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Versions: 00 01 02 03 04 05 06 RFC 2133

Internet Engineering Task Force                R. E. Gilligan (Freegate)
INTERNET-DRAFT                                     S. Thomson (Bellcore)
                                                      J. Bound (Digital)
                                              W. R. Stevens (Consultant)
                                                       November 23, 1996

               Basic Socket Interface Extensions for IPv6
                   <draft-ietf-ipngwg-bsd-api-06.txt>

Abstract

   The de facto standard application program interface (API) for TCP/IP
   applications is the "sockets" interface.  Although this API was
   developed for Unix in the early 1980s it has also been implemented on
   a wide variety of non-Unix systems.  TCP/IP applications written
   using the sockets API have in the past enjoyed a high degree of
   portability and we would like the same portability with IPv6
   applications.  But changes are required to the sockets API to support
   IPv6 and this memo describes these changes.  These include a new
   socket address structure to carry IPv6 addresses, new address
   conversion functions, and some new socket options.  These extensions
   are designed to provide access to the basic IPv6 features required by
   TCP and UDP applications, including multicasting, while introducing a
   minimum of change into the system and providing complete
   compatibility for existing IPv4 applications.  Additional extensions
   for advanced IPv6 features (raw sockets and access to the IPv6
   extension headers) are defined in another document [5].

Status of this Memo

   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.  This Internet Draft expires on May 23, 1997.  Internet
   Drafts may be updated, replaced, or obsoleted by other documents at
   any time.  It is not appropriate to use Internet Drafts as reference
   material or to cite them other than as a "working draft" or "work in
   progress."

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
   munnari.oz.au.

   Distribution of this memo is unlimited.



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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

   3.  Socket Interface .................................................  5
   3.1.  IPv6 Address Family and Protocol Family ........................  5
   3.2.  IPv6 Address Structure .........................................  5
   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.  Flow Information ............................................... 10
   3.9.  IPv6 Wildcard Address .......................................... 12
   3.10.  IPv6 Loopback Address ......................................... 13

   4.  Interface Identification ......................................... 14
   4.1.  Name-to-Index .................................................. 15
   4.2.  Index-to-Name .................................................. 15
   4.3.  Return All Interface Names and Indexes ......................... 15

   5.  Socket Options ................................................... 16
   5.1.  Changing Socket Type ........................................... 16
   5.2.  Unicast Hop Limit .............................................. 17
   5.3.  Sending and Receiving Multicast Packets ........................ 18

   6.  Library Functions ................................................ 20
   6.1.  Hostname-to-Address Translation ................................ 20
   6.2.  Address To Hostname Translation ................................ 22
   6.3.  Protocol-Independent Hostname and Service Name Translation ..... 23
   6.4.  Socket Address Structure to Hostname and Service Name .......... 26
   6.5.  Address Conversion Functions ................................... 27
   6.6.  IPv4-Mapped Addresses .......................................... 28

   7.  Security Considerations .......................................... 29

   8.  Change History ................................................... 29

   9.  Acknowledgments .................................................. 33

   10.  References ...................................................... 33

   11.  Authors' Addresses .............................................. 34




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1.  Introduction

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


2.  Design Considerations

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

    -  The API changes should provide both source and binary
       compatibility for programs written to the original API.  That is,
       existing program binaries should continue to operate when run on
       a system supporting the new API.  In addition, existing
       applications that are re-compiled and run on a system supporting
       the new API should continue to operate.  Simply put, the API
       changes for IPv6 should not break existing programs.

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




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   The socket interface API consists of a few distinct components:

    -  Core socket functions.

    -  Address data structures.

    -  Name-to-address translation functions.

    -  Address conversion functions.

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

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

   The name-to-address translation functions in the socket interface are
   gethostbyname() and gethostbyaddr().  These must be modified to
   support IPv6 and the semantics defined must provide 100% backward
   compatibility for all existing IPv4 applications, along with IPv6
   support for new applications.  Additionally, the POSIX 1003.g draft
   [4] specifies a new hostname-to-address translation function which is
   protocol independent.  This function can also be used with IPv6.

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

   Finally, a few miscellaneous features are needed to support IPv6.
   New interfaces are needed to support the IPv6 flow label, priority,
   and hop limit header fields.  New socket options are needed to



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   control the sending and receiving of IPv6 multicast packets.

   The socket interface may be enhanced in the future to provide access
   to other IPv6 features.  These extensions are described in [5].


2.2.  Data Types

   The data types of the structure elements given in this memo are
   intended to be examples, not absolute requirements.  Whenever
   possible, POSIX 1003.1g data types are used:  u_intN_t means an
   unsigned integer of exactly N bits (e.g., u_int16_t) and u_intNm_t
   means an unsigned integer of at least N bits (e.g., u_int32m_t).  We
   also assume the argument data types from 1003.1g when possible (e.g.,
   the final argument to setsockopt() is a size_t value).  Whenever
   buffer sizes are specified, the POSIX 1003.1 size_t data type is used
   (e.g., the two length arguments to getnameinfo()).


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 data structure to hold a single IPv6 address is defined as
   follows:





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       struct in6_addr {
           u_char  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.

   Applications obtain the declaration for this structure by including
   the header <netinet/in.h>.


3.3.  Socket Address Structure for 4.3BSD-Based Systems

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

   The sockaddr_in structure is the protocol-specific address data
   structure for IPv4.  It is used to pass addresses between
   applications and the system in the socket functions.  The following
   structure is defined to carry IPv6 addresses:

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

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

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

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

   The sin6_flowinfo field is a 32-bit field that contains two pieces of



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   information: the 24-bit IPv6 flow label and the 4-bit priority field.
   The IPv6 flow label is represented as the low-order 24 bits of the
   32-bit field.  The priority is represented in the next 4 bits above
   this.  The high-order 4 bits of this field are reserved.  The
   sin6_flowinfo field is stored in network byte order.  The use of the
   flow label and priority fields are explained in Section 3.8.

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

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

   Applications obtain the declaration of the sockaddr_in6 structure by
   including the header <netinet/in.h>.


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:

       #define SIN6_LEN

       struct sockaddr_in6 {
           u_char          sin6_len;       /* length of this struct */
           u_char          sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* Transport layer port # */
           u_int32m_t      sin6_flowinfo;  /* IPv6 flow information */
           struct in6_addr sin6_addr;      /* IPv6 address */
       };

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

   Systems that provide this version of the sockaddr_in6 data structure
   must also declare SIN6_LEN as a result of including the



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   <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.

   Note that the size of the sockaddr_in6 structure is larger than the
   size of the sockaddr structure.  Applications that use the
   sockaddr_in6 structure need to be aware that they cannot use
   sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6
   structure.  They should use sizeof(sockaddr_in6) instead.


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



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   are:

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

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


3.6.  Compatibility with IPv4 Applications

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

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


3.7.  Compatibility with IPv4 Nodes

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

       ::FFFF:<IPv4-address>

   These addresses are often generated automatically by the
   gethostbyname() function when the specified host has only IPv4
   addresses (as described in Section 6.1).




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   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 inet6_isipv4mapped() function, defined in Section 6.6, is
   provided.


3.8.  Flow Information

   The IPv6 header has a 24-bit field to hold a "flow label", and a 4-
   bit field to hold a "priority" value.  Applications must have control
   over what values for these fields are used in packets that they
   originate, and must have access to the field values of packets that
   they receive.

   The sin6_flowinfo field of the sockaddr_in6 structure encodes two
   pieces of information: IPv6 flow label and IPv6 priority.
   Applications use this field to set the flow label and priority in
   IPv6 headers of packets they generate, and to retrieve the flow label
   and priority from the packets they receive.  The header fields of an
   actively opened TCP connection are set by assigning in the
   sin6_flowinfo field of the destination address sockaddr_in6 structure
   passed in the connect() function.  The same technique can be used
   with the sockaddr_in6 structure passed to the sendto() or sendmsg()
   function to set the flow label and priority fields of UDP packets.
   Similarly, the flow label and priority values of received UDP packets
   and accepted TCP connections are reflected in the sin6_flowinfo field
   of the sockaddr_in6 structure returned to the application by the
   recvfrom(), recvmsg(), and accept() functions.  An application may
   specify the flow label and priority to use in transmitted packets of
   a passively accepted TCP connection, by setting the sin6_flowinfo
   field of the address passed to the bind() function.

   Implementations provide two bitmask constant declarations to help
   applications select out the flow label and priority fields.  These
   constants are:

       IPV6_FLOWINFO_FLOWLABEL
       IPV6_FLOWINFO_PRIORITY



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   These constants can be applied to the sin6_flowinfo field of
   addresses returned to the application, for example:

       int  recv_flow;            /* host byte ordered, 0-0x00ffffff */
       int  recv_prio;            /* host byte ordered, 0-15 */
       struct sockaddr_in6  sin6;
        . . .
       recvfrom(s, buf, buflen, flags, (struct sockaddr *) &sin6, &fromlen);
        . . .
       recv_flow = ntohl(sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL);
       recv_prio = ntohl(sin6.sin6_flowinfo & IPV6_FLOWINFO_PRIORITY) >> 24;
       printf("flow = %d, prio = %d\n", recv_flow, recv_prio);

   Recall that sin6_flowinfo is network byte ordered, as are the two
   IPV6_FLOWINFO_xxx constants.

   On the sending side, applications are responsible for selecting the
   flow label value and specifying a priority.  The headers provide
   constant declarations for the 16 IPv6 priority values defined in the
   IPv6 specification [1].  These constants are:

       IPV6_PRIORITY_UNCHARACTERIZED
       IPV6_PRIORITY_FILLER
       IPV6_PRIORITY_UNATTENDED
       IPV6_PRIORITY_RESERVED1
       IPV6_PRIORITY_BULK
       IPV6_PRIORITY_RESERVED2
       IPV6_PRIORITY_INTERACTIVE
       IPV6_PRIORITY_CONTROL
       IPV6_PRIORITY_8
       IPV6_PRIORITY_9
       IPV6_PRIORITY_10
       IPV6_PRIORITY_11
       IPV6_PRIORITY_12
       IPV6_PRIORITY_13
       IPV6_PRIORITY_14
       IPV6_PRIORITY_15

   Most applications will use these constants (e.g.,
   IPV6_PRIORITY_INTERACTIVE can be built into Telnet clients and
   servers).  Since these constants are defined in network byte order an
   example is:









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       int  send_flow;           /* host byte ordered, 0-0x00ffffff */
       struct sockaddr_in6  sin6;

       send_flow =      /* undefined at this time; perhaps a system call */
       sin6.sin6_flowinfo = htonl(send_flow) & IPV6_FLOWINFO_FLOWLABEL |
                            IPV6_PRIORITY_INTERACTIVE;
        . . .
       connect( ... )


   Some applications may specify the priority as a value between 0 and
   15 (perhaps a command-line argument) and the following example shows
   the required byte ordering and shifting:

       int  send_flow;           /* host byte ordered, 0-0x00ffffff */
       int  send_prio;           /* host byte ordered, 0-15 */
       struct sockaddr_in6  sin6;

       send_flow =      /* undefined at this time; perhaps a system call */
       send_prio = 12;  /* or some other host byte ordered value, 0-15 */
       sin6.sin6_flowinfo = htonl(send_flow) & IPV6_FLOWINFO_FLOWLABEL |
                            htonl(send_prio << 24) & IPV6_FLOWINFO_PRIORITY;
        . . .
       sendto( ... )

   The declarations for these constants are obtained by including the
   header <netinet/in.h>.


3.9.  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 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:

       extern const struct in6_addr in6addr_any;



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   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.
   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 type.  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 */

   The extern declaration for in6addr_any and the declaration for
   IN6ADDR_ANY_INIT are obtained by including the header <netinet/in.h>.

   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.


3.10.  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.




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   The global variable is an in6_addr structure named
   "in6addr_loopback."  The extern declaration for this variable is:

       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.  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.

   The extern declaration for in6addr_loopback and the declaration for
   IN6ADDR_LOOPBACK_INIT are obtained by including the header
   <netinet/in.h>.


4.  Interface Identification

   This API uses an interface index (a small positive integer) to
   identify the local interface on which a multicast group is joined
   (Section 5.3).  Additionally, the advanced API [5] uses these same
   interface indexes to identify the interface on which a datagram is
   received, or to specify the interface on which a datagram is to be
   sent.

   Interfaces are normally known by names such as "le0", "sl1", "ppp2",
   and the like.  On Berkeley-derived implementations, when an interface
   is made known to the system, the kernel assigns a unique positive
   integer value (called the interface index) to that interface.  These
   are small positive integers that start at 1.  (Note that 0 is never



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   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, and a third function that returns all the interface names and
   indexes.  How these three functions are implemented is left up to the
   implementation.  4.4BSD implementations can implement all three
   functions using the existing sysctl() function with the NET_RT_LIST
   command.  Other implementations may wish to use ioctl() for this
   purpose.  The function prototypes for these three functions, the
   constant IF_MAXNAME, and the if_nameindex structure are defined as a
   result of including the <sys/socket.h> header.


4.1.  Name-to-Index

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

       unsigned int  if_nametoindex(const char *ifname);

   If the specified interface does not exist, the return value is 0.


4.2.  Index-to-Name

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

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

   The ifname argument must point to a buffer of at least IF_MAXNAME
   bytes into which the interface name corresponding to the specified
   index is returned.  This pointer is also the return value of the
   function.  If there is no interface corresponding to the specified
   index, NULL is returned and the buffer pointed to by ifname is not
   modified.


4.3.  Return All Interface Names and Indexes

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








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

       struct if_nameindex  *if_nameindex(void);

   The end of the array of structures is indicated by a structure with
   an if_index of 0 and an if_name of NULL.  The memory used for this
   array of structures along with the interface names pointed to by the
   if_name members is obtained using one call to malloc() and can be
   returned by calling free() with an argument that is the pointer
   returned by if_nameindex().


5.  Socket Options

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

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


5.1.  Changing Socket Type

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

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



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

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

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

       int  addrform = PF_INET;

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

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

       int  addrform;
       size_t  len = sizeof(addrform);

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



5.2.  Unicast Hop Limit

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



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

       int  hoplimit;
       size_t  len = sizeof(hoplimit);

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



5.3.  Sending and Receiving Multicast Packets

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

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












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       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.)

           Argument type: unsigned int

       IPV6_MULTICAST_LOOP

           Controls whether outgoing multicast packets  sent  should  be
           delivered  back  to the local application.  A toggle.  If the
           option is set to 1, multicast packets are looped back.  If it
           is set to 0, they are not.

           Argument type: unsigned int

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

       IPV6_ADD_MEMBERSHIP

           Join a multicast group on a specified  local  interface.   If
           the interface index is specified as 0, the kernel chooses the
           local interface by looking up  the  multicast  group  in  the
           normal IPv6 routing table and using the resulting interface.

           Argument type: struct ipv6_mreq

       IPV6_DROP_MEMBERSHIP

           Leave a multicast group on a specified interface.

           Argument type: struct ipv6_mreq

   The argument type of both of these options is the ipv6_mreq
   structure, defined as follows:






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

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


6.  Library Functions

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


6.1.  Hostname-to-Address Translation

   The commonly used function gethostbyname() remains unchanged as does
   the hostent structure to which it returns a pointer.  Existing
   applications that call this function continue to receive only IPv4
   addresses that are the result of a query in the DNS for A records.
   (We assume the DNS is being used; some environments may be using a
   hosts file or some other name resolution system, either of which may
   impede renumbering.)

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

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


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

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

    -  If the af argument is AF_INET6, then a query is made for AAAA



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       records.  If successful, IPv6 addresses are returned and the
       h_length member of the hostent structure will be 16, else the
       function returns a NULL pointer.

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

    -  The first way is

           res_init();
           _res.options |= RES_USE_INET6;

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

    -  The second way to set this option is to set the environment
       variable RES_OPTIONS, as in RES_OPTIONS=inet6.  This method
       affects any processes that see this environment variable.

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

   When the RES_USE_INET6 option is set, two changes occur:

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

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

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

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





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


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

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


6.2.  Address To Hostname Translation

   The existing gethostbyaddr() function already requires an address
   family argument and can therefore work with IPv6 addresses:

       struct hostent *gethostbyaddr(const char *src, int len, int af);




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   One possible source of confusion is the handling of IPv4-mapped IPv6
   addresses and IPv4-compatible IPv6 addresses.  Current thinking
   involves the following logic:

    -  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.

    -  If af is AF_INET, then query for a PTR record in the in-addr.arpa
       domain.

    -  If af is AF_INET6, then query for a PTR record in the ip6.int
       domain.

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

   The same caveats regarding a thread-safe version of gethostbyname()
   that were made at the end of the previous section apply here as well.


6.3.  Protocol-Independent Hostname and Service Name Translation

   Hostname-to-address translation is done in a protocol-independent
   fashion using the getaddrinfo() function that is taken from the
   Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
   (Protocol Independent Interfaces) draft specification [4].

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

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

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

   The addrinfo structure is defined as:




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

   The return value from the function is 0 upon success or a nonzero
   error code.  The following names are the nonzero error codes from
   getaddrinfo():

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

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

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



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   IPv6, then the ai_family member of the hints structure should be set
   to PF_INET when getaddrinfo() is called.  If the third argument to
   getaddrinfo() is a NULL pointer, this is the same as if the caller
   had filled in an addrinfo structure initialized to zero with
   ai_family set to PF_UNSPEC.

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

   If the AI_PASSIVE bit is set in the ai_flags member of the hints
   structure, then the caller plans to use the returned socket address
   structure in a call to bind().  In this case, if the hostname
   argument is a NULL pointer, then the IP address portion of the socket
   address structure will be set to INADDR_ANY for an IPv4 address or
   IN6ADDR_ANY_INIT for an IPv6 address.  Notice that if the AI_PASSIVE
   bit is set and the hostname argument is a NULL pointer then the
   caller must also specify a nonzero ai_family, otherwise getaddrinfo()
   is unable to allocate and initialize a socket address structure of
   the correct type.

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

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

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





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       #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.


6.4.  Socket Address Structure to Hostname and Service Name

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

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

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

   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 first
   argument, sa, points to either a sockaddr_in structure (for IPv4) or
   a sockaddr_in6 structure (for IPv6) that holds the IP address and
   port number.  The salen argument gives the length of the sockaddr_in
   or sockaddr_in6 structure.  The function returns the hostname
   associated with the IP address in the buffer pointed to by the host
   argument.  The caller provides the size of this buffer via the
   hostlen argument.  The service name associated with the port number
   is returned in the buffer pointed to by serv, and the servlen
   argument gives the length of this buffer.  The caller specifies not
   to return either string by providing a zero value for the hostlen or
   servlen arguments.  Otherwise, the caller must provide buffers large
   enough to hold the fully qualified domain hostname, and the full
   service name, including the terminating null character.  The function
   indicates successful completion by a zero return value; a non-zero
   return value indicates failure.

   Note that this function does not know the protocol of the socket
   address structure.  Normally this is not a problem because the same
   port is assigned to a given service for both TCP and UDP.  But there
   exist historical artifacts that violate this rule (e.g., ports 512,



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   513, and 514).


6.5.  Address Conversion Functions

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

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


    and

       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 function does not
   modify the buffer pointed to by dst if the conversion fails.  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.

   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



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   argument points to a buffer holding an IPv4 address if the af
   argument is AF_INET, or an IPv6 address if the af argument is
   AF_INET6.  The dst argument points to a buffer where the function
   will store the resulting text string.  The size argument specifies
   the size of this buffer.  The application must specify a non-NULL dst
   argument.  For IPv6 addresses, the buffer must be at least 46-octets.
   For IPv4 addresses, the buffer must be at least 16-octets.  In order
   to allow applications to easily declare buffers of the proper size to
   store IPv4 and IPv6 addresses in string form, implementations should
   provide the following constants, made available to applications that
   include <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.  The
   function does not modify the storage pointed to by dst if the
   conversion fails.

   Applications obtain the prototype declarations for inet_ntop() and
   inet_pton() by including the header <arpa/inet.h>.


6.6.  IPv4-Mapped Addresses

   The IPv4-mapped IPv6 address format represents IPv4 addresses as IPv6
   addresses.  Most applications should be able to manipulate IPv6
   addresses as opaque 16-octet quantities, without needing to know
   whether they represent IPv4 addresses.  However, a few applications
   may need to determine whether an IPv6 address is an IPv4-mapped
   address or not.  The following function is provided for those
   applications:

       int inet6_isipv4mapped(const struct in6_addr *addr);

   The "addr" argument to this function points to a buffer holding an
   IPv6 address in network byte order.  The function returns non-zero if
   that address is an IPv4-mapped address, and returns 0 otherwise.

   This function could be used by server applications to determine
   whether the peer is an IPv4 node or an IPv6 node.  After accepting a
   TCP connection via accept(), or receiving a UDP packet via
   recvfrom(), the application can apply the inet6_isipv4mapped()
   function to the returned address.



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   Applications obtain the prototype for this function by including the
   header <arpa/inet.h>.


7.  Security Considerations

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


8.  Change History

   Changes from the April 1996 Edition (-05 draft)

    -  Rewrote Abstract.

    -  Added Table of Contents.

    -  New Section 2.2 (Data Types).

    -  Removed the example from Section 3.4 (Socket Address Structure
       for 4.4BSD-Based Systems) implying that the process must set the
       sin6_len field.  This field need not be set by the process before
       passing a socket address structure to the kernel:  bind(),
       connect(), sendto(), and sendmsg().

    -  The examples in Section 3.8 (Flow Information) on setting and
       fetching the flow label and priority have been expanded, since
       the byte ordering and shifting required to set and fetch these
       fields can be confusing.  It is also explicitly stated that the
       two IPV6_FLOWLABEL_xxx constants and the 16 IPV6_PRIORITY_xxx
       constants are all network byte ordered.

    -  Warning placed at the end of Section 3.9 concerning the byte
       ordering of the IPv4 INADDR_xxx constants versus the IPv6
       IN6ADDR_xxx constants and in6addr_xxx externals.

    -  Added a new Section 4 (Interface Identification).  This provides
       functions to map between an interface name and an interface
       index.

    -  In Section 5.1 (Changing Socket Type) the qualifier was added
       that you cannot downgrade an IPv6 socket to an IPv4 socket unless
       all nonwildcard addresses already associated with the IPv6 socket
       are IPv4-mapped IPv6 addresses.




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    -  In Section 5.3 (Sending and Receiving Multicast Packets) the
       method of specifying the local interface was changed from using a
       local IPv6 address to using the interface index.  This changes
       the argument type for IPV6_MULTICAST_IF and the second member of
       the ipv6_mreq structure.

    -  In Section 5.3 (Sending and Receiving Multicast Packets) the
       IPV6_ADD_MEMBERSHIP socket option description was corrected.  A
       note was also added at the end of this section concerning joining
       the group versus binding the group address to the socket.

    -  The old Sections 5.1, 5.2, and 5.3 are gone, and new Sections
       6.1, 6.2, 6.3, 6.4, and 6.5 are provided.  The new sections
       describe the BIND 4.9.4 implementation of the name-to-address
       functions (which support IPv6), a POSIX-free description of the
       getaddrinfo() function, a description of the new getnameinfo()
       function, and the inet_ntop() and inet_pton() functions.  The old
       Section 5.4 (Embedded IPv4 addresses) is now Section 6.6 (IPv4-
       Mapped Addresses).

    -  Renamed inet6_isipv4addr() to inet6_isipv4mapped() so the name
       better describes the function.

    -  Section 8 (Open Issues) was removed.

   Changes from the January 1996 Edition (-04 draft)

    -  Re-arranged the ipv6_hostent_addr structure, placing the IPv6
       address element first.

   Changes from the November 1995 Edition (-03 draft)

    -  Added the symbolic constants IN6ADDR_ANY_INIT and
       IN6ADDR_LOOPBACK_INIT for applications to use for
       initializations.

    -  Eliminated restrictions on the value of ipv6addr_any.  Systems
       may now choose any value, including all-zeros.

    -  Added a mechanism for returning time to live with the address in
       the name-to-address translation functions.

    -  Added a mechanism for applications to specify the interface in
       the setsockopt() options to join and leave a multicast group.

   Changes from the July 1995 Edition

    -  Changed u_long and u_short types in structures to u_int32_t and



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       u_int16_t for consistency and clarity.

    -  Added implementation-provided constants for IPv4 and IPv6 text
       address buffer length.

    -  Defined a set of constants for subfields of sin6_flowid and for
       priority values.

    -  Defined constants for getting and setting the source route flag.

    -  Define where ansi prototypes for hostname2addr(),
       addr2hostname(), addr2ascii(), ascii2addr(), and
       ipv6_isipv4addr() reside.

    -  Clarified the include file requirements.  Say that the structure
       definitions are defined as a result of including the header
       <netinet/in.h>, not that the structures are necessarily defined
       there.

    -  Removed underscore chars from is_ipv4_addr() function name for
       BSD compatibility.

    -  Added inet6_ prefix to is_ipv4_addr() function name to avoid name
       space conflicts.

    -  Changes setsockopt option naming convention to use IPV6_ prefix
       instead of IP_ so that there is clearly no ambiguity with IPv4
       options.  Also, use level IPPROTO_IPV6 for these options.

    -  Made hostname2addr() and addr2hostname() functions thread-safe.

    -  Added support for sendmsg() and recvmsg() in source routing
       section.

    -  Changed in_addr6 to in6_addr for consistency.

    -  Re-structured document into sub-sections.

    -  Deleted the implementation experience section.  It was too wordy.

    -  Added argument types to multicast socket options.

    -  Added constant for largest source route array buffer.

    -  Added the freehostent() function.

    -  Added receiving interface determination and sending interface
       selection options.



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    -  Added definitions of ipv6addr_any and ipv6addr_loopback.

    -  Added text making the lookup of IPv4 addresses by hostname2addr()
       optional.

   Changes from the June 1995 Edition

    -  Added capability for application to select loose or strict source
       routing.

   Changes from the March 1995 Edition

    -  Changed the definition of the ipv6_addr structure to be an array
       of sixteen chars instead of four longs.  This change is necessary
       to support machines that implement the socket interface, but do
       not have a 32-bit addressable word.  Virtually all machines that
       provide the socket interface do support an 8-bit addressable data
       type.

    -  Added a more detailed explanation that the data types defined in
       this documented are not intended to be hard and fast
       requirements.  Systems may use other data types if they wish.

    -  Added a note flagging the fact that the sockaddr_in6 structure is
       not the same size as the sockaddr structure.

    -  Changed the sin6_flowlabel field to sin6_flowinfo to accommodate
       the addition of the priority field to the IPv6 header.

   Changes from the October 1994 Edition

    -  Added variant of sockaddr_in6 for 4.4BSD-based systems (sa_len
       compatibility).

    -  Removed references to SIT transition specification, and added
       reference to addressing architecture document, for definition of
       IPv4-mapped addresses.

    -  Added a solution to the problem of the application not providing
       enough buffer space to hold a received source route.

    -  Moved discussion of IPv4 applications interoperating with IPv6
       nodes to open issues section.

    -  Added length parameter to addr2ascii() function to be consistent
       with addr2hostname().

    -  Changed IP_MULTICAST_TTL to IP_MULTICAST_HOPS to match IPv6



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       terminology, and added IP_UNICAST_HOPS option to match
       IP_MULTICAST_HOPS.

    -  Removed specification of numeric values for AF_INET6,
       IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on
       different implementations.

    -  Added a definition for the in_addr6 IPv6 address data structure.
       Added this so that applications could use sizeof(struct in_addr6)
       to get the size of an IPv6 address, and so that a structured type
       could be used in the is_ipv4_addr().


9.  Acknowledgments

   Thanks to the many people who made suggestions and provided feedback
   to to the numerous revisions of this document, including:  Werner
   Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson,
   Alex Conta, Alan Cox, Steve Deering, Francis Dupont, Robert Elz, Marc
   Hasson, Tom Herbert, Christian Huitema, Wan-Yen Hsu, Alan Lloyd,
   Charles Lynn, Dan McDonald, Craig Metz, Erik Nordmark, Josh Osborne,
   Craig Partridge, Matt Thomas, Dean D. Throop, Glenn Trewitt, Paul
   Vixie, David Waitzman, and Carl Williams.

   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".

   Ramesh Govindan made a number of contributions and co-authored an
   earlier version of this memo.


10.  References


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

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




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   [3] D. McDonald, "A Simple IP Security API Extension to BSD Sockets",
       Internet-Draft, <draft-mcdonald-simple-ipsec-api-00.txt>,
       November 1996.

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

   [5] W. R. Stevens, M. Thomas, "Advanced Sockets API for IPv6",
       Internet-Draft, <draft-stevens-advanced-api-00.txt>, October
       1996.


11.  Authors' Addresses

    Robert E. Gilligan
    Freegate Corporation
    710 Lakeway Dr.  STE 230
    Sunnyvale, CA 94086
    Phone: +1 408 524 4804
    Email: gilligan@freegate.net


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


    Jim Bound
    Digital Equipment Corporation
    110 Spitbrook Road ZK3-3/U14
    Nashua, NH 03062-2698
    Phone: +1 603 881 0400
    Email: bound@zk3.dec.com


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








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