IP6(4) BSD Kernel Interfaces Manual IP6(4)NAME
ip6 — Internet Protocol version 6 (IPv6)
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET6, SOCK_RAW, proto);
DESCRIPTION
IPv6 is the network layer protocol used by the Internet protocol version
6 family (AF_INET6). Options may be set at the IPv6 level when using
higher-level protocols that are based on IPv6 (such as TCP and UDP). It
may also be accessed through a “raw socket” when developing new proto‐
cols, or special-purpose applications.
There are several IPv6-level setsockopt(2)/getsockopt(2) options. They
are separated into the basic IPv6 sockets API (defined in RFC2553), and
the advanced API (defined in RFC2292). The basic API looks very similar
to the API presented in ip(4). Advanced API uses ancillary data and can
handle more complex cases.
To specify some of socket options, certain privilege (i.e. root privi‐
lege) is required.
Basic IPv6 sockets API
IPV6_UNICAST_HOPS may be used to set the hoplimit field in the IPv6
header. As symbol name suggests, the option controls 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
EINVAL will be returned. For example:
int hlim = 60; /* max = 255 */
setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &hlim, sizeof(hlim));
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.
The IPV6_MULTICAST_HOPS option changes the hoplimit for outgoing multi‐
cast datagrams in order to control the scope of the multicasts:
unsigned int hlim; /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hlim, sizeof(hlim));
Datagrams with a hoplimit of 1 are not forwarded beyond the local net‐
work. 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 hoplimit
greater than 1 may be forwarded to other networks if a multicast router
is attached to the local network.
For hosts with multiple interfaces, each multicast transmission is sent
from the primary network interface. The IPV6_MULTICAST_IF option over‐
rides the default for subsequent transmissions from a given socket:
unsigned int outif;
outif = if_nametoindex("ne0");
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_IF, &outif, sizeof(outif));
where "outif" is an interface index of the desired interface, or 0 to
specify the default interface.
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
IPV6_MULTICAST_LOOP option gives the sender explicit control over whether
or not subsequent datagrams are looped back:
u_char loop; /* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loop, sizeof(loop));
This option improves performance for applications that may have no more
than one instance on a single host (such as a router daemon), by elimi‐
nating the overhead of receiving their own transmissions. It should gen‐
erally 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 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.
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
IPV6_JOIN_GROUP option:
struct ipv6_mreq mreq6;
setsockopt(s, IPPROTO_IPV6, IPV6_JOIN_GROUP, &mreq6, sizeof(mreq6));
where mreq6 is the following structure:
struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr;
u_int ipv6mr_interface;
};
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.
To drop a membership, use:
struct ipv6_mreq mreq6;
setsockopt(s, IPPROTO_IPV6, IPV6_LEAVE_GROUP, &mreq6, sizeof(mreq6));
where mreq6 contains the same values as used to add the membership. Mem‐
berships are dropped when the socket is closed or the process exits.
IPV6_PORTRANGE controls how ephemeral ports are allocated for SOCK_STREAM
and SOCK_DGRAM sockets. For example,
int range = IPV6_PORTRANGE_LOW; /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IPV6, IPV6_PORTRANGE, &range, sizeof(range));
IPV6_V6ONLY controls behavior of AF_INET6 wildcard listening socket. The
following example sets the option to 1:
int on = 1;
setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on));
If set to 1, AF_INET6 wildcard listening socket will accept IPv6 traffic
only. If set to 0, it will accept IPv4 traffic as well, as if it was
from IPv4 mapped address like ::ffff:10.1.1.1. Note that if you set it
this to 0, IPv4 access control gets much more complicated. For example,
even if you have no listening AF_INET listening socket on port X, you
will end up accepting IPv4 traffic by AF_INET6 listening socket on the
same port. The default value for this flag is copied at socket instanti‐
ation time, from net.inet6.ip6.v6only sysctl(3) variable. The option
affects TCP and UDP sockets only.
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.
There are setsockopt(2)/getsockopt(2) options to get optional information
on incoming packets. They are IPV6_PKTINFO, IPV6_HOPLIMIT, IPV6_HOPOPTS,
IPV6_DSTOPTS, and IPV6_RTHDR.
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));
When any of these options are enabled, the corresponding data is returned
as control information by recvmsg(2), as one or more ancillary data
objects.
If IPV6_PKTINFO is enabled, the destination IPv6 address and the arriving
interface index will be available via struct in6_pktinfo on ancillary
data stream. You can pick the structure by checking for an ancillary
data item with cmsg_level equals to IPPROTO_IPV6, and cmsg_type equals to
IPV6_PKTINFO.
If IPV6_HOPLIMIT is enabled, hoplimit value on the packet will be made
available to the userland program. Ancillary data stream will contain an
integer data item with cmsg_level equals to IPPROTO_IPV6, and cmsg_type
equals to IPV6_HOPLIMIT.
inet6_option_space(3) and friends will help you parse ancillary data
items for IPV6_HOPOPTS and IPV6_DSTOPTS. Similarly, inet6_rthdr_space(3)
and friends will help you parse ancillary data items for IPV6_RTHDR.
IPV6_HOPOPTS and IPV6_DSTOPTS may appear multiple times on an ancillary
data stream (note that the behavior is slightly different than the speci‐
fication). Other ancillary data item will appear no more than once.
For outgoing direction, you can pass ancillary data items with normal
payload data, using sendmsg(2). Ancillary data items will be parsed by
the kernel, and used to construct the IPv6 header and extension headers.
For the 5 cmsg_level values listed above, ancillary data format is the
same as inbound case. Additionally, you can specify IPV6_NEXTHOP data
object. The IPV6_NEXTHOP ancillary data object specifies the next hop
for the datagram as a socket address structure. In the cmsghdr structure
containing this ancillary data, the cmsg_level member will be
IPPROTO_IPV6, the cmsg_type member will be IPV6_NEXTHOP, and the first
byte of cmsg_data[] will be the first byte of the socket address struc‐
ture.
If the socket address structure contains an IPv6 address (e.g., the
sin6_family member is AF_INET6), then the node identified by that address
must be a neighbor of the sending host. If that address equals the des‐
tination IPv6 address of the datagram, then this is equivalent to the
existing SO_DONTROUTE socket option.
For applications that do not, or unable to use sendmsg(2) or recvmsg(2),
IPV6_PKTOPTIONS socket option is defined. Setting the socket option
specifies any of the optional output fields:
setsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, len);
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 sendmsg(2) as control information.
The options set by calling setsockopt(2) for IPV6_PKTOPTIONS are called
"sticky" options because once set they apply to all packets sent on that
socket. The application can call setsockopt(2) again to change all the
sticky options, or it can call setsockopt(2) with a length of 0 to remove
all the sticky options for the socket.
The corresponding receive option
getsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, &len);
returns a buffer with one or more ancillary data objects for all the
optional receive information that the application has previously speci‐
fied 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 recvmsg(2) as control information.
Advanced API and TCP sockets
When using getsockopt(2) with the IPV6_PKTOPTIONS option and a 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. The application is not allowed to specify ancillary data in a
call to sendmsg(2) on a TCP socket, and none of the ancillary data that
we described above is ever returned as control information by recvmsg(2)
on a TCP socket.
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 IPV6_MULTICAST_IF in basic API,
IPV6_PKTINFO in advanced API, and sin6_scope_id field of the socket
address passed to sendto(2).
When conflicting options are given to the kernel, the kernel will get the
value in the following preference: (1) options specified by using ancil‐
lary 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 specifcation and implementation dependent.
Raw IPv6 Sockets
Raw IPv6 sockets are connectionless, and are normally used with the
sendto(2) and recvfrom(2) calls, though the connect(2) call may also be
used to fix the destination for future packets (in which case the read(2)
or recv(2) and write(2) or send(2) system calls may be used).
If proto is 0, the default protocol IPPROTO_RAW is used for outgoing
packets, and only incoming packets destined for that protocol are
received. If proto is non-zero, that protocol number will be used on
outgoing packets and to filter incoming packets.
Outgoing packets automatically have an IPv6 header prepended to them
(based on the destination address and the protocol number the socket is
created with). Incoming packets are received without IPv6 header nor
extension headers.
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 typ‐
ically used the host's byte order.
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 applica‐
tion need access to the complete IPv6 packet, some other technique, such
as the datalink interfaces, such as bpf(4), must be used.
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 IP_HDRINCL socket option.
When writing to a raw socket the kernel will automatically fragment the
packet if its size exceeds the path MTU, inserting the required fragmen‐
tation headers. On input the kernel reassembles received fragments, so
the reader of a raw socket never sees any fragment headers.
Most IPv4 implementations give special treatment to a raw socket created
with a third argument to socket(2) of IPPROTO_RAW, whose value is nor‐
mally 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).
For ICMPv6 raw sockets, the kernel will calculate and insert the ICMPv6
checksum for since this checksum is mandatory.
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 psuedo 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 incorpo‐
rate the IPv6 pseudo-header, defined in Section 8.1 of RFC2460. This new
socket option also specifies an integer offset into the user data of
where the checksum is located.
int offset = 2;
setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset, sizeof(offset));
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.
Note: Since the checksum is always calculated by the kernel for an ICMPv6
socket, applications are not able to generate ICMPv6 packets with incor‐
rect checksums (presumably for testing purposes) using this API.
ERRORS
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 data‐
gram 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 hasn't been con‐
nected;
[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:
[EINVAL] An unknown socket option name was given.
[EINVAL] The ancillary data items were improperly formed, or
option name was unknown.
SEE ALSOgetsockopt(2), recv(2), send(2), setsockopt(2), inet6_option_space(3),
inet6_rthdr_space(3), icmp6(4), inet6(4), intro(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.
STANDARDS
Most of the socket options are defined in RFC2292 and/or RFC2553.
IPV6_V6ONLY socket option is defined in draft-ietf-ipngwg-rfc2553bis-03.
IPV6_PORTRANGE socket option and conflict resolution rule are not defined
in the RFCs and should be considered implementation dependent.
HISTORY
The implementation is based on KAME stack (which is descendant of WIDE
hydrangea IPv6 stack kit).
Part of the document was shamelessly copied from RFC2553 and RFC2292.
BUGS
The IPV6_NEXTHOP object/option is not fully implemented as of writing
this.
BSD March 13, 2000 BSD
