IPV6(7)                   (2020-12-21)                    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, ver-
          sion 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

          The IPv6 API aims to be mostly compatible with the IPv4 API
          (see ip(7)).  Only differences are described in this man

          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 needs to sup-
          port 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 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) */

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              struct in6_addr {
                  unsigned char   s6_addr[16];   /* IPv6 address */

          sin6_family is always set to AF_INET6; sin6_port is the pro-
          tocol 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 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 hex-
          adecimal numbers, separated with a aq:aq.  "::" 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 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

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               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

               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 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 deliv-
               ers the destination options, IPV6_HOPOPTS delivers the
               hop options, IPV6_FLOWINFO delivers an integer contain-
               ing the flow ID, IPV6_HOPLIMIT delivers an integer con-
               taining 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 pack-
               ets 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.

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               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 dis-
               ables 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

          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

               The default value for this flag is defined by the con-
               tents of the file /proc/sys/net/ipv6/bindv6only. The
               default value for that file is 0 (false).

               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 trans-
          parently passed/read by the kernel when the passed address

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          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 com-
          plete support for receiving options, the macros for generat-
          ing 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

          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 5.10 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 https://www.kernel.org/doc/man-pages/.

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