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Versions: 00 01 02 03 04 05 06 RFC 2553

Internet Engineering Task Force                 R.E. Gilligan (FreeGate)
INTERNET-DRAFT                                     S. Thomson (Bellcore)
Obsoletes RFC 2133                                    Jim Bound (Compaq)
                                              W. R. Stevens (Consultant)
                                                        January 25, 1999






               Basic Socket Interface Extensions for IPv6

                 <draft-ietf-ipngwg-bsd-api-new-06.txt>


Status of this Memo

        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.

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

        Internet-Drafts are draft documents valid for a maximum of six
        months 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."

        To view the entire list of current Internet-Drafts, please check
        the "1id-abstracts.txt" listing contained in the Internet-Drafts
        Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
        (Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East
        Coast), or ftp.isi.edu (US West Coast).

        Distribution of this memo is unlimited.


Abstract

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


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Table of Contents:

1. Introduction.................................................3
2. Design Considerations........................................3
2.1 What Needs to be Changed....................................3
2.2 Data Types..................................................5
2.3 Headers.....................................................5
2.4 Structures..................................................5
3. Socket Interface.............................................5
3.1 IPv6 Address Family and Protocol Family.....................5
3.2 IPv6 Address Structure......................................6
3.3 Socket Address Structure for 4.3BSD-Based Systems...........6
3.4 Socket Address Structure for 4.4BSD-Based Systems...........7
3.5 The Socket Functions........................................8
3.6 Compatibility with IPv4 Applications........................9
3.7 Compatibility with IPv4 Nodes...............................9
3.8 IPv6 Wildcard Address......................................10
3.9 IPv6 Loopback Address......................................11
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.....................15
4.4 Free Memory................................................15
5. Socket Options..............................................15
5.1 Unicast Hop Limit..........................................16
5.2 Sending and Receiving Multicast Packets....................16
6. Library Functions...........................................18
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.22
6.5 Socket Address Structure to Nodename and Service Name......25
6.6 Address Conversion Functions...............................26
6.7 Address Testing Macros.....................................27
7. Summary of New Definitions..................................28
8. Security Considerations.....................................29
9. Year 2000 Considerations....................................29
Changes From RFC 2133..........................................30
Acknowledgments................................................32
References.....................................................33
Authors' Addresses.............................................33


















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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.



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   -  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 POSIX
1003.g draft [3] 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].







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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.





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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


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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.  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 */
   };


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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


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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()
function when the specified host has only IPv4 addresses (as described
in Section 6.1).

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

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






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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.








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

   - It has a large enough implementation specific maximum size to store
     the desired set of protocol specific socket address data structures.
     Specifically, it is at least large enough to accommodate sockaddr_in
     and sockaddr_in6 and possibly other protocol specific socket
     addresses too.
   - It is aligned at an appropriate boundary so protocol specific socket
     address data structure pointers can be cast to it and access their
     fields without alignment problems. (e.g. pointers to sockaddr_in6
     and/or sockaddr_in can be cast to it and access fields without alignment


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     problems).
   - It has the initial field(s) isomorphic to the fields of the
     "struct sockaddr" data structure on that implementation which
     can be used as a discriminants for deriving the protocol in use.
     These initial field(s) would on most implementations either be a
     single field of type "sa_family_t" (isomorphic to sa_family field,
     16 bits) or two fields of type uint8_t and sa_family_t respectively,
     (isomorphic to sa_len and sa_family_t, 8 bits each).

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 */
};

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 */


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                  /* 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 */
};

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.

The sockaddr_storage structure solves the problem of declaring storage
for automatic variables which is large enough and aligned enough for
storing socket address data structure of any family. For example, code
with a file descriptor and without the context of the address family can
pass a pointer to a variable of this type where a pointer to a socket
address structure is expected in calls such as getpeername() and
determine the address family by accessing the received content after the
call.

The sockaddr_storage structure may also be useful and applied to certain
other interfaces where a generic socket address large enough and aligned
for use with multiple address families may be needed. A discussion of
those interfaces is outside the scope of this document.

Also, much existing code assumes that any socket address structure can
fit in a generic sockaddr structure.  While this has been true for IPv4
socket address structures, it has always been false for Unix domain
socket address structures (but in practice this has not been a problem)
and it is also false for IPv6 socket address structures (which can be a
problem).

So now an application can do the following:

   struct sockaddr_storage __ss;
   struct sockaddr_in6 *sin6;
   sin6 = (struct sockaddr_in6 *) &__ss;



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


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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).







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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>.





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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



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   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 using 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 */


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       unsigned int    ipv6mr_interface; /* interface index */
   };

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



6. Library Functions

New library functions are needed to perform a variety of operations with
IPv6 addresses.  Functions are needed to lookup IPv6 addresses in the
Domain Name System (DNS).  Both forward lookup (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

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 function is new 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);

The name argument can 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.



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   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 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 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.



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   -  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 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 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 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 an IPv6 hex address containing the IPv4-mapped IPv6
address, 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.


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6.2 Address-To-Nodename Translation

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 domain).

   4.  If the function is returning success, then the single address that
       is returned in the hostent structure is a copy of the first argument
       to the function with 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


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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 this memory:

   #include <netdb.h>

   void freehostent(struct hostent *ptr);



6.4 Protocol-Independent Nodename and Service Name Translation

Nodename-to-address translation is done in a protocol-independent
fashion using the getaddrinfo() function that is taken from the
Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) draft specification [3].

The official specification for this function will be the final POSIX
standard, with the following additional requirements:

   -  getaddrinfo() (along with the getnameinfo() function described in
      the next section) must be thread safe.

   -  The AI_NUMERICHOST is new with this document.

   -  All fields in socket address structures returned by getaddrinfo()
      that are not filled in through an explicit argument (e.g.,
      sin6_flowinfo and sin_zero) must be set to 0.  (This makes it easier
      to compare socket address structures.)

   -  getaddrinfo() must fill in the length field of a socket address structure
      (e.g., sin6_len) on systems that support this field.

We are providing this independent description of the function because
POSIX standards are not freely available (as are IETF documents).

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

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

The addrinfo structure is defined as a result of including the <netdb.h>
header.

   struct addrinfo {


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     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 return value from the function is 0 upon success or a nonzero error
code.  The following names are the nonzero error codes from
getaddrinfo(), and are defined in <netdb.h>:

   EAI_ADDRFAMILY  address family for nodename not supported
   EAI_AGAIN       temporary failure in name resolution
   EAI_BADFLAGS    invalid value for ai_flags
   EAI_FAIL        non-recoverable failure in name resolution
   EAI_FAMILY      ai_family not supported
   EAI_MEMORY      memory allocation failure
   EAI_NODATA      no address associated with nodename
   EAI_NONAME      nodename nor servname provided, or not known
   EAI_SERVICE     servname not supported for ai_socktype
   EAI_SOCKTYPE    ai_socktype not supported
   EAI_SYSTEM      system error returned in errno

The nodename and servname arguments are pointers to null-terminated
strings or NULL.  One or both of these two arguments must be a non-NULL
pointer.  In the normal client scenario, both the nodename and servname
are specified.  In the normal server scenario, only the servname is
specified.  A non-NULL nodename string can be either a node name or a
numeric host address string (i.e., a dotted-decimal IPv4 address or an
IPv6 hex address).  A non-NULL servname string can be either a service
name or a decimal port number.

The caller can optionally pass an addrinfo structure, pointed to by the
third argument, to provide hints concerning the type of socket that the
caller supports.  In this hints structure all members other than
ai_flags, ai_family, ai_socktype, and ai_protocol must be zero or a NULL
pointer.  A value of PF_UNSPEC for ai_family means the caller will
accept any protocol family.  A value of 0 for ai_socktype means the
caller will accept any socket type.  A value of 0 for ai_protocol means
the caller will accept any protocol.  For example, if the caller handles
only TCP and not UDP, then the ai_socktype member of the hints structure
should be set to SOCK_STREAM when getaddrinfo() is called.  If the
caller handles only IPv4 and not IPv6, then the ai_family member of the
hints structure should be set to PF_INET when getaddrinfo() is called.
If the third argument to getaddrinfo() is a NULL pointer, this is the
same as if the caller had filled in an addrinfo structure initialized to
zero with ai_family set to PF_UNSPEC.

Upon successful return a pointer to a linked list of one or more
addrinfo structures is returned through the final argument.  The caller
can process each addrinfo structure in this list by following the
ai_next pointer, until a NULL pointer is encountered.  In each returned
addrinfo structure the three members ai_family, ai_socktype, and
ai_protocol are the corresponding arguments for a call to the socket()
function.  In each addrinfo structure the ai_addr member points to a


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filled-in socket address structure whose length is specified by the
ai_addrlen member.

If the AI_PASSIVE bit is set in the ai_flags member of the hints
structure, then the caller plans to use the returned socket address
structure in a call to bind().  In this case, if the nodename argument
is a NULL pointer, then the IP address portion of the socket address
structure will be set to INADDR_ANY for an IPv4 address or
IN6ADDR_ANY_INIT for an IPv6 address.

If the AI_PASSIVE bit is not set in the ai_flags member of the hints
structure, then the returned socket address structure will be ready for
a call to connect() (for a connection-oriented protocol) or either
connect(), sendto(), or sendmsg() (for a connectionless protocol).  In
this case, if the nodename argument is a NULL pointer, then the IP
address portion of the socket address structure will be set to the
loopback address.

If the AI_CANONNAME bit is set in the ai_flags member of the hints
structure, then upon successful return the ai_canonname member of the
first addrinfo structure in the linked list will point to a null-
terminated string containing the canonical name of the specified
nodename.

If the AI_NUMERICHOST bit is set in the ai_flags member of the hints
structure, 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 (e.g., the DNS) from
being called.

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

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

   void freeaddrinfo(struct addrinfo *ai);

The addrinfo structure pointed to by the ai argument is freed, along
with any dynamic storage pointed to by the structure.  This operation is
repeated until a NULL ai_next pointer is encountered.

To aid applications in printing error messages based on the EAI_xxx
codes returned by getaddrinfo(), the following function is defined.

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

   char *gai_strerror(int ecode);

The argument is one of the EAI_xxx values defined earlier and the 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.



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6.5 Socket Address Structure to Nodename and Service Name

The POSIX 1003.1g specification includes no function to perform the
reverse conversion from getaddrinfo(): to look up a nodename and service
name, given the binary address and port.  Therefore, we define the
following function:

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

   int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                   char *host, size_t hostlen,
                   char *serv, size_t servlen,
                   int flags);

This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text strings
for both in buffers provided by the caller.  The function indicates
successful completion by a zero return value; a non-zero return value
indicates failure.

The first argument, sa, points to either a sockaddr_in structure (for
IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address
and port number.  The salen argument gives the length of the sockaddr_in
or sockaddr_in6 structure.

The function returns the nodename associated with the IP address in the
buffer pointed to by the host argument.  The caller provides the size of
this buffer via the hostlen argument.  The service name associated with
the port number is returned in the buffer pointed to by serv, and the
servlen argument gives the length of this buffer.  The caller specifies
not to return either string by providing a zero value for the hostlen or
servlen arguments.  Otherwise, the caller must provide buffers large
enough to hold the nodename and the service name, including the
terminating null characters.

Unfortunately most systems do not provide constants that specify the
maximum size of either a fully-qualified domain name or a service name.
Therefore to aid the application in allocating buffers for these two
returned strings the following constants are defined in <netdb.h>:

   #define NI_MAXHOST  1025
   #define NI_MAXSERV    32

The first value is actually defined as the constant MAXDNAME in recent
versions of BIND's <arpa/nameser.h> header (older versions of BIND
define this constant to be 256) and the second is a guess based on the
services listed in the current Assigned Numbers RFC.

The final argument is a flag that changes the default actions of this
function.  By default the fully-qualified domain name (FQDN) for the
host is looked up in the DNS and returned.  If the flag bit NI_NOFQDN is
set, only the nodename portion of the FQDN is returned for local hosts.

If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
located in the DNS, the numeric form of the host's address is returned
instead of its name (e.g., by calling inet_ntop() instead of
getipnodebyaddr()).  If the flag bit NI_NAMEREQD is set, an error is


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returned if the host's name cannot be located in the DNS.

If the flag bit NI_NUMERICSERV is set, the numeric form of the service
address is returned (e.g., its port number) instead of its name.  The
two NI_NUMERICxxx flags are required to support the "-n" flag that many
commands provide.

A fifth flag bit, NI_DGRAM, specifies that the service is a datagram
service, and causes getservbyport() to be called with a second argument
of "udp" instead of its default of "tcp".  This is required for the few
ports (e.g. 512-514) that have different services for UDP and TCP.

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



6.6 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].


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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 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.








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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


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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.








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Changes From RFC 2133

Changes made in the March 1998 Edition (-01 draft):

   Changed all "hostname" to "nodename" for consistency with other IPv6
   documents.

   Section 3.3: changed comment for sin6_flowinfo to be "traffic class &
   flow info" and updated corresponding text description to current
   definition of these two fields.

   Section 3.10 ("Portability Additions") is new.

   Section 6: a new paragraph was added reiterating that the existing
   gethostbyname() and gethostbyaddr() are not changed.

   Section 6.1: change gethostbyname3() to getnodebyname().  Add
   AI_DEFAULT to handle majority of applications.  Renamed
   AI_V6ADDRCONFIG to AI_ADDRCONFIG and define it for A records and IPv4
   addresses too.  Defined exactly what getnodebyname() must return if
   the name argument is a numeric address string.

   Section 6.2: change gethostbyaddr() to getnodebyaddr().  Reword items
   2 and 3 in the description of how to handle IPv4-mapped and IPv4-
   compatible addresses to "lookup a name" for a given address, instead
   of specifying what type of DNS query to issue.

   Section 6.3: added two more requirements to getaddrinfo().

   Section 7: added the following constants to the list for <netdb.h>:
   AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED.  Add union sockaddr_union and
   SA_LEN to the lists for <sys/socket.h>.

   Updated references.

Changes made in the November 1997 Edition (-00 draft):

   The data types have been changed to conform with Draft 6.6 of the
   Posix 1003.1g standard.

   Section 3.2: data type of s6_addr changed to "uint8_t".

   Section 3.3: data type of sin6_family changed to "sa_family_t".  data
   type of sin6_port changed to "in_port_t", data type of sin6_flowinfo
   changed to "uint32_t".

   Section 3.4: same as Section 3.3, plus data type of sin6_len changed
   to "uint8_t".

   Section 6.2: first argument of gethostbyaddr() changed from "const
   char *" to "const void *" and second argument changed from "int" to
   "size_t".

   Section 6.4: second argument of getnameinfo() changed from "size_t"
   to "socklen_t".

   The wording was changed when new structures were defined, to be more
   explicit as to which header must be included to define the structure:


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   Section 3.2 (in6_addr{}), Section 3.3 (sockaddr_in6{}), Section 3.4
   (sockaddr_in6{}), Section 4.3 (if_nameindex{}), Section 5.3
   (ipv6_mreq{}), and Section 6.3 (addrinfo{}).

   Section 4: NET_RT_LIST changed to NET_RT_IFLIST.

   Section 5.1: The IPV6_ADDRFORM socket option was removed.

   Section 5.3: Added a note that an option value other than 0 or 1 for
   IPV6_MULTICAST_LOOP returns an error.  Added a note that
   IPV6_MULTICAST_IF, IPV6_MULTICAST_HOPS, and IPV6_MULTICAST_LOOP can
   also be used with getsockopt(), but IPV6_ADD_MEMBERSHIP and
   IPV6_DROP_MEMBERSHIP cannot be used with getsockopt().

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

   Section 6.2: Added requirement that gethostbyaddr() be thread safe.
   Reworded step 4 to avoid using the RES_USE_INET6 option.

   Section 6.3: Added the requirement that getaddrinfo() and
   getnameinfo() be thread safe.  Added the AI_NUMERICHOST flag.

   Section 6.6: Added clarification about IN6_IS_ADDR_LINKLOCAL and
   IN6_IS_ADDR_SITELOCAL macros.

Changes made to the draft -01 specification Sept 98

   Changed priority to traffic class in the spec.

   Added the need for scope identification in section 2.1.

   Added sin6_scope_id to struct sockaddr_in6 in sections 3.3 and 3.4.

   Changed 3.10 to use generic storage structure to support holding IPv6
   addresses and removed the SA_LEN macro.

   Distinguished between invalid input parameters and system failures
   for Interface Identification in Section 4.1 and 4.2.

   Added defaults for multicast operations in section 5.2 and changed
   the names from ADD to JOIN and DROP to LEAVE to be consistent with
   IPv6 multicast terminology.

   Changed getnodebyname to getipnodebyname, getnodebyaddr to
   getipnodebyaddr, and added MT safe error code to function parameters
   in section 6.

   Moved freehostent to its own sub-section after getipnodebyaddr now
   6.3 (so this bumps all remaining sections in section 6.

   Clarified the use of AI_ALL and AI_V4MAPPED that these are dependent
   on the AF parameter and must be used as a conjunction in section 6.1.

   Removed the restriction that literal addresses cannot be used with a
   flags argument in section 6.1.

   Added Year 2000 Section to the draft


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   Deleted Reference to the following because the attached is deleted from
   the ID directory and has expired.  But the logic from the aforementioned
   draft still applies, so that was kept in Section 6.2 bullets after 3rd
   paragraph.
   [7]  P. Vixie, "Reverse Name Lookups of Encapsulated IPv4 Addresses
        in IPv6", Internet-Draft, <draft-vixie-ipng-ipv4ptr-00.txt>,
        May 1996.

   Deleted the following reference as it is no longer referenced.
   And the draft has expired.
   [3]  D. McDonald, "A Simple IP Security API Extension to BSD Sockets",
        Internet-Draft, <draft-mcdonald-simple-ipsec-api-01.txt>,
        March 1997.

   Deleted the following reference as it is no longer referenced.
   [4]  C. Metz, "Network Security API for Sockets",
        Internet-Draft, <draft-metz-net-security-api-01.txt>,
        January 1998.

   Update current references to current status.

   Added alignment notes for in6_addr and sin6_addr.

   Clarified further that AI_V4MAPPED must be used with a dotted IPv4
   literal address for getipnodebyname(), when address family is
   AF_INET6.

   Added text to clarify "::" and "::1" when used by getipnodebyaddr().




Acknowledgments

Thanks to the many people who made suggestions and provided feedback to
this document, including: Werner Almesberger, Ran Atkinson, Fred Baker,
Dave Borman, Andrew Cherenson, Alex Conta, Alan Cox, Steve Deering,
Richard Draves, Francis Dupont, Robert Elz, Marc Hasson, 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, Erik Scoredos, Keith
Sklower, Matt Thomas, Harvey Thompson, Dean D. Throop, Karen Tracey,
Glenn Trewitt, Paul Vixie, David Waitzman, Carl Williams, and Kazu
Yamamoto,

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

Craig Metz, Jack McCann, Erik Nordmark, Tim Hartrick, and Mukesh Kacker
made many contributions to this document.  Ramesh Govindan made a number


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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.




Authors' Addresses

Robert E. Gilligan
FreeGate Corporation
1208 E. Arques Ave.
Sunnyvale, CA 94086
Phone: +1 408 617 1004
Email: gilligan@freegate.net

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

Jim Bound
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










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