ipv6 - 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 needs to support only 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
   supports it only for link-local 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  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

   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
          allowed only 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.

          getsockopt(): Retrieve the current known path MTU of the current
          socket.  Valid only when the socket has been connected.  Returns
          an integer.

          setsockopt(): 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  allowed  only 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.

   IPV6_RECVPKTINFO (since Linux 2.6.14)
          Set delivery of the IPV6_PKTINFO  control  message  on  incoming
          datagrams.   Such control messages contain a struct in6_pktinfo,
          as per RFC  3542.   Allowed  only  for  SOCK_DGRAM  or  SOCK_RAW
          sockets.   Argument  is  a  pointer  to  a  boolean  value in an

          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).  Allowed only 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.  Allowed only 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).


   ENODEV The user tried to bind(2) to a link-local IPv6 address, but  the
          sin6_scope_id  in  the  supplied sockaddr_in6 structure is not a
          valid interface index.


   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.

   SOL_IP,  SOL_IPV6,  SOL_ICMPV6  and  other  SOL_*  socket  options  are
   nonportable variants of IPPROTO_*.  See also ip(7).


   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.


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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