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NAME
inet6 — Internet protocol version 6 family
SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
DESCRIPTION
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.
ADDRESSING
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.
PROTOCOLS
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 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:10.1.1.1. 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 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.
SEE ALSO
ioctl(2), socket(2), sysctl(3), icmp6(4), intro(4), ip6(4), tcp(4), udp(4)
STANDARDS
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.
HISTORY
The inet6 protocol interfaces are defined in RFC2553 and RFC2292. The implementation
described herein appeared in the WIDE/KAME project.
BUGS
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.