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rfc:rfc2133

Network Working Group R. Gilligan Request for Comments: 2133 Freegate Category: Informational S. Thomson

                                                              Bellcore
                                                              J. Bound
                                                               Digital
                                                            W. Stevens
                                                            Consultant
                                                            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.

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 [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

Gilligan, et. al. Informational [Page 1] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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.

Gilligan, et. al. Informational [Page 2] RFC 2133 IPv6 Socket Interface Extensions April 1997

2. Design Considerations

 There are a number of important considerations in designing changes
 to this well-worn API:
  1. 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.
  1. The changes to the API should be as small as possible in order to

simplify the task of converting existing IPv4 applications to

     IPv6.
  1. 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.
  1. 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:
  1. Core socket functions.
  1. Address data structures.
  1. Name-to-address translation functions.
  1. 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

Gilligan, et. al. Informational [Page 3] RFC 2133 IPv6 Socket Interface Extensions April 1997

  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
  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
  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].

Gilligan, et. al. Informational [Page 4] RFC 2133 IPv6 Socket Interface Extensions April 1997

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
 structure.
 A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
 Like most of the other protocol family names, this will usually be
 defined to have the same value as the corresponding address family
 name:
     #define PF_INET6        AF_INET6
 The PF_INET6 is used in the first argument to the socket() function
 to indicate that an IPv6 socket is being created.

Gilligan, et. al. Informational [Page 5] RFC 2133 IPv6 Socket Interface Extensions April 1997

3.2. IPv6 Address Structure

 A new data structure to hold a single IPv6 address is defined as
  follows:
     #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
 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
 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.

Gilligan, et. al. Informational [Page 6] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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:
     #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 */
     };

Gilligan, et. al. Informational [Page 7] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 system.  The functions that the application uses to pass addresses
 into the system are:
     bind()
     connect()
     sendmsg()
     sendto()

Gilligan, et. al. Informational [Page 8] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 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).

Gilligan, et. al. Informational [Page 9] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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;

Gilligan, et. al. Informational [Page 10] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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.

Gilligan, et. al. Informational [Page 11] RFC 2133 IPv6 Socket Interface Extensions April 1997

 The global variable is an in6_addr structure named
 "in6addr_loopback."  The extern declaration for this variable is
 defined in <netinet/in.h>:
     extern const struct in6_addr in6addr_loopback;
 Applications use in6addr_loopback as they would use INADDR_LOOPBACK
 in IPv4 applications (but beware of the byte ordering difference
 mentioned at the end of the previous section).  For example, to open
 a TCP connection to the local telnet server, an application could use
 the following code:
     struct sockaddr_in6 sin6;
      . . .
     sin6.sin6_family = AF_INET6;
     sin6.sin6_flowinfo = 0;
     sin6.sin6_port = htons(23);
     sin6.sin6_addr = in6addr_loopback;  /* structure assignment */
      . . .
     if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
             . . .
 The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined
 in <netinet/in.h>.  It can be used at declaration time ONLY; for
 example:
     struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
 Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
 to a previously declared IPv6 address variable.

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
 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.

Gilligan, et. al. Informational [Page 12] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 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.

Gilligan, et. al. Informational [Page 13] RFC 2133 IPv6 Socket Interface Extensions April 1997

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
 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>.

Gilligan, et. al. Informational [Page 14] RFC 2133 IPv6 Socket Interface Extensions April 1997

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
 functions.
 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
 functions.
 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");

Gilligan, et. al. Informational [Page 15] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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");
     else
         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

Gilligan, et. al. Informational [Page 16] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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);

5.3. 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()
 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:
     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

Gilligan, et. al. Informational [Page 17] RFC 2133 IPv6 Socket Interface Extensions April 1997

     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.
         Argument type: unsigned int
 The reception of multicast packets is controlled by the two
 setsockopt() options summarized below:
     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:
     #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.

Gilligan, et. al. Informational [Page 18] RFC 2133 IPv6 Socket Interface Extensions April 1997

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:
  1. 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.
  1. 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.

Gilligan, et. al. Informational [Page 19] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 option.
  1. The first way is
         res_init();
         _res.options |= RES_USE_INET6;
     and then call either gethostbyname() or gethostbyname2().  This
     option then affects only the process that is calling the
     resolver.
  1. 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.
  1. 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:
  1. 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.
  1. 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
 structure.

Gilligan, et. al. Informational [Page 20] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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.

Gilligan, et. al. Informational [Page 21] RFC 2133 IPv6 Socket Interface Extensions April 1997

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
      domain.
  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
 precedence.

Gilligan, et. al. Informational [Page 22] RFC 2133 IPv6 Socket Interface Extensions April 1997

     #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.

Gilligan, et. al. Informational [Page 23] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 hostname.

Gilligan, et. al. Informational [Page 24] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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.

Gilligan, et. al. Informational [Page 25] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 <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.

Gilligan, et. al. Informational [Page 26] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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
 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.

Gilligan, et. al. Informational [Page 27] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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 *);

Gilligan, et. al. Informational [Page 28] RFC 2133 IPv6 Socket Interface Extensions April 1997

 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

Gilligan, et. al. Informational [Page 29] RFC 2133 IPv6 Socket Interface Extensions April 1997

   <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 *);

Gilligan, et. al. Informational [Page 30] RFC 2133 IPv6 Socket Interface Extensions April 1997

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.

Gilligan, et. al. Informational [Page 31] RFC 2133 IPv6 Socket Interface Extensions April 1997

 [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

Gilligan, et. al. Informational [Page 32]

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