# man > ipv6(7) [_ipv6_(7)](https://www.chedong.com/phpMan.php/man/ipv6/7/markdown) Miscellaneous Information Manual [_ipv6_(7)](https://www.chedong.com/phpMan.php/man/ipv6/7/markdown) ## NAME ipv6 - Linux IPv6 protocol implementation ## SYNOPSIS ### #include ### #include _tcp6_socket_ **= socket(AF_INET6, SOCK_STREAM, 0);** _raw6_socket_ **= socket(AF_INET6, SOCK_RAW, _**protocol_**);** _udp6_socket_ **= socket(AF_INET6, SOCK_DGRAM, _**protocol_**);** ## DESCRIPTION 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 in‐ terface is based on the BSD sockets interface; see [**socket**(7)](https://www.chedong.com/phpMan.php/man/socket/7/markdown). The IPv6 API aims to be mostly compatible with the IPv4 API (see [**ip**(7)](https://www.chedong.com/phpMan.php/man/ip/7/markdown)). 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 han‐ dled 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)](https://www.chedong.com/phpMan.php/man/ip/7/markdown)); _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)](https://www.chedong.com/phpMan.php/man/netdevice/7/markdown)) 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: 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)](https://www.chedong.com/phpMan.php/man/setsockopt/2/markdown) and read with [**getsockopt**(2)](https://www.chedong.com/phpMan.php/man/getsockopt/2/markdown). The socket option level for IPv6 is **IPPROTO_IPV6**. A boolean inte‐ ger 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 con‐ taining **AF_INET**. This is useful to pass v4-mapped sockets as file descriptors to pro‐ grams 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 in‐ teger. **IPV6_MTU_DISCOVER** Control path-MTU discovery on the socket. See **IP_MTU_DISCOVER **in [**ip**(7)](https://www.chedong.com/phpMan.php/man/ip/7/markdown) 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)](https://www.chedong.com/phpMan.php/man/netdevice/7/markdown)) 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 **deliv‐ ers 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 op‐ tions can also be set for outgoing packets by putting the appropriate control message into the control buffer of [**sendmsg**(2)](https://www.chedong.com/phpMan.php/man/sendmsg/2/markdown). Allowed only for **SOCK_DGRAM **or **SOCK_RAW **sock‐ ets. Argument is a pointer to a boolean value. **IPV6_RECVERR** Control receiving of asynchronous error options. See **IP_RECVERR **in [**ip**(7)](https://www.chedong.com/phpMan.php/man/ip/7/markdown) 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 ker‐ nel, 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). ## ERRORS **ENODEV **The user tried to [**bind**(2)](https://www.chedong.com/phpMan.php/man/bind/2/markdown) to a link-local IPv6 address, but the _sin6_scope_id_ in the supplied _sockaddr_in6_ structure is not a valid interface index. ## VERSIONS 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 inter‐ faces 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. ## NOTES 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 **IP‐** **PROTO_***. See also [**ip**(7)](https://www.chedong.com/phpMan.php/man/ip/7/markdown). ## BUGS 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 op‐ tions 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. ## SEE ALSO [**cmsg**(3)](https://www.chedong.com/phpMan.php/man/cmsg/3/markdown), [**ip**(7)](https://www.chedong.com/phpMan.php/man/ip/7/markdown) RFC 2553: IPv6 BASIC API; Linux tries to be compliant to this. RFC 2460: IPv6 specification. Linux man-pages 6.7 2023-10-31 [_ipv6_(7)](https://www.chedong.com/phpMan.php/man/ipv6/7/markdown)