ipv6(7) — Linux manual page


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

NAME         top

       ipv6 - Linux IPv6 protocol implementation

SYNOPSIS         top

       #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         top

       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 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
       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 an IPv4 host, and other reserved address

       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 cur‐
              rent 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_pkt‐
              info, 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 con‐
              taining extension headers from the received packet.  IPV6_RTH‐
              DR delivers the routing header, IPV6_AUTHHDR delivers the au‐
              thentication header, IPV6_DSTOPTS delivers the destination op‐
              tions, IPV6_HOPOPTS delivers the hop options, IPV6_FLOWINFO
              delivers an integer containing the flow ID, IPV6_HOPLIMIT de‐
              livers 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 buf‐
              fer 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

              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 de‐
              livery 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).

ERRORS         top

       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.

VERSIONS         top

       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         top

       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).

BUGS         top

       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.

SEE ALSO         top

       cmsg(3), ip(7)

       RFC 2553: IPv6 BASIC API; Linux tries to be compliant to this.
       RFC 2460: IPv6 specification.

COLOPHON         top

       This page is part of release 5.08 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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