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

       netlink - Communication between kernel and userspace (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 kernel and userspace processes.  It
       consists of a standard sockets-based  interface  for  userspace  processes  and  an
       internal kernel API for kernel modules.  The internal kernel interface is not docu-
       mented in this manual page.  There  is  also  an  obsolete  netlink  interface  via
       netlink  character  devices; this interface is not documented here and is only pro-
       vided for backwards 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

              Messages from 1-wire subsystem.

              Reserved for user-mode socket protocols.

              Transport IPv4 packets from netfilter to userspace.  Used by ip_queue kernel

              INET socket monitoring.

              Netfilter/iptables ULOG.


              SELinux event notifications.



              Access to FIB lookup from userspace.

              Kernel  connector.   See  Documentation/connector/* in the kernel source for
              further information.

              Netfilter subsystem.

              Transport IPv6 packets from netfilter to userspace.  Used by ip6_queue  ker-
              nel module.

              DECnet routing messages.

              Kernel messages to userspace.

              Generic netlink family for simplified netlink usage.

       Netlink messages consist of a byte stream with one or multiple nlmsghdr headers and
       associated payload.  The byte stream should only  be  accessed  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;    /* PID of the sending process. */

       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 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  mes-
                       sage 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 (NLM_F_ROOT|NLM_F_MATCH).

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

       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 acknowledgment is an NLMSG_ERROR
       packet with the error field set to 0.  The application must  generate  acknowledge-
       ments  for  received messages itself.  The kernel tries to send an NLMSG_ERROR mes-
       sage 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 userspace 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  ker-
       nel.   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;     /* Process 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 userspace process, nl_pid is usually the PID of the pro-
       cess 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 only be
       equal to the process ID 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 appli-
       cation 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.  Any replies to a message received for a multicast group
       should be sent back to the sending PID and the multicast group.

       The socket interface to netlink is a new feature of 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

       NETLINK_SELINUX appeared in Linux 2.6.4.

       NETLINK_AUDIT appeared in Linux 2.6.6.

       NETLINK_KOBJECT_UEVENT appeared in Linux 2.6.10.

       NETLINK_W1 and NETLINK_FIB_LOOKUP appeared in Linux 2.6.13.


       NETLINK_GENERIC and NETLINK_ISCSI appeared in Linux 2.6.15.

       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  RTM-
       GRP_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 application must take care of message sequence numbers in order to reli-
       ably track acknowledgements.

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

           msg = { (void *)&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 = { (void *)&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)* for information about libnetlink. for information about libnl.

       RFC 3549 "Linux Netlink as an IP Services Protocol"

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2008-11-11                        NETLINK(7)

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