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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 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 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

       The address notation for IPv6 is a group of 8 4-digit hexadecimal numbers, separated  with
       a  ':'.   "::"  stands for a string of 0 bits.  Special addresses are ::1 for loopback and
       ::FFFF:<IPv4 address> for IPv4-mapped-on-IPv6.

       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

              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

              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

              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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