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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
     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 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
            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  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 4.16 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)

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