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NETLINK(7)		   Linux Programmers Manual		   NETLINK(7)

       netlink - Communication between kernel and userspace (PF_NETLINK)


       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, 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 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,   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 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      Dont replace if the object already exists.
       NLM_F_CREATE    Create object if it doesnt 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 isnt 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 cant 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.  Its 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 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 multi
       cast 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 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 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) 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;

	   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. */

       cmsg(3), netlink(3), capabilities(7), rtnetlink(7)

       ftp://ftp.inr.ac.ru/ip-routing/iproute2*  for  information  about  lib

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

       RFC 3549 "Linux Netlink as an IP Services Protocol"

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2005-12-27			    NETLINK(7)

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