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Versions: 00 01 02 03 04 05 06 RFC 2553

Internet Engineering Task Force                R. E. Gilligan (Freegate)  |
INTERNET-DRAFT                                     S. Thomson (Bellcore)
                                                      J. Bound (Digital)
                                              W. R. Stevens (Consultant)
                                                       November 19, 1997  |

               Basic Socket Interface Extensions for IPv6
                 <draft-ietf-ipngwg-bsd-api-new-00.txt>                   |

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 [6].                |

Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months.  This Internet Draft expires on May 19, 1998.  Internet        |
   Drafts may be updated, replaced, or obsoleted by other documents at
   any time.  It is not appropriate to use Internet Drafts as reference
   material or to cite them other than as a "working draft" or "work in
   progress."

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
   munnari.oz.au.

   Distribution of this memo is unlimited.



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Table of Contents

   1.  Introduction .....................................................  4

   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 ..............  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

   4.  Interface Identification ......................................... 13
   4.1.  Name-to-Index .................................................. 14
   4.2.  Index-to-Name .................................................. 14
   4.3.  Return All Interface Names and Indexes ......................... 14
   4.4.  Free Memory .................................................... 15

   5.  Socket Options ................................................... 15
   5.1.  Unicast Hop Limit .............................................. 15
   5.2.  Sending and Receiving Multicast Packets ........................ 16

   6.  Library Functions ................................................ 18
   6.1.  Hostname-to-Address Translation ................................ 18
   6.2.  Address To Hostname Translation ................................ 20
   6.3.  Protocol-Independent Hostname and Service Name Translation ..... 21
   6.4.  Socket Address Structure to Hostname and Service Name .......... 24
   6.5.  Address Conversion Functions ................................... 26
   6.6.  Address Testing Macros ......................................... 27

   7.  Summary of New Definitions ....................................... 28

   8.  Security Considerations .......................................... 30

   9.  Changes From RFC 2133 ............................................ 30

   10.  Acknowledgments ................................................. 31

   11.  References ...................................................... 31



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   12.  Authors' Addresses .............................................. 32


















































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1.  Introduction

   While IPv4 addresses are 32 bits long, IPv6 interfaces are identified
   by 128-bit addresses.  The socket interface make 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., flow label and priority), 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.

    -  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




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   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.

   The name-to-address translation functions in the socket interface are
   gethostbyname() and gethostbyaddr().  These must be modified to
   support IPv6 and the semantics defined must provide 100% backward
   compatibility for all existing IPv4 applications, along with IPv6
   support for new applications.  Additionally, the POSIX 1003.g draft    |
   [5] specifies a new hostname-to-address translation function which is
   protocol independent.  This function can also be used with IPv6.

   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 flow label, priority,
   and hop limit header fields.  New socket options are needed to



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   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 [6].        |


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.

   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>.



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   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.


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 */
       };

   This structure is designed to be compatible with the sockaddr data



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   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 that contains two pieces of
   information: the 24-bit IPv6 flow label and the 4-bit priority field.
   The contents and interpretation of this member is unspecified at this
   time.

   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 the sin6_addr field will be aligned on a 64-bit boundary.
   This is done for optimum performance on 64-bit architectures.

   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 */
       };

   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);

   Once the application has created a PF_INET6 socket, it must use the
   sockaddr_in6 address structure when passing addresses in to the



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   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 are often generated automatically by the
   gethostbyname() 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 6.6, 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



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   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



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   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.


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 [6] 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



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   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 the specified interface does not exist, the return value is 0.


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 IFNAMSIZ bytes
   into which the interface name corresponding to the specified index is
   returned.  (IFNAMSIZ 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.


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", ... */ |
       };                                                                 |



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   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.

   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 argument to this function must be a pointer that was returned by
   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:



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       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;
       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().





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       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:

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


           Argument type: int

       IPV6_MULTICAST_LOOP

           Controls whether outgoing multicast packets  sent  should  be
           delivered  back  to the local application.  A toggle.  If the
           option is set to 1, multicast packets are looped back.  If it
           is  set  to  0,  they are not.  Other option values return an  |
           error of EINVAL.

           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().














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       IPV6_ADD_MEMBERSHIP

           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_DROP_MEMBERSHIP

           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 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 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 (hostname-to-
   address translation) and reverse lookup (address-to-hostname
   translation) need to be supported.  Functions are also needed to
   convert IPv6 addresses between their binary and textual form.


6.1.  Hostname-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,      |



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   IPv6 only, IPv4-mapped IPv6 are OK, etc.), and second 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 *gethostbyname3(const char *name, int af, int flags);|

   The name argument can be either a host name or a numeric host 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        |
   AF_INET6.  We define the flags argument shortly.

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

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

       void freehostent(struct hostent *ptr);                             |


   The flags argument specifies the types of addresses that are searched  |
   for, and the types of addresses that are returned.  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.



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    -  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.

    -  If the AI_ALL flag is specified along with an af of AF_INET6,      |
       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.  If both AI_ALL and AI_V4MAPPED are specified, AI_ALL    |
       takes precedence.                                                  |

    -  The AI_V6ADDRCONFIG flag specifies that a query for AAAA records   |
       should occur only if the node has at least one IPv6 source         |
       address configured.  This flag is ignored unless af equals         |
       AF_INET6.                                                          |

       If the node has no IPv6 source addresses configured, and af        |
       equals AF_INET6, and the host name being looked up has both AAAA   |
       and A records, then:  (a) if only AI_V6ADDRCONFIG is specified,    |
       the function returns a NULL pointer; (b) if AI_V6ADDRCONFIG |      |
       AI_MAPPED is specified, the A records are returned as IPv4-mapped  |
       IPv6 addresses; (c) if AI_V6ADDRCONFIG | AI_ALL is specified, the  |
       A records are returned as IPv4-mapped IPv6 addresses.



6.2.  Address To Hostname Translation                                     *

   The existing gethostbyaddr() function already requires an address
   family argument and can therefore work with IPv6 addresses:

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

       struct hostent *gethostbyaddr(const void *src, size_t len, int af);|


   As with gethostbyname3(), gethostbyaddr() must be thread safe.  The    |
   pointer that is returned points to dynamically allocated memory that   |
   is returned by calling freehostent().                                  |

   One possible source of confusion is the handling of IPv4-mapped IPv6



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   addresses and IPv4-compatible IPv6 addresses.  This is addressed in    |
   [7] and involves the following logic:

    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, then query for a PTR record in the in-
        addr.arpa domain.

    3.  If af is AF_INET6, then 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 with the same address family that   |
        was passed as an argument to this function.

   All four steps listed are performed, in order.                         *


6.3.  Protocol-Independent Hostname and Service Name Translation

   Hostname-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 [5].

   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.                      |

   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>

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

   The addrinfo structure is defined as a result of including the         |



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   <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 hostname */
         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 hostname 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 hostname
       EAI_NONAME      hostname 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 hostname 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 hostname and
   servname are specified.  In the normal server scenario, only the
   servname is specified.  A non-NULL hostname string can be either a
   host 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
   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



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   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 hostname
   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 hostname 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
   hostname.

   If the AI_NUMERICHOST bit is set in the ai_flags member of the hints   |
   structure, then a non-NULL hostname 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.                                                     |

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



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   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.


6.4.  Socket Address Structure to Hostname and Service Name

   The POSIX 1003.1g specification includes no function to perform the
   reverse conversion from getaddrinfo():  to look up a hostname 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.

   The first argument, sa, points to either a sockaddr_in structure (for
   IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP



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   address and port number.  The salen argument gives the length of the
   sockaddr_in or sockaddr_in6 structure.

   The function returns the hostname 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 hostname 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 hostname 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
   gethostbyaddr()).  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.

   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 (512-514) that have different services for UDP and



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   TCP.

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


6.5.  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.

   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



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   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 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.


6.6.  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



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

     <netdb.h>       AI_CANONNAME
     <netdb.h>       AI_PASSIVE
     <netdb.h>       AI_NUMERICHOST                                       |
     <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
     <netinet/in.h>  INET_ADDRSTRLEN
     <netinet/in.h>  IPPROTO_IPV6
     <netinet/in.h>  IPV6_ADD_MEMBERSHIP                                  *
     <netinet/in.h>  IPV6_DROP_MEMBERSHIP
     <netinet/in.h>  IPV6_MULTICAST_HOPS



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     <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


   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 *gethostbyname3(const char *, int, int);|
     <netdb.h>       struct hostent *gethostbyaddr(const void *, size_t, 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 *);
     <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 *);



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     <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 [3, 4].                                                  |


9.  Changes From RFC 2133

   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().

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



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    -  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.

    -  Updated references.                                                |


10.  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, Tim
   Hartrick, Tom Herbert, Bob Hinden, Wan-Yen Hsu, Christian Huitema,
   Koji Imada, Markus Jork, Ron Lee, Alan Lloyd, Charles Lynn, Jack
   McCann, Dan McDonald, Dave Mitton, Thomas Narten, Erik Nordmark, 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,

   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 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] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
       Specification", Internet Draft, <draft-ietf-ipngwg-ipv6-spec-v2-   |
       00.txt>, July 1997.



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   [2] R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",     |
       Internet Draft, <draft-ietf-ipngwg-addr-arch-v2-05.txt>, November  |
       1997.

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

   [4] C. Metz, "Network Security API for Sockets", Internet-Draft,       |
       <draft-metz-net-security-api-00.txt>, Aug. 1997.

   [5] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT   |
       6.6, March 1997.

   [6] W. R. Stevens, M. Thomas, "Advanced Sockets API for IPv6",         |
       Internet-Draft, <draft-stevens-advanced-api-04.txt>, July 1997.    |

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


12.  Authors' Addresses

    Robert E. Gilligan
    Freegate Corporation
    710 Lakeway Dr.  STE 230
    Sunnyvale, CA 94086
    Phone: +1 408 524 4804
    Email: gilligan@freegate.net


    Susan Thomson
    Bell Communications Research
    MRE 2P-343, 445 South Street
    Morristown, NJ 07960
    Telephone: +1 201 829 4514
    Email: set@thumper.bellcore.com


    Jim Bound
    Digital Equipment Corporation
    110 Spitbrook Road ZK3-3/U14
    Nashua, NH 03062-2698
    Phone: +1 603 881 0400
    Email: bound@zk3.dec.com





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    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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