Network Working Group
Request for Comments: 2553
Obsoletes: 2133
Category: Informational

R. Gilligan
FreeGate
S. Thomson
Bellcore
J. Bound
Compaq
W. Stevens
Consultant
March 1999

Basic Socket Interface Extensions for IPv6

Status of this Memo

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

Copyright Notice

Abstract

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

   1. Introduction.................................................3
   2. Design Considerations........................................3
   2.1 What Needs to be Changed....................................4
   2.2 Data Types..................................................5
   2.3 Headers.....................................................5
   2.4 Structures..................................................5
   3. Socket Interface.............................................6
   3.1 IPv6 Address Family and Protocol Family.....................6
   3.2 IPv6 Address Structure......................................6
   3.3 Socket Address Structure for 4.3BSD-Based Systems...........7
   3.4 Socket Address Structure for 4.4BSD-Based Systems...........8
   3.5 The Socket Functions........................................9
   3.6 Compatibility with IPv4 Applications.......................10
   3.7 Compatibility with IPv4 Nodes..............................10
   3.8 IPv6 Wildcard Address......................................11
   3.9 IPv6 Loopback Address......................................12
   3.10 Portability Additions.....................................13
   4. Interface Identification....................................16
   4.1 Name-to-Index..............................................16
   4.2 Index-to-Name..............................................17
   4.3 Return All Interface Names and Indexes.....................17
   4.4 Free Memory................................................18
   5. Socket Options..............................................18
   5.1 Unicast Hop Limit..........................................18
   5.2 Sending and Receiving Multicast Packets....................19
   6. Library Functions...........................................21
   6.1 Nodename-to-Address Translation............................21
   6.2 Address-To-Nodename Translation............................24
   6.3 Freeing memory for getipnodebyname and getipnodebyaddr.....26
   6.4 Protocol-Independent Nodename and Service Name Translation.26
   6.5 Socket Address Structure to Nodename and Service Name......29
   6.6 Address Conversion Functions...............................31
   6.7 Address Testing Macros.....................................32
   7. Summary of New Definitions..................................33
   8. Security Considerations.....................................35
   9. Year 2000 Considerations....................................35
   Changes From RFC 2133..........................................35
   Acknowledgments................................................38
   References.....................................................39
   Authors' Addresses.............................................40
   Full Copyright Statement.......................................41

1. Introduction

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

2. Design Considerations

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

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

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

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

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

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

2.1 What Needs to be Changed

The socket interface API consists of a few distinct components:

Core socket functions.

Address data structures.

Name-to-address translation functions.

Address conversion functions.

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

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

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

The name-to-address translation functions in the socket interface are gethostbyname() and gethostbyaddr(). These are left as is and new functions are defined to support IPv4 and IPv6. Additionally, the POSIX 1003.g draft [3] specifies a new nodename-to-address translation function which is protocol independent. This function can also be used with IPv4 and IPv6.

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

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

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

2.2 Data Types

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

2.3 Headers

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

2.4 Structures

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

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

3. Socket Interface

This section specifies the socket interface changes for IPv6.

3.1 IPv6 Address Family and Protocol Family

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

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

      #define PF_INET6        AF_INET6

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

3.2 IPv6 Address Structure

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

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

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

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

An example is as follows:

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

3.3 Socket Address Structure for 4.3BSD-Based Systems

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

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

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

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

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

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

The sin6_flowinfo field is a 32-bit field that contains two pieces of information: the traffic class and the flow label. The contents and interpretation of this member is specified in [1]. The sin6_flowinfo field SHOULD be set to zero by an implementation prior to using the sockaddr_in6 structure by an application on receive operations.

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

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

The sin6_scope_id field is a 32-bit integer that identifies a set of interfaces as appropriate for the scope of the address carried in the sin6_addr field. For a link scope sin6_addr sin6_scope_id would be an interface index. For a site scope sin6_addr, sin6_scope_id would be a site identifier. The mapping of sin6_scope_id to an interface or set of interfaces is left to implementation and future specifications on the subject of site identifiers.

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

3.4 Socket Address Structure for 4.4BSD-Based Systems

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

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

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

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

3.5 The Socket Functions

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

      s = socket(PF_INET, SOCK_STREAM, 0);

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

      s = socket(PF_INET, SOCK_DGRAM, 0);

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

      s = socket(PF_INET6, SOCK_STREAM, 0);

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

      s = socket(PF_INET6, SOCK_DGRAM, 0);

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

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

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

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

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

3.6 Compatibility with IPv4 Applications

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

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

3.7 Compatibility with IPv4 Nodes

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

      ::FFFF:<IPv4-address>

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

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

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

3.8 IPv6 Wildcard Address

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

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

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

extern const struct in6_addr in6addr_any;

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

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

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

struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

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

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

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

3.9 IPv6 Loopback Address

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

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

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

extern const struct in6_addr in6addr_loopback;

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

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

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

struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;

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

3.10 Portability Additions

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

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

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

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

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

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

/*
 * Definitions used for sockaddr_storage structure paddings design.
 */
#define _SS_PAD1SIZE (_SS_ALIGNSIZE -
                            (sizeof (uint8_t) + sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t)+

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

The above example implementation illustrates a data structure which will align on a 64 bit boundary. An implementation specific field "__ss_align" along "__ss_pad1" is used to force a 64-bit alignment which covers proper alignment good enough for needs of sockaddr_in6 (IPv6), sockaddr_in (IPv4) address data structures. The size of padding fields __ss_pad1 depends on the chosen alignment boundary. The size of padding field __ss_pad2 depends on the value of overall size chosen for the total size of the structure. This size and alignment are represented in the above example by implementation specific (not required) constants _SS_MAXSIZE (chosen value 128) and _SS_ALIGNMENT (with chosen value 8). Constants _SS_PAD1SIZE (derived value 6) and _SS_PAD2SIZE (derived value 112) are also for illustration and not required. The implementation specific definitions and structure field names above start with an underscore to denote implementation private namespace. Portable code is not expected to access or reference those fields or constants.

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

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

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

So now an application can do the following:

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

4. Interface Identification

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

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

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

4.1 Name-to-Index

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

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

If the specified interface name does not exist, the return value is 0, and errno is set to ENXIO. If there was a system error (such as running out of memory), the return value is 0 and errno is set to the proper value (e.g., ENOMEM).

4.2 Index-to-Name

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

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

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

4.3 Return All Interface Names and Indexes

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

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

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

      struct if_nameindex  *if_nameindex(void);

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

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

4.4 Free Memory

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

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

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

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

5. Socket Options

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

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

5.1 Unicast Hop Limit

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

      int  hoplimit = 10;

if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,

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

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

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

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

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

5.2 Sending and Receiving Multicast Packets

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

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

      IPV6_MULTICAST_IF

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

Argument type: unsigned int

      IPV6_MULTICAST_HOPS

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

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

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

If IPV6_MULTICAST_HOPS is not set, the default is 1 (same as IPv4 today)

         Argument type: int
      
      IPV6_MULTICAST_LOOP

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

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

Argument type: unsigned int

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

      IPV6_JOIN_GROUP

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

Argument type: struct ipv6_mreq

      IPV6_LEAVE_GROUP

Leave a multicast group on a specified interface.

Argument type: struct ipv6_mreq

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

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

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

6. Library Functions

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

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

6.1 Nodename-to-Address Translation

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

The following function is new and must be thread safe:

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

   struct hostent *getipnodebyname(const char *name, int af, int flags
                                       int *error_num);

The name argument can be either a node name or a numeric address string (i.e., a dotted-decimal IPv4 address or an IPv6 hex address). The af argument specifies the address family, either AF_INET or

AF_INET6. The error_num value is returned to the caller, via a pointer, with the appropriate error code in error_num, to support thread safe error code returns. error_num will be set to one of the following values:

      HOST_NOT_FOUND

No such host is known.

      NO_ADDRESS

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

      NO_RECOVERY

An unexpected server failure occurred which cannot be recovered.

      TRY_AGAIN

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

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

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

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

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

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

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

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

- If flags is 0 and if af is AF_INET6, then the caller wants only

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

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

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

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

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

For example, if the node has no IPv6 source addresses

configured, and af equals AF_INET6, and the node name being

looked up has both AAAA and A records, then:

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

The special flags value of AI_DEFAULT is defined as

      #define  AI_DEFAULT  (AI_V4MAPPED | AI_ADDRCONFIG)

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

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

The two simple cases are:

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

   When name is a dotted-decimal IPv4 address and af equals AF_INET6,
   and flags equals AI_V4MAPPED, an IPv4-mapped IPv6 address is
   returned:  h_name points to an IPv6 hex address containing the IPv4-
   mapped IPv6 address, h_aliases is a NULL pointer, h_addrtype is
   AF_INET6, h_length is 16, h_addr_list[0] is a pointer to the 16-byte
   binary address, and h_addr_list[1] is a NULL pointer.  If AI_V4MAPPED
   is set (with or without AI_ALL) return IPv4-mapped otherwise return
   NULL.

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

6.2 Address-To-Nodename Translation

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

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

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

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

      HOST_NOT_FOUND

No such host is known.

      NO_ADDRESS

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

      NO_RECOVERY

An unexpected server failure occurred which cannot be recovered.

      TRY_AGAIN

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

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

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

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

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

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

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

6.3 Freeing memory for getipnodebyname and getipnodebyaddr

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

      #include <netdb.h>

void freehostent(struct hostent *ptr);

6.4 Protocol-Independent Nodename and Service Name Translation

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

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

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

The AI_NUMERICHOST is new with this document.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

If the AI_NUMERICHOST bit is set in the ai_flags member of the hints structure, then a non-NULL nodename string must be a numeric host address string. Otherwise an error of EAI_NONAME is returned. This flag prevents any type of name resolution service (e.g., the DNS) from being called.

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

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

void freeaddrinfo(struct addrinfo *ai);

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

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

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

      char *gai_strerror(int ecode);

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

6.5 Socket Address Structure to Nodename and Service Name

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

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

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

char *host, size_t hostlen,

char *serv, size_t servlen,

int flags);

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

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

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

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

      #define NI_MAXHOST  1025
      #define NI_MAXSERV    32

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

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

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

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

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

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

6.6 Address Conversion Functions

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

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

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

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

      ddd.ddd.ddd.ddd

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

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

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

      #define INET_ADDRSTRLEN    16
      #define INET6_ADDRSTRLEN   46

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

6.7 Address Testing Macros

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

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

The first seven macros return true if the address is of the specified type, or false otherwise. The last five test the scope of a multicast address and return true if the address is a multicast address of the specified scope or false if the address is either not a multicast address or not of the specified scope. Note that IN6_IS_ADDR_LINKLOCAL and IN6_IS_ADDR_SITELOCAL return true only for the two local-use IPv6 unicast addresses. These two macros do not return true for IPv6 multicast addresses of either link-local scope or site-local scope.

7. Summary of New Definitions

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

      <net/if.h>      IF_NAMESIZE
      <net/if.h>      struct if_nameindex{};
      
      <netdb.h>       AI_ADDRCONFIG
      <netdb.h>       AI_DEFAULT
      <netdb.h>       AI_ALL
      <netdb.h>       AI_CANONNAME
      <netdb.h>       AI_NUMERICHOST
      <netdb.h>       AI_PASSIVE
      <netdb.h>       AI_V4MAPPED
      <netdb.h>       EAI_ADDRFAMILY
      <netdb.h>       EAI_AGAIN
      <netdb.h>       EAI_BADFLAGS
      <netdb.h>       EAI_FAIL
      <netdb.h>       EAI_FAMILY
      <netdb.h>       EAI_MEMORY
      <netdb.h>       EAI_NODATA
      <netdb.h>       EAI_NONAME
      <netdb.h>       EAI_SERVICE
      <netdb.h>       EAI_SOCKTYPE
      <netdb.h>       EAI_SYSTEM
      <netdb.h>       NI_DGRAM
      <netdb.h>       NI_MAXHOST
      <netdb.h>       NI_MAXSERV
      <netdb.h>       NI_NAMEREQD
      <netdb.h>       NI_NOFQDN
      <netdb.h>       NI_NUMERICHOST
      <netdb.h>       NI_NUMERICSERV
      <netdb.h>       struct addrinfo{};
      
      <netinet/in.h>  IN6ADDR_ANY_INIT
      <netinet/in.h>  IN6ADDR_LOOPBACK_INIT
      <netinet/in.h>  INET6_ADDRSTRLEN
      <netinet/in.h>  INET_ADDRSTRLEN
      <netinet/in.h>  IPPROTO_IPV6
      <netinet/in.h>  IPV6_JOIN_GROUP
      <netinet/in.h>  IPV6_LEAVE_GROUP
      <netinet/in.h>  IPV6_MULTICAST_HOPS
      <netinet/in.h>  IPV6_MULTICAST_IF
      <netinet/in.h>  IPV6_MULTICAST_LOOP
      <netinet/in.h>  IPV6_UNICAST_HOPS
      <netinet/in.h>  SIN6_LEN
      <netinet/in.h>  extern const struct in6_addr in6addr_any;
      <netinet/in.h>  extern const struct in6_addr in6addr_loopback;
      <netinet/in.h>  struct in6_addr{};
      <netinet/in.h>  struct ipv6_mreq{};
      <netinet/in.h>  struct sockaddr_in6{};
      
      <sys/socket.h>  AF_INET6
      <sys/socket.h>  PF_INET6
      <sys/socket.h>  struct sockaddr_storage;

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

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

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

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

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

8. Security Considerations

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

9. Year 2000 Considerations

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

Changes From RFC 2133

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

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

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

Section 3.10 ("Portability Additions") is new.

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

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

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

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

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

Updated references.

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

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

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

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

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

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

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

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

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

Section 4: NET_RT_LIST changed to NET_RT_IFLIST.

Section 5.1: The IPV6_ADDRFORM socket option was removed.

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

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

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

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

Section 6.6: Added clarification about IN6_IS_ADDR_LINKLOCAL and IN6_IS_ADDR_SITELOCAL macros.

Changes made to the draft -01 specification Sept 98

Changed priority to traffic class in the spec.

Added the need for scope identification in section 2.1.

Added sin6_scope_id to struct sockaddr_in6 in sections 3.3 and 3.4.

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

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

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

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

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

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

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

Added Year 2000 Section to the draft

Deleted Reference to the following because the attached is deleted from the ID directory and has expired. But the logic from the aforementioned draft still applies, so that was kept in Section 6.2 bullets after 3rd paragraph.

      [7]  P. Vixie, "Reverse Name Lookups of Encapsulated IPv4
           Addresses in IPv6", Internet-Draft, <draft-vixie-ipng-
           ipv4ptr-00.txt>, May 1996.

Deleted the following reference as it is no longer referenced. And the draft has expired.

      [3]  D. McDonald, "A Simple IP Security API Extension to BSD
           Sockets", Internet-Draft, <draft-mcdonald-simple-ipsec-api-
           01.txt>, March 1997.

Deleted the following reference as it is no longer referenced.

      [4]  C. Metz, "Network Security API for Sockets",
           Internet-Draft, <draft-metz-net-security-api-01.txt>, January
           1998.

Update current references to current status.

Added alignment notes for in6_addr and sin6_addr.

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

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

Acknowledgments

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

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

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

References

   [1]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
   
        Specification", RFC 2460, December 1998.
   
   [2]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.
   
   [3]  IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
        6.6, March 1997.
   
   [4]  Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6", RFC
        2292, February 1998.

Authors' Addresses

Robert E. Gilligan
FreeGate Corporation
1208 E. Arques Ave.
Sunnyvale, CA 94086

   Phone: +1 408 617 1004
   EMail: gilligan@freegate.com

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

   Phone: +1 201 829 4514
   EMail: set@thumper.bellcore.com

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

   Phone: +1 603 884 0400
   EMail: bound@zk3.dec.com

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

   Phone: +1 520 297 9416
   EMail: rstevens@kohala.com

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Basic Socket Interface Extensions for IPv6

Status of this Memo

Copyright Notice

Abstract

Table of Contents

1. Introduction

2. Design Considerations

2.1 What Needs to be Changed

2.2 Data Types

2.3 Headers

2.4 Structures

3. Socket Interface

3.1 IPv6 Address Family and Protocol Family

3.2 IPv6 Address Structure

3.3 Socket Address Structure for 4.3BSD-Based Systems

3.4 Socket Address Structure for 4.4BSD-Based Systems

3.5 The Socket Functions

3.6 Compatibility with IPv4 Applications

3.7 Compatibility with IPv4 Nodes

3.8 IPv6 Wildcard Address

3.9 IPv6 Loopback Address

3.10 Portability Additions

4. Interface Identification

4.1 Name-to-Index

4.2 Index-to-Name

4.3 Return All Interface Names and Indexes

4.4 Free Memory

5. Socket Options

5.1 Unicast Hop Limit

5.2 Sending and Receiving Multicast Packets

6. Library Functions

6.1 Nodename-to-Address Translation

For example, if the node has no IPv6 source addresses

configured, and af equals AF_INET6, and the node name being

looked up has both AAAA and A records, then:

6.2 Address-To-Nodename Translation

6.3 Freeing memory for getipnodebyname and getipnodebyaddr

6.4 Protocol-Independent Nodename and Service Name Translation

6.5 Socket Address Structure to Nodename and Service Name

char *host, size_t hostlen,

char *serv, size_t servlen,

int flags);

6.6 Address Conversion Functions

6.7 Address Testing Macros

7. Summary of New Definitions

8. Security Considerations

9. Year 2000 Considerations

Changes From RFC 2133

Acknowledgments

References

Authors' Addresses

Full Copyright Statement