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       netlink, PF_NETLINK - Communication between kernel and user


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

       netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);


       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
       backwards 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:

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

              Receives packets sent by the IPv4 firewall code.

              For managing the arp table in user space.

              Receives and sends IPv6 routing table updates.

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

              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.

              Reserved for ENskip.

              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 terminated 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 superuser rights.


       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 ID */
                  __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.


       This man page is not complete.


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


       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.


       cmsg(3), netlink(3), capabilities(7), rtnetlink(7)* for libnetlink