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

       netlink - Communication between kernel and userspace (PF_NETLINK)

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

       netlink_socket = socket(PF_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  back-
       wards 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 communi-
       cate 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 parame-
              ters, neighbour 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 pay-
       load 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  multi-
       part 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 appro-
       priate manual pages for that, e.g.  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 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 resynchronise.

       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 usu-
       ally 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 pro-
       cess opens and assigns a unique nl_pid to every netlink socket that the
       process subsequently creates.

       nl_groups is a bitmask with every bit representing a netlink group num-
       ber.  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 bitmask 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 multi-
       cast  groups  by setting nl_groups to a bitmask of the groups it wishes
       to send to when it calls sendmsg(2) or does a  connect(2).   Only  pro-
       cesses  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 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)  mul-
       ticast groups.

         struct sockaddr_nl sa;

         memset (&sa, 0, sizeof(sa));
         snl.nl_family = AF_NETLINK;
         snl.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 ker-
       nel (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. */

       This manual page is not complete.

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

       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.

       cmsg(3), netlink(3), capabilities(7), rtnetlink(7)*  for  information  about  lib-
       netlink. for information about libnl.

       RFC 3549 "Linux Netlink as an IP Services Protocol"

Linux Manual Page                 2005-12-27                        NETLINK(7)

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