IPNG Working Group                                         R.E. Gilligan
INTERNET-DRAFT: draft-ietf-ipngwg-rfc2553bis-06.txt draft-ietf-ipngwg-rfc2553bis-07.txt           Cache Flow
Obsoletes RFC 2553                                            S. Thomson
                                                                   Cisco
                                                                J. Bound
                                                               J. McCann
                                                         Hewlett-Packard
                                                           W. R. Stevens
                                                          September 2002

               Basic Socket Interface Extensions for IPv6

                 <draft-ietf-ipngwg-rfc2553bis-06.txt>

                 <draft-ietf-ipngwg-rfc2553bis-07.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   This document is a submission by the Internet Protocol IPv6 Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the ipng@sunroof.eng.sun.com mailing list.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   The de facto standard application program interface (API) for TCP/IP
   applications is the "sockets" interface.  Although this API was
   developed for Unix in the early 1980s it has also been implemented on
   a wide variety of non-Unix systems.  TCP/IP applications written
   using the sockets API have in the past enjoyed a high degree of
   portability and we would like the same portability with IPv6
   applications.  But changes are required to the sockets API to support
   IPv6 and this memo describes these changes.  These include a new
   socket address structure to carry IPv6 addresses, new address
   conversion functions, and some new socket options.  These extensions
   are designed to provide access to the basic IPv6 features required by
   TCP and UDP applications, including multicasting, while introducing a
   minimum of change into the system and providing complete
   compatibility for existing IPv4 applications.  Additional extensions
   for advanced IPv6 features (raw sockets and access to the IPv6
   extension headers) are defined in another document [4].

Table of Contents:

1. Introduction.................................................3
2. Design Considerations........................................3
2.1 What Needs to be Changed....................................4
2.2 Data Types..................................................5
2.3 Headers.....................................................5
2.4 Structures..................................................5
3. Socket Interface.............................................5
3.1 IPv6 Address Family and Protocol Family.....................6
3.2 IPv6 Address Structure......................................6
3.3 Socket Address Structure for 4.3BSD-Based Systems...........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
3.10 Portability Additions.....................................11
4. Interface Identification....................................13
4.1 Name-to-Index..............................................14
4.2 Index-to-Name..............................................14
4.3 Return All Interface Names and Indexes.....................14
4.4 Free Memory................................................15
5. Socket Options..............................................15
5.1 Unicast Hop Limit..........................................15
5.2 Sending and Receiving Multicast Packets....................16
5.3 IPV6_V6ONLY option for AF_INET6 Sockets....................18
6. Library Functions...........................................18
6.1 Protocol-Independent Nodename and Service Name Translation.19
6.2 Socket Address Structure to Node Name and Service Name.....23
6.3 Address Conversion Functions...............................25
6.4 Address Testing Macros.....................................26
7. Summary of New Definitions..................................27
8. Security Considerations.....................................29
Changes from RFC 2553..........................................29
Acknowledgments................................................29
References.....................................................30
Authors' Addresses.............................................31

1. Introduction

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

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

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

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

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

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

2. Design Considerations

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

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

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

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

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

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

2.1 What Needs to be Changed

The socket interface API consists of a few distinct components:

   -  Core socket functions.

   -  Address data structures.

   -  Name-to-address translation functions.

   -  Address conversion functions.

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

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

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

The IPv4 name-to-address translation functions in the socket interface

are gethostbyname() and gethostbyaddr().  These are left as is, and new
functions are defined which support both IPv4 and IPv6.

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

Finally, a few miscellaneous features are needed to support IPv6.  New
interfaces are needed to support the IPv6 traffic class, flow label, and
hop limit header fields.  New socket options are needed to control the
sending and receiving of IPv6 multicast packets.

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

2.2 Data Types

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

2.3 Headers

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

2.4 Structures

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

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

3. Socket Interface

This section specifies the socket interface changes for IPv6.

3.1 IPv6 Address Family and Protocol Family

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

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

   #define PF_INET6        AF_INET6

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

3.2 IPv6 Address Structure

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

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

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

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

An example is as follows:

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

3.3 Socket Address Structure for 4.3BSD-Based Systems

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

The sockaddr_in structure is the protocol-specific address data

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

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

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

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

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

The sin6_flowinfo field is a 32-bit field that contains two pieces of
information: the traffic class and the flow label.  The contents and
interpretation of this member is specified in [1].

The sin6_flowinfo
field SHOULD be set to zero by an implementation prior to using the
sockaddr_in6 structure by an application on receive operations.

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

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

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

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

3.4 Socket Address Structure for 4.4BSD-Based Systems

The 4.4BSD release includes a small, but incompatible change to the
socket interface.  The "sa_family" field of the sockaddr data structure
was changed from a 16-bit value to an 8-bit value, and the space saved
used to hold a length field, named "sa_len".  The sockaddr_in6 data
structure given in the previous section cannot be correctly cast into

the newer sockaddr data structure.  For this reason, the following
alternative IPv6 address data structure is provided to be used on
systems based on 4.4BSD.  It is defined as a result of including the
<netinet/in.h> header.

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

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

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

3.5 The Socket Functions

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

   s = socket(AF_INET, SOCK_STREAM, 0);

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

   s = socket(AF_INET, SOCK_DGRAM, 0);

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

   s = socket(AF_INET6, SOCK_STREAM, 0);

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

   s = socket(AF_INET6, SOCK_DGRAM, 0);

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

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

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

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

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

3.6 Compatibility with IPv4 Applications

In order to support the large base of applications using the original
API, system implementations must provide complete source and binary
compatibility with the original API.  This means that systems must
continue to support AF_INET sockets and the sockaddr_in address
structure.  Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the AF_INET constant in the socket() function, as
described in the previous section.  Applications should be able to hold
a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP sockets
simultaneously within the same process.

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

3.7 Compatibility with IPv4 Nodes

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

   ::FFFF:<IPv4-address>

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

Applications may use AF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and passing
that address, within a sockaddr_in6 structure, in the connect() or

sendto() call.  When applications use AF_INET6 sockets to accept TCP
connections from IPv4 nodes, or receive UDP packets from IPv4 nodes, the
system returns the peer's address to the application in the accept(),
recvfrom(), or getpeername() call using a sockaddr_in6 structure encoded
this way.

Few applications will likely need to know which type of node they are
interoperating with.  However, for those applications that do need to
know, the IN6_IS_ADDR_V4MAPPED() macro, defined in Section 6.7, is
provided.

3.8 IPv6 Wildcard Address

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

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

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

   extern const struct in6_addr in6addr_any;

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

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

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

   struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

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

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

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

3.9 IPv6 Loopback Address

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

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

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

   extern const struct in6_addr in6addr_loopback;

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

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

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

   struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;

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

3.10 Portability Additions

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

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

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

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

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

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

The above example implementation illustrates a data structure which will
align on a 64-bit boundary. An implementation-specific field "_ss_align"
"__ss_align" along with "_ss_pad1" "__ss_pad1" is used to force a 64-bit alignment
which covers proper alignment good enough for the needs of sockaddr_in6
(IPv6), sockaddr_in (IPv4) address data structures. The size of padding
field
_ss_pad1 __ss_pad1 depends on the chosen alignment boundary. The size of
padding field _ss_pad2 __ss_pad2 depends on the value of overall size chosen for
the total size of the structure. This size and alignment are represented
in the above example by implementation specific (not required) constants
_SS_MAXSIZE (chosen value 128) and _SS_ALIGNMENT _SS_ALIGNSIZE (with chosen value 8).
Constants _SS_PAD1SIZE (derived value 6) and _SS_PAD2SIZE (derived value

112) are also for illustration and not required. The derived values
assume sa_family_t is 2 bytes. The implementation specific definitions
and structure field names above start with an underscore to denote
implementation private namespace.  Portable code is not expected to
access or reference those fields or constants.

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

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

4. Interface Identification

This API uses an interface index (a small positive integer) to identify
the local interface on which a multicast group is joined (Section 5.3).
Additionally, the advanced API [4] uses these same interface indexes to
identify the interface on which a datagram is received, or to specify
the interface on which a datagram is to be sent.

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

This API defines two functions that map between an interface name and
index, a third function that returns all the interface names and
indexes, and a fourth function to return the dynamic memory allocated by
the previous function.  How these functions are implemented is left up
to the implementation.  4.4BSD implementations can implement these
functions using the existing sysctl() function with the NET_RT_IFLIST
command.  Other implementations may wish to use ioctl() for this

purpose.

4.1 Name-to-Index

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

   #include <net/if.h>

   unsigned int  if_nametoindex(const char *ifname);

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

4.2 Index-to-Name

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

   #include <net/if.h>

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

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

4.3 Return All Interface Names and Indexes

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

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

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

   #include <net/if.h>

   struct if_nameindex  *if_nameindex(void);

The end of the array of structures is indicated by a structure with an

if_index of 0 and an if_name of NULL.  The function returns a NULL
pointer upon an error, and would set errno to the appropriate value.

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

4.4 Free Memory

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

   #include <net/if.h>

   void  if_freenameindex(struct if_nameindex *ptr);

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

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

5. Socket Options

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

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

5.1 Unicast Hop Limit

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

   int  hoplimit = 10;

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

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

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

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

   int  hoplimit;
   socklen_t  len = sizeof(hoplimit);

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

5.2 Sending and Receiving Multicast Packets

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

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

   IPV6_MULTICAST_IF

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

      Argument type: unsigned int

   IPV6_MULTICAST_HOPS

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

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

         x < -1:        return an error of EINVAL
         x == -1:       use kernel default
         0 <= x <= 255: use x
         x >= 256:      return an error of EINVAL
         If IPV6_MULTICAST_HOPS is not set, the default is 1
         (same as IPv4 today)

      Argument type: int

   IPV6_MULTICAST_LOOP

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

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

      Argument type: unsigned int

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

   IPV6_JOIN_GROUP

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

      Argument type: struct ipv6_mreq

   IPV6_LEAVE_GROUP

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

      Argument type: struct ipv6_mreq

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

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

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

5.3 IPV6_V6ONLY option for AF_INET6 Sockets

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

       Here is an example of setting this option:

           int on = 1;

           if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY,
                          (char *)&on, sizeof(on)) == -1)
               perror("setsockopt IPV6_V6ONLY");
           else
               printf("IPV6_V6ONLY set0); set\n");

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

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

6. Library Functions

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

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

The commonly used function gethostbyname() is inadequate for many
applications, first because it provides no way for the caller to specify
anything about the types of addresses desired (IPv4 only, IPv6 only,
IPv4-mapped IPv6 are OK, etc.), and second because many implementations
of this function are not thread safe.  RFC 2133 defined a function named
gethostbyname2() but this function was also inadequate, first because
its use required setting a global option (RES_USE_INET6) when IPv6
addresses were required, and second because a flag argument is needed to
provide the caller with additional control over the types of addresses
required.  The gethostbyname2() function was deprecated in RFC 2553 and
is no longer part of the basic API.

6.1 Protocol-Independent Nodename and Service Name Translation

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

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

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

   void freeaddrinfo(struct addrinfo *ai);

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

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

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

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

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

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

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

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

       Note:

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

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

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

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

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

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

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

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

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

   The AI_V4MAPPED flag shall be ignored unless ai_family equals
   AF_INET6.

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

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

   The AI_ALL flag without the AI_V4MAPPED flag is ignored.

      Note:

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

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

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

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

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

   The freeaddrinfo() function frees one or more addrinfo structures
   returned by getaddrinfo(), along with any additional storage
   associated with those structures. structures (for example, storage pointed to by
   the ai_canonname and ai_addr fields; an application must not
   reference this storage after the associated addrinfo structure has
   been freed).  If the ai_next field of the structure is not null, the
   entire list of structures is freed. The freeaddrinfo() function must
   support the freeing of arbitrary sublists of an addrinfo list
   originally returned by getaddrinfo().

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

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

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

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

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

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

   Error Return Values:

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

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

      [EAI_BADFLAGS]  The flags parameter had an invalid value.

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

      [EAI_FAMILY]    The address family was not recognized.

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

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

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

      [EAI_SOCKTYPE]  The intended socket type was not recognized.

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

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

   #include <netdb.h>

   const char *gai_strerror(int ecode);

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

6.2 Socket Address Structure to Node Name and Service Name

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

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

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

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

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

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

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

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

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

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

The arguments node and service cannot both be NULL.

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

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

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

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

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

  - If the flag bit NI_NUMERICSCOPE is set, the numeric form of the
    scope identifier shall be returned (for example, interface index)
    instead of its name.  This flag is ignored if the sa argument is
    not an IPv6 address.

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

Note:

  1.  The three NI_NUMERICxxx flags are required to support the "-n"
      flags that many commands provide.
  2.  The NI_DGRAM flag is required for the few AF_INET and AF_INET6 port
      numbers (for example, [512,514]) that represent different services
      for UDP and TCP.

The getnameinfo() function shall be thread safe.

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

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

Error Return Values:

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

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

     [EAI_BADFLAGS] The flags had an invalid value.

     [EAI_FAIL]     A non-recoverable error occurred.

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

     [EAI_MEMORY]   There was a memory allocation failure.

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

     [EAI_OVERFLOW] An argument buffer overflowed.

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

6.3 Address Conversion Functions

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

   #include <arpa/inet.h>

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

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

The inet_pton() function shall convert an address in its standard text
presentation form into its numeric binary form.  The af argument shall
specify the family of the address.  The AF_INET and AF_INET6 address
families shall be supported.  The src argument points to the string

being passed in.  The dst argument points to a buffer into which the
function stores the numeric address; this shall be large enough to hold
the numeric address (32 bits for AF_INET, 128 bits for AF_INET6).  The
inet_pton() function shall return 1 if the conversion succeeds, with the
address pointed to by dst in network byte order.  It shall return 0 if
the input is not a valid IPv4 dotted-decimal string or a valid IPv6
address string, or -1 with errno set to EAFNOSUPPORT if the af argument
is unknown.

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

   ddd.ddd.ddd.ddd

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

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

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

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

   #define INET_ADDRSTRLEN    16
   #define INET6_ADDRSTRLEN   46

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

6.4 Address Testing Macros

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

   #include <netinet/in.h>

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

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

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

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

7. Summary of New Definitions

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

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

   <netdb.h>       AI_ADDRCONFIG
   <netdb.h>       AI_ALL
   <netdb.h>       AI_CANONNAME
   <netdb.h>       AI_NUMERICHOST
   <netdb.h>       AI_NUMERICSERV
   <netdb.h>       AI_PASSIVE
   <netdb.h>       AI_V4MAPPED
   <netdb.h>       EAI_AGAIN
   <netdb.h>       EAI_BADFLAGS
   <netdb.h>       EAI_FAIL
   <netdb.h>       EAI_FAMILY
   <netdb.h>       EAI_MEMORY
   <netdb.h>       EAI_NONAME
   <netdb.h>       EAI_OVERFLOW
   <netdb.h>       EAI_SERVICE
   <netdb.h>       EAI_SOCKTYPE
   <netdb.h>       EAI_SYSTEM
   <netdb.h>       NI_DGRAM
   <netdb.h>       NI_NAMEREQD
   <netdb.h>       NI_NOFQDN
   <netdb.h>       NI_NUMERICHOST
   <netdb.h>       NI_NUMERICSERV
   <netdb.h>       struct addrinfo{};

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

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

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

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

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

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

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

8. Security Considerations

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

Changes from RFC 2553

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

   2.  Alignments with [3].

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

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

   5.  Added SIIT to references and added new contributors.

Acknowledgments

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

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

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

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

Interface".

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

References

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

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

   [3]  IEEE Std. 1003.1-2001 Standard for Information Technology --
        Portable Operating System Interface (POSIX)

        Open Group Technical Standard: Base Specifications, Issue 6
        December 2001

        ISO 9945 (pending final approval by ISO)

        http://www.opengroup.org/austin

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

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

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

Authors' Addresses

Bob Gilligan
Cacheflow, Inc.
650 Almanor Ave.
Sunnyvale, CA 94086
Telephone: 408-220-2084 (voice)
           408-220-2250 (fax)
Email: gilligan@cacheflow.com

Susan Thomson
Cisco Systems
499 Thornall Street, 8th floor
Edison, NJ 08837
Telephone: 732-635-3086
Email:  sethomso@cisco.com

Jim Bound
Hewlett-Packard Company
110 Spitbrook Road ZKO3-3/W20
Nashua, NH 03062
Telephone: 603-884-0062
Email: Jim.Bound@hp.com

Jack McCann
Hewlett-Packard Company
110 Spitbrook Road ZKO3-3/W20
Nashua, NH 03062
Telephone: 603-884-2608
Email: Jack.McCann@hp.com