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       netlink - communication between kernel and user space (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 the kernel and user-space processes.  It
       consists of a standard sockets-based interface for user space 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 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
       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)).

       NETLINK_W1 (Linux 2.6.13 to 2.16.17)
              Messages from 1-wire subsystem.

              Reserved for user-mode socket protocols.

       NETLINK_FIREWALL (up to and including Linux 3.4)
              Transport IPv4 packets from netfilter to  user  space.   Used  by  ip_queue  kernel
              module.   After  a  long  period  of  being declared obsolete (in favor of the more
              advanced nfnetlink_queue feature), NETLINK_FIREWALL was removed in Linux 3.5.

       NETLINK_INET_DIAG (since Linux 2.6.14)
              Query information about sockets of various protocol families from the  kernel  (see

       NETLINK_SOCK_DIAG (since Linux 3.3)
              A synonym for NETLINK_INET_DIAG.

       NETLINK_NFLOG (up to and including Linux 3.16)
              Netfilter/iptables ULOG.


       NETLINK_SELINUX (since Linux 2.6.4)
              SELinux event notifications.

       NETLINK_ISCSI (since Linux 2.6.15)

       NETLINK_AUDIT (since Linux 2.6.6)

       NETLINK_FIB_LOOKUP (since Linux 2.6.13)
              Access to FIB lookup from user space.

       NETLINK_CONNECTOR (since Linux 2.6.14)
              Kernel  connector.   See  Documentation/connector/* in the Linux kernel source tree
              for further information.

       NETLINK_NETFILTER (since Linux 2.6.14)
              Netfilter subsystem.

       NETLINK_SCSITRANSPORT (since Linux 2.6.19)
              SCSI Transports.

       NETLINK_RDMA (since Linux 3.0)
              Infiniband RDMA.

       NETLINK_IP6_FW (up to and including Linux 3.4)
              Transport IPv6 packets from netfilter to user  space.   Used  by  ip6_queue  kernel

              DECnet routing messages.

       NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
              Kernel messages to user space.

       NETLINK_GENERIC (since Linux 2.6.15)
              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
       associated  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
                       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

       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 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
       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.   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.

   Socket options
       To set or get a netlink socket option, call getsockopt(2)  to  read  or  setsockopt(2)  to
       write  the  option  with  the  option level argument set to SOL_NETLINK.  Unless otherwise
       noted, optval is a pointer to an int.

       NETLINK_PKTINFO (since Linux 2.6.14)
              Enable nl_pktinfo control  messages  for  received  packets  to  get  the  extended
              destination group number.

              Join/leave a group specified by optval.

       NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
              Retrieve  all  groups  a  socket  is a member of.  optval is a pointer to __u32 and
              optlen is the size of the array.  The array is filled with the full membership  set
              of the socket, and the required array size is returned in optlen.

       NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
              When  not  set,  netlink_broadcast()  only reports ESRCH errors and silently ignore
              ENOBUFS errors.

       NETLINK_NO_ENOBUFS (since Linux 2.6.30)
              This flag can be used by unicast and broadcast listeners to avoid receiving ENOBUFS

       NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
              When  set,  this  socket  will  receive  netlink  notifications  from  all  network
              namespaces that have an nsid assigned into the network namespace where  the  socket
              has been opened.  The nsid is sent to user space via an ancillary data.

       NETLINK_CAP_ACK (since Linux 4.2)
              The  kernel  may fail to allocate the necessary room for the acknowledgment message
              back to user space.  This option trims off the  payload  of  the  original  netlink
              message.   The netlink message header is still included, so the user can guess from
              the sequence number which message triggered the acknowledgment.


       The socket interface to netlink first appeared 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.


       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 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;
           /* 8192 to avoid message truncation on platforms with
              page size > 4096 */
           struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
           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), sock_diag(7)

       information about libnetlink ⟨*⟩

       information about libnl ⟨⟩

       RFC 3549 "Linux Netlink as an IP Services Protocol"


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