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



NAME
       ipv6, AF_INET6 - Linux IPv6 protocol implementation

SYNOPSIS
       #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);

DESCRIPTION
       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 dif-
       ferences 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 vari-
       able.  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,  sepa-
       rated  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  setsock-
       opt(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.

       IPV6_ADDRFORM
              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 sock-
              ets 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.

       IPV6_ADD_MEMBERSHIP, IPV6_DROP_MEMBERSHIP
              Control  membership  in multicast groups.  Argument is a pointer to a struct
              ipv6_mreq structure.

       IPV6_MTU
              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.

       IPV6_MTU_DISCOVER
              Control path mtu discovery on the socket.  See IP_MTU_DISCOVER in ip(7)  for
              details.

       IPV6_MULTICAST_HOPS
              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.

       IPV6_MULTICAST_IF
              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.

       IPV6_MULTICAST_LOOP
              Control  whether  the socket sees multicast packets that it has send itself.
              Argument is a pointer to boolean.

       IPV6_PKTINFO
              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.

       IPV6_RTHDR, IPV6_AUTHHDR, IPV6_DSTOPS, IPV6_HOPOPTS, IPV6_FLOWINFO, IPV6_HOPLIMIT
              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  inte-
              ger  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 sock-
              ets.  Argument is a pointer to a boolean value.

       IPV6_RECVERR
              Control  receiving  of  asynchronous error options.  See IP_RECVERR in ip(7)
              for details.  Argument is a pointer to boolean.

       IPV6_ROUTER_ALERT
              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 for-
              warded 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 op-
              tion  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.

       IPV6_UNICAST_HOPS
              Set the unicast hop limit for the socket.  Argument is a pointer to an inte-
              ger.  -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 (non-zero), 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).

VERSIONS
       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.

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

BUGS
       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 gen-
       erating 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.

SEE ALSO
       cmsg(3), ip(7)

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

       RFC 2460: IPv6 specification.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-28                           IPV6(7)

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