netlink (7)


       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  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 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 (cur-
              rently not implemented).

              are the instances of the ethertap device. Ethertap is  a  pseudo
              network tunnel device that allows an ethernet driver to be simu-
              lated 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
       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 multi-
       cast  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 appro-
       priate 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 mes-
                       sage 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.


       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.


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

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