682 lines
26 KiB
ReStructuredText
682 lines
26 KiB
ReStructuredText
.. SPDX-License-Identifier: BSD-3-Clause
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=======================
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Introduction to Netlink
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=======================
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Netlink is often described as an ioctl() replacement.
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It aims to replace fixed-format C structures as supplied
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to ioctl() with a format which allows an easy way to add
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or extended the arguments.
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To achieve this Netlink uses a minimal fixed-format metadata header
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followed by multiple attributes in the TLV (type, length, value) format.
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Unfortunately the protocol has evolved over the years, in an organic
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and undocumented fashion, making it hard to coherently explain.
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To make the most practical sense this document starts by describing
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netlink as it is used today and dives into more "historical" uses
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in later sections.
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Opening a socket
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================
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Netlink communication happens over sockets, a socket needs to be
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opened first:
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.. code-block:: c
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fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
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The use of sockets allows for a natural way of exchanging information
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in both directions (to and from the kernel). The operations are still
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performed synchronously when applications send() the request but
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a separate recv() system call is needed to read the reply.
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A very simplified flow of a Netlink "call" will therefore look
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something like:
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.. code-block:: c
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fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
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/* format the request */
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send(fd, &request, sizeof(request));
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n = recv(fd, &response, RSP_BUFFER_SIZE);
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/* interpret the response */
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Netlink also provides natural support for "dumping", i.e. communicating
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to user space all objects of a certain type (e.g. dumping all network
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interfaces).
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.. code-block:: c
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fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
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/* format the dump request */
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send(fd, &request, sizeof(request));
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while (1) {
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n = recv(fd, &buffer, RSP_BUFFER_SIZE);
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/* one recv() call can read multiple messages, hence the loop below */
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for (nl_msg in buffer) {
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if (nl_msg.nlmsg_type == NLMSG_DONE)
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goto dump_finished;
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/* process the object */
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}
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}
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dump_finished:
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The first two arguments of the socket() call require little explanation -
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it is opening a Netlink socket, with all headers provided by the user
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(hence NETLINK, RAW). The last argument is the protocol within Netlink.
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This field used to identify the subsystem with which the socket will
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communicate.
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Classic vs Generic Netlink
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--------------------------
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Initial implementation of Netlink depended on a static allocation
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of IDs to subsystems and provided little supporting infrastructure.
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Let us refer to those protocols collectively as **Classic Netlink**.
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The list of them is defined on top of the ``include/uapi/linux/netlink.h``
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file, they include among others - general networking (NETLINK_ROUTE),
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iSCSI (NETLINK_ISCSI), and audit (NETLINK_AUDIT).
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**Generic Netlink** (introduced in 2005) allows for dynamic registration of
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subsystems (and subsystem ID allocation), introspection and simplifies
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implementing the kernel side of the interface.
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The following section describes how to use Generic Netlink, as the
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number of subsystems using Generic Netlink outnumbers the older
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protocols by an order of magnitude. There are also no plans for adding
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more Classic Netlink protocols to the kernel.
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Basic information on how communicating with core networking parts of
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the Linux kernel (or another of the 20 subsystems using Classic
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Netlink) differs from Generic Netlink is provided later in this document.
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Generic Netlink
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===============
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In addition to the Netlink fixed metadata header each Netlink protocol
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defines its own fixed metadata header. (Similarly to how network
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headers stack - Ethernet > IP > TCP we have Netlink > Generic N. > Family.)
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A Netlink message always starts with struct nlmsghdr, which is followed
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by a protocol-specific header. In case of Generic Netlink the protocol
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header is struct genlmsghdr.
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The practical meaning of the fields in case of Generic Netlink is as follows:
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.. code-block:: c
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struct nlmsghdr {
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__u32 nlmsg_len; /* Length of message including headers */
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__u16 nlmsg_type; /* Generic Netlink Family (subsystem) ID */
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__u16 nlmsg_flags; /* Flags - request or dump */
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__u32 nlmsg_seq; /* Sequence number */
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__u32 nlmsg_pid; /* Port ID, set to 0 */
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};
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struct genlmsghdr {
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__u8 cmd; /* Command, as defined by the Family */
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__u8 version; /* Irrelevant, set to 1 */
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__u16 reserved; /* Reserved, set to 0 */
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};
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/* TLV attributes follow... */
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In Classic Netlink :c:member:`nlmsghdr.nlmsg_type` used to identify
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which operation within the subsystem the message was referring to
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(e.g. get information about a netdev). Generic Netlink needs to mux
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multiple subsystems in a single protocol so it uses this field to
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identify the subsystem, and :c:member:`genlmsghdr.cmd` identifies
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the operation instead. (See :ref:`res_fam` for
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information on how to find the Family ID of the subsystem of interest.)
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Note that the first 16 values (0 - 15) of this field are reserved for
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control messages both in Classic Netlink and Generic Netlink.
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See :ref:`nl_msg_type` for more details.
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There are 3 usual types of message exchanges on a Netlink socket:
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- performing a single action (``do``);
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- dumping information (``dump``);
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- getting asynchronous notifications (``multicast``).
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Classic Netlink is very flexible and presumably allows other types
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of exchanges to happen, but in practice those are the three that get
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used.
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Asynchronous notifications are sent by the kernel and received by
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the user sockets which subscribed to them. ``do`` and ``dump`` requests
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are initiated by the user. :c:member:`nlmsghdr.nlmsg_flags` should
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be set as follows:
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- for ``do``: ``NLM_F_REQUEST | NLM_F_ACK``
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- for ``dump``: ``NLM_F_REQUEST | NLM_F_ACK | NLM_F_DUMP``
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:c:member:`nlmsghdr.nlmsg_seq` should be a set to a monotonically
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increasing value. The value gets echoed back in responses and doesn't
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matter in practice, but setting it to an increasing value for each
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message sent is considered good hygiene. The purpose of the field is
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matching responses to requests. Asynchronous notifications will have
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:c:member:`nlmsghdr.nlmsg_seq` of ``0``.
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:c:member:`nlmsghdr.nlmsg_pid` is the Netlink equivalent of an address.
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This field can be set to ``0`` when talking to the kernel.
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See :ref:`nlmsg_pid` for the (uncommon) uses of the field.
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The expected use for :c:member:`genlmsghdr.version` was to allow
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versioning of the APIs provided by the subsystems. No subsystem to
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date made significant use of this field, so setting it to ``1`` seems
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like a safe bet.
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.. _nl_msg_type:
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Netlink message types
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---------------------
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As previously mentioned :c:member:`nlmsghdr.nlmsg_type` carries
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protocol specific values but the first 16 identifiers are reserved
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(first subsystem specific message type should be equal to
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``NLMSG_MIN_TYPE`` which is ``0x10``).
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There are only 4 Netlink control messages defined:
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- ``NLMSG_NOOP`` - ignore the message, not used in practice;
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- ``NLMSG_ERROR`` - carries the return code of an operation;
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- ``NLMSG_DONE`` - marks the end of a dump;
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- ``NLMSG_OVERRUN`` - socket buffer has overflown, not used to date.
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``NLMSG_ERROR`` and ``NLMSG_DONE`` are of practical importance.
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They carry return codes for operations. Note that unless
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the ``NLM_F_ACK`` flag is set on the request Netlink will not respond
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with ``NLMSG_ERROR`` if there is no error. To avoid having to special-case
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this quirk it is recommended to always set ``NLM_F_ACK``.
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The format of ``NLMSG_ERROR`` is described by struct nlmsgerr::
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----------------------------------------------
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| struct nlmsghdr - response header |
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----------------------------------------------
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| int error |
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----------------------------------------------
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| struct nlmsghdr - original request header |
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----------------------------------------------
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| ** optionally (1) payload of the request |
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----------------------------------------------
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| ** optionally (2) extended ACK |
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----------------------------------------------
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There are two instances of struct nlmsghdr here, first of the response
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and second of the request. ``NLMSG_ERROR`` carries the information about
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the request which led to the error. This could be useful when trying
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to match requests to responses or re-parse the request to dump it into
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logs.
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The payload of the request is not echoed in messages reporting success
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(``error == 0``) or if ``NETLINK_CAP_ACK`` setsockopt() was set.
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The latter is common
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and perhaps recommended as having to read a copy of every request back
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from the kernel is rather wasteful. The absence of request payload
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is indicated by ``NLM_F_CAPPED`` in :c:member:`nlmsghdr.nlmsg_flags`.
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The second optional element of ``NLMSG_ERROR`` are the extended ACK
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attributes. See :ref:`ext_ack` for more details. The presence
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of extended ACK is indicated by ``NLM_F_ACK_TLVS`` in
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:c:member:`nlmsghdr.nlmsg_flags`.
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``NLMSG_DONE`` is simpler, the request is never echoed but the extended
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ACK attributes may be present::
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----------------------------------------------
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| struct nlmsghdr - response header |
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----------------------------------------------
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| int error |
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----------------------------------------------
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| ** optionally extended ACK |
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----------------------------------------------
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.. _res_fam:
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Resolving the Family ID
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-----------------------
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This section explains how to find the Family ID of a subsystem.
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It also serves as an example of Generic Netlink communication.
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Generic Netlink is itself a subsystem exposed via the Generic Netlink API.
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To avoid a circular dependency Generic Netlink has a statically allocated
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Family ID (``GENL_ID_CTRL`` which is equal to ``NLMSG_MIN_TYPE``).
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The Generic Netlink family implements a command used to find out information
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about other families (``CTRL_CMD_GETFAMILY``).
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To get information about the Generic Netlink family named for example
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``"test1"`` we need to send a message on the previously opened Generic Netlink
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socket. The message should target the Generic Netlink Family (1), be a
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``do`` (2) call to ``CTRL_CMD_GETFAMILY`` (3). A ``dump`` version of this
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call would make the kernel respond with information about *all* the families
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it knows about. Last but not least the name of the family in question has
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to be specified (4) as an attribute with the appropriate type::
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struct nlmsghdr:
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__u32 nlmsg_len: 32
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__u16 nlmsg_type: GENL_ID_CTRL // (1)
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__u16 nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK // (2)
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__u32 nlmsg_seq: 1
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__u32 nlmsg_pid: 0
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struct genlmsghdr:
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__u8 cmd: CTRL_CMD_GETFAMILY // (3)
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__u8 version: 2 /* or 1, doesn't matter */
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__u16 reserved: 0
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struct nlattr: // (4)
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__u16 nla_len: 10
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__u16 nla_type: CTRL_ATTR_FAMILY_NAME
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char data: test1\0
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(padding:)
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char data: \0\0
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The length fields in Netlink (:c:member:`nlmsghdr.nlmsg_len`
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and :c:member:`nlattr.nla_len`) always *include* the header.
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Attribute headers in netlink must be aligned to 4 bytes from the start
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of the message, hence the extra ``\0\0`` after ``CTRL_ATTR_FAMILY_NAME``.
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The attribute lengths *exclude* the padding.
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If the family is found kernel will reply with two messages, the response
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with all the information about the family::
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/* Message #1 - reply */
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struct nlmsghdr:
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__u32 nlmsg_len: 136
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__u16 nlmsg_type: GENL_ID_CTRL
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__u16 nlmsg_flags: 0
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__u32 nlmsg_seq: 1 /* echoed from our request */
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__u32 nlmsg_pid: 5831 /* The PID of our user space process */
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struct genlmsghdr:
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__u8 cmd: CTRL_CMD_GETFAMILY
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__u8 version: 2
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__u16 reserved: 0
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struct nlattr:
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__u16 nla_len: 10
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__u16 nla_type: CTRL_ATTR_FAMILY_NAME
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char data: test1\0
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(padding:)
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data: \0\0
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struct nlattr:
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__u16 nla_len: 6
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__u16 nla_type: CTRL_ATTR_FAMILY_ID
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__u16: 123 /* The Family ID we are after */
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(padding:)
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char data: \0\0
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struct nlattr:
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__u16 nla_len: 9
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__u16 nla_type: CTRL_ATTR_FAMILY_VERSION
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__u16: 1
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/* ... etc, more attributes will follow. */
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And the error code (success) since ``NLM_F_ACK`` had been set on the request::
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/* Message #2 - the ACK */
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struct nlmsghdr:
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__u32 nlmsg_len: 36
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__u16 nlmsg_type: NLMSG_ERROR
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__u16 nlmsg_flags: NLM_F_CAPPED /* There won't be a payload */
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__u32 nlmsg_seq: 1 /* echoed from our request */
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__u32 nlmsg_pid: 5831 /* The PID of our user space process */
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int error: 0
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struct nlmsghdr: /* Copy of the request header as we sent it */
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__u32 nlmsg_len: 32
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__u16 nlmsg_type: GENL_ID_CTRL
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__u16 nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK
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__u32 nlmsg_seq: 1
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__u32 nlmsg_pid: 0
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The order of attributes (struct nlattr) is not guaranteed so the user
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has to walk the attributes and parse them.
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Note that Generic Netlink sockets are not associated or bound to a single
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family. A socket can be used to exchange messages with many different
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families, selecting the recipient family on message-by-message basis using
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the :c:member:`nlmsghdr.nlmsg_type` field.
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.. _ext_ack:
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Extended ACK
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------------
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Extended ACK controls reporting of additional error/warning TLVs
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in ``NLMSG_ERROR`` and ``NLMSG_DONE`` messages. To maintain backward
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compatibility this feature has to be explicitly enabled by setting
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the ``NETLINK_EXT_ACK`` setsockopt() to ``1``.
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Types of extended ack attributes are defined in enum nlmsgerr_attrs.
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The most commonly used attributes are ``NLMSGERR_ATTR_MSG``,
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``NLMSGERR_ATTR_OFFS`` and ``NLMSGERR_ATTR_MISS_*``.
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``NLMSGERR_ATTR_MSG`` carries a message in English describing
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the encountered problem. These messages are far more detailed
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than what can be expressed thru standard UNIX error codes.
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``NLMSGERR_ATTR_OFFS`` points to the attribute which caused the problem.
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``NLMSGERR_ATTR_MISS_TYPE`` and ``NLMSGERR_ATTR_MISS_NEST``
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inform about a missing attribute.
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Extended ACKs can be reported on errors as well as in case of success.
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The latter should be treated as a warning.
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Extended ACKs greatly improve the usability of Netlink and should
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always be enabled, appropriately parsed and reported to the user.
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Advanced topics
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===============
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Dump consistency
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----------------
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Some of the data structures kernel uses for storing objects make
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it hard to provide an atomic snapshot of all the objects in a dump
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(without impacting the fast-paths updating them).
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Kernel may set the ``NLM_F_DUMP_INTR`` flag on any message in a dump
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(including the ``NLMSG_DONE`` message) if the dump was interrupted and
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may be inconsistent (e.g. missing objects). User space should retry
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the dump if it sees the flag set.
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Introspection
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-------------
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The basic introspection abilities are enabled by access to the Family
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object as reported in :ref:`res_fam`. User can query information about
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the Generic Netlink family, including which operations are supported
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by the kernel and what attributes the kernel understands.
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Family information includes the highest ID of an attribute kernel can parse,
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a separate command (``CTRL_CMD_GETPOLICY``) provides detailed information
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about supported attributes, including ranges of values the kernel accepts.
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Querying family information is useful in cases when user space needs
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to make sure that the kernel has support for a feature before issuing
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a request.
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.. _nlmsg_pid:
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nlmsg_pid
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---------
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:c:member:`nlmsghdr.nlmsg_pid` is the Netlink equivalent of an address.
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It is referred to as Port ID, sometimes Process ID because for historical
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reasons if the application does not select (bind() to) an explicit Port ID
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kernel will automatically assign it the ID equal to its Process ID
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(as reported by the getpid() system call).
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Similarly to the bind() semantics of the TCP/IP network protocols the value
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of zero means "assign automatically", hence it is common for applications
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to leave the :c:member:`nlmsghdr.nlmsg_pid` field initialized to ``0``.
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The field is still used today in rare cases when kernel needs to send
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a unicast notification. User space application can use bind() to associate
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its socket with a specific PID, it then communicates its PID to the kernel.
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This way the kernel can reach the specific user space process.
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This sort of communication is utilized in UMH (User Mode Helper)-like
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scenarios when kernel needs to trigger user space processing or ask user
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space for a policy decision.
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Multicast notifications
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-----------------------
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One of the strengths of Netlink is the ability to send event notifications
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to user space. This is a unidirectional form of communication (kernel ->
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user) and does not involve any control messages like ``NLMSG_ERROR`` or
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``NLMSG_DONE``.
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For example the Generic Netlink family itself defines a set of multicast
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notifications about registered families. When a new family is added the
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sockets subscribed to the notifications will get the following message::
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struct nlmsghdr:
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__u32 nlmsg_len: 136
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__u16 nlmsg_type: GENL_ID_CTRL
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__u16 nlmsg_flags: 0
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__u32 nlmsg_seq: 0
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__u32 nlmsg_pid: 0
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struct genlmsghdr:
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__u8 cmd: CTRL_CMD_NEWFAMILY
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__u8 version: 2
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__u16 reserved: 0
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struct nlattr:
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__u16 nla_len: 10
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__u16 nla_type: CTRL_ATTR_FAMILY_NAME
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char data: test1\0
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(padding:)
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data: \0\0
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struct nlattr:
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__u16 nla_len: 6
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__u16 nla_type: CTRL_ATTR_FAMILY_ID
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__u16: 123 /* The Family ID we are after */
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(padding:)
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char data: \0\0
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struct nlattr:
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__u16 nla_len: 9
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__u16 nla_type: CTRL_ATTR_FAMILY_VERSION
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__u16: 1
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/* ... etc, more attributes will follow. */
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The notification contains the same information as the response
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to the ``CTRL_CMD_GETFAMILY`` request.
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The Netlink headers of the notification are mostly 0 and irrelevant.
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The :c:member:`nlmsghdr.nlmsg_seq` may be either zero or a monotonically
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increasing notification sequence number maintained by the family.
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To receive notifications the user socket must subscribe to the relevant
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notification group. Much like the Family ID, the Group ID for a given
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multicast group is dynamic and can be found inside the Family information.
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The ``CTRL_ATTR_MCAST_GROUPS`` attribute contains nests with names
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(``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.
|
|
|
|
Notification echo
|
|
-----------------
|
|
|
|
``NLM_F_ECHO`` requests for notifications resulting from the request
|
|
to be queued onto the requesting socket. This is useful to discover
|
|
the impact of the request.
|
|
|
|
Note that this feature is not universally implemented.
|
|
|
|
Other request-type-specific flags
|
|
---------------------------------
|
|
|
|
Classic Netlink defined various flags for its ``GET``, ``NEW``
|
|
and ``DEL`` requests in the upper byte of nlmsg_flags in struct nlmsghdr.
|
|
Since request types have not been generalized the request type specific
|
|
flags are rarely used (and considered deprecated for new families).
|
|
|
|
For ``GET`` - ``NLM_F_ROOT`` and ``NLM_F_MATCH`` are combined into
|
|
``NLM_F_DUMP``, and not used separately. ``NLM_F_ATOMIC`` is never used.
|
|
|
|
For ``DEL`` - ``NLM_F_NONREC`` is only used by nftables and ``NLM_F_BULK``
|
|
only by FDB some operations.
|
|
|
|
The flags for ``NEW`` are used most commonly in classic Netlink. Unfortunately,
|
|
the meaning is not crystal clear. The following description is based on the
|
|
best guess of the intention of the authors, and in practice all families
|
|
stray from it in one way or another. ``NLM_F_REPLACE`` asks to replace
|
|
an existing object, if no matching object exists the operation should fail.
|
|
``NLM_F_EXCL`` has the opposite semantics and only succeeds if object already
|
|
existed.
|
|
``NLM_F_CREATE`` asks for the object to be created if it does not
|
|
exist, it can be combined with ``NLM_F_REPLACE`` and ``NLM_F_EXCL``.
|
|
|
|
A comment in the main Netlink uAPI header states::
|
|
|
|
4.4BSD ADD NLM_F_CREATE|NLM_F_EXCL
|
|
4.4BSD CHANGE NLM_F_REPLACE
|
|
|
|
True CHANGE NLM_F_CREATE|NLM_F_REPLACE
|
|
Append NLM_F_CREATE
|
|
Check NLM_F_EXCL
|
|
|
|
which seems to indicate that those flags predate request types.
|
|
``NLM_F_REPLACE`` without ``NLM_F_CREATE`` was initially used instead
|
|
of ``SET`` commands.
|
|
``NLM_F_EXCL`` without ``NLM_F_CREATE`` was used to check if object exists
|
|
without creating it, presumably predating ``GET`` commands.
|
|
|
|
``NLM_F_APPEND`` indicates that if one key can have multiple objects associated
|
|
with it (e.g. multiple next-hop objects for a route) the new object should be
|
|
added to the list rather than replacing the entire list.
|
|
|
|
uAPI reference
|
|
==============
|
|
|
|
.. kernel-doc:: include/uapi/linux/netlink.h
|