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

NAME

       netlink, PF_NETLINK - Communication between kernel and user.

SYNOPSIS

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

       netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);

DESCRIPTION

       Netlink is used to transfer information between kernel modules and user space processes.  It consists of a standard sockets based interface
       for user processes and an internal kernel API for kernel modules. The internal kernel interface is not documented in this  man  page.  Also
       there  is 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 communicate with.  The currently assigned netlink families are:

       NETLINK_ROUTE
	      Receives routing updates and may be used to modify the IPv4 routing table (see rtnetlink(7)).

       NETLINK_FIREWALL
	      Receives packets sent by the IPv4 firewall code.

       NETLINK_ARPD
	      For managing the arp table in user space.

       NETLINK_ROUTE6
	      Receives and sends IPv6 routing table updates.

       NETLINK_IP6_FW
	      to receive packets that failed the IPv6 firewall checks (currently not implemented).

       NETLINK_TAPBASE...NETLINK_TAPBASE+15
	      are  the instances of the ethertap device. Ethertap is a pseudo network tunnel device that allows an ethernet driver to be simulated
	      from user space.

       NETLINK_SKIP
	      Reserved for ENskip.

       NETLINK_USERSOCK
	      is reserved for future user space protocols.

       Netlink messages consist of a byte stream with one or multiple nlmsghdr headers and associated payload.	For multipart messages	the  first
       and  all following headers have the NLM_F_MULTI flag set, except for the last header which has the type NLMSG_DONE.  The byte stream should
       only be accessed with the standard NLMSG_* macros, see netlink(3).

       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 acks 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.

       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 multicast 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 users 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.

	      struct nlmsghdr
	      {
		  __u32    nlmsg_len;  /* Length of message including header */
		  __u16    nlmsg_type; /* Message content */
		  __u16    nlmsg_flags;/* Additional flags */
		  __u32    nlmsg_seq;  /* Sequence number */
		  __u32    nlmsg_pid;  /* PID of the process that opened the socket */
	      };

	      struct nlmsgerr
	      {
		  int	   error;      /* negative errno or 0 for acks. */
		  struct nlmsghdr msg; /* message header that caused the error */
	      };

       After  each  nlmsghdr  the  payload  follows.   nlmsg_type  can	be one of the standard message types: NLMSG_NOOP message is to be ignored,
       NLMSG_ERROR the message signals an error and the payload contains a nlmsgerr structure, NLMSG_DONE message terminates a multipart message,

       A netlink family usually specifies more message types, see the appropriate man pages for that, e.g.  rtnetlink(7) for NETLINK_ROUTE.

       Standard Flag bits in nlmsg_flags
       NLM_F_REQUEST   set on all request messages
       NLM_F_MULTI     the message is part of a multipart message termi-
		       nated by NLMSG_DONE
       NLM_F_ACK       reply with 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    Not implemented yet.
       NLM_F_ATOMIC   Return an atomic snapshot of the table.
       NLM_F_DUMP     not documented yet.

       Additional flag bits for NEW requests
       NLM_F_REPLACE   Override existing 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.

       Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.

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 send to one
       peer) or send to netlink 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 pid */
		  __u32       nl_groups;    /* multicast groups mask */
	      };

       nl_pid is the pid of the process owning the destination socket, or 0 if the destination is in the kernel.   nl_groups  is  a  bitmask  with
       every bit representing a netlink group number.

BUGS

       This man page is not complete.

NOTES

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

VERSIONS

       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.

SEE ALSO

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

       ftp://ftp.inr.ac.ru/ip-routing/iproute2* for libnetlink

Linux Man Page							    1999-04-27								NETLINK(7)