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       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 documented in this manual page.  There
       is  also  an  obsolete netlink interface via netlink character devices;
       this interface  is  not  documented  here  and  is  only  provided  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 rtnetlink(7)).

              Messages from 1-wire subsystem.

              Reserved for user-mode socket protocols.

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

              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 kernel 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

       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;    /* 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

       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 (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

       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  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 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;     /* 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 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 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 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.  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 here.

       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.

       Linux 2.6.14.

       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  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  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 = { (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"


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