Network Working Group
Request for Comments: 2133
Category: Informational
R. Gilligan
S. Thomson
J. Bound
W. Stevens
April 1997

Basic Socket Interface Extensions for IPv6

Status of this Memo

This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.


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

Table of Contents

   1.  Introduction ................................................  2
   2.  Design Considerations .......................................  3
   2.1.  What Needs to be Changed ..................................  3
   2.2.  Data Types ................................................  5
   2.3.  Headers ...................................................  5
   2.4.  Structures ................................................  5
   3.  Socket Interface ............................................  5
   3.1.  IPv6 Address Family and Protocol Family ...................  5
   3.2.  IPv6 Address Structure ....................................  6
   3.3.  Socket Address Structure for 4.3BSD-Based Systems .........  6
   3.4.  Socket Address Structure for 4.4BSD-Based Systems .........  7
   3.5.  The Socket Functions ......................................  8
   3.6.  Compatibility with IPv4 Applications ......................  9
   3.7.  Compatibility with IPv4 Nodes .............................  9
   3.8.  IPv6 Wildcard Address ..................................... 10
   3.9.  IPv6 Loopback Address ..................................... 11
   4.  Interface Identification .................................... 12
   4.1.  Name-to-Index ............................................. 13
   4.2.  Index-to-Name ............................................. 13
   4.3.  Return All Interface Names and Indexes .................... 14
   4.4.  Free Memory ............................................... 14
   5.  Socket Options .............................................. 14
   5.1.  Changing Socket Type ...................................... 15
   5.2.  Unicast Hop Limit ......................................... 16
   5.3.  Sending and Receiving Multicast Packets ................... 17
   6.  Library Functions ........................................... 19
   6.1.  Hostname-to-Address Translation ........................... 19
   6.2.  Address To Hostname Translation ........................... 22
   6.3.  Protocol-Independent Hostname and Service Name Translation  22
   6.4.  Socket Address Structure to Hostname and Service Name ..... 25
   6.5.  Address Conversion Functions .............................. 27
   6.6.  Address Testing Macros .................................... 28
   7.  Summary of New Definitions .................................. 29
   8.  Security Considerations ..................................... 31
   9.  Acknowledgments ............................................. 31
   10.  References ................................................. 31
   11.  Authors' Addresses ......................................... 32

1. Introduction

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

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

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 work in progress [4] specifies a new hostname-to-address translation function which is protocol independent. This function can also be used with IPv6.

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

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

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

2.2. Data Types

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

2.3. Headers

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

2.4. Structures

When structures are described the members shown are the ones that must appear in an implementation. Additional, nonstandard members may also be defined by an implementation.

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

3. Socket Interface

This section specifies the socket interface changes for IPv6.

3.1. IPv6 Address Family and Protocol Family

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

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

       #define PF_INET6        AF_INET6

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

3.2. IPv6 Address Structure

A new data structure to hold a single IPv6 address is defined as


       #include <netinet/in.h>
       struct in6_addr {
           u_int8_t  s6_addr[16];      /* IPv6 address */

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

3.3. Socket Address Structure for 4.3BSD-Based Systems

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

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

       #include <netinet/in.h>
       struct sockaddr_in6 {
           u_int16m_t      sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* transport layer port # */
           u_int32m_t      sin6_flowinfo;  /* IPv6 flow information */
           struct in6_addr sin6_addr;      /* IPv6 address */

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

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

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

The sin6_flowinfo field is a 32-bit field that contains two pieces of information: the 24-bit IPv6 flow label and the 4-bit priority field. The contents and interpretation of this member is unspecified at this time.

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

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

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

3.4. Socket Address Structure for 4.4BSD-Based Systems

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

       #include <netinet/in.h>
       #define SIN6_LEN
       struct sockaddr_in6 {
           u_char          sin6_len;       /* length of this struct */
           u_char          sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* transport layer port # */
           u_int32m_t      sin6_flowinfo;  /* IPv6 flow information */
           struct in6_addr sin6_addr;      /* IPv6 address */

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:


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:


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

3.6. Compatibility with IPv4 Applications

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

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

3.7. Compatibility with IPv4 Nodes

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


These addresses are often generated automatically by the gethostbyname() 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 6.6, is provided.

3.8. IPv6 Wildcard Address

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

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

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

extern const struct in6_addr in6addr_any;

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

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

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

struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

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

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

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

3.9. IPv6 Loopback Address

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

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

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

extern const struct in6_addr in6addr_loopback;

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

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

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

struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;

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

4. Interface Identification

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

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

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

4.1. Name-to-Index

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

       #include <net/if.h>
       unsigned int  if_nametoindex(const char *ifname);

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

4.2. Index-to-Name

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

       #include <net/if.h>
       char  *if_indextoname(unsigned int ifindex, char *ifname);

The ifname argument must point to a buffer of at least IFNAMSIZ bytes into which the interface name corresponding to the specified index is returned. (IFNAMSIZ is also defined in <net/if.h> and its value includes a terminating null byte at the end of the interface name.) This pointer is also the return value of the function. If there is no interface corresponding to the specified index, NULL is returned.

4.3. Return All Interface Names and Indexes

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

       #include <net/if.h>
       struct if_nameindex {
         unsigned int   if_index;  /* 1, 2, ... */
         char          *if_name;   /* null terminated name: "le0", ... */
       struct if_nameindex  *if_nameindex(void);

The end of the array of structures is indicated by a structure with an if_index of 0 and an if_name of NULL. The function returns a NULL pointer upon an error.

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

4.4. Free Memory

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

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

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

5. Socket Options

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

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

5.1. Changing Socket Type

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

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

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

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

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

       int  addrform = PF_INET;

if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,

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

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");
           printf("This system is broken.\n");

5.2. Unicast Hop Limit

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

       int  hoplimit = 10;

if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,

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

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

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

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");
           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:

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

Argument type: unsigned int


Set the hop limit to use for outgoing multicast packets. (Note a separate option - IPV6_UNICAST_HOPS - is provided to set the hop limit to use for outgoing unicast packets.) The interpretation of the argument is the same as for the IPV6_UNICAST_HOPS option:

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


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


Leave a multicast group on a specified interface.

Argument type: struct ipv6_mreq

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

       #include <netinet/in.h>
       struct ipv6_mreq {
           struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
           unsigned int    ipv6mr_interface; /* interface index */

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

6. Library Functions

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

6.1. Hostname-to-Address Translation

The commonly used function gethostbyname() remains unchanged as does the hostent structure to which it returns a pointer. Existing applications that call this function continue to receive only IPv4 addresses that are the result of a query in the DNS for A records. (We assume the DNS is being used; some environments may be using a hosts file or some other name resolution system, either of which may impede renumbering. We also assume that the RES_USE_INET6 resolver option is not set, which we describe in more detail shortly.)

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

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

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

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

  • 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 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.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 example is for the Bourne and Korn shells.) 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.

There is no priority among these three methods. 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.

|                  |              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:

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

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

One possible source of confusion is the handling of IPv4-mapped IPv6 addresses and IPv4-compatible IPv6 addresses. This is addressed in [6] and involves the following logic:

  1. If af is AF_INET6, and if len equals 16, and if the IPv6 address is an IPv4-mapped IPv6 address or an IPv4-compatible IPv6 address, then skip over the first 12 bytes of the IPv6 address, set af to AF_INET, and set len to 4.
  1. If af is AF_INET, then query for a PTR record in the in- domain.
  1. If af is AF_INET6, then query for a PTR record in the domain.
  1. If the function is returning success, and if af equals AF_INET, and if the RES_USE_INET6 option was set, then the single address that is returned in the hostent structure (a copy of the first argument to the function) is returned as an IPv4-mapped IPv6 address and the h_length member is set to 16.

All four steps listed are performed, in order. The same caveats regarding a thread-safe version of gethostbyname() that were made at the end of the previous section apply here as well.

6.3. Protocol-Independent Hostname and Service Name Translation

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

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

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

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

The addrinfo structure is defined as:

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

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

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

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

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

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

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

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

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

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

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

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

void freeaddrinfo(struct addrinfo *ai);

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

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

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

       char *gai_strerror(int ecode);

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

6.4. Socket Address Structure to Hostname and Service Name

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

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

int getnameinfo(const struct sockaddr *sa, size_t salen,

char *host, size_t hostlen,

char *serv, size_t servlen,

int flags);

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

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

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

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

       #define NI_MAXHOST  1025
       #define NI_MAXSERV    32

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

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

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

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

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

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

6.5. Address Conversion Functions

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

       #include <sys/socket.h>
       #include <arpa/inet.h>
       int inet_pton(int af, const char *src, void *dst);
       const char *inet_ntop(int af, const void *src,
                             char *dst, size_t size);

The inet_pton() function converts an address in its standard text presentation form into its numeric binary form. The af argument specifies the family of the address. Currently the AF_INET and AF_INET6 address families are supported. The src argument points to the string being passed in. The dst argument points to a buffer into which the function stores the numeric address. The address is returned in network byte order. Inet_pton() returns 1 if the conversion succeeds, 0 if the input is not a valid IPv4 dotted- decimal string or a valid IPv6 address string, or -1 with errno set to EAFNOSUPPORT if the af argument is unknown. The calling application must ensure that the buffer referred to by dst is large enough to hold the numeric address (e.g., 4 bytes for AF_INET or 16 bytes for AF_INET6).

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

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

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

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

       #define INET_ADDRSTRLEN    16
       #define INET6_ADDRSTRLEN   46

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

6.6. Address Testing Macros

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

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

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

7. Summary of New Definitions

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

     <net/if.h>      IFNAMSIZ
     <net/if.h>      struct if_nameindex{};
     <netdb.h>       AI_CANONNAME
     <netdb.h>       AI_PASSIVE
     <netdb.h>       EAI_ADDRFAMILY
     <netdb.h>       EAI_AGAIN
     <netdb.h>       EAI_BADFLAGS
     <netdb.h>       EAI_FAIL
     <netdb.h>       EAI_FAMILY
     <netdb.h>       EAI_MEMORY
     <netdb.h>       EAI_NODATA
     <netdb.h>       EAI_NONAME
     <netdb.h>       EAI_SERVICE
     <netdb.h>       EAI_SOCKTYPE
     <netdb.h>       EAI_SYSTEM
     <netdb.h>       NI_DGRAM
     <netdb.h>       NI_MAXHOST
     <netdb.h>       NI_MAXSERV
     <netdb.h>       NI_NAMEREQD
     <netdb.h>       NI_NOFQDN
     <netdb.h>       NI_NUMERICHOST
     <netdb.h>       NI_NUMERICSERV
     <netdb.h>       struct addrinfo{};
     <netinet/in.h>  IN6ADDR_ANY_INIT
     <netinet/in.h>  IN6ADDR_LOOPBACK_INIT
     <netinet/in.h>  INET6_ADDRSTRLEN
     <netinet/in.h>  INET_ADDRSTRLEN
     <netinet/in.h>  IPPROTO_IPV6
     <netinet/in.h>  IPV6_ADDRFORM
     <netinet/in.h>  IPV6_ADD_MEMBERSHIP
     <netinet/in.h>  IPV6_DROP_MEMBERSHIP
     <netinet/in.h>  IPV6_MULTICAST_HOPS
     <netinet/in.h>  IPV6_MULTICAST_IF
     <netinet/in.h>  IPV6_MULTICAST_LOOP
     <netinet/in.h>  IPV6_UNICAST_HOPS
     <netinet/in.h>  SIN6_LEN
     <netinet/in.h>  extern const struct in6_addr in6addr_any;
     <netinet/in.h>  extern const struct in6_addr in6addr_loopback;
     <netinet/in.h>  struct in6_addr{};
     <netinet/in.h>  struct ipv6_mreq{};
     <netinet/in.h>  struct sockaddr_in6{};
     <resolv.h>      RES_USE_INET6
     <sys/socket.h>  AF_INET6
     <sys/socket.h>  PF_INET6

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

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

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

<netdb.h>       int getaddrinfo(const char *, const char *,
                                const struct addrinfo *,
                                struct addrinfo **);
<netdb.h>       int getnameinfo(const struct sockaddr *, size_t,
                                char *, size_t, char *, size_t, int);
<netdb.h>       void freeaddrinfo(struct addrinfo *);
<netdb.h>       char *gai_strerror(int);
<netdb.h>       struct hostent *gethostbyname(const char *);
<netdb.h>       struct hostent *gethostbyaddr(const char *, int, int);
<netdb.h>       struct hostent *gethostbyname2(const char *, int);

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

8. Security Considerations

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

9. Acknowledgments

Thanks to the many people who made suggestions and provided feedback to to the numerous revisions of this document, including: Werner Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson, Alex Conta, Alan Cox, Steve Deering, Richard Draves, Francis Dupont, Robert Elz, Marc Hasson, Tim Hartrick, Tom Herbert, Bob Hinden, Wan- Yen Hsu, Christian Huitema, Koji Imada, Markus Jork, Ron Lee, Alan Lloyd, Charles Lynn, Jack McCann, Dan McDonald, Dave Mitton, Thomas Narten, Erik Nordmark, Josh Osborne, Craig Partridge, Jean-Luc Richier, Erik Scoredos, Keith Sklower, Matt Thomas, Harvey Thompson, Dean D. Throop, Karen Tracey, Glenn Trewitt, Paul Vixie, David Waitzman, Carl Williams, and Kazuhiko Yamamoto,

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

Craig Metz made many contributions to this document. Ramesh Govindan made a number of contributions and co-authored an earlier version of this memo.

10. References

   [1] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
       Specification", RFC 1883, December 1995.
   [2] Hinden, R., and S. Deering, "IP Version 6 Addressing Architecture",
       RFC 1884, December 1995.
   [3] McDonald, D., "A Simple IP Security API Extension to BSD Sockets",
       Work in Progress.
   [4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
       6.3, November 1995.
   [5] Stevens, W., and M. Thomas, "Advanced Sockets API for IPv6",
       Work in Progress.
   [6] Vixie, P., "Reverse Name Lookups of Encapsulated IPv4 Addresses in
       IPv6", Work in Progress.

11. Authors' Addresses

Robert E. Gilligan
Freegate Corporation
710 Lakeway Dr. STE 230
Sunnyvale, CA 94086

    Phone: +1 408 524 4804

Susan Thomson
Bell Communications Research
MRE 2P-343, 445 South Street
Morristown, NJ 07960

    Phone: +1 201 829 4514

Jim Bound
Digital Equipment Corporation
110 Spitbrook Road ZK3-3/U14
Nashua, NH 03062-2698

    Phone: +1 603 881 0400

W. Richard Stevens
1202 E. Paseo del Zorro
Tucson, AZ 85718-2826

    Phone: +1 520 297 9416