man/man4/inet6.4

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.Dd January 31, 2026
.Dt INET6 4
.Os
.Sh NAME
.Nm inet6
.Nd Internet protocol version 6 family
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.Sh DESCRIPTION
The
.Nm
family is an updated version of
.Xr inet 4
family.
While
.Xr inet 4
implements Internet Protocol version 4,
.Nm
implements Internet Protocol version 6.
.Pp
.Nm
is a collection of protocols layered atop the
.Em Internet Protocol version 6
.Pq Tn IPv6
transport layer, and utilizing the IPv6 address format.
The
.Nm
family provides protocol support for the
.Dv SOCK_STREAM , SOCK_DGRAM ,
and
.Dv SOCK_RAW
socket types; the
.Dv SOCK_RAW
interface provides access to the
.Tn IPv6
protocol.
.Sh ADDRESSING
IPv6 addresses are 16 byte quantities, stored in network standard byteorder.
The include file
.In netinet/in.h
defines this address
as a discriminated union.
.Pp
Sockets bound to the
.Nm
family utilize the following addressing structure:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
Sockets may be created with the local address
.Dq Dv ::
(which is equal to IPv6 address
.Dv 0:0:0:0:0:0:0:0 )
to effect
.Dq wildcard
matching on incoming messages.
.Pp
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
.Xr 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.
.Pp
The KAME implementation supports an extended numeric IPv6 address notation
for link-local addresses,
like
.Dq Li fe80::1%de0
to specify
.Do
.Li fe80::1
on
.Li de0
interface
.Dc .
This notation is supported by
.Xr getaddrinfo 3
and
.Xr getnameinfo 3 .
Some of normal userland programs, such as
.Xr telnet 1
or
.Xr ftp 1 ,
are able to use this notation.
With special programs
like
.Xr ping 8 ,
you can specify the outgoing interface by an extra command line option
to disambiguate scoped addresses.
.Pp
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
.Dv PF_ROUTE
socket, kernel memory accesses via
.Xr kvm 3
and on some other occasions.
HOWEVER, users should never use the embedded form.
For details please consult
.Pa IMPLEMENTATION
supplied with KAME kit.
.Sh PROTOCOLS
The
.Nm
family is comprised of the
.Tn IPv6
network protocol, Internet Control
Message Protocol version 6
.Pq Tn ICMPv6 ,
Transmission Control Protocol
.Pq Tn TCP ,
and User Datagram Protocol
.Pq Tn UDP .
.Tn TCP
is used to support the
.Dv SOCK_STREAM
abstraction while
.Tn UDP
is used to support the
.Dv SOCK_DGRAM
abstraction.
Note that
.Tn TCP
and
.Tn UDP
are common to
.Xr inet 4
and
.Nm .
A raw interface to
.Tn IPv6
is available
by creating an Internet socket of type
.Dv SOCK_RAW .
The
.Tn ICMPv6
message protocol is accessible from a raw socket.
.Ss MIB Variables
A number of variables are implemented in the
.Va net.inet6
branch of the
.Xr 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:
.Bl -tag -width IPV6CTL_MAXFRAGPACKETS
.It Dv IPV6CTL_FORWARDING
.Pq ip6.forwarding
Boolean: enable/disable forwarding of
.Tn IPv6
packets.
Also, identify if the node is acting as a router.
Defaults to off.
.It Dv IPV6CTL_SENDREDIRECTS
.Pq ip6.redirect
Boolean: enable/disable sending of
.Tn ICMPv6
redirects in response to unforwardable
.Tn IPv6
packets.
This option is ignored unless the node is routing
.Tn IPv6
packets,
and should normally be enabled on all systems.
Defaults to on.
.It Dv IPV6CTL_DEFHLIM
.Pq ip6.hlim
Integer: default hop limit value to use for outgoing
.Tn IPv6
packets.
This value applies to all the transport protocols on top of
.Tn IPv6 .
There are APIs to override the value.
.It Dv IPV6CTL_MAXFRAGS
.Pq 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.
.It Dv IPV6CTL_MAXFRAGPACKETS
.Pq 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.
.It Dv IPV6CTL_MAXFRAGBUCKETSIZE
.Pq 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
.Va ip6.maxfragpackets
changes.
This is a per-VNET limit.
.It Dv IPV6CTL_MAXFRAGSPERPACKET
.Pq 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.
.It Dv IPV6CTL_ACCEPT_RTADV
.Pq ip6.accept_rtadv
Boolean: the default value of a per-interface flag to
enable/disable receiving of
.Tn 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.
.It Dv IPV6CTL_AUTO_LINKLOCAL
.Pq ip6.auto_linklocal
Boolean: the default value of a per-interface flag to
enable/disable performing automatic link-local address configuration.
Defaults to on.
.It Dv IPV6CTL_LOG_INTERVAL
.Pq ip6.log_interval
Integer: default interval between
.Tn IPv6
packet forwarding engine log output
(in seconds).
.It Dv IPV6CTL_HDRNESTLIMIT
.Pq ip6.hdrnestlimit
Integer: default number of the maximum
.Tn IPv6
extension headers
permitted on incoming
.Tn IPv6
packets.
If set to 0, the node will accept as many extension headers as possible.
.It Dv IPV6CTL_DAD_COUNT
.Pq ip6.dad_count
Integer: default number of
.Tn IPv6
DAD
.Pq duplicated address detection
probe packets.
The packets will be generated when
.Tn IPv6
interface addresses are configured.
.It Dv IPV6CTL_GRAND_COUNT
.Pq ip6.grand_count
Integer: default number of
.Tn IPv6
GRAND
.Pq gratuitous neighbor discovery
unsolicited NA packets.
The packets will be generated when
.Tn IPv6
interface addresses are configured or when there are changes to
link-layer interface addresses.
.It Dv IPV6CTL_AUTO_FLOWLABEL
.Pq ip6.auto_flowlabel
Boolean: enable/disable automatic filling of
.Tn IPv6
flowlabel field, for outstanding connected transport protocol packets.
The field might be used by intermediate routers to identify packet flows.
Defaults to on.
.It Dv IPV6CTL_DEFMCASTHLIM
.Pq ip6.defmcasthlim
Integer: default hop limit value for an
.Tn IPv6
multicast packet sourced by the node.
This value applies to all the transport protocols on top of
.Tn IPv6 .
There are APIs to override the value as documented in
.Xr ip6 4 .
.It Dv IPV6CTL_GIF_HLIM
.Pq ip6.gifhlim
Integer: default maximum hop limit value for an
.Tn IPv6
packet generated by
.Xr gif 4
tunnel interface.
.It Dv IPV6CTL_KAME_VERSION
.Pq ip6.kame_version
String: identifies the version of KAME
.Tn IPv6
stack implemented in the kernel.
.It Dv IPV6CTL_USE_DEPRECATED
.Pq ip6.use_deprecated
Boolean: enable/disable use of deprecated address,
specified in RFC2462 5.5.4.
Defaults to on.
.It Dv IPV6CTL_RR_PRUNE
.Pq ip6.rr_prune
Integer: default interval between
.Tn IPv6
router renumbering prefix babysitting, in seconds.
.It Dv IPV6CTL_V6ONLY
.Pq ip6.v6only
Boolean: enable/disable the prohibited use of
.Tn IPv4
mapped address on
.Dv AF_INET6
sockets.
Defaults to on.
.It Va ip6.log_cannot_forward
Boolean: log packets that can't be forwarded because of unspecified source
address or destination address beyond the scope of the source address as
described in RFC4443.
Enabled by default.
.It Va ip6.source_address_validation
Boolean: perform source address validation for packets destined for the local
host.
Consider this as following Section 3.2 of RFC3704/BCP84, where we treat local
host as our own infrastructure.
This has no effect on packets to be forwarded, so don't consider it as
anti-spoof feature for a router.
Enabled by default.
.El
.Ss Interaction between IPv4/v6 sockets
By default,
.Fx
does not route IPv4 traffic to
.Dv 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
.Xr ip6 4
for details.
.Pp
The behavior of
.Dv AF_INET6
TCP/UDP socket is documented in RFC2553.
Basically, it says this:
.Bl -bullet -compact
.It
A specific bind on an
.Dv AF_INET6
socket
.Xr ( bind 2
with an address specified)
should accept IPv6 traffic to that address only.
.It
If you perform a wildcard bind
on an
.Dv AF_INET6
socket
.Xr ( bind 2
to IPv6 address
.Li :: ) ,
and there is no wildcard bind
.Dv AF_INET
socket on that TCP/UDP port, IPv6 traffic as well as IPv4 traffic
should be routed to that
.Dv AF_INET6
socket.
IPv4 traffic should be seen as if it came from an IPv6 address like
.Li ::ffff:10.1.1.1 .
This is called an IPv4 mapped address.
.It
If there are both a wildcard bind
.Dv AF_INET
socket and a wildcard bind
.Dv AF_INET6
socket on one TCP/UDP port, they should behave separately.
IPv4 traffic should be routed to the
.Dv AF_INET
socket and IPv6 should be routed to the
.Dv AF_INET6
socket.
.El
.Pp
However, RFC2553 does not define the ordering constraint between calls to
.Xr 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
.Dv AF_INET6
wildcard bind sockets.
It is recommended to listen to two sockets, one for
.Dv AF_INET
and another for
.Dv AF_INET6 ,
when you would like to accept both IPv4 and IPv6 traffic.
.Pp
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
.Dv AF_INET6
socket.
Users are advised to take care handling connections
from IPv4 mapped address to
.Dv AF_INET6
sockets.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr icmp6 4 ,
.Xr intro 4 ,
.Xr ip6 4 ,
.Xr tcp 4 ,
.Xr udp 4
.Rs
.%A A. Conta
.%A S. Deering
.%A M. Gupta
.%T "Internet Control Message Protocol (ICMPv6) for the Internet" \
    "Protocol Version 6 (IPv6) Specification"
.%R RFC 4443
.%D March 2006
.Re
.Sh STANDARDS
.Rs
.%A Tatsuya Jinmei
.%A Atsushi Onoe
.%T "An Extension of Format for IPv6 Scoped Addresses"
.%R internet draft
.%D June 2000
.%N draft-ietf-ipngwg-scopedaddr-format-02.txt
.%O work in progress material
.Re
.Sh HISTORY
The
.Nm
protocol interfaces are defined in RFC2553 and RFC2292.
The implementation described herein appeared in the WIDE/KAME project.
.Sh 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.
.Pp
Users are suggested to implement
.Dq version independent
code as much as possible, as you will need to support both
.Xr inet 4
and
.Nm .