IP6(4)                    OpenBSD Programmer's Manual                   IP6(4)

NAME
     ip6 - Internet Protocol version 6 (IPv6)

SYNOPSIS
     #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 head-
     er.  As symbol name suggests, the option controls hoplimit field on uni-
     cast 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 it-
     self 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 demon), by eliminat-
     ing the overhead of receiving their own transmissions.  It should gener-
     ally not be used by applications for which there may be more than one in-
     stance 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;
               unsigned 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));

   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 ob-
     jects.

     If IPV6_PKTINFO is enabled, the destination IPv6 address and the arriving
     interface index will be available via struct in6_pktinfo on ancillary da-
     ta 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 ad-
     dress 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 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 ob-
     jects.  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 op-
     tional 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-re-
     sult integer: when the function is called the integer specifies the size
     of the buffer pointed to by the fourth argument, and on return this inte-
     ger contains the actual number of bytes that were returned.  The applica-
     tion 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 sock-
     et, 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 de-
     scribed 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 ad-
     dress 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 un-
     defined in the API specification and implementation dependent.

   Raw IPv6 Sockets
     Raw IPv6 sockets are connectionless, and are normally used with the send-
     to(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 re-
     ceived.  If proto is non-zero, that protocol number will be used on out-
     going 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 ex-
     tension 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 normal-
     ly 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 pseudo header checksum for output, and (2) verify the received pseudo
     header checksum on input, discarding the packet if the checksum is in er-
     ror.  This option prevents applications from having to perform source ad-
     dress selection on the packets they send.  The checksum will incorporate
     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 al-
     so disables the option.  By disabled we mean (1) the kernel will not cal-
     culate 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.

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 al-
                      ready connected;

     [ENOTCONN]       when trying to send a datagram, but no destination ad-
                      dress is specified, and the socket hasn't been connect-
                      ed;

     [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 op-
                      tion name was unknown.

SEE ALSO
     getsockopt(2), send(2), setsockopt(2), recv(2), inet6_option_space(3),
     inet6_rthdr_space(3), intro(4), icmp6(4), inet6(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_PORTRANGE and conflict resolution rule is 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.

OpenBSD 2.9                    December 17, 1999                             6
