netlink(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | VERSIONS | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

NETLINK(7)              Linux Programmer's Manual             NETLINK(7)

NAME         top

       netlink - communication between kernel and user space
       (AF_NETLINK)

SYNOPSIS         top

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

       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

DESCRIPTION         top

       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 provided 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.

       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)
              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 multiple
       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 associated
       payload in one byte stream) the first and all following 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 structure, NLMSG_DONE
       message terminates a multipart message. Error messages get the
       original request appened, unless the user requests to cap the
       error message, and get extra error data if requested.

           struct nlmsgerr {
               int error;        /* Negative errno or 0 for acknowledgements */
               struct nlmsghdr msg;  /* Message header that caused the error */
               /*
                * followed by the message contents unless NETLINK_CAP_ACK was set
                * or the ACK indicates success (error == 0).
                * For example Generic Netlink message with attributes.
                * message length is aligned with NLMSG_ALIGN()
                */
               /*
                * followed by TLVs defined in enum nlmsgerr_attrs
                * if NETLINK_EXT_ACK was set
                */
           };

       A netlink family usually specifies more message types, see the
       appropriate manual pages for that, for example, rtnetlink(7) for
       NETLINK_ROUTE.

       Standard flag bits in nlmsg_flags
       ──────────────────────────────────────────────────────────────────
       NLM_F_REQUEST           Must be set on all request messages.
       NLM_F_MULTI             The message is part of a multipart
                               message terminated by NLMSG_DONE.
       NLM_F_ACK               Request for an acknowledgement on
                               success.
       NLM_F_ECHO              Echo this request.

       Additional flag bits for GET requests
       ──────────────────────────────────────────────────────────────────
       NLM_F_ROOT               Return the complete table instead of a
                                single entry.
       NLM_F_MATCH              Return all entries matching criteria
                                passed in message content.  Not
                                implemented yet.
       NLM_F_ATOMIC             Return an atomic snapshot of the table.
       NLM_F_DUMP               Convenience macro; equivalent to
                                (NLM_F_ROOT|NLM_F_MATCH).

       Note that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability or
       an effective UID of 0.

       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 messages when an
       out-of-memory condition or other error occurs.  For reliable
       transfer the sender can request an acknowledgement from the
       receiver by setting the NLM_F_ACK flag.  An acknowledgement 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 owning the destination
       socket.  However, nl_pid identifies a 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
       process 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 netlink
       group number.  Each netlink family has a set of 32 multicast
       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 messages 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 messages.  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.

       NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux
       2.6.14)
              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.

       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 listeners
              to avoid receiving ENOBUFS errors.

       NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
              When set, this socket will receive netlink notifications
              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 4.3)
              The kernel may fail to allocate the necessary room for the
              acknowledgement 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 acknowledgement.

VERSIONS         top

       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         top

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

BUGS         top

       This manual page is not complete.

EXAMPLES         top

       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));

       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         top

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

       information about libnetlink 
       ⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩

       information about libnl ⟨http://www.infradead.org/~tgr/libnl/⟩

       RFC 3549 "Linux Netlink as an IP Services Protocol"

COLOPHON         top

       This page is part of release 5.11 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/.

Linux                          2021-03-22                     NETLINK(7)

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