Network Working Group                                        R. Gilligan
Request for Comments: 3493                                Intransa, Inc.
Obsoletes: 2553                                               S. Thomson
Category: Informational                                            Cisco
                                                                J. Bound
                                                               J. McCann
                                                         Hewlett-Packard
                                                              W. Stevens
                                                           February 2003

Basic Socket Interface Extensions for IPv6

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

Abstract

   The de facto standard Application Program Interface (API) for TCP/IP
   applications is the "sockets" interface.  Although this API was
   developed for Unix in the early 1980s it has also been implemented on
   a wide variety of non-Unix systems.  TCP/IP applications written
   using the sockets API have in the past enjoyed a high degree of
   portability and we would like the same portability with IPv6
   applications.  But changes are required to the sockets API to support
   IPv6 and this memo describes these changes.  These include a new
   socket address structure to carry IPv6 addresses, new address
   conversion functions, and some new socket options.  These extensions
   are designed to provide access to the basic IPv6 features required by
   TCP and UDP applications, including multicasting, while introducing a
   minimum of change into the system and providing complete
   compatibility for existing IPv4 applications.  Additional extensions
   for advanced IPv6 features (raw sockets and access to the IPv6
   extension headers) are defined in another document.

   1.  Introduction................................................3
   2.  Design Considerations.......................................4
       2.1  What Needs to be Changed...............................4
       2.2  Data Types.............................................6
       2.3  Headers................................................6
       2.4  Structures.............................................6
   3.  Socket Interface............................................6
       3.1  IPv6 Address Family and Protocol Family................6
       3.2  IPv6 Address Structure.................................7
       3.3  Socket Address Structure for 4.3BSD-Based Systems......7
       3.4  Socket Address Structure for 4.4BSD-Based Systems......9
       3.5  The Socket Functions...................................9
       3.6  Compatibility with IPv4 Applications..................10
       3.7  Compatibility with IPv4 Nodes.........................11
       3.8  IPv6 Wildcard Address.................................11
       3.9  IPv6 Loopback Address.................................13
       3.10 Portability Additions.................................14
   4.  Interface Identification...................................16
       4.1  Name-to-Index.........................................17
       4.2  Index-to-Name.........................................17
       4.3  Return All Interface Names and Indexes................18
       4.4  Free Memory...........................................18
   5.  Socket Options.............................................18
       5.1  Unicast Hop Limit.....................................19
       5.2  Sending and Receiving Multicast Packets...............19
       5.3  IPV6_V6ONLY option for AF_INET6 Sockets...............22
   6.  Library Functions..........................................22
       6.1  Protocol-Independent Nodename and
            Service Name Translation..............................23
       6.2  Socket Address Structure to Node Name
            and Service Name......................................28
       6.3  Address Conversion Functions..........................31
       6.4  Address Testing Macros................................33
   7.  Summary of New Definitions.................................33
   8.  Security Considerations....................................35
   9.  Changes from RFC 2553......................................35
   10. Acknowledgments............................................36
   11. References.................................................37
   12. Authors' Addresses.........................................38
   13. Full Copyright Statement...................................39

1. Introduction

   While IPv4 addresses are 32 bits long, IPv6 addresses are 128 bits
   long.  The socket interface makes the size of an IP address quite
   visible to an application; virtually all TCP/IP applications for
   BSD-based systems have knowledge of the size of an IP address.  Those
   parts of the API that expose the addresses must be changed to
   accommodate the larger IPv6 address size.  IPv6 also introduces new
   features, some of which must be made visible to applications via the
   API.  This memo defines a set of extensions to the socket interface
   to support the larger address size and new features of IPv6.  It
   defines "basic" extensions that are of use to a broad range of
   applications.  A companion document, the "advanced" API [4], covers
   extensions that are of use to more specialized applications, examples
   of which include routing daemons, and the "ping" and "traceroute"
   utilities.

   The development of this API was started in 1994 in the IETF IPng
   working group.  The API has evolved over the years, published first
   in RFC 2133, then again in RFC 2553, and reaching its final form in
   this document.

   As the API matured and stabilized, it was incorporated into the Open
   Group's Networking Services (XNS) specification, issue 5.2, which was
   subsequently incorporated into a joint Open Group/IEEE/ISO standard
   [3].

   Effort has been made to ensure that this document and [3] contain the
   same information with regard to the API definitions.  However, the
   reader should note that this document is for informational purposes
   only, and that the official standard specification of the sockets API
   is [3].

   It is expected that any future standardization work on this API would
   be done by the Open Group Base Working Group [6].

   It should also be noted that this document describes only those
   portions of the API needed for IPv4 and IPv6 communications.  Other
   potential uses of the API, for example the use of getaddrinfo() and
   getnameinfo() with the AF_UNIX address family, are beyond the scope
   of this document.

2. Design Considerations

   There are a number of important considerations in designing changes
   to this well-worn API:

   -  The API changes should provide both source and binary
      compatibility for programs written to the original API.  That is,
      existing program binaries should continue to operate when run on a
      system supporting the new API.  In addition, existing applications
      that are re-compiled and run on a system supporting the new API
      should continue to operate.  Simply put, the API changes for IPv6
      should not break existing programs.  An additional mechanism for
      implementations to verify this is to verify the new symbols are
      protected by Feature Test Macros as described in [3].  (Such
      Feature Test Macros are not defined by this RFC.)

   -  The changes to the API should be as small as possible in order to
      simplify the task of converting existing IPv4 applications to
      IPv6.

   -  Where possible, applications should be able to use this API to
      interoperate with both IPv6 and IPv4 hosts.  Applications should
      not need to know which type of host they are communicating with.

   -  IPv6 addresses carried in data structures should be 64-bit
      aligned.  This is necessary in order to obtain optimum performance
      on 64-bit machine architectures.

   Because of the importance of providing IPv4 compatibility in the API,
   these extensions are explicitly designed to operate on machines that
   provide complete support for both IPv4 and IPv6.  A subset of this
   API could probably be designed for operation on systems that support
   only IPv6.  However, this is not addressed in this memo.

2.1 What Needs to be Changed

   The socket interface API consists of a few distinct components:

   -  Core socket functions.

   -  Address data structures.

   -  Name-to-address translation functions.

   -  Address conversion functions.

   The core socket functions -- those functions that deal with such
   things as setting up and tearing down TCP connections, and sending
   and receiving UDP packets -- were designed to be transport
   independent.  Where protocol addresses are passed as function
   arguments, they are carried via opaque pointers.  A protocol-specific
   address data structure is defined for each protocol that the socket
   functions support.  Applications must cast pointers to these
   protocol-specific address structures into pointers to the generic
   "sockaddr" address structure when using the socket functions.  These
   functions need not change for IPv6, but a new IPv6-specific address
   data structure is needed.

   The "sockaddr_in" structure is the protocol-specific data structure
   for IPv4.  This data structure actually includes 8-octets of unused
   space, and it is tempting to try to use this space to adapt the
   sockaddr_in structure to IPv6.  Unfortunately, the sockaddr_in
   structure is not large enough to hold the 16-octet IPv6 address as
   well as the other information (address family and port number) that
   is needed.  So a new address data structure must be defined for IPv6.

   IPv6 addresses are scoped [2] so they could be link-local, site,
   organization, global, or other scopes at this time undefined.  To
   support applications that want to be able to identify a set of
   interfaces for a specific scope, the IPv6 sockaddr_in structure must
   support a field that can be used by an implementation to identify a
   set of interfaces identifying the scope for an IPv6 address.

   The IPv4 name-to-address translation functions in the socket
   interface are gethostbyname() and gethostbyaddr().  These are left as
   is, and new functions are defined which support both IPv4 and IPv6.

   The IPv4 address conversion functions -- inet_ntoa() and inet_addr()
   -- convert IPv4 addresses between binary and printable form.  These
   functions are quite specific to 32-bit IPv4 addresses.  We have
   designed two analogous functions that convert both IPv4 and IPv6
   addresses, and carry an address type parameter so that they can be
   extended to other protocol families as well.

   Finally, a few miscellaneous features are needed to support IPv6.  A
   new interface is needed to support the IPv6 hop limit header field.
   New socket options are needed to control the sending and receiving of
   IPv6 multicast packets.

   The socket interface will be enhanced in the future to provide access
   to other IPv6 features.  Some of these extensions are described in
   [4].

2.2 Data Types

   The data types of the structure elements given in this memo are
   intended to track the relevant standards.  uintN_t means an unsigned
   integer of exactly N bits (e.g., uint16_t).  The sa_family_t and
   in_port_t types are defined in [3].

2.3 Headers

When function prototypes and structures are shown we show the headers
that must be #included to cause that item to be defined.

2.4 Structures

   When structures are described the members shown are the ones that
   must appear in an implementation.  Additional, nonstandard members
   may also be defined by an implementation.  As an additional
   precaution nonstandard members could be verified by Feature Test
   Macros as described in [3].  (Such Feature Test Macros are not
   defined by this RFC.)

   The ordering shown for the members of a structure is the recommended
   ordering, given alignment considerations of multibyte members, but an
   implementation may order the members differently.

3. Socket Interface

This section specifies the socket interface changes for IPv6.

3.1 IPv6 Address Family and Protocol Family

   A new address family name, AF_INET6, is defined in <sys/socket.h>.
   The AF_INET6 definition distinguishes between the original
   sockaddr_in address data structure, and the new sockaddr_in6 data
   structure.

   A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
   Like most of the other protocol family names, this will usually be
   defined to have the same value as the corresponding address family
   name:

      #define PF_INET6        AF_INET6

   The AF_INET6 is used in the first argument to the socket() function
   to indicate that an IPv6 socket is being created.

3.2 IPv6 Address Structure

   A new in6_addr structure holds a single IPv6 address and is defined
   as a result of including <netinet/in.h>:

      struct in6_addr {
          uint8_t  s6_addr[16];      /* IPv6 address */
      };

   This data structure contains an array of sixteen 8-bit elements,
   which make up one 128-bit IPv6 address.  The IPv6 address is stored
   in network byte order.

   The structure in6_addr above is usually implemented with an embedded
   union with extra fields that force the desired alignment level in a
   manner similar to BSD implementations of "struct in_addr".  Those
   additional implementation details are omitted here for simplicity.

   An example is as follows:

   struct in6_addr {
        union {
            uint8_t  _S6_u8[16];
            uint32_t _S6_u32[4];
            uint64_t _S6_u64[2];
        } _S6_un;
   };
   #define s6_addr _S6_un._S6_u8

3.3 Socket Address Structure for 4.3BSD-Based Systems

   In the socket interface, a different protocol-specific data structure
   is defined to carry the addresses for each protocol suite.  Each
   protocol-specific data structure is designed so it can be cast into a
   protocol-independent data structure -- the "sockaddr" structure.
   Each has a "family" field that overlays the "sa_family" of the
   sockaddr data structure.  This field identifies the type of the data
   structure.

   The sockaddr_in structure is the protocol-specific address data
   structure for IPv4.  It is used to pass addresses between
   applications and the system in the socket functions.  The following
   sockaddr_in6 structure holds IPv6 addresses and is defined as a
   result of including the <netinet/in.h> header:

struct sockaddr_in6 {
    sa_family_t     sin6_family;    /* AF_INET6 */
    in_port_t       sin6_port;      /* transport layer port # */
    uint32_t        sin6_flowinfo;  /* IPv6 flow information */
    struct in6_addr sin6_addr;      /* IPv6 address */
    uint32_t        sin6_scope_id;  /* set of interfaces for a scope */
};

   This structure is designed to be compatible with the sockaddr data
   structure used in the 4.3BSD release.

   The sin6_family field identifies this as a sockaddr_in6 structure.
   This field overlays the sa_family field when the buffer is cast to a
   sockaddr data structure.  The value of this field must be AF_INET6.

   The sin6_port field contains the 16-bit UDP or TCP port number.  This
   field is used in the same way as the sin_port field of the
   sockaddr_in structure.  The port number is stored in network byte
   order.

   The sin6_flowinfo field is a 32-bit field intended to contain flow-
   related information.  The exact way this field is mapped to or from a
   packet is not currently specified.  Until such time as its use is
   specified, applications should set this field to zero when
   constructing a sockaddr_in6, and ignore this field in a sockaddr_in6
   structure constructed by the system.

   The sin6_addr field is a single in6_addr structure (defined in the
   previous section).  This field holds one 128-bit IPv6 address.  The
   address is stored in network byte order.

   The ordering of elements in this structure is specifically designed
   so that when sin6_addr field is aligned on a 64-bit boundary, the
   start of the structure will also be aligned on a 64-bit boundary.
   This is done for optimum performance on 64-bit architectures.

   The sin6_scope_id field is a 32-bit integer that identifies a set of
   interfaces as appropriate for the scope [2] of the address carried in
   the sin6_addr field.  The mapping of sin6_scope_id to an interface or
   set of interfaces is left to implementation and future specifications
   on the subject of scoped addresses.

   Notice that the sockaddr_in6 structure will normally be larger than
   the generic sockaddr structure.  On many existing implementations the
   sizeof(struct sockaddr_in) equals sizeof(struct sockaddr), with both
   being 16 bytes.  Any existing code that makes this assumption needs
   to be examined carefully when converting to IPv6.

3.4 Socket Address Structure for 4.4BSD-Based Systems

   The 4.4BSD release includes a small, but incompatible change to the
   socket interface.  The "sa_family" field of the sockaddr data
   structure was changed from a 16-bit value to an 8-bit value, and the
   space saved used to hold a length field, named "sa_len".  The
   sockaddr_in6 data structure given in the previous section cannot be
   correctly cast into the newer sockaddr data structure.  For this
   reason, the following alternative IPv6 address data structure is
   provided to be used on systems based on 4.4BSD.  It is defined as a
   result of including the <netinet/in.h> header.

struct sockaddr_in6 {
    uint8_t         sin6_len;       /* length of this struct */
    sa_family_t     sin6_family;    /* AF_INET6 */
    in_port_t       sin6_port;      /* transport layer port # */
    uint32_t        sin6_flowinfo;  /* IPv6 flow information */
    struct in6_addr sin6_addr;      /* IPv6 address */
    uint32_t        sin6_scope_id;  /* set of interfaces for a scope */
};

   The only differences between this data structure and the 4.3BSD
   variant are the inclusion of the length field, and the change of the
   family field to a 8-bit data type.  The definitions of all the other
   fields are identical to the structure defined in the previous
   section.

   Systems that provide this version of the sockaddr_in6 data structure
   must also declare SIN6_LEN as a result of including the
   <netinet/in.h> header.  This macro allows applications to determine
   whether they are being built on a system that supports the 4.3BSD or
   4.4BSD variants of the data structure.

3.5 The Socket Functions

   Applications call the socket() function to create a socket descriptor
   that represents a communication endpoint.  The arguments to the
   socket() function tell the system which protocol to use, and what
   format address structure will be used in subsequent functions.  For
   example, to create an IPv4/TCP socket, applications make the call:

      s = socket(AF_INET, SOCK_STREAM, 0);

   To create an IPv4/UDP socket, applications make the call:

      s = socket(AF_INET, SOCK_DGRAM, 0);

   Applications may create IPv6/TCP and IPv6/UDP sockets (which may also
   handle IPv4 communication as described in section 3.7) by simply
   using the constant AF_INET6 instead of AF_INET in the first argument.
   For example, to create an IPv6/TCP socket, applications make the
   call:

      s = socket(AF_INET6, SOCK_STREAM, 0);

   To create an IPv6/UDP socket, applications make the call:

      s = socket(AF_INET6, SOCK_DGRAM, 0);

   Once the application has created a AF_INET6 socket, it must use the
   sockaddr_in6 address structure when passing addresses in to the
   system.  The functions that the application uses to pass addresses
   into the system are:

      bind()
      connect()
      sendmsg()
      sendto()

   The system will use the sockaddr_in6 address structure to return
   addresses to applications that are using AF_INET6 sockets.  The
   functions that return an address from the system to an application
   are:

      accept()
      recvfrom()
      recvmsg()
      getpeername()
      getsockname()

   No changes to the syntax of the socket functions are needed to
   support IPv6, since all of the "address carrying" functions use an
   opaque address pointer, and carry an address length as a function
   argument.

3.6 Compatibility with IPv4 Applications

   In order to support the large base of applications using the original
   API, system implementations must provide complete source and binary
   compatibility with the original API.  This means that systems must
   continue to support AF_INET sockets and the sockaddr_in address
   structure.  Applications must be able to create IPv4/TCP and IPv4/UDP
   sockets using the AF_INET constant in the socket() function, as

   described in the previous section.  Applications should be able to
   hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
   sockets simultaneously within the same process.

   Applications using the original API should continue to operate as
   they did on systems supporting only IPv4.  That is, they should
   continue to interoperate with IPv4 nodes.

3.7 Compatibility with IPv4 Nodes

   The API also provides a different type of compatibility: the ability
   for IPv6 applications to interoperate with IPv4 applications.  This
   feature uses the IPv4-mapped IPv6 address format defined in the IPv6
   addressing architecture specification [2].  This address format
   allows the IPv4 address of an IPv4 node to be represented as an IPv6
   address.  The IPv4 address is encoded into the low-order 32 bits of
   the IPv6 address, and the high-order 96 bits hold the fixed prefix
   0:0:0:0:0:FFFF.  IPv4-mapped addresses are written as follows:

      ::FFFF:<IPv4-address>

   These addresses can be generated automatically by the getaddrinfo()
   function, as described in Section 6.1.

   Applications may use AF_INET6 sockets to open TCP connections to IPv4
   nodes, or send UDP packets to IPv4 nodes, by simply encoding the
   destination's IPv4 address as an IPv4-mapped IPv6 address, and
   passing that address, within a sockaddr_in6 structure, in the
   connect() or sendto() call.  When applications use AF_INET6 sockets
   to accept TCP connections from IPv4 nodes, or receive UDP packets
   from IPv4 nodes, the system returns the peer's address to the
   application in the accept(), recvfrom(), or getpeername() call using
   a sockaddr_in6 structure encoded this way.

   Few applications will likely need to know which type of node they are
   interoperating with.  However, for those applications that do need to
   know, the IN6_IS_ADDR_V4MAPPED() macro, defined in Section 6.4, is
   provided.

3.8 IPv6 Wildcard Address

   While the bind() function allows applications to select the source IP
   address of UDP packets and TCP connections, applications often want
   the system to select the source address for them.  With IPv4, one
   specifies the address as the symbolic constant INADDR_ANY (called the
   "wildcard" address) in the bind() call, or simply omits the bind()
   entirely.

   Since the IPv6 address type is a structure (struct in6_addr), a
   symbolic constant can be used to initialize an IPv6 address variable,
   but cannot be used in an assignment.  Therefore systems provide the
   IPv6 wildcard address in two forms.

   The first version is a global variable named "in6addr_any" that is an
   in6_addr structure.  The extern declaration for this variable is
   defined in <netinet/in.h>:

      extern const struct in6_addr in6addr_any;

   Applications use in6addr_any similarly to the way they use INADDR_ANY
   in IPv4.  For example, to bind a socket to port number 23, but let
   the system select the source address, an application could use the
   following code:

      struct sockaddr_in6 sin6;
       . . .
      sin6.sin6_family = AF_INET6;
      sin6.sin6_flowinfo = 0;
      sin6.sin6_port = htons(23);
      sin6.sin6_addr = in6addr_any;  /* structure assignment */
       . . .
      if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
              . . .

   The other version is a symbolic constant named IN6ADDR_ANY_INIT and
   is defined in <netinet/in.h>.  This constant can be used to
   initialize an in6_addr structure:

      struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

   Note that this constant can be used ONLY at declaration time.  It can
   not be used to assign a previously declared in6_addr structure.  For
   example, the following code will not work:

      /* This is the WRONG way to assign an unspecified address */
      struct sockaddr_in6 sin6;
       . . .
      sin6.sin6_addr = IN6ADDR_ANY_INIT; /* will NOT compile */

   Be aware that the IPv4 INADDR_xxx constants are all defined in host
   byte order but the IPv6 IN6ADDR_xxx constants and the IPv6
   in6addr_xxx externals are defined in network byte order.

3.9 IPv6 Loopback Address

   Applications may need to send UDP packets to, or originate TCP
   connections to, services residing on the local node.  In IPv4, they
   can do this by using the constant IPv4 address INADDR_LOOPBACK in
   their connect(), sendto(), or sendmsg() call.

   IPv6 also provides a loopback address to contact local TCP and UDP
   services.  Like the unspecified address, the IPv6 loopback address is
   provided in two forms -- a global variable and a symbolic constant.

   The global variable is an in6_addr structure named
   "in6addr_loopback."  The extern declaration for this variable is
   defined in <netinet/in.h>:

      extern const struct in6_addr in6addr_loopback;

   Applications use in6addr_loopback as they would use INADDR_LOOPBACK
   in IPv4 applications (but beware of the byte ordering difference
   mentioned at the end of the previous section).  For example, to open
   a TCP connection to the local telnet server, an application could use
   the following code:

   struct sockaddr_in6 sin6;
    . . .
   sin6.sin6_family = AF_INET6;
   sin6.sin6_flowinfo = 0;
   sin6.sin6_port = htons(23);
   sin6.sin6_addr = in6addr_loopback;  /* structure assignment */
    . . .
   if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
           . . .

   The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined
   in <netinet/in.h>.  It can be used at declaration time ONLY; for
   example:

      struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;

   Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
   to a previously declared IPv6 address variable.

3.10 Portability Additions

   One simple addition to the sockets API that can help application
   writers is the "struct sockaddr_storage".  This data structure can
   simplify writing code that is portable across multiple address
   families and platforms.  This data structure is designed with the
   following goals.

   - Large enough to accommodate all supported protocol-specific address
      structures.

   - Aligned at an appropriate boundary so that pointers to it can be
      cast as pointers to protocol specific address structures and used
      to access the fields of those structures without alignment
      problems.

   The sockaddr_storage structure contains field ss_family which is of
   type sa_family_t.  When a sockaddr_storage structure is cast to a
   sockaddr structure, the ss_family field of the sockaddr_storage
   structure maps onto the sa_family field of the sockaddr structure.
   When a sockaddr_storage structure is cast as a protocol specific
   address structure, the ss_family field maps onto a field of that
   structure that is of type sa_family_t and that identifies the
   protocol's address family.

   An example implementation design of such a data structure would be as
   follows.

/*
* Desired design of maximum size and alignment
*/
#define _SS_MAXSIZE    128  /* Implementation specific max size */
#define _SS_ALIGNSIZE  (sizeof (int64_t))
                         /* Implementation specific desired alignment */
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE   (_SS_ALIGNSIZE - sizeof (sa_family_t))
#define _SS_PAD2SIZE   (_SS_MAXSIZE - (sizeof (sa_family_t) +
                              _SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
    sa_family_t  ss_family;     /* address family */
    /* Following fields are implementation specific */
    char      __ss_pad1[_SS_PAD1SIZE];
              /* 6 byte pad, this is to make implementation
              /* specific pad up to alignment field that */
              /* follows explicit in the data structure */
    int64_t   __ss_align;     /* field to force desired structure */
               /* storage alignment */
    char      __ss_pad2[_SS_PAD2SIZE];
              /* 112 byte pad to achieve desired size, */
              /* _SS_MAXSIZE value minus size of ss_family */
              /* __ss_pad1, __ss_align fields is 112 */
};

   The above example implementation illustrates a data structure which
   will align on a 64-bit boundary.  An implementation-specific field
   "__ss_align" along with "__ss_pad1" is used to force a 64-bit
   alignment which covers proper alignment good enough for the needs of
   sockaddr_in6 (IPv6), sockaddr_in (IPv4) address data structures.  The
   size of padding field __ss_pad1 depends on the chosen alignment
   boundary.  The size of padding field __ss_pad2 depends on the value
   of overall size chosen for the total size of the structure.  This
   size and alignment are represented in the above example by
   implementation specific (not required) constants _SS_MAXSIZE (chosen
   value 128) and _SS_ALIGNSIZE (with chosen value 8).  Constants
   _SS_PAD1SIZE (derived value 6) and _SS_PAD2SIZE (derived value 112)
   are also for illustration and not required.  The derived values
   assume sa_family_t is 2 bytes.  The implementation specific
   definitions and structure field names above start with an underscore
   to denote implementation private namespace.  Portable code is not
   expected to access or reference those fields or constants.

   On implementations where the sockaddr data structure includes a
   "sa_len" field this data structure would look like this:

/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE -
                            (sizeof (uint8_t) + sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE -
                            (sizeof (uint8_t) + sizeof (sa_family_t) +
                             _SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
    uint8_t      ss_len;        /* address length */
    sa_family_t  ss_family;     /* address family */
    /* Following fields are implementation specific */
    char         __ss_pad1[_SS_PAD1SIZE];
                  /* 6 byte pad, this is to make implementation
                  /* specific pad up to alignment field that */
                  /* follows explicit in the data structure */
    int64_t      __ss_align;  /* field to force desired structure */
                  /* storage alignment */
    char         __ss_pad2[_SS_PAD2SIZE];
                  /* 112 byte pad to achieve desired size, */
                  /* _SS_MAXSIZE value minus size of ss_len, */
                  /* __ss_family, __ss_pad1, __ss_align fields is 112 */
};

4. Interface Identification

   This API uses an interface index (a small positive integer) to
   identify the local interface on which a multicast group is joined
   (Section 5.2).  Additionally, the advanced API [4] uses these same
   interface indexes to identify the interface on which a datagram is
   received, or to specify the interface on which a datagram is to be
   sent.

   Interfaces are normally known by names such as "le0", "sl1", "ppp2",
   and the like.  On Berkeley-derived implementations, when an interface
   is made known to the system, the kernel assigns a unique positive
   integer value (called the interface index) to that interface.  These
   are small positive integers that start at 1.  (Note that 0 is never
   used for an interface index.)  There may be gaps so that there is no
   current interface for a particular positive interface index.

   This API defines two functions that map between an interface name and
   index, a third function that returns all the interface names and
   indexes, and a fourth function to return the dynamic memory allocated
   by the previous function.  How these functions are implemented is

   left up to the implementation.  4.4BSD implementations can implement
   these functions using the existing sysctl() function with the
   NET_RT_IFLIST command.  Other implementations may wish to use ioctl()
   for this purpose.

4.1 Name-to-Index

   The first function maps an interface name into its corresponding
   index.

      #include <net/if.h>

      unsigned int  if_nametoindex(const char *ifname);

   If ifname is the name of an interface, the if_nametoindex() function
   shall return the interface index corresponding to name ifname;
   otherwise, it shall return zero.  No errors are defined.

4.2 Index-to-Name

   The second function maps an interface index into its corresponding
   name.

      #include <net/if.h>

      char  *if_indextoname(unsigned int ifindex, char *ifname);

   When this function is called, the ifname argument shall point to a
   buffer of at least IF_NAMESIZE bytes.  The function shall place in
   this buffer the name of the interface with index ifindex.
   (IF_NAMESIZE is also defined in <net/if.h> and its value includes a
   terminating null byte at the end of the interface name.)  If ifindex
   is an interface index, then the function shall return the value
   supplied in ifname, which points to a buffer now containing the
   interface name.  Otherwise, the function shall return a NULL pointer
   and set errno to indicate the error.  If there is no interface
   corresponding to the specified index, errno is set to ENXIO.  If
   there was a system error (such as running out of memory), errno would
   be set to the proper value (e.g., ENOMEM).

4.3 Return All Interface Names and Indexes

   The if_nameindex structure holds the information about a single
   interface and is defined as a result of including the <net/if.h>
   header.

   struct if_nameindex {
     unsigned int   if_index;  /* 1, 2, ... */
     char          *if_name;   /* null terminated name: "le0", ... */
   };

   The final function returns an array of if_nameindex structures, one
   structure per interface.

      #include <net/if.h>

      struct if_nameindex  *if_nameindex(void);

   The end of the array of structures is indicated by a structure with
   an if_index of 0 and an if_name of NULL.  The function returns a NULL
   pointer upon an error, and would set errno to the appropriate value.

   The memory used for this array of structures along with the interface
   names pointed to by the if_name members is obtained dynamically.
   This memory is freed by the next function.

4.4 Free Memory

   The following function frees the dynamic memory that was allocated by
   if_nameindex().

      #include <net/if.h>

      void  if_freenameindex(struct if_nameindex *ptr);

   The ptr argument shall be a pointer that was returned by
   if_nameindex().  After if_freenameindex() has been called, the
   application shall not use the array of which ptr is the address.

5. Socket Options

   A number of new socket options are defined for IPv6.  All of these
   new options are at the IPPROTO_IPV6 level.  That is, the "level"
   parameter in the getsockopt() and setsockopt() calls is IPPROTO_IPV6
   when using these options.  The constant name prefix IPV6_ is used in
   all of the new socket options.  This serves to clearly identify these
   options as applying to IPv6.

   The declaration for IPPROTO_IPV6, the new IPv6 socket options, and
   related constants defined in this section are obtained by including
   the header <netinet/in.h>.

5.1 Unicast Hop Limit

   A new setsockopt() option controls the hop limit used in outgoing
   unicast IPv6 packets.  The name of this option is IPV6_UNICAST_HOPS,
   and it is used at the IPPROTO_IPV6 layer.  The following example
   illustrates how it is used:

   int  hoplimit = 10;

   if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
                  (char *) &hoplimit, sizeof(hoplimit)) == -1)
       perror("setsockopt IPV6_UNICAST_HOPS");

   When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
   option value given is used as the hop limit for all subsequent
   unicast packets sent via that socket.  If the option is not set, the
   system selects a default value.  The integer hop limit value (called
   x) is interpreted as follows:

      x < -1:        return an error of EINVAL
      x == -1:       use kernel default
      0 <= x <= 255: use x
      x >= 256:      return an error of EINVAL

   The IPV6_UNICAST_HOPS option may be used with getsockopt() to
   determine the hop limit value that the system will use for subsequent
   unicast packets sent via that socket.  For example:

      int  hoplimit;
      socklen_t  len = sizeof(hoplimit);

      if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
                     (char *) &hoplimit, &len) == -1)
          perror("getsockopt IPV6_UNICAST_HOPS");
      else
          printf("Using %d for hop limit.\n", hoplimit);

5.2 Sending and Receiving Multicast Packets

   IPv6 applications may send multicast packets by simply specifying an
   IPv6 multicast address as the destination address, for example in the
   destination address argument of the sendto() function.

   Three socket options at the IPPROTO_IPV6 layer control some of the
   parameters for sending multicast packets.  Setting these options is
   not required: applications may send multicast packets without using
   these options.  The setsockopt() options for controlling the sending
   of multicast packets are summarized below.  These three options can
   also be used with getsockopt().

      IPV6_MULTICAST_IF

         Set the interface to use for outgoing multicast packets.  The
         argument is the index of the interface to use.  If the
         interface index is specified as zero, the system selects the
         interface (for example, by looking up the address in a routing
         table and using the resulting interface).

         Argument type: unsigned int

      IPV6_MULTICAST_HOPS

         Set the hop limit to use for outgoing multicast packets.  (Note
         a separate option - IPV6_UNICAST_HOPS - is provided to set the
         hop limit to use for outgoing unicast packets.)

         The interpretation of the argument is the same as for the
         IPV6_UNICAST_HOPS option:

            x < -1:        return an error of EINVAL
            x == -1:       use kernel default
            0 <= x <= 255: use x
            x >= 256:      return an error of EINVAL

            If IPV6_MULTICAST_HOPS is not set, the default is 1
            (same as IPv4 today)

         Argument type: int

      IPV6_MULTICAST_LOOP

         If a multicast datagram is sent to a group to which the sending
         host itself belongs (on the outgoing interface), a copy of the
         datagram is looped back by the IP layer for local delivery if
         this option is set to 1.  If this option is set to 0 a copy is
         not looped back.  Other option values return an error of
         EINVAL.

         If IPV6_MULTICAST_LOOP is not set, the default is 1 (loopback;
         same as IPv4 today).

         Argument type: unsigned int

   The reception of multicast packets is controlled by the two
   setsockopt() options summarized below.  An error of EOPNOTSUPP is
   returned if these two options are used with getsockopt().

      IPV6_JOIN_GROUP

         Join a multicast group on a specified local interface.
         If the interface index is specified as 0,
         the kernel chooses the local interface.
         For example, some kernels look up the multicast group
         in the normal IPv6 routing table and use the resulting
         interface.

         Argument type: struct ipv6_mreq

      IPV6_LEAVE_GROUP

         Leave a multicast group on a specified interface.
         If the interface index is specified as 0, the system
         may choose a multicast group membership to drop by
         matching the multicast address only.

         Argument type: struct ipv6_mreq

   The argument type of both of these options is the ipv6_mreq
   structure, defined as a result of including the <netinet/in.h>
   header;

   struct ipv6_mreq {
       struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
       unsigned int    ipv6mr_interface; /* interface index */
   };

   Note that to receive multicast datagrams a process must join the
   multicast group to which datagrams will be sent.  UDP applications
   must also bind the UDP port to which datagrams will be sent.  Some
   processes also bind the multicast group address to the socket, in
   addition to the port, to prevent other datagrams destined to that
   same port from being delivered to the socket.

5.3 IPV6_V6ONLY option for AF_INET6 Sockets

   This socket option restricts AF_INET6 sockets to IPv6 communications
   only.  As stated in section <3.7 Compatibility with IPv4 Nodes>,
   AF_INET6 sockets may be used for both IPv4 and IPv6 communications.
   Some applications may want to restrict their use of an AF_INET6
   socket to IPv6 communications only.  For these applications the
   IPV6_V6ONLY socket option is defined.  When this option is turned on,
   the socket can be used to send and receive IPv6 packets only.  This
   is an IPPROTO_IPV6 level option.  This option takes an int value.
   This is a boolean option.  By default this option is turned off.

   Here is an example of setting this option:

      int on = 1;

      if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY,
                     (char *)&on, sizeof(on)) == -1)
          perror("setsockopt IPV6_V6ONLY");
      else
          printf("IPV6_V6ONLY set\n");

   Note - This option has no effect on the use of IPv4 Mapped addresses
   which enter a node as a valid IPv6 addresses for IPv6 communications
   as defined by Stateless IP/ICMP Translation Algorithm (SIIT) [5].

   An example use of this option is to allow two versions of the same
   server process to run on the same port, one providing service over
   IPv6, the other providing the same service over IPv4.

6. Library Functions

   New library functions are needed to perform a variety of operations
   with IPv6 addresses.  Functions are needed to lookup IPv6 addresses
   in the Domain Name System (DNS).  Both forward lookup (nodename-to-
   address translation) and reverse lookup (address-to-nodename
   translation) need to be supported.  Functions are also needed to
   convert IPv6 addresses between their binary and textual form.

   We note that the two existing functions, gethostbyname() and
   gethostbyaddr(), are left as-is.  New functions are defined to handle
   both IPv4 and IPv6 addresses.

   The commonly used function gethostbyname() is inadequate for many
   applications, first because it provides no way for the caller to
   specify anything about the types of addresses desired (IPv4 only,
   IPv6 only, IPv4-mapped IPv6 are OK, etc.), and second because many
   implementations of this function are not thread safe.  RFC 2133

   defined a function named gethostbyname2() but this function was also
   inadequate, first because its use required setting a global option
   (RES_USE_INET6) when IPv6 addresses were required, and second because
   a flag argument is needed to provide the caller with additional
   control over the types of addresses required.  The gethostbyname2()
   function was deprecated in RFC 2553 and is no longer part of the
   basic API.

6.1 Protocol-Independent Nodename and Service Name Translation

   Nodename-to-address translation is done in a protocol-independent
   fashion using the getaddrinfo() function.

#include <sys/socket.h>
#include <netdb.h>

int getaddrinfo(const char *nodename, const char *servname,
                const struct addrinfo *hints, struct addrinfo **res);

void freeaddrinfo(struct addrinfo *ai);

struct addrinfo {
  int     ai_flags;     /* AI_PASSIVE, AI_CANONNAME,
                           AI_NUMERICHOST, .. */
  int     ai_family;    /* AF_xxx */
  int     ai_socktype;  /* SOCK_xxx */
  int     ai_protocol;  /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
  socklen_t  ai_addrlen;   /* length of ai_addr */
  char   *ai_canonname; /* canonical name for nodename */
  struct sockaddr  *ai_addr; /* binary address */
  struct addrinfo  *ai_next; /* next structure in linked list */
};

   The getaddrinfo() function translates the name of a service location
   (for example, a host name) and/or a service name and returns a set of
   socket addresses and associated information to be used in creating a
   socket with which to address the specified service.

   The nodename and servname arguments are either null pointers or
   pointers to null-terminated strings.  One or both of these two
   arguments must be a non-null pointer.

   The format of a valid name depends on the address family or families.
   If a specific family is not given and the name could be interpreted
   as valid within multiple supported families, the implementation will
   attempt to resolve the name in all supported families and, in absence
   of errors, one or more results shall be returned.

   If the nodename argument is not null, it can be a descriptive name or
   can be an address string.  If the specified address family is
   AF_INET, AF_INET6, or AF_UNSPEC, valid descriptive names include host
   names. If the specified address family is AF_INET or AF_UNSPEC,
   address strings using Internet standard dot notation as specified in
   inet_addr() are valid.  If the specified address family is AF_INET6
   or AF_UNSPEC, standard IPv6 text forms described in inet_pton() are
   valid.

   If nodename is not null, the requested service location is named by
   nodename; otherwise, the requested service location is local to the
   caller.

   If servname is null, the call shall return network-level addresses
   for the specified nodename.  If servname is not null, it is a null-
   terminated character string identifying the requested service.  This
   can be either a descriptive name or a numeric representation suitable
   for use with the address family or families.  If the specified
   address family is AF_INET, AF_INET6 or AF_UNSPEC, the service can be
   specified as a string specifying a decimal port number.

   If the argument hints is not null, it refers to a structure
   containing input values that may direct the operation by providing
   options and by limiting the returned information to a specific socket
   type, address family and/or protocol.  In this hints structure every
   member other than ai_flags, ai_family, ai_socktype and ai_protocol
   shall be set to zero or a null pointer.  A value of AF_UNSPEC for
   ai_family means that the caller shall accept any address family.  A
   value of zero for ai_socktype means that the caller shall accept any
   socket type.  A value of zero for ai_protocol means that the caller
   shall accept any protocol.  If hints is a null pointer, the behavior
   shall be as if it referred to a structure containing the value zero
   for the ai_flags, ai_socktype and ai_protocol fields, and AF_UNSPEC
   for the ai_family field.

   Note:

   1. If the caller handles only TCP and not UDP, for example, then the
      ai_protocol member of the hints structure should be set to
      IPPROTO_TCP when getaddrinfo() is called.

   2. If the caller handles only IPv4 and not IPv6, then the ai_family
      member of the hints structure should be set to AF_INET when
      getaddrinfo() is called.

   The ai_flags field to which hints parameter points shall be set to
   zero or be the bitwise-inclusive OR of one or more of the values
   AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST, AI_NUMERICSERV,
   AI_V4MAPPED, AI_ALL, and AI_ADDRCONFIG.

   If the AI_PASSIVE flag is specified, the returned address information
   shall be suitable for use in binding a socket for accepting incoming
   connections for the specified service (i.e., a call to bind()).  In
   this case, if the nodename argument is null, then the IP address
   portion of the socket address structure shall be set to INADDR_ANY
   for an IPv4 address or IN6ADDR_ANY_INIT for an IPv6 address.  If the
   AI_PASSIVE flag is not specified, the returned address information
   shall be suitable for a call to connect() (for a connection-mode
   protocol) or for a call to connect(), sendto() or sendmsg() (for a
   connectionless protocol).  In this case, if the nodename argument is
   null, then the IP address portion of the socket address structure
   shall be set to the loopback address.  This flag is ignored if the
   nodename argument is not null.

   If the AI_CANONNAME flag is specified and the nodename argument is
   not null, the function shall attempt to determine the canonical name
   corresponding to nodename (for example, if nodename is an alias or
   shorthand notation for a complete name).

   If the AI_NUMERICHOST flag is specified, then a non-null nodename
   string supplied shall be a numeric host address string.  Otherwise,
   an [EAI_NONAME] error is returned.  This flag shall prevent any type
   of name resolution service (for example, the DNS) from being invoked.

   If the AI_NUMERICSERV flag is specified, then a non-null servname
   string supplied shall be a numeric port string.  Otherwise, an
   [EAI_NONAME] error shall be returned.  This flag shall prevent any
   type of name resolution service (for example, NIS+) from being
   invoked.

   If the AI_V4MAPPED flag is specified along with an ai_family of
   AF_INET6, then getaddrinfo() shall return IPv4-mapped IPv6 addresses
   on finding no matching IPv6 addresses (ai_addrlen shall be 16).

      For example, when using the DNS, if no AAAA records are found then
      a query is made for A records and any found are returned as IPv4-
      mapped IPv6 addresses.

   The AI_V4MAPPED flag shall be ignored unless ai_family equals
   AF_INET6.

   If the AI_ALL flag is used with the AI_V4MAPPED flag, then
   getaddrinfo() shall return all matching IPv6 and IPv4 addresses.

      For example, when using the DNS, queries are made for both AAAA
      records and A records, and getaddrinfo() returns the combined
      results of both queries.  Any IPv4 addresses found are returned as
      IPv4-mapped IPv6 addresses.

   The AI_ALL flag without the AI_V4MAPPED flag is ignored.

      Note:

      When ai_family is not specified (AF_UNSPEC), AI_V4MAPPED and
      AI_ALL flags will only be used if AF_INET6 is supported.

   If the AI_ADDRCONFIG flag is specified, IPv4 addresses shall be
   returned only if an IPv4 address is configured on the local system,
   and IPv6 addresses shall be returned only if an IPv6 address is
   configured on the local system.  The loopback address is not
   considered for this case as valid as a configured address.

      For example, when using the DNS, a query for AAAA records should
      occur only if the node has at least one IPv6 address configured
      (other than IPv6 loopback) and a query for A records should occur
      only if the node has at least one IPv4 address configured (other
      than the IPv4 loopback).

   The ai_socktype field to which argument hints points specifies the
   socket type for the service, as defined for socket().  If a specific
   socket type is not given (for example, a value of zero) and the
   service name could be interpreted as valid with multiple supported
   socket types, the implementation shall attempt to resolve the service
   name for all supported socket types and, in the absence of errors,
   all possible results shall be returned.  A non-zero socket type value
   shall limit the returned information to values with the specified
   socket type.

   If the ai_family field to which hints points has the value AF_UNSPEC,
   addresses shall be returned for use with any address family that can
   be used with the specified nodename and/or servname.  Otherwise,
   addresses shall be returned for use only with the specified address
   family.  If ai_family is not AF_UNSPEC and ai_protocol is not zero,
   then addresses are returned for use only with the specified address
   family and protocol; the value of ai_protocol shall be interpreted as
   in a call to the socket() function with the corresponding values of
   ai_family and ai_protocol.

   The freeaddrinfo() function frees one or more addrinfo structures
   returned by getaddrinfo(), along with any additional storage
   associated with those structures (for example, storage pointed to by
   the ai_canonname and ai_addr fields; an application must not

   reference this storage after the associated addrinfo structure has
   been freed).  If the ai_next field of the structure is not null, the
   entire list of structures is freed.  The freeaddrinfo() function must
   support the freeing of arbitrary sublists of an addrinfo list
   originally returned by getaddrinfo().

   Functions getaddrinfo() and freeaddrinfo() must be thread-safe.

   A zero return value for getaddrinfo() indicates successful
   completion; a non-zero return value indicates failure.  The possible
   values for the failures are listed below under Error Return Values.

   Upon successful return of getaddrinfo(), the location to which res
   points shall refer to a linked list of addrinfo structures, each of
   which shall specify a socket address and information for use in
   creating a socket with which to use that socket address.  The list
   shall include at least one addrinfo structure.  The ai_next field of
   each structure contains a pointer to the next structure on the list,
   or a null pointer if it is the last structure on the list.  Each
   structure on the list shall include values for use with a call to the
   socket() function, and a socket address for use with the connect()
   function or, if the AI_PASSIVE flag was specified, for use with the
   bind() function.  The fields ai_family, ai_socktype, and ai_protocol
   shall be usable as the arguments to the socket() function to create a
   socket suitable for use with the returned address.  The fields
   ai_addr and ai_addrlen are usable as the arguments to the connect()
   or bind() functions with such a socket, according to the AI_PASSIVE
   flag.

   If nodename is not null, and if requested by the AI_CANONNAME flag,
   the ai_canonname field of the first returned addrinfo structure shall
   point to a null-terminated string containing the canonical name
   corresponding to the input nodename; if the canonical name is not
   available, then ai_canonname shall refer to the nodename argument or
   a string with the same contents.  The contents of the ai_flags field
   of the returned structures are undefined.

   All fields in socket address structures returned by getaddrinfo()
   that are not filled in through an explicit argument (for example,
   sin6_flowinfo) shall be set to zero.

   Note: This makes it easier to compare socket address structures.

   Error Return Values:

   The getaddrinfo() function shall fail and return the corresponding
   value if:

   [EAI_AGAIN]     The name could not be resolved at this time.  Future
                   attempts may succeed.

   [EAI_BADFLAGS]  The flags parameter had an invalid value.

   [EAI_FAIL]      A non-recoverable error occurred when attempting to
                   resolve the name.

   [EAI_FAMILY]    The address family was not recognized.

   [EAI_MEMORY]    There was a memory allocation failure when trying to
                   allocate storage for the return value.

   [EAI_NONAME]    The name does not resolve for the supplied
                   parameters.  Neither nodename nor servname were
                   supplied.  At least one of these must be supplied.

   [EAI_SERVICE]   The service passed was not recognized for the
                   specified socket type.

   [EAI_SOCKTYPE]  The intended socket type was not recognized.

   [EAI_SYSTEM]    A system error occurred; the error code can be found
                   in errno.

   The gai_strerror() function provides a descriptive text string
   corresponding to an EAI_xxx error value.

      #include <netdb.h>

      const char *gai_strerror(int ecode);

   The argument is one of the EAI_xxx values defined for the
   getaddrinfo() and getnameinfo() functions.  The return value points
   to a string describing the error.  If the argument is not one of the
   EAI_xxx values, the function still returns a pointer to a string
   whose contents indicate an unknown error.

6.2 Socket Address Structure to Node Name and Service Name

The getnameinfo() function is used to translate the contents of a
socket address structure to a node name and/or service name.

   #include <sys/socket.h>
   #include <netdb.h>

   int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                       char *node, socklen_t nodelen,
                       char *service, socklen_t servicelen,
                         int flags);

   The getnameinfo() function shall translate a socket address to a node
   name and service location, all of which are defined as in
   getaddrinfo().

   The sa argument points to a socket address structure to be
   translated.

   The salen argument holds the size of the socket address structure
   pointed to by sa.

   If the socket address structure contains an IPv4-mapped IPv6 address
   or an IPv4-compatible IPv6 address, the implementation shall extract
   the embedded IPv4 address and lookup the node name for that IPv4
   address.

      Note: The IPv6 unspecified address ("::") and the IPv6 loopback
      address ("::1") are not IPv4-compatible addresses.  If the address
      is the IPv6 unspecified address ("::"), a lookup is not performed,
      and the [EAI_NONAME] error is returned.

   If the node argument is non-NULL and the nodelen argument is nonzero,
   then the node argument points to a buffer able to contain up to
   nodelen characters that receives the node name as a null-terminated
   string.  If the node argument is NULL or the nodelen argument is
   zero, the node name shall not be returned.  If the node's name cannot
   be located, the numeric form of the node's address is returned
   instead of its name.

   If the service argument is non-NULL and the servicelen argument is
   non-zero, then the service argument points to a buffer able to
   contain up to servicelen bytes that receives the service name as a
   null-terminated string.  If the service argument is NULL or the
   servicelen argument is zero, the service name shall not be returned.
   If the service's name cannot be located, the numeric form of the
   service address (for example, its port number) shall be returned
   instead of its name.

   The arguments node and service cannot both be NULL.

   The flags argument is a flag that changes the default actions of the
   function.  By default the fully-qualified domain name (FQDN) for the
   host shall be returned, but:

   -  If the flag bit NI_NOFQDN is set, only the node name portion of
      the FQDN shall be returned for local hosts.

   -  If the flag bit NI_NUMERICHOST is set, the numeric form of the
      host's address shall be returned instead of its name, under all
      circumstances.

   -  If the flag bit NI_NAMEREQD is set, an error shall be returned if
      the host's name cannot be located.

   -  If the flag bit NI_NUMERICSERV is set, the numeric form of the
      service address shall be returned (for example, its port number)
      instead of its name, under all circumstances.

   -  If the flag bit NI_DGRAM is set, this indicates that the service
      is a datagram service (SOCK_DGRAM).  The default behavior shall
      assume that the service is a stream service (SOCK_STREAM).

   Note:

   1. The NI_NUMERICxxx flags are required to support the "-n" flags
      that many commands provide.

   2. The NI_DGRAM flag is required for the few AF_INET and AF_INET6
      port numbers (for example, [512,514]) that represent different
      services for UDP and TCP.

   The getnameinfo() function shall be thread safe.

   A zero return value for getnameinfo() indicates successful
   completion; a non-zero return value indicates failure.

   Upon successful completion, getnameinfo() shall return the node and
   service names, if requested, in the buffers provided.  The returned
   names are always null-terminated strings.

   Error Return Values:

   The getnameinfo() function shall fail and return the corresponding
   value if:

   [EAI_AGAIN]    The name could not be resolved at this time.
                  Future attempts may succeed.

   [EAI_BADFLAGS] The flags had an invalid value.

   [EAI_FAIL]     A non-recoverable error occurred.

   [EAI_FAMILY]   The address family was not recognized or the address
                  length was invalid for the specified family.

   [EAI_MEMORY]   There was a memory allocation failure.

   [EAI_NONAME]   The name does not resolve for the supplied parameters.
                  NI_NAMEREQD is set and the host's name cannot be
                  located, or both nodename and servname were null.

   [EAI_OVERFLOW] An argument buffer overflowed.

   [EAI_SYSTEM]   A system error occurred.  The error code can be found
                  in errno.

6.3 Address Conversion Functions

   The two IPv4 functions inet_addr() and inet_ntoa() convert an IPv4
   address between binary and text form.  IPv6 applications need similar
   functions.  The following two functions convert both IPv6 and IPv4
   addresses:

   #include <arpa/inet.h>

   int inet_pton(int af, const char *src, void *dst);

   const char *inet_ntop(int af, const void *src,
                            char *dst, socklen_t size);

   The inet_pton() function shall convert an address in its standard
   text presentation form into its numeric binary form.  The af argument
   shall specify the family of the address.  The AF_INET and AF_INET6
   address families shall be supported.  The src argument points to the
   string being passed in.  The dst argument points to a buffer into
   which the function stores the numeric address; this shall be large
   enough to hold the numeric address (32 bits for AF_INET, 128 bits for
   AF_INET6).  The inet_pton() function shall return 1 if the conversion

   succeeds, with the address pointed to by dst in network byte order.
   It shall return 0 if the input is not a valid IPv4 dotted-decimal
   string or a valid IPv6 address string, or -1 with errno set to
   EAFNOSUPPORT if the af argument is unknown.

   If the af argument of inet_pton() is AF_INET, the src string shall be
   in the standard IPv4 dotted-decimal form:

      ddd.ddd.ddd.ddd

   where "ddd" is a one to three digit decimal number between 0 and 255.
   The inet_pton() function does not accept other formats (such as the
   octal numbers, hexadecimal numbers, and fewer than four numbers that
   inet_addr() accepts).

   If the af argument of inet_pton() is AF_INET6, the src string shall
   be in one of the standard IPv6 text forms defined in Section 2.2 of
   the addressing architecture specification [2].

   The inet_ntop() function shall convert a numeric address into a text
   string suitable for presentation.  The af argument shall specify the
   family of the address.  This can be AF_INET or AF_INET6.  The src
   argument points to a buffer holding an IPv4 address if the af
   argument is AF_INET, or an IPv6 address if the af argument is
   AF_INET6; the address must be in network byte order.  The dst
   argument points to a buffer where the function stores the resulting
   text string; it shall not be NULL.  The size argument specifies the
   size of this buffer, which shall be large enough to hold the text
   string (INET_ADDRSTRLEN characters for IPv4, INET6_ADDRSTRLEN
   characters for IPv6).

   In order to allow applications to easily declare buffers of the
   proper size to store IPv4 and IPv6 addresses in string form, the
   following two constants are defined in <netinet/in.h>:

      #define INET_ADDRSTRLEN    16
      #define INET6_ADDRSTRLEN   46

   The inet_ntop() function shall return a pointer to the buffer
   containing the text string if the conversion succeeds, and NULL
   otherwise.  Upon failure, errno is set to EAFNOSUPPORT if the af
   argument is invalid or ENOSPC if the size of the result buffer is
   inadequate.

6.4 Address Testing Macros

   The following macros can be used to test for special IPv6 addresses.

   #include <netinet/in.h>

   int  IN6_IS_ADDR_UNSPECIFIED (const struct in6_addr *);
   int  IN6_IS_ADDR_LOOPBACK    (const struct in6_addr *);
   int  IN6_IS_ADDR_MULTICAST   (const struct in6_addr *);
   int  IN6_IS_ADDR_LINKLOCAL   (const struct in6_addr *);
   int  IN6_IS_ADDR_SITELOCAL   (const struct in6_addr *);
   int  IN6_IS_ADDR_V4MAPPED    (const struct in6_addr *);
   int  IN6_IS_ADDR_V4COMPAT    (const struct in6_addr *);

   int  IN6_IS_ADDR_MC_NODELOCAL(const struct in6_addr *);
   int  IN6_IS_ADDR_MC_LINKLOCAL(const struct in6_addr *);
   int  IN6_IS_ADDR_MC_SITELOCAL(const struct in6_addr *);
   int  IN6_IS_ADDR_MC_ORGLOCAL (const struct in6_addr *);
   int  IN6_IS_ADDR_MC_GLOBAL   (const struct in6_addr *);

   The first seven macros return true if the address is of the specified
   type, or false otherwise.  The last five test the scope of a
   multicast address and return true if the address is a multicast
   address of the specified scope or false if the address is either not
   a multicast address or not of the specified scope.

   Note that IN6_IS_ADDR_LINKLOCAL and IN6_IS_ADDR_SITELOCAL return true
   only for the two types of local-use IPv6 unicast addresses (Link-
   Local and Site-Local) defined in [2], and that by this definition,
   the IN6_IS_ADDR_LINKLOCAL macro returns false for the IPv6 loopback
   address (::1).  These two macros do not return true for IPv6
   multicast addresses of either link-local scope or site-local scope.

7. Summary of New Definitions

   The following list summarizes the constants, structure, and extern
   definitions discussed in this memo, sorted by header.

<net/if.h>      IF_NAMESIZE
<net/if.h>      struct if_nameindex{};

<netdb.h>       AI_ADDRCONFIG
<netdb.h>       AI_ALL
<netdb.h>       AI_CANONNAME
<netdb.h>       AI_NUMERICHOST
<netdb.h>       AI_NUMERICSERV
<netdb.h>       AI_PASSIVE
<netdb.h>       AI_V4MAPPED

<netdb.h>       EAI_AGAIN
<netdb.h>       EAI_BADFLAGS
<netdb.h>       EAI_FAIL
<netdb.h>       EAI_FAMILY
<netdb.h>       EAI_MEMORY
<netdb.h>       EAI_NONAME
<netdb.h>       EAI_OVERFLOW
<netdb.h>       EAI_SERVICE
<netdb.h>       EAI_SOCKTYPE
<netdb.h>       EAI_SYSTEM
<netdb.h>       NI_DGRAM
<netdb.h>       NI_NAMEREQD
<netdb.h>       NI_NOFQDN
<netdb.h>       NI_NUMERICHOST
<netdb.h>       NI_NUMERICSERV
<netdb.h>       struct addrinfo{};

<netinet/in.h>  IN6ADDR_ANY_INIT
<netinet/in.h>  IN6ADDR_LOOPBACK_INIT
<netinet/in.h>  INET6_ADDRSTRLEN
<netinet/in.h>  INET_ADDRSTRLEN
<netinet/in.h>  IPPROTO_IPV6
<netinet/in.h>  IPV6_JOIN_GROUP
<netinet/in.h>  IPV6_LEAVE_GROUP
<netinet/in.h>  IPV6_MULTICAST_HOPS
<netinet/in.h>  IPV6_MULTICAST_IF
<netinet/in.h>  IPV6_MULTICAST_LOOP
<netinet/in.h>  IPV6_UNICAST_HOPS
<netinet/in.h>  IPV6_V6ONLY
<netinet/in.h>  SIN6_LEN
<netinet/in.h>  extern const struct in6_addr in6addr_any;
<netinet/in.h>  extern const struct in6_addr in6addr_loopback;
<netinet/in.h>  struct in6_addr{};
<netinet/in.h>  struct ipv6_mreq{};
<netinet/in.h>  struct sockaddr_in6{};

<sys/socket.h>  AF_INET6
<sys/socket.h>  PF_INET6
<sys/socket.h>  struct sockaddr_storage;

   The following list summarizes the function and macro prototypes
   discussed in this memo, sorted by header.

<arpa/inet.h>   int inet_pton(int, const char *, void *);
<arpa/inet.h>   const char *inet_ntop(int, const void *,
                               char *, socklen_t);

<net/if.h>      char *if_indextoname(unsigned int, char *);
<net/if.h>      unsigned int if_nametoindex(const char *);
<net/if.h>      void if_freenameindex(struct if_nameindex *);
<net/if.h>      struct if_nameindex *if_nameindex(void);

<netdb.h>       int getaddrinfo(const char *, const char *,
                                const struct addrinfo *,
                                struct addrinfo **);
<netdb.h>       int getnameinfo(const struct sockaddr *, socklen_t,
                  char *, socklen_t, char *, socklen_t, int);
<netdb.h>       void freeaddrinfo(struct addrinfo *);
<netdb.h>       const char *gai_strerror(int);

<netinet/in.h>  int IN6_IS_ADDR_LINKLOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_LOOPBACK(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MC_GLOBAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MC_LINKLOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MC_NODELOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MC_ORGLOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MC_SITELOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_MULTICAST(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_SITELOCAL(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_UNSPECIFIED(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_V4COMPAT(const struct in6_addr *);
<netinet/in.h>  int IN6_IS_ADDR_V4MAPPED(const struct in6_addr *);

8. Security Considerations

   IPv6 provides a number of new security mechanisms, many of which need
   to be accessible to applications.  Companion memos detailing the
   extensions to the socket interfaces to support IPv6 security are
   being written.

9. Changes from RFC 2553

   1. Add brief description of the history of this API and its relation
      to the Open Group/IEEE/ISO standards.

   2. Alignments with [3].

   3. Removed all references to getipnodebyname() and getipnodebyaddr(),
      which are deprecated in favor of getaddrinfo() and getnameinfo().

   4. Added IPV6_V6ONLY IP level socket option to permit nodes to not
      process IPv4 packets as IPv4 Mapped addresses in implementations.

   5. Added SIIT to references and added new contributors.

   6. In previous versions of this specification, the sin6_flowinfo
      field was associated with the IPv6 traffic class and flow label,
      but its usage was not completely specified.  The complete
      definition of the sin6_flowinfo field, including its association
      with the traffic class or flow label, is now deferred to a future
      specification.

10. Acknowledgments

   This specification's evolution and completeness were significantly
   influenced by the efforts of Richard Stevens, who has passed on.
   Richard's wisdom and talent made the specification what it is today.
   The co-authors will long think of Richard with great respect.

   Thanks to the many people who made suggestions and provided feedback
   to this document, including:

   Werner Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew
   Cherenson, Alex Conta, Alan Cox, Steve Deering, Richard Draves,
   Francis Dupont, Robert Elz, Brian Haberman, Jun-ichiro itojun Hagino,
   Marc Hasson, Tom Herbert, Bob Hinden, Wan-Yen Hsu, Christian Huitema,
   Koji Imada, Markus Jork, Ron Lee, Alan Lloyd, Charles Lynn, Dan
   McDonald, Dave Mitton, Finnbarr Murphy, Thomas Narten, Josh Osborne,
   Craig Partridge, Jean-Luc Richier, Bill Sommerfield, Erik Scoredos,
   Keith Sklower, JINMEI Tatuya, Dave Thaler, Matt Thomas, Harvey
   Thompson, Dean D. Throop, Karen Tracey, Glenn Trewitt, Paul Vixie,
   David Waitzman, Carl Williams, Kazu Yamamoto, Vlad Yasevich, Stig
   Venaas, and Brian Zill.

   The getaddrinfo() and getnameinfo() functions are taken from an
   earlier document by Keith Sklower.  As noted in that document,
   William Durst, Steven Wise, Michael Karels, and Eric Allman provided
   many useful discussions on the subject of protocol-independent name-
   to-address translation, and reviewed early versions of Keith
   Sklower's original proposal.  Eric Allman implemented the first
   prototype of getaddrinfo().  The observation that specifying the pair
   of name and service would suffice for connecting to a service
   independent of protocol details was made by Marshall Rose in a
   proposal to X/Open for a "Uniform Network Interface".

   Craig Metz, Jack McCann, Erik Nordmark, Tim Hartrick, and Mukesh
   Kacker made many contributions to this document.  Ramesh Govindan
   made a number of contributions and co-authored an earlier version of
   this memo.

11. References

   [1]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.

   [2]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

   [3]  IEEE Std. 1003.1-2001 Standard for Information Technology --
        Portable Operating System Interface (POSIX). Open Group
        Technical Standard: Base Specifications, Issue 6, December 2001.
        ISO/IEC 9945:2002.  http://www.opengroup.org/austin

   [4]  Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6", RFC
        2292, February 1998.

   [5]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
        RFC 2765, February 2000.

   [6]  The Open Group Base Working Group
        http://www.opengroup.org/platform/base.html

12. Authors' Addresses

   Bob Gilligan
   Intransa, Inc.
   2870 Zanker Rd.
   San Jose, CA 95134

   Phone: 408-678-8647
   EMail: gilligan@intransa.com

   Susan Thomson
   Cisco Systems
   499 Thornall Street, 8th floor
   Edison, NJ 08837

   Phone: 732-635-3086
   EMail:  sethomso@cisco.com

   Jim Bound
   Hewlett-Packard Company
   110 Spitbrook Road ZKO3-3/W20
   Nashua, NH 03062

   Phone: 603-884-0062
   EMail: Jim.Bound@hp.com

   Jack McCann
   Hewlett-Packard Company
   110 Spitbrook Road ZKO3-3/W20
   Nashua, NH 03062

   Phone: 603-884-2608
   EMail: Jack.McCann@hp.com

13. Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
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   The limited permissions granted above are perpetual and will not be
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

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Internet Society.