IPNG Working Group                                         R.E. Gilligan
INTERNET-DRAFT: draft-ietf-ipngwg-rfc2553bis-00.txt             Freegate draft-ietf-ipngwg-rfc2553bis-01.txt           Cache Flow
Obsoletes RFC 2553                                            S. Thomson
                                                                Bellcore
                                                                   Cisco
                                                                J. Bound
                                                                  Compaq
                                                           W. R. Stevens
                                                              Consultant
                                                            October 2000

               Basic Socket Interface Extensions for IPv6

                 <draft-ietf-ipngwg-rfc2553bis-00.txt>

                 <draft-ietf-ipngwg-rfc2553bis-01.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
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   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.................................................4
2. Design Considerations........................................4
2.1 What Needs to be Changed....................................4
2.2 Data Types..................................................6
2.3 Headers.....................................................6
2.4 Structures..................................................6
3. Socket Interface.............................................6
3.1 IPv6 Address Family and Protocol Family.....................6
3.2 IPv6 Address Structure......................................7
3.3 Socket Address Structure for 4.3BSD-Based Systems...........7
3.4 Socket Address Structure for 4.4BSD-Based Systems...........8
3.5 The Socket Functions........................................9
3.6 Compatibility with IPv4 Applications.......................10
3.7 Compatibility with IPv4 Nodes..............................10
3.8 IPv6 Wildcard Address......................................11
3.9 IPv6 Loopback Address......................................12
3.10 Portability Additions.....................................12
4. Interface Identification....................................14
4.1 Name-to-Index..............................................14
4.2 Index-to-Name..............................................15
4.3 Return All Interface Names and Indexes.....................15
4.4 Free Memory................................................15
5. Socket Options..............................................16
5.1 Unicast Hop Limit..........................................16
5.2 Sending and Receiving Multicast Packets....................17
5.3 IPV6_ONLY option for AF_INET6 Sockets......................18
6. Library Functions...........................................18 Functions...........................................19
6.1 Nodename-to-Address Translation............................18
6.2 Address-To-Nodename Translation............................21
6.3 Freeing memory for getipnodebyname and getipnodebyaddr.....22
6.4 Protocol-Independent Nodename and Service Name Translation.23
6.5 Translation.19
6.2 Socket Address Structure to Nodename and Service Name......27
6.6 Name......23
6.3 Address Conversion Functions...............................29
6.7 Functions...............................25
6.4 Address Testing Macros.....................................30 Macros.....................................26
7. Summary of New Definitions..................................30 Definitions..................................27
8. Security Considerations.....................................32 Considerations.....................................29
9. Year 2000 Considerations....................................32 Considerations....................................29
Changes made to rfc2553bis-00 to rfc2553bis-01.................29
Changes made rfc2553 to rfc2553bis-00:.........................32
Acknowledgments................................................33
References.....................................................33 rfc2553bis-00:.........................29
Acknowledgments................................................30
References.....................................................30
Authors' Addresses.............................................34 Addresses.............................................31

1. Introduction

While IPv4 addresses are 32 bits long, IPv6 interfaces are identified by
128-bit addresses.  The socket interface makes the size of an IP address
quite visible to an application; virtually all TCP/IP applications for
BSD-based systems have knowledge of the size of an IP address.  Those
parts of the API that expose the addresses must be changed to
accommodate the larger IPv6 address size.  IPv6 also introduces new
features (e.g., traffic class and flowlabel), some of which must be made
visible to applications via the API.  This memo defines a set of
extensions to the socket interface to support the larger address size
and new features of IPv6.

2. Design Considerations

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

   - The API changes should provide both source and binary
     compatibility for programs written to the original API.  That
     is, existing program binaries should continue to operate when
     run on a system supporting the new API.  In addition, existing
     applications that are re-compiled and run on a system supporting
     the new API should continue to operate.  Simply put, the API
     changes for IPv6 should not break existing programs.  An additonal
     mechanism for implementations to verify this is to verify the new
     symbols are protected by Feature Test Macros as described in IEEE Std
     1003.1.  (Such Feature Test Macros are not defined by this RFC.)

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

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

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

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

2.1 What Needs to be Changed

The socket interface API consists of a few distinct components:

   -  Core socket functions.

   -  Address data structures.

   -  Name-to-address translation functions.

   -  Address conversion functions.

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

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

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

The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr().  These are left as is and new
functions are defined to support IPv4 and IPv6.  Additionally,  The new API is based on
the POSIX 1003.g draft [3] and specifies a new nodename-to-address
translation function which is protocol independent.  This function can
also be used with IPv4 and 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 traffic class, flow label, and
hop limit header fields.  New socket options are needed to control the
sending and receiving of IPv6 multicast packets.

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

2.2 Data Types

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

2.3 Headers

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

2.4 Structures

When structures are described the members shown are the ones that must
appear in an implementation.  Additional, nonstandard members may also
be defined by an implementation.  As an additional precaution
nonstandard members could be verified by Feature Test Macros as
described in IEEE Std 1003.1.  (Such Feature Test Macros are not defined
by this RFC.)

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

3. Socket Interface

This section specifies the socket interface changes for IPv6.

3.1 IPv6 Address Family and Protocol Family

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

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

   #define PF_INET6        AF_INET6

The PF_INET6 is used in the first argument to the socket() function to
indicate that an IPv6 socket is being created.

3.2 IPv6 Address Structure

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

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

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

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

An example is as follows:

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 of the address carried in the
sin6_addr field [2,5,6,7].  For a link scope sin6_addr, sin6_scope_id
would be an interface index.  For a site scope sin6_addr, sin6_scope_id
would be a site identifier.  The mapping of sin6_scope_id to an
interface or set of interfaces is left to implementation and future
specifications on the subject of site identifiers.

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

3.4 Socket Address Structure for 4.4BSD-Based Systems

The 4.4BSD release includes a small, but incompatible change to the
socket interface.  The "sa_family" field of the sockaddr data structure
was changed from a 16-bit value to an 8-bit value, and the space saved
used to hold a length field, named "sa_len".  The sockaddr_in6 data
structure given in the previous section cannot be correctly cast into
the newer sockaddr data structure.  For this reason, the following
alternative IPv6 address data structure is provided to be used on
systems based on 4.4BSD.  It is defined as a result of including the
<netinet/in.h> header.

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

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

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

3.5 The Socket Functions

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

   s = socket(PF_INET, SOCK_STREAM, 0);

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

   s = socket(PF_INET, SOCK_DGRAM, 0);

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

   s = socket(PF_INET6, SOCK_STREAM, 0);

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

   s = socket(PF_INET6, SOCK_DGRAM, 0);

Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the system.
The functions that the application uses to pass addresses into the
system are:

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

The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets.  The
functions that return an address from the system to an application are:

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

No changes to the syntax of the socket functions are needed to support

IPv6, since all of the "address carrying" functions use an opaque
address pointer, and carry an address length as a function argument.

3.6 Compatibility with IPv4 Applications

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

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

3.7 Compatibility with IPv4 Nodes

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

   ::FFFF:<IPv4-address>

These addresses can be generated automatically by the getipnodebyname()
and getaddrinfo() functions,
function, when the specified host has only IPv4 addresses (as described
in Section 6.1 and 6.4). 6.2).

Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and passing
that address, within a sockaddr_in6 structure, in the connect() or
sendto() call.  When applications use PF_INET6 sockets to accept TCP
connections from IPv4 nodes, or receive UDP packets from IPv4 nodes, the
system returns the peer's address to the application in the accept(),
recvfrom(), or getpeername() call using a sockaddr_in6 structure encoded
this way.

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

3.8 IPv6 Wildcard Address

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

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

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

   extern const struct in6_addr in6addr_any;

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

   - Large enough to accomodate 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)+
                              _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" along "_ss_pad1" is used to force a 64-bit alignment which
covers proper alignment good enough for needs of sockaddr_in6 (IPv6),
sockaddr_in (IPv4) address data structures. The size of padding fields
_ss_pad1 depends on the chosen alignment boundary. The size of padding
field _ss_pad2 depends on the value of overall size chosen for the total
size of the structure. This size and alignment are represented in the
above example by implementation specific (not required) constants
_SS_MAXSIZE (chosen value 128) and _SS_ALIGNMENT (with chosen value 8).
Constants _SS_PAD1SIZE (derived value 6) and _SS_PAD2SIZE (derived value
112) are also for illustration and not required. The implementation specific
definitions and structure field names above start with an underscore to
denote implementation private namespace.  Portable code is not expected to
access or reference those fields or constants.

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

/*
 * Definitions used for sockaddr_storage structure paddings design.

 */
#define _SS_PAD1SIZE (_SS_ALIGNSIZE -
                            (sizeof (uint8_t) + sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t)+
                              _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 the specified interface name does not exist, the return value is 0,
and errno is set to ENXIO.   If there was a system error (such as
running out of memory), the return value is 0 and errno is set to the
proper value (e.g., ENOMEM).

4.2 Index-to-Name

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

   #include <net/if.h>

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

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

4.3 Return All Interface Names and Indexes

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

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

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

   struct if_nameindex  *if_nameindex(void);

The end of the array of structures is indicated by a structure with an
if_index of 0 and an if_name of NULL.  The function returns a NULL
pointer upon an error, and would set errno to the appropriate value.

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

4.4 Free Memory

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

   #include <net/if.h>
   void  if_freenameindex(struct if_nameindex *ptr);

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

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

5. Socket Options

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

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

5.1 Unicast Hop Limit

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

   int  hoplimit = 10;

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

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

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

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

   int  hoplimit;
   size_t  len = sizeof(hoplimit);

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

5.2 Sending and Receiving Multicast Packets

IPv6 applications may send UDP multicast packets by simply specifying an
IPv6 multicast address in the address argument of the sendto() function.

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

   IPV6_MULTICAST_IF

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

      Argument type: unsigned int

   IPV6_MULTICAST_HOPS

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

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

         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.

      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 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.

5.3 IPV6_ONLY 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);

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) [8].

6. Library Functions

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

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

6.1 Nodename-to-Address Translation

Note: This function at this time MUST not be used by any applications
that will want to support scope_ids [5,6,7] within an application, and
for those applications the getaddrinfo() function should be used as
specified in section 6.4 of this specification.

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

The following

6.1 Protocol-Independent Nodename and Service Name Translation

Nodename-to-address translation is done in a protocol-independent
fashion using the getaddrinfo() function that is new taken from the
Institute of Electrical and must be thread safe:

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

   struct hostent *getipnodebyname(const char *name, int af, int flags
                                       int *error_num); Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) draft specification [3].

The name argument can official specification for this function will be either a node name or a numeric address string
(i.e., a dotted-decimal IPv4 address or an IPv6 hex address).  The af
argument specifies the address family, either AF_INET or AF_INET6. The
error_num value is returned to the caller, via a pointer, with the
appropriate error code in error_num, to support thread safe error code
returns.  error_num will be set to one of the following values:

   HOST_NOT_FOUND

     No such host is known.

   NO_ADDRESS

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

   NO_RECOVERY

     An unexpected server failure occurred which cannot be recovered.

   TRY_AGAIN

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

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

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

   hptr = gethostbyname(name);

with

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

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

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

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

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

   -  If flags is 0 and if af is AF_INET6, then the caller wants only
      IPv6 addresses.  A query is made for AAAA/A6 records.  If successful,
      the IPv6 addresses are returned and the h_length member of the
      hostent structure will be 16, else the function returns a NULL
      pointer.

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

   -  If the AI_V4MAPPED flag is specified along with an af of
      AF_INET6, then the caller will accept IPv4-mapped IPv6
      addresses.  That is, if no AAAA or A6 records are found then a query
      is made for A records and any found are returned as IPv4-mapped
      IPv6 addresses (h_length will be 16).  The AI_V4MAPPED flag is
      ignored unless af equals AF_INET6.

   -  The AI_ALL flag is used in conjunction with the AI_V4MAPPED
      flag, and is only used with the IPv6 address family.  When AI_ALL
      is logically or'd with AI_V4MAPPED flag then the caller wants
      all addresses: IPv6 and IPv4-mapped IPv6.  A query is first made
      for AAAA/A6 records and if successful, the IPv6 addresses are returned.
      Another query is then made for A records and any found are returned
      as IPv4-mapped IPv6 addresses. h_length will be 16.  Only if both
      queries fail does the function return a NULL pointer.  This flag is
      ignored unless af equals AF_INET6.

   -  The AI_ADDRCONFIG flag specifies that a query for AAAA/A6 records
      should occur only if the node has at least one IPv6 source address
      configured and a query for A records should occur only if the
      node has at least one IPv4 source address configured.

      For example, if the node has no IPv6 source addresses configured,
      and af equals AF_INET6, and the node name being looked up has both
      AAAA/A6 and A records, then:

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

The special flags value of AI_DEFAULT is defined as

   #define  AI_DEFAULT  (AI_V4MAPPED | AI_ADDRCONFIG)

We noted that the getipnodebyname() function must allow the name

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

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

The two simple cases are:

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

When name is a dotted-decimal IPv4 address and af equals AF_INET6, and
flags equals AI_V4MAPPED, an IPv4-mapped IPv6 address is returned:
h_name points to a copy of the name argument, h_aliases is a NULL
pointer, h_addrtype is AF_INET6, h_length is 16, h_addr_list[0] is a
pointer to the 16-byte binary address, and h_addr_list[1] is a NULL
pointer.

If AI_V4MAPPED is set (with or without AI_ALL) return IPv4-mapped
otherwise return NULL.

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

6.2 Address-To-Nodename Translation

Note:  Applications using scope identifiers should use getnameinfo() as
spepcified in section 6.5.

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

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

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

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

   HOST_NOT_FOUND

     No such host is known.

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

   NO_RECOVERY

     An unexpected server failure occurred which cannot be recovered.

   TRY_AGAIN

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

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

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

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

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

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

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

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

6.3 Freeing memory for getipnodebyname and getipnodebyaddr

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

   #include <netdb.h>

   void freehostent(struct hostent *ptr);

6.4 Protocol-Independent Nodename and Service Name Translation

Nodename-to-address translation specification is done in a protocol-independent
fashion using not specifying all
parameter possibilities for this function, but only the getaddrinfo() function parameters that
can be provided to support IPv4 and IPv6 communications to support this
specification.  This is taken from beyond the
Institute scope of Electrical this document and Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) draft specification [3].

The official specification for additional
work on this function will be done by the final IEEE POSIX
standard. group.

      #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;    /* PF_xxx */
     int     ai_socktype;  /* SOCK_xxx */
     int     ai_protocol;  /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
     size_t  ai_addrlen;   /* length of ai_addr */
     char   *ai_canonname; /* canonical name for nodename */
     struct sockaddr  *ai_addr; /* binary address */
     struct addrinfo  *ai_next; /* next structure in linked list */
   };

   The 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 protocol 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, all successful results will 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,
   or AF_UNSPEC, the permisssable nodename argument is specified as
   defined in inet_pton().  If the specified address family is AF_INET6
   or AF_UNSPEC, the permisssable nodename argument is specified as
   defined in [5].

   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 returns 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
   must be zero or a null pointer. A value of AF_UNSPEC for ai_family
   means that the caller will accept any protocol family. A value of
   zero for ai_socktype means that the caller will accept any socket
   type. A value of zero for ai_protocol means that the caller will
   accept any protocol. If hints is a null pointer, the behavior must 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 PF_INET when
          getaddrinfo ( ) is called.

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

   The AI_PASSIVE flag in the ai_flags member of the hints structure
   specifies how to fill in the IP address portion of the socket address
   structure. If the AI_PASSIVE flag is specified, then the returned
   address information must 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 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, the returned address
   information must 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 will be set to the loopback address.  This flag is
   ignored if the nodename argument is not null.

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

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

   If the flag AI_NUMERICSERV is specified then a non-null servname
   string must be a numeric port string. Otherwise an error [EAI_NONAME]
   is returned. This flag prevents 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 the caller will accept IPv4-mapped IPv6 addresses.
   That is, if no AAAA or A6, records are found then a query is made for
   A records and any found are returned as IPv4-mapped IPv6 addresses
   (ai_addrlen will be 16). The AI_V4MAPPED flag is ignored unless
   ai_family equals AF_INET6.

   The AI_ALL flag is used in conjunction with the AI_V4MAPPED flag, and
   is only used with an ai_family of AF_INET6.  When AI_ALL is logically
   or'd with AI_V4MAPPED flag then the caller will accept all addresses:
   IPv6 and IPv4-mapped IPv6.  A query is first made for AAAA/A6 records
   and if successful, the IPv6 addresses are returned.  Another query is
   then made for A records and any found are returned as IPv4-mapped
   IPv6 addresses (ai_addrlen will be 16).  This flag is ignored unless
   ai_family equals AF_INET6.

      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 then a query for AAAA or A6
   records should occur only if the node has at least one IPv6 source
   address configured and a query for A records should occur only if the
   node has at least one IPv4 source address configured.  The loopback
   address is not considered for this case as valid as a configured
   sources address.

   The ai_socktype field to which argument hints points specifies the
   socket type for the service. 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 will attempt to resolve the service name for all
   supported socket types and, all successful results will be returned.
   A non-zero socket type value will limit the returned information to
   values with the specified socket type.

   The freeaddrinfo ( ) function frees one or more addrinfo structures
   returned by getaddrinfo ( ), along with any additional storage
   associated with those structures. 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.

   Upon successful return of getaddrinfo ( ), the location to which res
   points refers to a linked list of addrinfo structures, each of which
   specifies a socket address and information for use in creating a
   socket with which to use that socket address. The list must 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 includes 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 are
   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
   points to a null-terminated string containing the canonical name
   corresponding to the input nodename; if the canonical name is not
   available, then ai_canonname refers to the argument nodename or a
   string with the same contents. The contents of the ai_flags field of
   the returned structures is undefined.

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

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

   Error Return Values:

      [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 passed. At least one
                      of these must be passed.

      [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.

   #include <sys/socket.h>

   #include <netdb.h>

   char *gai_strerror(int ecode);

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

6.5

6.2 Socket Address Structure to Nodename and Service Name

The official specification for this function will be the final POSIX
standard update to getaddrinfo(), and will incorporate this function.
In addition this specification is not specifying all parameter
possibilities for this function, but only the parameters that can be
provided to support IPv4 and IPv6 communications to support this
specification.  This is beyond the scope of this document and additional
work on this function will be done by the IEEE POSIX group.

   #include <sys/socket.h>
   #include <netdb.h>
   int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                   char *host, size_t hostlen,
                   char *serv, size_t servlen,
                   int flags);

The getnameinfo( ) translates a socket address to a node name and
service location, all of which are defined as with getaddrinfo ().

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

If the argument node is non-NULL and the argument nodelen is nonzero,
then the argument node points to a buffer able to contain up to nodelen
characters that will receive the node name as a null-terminated string.
If the argument node is NULL or the argument nodelen is zero, the node
name will not be returned. If the nodeÆs name cannot be located, the
numeric form of the nodes address is returned instead of its name. If
the sa argument is an IPv6 address the returned nodename may be in the
format as defined in [5].

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

  - If the flag bit NI_NOFQDN is set, only the nodename portion of the
    FQDN is returned for local hosts.

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

  - If the flag bit NI_NAMEREQD is set, an error is returned if the
    hostÆs name cannot be located.

  - If the flag bit NI_NUMERICSERV is set, the numeric form of the
    service address is 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 is 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 is to 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 support.
  2.  The NI_DGRAM flag is required for the new AF_INET/AF_INET6 port
      numbers (for example, 512-514) that represent different services
      for UDP and TCP.

Function getnameinfo() must be thread safe.

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

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

Error Return Values:

     [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 hostÆs name cannot be located, or
                    both nodename and servname were null.

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

6.6

6.3 Address Conversion Functions

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

   #include <sys/socket.h>
   #include <arpa/inet.h>

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

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

The inet_pton() function converts an address in its standard text
presentation form into its numeric binary form.  The af argument
specifies the family of the address.  Currently the AF_INET and AF_INET6
address families are supported.  The src argument points to the string
being passed in.  The dst argument points to a buffer into which the

function stores the numeric address.  The address is returned in network
byte order.  Inet_pton() returns 1 if the conversion succeeds, 0 if the
input is not a valid IPv4 dotted-decimal string or a valid IPv6 address
string, or -1 with errno set to EAFNOSUPPORT if the af argument is
unknown.  The calling application must ensure that the buffer referred
to by dst is large enough to hold the numeric address (e.g., 4 bytes for
AF_INET or 16 bytes for AF_INET6).

If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted-decimal form:

   ddd.ddd.ddd.ddd

where ddd is a one to three digit decimal number between 0 and 255.
Note that many implementations of the existing inet_addr() and
inet_aton() functions accept nonstandard input: octal numbers,
hexadecimal numbers, and fewer than four numbers.  inet_pton() does not
accept these formats.

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

The inet_ntop() function converts a numeric address into a text string
suitable for presentation.  The af argument specifies the family of the
address.  This can be AF_INET or AF_INET6.  The src argument points to a
buffer holding an IPv4 address if the af argument is AF_INET, or an IPv6
address if the af argument is AF_INET6, the address must be in network
byte order.  The dst argument points to a buffer where the function will
store the resulting text string.  The size argument specifies the size
of this buffer.  The application must specify a non-NULL dst argument.
For IPv6 addresses, the buffer must be at least 46-octets.  For IPv4
addresses, the buffer must be at least 16-octets.  In order to allow
applications to easily declare buffers of the proper size to store IPv4
and IPv6 addresses in string form, the following two constants are
defined in <netinet/in.h>:

   #define INET_ADDRSTRLEN    16
   #define INET6_ADDRSTRLEN   46

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

6.7

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 local-use IPv6
unicast addresses.  These two macros do not return true for IPv6
multicast addresses of either link-local scope or site-local scope.

7. Summary of New Definitions

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

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

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

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

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

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

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

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

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

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

8. Security Considerations

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

9. Year 2000 Considerations

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

Changes made to rfc2553bis-00 to rfc2553bis-01

   1.  Removed all references to getipnodebyname() and
       getipnodebyaddr().

   2.  Added IPV6_ONLY Socket IP level option to permit nodes
       to not process IPv4 packets as IPv4 Mapped addresses
       in implementations.

   3.  Added note to getaddrinfo() and getnameinfo()
       that final specification of paramter associations for
       these functions will be done by POSIX.

   4.  Added SIIT to references and added new contributors.

Changes made rfc2553 to rfc2553bis-00:

   1.  Updated Portability Section 3.10 to conform to XNS 5.2.

   2.  Updated getaddrinfo(), getnameinfo(), to conform to XNS 5.2.

   3.  Added references to Scope Architecture, Scope Routing, and
       Extension Format for Scoped Addresses work in progress.

   4.  Added NI_NUMERICSCOPE flag to getnameinfo().

   5.  Added qualification to getipnodebyname/addr() functions that
       they will not work as is with scope identifiers with IPv6, and
       getaddrinfo/getnameinfo should be used.

   6.  Added DNS A6 record notation to AAAA and added ip6.arpa as new
       PTR record domain.

Acknowledgments

This specification's evolution and completeness were siginficantly
influenced by the efforts of Richard Stevens, who has passed on.  Rich's
wisdom and talent made the specification what it is today.  The 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, 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, "Protocol Independent Interfaces",
        IEEE Std 1003.1g, DRAFT 6.6,
        March 1997.

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

   [5]  T. Jinmei, A. Onoe, "An Extension of Format for IPv6 Scoped
        Addresses", Work-in-Progress.

   [6]  S. Deering, B. Haberman, B. Zill "IP Version 6 Scoped Address
        Architecture", Work-in-Progress.

   [7]  B. Haberman " Routing of Scoped Addresses in the Internet Protocol
        Version 6 (IPv6)", Work-in-Progress.

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

Authors' Addresses

Robert E.

Bob Gilligan
FreeGate Corporation
1208 E. Arques
Cacheflow, Inc.
650 Almanor Ave.
Sunnyvale, CA 94086
Phone: +1 408 617 1004
Telephone: 408-220-2084 (voice)
           408-220-2250 (fax)
Email: gilligan@freegate.net gilligan@cacheflow.com

Susan Thomson
Bell Communications Research
MRE 2P-343, 445 South Street
Morristown,
Cisco Systems
499 Thornall Street, 8th floor
Edison, NJ 07960 08837
Telephone: +1 201 829 4514 732-635-3086
Email: set@thumper.bellcore.com  sethomso@cisco.com

Jim Bound
Compaq Computer Corporation
110 Spitbrook Road ZK3-3/U14
Nashua, NH 03062-2698
Phone: +1 603 884 0400
Email: bound@zk3.dec.com

W. Richard Stevens
1202 E. Paseo del Zorro
Tucson, AZ 85718-2826
Phone: +1 520 297 9416
Email: rstevens@kohala.com