Header

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 {
               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 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 {
     u_int8_t ip6e_nxt;
     u_int8_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. A value of -1 resets to the default value.

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)) 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 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 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 {
	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 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 {
	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 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 {
	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 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_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.

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.