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Network Working Group                                        R. Gilligan
Request for Comments: 2553                                      FreeGate
Obsoletes: 2133                                               S. Thomson
Category: Informational                                         Bellcore
                                                                J. Bound
                                                                  Compaq
                                                              W. Stevens
                                                              Consultant
                                                              March 1999


               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

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

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 [4].










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Table of Contents

   1. Introduction.................................................3
   2. Design Considerations........................................3
   2.1 What Needs to be Changed....................................4
   2.2 Data Types..................................................5
   2.3 Headers.....................................................5
   2.4 Structures..................................................5
   3. Socket Interface.............................................6
   3.1 IPv6 Address Family and Protocol Family.....................6
   3.2 IPv6 Address Structure......................................6
   3.3 Socket Address Structure for 4.3BSD-Based Systems...........7
   3.4 Socket Address Structure for 4.4BSD-Based Systems...........8
   3.5 The Socket Functions........................................9
   3.6 Compatibility with IPv4 Applications.......................10
   3.7 Compatibility with IPv4 Nodes..............................10
   3.8 IPv6 Wildcard Address......................................11
   3.9 IPv6 Loopback Address......................................12
   3.10 Portability Additions.....................................13
   4. Interface Identification....................................16
   4.1 Name-to-Index..............................................16
   4.2 Index-to-Name..............................................17
   4.3 Return All Interface Names and Indexes.....................17
   4.4 Free Memory................................................18
   5. Socket Options..............................................18
   5.1 Unicast Hop Limit..........................................18
   5.2 Sending and Receiving Multicast Packets....................19
   6. Library Functions...........................................21
   6.1 Nodename-to-Address Translation............................21
   6.2 Address-To-Nodename Translation............................24
   6.3 Freeing memory for getipnodebyname and getipnodebyaddr.....26
   6.4 Protocol-Independent Nodename and Service Name Translation.26
   6.5 Socket Address Structure to Nodename and Service Name......29
   6.6 Address Conversion Functions...............................31
   6.7 Address Testing Macros.....................................32
   7. Summary of New Definitions..................................33
   8. Security Considerations.....................................35
   9. Year 2000 Considerations....................................35
   Changes From RFC 2133..........................................35
   Acknowledgments................................................38
   References.....................................................39
   Authors' Addresses.............................................40
   Full Copyright Statement.......................................41








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1. Introduction

   While IPv4 addresses are 32 bits long, IPv6 interfaces are identified
   by 128-bit addresses.  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 (e.g., traffic class and flowlabel), 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.

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
        additonal mechanism for implementations to verify this is to
        verify the new symbols are protected by Feature Test Macros as
        described in IEEE Std 1003.1.  (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.




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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 name-to-address translation functions in the socket interface are
   gethostbyname() and gethostbyaddr().  These are left as is and new
   functions are defined to support IPv4 and IPv6.  Additionally, the
   POSIX 1003.g draft [3] specifies a new nodename-to-address
   translation function which is protocol independent.  This function
   can also be used with IPv4 and IPv6.






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   The 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.
   New interfaces are needed to support the IPv6 traffic class, flow
   label, and hop limit header fields.  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.  These extensions are described in [4].

2.2 Data Types

   The data types of the structure elements given in this memo are
   intended to be examples, not absolute requirements.  Whenever
   possible, data types from Draft 6.6 (March 1997) of POSIX 1003.1g are
   used: uintN_t means an unsigned integer of exactly N bits (e.g.,
   uint16_t).  We also assume the argument data types from 1003.1g when
   possible (e.g., the final argument to setsockopt() is a size_t
   value).  Whenever buffer sizes are specified, the POSIX 1003.1 size_t
   data type is used (e.g., the two length arguments to getnameinfo()).

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 IEEE Std 1003.1.  (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.








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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 PF_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:











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   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 traffic class & flow info */
    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.







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   The sin6_flowinfo field is a 32-bit field that contains two pieces of
   information: the traffic class and the flow label.  The contents and
   interpretation of this member is specified in [1].  The sin6_flowinfo
   field SHOULD be set to zero by an implementation prior to using the
   sockaddr_in6 structure by an application on receive operations.

   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 of the address carried in the
   sin6_addr field.  For a link scope sin6_addr sin6_scope_id would be
   an interface index.  For a site scope sin6_addr, sin6_scope_id would
   be a site identifier.  The mapping of sin6_scope_id to an interface
   or set of interfaces is left to implementation and future
   specifications on the subject of site identifiers.

   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.











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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(PF_INET, SOCK_STREAM, 0);

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

      s = socket(PF_INET, SOCK_DGRAM, 0);

   Applications may create IPv6/TCP and IPv6/UDP sockets by simply using
   the constant PF_INET6 instead of PF_INET in the first argument.  For
   example, to create an IPv6/TCP socket, applications make the call:

      s = socket(PF_INET6, SOCK_STREAM, 0);

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

      s = socket(PF_INET6, SOCK_DGRAM, 0);







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   Once the application has created a PF_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 PF_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 PF_INET sockets and the sockaddr_in address
   structure.  Applications must be able to create IPv4/TCP and IPv4/UDP
   sockets using the PF_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



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   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
   getipnodebyname() function when the specified host has only IPv4
   addresses (as described in Section 6.1).

   Applications may use PF_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 PF_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 6.7, 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;






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   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.







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   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 portable across multiple address families and
   platforms.  This data structure is designed with the following goals.

      - It has a large enough implementation specific maximum size to
        store the desired set of protocol specific socket address data
        structures. Specifically, it is at least large enough to
        accommodate sockaddr_in and sockaddr_in6 and possibly other
        protocol specific socket addresses too.
      - It is aligned at an appropriate boundary so protocol specific
        socket address data structure pointers can be cast to it and
        access their fields without alignment problems. (e.g. pointers
        to sockaddr_in6 and/or sockaddr_in can be cast to it and access
        fields without alignment problems).



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      - It has the initial field(s) isomorphic to the fields of the
        "struct sockaddr" data structure on that implementation which
        can be used as a discriminants for deriving the protocol in use.
        These initial field(s) would on most implementations either be a
        single field of type "sa_family_t" (isomorphic to sa_family
        field, 16 bits) or two fields of type uint8_t and sa_family_t
        respectively, (isomorphic to sa_len and sa_family_t, 8 bits
        each).

   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 */
};

   On implementations where 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 (sa_family_t)+



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                              _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 */
};

   The above example implementation illustrates a data structure which
   will align on a 64 bit boundary. An implementation specific field
   "__ss_align" along "__ss_pad1" is used to force a 64-bit alignment
   which covers proper alignment good enough for needs of sockaddr_in6
   (IPv6), sockaddr_in (IPv4) address data structures.  The size of
   padding fields __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_ALIGNMENT (with chosen value 8).  Constants _SS_PAD1SIZE (derived
   value 6) and _SS_PAD2SIZE (derived value 112) are also for
   illustration and not required.  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.

   The sockaddr_storage structure solves the problem of declaring
   storage for automatic variables which is large enough and aligned
   enough for storing socket address data structure of any family. For
   example, code with a file descriptor and without the context of the
   address family can pass a pointer to a variable of this type where a
   pointer to a socket address structure is expected in calls such as
   getpeername() and determine the address family by accessing the
   received content after the call.

   The sockaddr_storage structure may also be useful and applied to
   certain other interfaces where a generic socket address large enough
   and aligned for use with multiple address families may be needed. A
   discussion of those interfaces is outside the scope of this document.




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   Also, much existing code assumes that any socket address structure
   can fit in a generic sockaddr structure.  While this has been true
   for IPv4 socket address structures, it has always been false for Unix
   domain socket address structures (but in practice this has not been a
   problem) and it is also false for IPv6 socket address structures
   (which can be a problem).

   So now an application can do the following:

      struct sockaddr_storage __ss;
      struct sockaddr_in6 *sin6;
      sin6 = (struct sockaddr_in6 *) &__ss;

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 5.3).  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);




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   If the specified interface name does not exist, the return value is
   0, and errno is set to ENXIO.  If there was a system error (such as
   running out of memory), the return value is 0 and errno is set to the
   proper value (e.g., ENOMEM).

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);

   The ifname argument must point to a buffer of at least IF_NAMESIZE
   bytes into which the interface name corresponding to the specified
   index is returned.  (IF_NAMESIZE is also defined in <net/if.h> and
   its value includes a terminating null byte at the end of the
   interface name.) This pointer is also the return value of the
   function.  If there is no interface corresponding to the specified
   index, NULL is returned, and errno is set to ENXIO, if there was a
   system error (such as running out of memory), if_indextoname returns
   NULL and 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.

      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.




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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 argument to this function must be a pointer that was returned by
   if_nameindex().

   Currently net/if.h doesn't have prototype definitions for functions
   and it is recommended that these definitions be defined in net/if.h
   as well and the struct if_nameindex{}.

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:




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      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;
      size_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 UDP multicast packets by simply specifying
   an IPv6 multicast address in the 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.

         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:





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           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 using the resulting
         interface.

         Argument type: struct ipv6_mreq

      IPV6_LEAVE_GROUP

         Leave a multicast group on a specified interface.

         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;





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   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 and 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.

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.

6.1 Nodename-to-Address Translation

   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 following function is new and must be thread safe:

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

   struct hostent *getipnodebyname(const char *name, int af, int flags
                                       int *error_num);

   The name argument can be either a node name or a numeric address
   string (i.e., a dotted-decimal IPv4 address or an IPv6 hex address).
   The af argument specifies the address family, either AF_INET or



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   AF_INET6. The error_num value is returned to the caller, via a
   pointer, with the appropriate error code in error_num, to support
   thread safe error code returns.  error_num will be set to one of the
   following values:

      HOST_NOT_FOUND

         No such host is known.

      NO_ADDRESS

         The server recognised the request and the name but no address is
         available.  Another type of request to the name server for the
         domain might return an answer.

      NO_RECOVERY

         An unexpected server failure occurred which cannot be recovered.

      TRY_AGAIN

         A temporary and possibly transient error occurred, such as a
         failure of a server to respond.

   The flags argument specifies the types of addresses that are searched
   for, and the types of addresses that are returned.  We note that a
   special flags value of AI_DEFAULT (defined below) should handle most
   applications.

   That is, porting simple applications to use IPv6 replaces the call

      hptr = gethostbyname(name);

   with

      hptr = getipnodebyname(name, AF_INET6, AI_DEFAULT, &error_num);

   and changes any subsequent error diagnosis code to use error_num
   instead of externally declared variables, such as h_errno.

   Applications desiring finer control over the types of addresses
   searched for and returned, can specify other combinations of the
   flags argument.








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   A flags of 0 implies a strict interpretation of the af argument:

      - If flags is 0 and af is AF_INET, then the caller wants only
        IPv4 addresses.  A query is made for A records.  If successful,
        the IPv4 addresses are returned and the h_length member of the
        hostent structure will be 4, else the function returns a NULL
        pointer.

      -  If flags is 0 and if af is AF_INET6, then the caller wants only
        IPv6 addresses.  A query is made for AAAA records.  If
        successful, the IPv6 addresses are returned and the h_length
        member of the hostent structure will be 16, else the function
        returns a NULL pointer.

   Other constants can be logically-ORed into the flags argument, to
   modify the behavior of the function.

      - If the AI_V4MAPPED flag is specified along with an af of
        AF_INET6, then the caller will accept IPv4-mapped IPv6
        addresses.  That is, if no AAAA records are found then a query
        is made for A records and any found are returned as IPv4-mapped
        IPv6 addresses (h_length will be 16).  The AI_V4MAPPED flag is
        ignored unless af equals AF_INET6.

      - The AI_ALL flag is used in conjunction with the AI_V4MAPPED
        flag, and is only used with the IPv6 address family.  When AI_ALL
        is logically or'd with AI_V4MAPPED flag then the caller wants
        all addresses: IPv6 and IPv4-mapped IPv6.  A query is first made
        for AAAA records and if successful, the IPv6 addresses are
        returned. Another query is then made for A records and any found
        are returned as IPv4-mapped IPv6 addresses. h_length will be 16.
        Only if both queries fail does the function return a NULL pointer.
        This flag is ignored unless af equals AF_INET6.

      - The AI_ADDRCONFIG flag specifies that a query for AAAA records
        should occur only if the node has at least one IPv6 source
        address configured and a query for A records should occur only
        if the node has at least one IPv4 source address configured.

        For example, if the node has no IPv6 source addresses
        configured, and af equals AF_INET6, and the node name being
        looked up has both AAAA and A records, then:

            (a) if only AI_ADDRCONFIG is specified, the function
                returns a NULL pointer;
            (b) if AI_ADDRCONFIG | AI_V4MAPPED is specified, the A
                records are returned as IPv4-mapped IPv6 addresses;




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   The special flags value of AI_DEFAULT is defined as

      #define  AI_DEFAULT  (AI_V4MAPPED | AI_ADDRCONFIG)

   We noted that the getipnodebyname() function must allow the name
   argument to be either a node name or a literal address string (i.e.,
   a dotted-decimal IPv4 address or an IPv6 hex address).  This saves
   applications from having to call inet_pton() to handle literal
   address strings.

   There are four scenarios based on the type of literal address string
   and the value of the af argument.

   The two simple cases are:

   When name is a dotted-decimal IPv4 address and af equals AF_INET, or
   when name is an IPv6 hex address and af equals AF_INET6.  The members
   of the returned hostent structure are: h_name points to a copy of the
   name argument, h_aliases is a NULL pointer, h_addrtype is a copy of
   the af argument, h_length is either 4 (for AF_INET) or 16 (for
   AF_INET6), h_addr_list[0] is a pointer to the 4-byte or 16-byte
   binary address, and h_addr_list[1] is a NULL pointer.

   When name is a dotted-decimal IPv4 address and af equals AF_INET6,
   and flags equals AI_V4MAPPED, an IPv4-mapped IPv6 address is
   returned:  h_name points to an IPv6 hex address containing the IPv4-
   mapped IPv6 address, h_aliases is a NULL pointer, h_addrtype is
   AF_INET6, h_length is 16, h_addr_list[0] is a pointer to the 16-byte
   binary address, and h_addr_list[1] is a NULL pointer.  If AI_V4MAPPED
   is set (with or without AI_ALL) return IPv4-mapped otherwise return
   NULL.

   It is an error when name is an IPv6 hex address and af equals
   AF_INET.  The function's return value is a NULL pointer and error_num
   equals HOST_NOT_FOUND.

6.2 Address-To-Nodename Translation

   The following function has the same arguments as the existing
   gethostbyaddr() function, but adds an error number.

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

      struct hostent *getipnodebyaddr(const void *src, size_t len,
                                          int af, int *error_num);






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   As with getipnodebyname(), getipnodebyaddr() must be thread safe.
   The error_num value is returned to the caller with the appropriate
   error code, to support thread safe error code returns.  The following
   error conditions may be returned for error_num:

      HOST_NOT_FOUND

         No such host is known.

      NO_ADDRESS

         The server recognized the request and the name but no address
         is available.  Another type of request to the name server for
         the domain might return an answer.

      NO_RECOVERY

         An unexpected server failure occurred which cannot be
         recovered.

      TRY_AGAIN

         A temporary and possibly transient error occurred, such as a
         failure of a server to respond.

   One possible source of confusion is the handling of IPv4-mapped IPv6
   addresses and IPv4-compatible IPv6 addresses, but the following logic
   should apply.

      1.  If af is AF_INET6, and if len equals 16, and if the IPv6
          address is an IPv4-mapped IPv6 address or an IPv4-compatible
          IPv6 address, then skip over the first 12 bytes of the IPv6
          address, set af to AF_INET, and set len to 4.

      2.  If af is AF_INET, lookup the name for the given IPv4 address
          (e.g., query for a PTR record in the in-addr.arpa domain).

      3.  If af is AF_INET6, lookup the name for the given IPv6 address
          (e.g., query for a PTR record in the ip6.int domain).

      4.  If the function is returning success, then the single address
          that is returned in the hostent structure is a copy of the
          first argument to the function with the same address family
          that was passed as an argument to this function.







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   All four steps listed are performed, in order.  Also note that the
   IPv6 hex addresses "::" and "::1" MUST NOT be treated as IPv4-
   compatible addresses, and if the address is "::", HOST_NOT_FOUND MUST
   be returned and a query of the address not performed.

   Also for the macro in section 6.7 IN6_IS_ADDR_V4COMPAT MUST return
   false for "::" and "::1".

6.3 Freeing memory for getipnodebyname and getipnodebyaddr

   The hostent structure does not change from its existing definition.
   This structure, and the information pointed to by this structure, are
   dynamically allocated by getipnodebyname and getipnodebyaddr.  The
   following function frees this memory:

      #include <netdb.h>

      void freehostent(struct hostent *ptr);

6.4 Protocol-Independent Nodename and Service Name Translation

   Nodename-to-address translation is done in a protocol-independent
   fashion using the getaddrinfo() function that is taken from the
   Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
   (Protocol Independent Interfaces) draft specification [3].

   The official specification for this function will be the final POSIX
   standard, with the following additional requirements:

      -  getaddrinfo() (along with the getnameinfo() function described
         in the next section) must be thread safe.

      -  The AI_NUMERICHOST is new with this document.

      -  All fields in socket address structures returned by
         getaddrinfo() that are not filled in through an explicit
         argument (e.g., sin6_flowinfo and sin_zero) must be set to 0.
         (This makes it easier to compare socket address structures.)

      -  getaddrinfo() must fill in the length field of a socket address
         structure (e.g., sin6_len) on systems that support this field.

   We are providing this independent description of the function because
   POSIX standards are not freely available (as are IETF documents).

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




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

   The addrinfo structure is defined as a result of including the
   <netdb.h> header.

  struct addrinfo {
    int     ai_flags;     /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
    int     ai_family;    /* PF_xxx */
    int     ai_socktype;  /* SOCK_xxx */
    int     ai_protocol;  /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
    size_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 return value from the function is 0 upon success or a nonzero
   error code.  The following names are the nonzero error codes from
   getaddrinfo(), and are defined in <netdb.h>:

      EAI_ADDRFAMILY  address family for nodename not supported
      EAI_AGAIN       temporary failure in name resolution
      EAI_BADFLAGS    invalid value for ai_flags
      EAI_FAIL        non-recoverable failure in name resolution
      EAI_FAMILY      ai_family not supported
      EAI_MEMORY      memory allocation failure
      EAI_NODATA      no address associated with nodename
      EAI_NONAME      nodename nor servname provided, or not known
      EAI_SERVICE     servname not supported for ai_socktype
      EAI_SOCKTYPE    ai_socktype not supported
      EAI_SYSTEM      system error returned in errno

   The nodename and servname arguments are pointers to null-terminated
   strings or NULL.  One or both of these two arguments must be a non-
   NULL pointer.  In the normal client scenario, both the nodename and
   servname are specified.  In the normal server scenario, only the
   servname is specified.  A non-NULL nodename string can be either a
   node name or a numeric host address string (i.e., a dotted-decimal
   IPv4 address or an IPv6 hex address).  A non-NULL servname string can
   be either a service name or a decimal port number.

   The caller can optionally pass an addrinfo structure, pointed to by
   the third argument, to provide hints concerning the type of socket
   that the caller supports.  In this hints structure all members other
   than ai_flags, ai_family, ai_socktype, and ai_protocol must be zero
   or a NULL pointer.  A value of PF_UNSPEC for ai_family means the



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   caller will accept any protocol family.  A value of 0 for ai_socktype
   means the caller will accept any socket type.  A value of 0 for
   ai_protocol means the caller will accept any protocol.  For example,
   if the caller handles only TCP and not UDP, then the ai_socktype
   member of the hints structure should be set to SOCK_STREAM when
   getaddrinfo() is called.  If the caller handles only IPv4 and not
   IPv6, then the ai_family member of the hints structure should be set
   to PF_INET when getaddrinfo() is called.  If the third argument to
   getaddrinfo() is a NULL pointer, this is the same as if the caller
   had filled in an addrinfo structure initialized to zero with
   ai_family set to PF_UNSPEC.

   Upon successful return a pointer to a linked list of one or more
   addrinfo structures is returned through the final argument.  The
   caller can process each addrinfo structure in this list by following
   the ai_next pointer, until a NULL pointer is encountered.  In each
   returned addrinfo structure the three members ai_family, ai_socktype,
   and ai_protocol are the corresponding arguments for a call to the
   socket() function.  In each addrinfo structure the ai_addr member
   points to a filled-in socket address structure whose length is
   specified by the ai_addrlen member.

   If the AI_PASSIVE bit is set in the ai_flags member of the hints
   structure, then the caller plans to use the returned socket address
   structure in a call to bind().  In this case, if the nodename
   argument is a NULL pointer, then the IP address portion of the socket
   address structure will be set to INADDR_ANY for an IPv4 address or
   IN6ADDR_ANY_INIT for an IPv6 address.

   If the AI_PASSIVE bit is not set in the ai_flags member of the hints
   structure, then the returned socket address structure will be ready
   for a call to connect() (for a connection-oriented protocol) or
   either connect(), sendto(), or sendmsg() (for a connectionless
   protocol).  In this case, if the nodename argument is a NULL pointer,
   then the IP address portion of the socket address structure will be
   set to the loopback address.

   If the AI_CANONNAME bit is set in the ai_flags member of the hints
   structure, then upon successful return the ai_canonname member of the
   first addrinfo structure in the linked list will point to a null-
   terminated string containing the canonical name of the specified
   nodename.

   If the AI_NUMERICHOST bit is set in the ai_flags member of the hints
   structure, then a non-NULL nodename string must be a numeric host
   address string.  Otherwise an error of EAI_NONAME is returned.  This
   flag prevents any type of name resolution service (e.g., the DNS)
   from being called.



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   All of the information returned by getaddrinfo() is dynamically
   allocated: the addrinfo structures, and the socket address structures
   and canonical node name strings pointed to by the addrinfo
   structures.  To return this information to the system the function
   freeaddrinfo() is called:

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

      void freeaddrinfo(struct addrinfo *ai);

   The addrinfo structure pointed to by the ai argument is freed, along
   with any dynamic storage pointed to by the structure.  This operation
   is repeated until a NULL ai_next pointer is encountered.

   To aid applications in printing error messages based on the EAI_xxx
   codes returned by getaddrinfo(), the following function is defined.

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

      char *gai_strerror(int ecode);

   The argument is one of the EAI_xxx values defined earlier and 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.

6.5 Socket Address Structure to Nodename and Service Name

   The POSIX 1003.1g specification includes no function to perform the
   reverse conversion from getaddrinfo(): to look up a nodename and
   service name, given the binary address and port.  Therefore, we
   define the following function:

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

      int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                      char *host, size_t hostlen,
                      char *serv, size_t servlen,
                      int flags);

   This function looks up an IP address and port number provided by the
   caller in the DNS and system-specific database, and returns text
   strings for both in buffers provided by the caller.  The function
   indicates successful completion by a zero return value; a non-zero
   return value indicates failure.





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   The first argument, sa, points to either a sockaddr_in structure (for
   IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP
   address and port number.  The salen argument gives the length of the
   sockaddr_in or sockaddr_in6 structure.

   The function returns the nodename associated with the IP address in
   the buffer pointed to by the host argument.  The caller provides the
   size of this buffer via the hostlen argument.  The service name
   associated with the port number is returned in the buffer pointed to
   by serv, and the servlen argument gives the length of this buffer.
   The caller specifies not to return either string by providing a zero
   value for the hostlen or servlen arguments.  Otherwise, the caller
   must provide buffers large enough to hold the nodename and the
   service name, including the terminating null characters.

   Unfortunately most systems do not provide constants that specify the
   maximum size of either a fully-qualified domain name or a service
   name.  Therefore to aid the application in allocating buffers for
   these two returned strings the following constants are defined in
   <netdb.h>:

      #define NI_MAXHOST  1025
      #define NI_MAXSERV    32

   The first value is actually defined as the constant MAXDNAME in recent
   versions of BIND's <arpa/nameser.h> header (older versions of BIND
   define this constant to be 256) and the second is a guess based on the
   services listed in the current Assigned Numbers RFC.

   The final argument is a flag that changes the default actions of this
   function.  By default the fully-qualified domain name (FQDN) for the
   host is looked up in the DNS and returned.  If the flag bit NI_NOFQDN
   is set, only the nodename portion of the FQDN is returned for local
   hosts.

   If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
   located in the DNS, the numeric form of the host's address is returned
   instead of its name (e.g., by calling inet_ntop() instead of
   getipnodebyaddr()).  If the flag bit NI_NAMEREQD is set, an error is
   returned if the host's name cannot be located in the DNS.

   If the flag bit NI_NUMERICSERV is set, the numeric form of the service
   address is returned (e.g., its port number) instead of its name.  The
   two NI_NUMERICxxx flags are required to support the "-n" flag that
   many commands provide.






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   A fifth flag bit, NI_DGRAM, specifies that the service is a datagram
   service, and causes getservbyport() to be called with a second
   argument of "udp" instead of its default of "tcp".  This is required
   for the few ports (e.g. 512-514) that have different services for UDP
   and TCP.

   These NI_xxx flags are defined in <netdb.h> along with the AI_xxx
   flags already defined for getaddrinfo().

6.6 Address Conversion Functions

   The two 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 <sys/socket.h>
      #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, size_t size);

   The inet_pton() function converts an address in its standard text
   presentation form into its numeric binary form.  The af argument
   specifies the family of the address.  Currently the AF_INET and
   AF_INET6 address families are 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.  The address is
   returned in network byte order.  Inet_pton() returns 1 if the
   conversion succeeds, 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.  The calling
   application must ensure that the buffer referred to by dst is large
   enough to hold the numeric address (e.g., 4 bytes for AF_INET or 16
   bytes for AF_INET6).

   If the af argument is AF_INET, the function accepts a string 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.
   Note that many implementations of the existing inet_addr() and
   inet_aton() functions accept nonstandard input: octal numbers,
   hexadecimal numbers, and fewer than four numbers.  inet_pton() does
   not accept these formats.



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   If the af argument is AF_INET6, then the function accepts a string in
   one of the standard IPv6 text forms defined in Section 2.2 of the
   addressing architecture specification [2].

   The inet_ntop() function converts a numeric address into a text
   string suitable for presentation.  The af argument specifies 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 will store the
   resulting text string.  The size argument specifies the size of this
   buffer.  The application must specify a non-NULL dst argument.  For
   IPv6 addresses, the buffer must be at least 46-octets.  For IPv4
   addresses, the buffer must be at least 16-octets.  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 returns 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.

6.7 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 *);





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   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 local-use IPv6 unicast addresses.  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_DEFAULT
      <netdb.h>       AI_ALL
      <netdb.h>       AI_CANONNAME
      <netdb.h>       AI_NUMERICHOST
      <netdb.h>       AI_PASSIVE
      <netdb.h>       AI_V4MAPPED
      <netdb.h>       EAI_ADDRFAMILY
      <netdb.h>       EAI_AGAIN
      <netdb.h>       EAI_BADFLAGS
      <netdb.h>       EAI_FAIL
      <netdb.h>       EAI_FAMILY
      <netdb.h>       EAI_MEMORY
      <netdb.h>       EAI_NODATA
      <netdb.h>       EAI_NONAME
      <netdb.h>       EAI_SERVICE
      <netdb.h>       EAI_SOCKTYPE
      <netdb.h>       EAI_SYSTEM
      <netdb.h>       NI_DGRAM
      <netdb.h>       NI_MAXHOST
      <netdb.h>       NI_MAXSERV
      <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



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      <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>  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 *, size_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 *, size_t, char *, size_t, int);
<netdb.h>       void freeaddrinfo(struct addrinfo *);
<netdb.h>       char *gai_strerror(int);
<netdb.h>       struct hostent *getipnodebyname(const char *, int, int,
                                       int *);
<netdb.h>       struct hostent *getipnodebyaddr(const void *, size_t,
                                       int, int *);
<netdb.h>       void freehostent(struct hostent *);

<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 *);



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<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. Year 2000 Considerations

   There are no issues for this memo concerning the Year 2000 issue
   regarding the use of dates.

Changes From RFC 2133

   Changes made in the March 1998 Edition (-01 draft):

      Changed all "hostname" to "nodename" for consistency with other
      IPv6 documents.

      Section 3.3: changed comment for sin6_flowinfo to be "traffic
      class & flow info" and updated corresponding text description to
      current definition of these two fields.

      Section 3.10 ("Portability Additions") is new.

      Section 6: a new paragraph was added reiterating that the existing
      gethostbyname() and gethostbyaddr() are not changed.

      Section 6.1: change gethostbyname3() to getnodebyname().  Add
      AI_DEFAULT to handle majority of applications.  Renamed
      AI_V6ADDRCONFIG to AI_ADDRCONFIG and define it for A records and
      IPv4 addresses too.  Defined exactly what getnodebyname() must
      return if the name argument is a numeric address string.

      Section 6.2: change gethostbyaddr() to getnodebyaddr().  Reword
      items 2 and 3 in the description of how to handle IPv4-mapped and
      IPv4- compatible addresses to "lookup a name" for a given address,
      instead of specifying what type of DNS query to issue.




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      Section 6.3: added two more requirements to getaddrinfo().

      Section 7: added the following constants to the list for
      <netdb.h>:  AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED.  Add union
      sockaddr_union and SA_LEN to the lists for <sys/socket.h>.

      Updated references.

   Changes made in the November 1997 Edition (-00 draft):

      The data types have been changed to conform with Draft 6.6 of the
      Posix 1003.1g standard.

      Section 3.2: data type of s6_addr changed to "uint8_t".

      Section 3.3: data type of sin6_family changed to "sa_family_t".
      data type of sin6_port changed to "in_port_t", data type of
      sin6_flowinfo changed to "uint32_t".

      Section 3.4: same as Section 3.3, plus data type of sin6_len
      changed to "uint8_t".

      Section 6.2: first argument of gethostbyaddr() changed from "const
      char *" to "const void *" and second argument changed from "int"
      to "size_t".

      Section 6.4: second argument of getnameinfo() changed from
      "size_t" to "socklen_t".

      The wording was changed when new structures were defined, to be
      more explicit as to which header must be included to define the
      structure:

      Section 3.2 (in6_addr{}), Section 3.3 (sockaddr_in6{}), Section
      3.4 (sockaddr_in6{}), Section 4.3 (if_nameindex{}), Section 5.3
      (ipv6_mreq{}), and Section 6.3 (addrinfo{}).

      Section 4: NET_RT_LIST changed to NET_RT_IFLIST.

      Section 5.1: The IPV6_ADDRFORM socket option was removed.

      Section 5.3: Added a note that an option value other than 0 or 1
      for IPV6_MULTICAST_LOOP returns an error.  Added a note that
      IPV6_MULTICAST_IF, IPV6_MULTICAST_HOPS, and IPV6_MULTICAST_LOOP
      can also be used with getsockopt(), but IPV6_ADD_MEMBERSHIP and
      IPV6_DROP_MEMBERSHIP cannot be used with getsockopt().





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      Section 6.1: Removed the description of gethostbyname2() and its
      associated RES_USE_INET6 option, replacing it with
      gethostbyname3().

      Section 6.2: Added requirement that gethostbyaddr() be thread
      safe.  Reworded step 4 to avoid using the RES_USE_INET6 option.

      Section 6.3: Added the requirement that getaddrinfo() and
      getnameinfo() be thread safe.  Added the AI_NUMERICHOST flag.

      Section 6.6: Added clarification about IN6_IS_ADDR_LINKLOCAL and
      IN6_IS_ADDR_SITELOCAL macros.

   Changes made to the draft -01 specification Sept 98

      Changed priority to traffic class in the spec.

      Added the need for scope identification in section 2.1.

      Added sin6_scope_id to struct sockaddr_in6 in sections 3.3 and
      3.4.

      Changed 3.10 to use generic storage structure to support holding
      IPv6 addresses and removed the SA_LEN macro.

      Distinguished between invalid input parameters and system failures
      for Interface Identification in Section 4.1 and 4.2.

      Added defaults for multicast operations in section 5.2 and changed
      the names from ADD to JOIN and DROP to LEAVE to be consistent with
      IPv6 multicast terminology.

      Changed getnodebyname to getipnodebyname, getnodebyaddr to
      getipnodebyaddr, and added MT safe error code to function
      parameters in section 6.

      Moved freehostent to its own sub-section after getipnodebyaddr now
      6.3 (so this bumps all remaining sections in section 6.

      Clarified the use of AI_ALL and AI_V4MAPPED that these are
      dependent on the AF parameter and must be used as a conjunction in
      section 6.1.

      Removed the restriction that literal addresses cannot be used with
      a flags argument in section 6.1.

      Added Year 2000 Section to the draft




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      Deleted Reference to the following because the attached is deleted
      from the ID directory and has expired.  But the logic from the
      aforementioned draft still applies, so that was kept in Section
      6.2 bullets after 3rd paragraph.

      [7]  P. Vixie, "Reverse Name Lookups of Encapsulated IPv4
           Addresses in IPv6", Internet-Draft, <draft-vixie-ipng-
           ipv4ptr-00.txt>, May 1996.

      Deleted the following reference as it is no longer referenced.
      And the draft has expired.

      [3]  D. McDonald, "A Simple IP Security API Extension to BSD
           Sockets", Internet-Draft, <draft-mcdonald-simple-ipsec-api-
           01.txt>, March 1997.

      Deleted the following reference as it is no longer referenced.

      [4]  C. Metz, "Network Security API for Sockets",
           Internet-Draft, <draft-metz-net-security-api-01.txt>, January
           1998.

      Update current references to current status.

      Added alignment notes for in6_addr and sin6_addr.

      Clarified further that AI_V4MAPPED must be used with a dotted IPv4
      literal address for getipnodebyname(), when address family is
      AF_INET6.

      Added text to clarify "::" and "::1" when used by
      getipnodebyaddr().

Acknowledgments

   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, 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, Thomas Narten, Josh Osborne, Craig Partridge, Jean-Luc
   Richier, Erik Scoredos, Keith Sklower, Matt Thomas, Harvey Thompson,
   Dean D. Throop, Karen Tracey, Glenn Trewitt, Paul Vixie, David
   Waitzman, Carl Williams, and Kazu Yamamoto,






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   The getaddrinfo() and getnameinfo() functions are taken from an
   earlier Internet Draft by Keith Sklower.  As noted in that draft,
   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.

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, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
        6.6, March 1997.

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






















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Authors' Addresses

   Robert E. Gilligan
   FreeGate Corporation
   1208 E. Arques Ave.
   Sunnyvale, CA 94086

   Phone: +1 408 617 1004
   EMail: gilligan@freegate.com


   Susan Thomson
   Bell Communications Research
   MRE 2P-343, 445 South Street
   Morristown, NJ 07960

   Phone: +1 201 829 4514
   EMail: set@thumper.bellcore.com


   Jim Bound
   Compaq Computer Corporation
   110 Spitbrook Road ZK3-3/U14
   Nashua, NH 03062-2698

   Phone: +1 603 884 0400
   EMail: bound@zk3.dec.com


   W. Richard Stevens
   1202 E. Paseo del Zorro
   Tucson, AZ 85718-2826

   Phone: +1 520 297 9416
   EMail: rstevens@kohala.com
















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RFC 2553       Basic Socket Interface Extensions for IPv6     March 1999


Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   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
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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