man/man4/ip6.4

.\"	$KAME: ip6.4,v 1.23 2005/01/11 05:56:25 itojun Exp $
.\"	$OpenBSD: ip6.4,v 1.21 2005/01/06 03:50:46 itojun Exp $
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.Dd December 31, 2019
.Dt IP6 4
.Os
.Sh NAME
.Nm ip6
.Nd Internet Protocol version 6 (IPv6) network layer
.Sh SYNOPSIS
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET6 SOCK_RAW proto
.Sh DESCRIPTION
The IPv6 network layer is used by the IPv6 protocol family for
transporting data.
IPv6 packets contain an IPv6 header that is not provided as part of the
payload contents when passed to an application.
IPv6 header options affect the behavior of this protocol and may be used
by high-level protocols (such as the
.Xr tcp 4
and
.Xr udp 4
protocols) as well as directly by
.Dq raw sockets ,
which process IPv6 messages at a lower-level and may be useful for
developing new protocols and special-purpose applications.
.Ss Header
All IPv6 packets begin with an IPv6 header.
When data received by the kernel are passed to the application, this
header is not included in buffer, even when raw sockets are being used.
Likewise, when data are sent to the kernel for transmit from the
application, the buffer is not examined for an IPv6 header:
the kernel always constructs the header.
To directly access IPv6 headers from received packets and specify them
as part of the buffer passed to the kernel, link-level access
.Po
.Xr bpf 4 ,
for example
.Pc
must instead be utilized.
.Pp
The header has the following definition:
.Bd -literal -offset indent
struct ip6_hdr {
     union {
          struct ip6_hdrctl {
               u_int32_t ip6_un1_flow;	/* 20 bits of flow ID */
               u_int16_t ip6_un1_plen;	/* payload length */
               u_int8_t	 ip6_un1_nxt;	/* next header */
               u_int8_t	 ip6_un1_hlim;	/* hop limit */
          } ip6_un1;
          u_int8_t ip6_un2_vfc;   /* version and class */
     } ip6_ctlun;
     struct in6_addr ip6_src;	/* source address */
     struct in6_addr ip6_dst;	/* destination address */
} __packed;

#define ip6_vfc		ip6_ctlun.ip6_un2_vfc
#define ip6_flow	ip6_ctlun.ip6_un1.ip6_un1_flow
#define ip6_plen	ip6_ctlun.ip6_un1.ip6_un1_plen
#define ip6_nxt		ip6_ctlun.ip6_un1.ip6_un1_nxt
#define ip6_hlim	ip6_ctlun.ip6_un1.ip6_un1_hlim
#define ip6_hops	ip6_ctlun.ip6_un1.ip6_un1_hlim
.Ed
.Pp
All fields are in network-byte order.
Any options specified (see
.Sx Options
below) must also be specified in network-byte order.
.Pp
.Va ip6_flow
specifies the flow ID.
.Va ip6_plen
specifies the payload length.
.Va ip6_nxt
specifies the type of the next header.
.Va ip6_hlim
specifies the hop limit.
.Pp
The top 4 bits of
.Va ip6_vfc
specify the class and the bottom 4 bits specify the version.
.Pp
.Va ip6_src
and
.Va ip6_dst
specify the source and destination addresses.
.Pp
The IPv6 header may be followed by any number of extension headers that start
with the following generic definition:
.Bd -literal -offset indent
struct ip6_ext {
     u_int8_t ip6e_nxt;
     u_int8_t ip6e_len;
} __packed;
.Ed
.Ss Options
IPv6 allows header options on packets to manipulate the behavior of the
protocol.
These options and other control requests are accessed with the
.Xr getsockopt 2
and
.Xr setsockopt 2
system calls at level
.Dv IPPROTO_IPV6
and by using ancillary data in
.Xr recvmsg 2
and
.Xr sendmsg 2 .
They can be used to access most of the fields in the IPv6 header and
extension headers.
.Pp
The following socket options are supported:
.Bl -tag -width Ds
.\" .It Dv IPV6_OPTIONS
.It Dv IPV6_UNICAST_HOPS Fa "int *"
Get or set the default hop limit header field for outgoing unicast
datagrams sent on this socket.
A value of \-1 resets to the default value.
.\" .It Dv IPV6_RECVOPTS Fa "int *"
.\" Get or set the status of whether all header options will be
.\" delivered along with the datagram when it is received.
.\" .It Dv IPV6_RECVRETOPTS Fa "int *"
.\" Get or set the status of whether header options will be delivered
.\" for reply.
.\" .It Dv IPV6_RECVDSTADDR Fa "int *"
.\" Get or set the status of whether datagrams are received with
.\" destination addresses.
.\" .It Dv IPV6_RETOPTS
.\" Get or set IPv6 options.
.It Dv IPV6_MULTICAST_IF Fa "u_int *"
Get or set the interface from which multicast packets will be sent.
For hosts with multiple interfaces, each multicast transmission is sent
from the primary network interface.
The interface is specified as its index as provided by
.Xr if_nametoindex 3 .
A value of zero specifies the default interface.
.It Dv IPV6_MULTICAST_HOPS Fa "int *"
Get or set the default hop limit header field for outgoing multicast
datagrams sent on this socket.
This option controls the scope of multicast datagram transmissions.
.Pp
Datagrams with a hop limit of 1 are not forwarded beyond the local
network.
Multicast datagrams with a hop limit of zero 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 (see below) has not been
disabled on the sending socket.
Multicast datagrams with a hop limit greater than 1 may be forwarded to
the other networks if a multicast router (such as
.Xr mrouted 8 Pq Pa net/mrouted )
is attached to the local network.
.It Dv IPV6_MULTICAST_LOOP Fa "u_int *"
Get or set the status of whether multicast datagrams will be looped back
for local delivery when a multicast datagram is sent to a group to which
the sending host belongs.
.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 hop limit 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 multicast loopback control option has no effect on such delivery.
.It Dv IPV6_JOIN_GROUP Fa "struct ipv6_mreq *"
Join a multicast group.
A host must become a member of a multicast group before it can receive
datagrams sent to the group.
.Bd -literal
struct ipv6_mreq {
	struct in6_addr	ipv6mr_multiaddr;
	unsigned int	ipv6mr_interface;
};
.Ed
.Pp
.Va ipv6mr_interface
may be set to zeroes to choose the default multicast interface or to the
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
If the multicast address is unspecified (i.e., all zeroes), messages
from all multicast addresses will be accepted by this group.
Note that setting to this value requires superuser privileges.
.It Dv IPV6_LEAVE_GROUP Fa "struct ipv6_mreq *"
Drop membership from the associated multicast group.
Memberships are automatically dropped when the socket is closed or when
the process exits.
.It Dv IPV6_PORTRANGE Fa "int *"
Get or set the allocation policy of ephemeral ports for when the kernel
automatically binds a local address to this socket.
The following values are available:
.Pp
.Bl -tag -width IPV6_PORTRANGE_DEFAULT -compact
.It Dv IPV6_PORTRANGE_DEFAULT
Use the regular range of non-reserved ports (varies, see
.Xr sysctl 8 ) .
.It Dv IPV6_PORTRANGE_HIGH
Use a high range (varies, see
.Xr sysctl 8 ) .
.It Dv IPV6_PORTRANGE_LOW
Use a low, reserved range (600\-1023).
.El
.It Dv IPV6_PKTINFO Fa "int *"
Get or set whether additional information about subsequent packets will
be provided as ancillary data along with the payload in subsequent
.Xr recvmsg 2
calls.
The information is stored in the following structure in the ancillary
data returned:
.Bd -literal
struct in6_pktinfo {
	struct in6_addr ipi6_addr;    /* src/dst IPv6 address */
	unsigned int    ipi6_ifindex; /* send/recv if index */
};
.Ed
.It Dv IPV6_HOPLIMIT Fa "int *"
Get or set whether the hop limit header field from subsequent packets
will be provided as ancillary data along with the payload in subsequent
.Xr recvmsg 2
calls.
The value is stored as an
.Vt int
in the ancillary data returned.
.\" .It Dv IPV6_NEXTHOP Fa "int *"
.\" Get or set whether the address of the next hop for subsequent
.\" packets will be provided as ancillary data along with the payload in
.\" subsequent
.\" .Xr recvmsg 2
.\" calls.
.\" The option is stored as a
.\" .Vt sockaddr
.\" structure in the ancillary data returned.
.\" .Pp
.\" This option requires superuser privileges.
.It Dv IPV6_HOPOPTS Fa "int *"
Get or set whether the hop-by-hop options from subsequent packets will be
provided as ancillary data along with the payload in subsequent
.Xr recvmsg 2
calls.
The option is stored in the following structure in the ancillary data
returned:
.Bd -literal
struct ip6_hbh {
	u_int8_t ip6h_nxt;	/* next header */
	u_int8_t ip6h_len;	/* length in units of 8 octets */
/* followed by options */
} __packed;
.Ed
.Pp
The
.Fn inet6_option_space
routine and family of routines may be used to manipulate this data.
.Pp
This option requires superuser privileges.
.It Dv IPV6_DSTOPTS Fa "int *"
Get or set whether the destination options from subsequent packets will
be provided as ancillary data along with the payload in subsequent
.Xr recvmsg 2
calls.
The option is stored in the following structure in the ancillary data
returned:
.Bd -literal
struct ip6_dest {
	u_int8_t ip6d_nxt;	/* next header */
	u_int8_t ip6d_len;	/* length in units of 8 octets */
/* followed by options */
} __packed;
.Ed
.Pp
The
.Fn inet6_option_space
routine and family of routines may be used to manipulate this data.
.Pp
This option requires superuser privileges.
.It Dv IPV6_RTHDR Fa "int *"
Get or set whether the routing header from subsequent packets will be
provided as ancillary data along with the payload in subsequent
.Xr recvmsg 2
calls.
The header is stored in the following structure in the ancillary data
returned:
.Bd -literal
struct ip6_rthdr {
	u_int8_t ip6r_nxt;	/* next header */
	u_int8_t ip6r_len;	/* length in units of 8 octets */
	u_int8_t ip6r_type;	/* routing type */
	u_int8_t ip6r_segleft;	/* segments left */
/* followed by routing-type-specific data */
} __packed;
.Ed
.Pp
The
.Fn inet6_option_space
routine and family of routines may be used to manipulate this data.
.Pp
This option requires superuser privileges.
.It Dv IPV6_PKTOPTIONS Fa "struct cmsghdr *"
Get or set all header options and extension headers at one time on the
last packet sent or received on the socket.
All options must fit within the size of an mbuf (see
.Xr mbuf 9 ) .
Options are specified as a series of
.Vt cmsghdr
structures followed by corresponding values.
.Va cmsg_level
is set to
.Dv IPPROTO_IPV6 ,
.Va cmsg_type
to one of the other values in this list, and trailing data to the option
value.
When setting options, if the length
.Va optlen
to
.Xr setsockopt 2
is zero, all header options will be reset to their default values.
Otherwise, the length should specify the size the series of control
messages consumes.
.Pp
Instead of using
.Xr sendmsg 2
to specify option values, the ancillary data used in these calls that
correspond to the desired header options may be directly specified as
the control message in the series of control messages provided as the
argument to
.Xr setsockopt 2 .
.It Dv IPV6_CHECKSUM Fa "int *"
Get or set the byte offset into a packet where the 16-bit checksum is
located.
When set, this byte offset is where incoming packets will be expected
to have checksums of their data stored and where outgoing packets will
have checksums of their data computed and stored by the kernel.
A value of \-1 specifies that no checksums will be checked on incoming
packets and that no checksums will be computed or stored on outgoing
packets.
The offset of the checksum for ICMPv6 sockets cannot be relocated or
turned off.
.It Dv IPV6_V6ONLY Fa "int *"
Get or set whether only IPv6 connections can be made to this socket.
For wildcard sockets, this can restrict connections to IPv6 only.
With
.Dx ,
the socket option cannot be set to 0 (which will result in returning
.Er EOPNOTSUPP ) .
.It Dv IPV6_USE_MIN_MTU Fa "int *"
Get or set whether the minimal IPv6 maximum transmission unit (MTU) size
will be used to avoid fragmentation from occurring for subsequent
outgoing datagrams.
.El
.Pp
The
.Dv IPV6_PKTINFO ,
.\" .Dv IPV6_NEXTHOP ,
.Dv IPV6_HOPLIMIT ,
.Dv IPV6_HOPOPTS ,
.Dv IPV6_DSTOPTS ,
and
.Dv IPV6_RTHDR
options will return ancillary data along with payload contents in subsequent
.Xr recvmsg 2
calls with
.Va cmsg_level
set to
.Dv IPPROTO_IPV6
and
.Va cmsg_type
set to respective option name value (e.g.,
.Dv IPV6_HOPLIMIT ) .
These options may also be used directly as ancillary
.Va cmsg_type
values in
.Xr sendmsg 2
to set options on the packet being transmitted by the call.
The
.Va cmsg_level
value must be
.Dv IPPROTO_IPV6 .
For these options, the ancillary data object value format is the same
as the value returned as explained for each when received with
.Xr recvmsg 2 .
.Pp
Note that using
.Xr sendmsg 2
to specify options on particular packets works only on UDP and raw sockets.
To manipulate header options for packets on TCP sockets, only the socket
options may be used.
.Pp
In some cases, there are multiple APIs defined for manipulating an IPv6
header field.
A good example is the outgoing interface for multicast datagrams, which
can be set by the
.Dv IPV6_MULTICAST_IF
socket option, through the
.Dv IPV6_PKTINFO
option, and through the
.Va sin6_scope_id
field of the socket address passed to the
.Xr sendto 2
system call.
.Pp
Resolving these conflicts is implementation dependent.
This implementation determines the value in the following way:
options specified by using ancillary data (i.e.,
.Xr sendmsg 2 )
are considered first,
options specified by using
.Dv IPV6_PKTOPTIONS
to set
.Dq sticky
options are considered second,
options specified by using the individual, basic, and direct socket
options (e.g.,
.Dv IPV6_UNICAST_HOPS )
are considered third,
and options specified in the socket address supplied to
.Xr sendto 2
are the last choice.
.Ss Multicasting
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.
Socket options (see above) that manipulate membership of
multicast groups and other multicast options include
.Dv IPV6_MULTICAST_IF ,
.Dv IPV6_MULTICAST_HOPS ,
.Dv IPV6_MULTICAST_LOOP ,
.Dv IPV6_LEAVE_GROUP ,
and
.Dv IPV6_JOIN_GROUP .
.Ss Raw Sockets
Raw IPv6 sockets are connectionless and are normally used with the
.Xr sendto 2
and
.Xr recvfrom 2
calls, although the
.Xr connect 2
call may be used to fix the destination address for future outgoing
packets so that
.Xr send 2
may instead be used and the
.Xr bind 2
call may be used to fix the source address for future outgoing
packets instead of having the kernel choose a source address.
.Pp
By using
.Xr connect 2
or
.Xr bind 2 ,
raw socket input is constrained to only packets with their
source address matching the socket destination address if
.Xr connect 2
was used and to packets with their destination address
matching the socket source address if
.Xr bind 2
was used.
.Pp
If the
.Ar proto
argument to
.Xr socket 2
is zero, the default protocol
.Pq Dv IPPROTO_RAW
is used for outgoing packets.
For incoming packets, protocols recognized by kernel are
.Sy not
passed to the application socket (e.g.,
.Xr tcp 4
and
.Xr udp 4 )
except for some ICMPv6 messages.
The ICMPv6 messages not passed to raw sockets include echo, timestamp,
and address mask requests.
If
.Ar proto
is non-zero, only packets with this protocol will be passed to the
socket.
.Pp
IPv6 fragments are also not passed to application sockets until
they have been reassembled.
If reception of all packets is desired, link-level access (such as
.Xr bpf 4 )
must be used instead.
.Pp
Outgoing packets automatically have an IPv6 header prepended to them
(based on the destination address and the protocol number the socket
was created with).
Incoming packets are received by an application without the IPv6 header
or any extension headers.
.Pp
Outgoing packets will be fragmented automatically by the kernel if they
are too large.
Incoming packets will be reassembled before being sent to the raw socket,
so packet fragments or fragment headers will never be seen on a raw socket.
.Sh EXAMPLES
The following determines the hop limit on the next packet received:
.Bd -literal
struct iovec iov[2];
u_char buf[BUFSIZ];
struct cmsghdr *cm;
struct msghdr m;
int found, optval;
u_char data[2048];

/* Create socket. */

(void)memset(&m, 0, sizeof(m));
(void)memset(&iov, 0, sizeof(iov));

iov[0].iov_base = data;		/* buffer for packet payload */
iov[0].iov_len = sizeof(data);	/* expected packet length */

m.msg_name = &from;		/* sockaddr_in6 of peer */
m.msg_namelen = sizeof(from);
m.msg_iov = iov;
m.msg_iovlen = 1;
m.msg_control = (caddr_t)buf;	/* buffer for control messages */
m.msg_controllen = sizeof(buf);

/*
 * Enable the hop limit value from received packets to be
 * returned along with the payload.
 */
optval = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_HOPLIMIT, &optval,
    sizeof(optval)) == -1)
	err(1, "setsockopt");

found = 0;
while (!found) {
	if (recvmsg(s, &m, 0) == -1)
		err(1, "recvmsg");
	for (cm = CMSG_FIRSTHDR(&m); cm != NULL;
	     cm = CMSG_NXTHDR(&m, cm)) {
		if (cm->cmsg_level == IPPROTO_IPV6 &&
		    cm->cmsg_type == IPV6_HOPLIMIT &&
		    cm->cmsg_len == CMSG_LEN(sizeof(int))) {
			found = 1;
			(void)printf("hop limit: %d\en",
			    *(int *)CMSG_DATA(cm));
			break;
		}
	}
}
.Ed
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width EADDRNOTAVAILxx
.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 IPv6 may occur when setting or getting
header options:
.Bl -tag -width EADDRNOTAVAILxx
.It Bq Er EINVAL
An unknown socket option name was given.
.It Bq Er EINVAL
An ancillary data object was improperly formed.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr setsockopt 2 ,
.Xr socket 2 ,
.\" .Xr inet6_option_space 3 ,
.\" .Xr inet6_rthdr_space 3 ,
.Xr if_nametoindex 3 ,
.Xr bpf 4 ,
.Xr icmp6 4 ,
.Xr inet6 4 ,
.Xr netintro 4 ,
.Xr tcp 4 ,
.Xr udp 4
.Rs
.%A W. Stevens
.%A M. Thomas
.%T Advanced Sockets API for IPv6
.%R RFC 2292
.%D February 1998
.Re
.Rs
.%A S. Deering
.%A R. Hinden
.%T Internet Protocol, Version 6 (IPv6) Specification
.%R RFC 2460
.%D December 1998
.Re
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A W. Stevens
.%T Basic Socket Interface Extensions for IPv6
.%R RFC 2553
.%D March 1999
.Re
.Rs
.%A W. Stevens
.%A B. Fenner
.%A A. Rudoff
.%T UNIX Network Programming, third edition
.Re
.Sh STANDARDS
Most of the socket options are defined in RFC 2292 or RFC 2553.
The
.Dv IPV6_V6ONLY
socket option is defined in RFC 3542.
The
.Dv IPV6_PORTRANGE
socket option and the conflict resolution rule are not defined in the
RFCs and should be considered implementation dependent.