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 protocols, 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 privilege) 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 multicast 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 network. 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 overrides 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 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 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. Memberships 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 instantiation 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 specification). 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 structure.

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 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 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 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-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 ancillary 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 typically 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 application 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 fragmentation 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 normally 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 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 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 incorrect 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 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 hasn't 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:

[EINVAL]
    An unknown socket option name was given.

[EINVAL]
    The ancillary data items were improperly formed, or option name was unknown.

SEE ALSO
getsockopt(2), recv(2), send(2), setsockopt(2), inet6_option_space(3), inet6_rthdr_space(3), icmp6(4), inet6(4), intro(4) W. Stevens, M. Thomas, RFC, 2292, February 1998, Advanced Sockets API for IPv6. S. Deering, R. Hinden, RFC, 2460, December 1998, Internet Protocol, Version 6 (IPv6) Specification. R. Gilligan, S. Thomson, J. Bound, W. Stevens, RFC, 2553, March 1999, Basic Socket Interface Extensions for IPv6.

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.