SEND(4)                   BSD Kernel Interfaces Manual                   SEND(4)

     send — Kernel side support for Secure Neighbor Discovery (SeND)

     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netinet6/send.h>


     To load the driver as a module at boot time, place the following line in


     IPv6 nodes use the Neighbor Discovery Protocol (NDP) to discover other
     nodes on the link, to determine their link-layer addresses to find routers,
     and to maintain reachability information about the paths to active members.
     NDP is vulnerable to various attacks [RFC3756].  Secure Neighbor Discovery
     is a set of extensions to NDP that counter threats to NDP [RFC3971].

     Kernel side support for SeND consists of a kernel module with hooks that
     divert relevant packets (Neighbor Solicitations, Neighbor Advertisements,
     Router Solicitations, Router Advertisements and Redirects) from the NDP
     stack, send them to user space on a dedicated socket and reinject them back
     for further processing.  Hooks are triggered only if the send module is

     The native SeND socket is similar to a raw IP socket, but with its own,
     internal pseudo-protocol (IPPROTO_SEND).  Struct sockaddr_send is defined
     in <netinet6/send.h>.  It defines the total length of the structure, the
     address family, packet's incoming or outgoing direction from the
     interface's point of view, and the interface index.

     struct sockaddr_send {
             unsigned char           send_len;       /* total length */
             sa_family_t             send_family;    /* address family */
             int                     send_direction;
             int                     send_ifidx;
             char                    send_zero[8];

     The address family is always AF_INET6.  The send_direction variable denotes
     the direction of the packet from the interface's point of view and has
     either the value SND_IN or SND_OUT.  The send_ifidx variable is the
     interface index of the receiving or sending interface.  The send_zero
     variable is padding and must always be zero.

     In case that no user space application is connected to the send socket,
     processing continues normally as if the module was not loaded.

     The input hook is named after the input path of the incoming or outgoing
     NDP packets, on the way from the wire, through the nd6 stack, to user
     space.  Relevant packets are identified by adding an mbuf_tag (see
     mbuf_tags(9)) to the mbuf(9), if the send module is loaded.  It is then
     passed on to the kernel-userland interface for either cryptographic
     protection or validation by the SeND application.  The hook takes an
     argument that describes the direction of the packet, both in case of
     incoming and outgoing packets.  SND_IN is the direction of the incoming
     packets that are usually protected by the SeND options and then sent to
     user space for cryptographic validation.  SND_OUT is the outgoing
     direction.  It describes both reply and locally originated outgoing packets
     that are sent to user space for the addition of SeND options.

     The incoming ND packet from the wire:

                                             kernelspace ( userspace
      incoming SeND/ND packet                            (
                 |                                       )
                 v                 ( SND_IN )            (
                icmp6_input() -> send_input_hook ---> send socket ----+
                 :                                       )            |
                 :             #                 #       (            |
        normal   :             #                 #       )            v
      processing :             #     send.ko     #       (    SeND application
         path    :             #                 #       )            |
                 :             #                 #       (            |
                 v                                       )            |
        icmp6/nd6_??_input() <- protocol switch  <--- send socket <---+
                 |         structure (IPPPROTO_SEND)     )
                 |                ( SND_IN )             (
                 v                                       )
      continue normal ND processing                      (

     Outgoing ND packet (reply or locally triggered):

                                             kernelspace ( userspace
      nd6_na_input()                                     (
      +PACKET_TAG_ND_OUTGOING                            )
      |                                                  )
      |   outgoing packet                                (
      |          |                                       )
      |          v                                       (
      |   icmp6_redirect_output()                        )
      |   nd6_ns_output()                                (
      |   nd6_na_output()                                )
      |   +PACKET_TAG_ND_OUTGOING                        (
      |          |                                       )
      |          +-----------<- rip6_output() <----------)----- rtsol/rtadvd/..
      |          |              +PACKET_TAG_ND_OUTGOING  (
      |          v                                       )
      |       ip6_output()                               (
      |          |                                       )
      +-------->-+                                       (
                 |                                       )
                 v                ( SND_OUT )            (
             nd6_output_lle() -> send_input_hook ---> send socket ----+
      -PACKET_TAG_ND_OUTGOING                            )            |
                 :             #                 #       (            |
        normal   :             #                 #       )            v
      processing :             #     send.ko     #       (    SeND application
         path    :             #                 #       )            |
                 :             #                 #       (            |
                 v                                       )            |
         (*ifp->if_output)() <- protocol switch  <--- send socket <---+
                 |         structure (IPPPROTO_SEND)     )
                 |                ( SND_OUT )            (
                 v                                       )
      continue with normal packet output                 (

     A socket operation may fail with one of the following errors returned:

     [EEXIST]           Another user space SeND application is bound to the

     [ENOBUFS]          Shortage of space to receive the incoming (SeND-
                        protected) or outgoing (SeND-validated) packet from the
                        SeND application.

     [ENOSYS]           A packet received from user space and passed to the NDP
                        stack for further processing is neither Neighbor
                        Solicitation, Neighbor Advertisement, Router
                        Solicitation, Router Advertisement nor Redirect.

     [ENOENT]           Occurs if interface output routines fail to send the
                        packet out of the interface.

     recvfrom(2), sendto(2), socket(2), loader.conf(5)

     The send module first appeared in FreeBSD 9.0.

     Ana Kukec <>, University of Zagreb

     Due to the lack of NDP locking, it is currently not possible to unload the
     send module.

BSD                            September 19, 2010                            BSD