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_KEEPFAITH
- (ip6.keepfaith) Boolean: enable/disable “FAITH” TCP relay
IPv6-to-IPv4 translator code in the kernel. Refer
faith(4) and
faithd(8) for detail. Defaults to off.
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.
IPV6CTL_RTEXPIRE
- (ip6.rtexpire) Integer: lifetime in seconds of protocol-cloned IP routes
after the last reference drops (default one hour).
IPV6CTL_RTMINEXPIRE
- (ip6.rtminexpire) Integer: minimum value of ip.rtexpire (default ten
seconds).
IPV6CTL_RTMAXCACHE
- (ip6.rtmaxcache) Integer: trigger level of cached, unreferenced,
protocol-cloned routes which initiates dynamic adaptation (default
128).
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
.