ipv6(7) — Linux manual page


ipv6(7)             Miscellaneous Information 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 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 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.

   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 Linux 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

       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
       numbers, separated 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 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 when path
              MTU discovery is enabled.  Argument is a pointer to

              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 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 delivers
              the destination options, IPV6_HOPOPTS delivers the hop
              options, IPV6_FLOWINFO delivers an integer containing the
              flow ID, IPV6_HOPLIMIT delivers 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 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

              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 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
              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 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.

Linux man-pages (unreleased)     (date)                          ipv6(7)

Pages that refer to this page: bind(2)recv(2)send(2)socket(2)sd_is_fifo(3)sockaddr(3type)nfs(5)systemd.socket(5)address_families(7)ip(7)socket(7)udplite(7)