INET6(4) AerieBSD 1.0 Refernce Manual INET6(4)


inet6 — Internet protocol version 6 family


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


The inet6 family is an updated version of the inet(4) family. While inet(4) 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 byteorder. 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 {
	u_int8_t	sin6_len;
	sa_family_t	sin6_family;
	in_port_t	sin6_port;
	u_int32_t	sin6_flowinfo;
	struct in6_addr	sin6_addr;
	u_int32_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 effect “wildcard” matching on incoming messages.

The IPv6 specification defines scoped address, like link-local or site-local address. A scoped address is ambiguous to the kernel, if it is specified without a scope identifier. To manipulate scoped addresses properly from userland, programs must use the advanced API defined in RFC 2292. A compact description of the advanced API is available in ip6(4). If scoped addresses are specified without 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.

KAME implementation supports extended numeric IPv6 address notation for link-local addresses, like “fe80::1%de0” to specify “ fe80::1 on de0 interface ”. The notation is supported by getaddrinfo(3) and getnameinfo(3). Some normal userland programs, such as telnet(1) or ftp(1), are able to use the notation. With special programs like ping6(8), an outgoing interface can be specified with an extra command line option to disambiguate scoped addresses.

Scoped addresses are handled specially in the kernel. In the kernel structures like routing tables or interface structure, scoped addresses will have their interface index embedded into the address. Therefore, the address on some of the kernel structure is not the same as that on the wire. The embedded index will become visible on PF_ROUTE socket, kernel memory accesses via kvm(3) and some other occasions. HOWEVER, users should never use the embedded form. For details please consult Note that the above URL describes the situation with the latest KAME tree, not the OpenBSD tree.


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(4) 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.

Interaction between IPv4/v6 sockets

OpenBSD does not route IPv4 traffic to an AF_INET6 socket, for security reasons. If both IPv4 and IPv6 traffic need to be accepted, listen on two sockets.

The behavior of AF_INET6 TCP/UDP socket is documented in RFC 2553. Basically, it says the following:

However, RFC 2553 does not define the constraint between the order of bind(2), 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 socket. It is recommended to listen to two sockets, one for AF_INET and another for AF_INET6, if both IPv4 and IPv6 traffic are to be accepted.

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. Caution should be taken when handling connections from IPv4 mapped addresses to AF_INET6 sockets.


ioctl(2), socket(2), sysctl(3), icmp6(4), intro(4), ip6(4), tcp(4), udp(4)


.Rs .%A Tatsuya Jinmei .%A Atsushi Onoe .%T "An Extension of Format for IPv6 Scoped Addresses" .%R internet draft .%D June 2000 .%N draft-ietf-ipngwg-scopedaddr-format-02.txt .%O work in progress material .Re


The inet6 protocol interface is defined in RFC 2553 and RFC 2292. The implementation described herein appeared in 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.

“Version independent” code should be implemented as much as possible in order to support both inet(4) and inet6.

AerieBSD 1.0 Reference Manual August 26 2008 INET6(4)