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

       netlink - communication between kernel and user space (AF_NETLINK)


       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

       Netlink  is used to transfer information between kernel and user-space processes.  It con‐
       sists of a standard sockets-based interface for user space processes and an internal  ker‐
       nel  API for kernel modules.  The internal kernel interface is not documented in this man‐
       ual 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

       netlink_family  selects  the kernel module or netlink group to communicate with.  The cur‐
       rently 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 dis‐
              ciplines, 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 user space.  Used by ip_queue kernel  mod‐

              INET socket monitoring.

              Netfilter/iptables ULOG.


              SELinux event notifications.



              Access to FIB lookup from user space.

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

              Netfilter subsystem.

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

              DECnet routing messages.

              Kernel messages to user space.

              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  asso‐
       ciated payload.  The byte stream should be accessed only 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;    /* 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 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  mes‐
                      sage 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 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  desti‐
       nation(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  set‐
       ting  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  ker‐
       nel 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

       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  multi‐
       casts 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 Linux 3.0, the  NETLINK_KOBJECT_UEVENT,  NETLINK_GENERIC,  NETLINK_ROUTE,
       and  NETLINK_SELINUX  groups allow other users to receive messages.  No groups allow other
       users to send messages.

       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 avail‐
       able 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 RTM‐
       GRP_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

           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)

       information about libnetlink ⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩

       information about libnl ⟨http://people.suug.ch/~tgr/libnl/⟩

       RFC 3549 "Linux Netlink as an IP Services Protocol"

       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-01-10                                 NETLINK(7)


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