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       ipv6, PF_INET6 - Linux IPv6 protocol implementation


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

       tcp6_socket = socket(PF_INET6, SOCK_STREAM, 0);
       raw6_socket = socket(PF_INET6, SOCK_RAW, protocol);
       udp6_socket = socket(PF_INET6, SOCK_DGRAM, protocol);


       Linux 2.2 optionally implements the Internet Protocol, version 6.  This
       man page contains a description of the IPv6 basic API as implemented by
       the  Linux  kernel  and  glibc  2.1.  The interface is based on the BSD
       sockets interface; see socket(7).

       The IPv6 API aims to be mostly compatible with the ip(7) v4 API.   Only
       differences are described in this man page.

       To  bind  an AF_INET6 socket to any process the local address should be
       copied from the in6addr_any  variable  which  has  in6_addr  type.   In
       static initializations IN6ADDR_ANY_INIT may also be used, which expands
       to a constant expression.  Both of them are in network order.

       The  IPv6  loopback  address  (::1)  is   available   in   the   global
       in6addr_loopback  variable.   For initializations IN6ADDR_LOOPBACK_INIT
       should be used.

       IPv4 connections can be handled with the v6 API by using the v4-mapped-
       on-v6  address type; thus a program only needs only to support this API
       type to support both protocols.  This is handled transparently  by  the
       address handling functions in libc.

       IPv4  and  IPv6  share  the  local  port  space.   When you get an IPv4
       connection or packet to a IPv6 socket its source address will be mapped
       to v6 and it will be mapped to v6.

   Address Format
           struct sockaddr_in6 {
               uint16_t        sin6_family;   /* AF_INET6 */
               uint16_t        sin6_port;     /* port number */
               uint32_t        sin6_flowinfo; /* IPv6 flow information */
               struct in6_addr sin6_addr;     /* IPv6 address */
               uint32_t        sin6_scope_id; /* Scope ID (new in 2.4) */

           struct in6_addr {
               unsigned char   s6_addr[16];   /* IPv6 address */

       sin6_family  is  always set to AF_INET6; sin6_port is the protocol port
       (see sin_port in ip(7)); sin6_flowinfo is  the  IPv6  flow  identifier;
       sin6_addr  is  the  128-bit  IPv6  address.   sin6_scope_id is an ID of
       depending of on the scope of the address.  It  is  new  in  Linux  2.4.
       Linux  only  supports  it  for  link  scope  addresses,  in  that  case
       sin6_scope_id contains the interface index (see netdevice(7))

       IPv6 supports several address types: unicast to address a single  host,
       multicast  to  address a group of hosts, anycast to address the nearest
       member of a group of hosts (not implemented in Linux), IPv4-on-IPv6  to
       address a IPv4 host, and other reserved address types.

       The  address  notation  for  IPv6  is a group of 16 2 digit hexadecimal
       numbers, separated with a ’:’. ’::’ stands for  a  string  of  0  bits.
       Special  addresses  are  ::1 for loopback and ::FFFF:<IPv4 address> for

       The port space of IPv6 is shared with IPv4.

   Socket Options
       IPv6 supports some protocol-specific socket options  that  can  be  set
       with  setsockopt(2)  and  read  with  getsockopt(2).  The socket option
       level for IPv6 is IPPROTO_IPV6.  A boolean integer flag is zero when it
       is false, otherwise true.

              Turn  an  AF_INET6  socket  into a socket of a different address
              family.  Only AF_INET is currently supported for  that.   It  is
              only  allowed for IPv6 sockets that are connected and bound to a
              v4-mapped-on-v6 address.   The  argument  is  a  pointer  to  an
              integer  containing  AF_INET.   This is useful to pass v4-mapped
              sockets as file descriptors to programs that don’t know  how  to
              deal with the IPv6 API.

              Control  membership  in multicast groups.  Argument is a pointer
              to a struct ipv6_mreq structure.

              Set the MTU to be used for the socket.  The MTU  is  limited  by
              the  device  MTU  or  the  path  mtu  when path mtu discovery is
              enabled.  Argument is a pointer to integer.

              Control path mtu discovery on the socket.   See  IP_MTU_DISCOVER
              in ip(7) for details.

              Set  the  multicast  hop  limit  for  the socket.  Argument is a
              pointer to an integer.  -1 in the  value  means  use  the  route
              default, otherwise it should be between 0 and 255.

              Set  the  device  for  outgoing multicast packets on the socket.
              This is only allowed for SOCK_DGRAM and  SOCK_RAW  socket.   The
              argument  is  a pointer to an interface index (see netdevice(7))
              in an integer.

              Control whether the socket sees multicast packets  that  it  has
              send itself.  Argument is a pointer to boolean.

              Set  delivery  of  the  IPV6_PKTINFO control message on incoming
              datagrams.  Only allowed for  SOCK_DGRAM  or  SOCK_RAW  sockets.
              Argument is a pointer to a boolean value in an integer.

              Set   delivery   of  control  messages  for  incoming  datagrams
              containing  extension  headers   from   the   received   packet.
              IPV6_RTHDR  delivers  the  routing header, IPV6_AUTHHDR delivers
              the authentication header, IPV6_DSTOPTS delivers the destination
              options,  IPV6_HOPOPTS  delivers  the hop options, IPV6_FLOWINFO
              delivers  an  integer  containing  the  flow  ID,  IPV6_HOPLIMIT
              delivers an integer containing the hop count of the packet.  The
              control messages have the same type as the socket  option.   All
              these  header  options  can  also be set for outgoing packets by
              putting the appropriate control message into the control  buffer
              of sendmsg(2).  Only allowed for SOCK_DGRAM or SOCK_RAW sockets.
              Argument is a pointer to a boolean value.

              Control receiving of asynchronous error options.  See IP_RECVERR
              in ip(7) for details.  Argument is a pointer to boolean.

              Pass  forwarded  packets  containing  a  router alert hop-by-hop
              option to this socket.  Only allowed for SOCK_RAW sockets.   The
              tapped packets are not forwarded by the kernel, it is the user’s
              responsibility to send them out again.  Argument is a pointer to
              an  integer.  A positive integer indicates a router alert option
              value to intercept.  Packets carrying a router alert option with
              a  value  field containing this integer will be delivered to the
              socket.  A negative integer disables delivery  of  packets  with
              router alert options to this socket.

              Set the unicast hop limit for the socket.  Argument is a pointer
              to an integer.  -1 in the value means  use  the  route  default,
              otherwise it should be between 0 and 255.


       The older libinet6 libc5 based IPv6 API implementation for Linux is not
       described here and may vary in details.

       Linux 2.4 will break binary  compatibility  for  the  sockaddr_in6  for
       64-bit  hosts  by  changing  the  alignment  of  in6_addr and adding an
       additional sin6_scope_id field.  The kernel interfaces stay compatible,
       but  a program including sockaddr_in6 or in6_addr into other structures
       may not be.  This is not a problem for 32-bit hosts like i386.

       The sin6_flowinfo field is new  in  Linux  2.4.   It  is  transparently
       passed/read  by  the kernel when the passed address length contains it.
       Some programs that pass a longer address  buffer  and  then  check  the
       outgoing address length may break.


       The  sockaddr_in6  structure  is  bigger  than  the  generic  sockaddr.
       Programs that assume that all address types can be stored safely  in  a
       struct  sockaddr  need to be changed to use struct sockaddr_storage for
       that instead.


       The  IPv6  extended  API  as  in  RFC 2292  is  currently  only  partly
       implemented;  although  the  2.2  kernel  has near complete support for
       receiving options, the macros for generating IPv6 options  are  missing
       in glibc 2.1.

       IPSec support for EH and AH headers is missing.

       Flow label management is not complete and not documented here.

       This man page is not complete.


       cmsg(3), ip(7)

       RFC 2553: IPv6 BASIC API.  Linux tries to be compliant to this.

       RFC 2460: IPv6 specification.


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