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INET6(4)               FreeBSD Kernel Interfaces Manual               INET6(4)

     inet6 - Internet protocol version 6 family

     #include <sys/types.h>
     #include <netinet/in.h>

     The inet6 family is an updated version of inet(4) family.  While inet(4)
     implements Internet Protocol version 4, inet6 implements Internet
     Protocol version 6.

     inet6 is a collection of protocols layered atop the Internet Protocol
     version 6 (IPv6) transport layer, and utilizing the IPv6 address format.
     The inet6 family provides protocol support for the SOCK_STREAM,
     SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides
     access to the IPv6 protocol.

     IPv6 addresses are 16 byte quantities, stored in network standard
     byteorder.  The include file <netinet/in.h> defines this address as a
     discriminated union.

     Sockets bound to the inet6 family utilize the following addressing

           struct sockaddr_in6 {
                   uint8_t         sin6_len;
                   sa_family_t     sin6_family;
                   in_port_t       sin6_port;
                   uint32_t        sin6_flowinfo;
                   struct in6_addr sin6_addr;
                   uint32_t        sin6_scope_id;

     Sockets may be created with the local address ``::'' (which is equal to
     IPv6 address 0:0:0:0:0:0:0:0) to affect ``wildcard'' matching on incoming

     The IPv6 specification defines scoped addresses, like link-local or site-
     local addresses.  A scoped address is ambiguous to the kernel, if it is
     specified without a scope identifier.  To manipulate scoped addresses
     properly from the userland, programs must use the advanced API defined in
     RFC2292.  A compact description of the advanced API is available in
     ip6(4).  If a scoped address is specified without an explicit scope, the
     kernel may raise an error.  Note that scoped addresses are not for daily
     use at this moment, both from a specification and an implementation point
     of view.

     The KAME implementation supports an extended numeric IPv6 address
     notation for link-local addresses, like ``fe80::1%de0'' to specify
     ``fe80::1 on de0 interface''.  This notation is supported by
     getaddrinfo(3) and getnameinfo(3).  Some of normal userland programs,
     such as telnet(1) or ftp(1), are able to use this notation.  With special
     programs like ping6(8), you can specify the outgoing interface by an
     extra command line option to disambiguate scoped addresses.

     Scoped addresses are handled specially in the kernel.  In kernel
     structures like routing tables or interface structures, a scoped address
     will have its interface index embedded into the address.  Therefore, the
     address in some kernel structures is not the same as that on the wire.
     The embedded index will become visible through a PF_ROUTE socket, kernel
     memory accesses via kvm(3) and on some other occasions.  HOWEVER, users
     should never use the embedded form.  For details please consult
     IMPLEMENTATION supplied with KAME kit.

     The inet6 family is comprised of the IPv6 network protocol, Internet
     Control Message Protocol version 6 (ICMPv6), Transmission Control
     Protocol (TCP), and User Datagram Protocol (UDP).  TCP is used to support
     the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM
     abstraction.  Note that TCP and UDP are common to inet(4) and inet6.  A
     raw interface to IPv6 is available by creating an Internet socket of type
     SOCK_RAW.  The ICMPv6 message protocol is accessible from a raw socket.

   MIB Variables
     A number of variables are implemented in the net.inet6 branch of the
     sysctl(3) MIB.  In addition to the variables supported by the transport
     protocols (for which the respective manual pages may be consulted), the
     following general variables are defined:

     IPV6CTL_FORWARDING      (ip6.forwarding) Boolean: enable/disable
                             forwarding of IPv6 packets.  Also, identify if
                             the node is acting as a router.  Defaults to off.

     IPV6CTL_SENDREDIRECTS   (ip6.redirect) Boolean: enable/disable sending of
                             ICMPv6 redirects in response to unforwardable
                             IPv6 packets.  This option is ignored unless the
                             node is routing IPv6 packets, and should normally
                             be enabled on all systems.  Defaults to on.

     IPV6CTL_DEFHLIM         (ip6.hlim) Integer: default hop limit value to
                             use for outgoing IPv6 packets.  This value
                             applies to all the transport protocols on top of
                             IPv6.  There are APIs to override the value.

     IPV6CTL_MAXFRAGPACKETS  (ip6.maxfragpackets) Integer: default maximum
                             number of fragmented packets the node will
                             accept.  0 means that the node will not accept
                             any fragmented packets.  -1 means that the node
                             will accept as many fragmented packets as it
                             receives.  The flag is provided basically for
                             avoiding possible DoS attacks.

     IPV6CTL_ACCEPT_RTADV    (ip6.accept_rtadv) Boolean: the default value of
                             a per-interface flag to enable/disable receiving
                             of ICMPv6 router advertisement packets, and
                             autoconfiguration of address prefixes and default
                             routers.  The node must be a host (not a router)
                             for the option to be meaningful.  Defaults to

     IPV6CTL_AUTO_LINKLOCAL  (ip6.auto_linklocal) Boolean: the default value
                             of a per-interface flag to enable/disable
                             performing automatic link-local address
                             configuration.  Defaults to on.

     IPV6CTL_LOG_INTERVAL    (ip6.log_interval) Integer: default interval
                             between IPv6 packet forwarding engine log output
                             (in seconds).

     IPV6CTL_HDRNESTLIMIT    (ip6.hdrnestlimit) Integer: default number of the
                             maximum IPv6 extension headers permitted on
                             incoming IPv6 packets.  If set to 0, the node
                             will accept as many extension headers as

     IPV6CTL_DAD_COUNT       (ip6.dad_count) Integer: default number of IPv6
                             DAD (duplicated address detection) probe packets.
                             The packets will be generated when IPv6 interface
                             addresses are configured.

     IPV6CTL_AUTO_FLOWLABEL  (ip6.auto_flowlabel) Boolean: enable/disable
                             automatic filling of IPv6 flowlabel field, for
                             outstanding connected transport protocol packets.
                             The field might be used by intermediate routers
                             to identify packet flows.  Defaults to on.

     IPV6CTL_DEFMCASTHLIM    (ip6.defmcasthlim) Integer: default hop limit
                             value for an IPv6 multicast packet sourced by the
                             node.  This value applies to all the transport
                             protocols on top of IPv6.  There are APIs to
                             override the value as documented in ip6(4).

     IPV6CTL_GIF_HLIM        (ip6.gifhlim) Integer: default maximum hop limit
                             value for an IPv6 packet generated by gif(4)
                             tunnel interface.

     IPV6CTL_KAME_VERSION    (ip6.kame_version) String: identifies the version
                             of KAME IPv6 stack implemented in the kernel.

     IPV6CTL_USE_DEPRECATED  (ip6.use_deprecated) Boolean: enable/disable use
                             of deprecated address, specified in RFC2462
                             5.5.4.  Defaults to on.

     IPV6CTL_RR_PRUNE        (ip6.rr_prune) Integer: default interval between
                             IPv6 router renumbering prefix babysitting, in

     IPV6CTL_V6ONLY          (ip6.v6only) Boolean: enable/disable the
                             prohibited use of IPv4 mapped address on AF_INET6
                             sockets.  Defaults to on.

   Interaction between IPv4/v6 sockets
     By default, FreeBSD does not route IPv4 traffic to AF_INET6 sockets.  The
     default behavior intentionally violates RFC2553 for security reasons.
     Listen to two sockets if you want to accept both IPv4 and IPv6 traffic.
     IPv4 traffic may be routed with certain per-socket/per-node
     configuration, however, it is not recommended to do so.  Consult ip6(4)
     for details.

     The behavior of AF_INET6 TCP/UDP socket is documented in RFC2553.
     Basically, it says this:
        A specific bind on an AF_INET6 socket (bind(2) with an address
         specified) should accept IPv6 traffic to that address only.
        If you perform a wildcard bind on an AF_INET6 socket (bind(2) to IPv6
         address ::), and there is no wildcard bind AF_INET socket on that
         TCP/UDP port, IPv6 traffic as well as IPv4 traffic should be routed
         to that AF_INET6 socket.  IPv4 traffic should be seen as if it came
         from an IPv6 address like ::ffff:  This is called an IPv4
         mapped address.
        If there are both a wildcard bind AF_INET socket and a wildcard bind
         AF_INET6 socket on one TCP/UDP port, they should behave separately.
         IPv4 traffic should be routed to the AF_INET socket and IPv6 should
         be routed to the AF_INET6 socket.

     However, RFC2553 does not define the ordering constraint between calls to
     bind(2), nor how IPv4 TCP/UDP port numbers and IPv6 TCP/UDP port numbers
     relate to each other (should they be integrated or separated).
     Implemented behavior is very different from kernel to kernel.  Therefore,
     it is unwise to rely too much upon the behavior of AF_INET6 wildcard bind
     sockets.  It is recommended to listen to two sockets, one for AF_INET and
     another for AF_INET6, when you would like to accept both IPv4 and IPv6

     It should also be noted that malicious parties can take advantage of the
     complexity presented above, and are able to bypass access control, if the
     target node routes IPv4 traffic to AF_INET6 socket.  Users are advised to
     take care handling connections from IPv4 mapped address to AF_INET6

     ioctl(2), socket(2), sysctl(3), icmp6(4), intro(4), ip6(4), tcp(4),

     Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
     Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-02.txt,
     June 2000, work in progress material.

     The inet6 protocol interfaces are defined in RFC2553 and RFC2292.  The
     implementation described herein appeared in the WIDE/KAME project.

     The IPv6 support is subject to change as the Internet protocols develop.
     Users should not depend on details of the current implementation, but
     rather the services exported.

     Users are suggested to implement ``version independent'' code as much as
     possible, as you will need to support both inet(4) and inet6.

FreeBSD 11.1-RELEASE-p4        September 2, 2009       FreeBSD 11.1-RELEASE-p4
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