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INET6 (Internet protocol version 6)


#include <sys/types.h>
#include <netinet/in.h>

The inet6 (Internet protocol version 6) family is an updated version of inet(4) family. While inet implements Internet Protocol version 4, inet6 implements Internet Protocol version 6.

inet6 is a collection of protocols layered atop the Internet Protocol version 6 (IPv6) transport layer, and utilizing the IPv6 address format. The inet6 family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IPv6 protocol.


IPv6 addresses are 16 byte quantities, stored in network standard byte-order. The include file <netinet/in.h> defines this address as a discriminated union.

Sockets bound to the inet6 family utilize the following addressing structure:

struct sockaddr_in6 {
    uint8_t        sin6_len;
    sa_family_t    sin6_family;
    in_port_t      sin6_port;
    uint32_t       sin6_flowinfo;
    struct in6_addr sin6_addr;
    uint32_t       sin6_scope_id;

Sockets may be created with the local address '::' (which is equal to IPv6 address 0:0:0:0:0:0:0:0) to affect 'wildcard' matching on incoming messages.

The IPv6 specification defines scoped addresses, like link-local or site-local addresses. A scoped address is ambiguous to the kernel, if it is specified without a scope identifier. To manipulate scoped addresses properly from the userland, programs must use the advanced API defined in RFC2292. A compact description of the advanced API is available in ip6(4). If a scoped address is specified without an explicit scope, the kernel may raise an error. Note that scoped addresses are not for daily use at this moment, both from a specification and an implementation point of view.

The KAME implementation supports an extended numeric IPv6 address notation for link-local addresses, like 'fe80::1%de0' to specify 'fe80::1 on de0 interface'. This notation is supported by getaddrinfo and getnameinfo. Some of normal userland programs, such as telnet(1) or ftp(1), are able to use this notation. With special programs like ping6, you can specify the outgoing interface by an extra command line option to disambiguate scoped addresses.

Scoped addresses are handled specially in the kernel. In kernel structures like routing tables or interface structures, a scoped address will have its interface index embedded into the address. Therefore, the address in some kernel structures is not the same as that on the wire. The embedded index will become visible through a PF_ROUTE socket, kernel memory accesses via kvm(3) and on some other occasions. HOWEVER, users should never use the embedded form. For details please consult IMPLEMENTATION supplied with KAME kit.


The inet6 family is comprised of the IPv6 network protocol, Internet Control Message Protocol version 6 (ICMPv6), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM abstraction. Note that TCP and UDP are common to inet and inet6. A raw interface to IPv6 is available by creating an Internet socket of type SOCK_RAW. The ICMPv6 message protocol is accessible from a raw socket.

MIB Variables

A number of variables are implemented in the net.inet6 branch of the sysctl MIB. In addition to the variables supported by the transport protocols (for which the respective manual pages may be consulted), the following general variables are defined:

Interaction between IPv4/v6 sockets

By default, FreeBSD does not route IPv4 traffic to AF_INET6 sockets. The default behavior intentionally violates RFC2553 for security reasons. Listen to two sockets if you want to accept both IPv4 and IPv6 traffic. IPv4 traffic may be routed with certain per-socket/per-node configuration, however, it is not recommended to do so. Consult IP6 for details.

The behavior of AF_INET6 TCP/UDP socket is documented in RFC2553. Basically, it says this:

However, RFC2553 does not define the ordering constraint between calls to bind, nor how IPv4 TCP/UDP port numbers and IPv6 TCP/UDP port numbers relate to each other (should they be integrated or separated). Implemented behavior is very different from kernel to kernel. Therefore, it is unwise to rely too much upon the behavior of AF_INET6 wildcard bind sockets. It is recommended to listen to two sockets, one for AF_INET and another for AF_INET6, when you would like to accept both IPv4 and IPv6 traffic.

It should also be noted that malicious parties can take advantage of the complexity presented above, and are able to bypass access control, if the target node routes IPv4 traffic to AF_INET6 socket. Users are advised to take care handling connections from IPv4 mapped address to AF_INET6 sockets.


Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-02.txt, June 2000, work in progress material.


The inet6 protocol interfaces are defined in RFC2553 and RFC2292. The implementation described herein appeared in the WIDE/KAME project.


The IPv6 support is subject to change as the Internet protocols develop. Users should not depend on details of the current implementation, but rather the services exported.

Users are suggested to implement 'version independent' code as much as possible, as you will need to support both inet and inet6.


Versions Link to
INtime 4.0 netlib.lib
See Also