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RFC 2133 - Basic Socket Interface Extensions for IPv6

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Network Working Group                                        R. Gilligan
Request for Comments: 2133                                      Freegate
Category: Informational                                       S. Thomson
                                                                J. Bound
                                                              W. Stevens
                                                              April 1997

               Basic Socket Interface Extensions for IPv6

Status of this Memo

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


   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 [5].

Table of Contents

   1.  Introduction ................................................  2
   2.  Design Considerations .......................................  3
   2.1.  What Needs to be Changed ..................................  3
   2.2.  Data Types ................................................  5
   2.3.  Headers ...................................................  5
   2.4.  Structures ................................................  5
   3.  Socket Interface ............................................  5
   3.1.  IPv6 Address Family and Protocol Family ...................  5
   3.2.  IPv6 Address Structure ....................................  6

   3.3.  Socket Address Structure for 4.3BSD-Based Systems .........  6
   3.4.  Socket Address Structure for 4.4BSD-Based Systems .........  7
   3.5.  The Socket Functions ......................................  8
   3.6.  Compatibility with IPv4 Applications ......................  9
   3.7.  Compatibility with IPv4 Nodes .............................  9
   3.8.  IPv6 Wildcard Address ..................................... 10
   3.9.  IPv6 Loopback Address ..................................... 11
   4.  Interface Identification .................................... 12
   4.1.  Name-to-Index ............................................. 13
   4.2.  Index-to-Name ............................................. 13
   4.3.  Return All Interface Names and Indexes .................... 14
   4.4.  Free Memory ............................................... 14
   5.  Socket Options .............................................. 14
   5.1.  Changing Socket Type ...................................... 15
   5.2.  Unicast Hop Limit ......................................... 16
   5.3.  Sending and Receiving Multicast Packets ................... 17
   6.  Library Functions ........................................... 19
   6.1.  Hostname-to-Address Translation ........................... 19
   6.2.  Address To Hostname Translation ........................... 22
   6.3.  Protocol-Independent Hostname and Service Name Translation  22
   6.4.  Socket Address Structure to Hostname and Service Name ..... 25
   6.5.  Address Conversion Functions .............................. 27
   6.6.  Address Testing Macros .................................... 28
   7.  Summary of New Definitions .................................. 29
   8.  Security Considerations ..................................... 31
   9.  Acknowledgments ............................................. 31
   10.  References ................................................. 31
   11.  Authors' Addresses ......................................... 32

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

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

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

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

2.1.  What Needs to be Changed

   The socket interface API consists of a few distinct components:

    -  Core socket functions.

    -  Address data structures.

    -  Name-to-address translation functions.

    -  Address conversion functions.

    The core socket functions -- those functions that deal with such
    things as setting up and tearing down TCP connections, and sending
    and receiving UDP packets -- were designed to be transport
    independent.  Where protocol addresses are passed as function
    arguments, they are carried via opaque pointers.  A protocol-specific

    address data structure is defined for each protocol that the socket
    functions support.  Applications must cast pointers to these
    protocol-specific address structures into pointers to the generic
    "sockaddr" address structure when using the socket functions.  These
    functions need not change for IPv6, but a new IPv6-specific address
    data structure is needed.

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

    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 work in
    progress [4] specifies a new hostname-to-address translation function
    which is protocol independent.  This function can also be used with

    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
    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 [5].

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, POSIX 1003.1g data types are used:  u_intN_t means an
   unsigned integer of exactly N bits (e.g., u_int16_t) and u_intNm_t
   means an unsigned integer of at least N bits (e.g., u_int32m_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

   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

       #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 data structure to hold a single IPv6 address is defined as

       #include <netinet/in.h>

       struct in6_addr {
           u_int8_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

   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
   structure is defined to carry IPv6 addresses:

       #include <netinet/in.h>

       struct sockaddr_in6 {
           u_int16m_t      sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* transport layer port # */
           u_int32m_t      sin6_flowinfo;  /* IPv6 flow information */
           struct in6_addr sin6_addr;      /* IPv6 address */

   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

   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

   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:

       #include <netinet/in.h>

       #define SIN6_LEN

       struct sockaddr_in6 {
           u_char          sin6_len;       /* length of this struct */
           u_char          sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* transport layer port # */
           u_int32m_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

   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
   system.  The functions that the application uses to pass addresses
   into the system are:


   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


   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

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
   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:


   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

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()

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

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

       extern const struct in6_addr in6addr_any;

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

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

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

       struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

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

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

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

3.9.  IPv6 Loopback Address

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

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

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

       extern const struct in6_addr in6addr_loopback;

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

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

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

       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 [5] 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

   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_LIST 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

       #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

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

       #include <net/if.h>

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

       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

       #include <net/if.h>

       void  if_freenameindex(struct if_nameindex *ptr);

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

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.  Changing Socket Type

   Unix allows open sockets to be passed between processes via the
   exec() call and other means.  It is a relatively common application
   practice to pass open sockets across exec() calls.  Thus it is
   possible for an application using the original API to pass an open
   PF_INET socket to an application that is expecting to receive a
   PF_INET6 socket.  Similarly, it is possible for an application using
   the extended API to pass an open PF_INET6 socket to an application
   using the original API, which would be equipped only to deal with
   PF_INET sockets.  Either of these cases could cause problems, because
   the application that is passed the open socket might not know how to
   decode the address structures returned in subsequent socket

   To remedy this problem, a new setsockopt() option is defined that
   allows an application to "convert" a PF_INET6 socket into a PF_INET
   socket and vice versa.

   An IPv6 application that is passed an open socket from an unknown
   process may use the IPV6_ADDRFORM setsockopt() option to "convert"
   the socket to PF_INET6.  Once that has been done, the system will
   return sockaddr_in6 address structures in subsequent socket

   An IPv6 application that is about to pass an open PF_INET6 socket to
   a program that is not be IPv6 capable can "downgrade" the socket to
   PF_INET before calling exec().  After that, the system will return
   sockaddr_in address structures to the application that was exec()'ed.
   Be aware that you cannot downgrade an IPv6 socket to an IPv4 socket
   unless all nonwildcard addresses already associated with the IPv6
   socket are IPv4-mapped IPv6 addresses.

   The IPV6_ADDRFORM option is valid at both the IPPROTO_IP and
   IPPROTO_IPV6 levels.  The only valid option values are PF_INET6 and
   PF_INET.  For example, to convert a PF_INET6 socket to PF_INET, a
   program would call:

       int  addrform = PF_INET;

       if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
                      (char *) &addrform, sizeof(addrform)) == -1)
           perror("setsockopt IPV6_ADDRFORM");

   An application may use IPV6_ADDRFORM with getsockopt() to learn
   whether an open socket is a PF_INET of PF_INET6 socket.  For example:

       int  addrform;
       size_t  len = sizeof(addrform);

       if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
                      (char *) &addrform, &len) == -1)
           perror("getsockopt IPV6_ADDRFORM");
       else if (addrform == PF_INET)
           printf("This is an IPv4 socket.\n");
       else if (addrform == PF_INET6)
           printf("This is an IPv6 socket.\n");
           printf("This system is broken.\n");

5.2.  Unicast Hop Limit

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

       int  hoplimit = 10;

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

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

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

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

       int  hoplimit;
       size_t  len = sizeof(hoplimit);

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

5.3.  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()

   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:


           Set the interface to use for outgoing multicast packets.  The
           argument is the index of the interface to use.

           Argument type: unsigned int


           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


           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.

           Argument type: unsigned int

   The reception of multicast packets is controlled by the two
   setsockopt() options summarized below:


           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


           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:

       #include <netinet/in.h>

       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() remains unchanged as does
   the hostent structure to which it returns a pointer.  Existing
   applications that call this function continue to receive only IPv4
   addresses that are the result of a query in the DNS for A records.
   (We assume the DNS is being used; some environments may be using a
   hosts file or some other name resolution system, either of which may
   impede renumbering.  We also assume that the RES_USE_INET6 resolver
   option is not set, which we describe in more detail shortly.)

   Two new changes are made to support IPv6 addresses.  First, the
   following function is new:

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

       struct hostent *gethostbyname2(const char *name, int af);

   The af argument specifies the address family.  The default operation
   of this function is simple:

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

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

   The second change, that provides additional functionality, is a new
   resolver option RES_USE_INET6, which is defined as a result of
   including the <resolv.h> header.  (This option is provided starting
   with the BIND 4.9.4 release.)  There are three ways to set this

    -  The first way is

           _res.options |= RES_USE_INET6;

       and then call either gethostbyname() or gethostbyname2().  This
       option then affects only the process that is calling the

    -  The second way to set this option is to set the environment
       variable RES_OPTIONS, as in RES_OPTIONS=inet6.  (This example is
       for the Bourne and Korn shells.)  This method affects any
       processes that see this environment variable.

    -  The third way is to set this option in the resolver configuration
       file (normally /etc/resolv.conf) and the option then affects all
       applications on the host.  This final method should not be done
       until all applications on the host are capable of dealing with
       IPv6 addresses.

   There is no priority among these three methods.  When the
   RES_USE_INET6 option is set, two changes occur:

    -  gethostbyname(host) first calls gethostbyname2(host, AF_INET6)
       looking for AAAA records, and if this fails it then calls
       gethostbyname2(host, AF_INET) looking for A records.

    -  gethostbyname2(host, AF_INET) always returns IPv4-mapped IPv6
       addresses with the h_length member of the hostent structure set
       to 16.

   An application must not enable the RES_USE_INET6 option until it is
   prepared to deal with 16-byte addresses in the returned hostent

   The following table summarizes the operation of the existing
   gethostbyname() function, the new function gethostbyname2(), along
   with the new resolver option RES_USE_INET6.

|                  |              RES_USE_INET6 option                 |
|                  +-------------------------+-------------------------+
|                  |          off            |           on            |
|                  |Search for A records.    |Search for AAAA records. |
| gethostbyname    | If found, return IPv4   | If found, return IPv6   |
| (host)           | addresses (h_length=4). | addresses (h_length=16).|
|                  | Else error.             | Else search for A       |
|                  |                         | records.  If found,     |
|                  |Provides backward        | return IPv4-mapped IPv6 |
|                  | compatibility with all  | addresses (h_length=16).|
|                  | existing IPv4 appls.    | Else error.             |
|                  |Search for A records.    |Search for A records.    |
| gethostbyname2   | If found, return IPv4   | If found, return        |
| (host, AF_INET)  | addresses (h_length=4). | IPv4-mapped IPv6        |
|                  | Else error.             | addresses (h_length=16).|
|                  |                         | Else error.             |
|                  |Search for AAAA records. |Search for AAAA records. |
| gethostbyname2   | If found, return IPv6   | If found, return IPv6   |
| (host, AF_INET6) | addresses (h_length=16).| addresses (h_length=16).|
|                  | Else error.             | Else error.             |

   It is expected that when a typical naive application that calls
   gethostbyname() today is modified to use IPv6, it simply changes the
   program to use IPv6 sockets and then enables the RES_USE_INET6
   resolver option before calling gethostbyname().  This application
   will then work with either IPv4 or IPv6 peers.

   Note that gethostbyname() and gethostbyname2() are not thread-safe,
   since both return a pointer to a static hostent structure.  But
   several vendors have defined a thread-safe gethostbyname_r() function
   that requires four additional arguments.  We expect these vendors to
   also define a gethostbyname2_r() function.

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 char *src, int len, int af);

   One possible source of confusion is the handling of IPv4-mapped IPv6
   addresses and IPv4-compatible IPv6 addresses.  This is addressed in
   [6] 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

    4.  If the function is returning success, and if af equals AF_INET,
        and if the RES_USE_INET6 option was set, then the single address
        that is returned in the hostent structure (a copy of the first
        argument to the function) is returned as an IPv4-mapped IPv6
        address and the h_length member is set to 16.

   All four steps listed are performed, in order.  The same caveats
   regarding a thread-safe version of gethostbyname() that were made at
   the end of the previous section apply here as well.

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) work in progress specification [4].

   The official specification for this function will be the final POSIX
   standard.  We are providing this independent description of the
   function because POSIX standards are not freely available (as are
   IETF documents).  Should there be any discrepancies between this
   description and the POSIX description, the POSIX description takes

       #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:

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

       struct addrinfo {
         int     ai_flags;     /* AI_PASSIVE, AI_CANONNAME */
         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
   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

   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
   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
   eturn 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, size_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
   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

       #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

   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

   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

       #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:


   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
   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
   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.

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>       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_ADDRFORM
     <netinet/in.h>  IPV6_ADD_MEMBERSHIP
     <netinet/in.h>  IPV6_DROP_MEMBERSHIP
     <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{};

     <resolv.h>      RES_USE_INET6

     <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 *, size_t,
                                char *, size_t, char *, size_t, int);
<netdb.h>       void freeaddrinfo(struct addrinfo *);
<netdb.h>       char *gai_strerror(int);
<netdb.h>       struct hostent *gethostbyname(const char *);
<netdb.h>       struct hostent *gethostbyaddr(const char *, int, int);
<netdb.h>       struct hostent *gethostbyname2(const char *, int);

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

8.  Security Considerations

   IPv6 provides a number of new security mechanisms, many of which need
   to be accessible to applications.  A companion memo detailing the
   extensions to the socket interfaces to support IPv6 security is being
   written [3].

9.  Acknowledgments

   Thanks to the many people who made suggestions and provided feedback
   to to the numerous revisions of 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 Kazuhiko Yamamoto,

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

   Craig Metz made many contributions to this document.  Ramesh Govindan
   made a number of contributions and co-authored an earlier version of
   this memo.

10.  References

   [1] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
       Specification", RFC 1883, December 1995.

   [2] Hinden, R., and S. Deering, "IP Version 6 Addressing Architecture",
       RFC 1884, December 1995.

   [3] McDonald, D., "A Simple IP Security API Extension to BSD Sockets",
       Work in Progress.

   [4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
       6.3, November 1995.

   [5] Stevens, W., and M. Thomas, "Advanced Sockets API for IPv6",
       Work in Progress.

   [6] Vixie, P., "Reverse Name Lookups of Encapsulated IPv4 Addresses in
       IPv6", Work in Progress.

11.  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

    Phone: +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

    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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