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NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON |
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ipv6(7) Miscellaneous Information Manual ipv6(7)
ipv6 - Linux IPv6 protocol implementation
#include <sys/socket.h>
#include <netinet/in.h>
tcp6_socket = socket(AF_INET6, SOCK_STREAM, 0);
raw6_socket = socket(AF_INET6, SOCK_RAW, protocol);
udp6_socket = socket(AF_INET6, SOCK_DGRAM, protocol);
Linux 2.2 optionally implements the Internet Protocol, version 6.
This man page contains a description of the IPv6 basic API as
implemented by the Linux kernel and glibc 2.1. The interface is
based on the BSD sockets interface; see socket(7).
The IPv6 API aims to be mostly compatible with the IPv4 API (see
ip(7)). Only differences are described in this man page.
To bind an AF_INET6 socket to any process, the local address
should be copied from the in6addr_any variable which has in6_addr
type. In static initializations, IN6ADDR_ANY_INIT may also be
used, which expands to a constant expression. Both of them are in
network byte order.
The IPv6 loopback address (::1) is available in the global
in6addr_loopback variable. For initializations,
IN6ADDR_LOOPBACK_INIT should be used.
IPv4 connections can be handled with the v6 API by using the
v4-mapped-on-v6 address type; thus a program needs to support only
this API type to support both protocols. This is handled
transparently by the address handling functions in the C library.
IPv4 and IPv6 share the local port space. When you get an IPv4
connection or packet to an IPv6 socket, its source address will be
mapped to v6.
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).
ENODEV The user tried to bind(2) to a link-local IPv6 address, but
the sin6_scope_id in the supplied sockaddr_in6 structure is
not a valid interface index.
Linux 2.4 will break binary compatibility for the sockaddr_in6 for
64-bit hosts by changing the alignment of in6_addr and adding an
additional sin6_scope_id field. The kernel interfaces stay
compatible, but a program including sockaddr_in6 or in6_addr into
other structures may not be. This is not a problem for 32-bit
hosts like i386.
The sin6_flowinfo field is new in Linux 2.4. It is transparently
passed/read by the kernel when the passed address length contains
it. Some programs that pass a longer address buffer and then
check the outgoing address length may break.
The sockaddr_in6 structure is bigger than the generic sockaddr.
Programs that assume that all address types can be stored safely
in a struct sockaddr need to be changed to use struct
sockaddr_storage for that instead.
SOL_IP, SOL_IPV6, SOL_ICMPV6, and other SOL_* socket options are
nonportable variants of IPPROTO_*. See also ip(7).
The IPv6 extended API as in RFC 2292 is currently only partly
implemented; although the 2.2 kernel has near complete support for
receiving options, the macros for generating IPv6 options are
missing in glibc 2.1.
IPSec support for EH and AH headers is missing.
Flow label management is not complete and not documented here.
This man page is not complete.
cmsg(3), ip(7)
RFC 2553: IPv6 BASIC API; Linux tries to be compliant to this.
RFC 2460: IPv6 specification.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 ipv6(7)
Pages that refer to this page: bind(2), recv(2), send(2), socket(2), sd_is_fifo(3), sockaddr(3type), nfs(5), systemd.socket(5), address_families(7), ip(7), socket(7), udplite(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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