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

       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 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 in-
       ternal 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

       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 user space.  Used by
	      ip_queue kernel module.

	      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 module.

	      DECnet routing messages.

	      Kernel messages to user space.

	      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 be accessed
       only with the standard NLMSG_* macros.  See netlink(3) for further  in-

       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;	   /* 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	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  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 ef-
       fective 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 in-

       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  re-
       turned 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 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 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	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.  Since Linux 2.6.13, mes-
       sages 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,
       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  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));
	   sa.nl_family	= AF_NETLINK;
	   sa.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 the application must take care of  message  se-
       quence numbers in order to reliably track acknowledgements.

	   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 <*>

       information about libnl <>

       RFC 3549	"Linux Netlink as an IP	Services Protocol"

       This  page  is  part of release 3.74 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

Linux				  2013-03-15			    NETLINK(7)


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