IPV6(7)                    Linux Programmer's Manual                   IPV6(7)

       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 ex-
       pands to a constant expression.  Both of them are in network  byte  or-

       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 ad-
       dress handling functions in the C library.

       IPv4 and IPv6 share the local port space.  When you get an IPv4 connec-
       tion  or packet to an 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  depend-
       ing  on  the scope of the address.  It is new in Linux 2.4.  Linux sup-
       ports it only for link-local addresses, in that case sin6_scope_id con-
       tains 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 an IPv4 host, and other reserved address types.

       The address notation for IPv6 is a group of 8 4-digit hexadecimal  num-
       bers,  separated with a ':'.  "::" stands for a string of 0 bits.  Spe-
       cial 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 inte-
              ger containing AF_INET.  This is useful to pass v4-mapped  sock-
              ets  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  discov-
              ery 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 de-
              fault, 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  sock-
              ets.  Argument is a pointer to a boolean value in an integer.

              Set delivery of control messages for incoming datagrams contain-
              ing  extension headers from the received packet.  IPV6_RTHDR de-
              livers the routing header, IPV6_AUTHHDR delivers the authentica-
              tion  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 inte-
              ger containing the hop count of the packet.   The  control  mes-
              sages have the same type as the socket option.  All these header
              options can also be set for outgoing packets by putting the  ap-
              propriate 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  op-
              tion  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  re-
              stricted  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  addi-
       tional 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 out-
       going address length may break.

       The sockaddr_in6 structure is bigger than the generic  sockaddr.   Pro-
       grams  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 non-
       portable variants of IPPROTO_*.  See also ip(7).

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

       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 5.05 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

Linux                             2017-09-15                           IPV6(7)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2024 Hurricane Electric. All Rights Reserved.