.\"	$OpenBSD: ip6.4,v 1.16 2003/08/08 09:51:53 jmc Exp $
.\"	$KAME: ip6.4,v 1.12 2000/06/08 21:19:39 itojun Exp $
.\"
.\" Copyright (C) 1999 WIDE Project.
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.Dd December 17, 1999
.Dt IP6 4
.Os
.Sh NAME
.Nm ip6
.Nd Internet Protocol version 6 (IPv6)
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <netinet/in.h>
.Ft int
.Fn socket AF_INET6 SOCK_RAW proto
.Sh DESCRIPTION
.Tn IPv6
is the network layer protocol used by the Internet protocol version 6 family
.Pq Dv AF_INET6 .
Options may be set at the
.Tn IPv6
level when using higher-level protocols that are based on
.Tn IPv6
(such as
.Tn TCP
and
.Tn UDP ) .
It may also be accessed through a
.Dq raw socket
when developing new protocols, or special-purpose applications.
.Pp
There are several
.Tn IPv6-level
.Xr setsockopt 2 / Ns Xr getsockopt 2
options.
They are separated into the basic IPv6 sockets API
.Pq defined in RFC 2553 ,
and the advanced API
.Pq defined in RFC 2292 .
The basic API looks very similar to the API presented in
.Xr ip 4 .
Advanced API uses ancillary data and can handle more complex cases.
.Pp
To specify some socket options, a certain level of privilege
(i.e. root privilege) is required.
.\"
.Ss Basic IPv6 sockets API
.Dv IPV6_UNICAST_HOPS
may be used to set the hoplimit field in the
.Tn IPv6
header.
As the symbol name suggests, the option controls the hoplimit field
on unicast packets.
If -1 is specified, the kernel will use a default value.
If a value of 0 to 255 is specified, the packet will have the specified
value as hoplimit.
Other values are considered invalid, and
.Dv EINVAL
will be returned.
For example:
.Bd -literal -offset indent
int hlim = 60;                   /* max = 255 */
setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &hlim, sizeof(hlim));
.Ed
.Pp
.Tn IPv6
multicasting is supported only on
.Dv AF_INET6
sockets of type
.Dv SOCK_DGRAM
and
.Dv SOCK_RAW ,
and only on networks where the interface driver supports multicasting.
.Pp
The
.Dv IPV6_MULTICAST_HOPS
option changes the hoplimit for outgoing multicast datagrams
in order to control the scope of the multicasts:
.Bd -literal -offset indent
unsigned int hlim;	/* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hlim, sizeof(hlim));
.Ed
.Pp
Datagrams with a hoplimit of 1 are not forwarded beyond the local network.
Multicast datagrams with a hoplimit of 0 will not be transmitted on any network,
but may be delivered locally if the sending host belongs to the destination
group and if multicast loopback has not been disabled on the sending socket
(see below).
Multicast datagrams with a hoplimit greater than 1 may be forwarded
to other networks if a multicast router is attached to the local network.
.Pp
For hosts with multiple interfaces, each multicast transmission is
sent from the primary network interface.
The
.Dv IPV6_MULTICAST_IF
option overrides the default for
subsequent transmissions from a given socket:
.Bd -literal -offset indent
unsigned int outif;
outif = if_nametoindex("ne0");
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_IF, &outif, sizeof(outif));
.Ed
.Pp
where "outif" is an interface index of the desired interface,
or 0 to specify the default interface.
.Pp
If a multicast datagram is sent to a group to which the sending host itself
belongs (on the outgoing interface), a copy of the datagram is, by default,
looped back by the IPv6 layer for local delivery.
The
.Dv IPV6_MULTICAST_LOOP
option gives the sender explicit control
over whether or not subsequent datagrams are looped back:
.Bd -literal -offset indent
u_char loop;	/* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loop, sizeof(loop));
.Ed
.Pp
This option
improves performance for applications that may have no more than one
instance on a single host (such as a router daemon), by eliminating
the overhead of receiving their own transmissions.
It should generally not be used by applications for which there
may be more than one instance on a single host (such as a conferencing
program) or for which the sender does not belong to the destination
group (such as a time querying program).
.Pp
A multicast datagram sent with an initial hoplimit greater than 1 may be
delivered to the sending host on a different interface from that on which
it was sent, if the host belongs to the destination group on that other
interface.
The loopback control option has no effect on such delivery.
.Pp
A host must become a member of a multicast group before it can receive
datagrams sent to the group.
To join a multicast group, use the
.Dv IPV6_JOIN_GROUP
option:
.Bd -literal -offset indent
struct ipv6_mreq mreq6;
setsockopt(s, IPPROTO_IPV6, IPV6_JOIN_GROUP, &mreq6, sizeof(mreq6));
.Ed
.Pp
where
.Fa mreq6
is the following structure:
.Bd -literal -offset indent
struct ipv6_mreq {
    struct in6_addr ipv6mr_multiaddr;
    unsigned int ipv6mr_interface;
};
.Ed
.Pp
.Dv ipv6mr_interface
should be 0 to choose the default multicast interface, or the
interface index of a particular multicast-capable interface if
the host is multihomed.
Membership is associated with a single interface;
programs running on multihomed hosts may need to
join the same group on more than one interface.
.Pp
To drop a membership, use:
.Bd -literal -offset indent
struct ipv6_mreq mreq6;
setsockopt(s, IPPROTO_IPV6, IPV6_LEAVE_GROUP, &mreq6, sizeof(mreq6));
.Ed
.Pp
where
.Fa mreq6
contains the same values as used to add the membership.
Memberships are dropped when the socket is closed or the process exits.
.Pp
.Dv IPV6_PORTRANGE
controls how ephemeral ports are allocated for
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
sockets.
For example,
.Bd -literal -offset indent
int range = IPV6_PORTRANGE_LOW;       /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IPV6, IPV6_PORTRANGE, &range, sizeof(range));
.Ed
.\"
.Ss Advanced IPv6 sockets API
The advanced IPv6 sockets API lets userland programs specify or obtain
details about the IPv6 header and the IPv6 extension headers on packets.
The advanced API uses ancillary data for passing data from/to the kernel.
.Pp
There are
.Xr setsockopt 2 / Ns Xr getsockopt 2
options to get optional information on incoming packets.
They are
.Dv IPV6_PKTINFO ,
.Dv IPV6_HOPLIMIT ,
.Dv IPV6_HOPOPTS ,
.Dv IPV6_DSTOPTS ,
and
.Dv IPV6_RTHDR .
.Bd -literal -offset indent
int  on = 1;

setsockopt(fd, IPPROTO_IPV6, IPV6_PKTINFO,  &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_HOPLIMIT, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_HOPOPTS,  &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_DSTOPTS,  &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RTHDR,    &on, sizeof(on));
.Ed
.Pp
When any of these options are enabled, the corresponding data is
returned as control information by
.Xr recvmsg 2 ,
as one or more ancillary data objects.
.Pp
If
.Dv IPV6_PKTINFO
is enabled, the destination IPv6 address and the arriving interface index
will be available via
.Li struct in6_pktinfo
on ancillary data stream.
You can pick the structure by checking for an ancillary data item with
.Li cmsg_level
equal to
.Dv IPPROTO_IPV6 ,
and
.Li cmsg_type
equal to
.Dv IPV6_PKTINFO .
.Pp
If
.Dv IPV6_HOPLIMIT
is enabled, the hoplimit value on the packet will be made available to the
userland program.
The ancillary data stream will contain an integer data item with
.Li cmsg_level
equal to
.Dv IPPROTO_IPV6 ,
and
.Li cmsg_type
equal to
.Dv IPV6_HOPLIMIT .
.Pp
.Xr inet6_option_space 3
and friends help parse ancillary data items for
.Dv IPV6_HOPOPTS
and
.Dv IPV6_DSTOPTS .
Similarly,
.Xr inet6_rthdr_space 3
and friends help parse ancillary data items for
.Dv IPV6_RTHDR .
.Pp
.Dv IPV6_HOPOPTS
and
.Dv IPV6_DSTOPTS
may appear multiple times on an ancillary data stream
(note that the behavior is slightly different than the specification).
Other ancillary data items can appear no more than once.
.Pp
For outgoing direction,
ancillary data items with normal payload data can be passed using
.Xr sendmsg 2 .
Ancillary data items will be parsed by the kernel, and used to construct
the IPv6 header and extension headers.
For the 5
.Li cmsg_level
values listed above, the ancillary data format is the same as the inbound case.
Additionally, the
.Dv IPV6_NEXTHOP
data object can also be specified.
The
.Dv IPV6_NEXTHOP
ancillary data object specifies the next hop for the
datagram as a socket address structure.
In the
.Li cmsghdr
structure
containing this ancillary data, the
.Li cmsg_level
member will be
.Dv IPPROTO_IPV6 ,
the
.Li cmsg_type
member will be
.Dv IPV6_NEXTHOP ,
and the first byte of
.Li cmsg_data[]
will be the first byte of the socket address structure.
.Pp
If the socket address structure contains an IPv6 address (i.e., the
sin6_family member is
.Dv AF_INET6
), then the node identified by that
address must be a neighbor of the sending host.
If that address
equals the destination IPv6 address of the datagram, then this is
equivalent to the existing
.Dv SO_DONTROUTE
socket option.
.Pp
For applications that do not, or are unable to use
.Xr sendmsg 2
or
.Xr recvmsg 2 ,
the
.Dv IPV6_PKTOPTIONS
socket option is defined.
Setting the socket option specifies any of the optional output fields:
.Bd -literal -offset indent
setsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, len);
.Ed
.Pp
The fourth argument points to a buffer containing one or more
ancillary data objects, and the fifth argument is the total length of
all these objects.
The application fills in this buffer exactly as
if the buffer were being passed to
.Xr sendmsg 2
as control information.
.Pp
The options set by calling
.Xr setsockopt 2
for
.Dv IPV6_PKTOPTIONS
are
called "sticky" options because once set they apply to all packets
sent on that socket.
The application can call
.Xr setsockopt 2
again to
change all the sticky options, or it can call
.Xr setsockopt 2
with a
length of 0 to remove all the sticky options for the socket.
.Pp
The corresponding receive option
.Bd -literal -offset indent
getsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, &len);
.Ed
.Pp
returns a buffer with one or more ancillary data objects for all the
optional receive information that the application has previously
specified that it wants to receive.
The fourth argument points to
the buffer that is filled in by the call.
The fifth argument is a
pointer to a value-result integer: when the function is called the
integer specifies the size of the buffer pointed to by the fourth
argument, and on return this integer contains the actual number of
bytes that were returned.
The application processes this buffer
exactly as if the buffer were returned by
.Xr recvmsg 2
as control information.
.\"
.Ss Advanced API and TCP sockets
When using
.Xr getsockopt 2
with the
.Dv IPV6_PKTOPTIONS
option and a
.Tn TCP
socket, only the options from the most recently received segment are
retained and returned to the caller, and only after the socket option
has been set.
.\" That is,
.\" .Tn TCP
.\" need not start saving a copy of the options until the application says
.\" to do so.
The application is not allowed to specify ancillary data in a call to
.Xr sendmsg 2
on a
.Tn TCP
socket, and none of the ancillary data that we
described above is ever returned as control information by
.Xr recvmsg 2
on a
.Tn TCP
socket.
.\"
.Ss Conflict resolution
In some cases, there are multiple APIs defined for manipulating
a IPv6 header field.
A good example is the outgoing interface for multicast datagrams:
it can be manipulated by
.Dv IPV6_MULTICAST_IF
in basic API,
.Dv IPV6_PKTINFO
in advanced API, and the
.Li sin6_scope_id
field of the socket address passed to
.Xr sendto 2 .
.Pp
When conflicting options are given to the kernel,
the kernel will get the value in the following order of preference:
(1) options specified by using ancillary data,
(2) options specified by a sticky option of the advanced API,
(3) options specified by using the basic API, and lastly
(4) options specified by a socket address.
Note that the conflict resolution is undefined in the API specification
and implementation dependent.
.\"
.Ss "Raw IPv6 Sockets"
Raw
.Tn IPv6
sockets are connectionless, and are normally used with the
.Xr sendto 2
and
.Xr recvfrom 2
calls, though the
.Xr connect 2
call may also be used to fix the destination for future
packets (in which case the
.Xr read 2
or
.Xr recv 2
and
.Xr write 2
or
.Xr send 2
system calls may be used).
.Pp
If
.Fa proto
is 0, the default protocol
.Dv IPPROTO_RAW
is used for outgoing packets, and only incoming packets destined
for that protocol are received.
If
.Fa proto
is non-zero, that protocol number will be used on outgoing packets
and to filter incoming packets.
.Pp
Outgoing packets automatically have an
.Tn IPv6
header prepended to them (based on the destination address and the
protocol number the socket is created with).
Incoming packets are received without an
.Tn IPv6
header or extension headers.
.Pp
All data sent via raw sockets MUST be in network byte order and all
data received via raw sockets will be in network byte order.
This differs from the IPv4 raw sockets, which did not specify a byte
ordering and typically used the host's byte order.
.Pp
Another difference from IPv4 raw sockets is that complete packets
(that is, IPv6 packets with extension headers) cannot be read or
written using the IPv6 raw sockets API.
Instead, ancillary data
objects are used to transfer the extension headers, as described above.
Should an application need access to the
complete IPv6 packet, some other technique, such as the datalink
interfaces, such as
.Xr bpf 4 ,
must be used.
.Pp
All fields in the IPv6 header that an application might want to
change (i.e., everything other than the version number) can be
modified using ancillary data and/or socket options by the
application for output.
All fields in a received IPv6 header (other
than the version number and Next Header fields) and all extension
headers are also made available to the application as ancillary data
on input.
Hence there is no need for a socket option similar to the
IPv4
.Dv IP_HDRINCL
socket option.
.Pp
When writing to a raw socket the kernel will automatically fragment
the packet if its size exceeds the path MTU, inserting the required
fragmentation headers.
On input the kernel reassembles received fragments, so the reader
of a raw socket never sees any fragment headers.
.Pp
Most IPv4 implementations give special treatment to a raw socket
created with a third argument to
.Xr socket 2
of
.Dv IPPROTO_RAW ,
whose value is normally 255.
We note that this value has no special meaning to
an IPv6 raw socket (and the IANA currently reserves the value of 255
when used as a next-header field).
.\" Note: This feature was added to
.\" IPv4 in 1988 by Van Jacobson to support traceroute, allowing a
.\" complete IP header to be passed by the application, before the
.\" .Dv IP_HDRINCL
.\" socket option was added.
.Pp
For ICMPv6 raw sockets,
the kernel will calculate and insert the ICMPv6 checksum
since this checksum is mandatory.
.Pp
For other raw IPv6 sockets (that is, for raw IPv6 sockets created
with a third argument other than IPPROTO_ICMPV6), the application
must set the new IPV6_CHECKSUM socket option to have the kernel (1)
compute and store a pseudo-header checksum for output,
and (2) verify the received
pseudo-header checksum on input,
discarding the packet if the checksum is in error.
This option prevents applications from having to perform source
address selection on the packets they send.
The checksum will
incorporate the IPv6 pseudo-header, defined in Section 8.1 of RFC 2460.
This new socket option also specifies an integer offset into
the user data of where the checksum is located.
.Bd -literal -offset indent
int offset = 2;
setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset, sizeof(offset));
.Ed
.Pp
By default, this socket option is disabled.
Setting the offset to -1 also disables the option.
By disabled we mean (1) the kernel will
not calculate and store a checksum for outgoing packets, and (2) the
kernel will not verify a checksum for received packets.
.Pp
Note: Since the checksum is always calculated by the kernel for an
ICMPv6 socket, applications are not able to generate ICMPv6 packets
with incorrect checksums (presumably for testing purposes) using this
API.
.\"
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EISCONN
when trying to establish a connection on a socket which already
has one, or when trying to send a datagram with the destination
address specified and the socket is already connected.
.It Bq Er ENOTCONN
when trying to send a datagram, but no destination address is
specified, and the socket hasn't been connected.
.It Bq Er ENOBUFS
when the system runs out of memory for an internal data structure.
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a socket with a network address
for which no network interface exists.
.It Bq Er EACCES
when an attempt is made to create a raw IPv6 socket by a non-privileged process.
.El
.Pp
The following errors specific to
.Tn IPv6
may occur:
.Bl -tag -width EADDRNOTAVAILxx
.It Bq Er EINVAL
An unknown socket option name was given.
.It Bq Er EINVAL
The ancillary data items were improperly formed, or option name was unknown.
.El
.\"
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr setsockopt 2 ,
.Xr inet6_option_space 3 ,
.Xr inet6_rthdr_space 3 ,
.Xr icmp6 4 ,
.Xr inet6 4
.Rs
.%A W. Stevens
.%A M. Thomas
.%R RFC 2292
.%D February 1998
.%T "Advanced Sockets API for IPv6"
.Re
.Rs
.%A S. Deering
.%A R. Hinden
.%R RFC 2460
.%D December 1998
.%T "Internet Protocol, Version 6 (IPv6) Specification"
.Re
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A W. Stevens
.%R RFC 2553
.%D March 1999
.%T "Basic Socket Interface Extensions for IPv6"
.Re
.\"
.Sh STANDARDS
Most of the socket options are defined in
RFC 2292 and/or RFC 2553.
.Dv IPV6_PORTRANGE
and conflict resolution rule
is not defined in the RFCs and should be considered implementation dependent.
.\"
.Sh HISTORY
The implementation is based on KAME stack
.Po
which is a descendant of WIDE hydrangea IPv6 stack kit
.Pc .
.Pp
Part of the document was shamelessly copied from RFC 2553 and RFC 2292.
.\"
.Sh BUGS
The
.Dv IPV6_NEXTHOP
object/option is not fully implemented as of writing this.