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_IPSEC_POLICY struct sadb_x_policy *
Get or set IPSec policy for sockets. For example,
const char *policy = "in ipsec ah/transport//require";
char *buf = ipsec_set_policy(policy, strlen(policy));
setsockopt(s, IPPROTO_IPV6, IPV6_IPSEC_POLICY, buf, ipsec_get_policylen(buf));
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_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 {
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_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.
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.