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       ipv6, AF_INET6 - Linux IPv6 protocol implementation


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

       tcp6_socket = socket(AF_INET6, SOCK_STREAM, 0);
       raw6_socket = socket(AF_INET6, SOCK_RAW, protocol);
       udp6_socket = socket(AF_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  IPv4  API  (see
       ip(7)).  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  byte

       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 to support this API  type
       to  support  both  protocols.   This  is  handled  transparently by the
       address handling functions in the C library.

       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 {
               sa_family_t     sin6_family;   /* AF_INET6 */
               in_port_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
       depending  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.

       IPV6_V6ONLY (since Linux 2.4.21 and 2.6)
              If this flag is set  to  true  (nonzero),  then  the  socket  is
              restricted  to sending and receiving IPv6 packets only.  In this
              case, an IPv4 and an IPv6 application can bind to a single  port
              at the same time.

              If this flag is set to false (zero), then the socket can be used
              to send and receive packets to and from an IPv6  address  or  an
              IPv4-mapped IPv6 address.

              The argument is a pointer to a boolean value in an integer.

              The  default  value  for this flag is defined by the contents of
              the file /proc/sys/net/ipv6/bindv6only.  The default  value  for
              that file is 0 (false).


       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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       description of the project, and information about reporting  bugs,  can
       be found at