NETLINK(7)                (2020-06-09)                 NETLINK(7)

     NAME
          netlink - communication between kernel and user space
          (AF_NETLINK)

     SYNOPSIS
          #include <asm/types.h>
          #include <sys/socket.h>
          #include <linux/netlink.h>

          netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

     DESCRIPTION
          Netlink is used to transfer information between the kernel
          and user-space processes.  It consists of a standard
          sockets-based interface for user space processes and an
          internal kernel API for kernel modules.  The internal kernel
          interface is not documented in this manual page.  There is
          also an obsolete netlink interface via netlink character
          devices; this interface is not documented here and is pro-
          vided only for backward compatibility.

          Netlink is a datagram-oriented service.  Both SOCK_RAW and
          SOCK_DGRAM are valid values for socket_type. However, the
          netlink protocol does not distinguish between datagram and
          raw sockets.

          netlink_family selects the kernel module or netlink group to
          communicate with.  The currently assigned netlink families
          are:

          NETLINK_ROUTE
               Receives routing and link updates and may be used to
               modify the routing tables (both IPv4 and IPv6), IP
               addresses, link parameters, neighbor setups, queueing
               disciplines, traffic classes and packet classifiers
               (see rtnetlink(7)).

          NETLINK_W1 (Linux 2.6.13 to 2.16.17)
               Messages from 1-wire subsystem.

          NETLINK_USERSOCK
               Reserved for user-mode socket protocols.

          NETLINK_FIREWALL (up to and including Linux 3.4)
               Transport IPv4 packets from netfilter to user space.
               Used by ip_queue kernel module.  After a long period of
               being declared obsolete (in favor of the more advanced
               nfnetlink_queue feature), NETLINK_FIREWALL was removed
               in Linux 3.5.

     Page 1                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          NETLINK_SOCK_DIAG (since Linux 3.3)
               Query information about sockets of various protocol
               families from the kernel (see sock_diag(7)).

          NETLINK_INET_DIAG (since Linux 2.6.14)
               An obsolete synonym for NETLINK_SOCK_DIAG.

          NETLINK_NFLOG (up to and including Linux 3.16)
               Netfilter/iptables ULOG.

          NETLINK_XFRM
               IPsec.

          NETLINK_SELINUX (since Linux 2.6.4)
               SELinux event notifications.

          NETLINK_ISCSI (since Linux 2.6.15)
               Open-iSCSI.

          NETLINK_AUDIT (since Linux 2.6.6)
               Auditing.

          NETLINK_FIB_LOOKUP (since Linux 2.6.13)
               Access to FIB lookup from user space.

          NETLINK_CONNECTOR (since Linux 2.6.14)
               Kernel connector.  See Documentation/driver-
               api/connector.rst (or
               /Documentation/connector/connector.* in kernel 5.2 and
               earlier) in the Linux kernel source tree for further
               information.

          NETLINK_NETFILTER (since Linux 2.6.14)
               Netfilter subsystem.

          NETLINK_SCSITRANSPORT (since Linux 2.6.19)
               SCSI Transports.

          NETLINK_RDMA (since Linux 3.0)
               Infiniband RDMA.

          NETLINK_IP6_FW (up to and including Linux 3.4)
               Transport IPv6 packets from netfilter to user space.
               Used by ip6_queue kernel module.

          NETLINK_DNRTMSG
               DECnet routing messages.

          NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
               Kernel messages to user space.

          NETLINK_GENERIC (since Linux 2.6.15)

     Page 2                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

               Generic netlink family for simplified netlink usage.

          NETLINK_CRYPTO (since Linux 3.2)
               Netlink interface to request information about ciphers
               registered with the kernel crypto API as well as allow
               configuration of the kernel crypto API.

          Netlink messages consist of a byte stream with one or multi-
          ple nlmsghdr headers and associated payload.  The byte
          stream should be accessed only with the standard NLMSG_*
          macros.  See netlink(3) for further information.

          In multipart messages (multiple nlmsghdr headers with asso-
          ciated payload in one byte stream) the first and all follow-
          ing headers have the NLM_F_MULTI flag set, except for the
          last header which has the type NLMSG_DONE.

          After each nlmsghdr the payload follows.

              struct nlmsghdr {
                  __u32 nlmsg_len;    /* Length of message including header */
                  __u16 nlmsg_type;   /* Type of message content */
                  __u16 nlmsg_flags;  /* Additional flags */
                  __u32 nlmsg_seq;    /* Sequence number */
                  __u32 nlmsg_pid;    /* Sender port ID */
              };

          nlmsg_type can be one of the standard message types:
          NLMSG_NOOP message is to be ignored, NLMSG_ERROR message
          signals an error and the payload contains an nlmsgerr struc-
          ture, NLMSG_DONE message terminates a multipart message.

              struct nlmsgerr {
                  int error;        /* Negative errno or 0 for acknowledgements */
                  struct nlmsghdr msg;  /* Message header that caused the error */
              };

          A netlink family usually specifies more message types, see
          the appropriate manual pages for that, for example,
          rtnetlink(7) for NETLINK_ROUTE.  tab(:); l s lB l.  Standard
          flag bits in nlmsg_flags _ NLM_F_REQUEST:Must be set on all
          request messages.  NLM_F_MULTI:T{ The message is part of a
          multipart message terminated by NLMSG_DONE.  T}
          NLM_F_ACK:Request for an acknowledgment on success.
          NLM_F_ECHO:Echo this request.  tab(:); l s lB l.  Additional
          flag bits for GET requests _ NLM_F_ROOT:Return the complete
          table instead of a single entry.  NLM_F_MATCH:T{ Return all
          entries matching criteria passed in message content.  Not
          implemented yet.  T} NLM_F_ATOMIC:Return an atomic snapshot
          of the table.  NLM_F_DUMP:T{ Convenience macro; equivalent
          to
          (NLM_F_ROOT|NLM_F_MATCH).  T}

     Page 3                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          Note that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability
          or an effective UID of 0.  tab(:); l s lB l.  Additional
          flag bits for NEW requests _ NLM_F_REPLACE:Replace existing
          matching object.  NLM_F_EXCL:Don't replace if the object
          already exists.  NLM_F_CREATE:Create object if it doesn't
          already exist.  NLM_F_APPEND:Add to the end of the object
          list.

          nlmsg_seq and nlmsg_pid are used to track messages.
          nlmsg_pid shows the origin of the message.  Note that there
          isn't a 1:1 relationship between nlmsg_pid and the PID of
          the process if the message originated from a netlink socket.
          See the ADDRESS FORMATS section for further information.

          Both nlmsg_seq and nlmsg_pid are opaque to netlink core.

          Netlink is not a reliable protocol.  It tries its best to
          deliver a message to its destination(s), but may drop mes-
          sages when an out-of-memory condition or other error occurs.
          For reliable transfer the sender can request an acknowledge-
          ment from the receiver by setting the NLM_F_ACK flag.  An
          acknowledgment is an NLMSG_ERROR packet with the error field
          set to 0.  The application must generate acknowledgements
          for received messages itself.  The kernel tries to send an
          NLMSG_ERROR message for every failed packet.  A user process
          should follow this convention too.

          However, reliable transmissions from kernel to user are
          impossible in any case.  The kernel can't send a netlink
          message if the socket buffer is full: the message will be
          dropped and the kernel and the user-space process will no
          longer have the same view of kernel state.  It is up to the
          application to detect when this happens (via the ENOBUFS
          error returned by recvmsg(2)) and resynchronize.

        Address formats
          The sockaddr_nl structure describes a netlink client in user
          space or in the kernel.  A sockaddr_nl can be either unicast
          (only sent to one peer) or sent to netlink multicast groups
          (nl_groups not equal 0).

              struct sockaddr_nl {
                  sa_family_t     nl_family;  /* AF_NETLINK */
                  unsigned short  nl_pad;     /* Zero */
                  pid_t           nl_pid;     /* Port ID */
                  __u32           nl_groups;  /* Multicast groups mask */
              };

          nl_pid is the unicast address of netlink socket.  It's
          always 0 if the destination is in the kernel.  For a user-
          space process, nl_pid is usually the PID of the process own-
          ing the destination socket.  However, nl_pid identifies a

     Page 4                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          netlink socket, not a process.  If a process owns several
          netlink sockets, then nl_pid can be equal to the process ID
          only for at most one socket.  There are two ways to assign
          nl_pid to a netlink socket.  If the application sets nl_pid
          before calling bind(2), then it is up to the application to
          make sure that nl_pid is unique.  If the application sets it
          to 0, the kernel takes care of assigning it.  The kernel
          assigns the process ID to the first netlink socket the pro-
          cess opens and assigns a unique nl_pid to every netlink
          socket that the process subsequently creates.

          nl_groups is a bit mask with every bit representing a net-
          link group number.  Each netlink family has a set of 32 mul-
          ticast groups.  When bind(2) is called on the socket, the
          nl_groups field in the sockaddr_nl should be set to a bit
          mask of the groups which it wishes to listen to.  The
          default value for this field is zero which means that no
          multicasts will be received.  A socket may multicast mes-
          sages to any of the multicast groups by setting nl_groups to
          a bit mask of the groups it wishes to send to when it calls
          sendmsg(2) or does a connect(2).  Only processes with an
          effective UID of 0 or the CAP_NET_ADMIN capability may send
          or listen to a netlink multicast group.  Since Linux 2.6.13,
          messages can't be broadcast to multiple groups.  Any replies
          to a message received for a multicast group should be sent
          back to the sending PID and the multicast group.  Some Linux
          kernel subsystems may additionally allow other users to send
          and/or receive messages.  As at Linux 3.0, the
          NETLINK_KOBJECT_UEVENT, NETLINK_GENERIC, NETLINK_ROUTE, and
          NETLINK_SELINUX groups allow other users to receive mes-
          sages.  No groups allow other users to send messages.

        Socket options
          To set or get a netlink socket option, call getsockopt(2) to
          read or setsockopt(2) to write the option with the option
          level argument set to SOL_NETLINK.  Unless otherwise noted,
          optval is a pointer to an int.

          NETLINK_PKTINFO (since Linux 2.6.14)
               Enable nl_pktinfo control messages for received packets
               to get the extended destination group number.

     2.6.14)
          NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux
               Join/leave a group specified by optval.

          NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
               Retrieve all groups a socket is a member of.  optval is
               a pointer to __u32 and optlen is the size of the array.
               The array is filled with the full membership set of the
               socket, and the required array size is returned in
               optlen.

     Page 5                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
               When not set, netlink_broadcast() only reports ESRCH
               errors and silently ignore ENOBUFS errors.

          NETLINK_NO_ENOBUFS (since Linux 2.6.30)
               This flag can be used by unicast and broadcast listen-
               ers to avoid receiving ENOBUFS errors.

          NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
               When set, this socket will receive netlink notifica-
               tions from all network namespaces that have an nsid
               assigned into the network namespace where the socket
               has been opened.  The nsid is sent to user space via an
               ancillary data.

          NETLINK_CAP_ACK (since Linux 4.2)
               The kernel may fail to allocate the necessary room for
               the acknowledgment message back to user space.  This
               option trims off the payload of the original netlink
               message.  The netlink message header is still included,
               so the user can guess from the sequence number which
               message triggered the acknowledgment.

     VERSIONS
          The socket interface to netlink first appeared Linux 2.2.

          Linux 2.0 supported a more primitive device-based netlink
          interface (which is still available as a compatibility
          option).  This obsolete interface is not described here.

     NOTES
          It is often better to use netlink via libnetlink or libnl
          than via the low-level kernel interface.

     BUGS
          This manual page is not complete.

     EXAMPLES
          The following example creates a NETLINK_ROUTE netlink socket
          which will listen to the RTMGRP_LINK (network interface
          create/delete/up/down events) and RTMGRP_IPV4_IFADDR (IPv4
          addresses add/delete events) multicast groups.

              struct sockaddr_nl sa;

              memset(&sa, 0, sizeof(sa));
              sa.nl_family = AF_NETLINK;
              sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;

              fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
              bind(fd, (struct sockaddr *) &sa, sizeof(sa));

     Page 6                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          The next example demonstrates how to send a netlink message
          to the kernel (pid 0).  Note that the application must take
          care of message sequence numbers in order to reliably track
          acknowledgements.

              struct nlmsghdr *nh;    /* The nlmsghdr with payload to send */
              struct sockaddr_nl sa;
              struct iovec iov = { nh, nh->nlmsg_len };
              struct msghdr msg;

              msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
              memset(&sa, 0, sizeof(sa));
              sa.nl_family = AF_NETLINK;
              nh->nlmsg_pid = 0;
              nh->nlmsg_seq = ++sequence_number;
              /* Request an ack from kernel by setting NLM_F_ACK */
              nh->nlmsg_flags |= NLM_F_ACK;

              sendmsg(fd, &msg, 0);

          And the last example is about reading netlink message.

              int len;
              /* 8192 to avoid message truncation on platforms with
                 page size > 4096 */
              struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
              struct iovec iov = { buf, sizeof(buf) };
              struct sockaddr_nl sa;
              struct msghdr msg;
              struct nlmsghdr *nh;

              msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
              len = recvmsg(fd, &msg, 0);

              for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
                   nh = NLMSG_NEXT (nh, len)) {
                  /* The end of multipart message */
                  if (nh->nlmsg_type == NLMSG_DONE)
                      return;

                  if (nh->nlmsg_type == NLMSG_ERROR)
                      /* Do some error handling */
                  ...

                  /* Continue with parsing payload */
                  ...
              }

     SEE ALSO
          cmsg(3), netlink(3), capabilities(7), rtnetlink(7),
          sock_diag(7)

     Page 7                        Linux             (printed 5/21/22)

     NETLINK(7)                (2020-06-09)                 NETLINK(7)

          information about libnetlink

          information about libnl

          RFC 3549 "Linux Netlink as an IP Services Protocol"

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 8                        Linux             (printed 5/21/22)