ip6(4)

NAME

ip6 - Internet Protocol version 6 (IPv6) network layer

SYNOPSIS

#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET6, SOCK_RAW, proto);

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 highlevel protocols (such as the tcp(4) and udp(4) protocols) as
well as directly by ``raw sockets'', which process IPv6 messages at
a lower-level and may be useful for developing new protocols and special
purpose applications.
Header: All IPv6 packets begin with an IPv6 header. When data re; ceived by the
kernel are passed to the application, this header is not in; cluded 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 spec; ify them as
part of the buffer passed to the kernel, link-level access; (bpf(4), for
example) must instead be utilized.; The header has the following definition:

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; All fields are in network-byte order. Any options specified; (see Options
below) must also be specified in network-byte order.; ip6_flow specifies the flow ID. ip6_plen specifies the pay; load length.
ip6_nxt specifies the type of the next header. ip6_hlim; specifies the
hop limit.; The top 4 bits of ip6_vfc specify the class and the bottom 4; bits specify
the version.; ip6_src and ip6_dst specify the source and destination ad; dresses.; The IPv6 header may be followed by any number of extension; headers that
start with the following generic definition:

struct ip6_ext {
u_int8_t ip6e_nxt;
u_int8_t ip6e_len;

} __packed;
Options: IPv6 allows header options on packets to manipulate the be; havior of the
protocol. These options and other control requests are ac; cessed with the
getsockopt(2) and setsockopt(2) system calls at level IPPRO; TO_IPV6 and by
using ancillary data in recvmsg(2) and sendmsg(2). They can; be used to
access most of the fields in the IPv6 header and extension; headers.; The following socket options are supported:; IPV6_UNICAST_HOPS 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.
IPV6_MULTICAST_IF u_int *: Get or set the interface from which multicast pack; ets will be
sent. For hosts with multiple interfaces, each mul; ticast transmission is sent from the primary network interface.; The interface is specified as its index as provided by; if_nametoindex(3).
A value of zero specifies the default interface.
IPV6_MULTICAST_HOPS 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.; 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 deliv; ered 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
mrouted(8)) is attached to the local network.
IPV6_MULTICAST_LOOP 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.; 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 applica; tions 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 pro; gram).; A multicast datagram sent with an initial hop limit; greater than
1 may be delivered to the sending host on a differ; ent 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.
IPV6_JOIN_GROUP 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.; struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr;
unsigned int ipv6mr_interface;; };; 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.; 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 su; peruser privileges.
IPV6_LEAVE_GROUP struct ipv6_mreq *: Drop membership from the associated multicast group.; Memberships
are automatically dropped when the socket is closed; or when the
process exits.
IPV6_PORTRANGE 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:; IPV6_PORTRANGE_DEFAULT Use the regular range of; non-reserved
ports (varies, see

sysctl(8)).; IPV6_PORTRANGE_HIGH Use a high range (varies,; see sysctl(8)).
IPV6_PORTRANGE_LOW Use a low, reserved range; (600-1023).
IPV6_PKTINFO int *: Get or set whether additional information about sub; sequent packets will be provided as ancillary data along with; the payload in
subsequent recvmsg(2) calls. The information is; stored in the
following structure in the ancillary data returned:; struct in6_pktinfo {
struct in6_addr ipi6_addr; /* src/dst

IPv6 address */
unsigned int ipi6_ifindex; /* send/recv

if index */; };
IPV6_HOPLIMIT 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 recvmsg(2) calls. The value is stored; as an int in
the ancillary data returned.
IPV6_HOPOPTS int *: Get or set whether the hop-by-hop options from sub; sequent packets
will be provided as ancillary data along with the; payload in subsequent recvmsg(2) calls. The option is stored in; the following
structure in the ancillary data returned:; 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;; The inet6_option_space() routine and family of rou; tines may be
used to manipulate this data.; This option requires superuser privileges.
IPV6_DSTOPTS int *: Get or set whether the destination options from sub; sequent packets will be provided as ancillary data along with; the payload in
subsequent recvmsg(2) calls. The option is stored; in the following structure in the ancillary data returned:; 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;; The inet6_option_space() routine and family of rou; tines may be
used to manipulate this data.; This option requires superuser privileges.
IPV6_RTHDR int *: Get or set whether the routing header from subse; quent packets
will be provided as ancillary data along with the; payload in subsequent recvmsg(2) calls. The header is stored in; the following
structure in the ancillary data returned:; 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;; The inet6_option_space() routine and family of rou; tines may be
used to manipulate this data.; This option requires superuser privileges.
IPV6_PKTOPTIONS 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 mbuf(9)).; Options are
specified as a series of cmsghdr structures followed; by corresponding values. cmsg_level is set to IPPROTO_IPV6,; cmsg_type to
one of the other values in this list, and trailing; data to the
option value. When setting options, if the length; optlen to
setsockopt(2) is zero, all header options will be; reset to their
default values. Otherwise, the length should speci; fy the size
the series of control messages consumes.; Instead of using sendmsg(2) to specify option val; ues, the ancillary data used in these calls that correspond to the; desired
header options may be directly specified as the con; trol message
in the series of control messages provided as the; argument to
setsockopt(2).
IPV6_CHECKSUM 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 spec; ifies 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 relo; cated or turned
off.
IPV6_V6ONLY int *: Get or set whether only IPv6 connections can be made; to this
socket. For wildcard sockets, this can restrict; connections to
IPv6 only.
IPV6_FAITH int *: Get or set the status of whether faith(4) connec; tions can be made
to this socket.
IPV6_USE_MIN_MTU int *: Get or set whether the minimal IPv6 maximum trans; mission unit
(MTU) size will be used to avoid fragmentation from; occurring for
subsequent outgoing datagrams.
IPV6_AUTH_LEVEL int *: Get or set the ipsec(4) authentication level.
IPV6_ESP_TRANS_LEVEL int *: Get or set the ESP transport level.
IPV6_ESP_NETWORK_LEVEL int *: Get or set the ESP encapsulation level.
IPV6_IPCOMP_LEVEL int *: Get or set the ipcomp(4) level.
The IPV6_PKTINFO, IPV6_HOPLIMIT, IPV6_HOPOPTS, IPV6_DSTOPTS,
and IPV6_RTHDR options will return ancillary data along with
payload contents
in subsequent recvmsg(2) calls with cmsg_level set to IPPRO
TO_IPV6 and
cmsg_type set to respective option name value (e.g.,
IPV6_HOPTLIMIT). These options may also be used directly as ancillary
cmsg_type values in
sendmsg(2) to set options on the packet being transmitted by
the call.
The cmsg_level value must be IPPROTO_IPV6. For these op
tions, the ancillary data object value format is the same as the value re
turned as explained for each when received with recvmsg(2).
Note that using 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.
In some cases, there are multiple APIs defined for manipu
lating an IPv6 header field. A good example is the outgoing interface for
multicast datagrams, which can be set by the IPV6_MULTICAST_IF socket
option, through the IPV6_PKTINFO option, and through the
sin6_scope_id field of
the socket address passed to the sendto(2) system call.
Resolving these conflicts is implementation dependent. This
implementation determines the value in the following way: options
specified by using ancillary data (i.e., sendmsg(2)) are considered
first, options specified by using IPV6_PKTOPTIONS to set ``sticky'' options
are considered second, options specified by using the individual, ba
sic, and direct socket options (e.g., IPV6_UNICAST_HOPS) are considered
third, and options specified in the socket address supplied to send
to(2) are the last choice.
Multicasting: IPv6 multicasting is supported only on AF_INET6 sockets of; type
SOCK_DGRAM and SOCK_RAW, and only on networks where the in; terface driver
supports multicasting. Socket options (see above) that ma; nipulate membership of multicast groups and other multicast options in; clude
IPV6_MULTICAST_IF, IPV6_MULTICAST_HOPS, IPV6_MULTICAST_LOOP,
IPV6_LEAVE_GROUP, and IPV6_JOIN_GROUP.
Raw Sockets: Raw IPv6 sockets are connectionless and are normally used; with the
sendto(2) and recvfrom(2) calls, although the connect(2); call may be used
to fix the destination address for future outgoing packets; so that
send(2) may instead be used and the bind(2) call may be used; to fix the
source address for future outgoing packets instead of having; the kernel
choose a source address.; By using connect(2) or bind(2), raw socket input is con; strained to only
packets with their source address matching the socket desti; nation address
if connect(2) was used and to packets with their destination; address
matching the socket source address if bind(2) was used.; If the proto argument to socket(2) is zero, the default pro; tocol
(IPPROTO_RAW) is used for outgoing packets. For incoming; packets, protocols recognized by kernel are not passed to the application; socket (e.g.,
tcp(4) and udp(4)) except for some ICMPv6 messages. The; ICMPv6 messages
not passed to raw sockets include echo, timestamp, and ad; dress mask
requests. If proto is non-zero, only packets with this pro; tocol will be
passed to the socket.; 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 bpf(4)) must be used instead.; 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 applica; tion without
the IPv6 header or any extension headers.; 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.

EXAMPLES

The following determines the hop limit on the next packet
received:
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 mes
sages */ 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: %d0,
*(int *)CMSG_DATA(cm));

break;

}

}

}

DIAGNOSTICS

A socket operation may fail with one of the following errors
returned:
[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.
[ENOTCONN] when trying to send a datagram, but no des
tination: address is specified, and the socket has; not been connected.
[ENOBUFS] when the system runs out of memory for an
internal data: structure.
[EADDRNOTAVAIL] when an attempt is made to create a socket
with a net: work address for which no network interface; exists.
[EACCES] when an attempt is made to create a raw
IPv6 socket by a: non-privileged process.
The following errors specific to IPv6 may occur when setting
or getting header options:
[EINVAL] An unknown socket option name was given.
[EINVAL] An ancillary data object was improperly
formed.

STANDARDS

Most of the socket options are defined in RFC 2292 or RFC
2553. The IPV6_V6ONLY socket option is defined in RFC 3542. The
IPV6_PORTRANGE socket option and the conflict resolution rule are not de
fined in the RFCs and should be considered implementation dependent.
BSD December 29, 2004

docs.sk

comprehensive documentation repository

See also

ip6(4)

NAME

SYNOPSIS

DESCRIPTION

EXAMPLES

DIAGNOSTICS

SEE ALSO

STANDARDS