netlink - communication between kernel and user space (AF_NETLINK)


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

   netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);


   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:

          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 (since Linux 2.6.13)
          Messages from 1-wire subsystem.

          Reserved for user-mode socket protocols.

          Transport  IPv4  packets  from netfilter to user space.  Used by
          ip_queue kernel module.

   NETLINK_INET_DIAG (since Linux 2.6.14)
          Query information about sockets  of  various  protocol  families
          from the kernel (see sock_diag(7)).

   NETLINK_SOCK_DIAG (since Linux 3.3)
          A synonym for NETLINK_INET_DIAG.

          Netfilter/iptables ULOG.


   NETLINK_SELINUX (since Linux 2.6.4)
          SELinux event notifications.

   NETLINK_ISCSI (since Linux 2.6.15)

   NETLINK_AUDIT (since Linux 2.6.6)

   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/connector/* in the Linux
          kernel source tree for further information.

   NETLINK_NETFILTER (since Linux 2.6.14)
          Netfilter subsystem.

          Transport IPv6 packets from netfilter to user  space.   Used  by
          ip6_queue kernel module.

          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

   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

   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.

       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

   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 acknowledgment 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

   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

   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  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 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
   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.

          Join/leave a group specified by optval.

          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 NOBUFS 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 4.2)
          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.


   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.


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


   This manual page is not complete.


   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;
       char buf[4096];
       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)

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

           /* Continue with parsing payload */


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

   information about libnetlink*

   information about libnl

   RFC 3549 "Linux Netlink as an IP Services Protocol"


   This page is part of release 4.09 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


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