Address format

struct sockaddr_in6 {


    sa_family_t     sin6_family;   /* AF_INET6 */


    in_port_t       sin6_port;     /* port number */


    uint32_t        sin6_flowinfo; /* IPv6 flow information */


    struct in6_addr sin6_addr;     /* IPv6 address */


    uint32_t        sin6_scope_id; /* Scope ID (new in Linux 2.4) */
};
struct in6_addr {


    unsigned char   s6_addr[16];   /* IPv6 address */
};

sin6_family is always set to AF_INET6; sin6_port is the protocol port (see sin_port in ip(7)); sin6_flowinfo is the IPv6 flow identifier; sin6_addr is the 128-bit IPv6 address. sin6_scope_id is an ID depending on the scope of the address. It is new in Linux 2.4. Linux supports it only for link-local addresses, in that case sin6_scope_id contains the interface index (see netdevice(7))

IPv6 supports several address types: unicast to address a single host, multicast to address a group of hosts, anycast to address the nearest member of a group of hosts (not implemented in Linux), IPv4-on-IPv6 to address an IPv4 host, and other reserved address types.

The address notation for IPv6 is a group of 8 4-digit hexadecimal numbers, separated with a ':'. "::" stands for a string of 0 bits. Special addresses are ::1 for loopback and ::FFFF:<IPv4 address> for IPv4-mapped-on-IPv6.

The port space of IPv6 is shared with IPv4.

Socket options

IPv6 supports some protocol-specific socket options that can be set with setsockopt(2) and read with getsockopt(2). The socket option level for IPv6 is IPPROTO_IPV6. A boolean integer flag is zero when it is false, otherwise true.

IPV6_ADDRFORM

Turn an AF_INET6 socket into a socket of a different address family. Only AF_INET is currently supported for that. It is allowed only for IPv6 sockets that are connected and bound to a v4-mapped-on-v6 address. The argument is a pointer to an integer containing AF_INET. This is useful to pass v4-mapped sockets as file descriptors to programs that don't know how to deal with the IPv6 API.

IPV6_ADD_MEMBERSHIP, IPV6_DROP_MEMBERSHIP

Control membership in multicast groups. Argument is a pointer to a struct ipv6_mreq.

IPV6_MTU

getsockopt(): Retrieve the current known path MTU of the current socket. Valid only when the socket has been connected. Returns an integer.

setsockopt(): Set the MTU to be used for the socket. The MTU is limited by the device MTU or the path MTU when path MTU discovery is enabled. Argument is a pointer to integer.

IPV6_MTU_DISCOVER

Control path-MTU discovery on the socket. See IP_MTU_DISCOVER in ip(7) for details.

IPV6_MULTICAST_HOPS

Set the multicast hop limit for the socket. Argument is a pointer to an integer. -1 in the value means use the route default, otherwise it should be between 0 and 255.

IPV6_MULTICAST_IF

Set the device for outgoing multicast packets on the socket. This is allowed only for SOCK_DGRAM and SOCK_RAW socket. The argument is a pointer to an interface index (see netdevice(7)) in an integer.

IPV6_MULTICAST_LOOP

Control whether the socket sees multicast packets that it has send itself. Argument is a pointer to boolean.

IPV6_RECVPKTINFO (since Linux 2.6.14)

Set delivery of the IPV6_PKTINFO control message on incoming datagrams. Such control messages contain a struct in6_pktinfo, as per RFC 3542. Allowed only for SOCK_DGRAM or SOCK_RAW sockets. Argument is a pointer to a boolean value in an integer.

IPV6_RTHDR, IPV6_AUTHHDR, IPV6_DSTOPTS, IPV6_HOPOPTS, IPV6_FLOWINFO, IPV6_HOPLIMIT

Set delivery of control messages for incoming datagrams containing extension headers from the received packet. IPV6_RTHDR delivers the routing header, IPV6_AUTHHDR delivers the authentication header, IPV6_DSTOPTS delivers the destination options, IPV6_HOPOPTS delivers the hop options, IPV6_FLOWINFO delivers an integer containing the flow ID, IPV6_HOPLIMIT delivers an integer containing the hop count of the packet. The control messages have the same type as the socket option. All these header options can also be set for outgoing packets by putting the appropriate control message into the control buffer of sendmsg(2). Allowed only for SOCK_DGRAM or SOCK_RAW sockets. Argument is a pointer to a boolean value.

IPV6_RECVERR

Control receiving of asynchronous error options. See IP_RECVERR in ip(7) for details. Argument is a pointer to boolean.

IPV6_ROUTER_ALERT

Pass forwarded packets containing a router alert hop-by-hop option to this socket. Allowed only for SOCK_RAW sockets. The tapped packets are not forwarded by the kernel, it is the user's responsibility to send them out again. Argument is a pointer to an integer. A positive integer indicates a router alert option value to intercept. Packets carrying a router alert option with a value field containing this integer will be delivered to the socket. A negative integer disables delivery of packets with router alert options to this socket.

IPV6_UNICAST_HOPS

Set the unicast hop limit for the socket. Argument is a pointer to an integer. -1 in the value means use the route default, otherwise it should be between 0 and 255.

IPV6_V6ONLY (since Linux 2.4.21 and 2.6)

If this flag is set to true (nonzero), then the socket is restricted to sending and receiving IPv6 packets only. In this case, an IPv4 and an IPv6 application can bind to a single port at the same time.

If this flag is set to false (zero), then the socket can be used to send and receive packets to and from an IPv6 address or an IPv4-mapped IPv6 address.

The argument is a pointer to a boolean value in an integer.

The default value for this flag is defined by the contents of the file /proc/sys/net/ipv6/bindv6only. The default value for that file is 0 (false).