a separate command (``CTRL_CMD_GETPOLICY``) provides detailed information
+about supported attributes, including ranges of values the kernel accepts.
+
+Querying family information is useful in cases when user space needs
+to make sure that the kernel has support for a feature before issuing
+a request.
+
+.. _nlmsg_pid:
+
+nlmsg_pid
+---------
+
+:c:member:`nlmsghdr.nlmsg_pid` is the Netlink equivalent of an address.
+It is referred to as Port ID, sometimes Process ID because for historical
+reasons if the application does not select (bind() to) an explicit Port ID
+kernel will automatically assign it the ID equal to its Process ID
+(as reported by the getpid() system call).
+
+Similarly to the bind() semantics of the TCP/IP network protocols the value
+of zero means "assign automatically", hence it is common for applications
+to leave the :c:member:`nlmsghdr.nlmsg_pid` field initialized to ``0``.
+
+The field is still used today in rare cases when kernel needs to send
+a unicast notification. User space application can use bind() to associate
+its socket with a specific PID, it then communicates its PID to the kernel.
+This way the kernel can reach the specific user space process.
+
+This sort of communication is utilized in UMH (User Mode Helper)-like
+scenarios when kernel needs to trigger user space processing or ask user
+space for a policy decision.
+
+Multicast notifications
+-----------------------
+
+One of the strengths of Netlink is the ability to send event notifications
+to user space. This is a unidirectional form of communication (kernel ->
+user) and does not involve any control messages like ``NLMSG_ERROR`` or
+``NLMSG_DONE``.
+
+For example the Generic Netlink family itself defines a set of multicast
+notifications about registered families. When a new family is added the
+sockets subscribed to the notifications will get the following message::
+
+ struct nlmsghdr:
+ __u32 nlmsg_len: 136
+ __u16 nlmsg_type: GENL_ID_CTRL
+ __u16 nlmsg_flags: 0
+ __u32 nlmsg_seq: 0
+ __u32 nlmsg_pid: 0
+
+ struct genlmsghdr:
+ __u8 cmd: CTRL_CMD_NEWFAMILY
+ __u8 version: 2
+ __u16 reserved: 0
+
+ struct nlattr:
+ __u16 nla_len: 10
+ __u16 nla_type: CTRL_ATTR_FAMILY_NAME
+ char data: test1\0
+
+ (padding:)
+ data: \0\0
+
+ struct nlattr:
+ __u16 nla_len: 6
+ __u16 nla_type: CTRL_ATTR_FAMILY_ID
+ __u16: 123 /* The Family ID we are after */
+
+ (padding:)
+ char data: \0\0
+
+ struct nlattr:
+ __u16 nla_len: 9
+ __u16 nla_type: CTRL_ATTR_FAMILY_VERSION
+ __u16: 1
+
+ /* ... etc, more attributes will follow. */
+
+The notification contains the same information as the response
+to the ``CTRL_CMD_GETFAMILY`` request.
+
+The Netlink headers of the notification are mostly 0 and irrelevant.
+The :c:member:`nlmsghdr.nlmsg_seq` may be either zero or a monotonically
+increasing notification sequence number maintained by the family.
+
+To receive notifications the user socket must subscribe to the relevant
+notification group. Much like the Family ID, the Group ID for a given
+multicast group is dynamic and can be found inside the Family information.
+The ``CTRL_ATTR_MCAST_GROUPS`` attribute contains nests with names
+(``CTRL_ATTR_MCAST_GRP_NAME``) and IDs (``CTRL_ATTR_MCAST_GRP_ID``) of
+the groups family.
+
+Once the Group ID is known a setsockopt() call adds the socket to the group:
+
+.. code-block:: c
+
+ unsigned int group_id;
+
+ /* .. find the group ID... */
+
+ setsockopt(fd, SOL_NETLINK, NETLINK_ADD_MEMBERSHIP,
+ &group_id, sizeof(group_id));
+
+The socket will now receive notifications.
+
+It is recommended to use separate sockets for receiving notifications
+and sending requests to the kernel. The asynchronous nature of notifications
+means that they may get mixed in with the responses making the message
+handling much harder.
+
+Buffer sizing
+-------------
+
+Netlink sockets are datagram sockets rather than stream sockets,
+meaning that each message must be received in its entirety by a single
+recv()/recvmsg() system call. If the buffer provided by the user is too
+short, the message will be truncated and the ``MSG_TRUNC`` flag set
+in struct msghdr (struct msghdr is the second argument
+of the recvmsg() system call, *not* a Netlink header).
+
+Upon truncation the remaining part of the message is discarded.
+
+Netlink expects that the user buffer will be at least 8kB or a page
+size of the CPU architecture, whichever is bigger. Particular Netlink
+families may, however, require a larger buffer. 32kB buffer is recommended
+for most efficient handling of dumps (larger buffer fits more dumped
+objects and therefore fewer recvmsg() calls are needed).
+
+Classic Netlink
+===============
+
+The main differences between Classic and Generic Netlink are the dynamic
+allocation of subsystem identifiers and availability of introspection.
+In theory the protocol does not differ significantly, however, in practice
+Classic Netlink experimented with concepts which were abandoned in Generic
+Netlink (really, they usually only found use in a small corner of a single
+subsystem). This section is meant as an explainer of a few of such concepts,
+with the explicit goal of giving the Generic Netlink
+users the confidence to ignore them when reading the uAPI headers.
+
+Most of the concepts and examples here refer to the ``NETLINK_ROUTE`` family,
+which covers much of the configuration of the Linux networking stack.
+Real documentation of that family, deserves a chapter (or a book) of its own.
+
+Families
+--------
+
+Netlink refers to subsystems as families. This is a remnant of using
+sockets and the concept of protocol families, which are part of message
+demultiplexing in ``NETLINK_ROUTE``.
+
+Sadly every layer of encapsulation likes to refer to whatever it's carrying
+as "families" making the term very confusing:
+
+ 1. AF_NETLINK is a bona fide socket protocol family
+ 2. AF_NETLINK's documentation refers to what comes after its own
+ header (struct nlmsghdr) in a message as a "Family Header"
+ 3. Generic Netlink is a family for AF_NETLINK (struct genlmsghdr follows
+ struct nlmsghdr), yet it also calls its users "Families".
+
+Note that the Generic Netlink Family IDs are in a different "ID space"
+and overlap with Classic Netlink protocol numbers (e.g. ``NETLINK_CRYPTO``
+has the Classic Netlink protocol ID of 21 which Generic Netlink will
+happily allocate to one of its families as well).
+
+Strict checking
+---------------
+
+The ``NETLINK_GET_STRICT_CHK`` socket option enables strict input checking
+in ``NETLINK_ROUTE``. It was needed because historically kernel did not
+validate the fields of structures it didn't process. This made it impossible
+to start using those fields later without risking regressions in applications
+which initialized them incorrectly or not at all.
+
+``NETLINK_GET_STRICT_CHK`` declares that the application is initializing
+all fields correctly. It also opts into validating that message does not
+contain trailing data and requests that kernel rejects attributes with
+type higher than largest attribute type known to the kernel.
+
+``NETLINK_GET_STRICT_CHK`` is not used outside of ``NETLINK_ROUTE``.
+
+Unknown attributes
+------------------
+
+Historically Netlink ignored all unknown attributes. The thinking was that
+it would free the application from having to probe what kernel supports.
+The application could make a request to change the state and check which
+parts of the request "stuck".
+
+This is no longer the case for new Generic Netlink families and those opting
+in to strict checking. See enum netlink_validation for validation types
+performed.
+
+Fixed metadata and structures
+-----------------------------
+
+Classic Netlink made liberal use of fixed-format structures within
+the messages. Messages would commonly have a structure with
+a considerable number of fields after struct nlmsghdr. It was also
+common to put structures with multiple members inside attributes,
+without breaking each member into an attribute of its own.
+
+This has caused problems with validation and extensibility and
+therefore using binary structures is actively discouraged for new
+attributes.
+
+Request types
+-------------
+
+``NETLINK_ROUTE`` categorized requests into 4 types ``NEW``, ``DEL``, ``GET``,
+and ``SET``. Each object can handle all or some of those requests
+(objects being netdevs, routes, addresses, qdiscs etc.) Request type
+is defined by the 2 lowest bits of the message type, so commands for
+new objects would always be allocated with a stride of 4.
+
+Each object would also have it's own fixed metadata shared by all request
+types (e.g. struct ifinfomsg for netdev requests, struct ifaddrmsg for address
+requests, struct tcmsg for qdisc requests).
+
+Even though other protocols and Generic Netlink commands often use
+the same verbs in their message names (``GET``, ``SET``) the concept
+of request types did not find wider adoption.
+
+Message flags
+-------------
+
+The earlier section has already covered the basic request flags
+(``NLM_F_REQUEST``, ``NLM_F_ACK``, ``NLM_F_DUMP``) and the ``NLMSG_ERROR`` /
+``NLMSG_DONE`` flags (``NLM_F_CAPPED``, ``NLM_F_ACK_TLVS``).
+Dump flags were also mentioned (``NLM_F_MULTI``, ``NLM_F_DUMP_INTR``).
+
+Those are the main flags of note, with a small exception (of ``ieee802154``)
+Generic Netlink does not make use of other flags. If the protocol needs
+to communicate special constraints for a request it should use
+an attribute, not the flags in struct nlmsghdr.
+
+Classic Netlink, however, defined various flags for its ``GET``, ``NEW``
+and ``DEL`` requests. Since request types have not been generalized
+the request type specific flags should not be used either.
+
+uAPI reference
+==============
+
+.. kernel-doc:: include/uapi/linux/netlink.h
projects / mirror_ubuntu-kernels.git / commitdiff