Manual Pages  — SEND

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 loaded.

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.

                                        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                      (

                                        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                 (