Provided by: freebsd-manpages_12.2-1_all bug


     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 structure:

           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 messages.

     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_MAXFRAGS        (ip6.maxfrags) Integer: maximum number of fragments the host will
                             accept and simultaneously hold across all reassembly queues in all
                             VNETs.  If set to 0, fragment reassembly is disabled.  If set to -1,
                             this limit is not applied.  This limit is recalculated when the
                             number of mbuf clusters is changed.  This is a global limit.

     IPV6CTL_MAXFRAGPACKETS  (ip6.maxfragpackets) Integer: maximum number of fragmented packets
                             the node will accept and simultaneously hold in the reassembly queue
                             for a particular VNET.  0 means that the node will not accept any
                             fragmented packets for that VNET.  -1 means that the node will not
                             apply this limit for that VNET.  This limit is recalculated when the
                             number of mbuf clusters is changed.  This is a per-VNET limit.

                             (ip6.maxfragbucketsize) Integer: maximum number of reassembly queues
                             per bucket.  Fragmented packets are hashed to buckets.  Each bucket
                             has a list of reassembly queues.  The system must compare the
                             incoming packets to the existing reassembly queues in the bucket to
                             find a matching reassembly queue.  To preserve system resources, the
                             system limits the number of reassembly queues allowed in each
                             bucket.  This limit is recalculated when the number of mbuf clusters
                             is changed or when the value of ip6.maxfragpackets changes.  This is
                             a per-VNET limit.

                             (ip6.maxfragsperpacket) Integer: maximum number of fragments the
                             host will accept and hold in the ressembly queue for a packet.  This
                             is a per-VNET limit.

     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 off.

     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 possible.

     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 seconds.

     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
        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 traffic.

     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 sockets.


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