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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 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

       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

              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

              Netfilter subsystem.

              Transport IPv6 packets from netfilter  to  userspace.   Used  by  ip6_queue  kernel

              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
                       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 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 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

       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.


       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


       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

           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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