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 high-level 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.
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 (
bpf(4), for example) must
instead be utilized.
The header has the following definition:
struct ip6_hdr {
union {
struct ip6_hdrctl {
uint32_t ip6_un1_flow; /* 20 bits of flow ID */
uint16_t ip6_un1_plen; /* payload length */
uint8_t ip6_un1_nxt; /* next header */
uint8_t ip6_un1_hlim; /* hop limit */
} ip6_un1;
uint8_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 payload 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
addresses.
The IPv6 header may be followed by any number of extension headers that start
with the following generic definition:
struct ip6_ext {
uint8_t ip6e_nxt;
uint8_t ip6e_len;
} __packed;
Options¶
IPv6 allows header options on packets to manipulate the behavior of the
protocol. These options and other control requests are accessed with the
getsockopt(2) and
setsockopt(2) system calls at level
IPPROTO_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.
IPV6_MULTICAST_IF
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 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 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 mrouted(8)
(ports/net/mrouted)) 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 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).
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.
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 superuser 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
ip(4)).
IPV6_PORTRANGE_HIGH
- Use a high range (varies, see
ip(4)).
IPV6_PORTRANGE_LOW
- Use a low, reserved range (600-1023, see
ip(4)).
IPV6_PKTINFO
int *
- Get or set whether additional information about subsequent 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 subsequent 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 {
uint8_t ip6h_nxt; /* next header */
uint8_t ip6h_len; /* length in units of 8 octets */
/* followed by options */
} __packed;
The inet6_option_space
() routine and
family of routines may be used to manipulate this data.
This option requires superuser privileges.
IPV6_DSTOPTS
int *
- Get or set whether the destination options from subsequent 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 {
uint8_t ip6d_nxt; /* next header */
uint8_t ip6d_len; /* length in units of 8 octets */
/* followed by options */
} __packed;
The inet6_option_space
() routine and
family of routines may be used to manipulate this data.
This option requires superuser privileges.
IPV6_TCLASS
int *
- Get or set the value of the traffic class field used for outgoing
datagrams on this socket. The value must be between -1 and 255. A value of
-1 resets to the default value.
IPV6_RECVTCLASS
int *
- Get or set the status of whether the traffic class header field will be
provided as ancillary data along with the payload in subsequent
recvmsg(2) calls. The header field is stored
as a single value of type int.
IPV6_RTHDR
int *
- Get or set whether the routing header from subsequent 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 {
uint8_t ip6r_nxt; /* next header */
uint8_t ip6r_len; /* length in units of 8 octets */
uint8_t ip6r_type; /* routing type */
uint8_t ip6r_segleft; /* segments left */
/* followed by routing-type-specific data */
} __packed;
The inet6_option_space
() routine and
family of routines 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
specify the size the series of control messages consumes.
Instead of using 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
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 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.
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)
connections can be made to this socket.
IPV6_USE_MIN_MTU
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.
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
IPPROTO_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
options, the ancillary data object value format is the same as the value
returned 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 manipulating 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, basic, and direct socket options (e.g.,
IPV6_UNICAST_HOPS
) are considered third,
and options specified in the socket address supplied to
sendto(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
interface driver supports multicasting. Socket options (see above) that
manipulate membership of multicast groups and other multicast options include
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 constrained to only
packets with their source address matching the socket destination 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 protocol
(
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 address mask requests. If
proto is non-zero, only packets with this
protocol 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 application 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 optval;
bool found;
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 = false;
do {
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 = true;
(void)printf("hop limit: %d\n",
*(int *)CMSG_DATA(cm));
break;
}
}
} while (!found);
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 destination 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 network 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.
SEE ALSO¶
getsockopt(2),
recv(2),
send(2),
setsockopt(2),
socket(2),
if_nametoindex(3),
bpf(4),
icmp6(4),
ip(4),
inet6(4),
netintro(4),
tcp(4),
udp(4)
W. Stevens and
M. Thomas, Advanced Sockets API for
IPv6, RFC 2292, February
1998.
S. Deering and
R. Hinden, Internet Protocol,
Version 6 (IPv6) Specification, RFC 2460,
December 1998.
R. Gilligan,
S. Thomson, J. Bound, and
W. Stevens, Basic Socket Interface
Extensions for IPv6, RFC 2553,
March 1999.
W. Stevens,
B. Fenner, and A. Rudoff,
UNIX Network Programming, third edition.
STANDARDS¶
Most of the socket options are defined in RFC 2292 or RFC 2553. The
IPV6_V6ONLY
socket option is defined in RFC
3493 Section 5.3. The
IPV6_PORTRANGE
socket
option and the conflict resolution rule are not defined in the RFCs and should
be considered implementation dependent.