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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 |