Internet Engineering Task Force R. E. Gilligan (Freegate)
INTERNET-DRAFT S. Thomson (Bellcore)
J. Bound (Digital)
W. R. Stevens (Consultant)
November 23, 1996
Basic Socket Interface Extensions for IPv6
<draft-ietf-ipngwg-bsd-api-06.txt>
Abstract
The de facto standard application program interface (API) for TCP/IP
applications is the "sockets" interface. Although this API was
developed for Unix in the early 1980s it has also been implemented on
a wide variety of non-Unix systems. TCP/IP applications written
using the sockets API have in the past enjoyed a high degree of
portability and we would like the same portability with IPv6
applications. But changes are required to the sockets API to support
IPv6 and this memo describes these changes. These include a new
socket address structure to carry IPv6 addresses, new address
conversion functions, and some new socket options. These extensions
are designed to provide access to the basic IPv6 features required by
TCP and UDP applications, including multicasting, while introducing a
minimum of change into the system and providing complete
compatibility for existing IPv4 applications. Additional extensions
for advanced IPv6 features (raw sockets and access to the IPv6
extension headers) are defined in another document [5].
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. This Internet Draft expires on May 23, 1997. Internet
Drafts may be updated, replaced, or obsoleted by other documents at
any time. It is not appropriate to use Internet Drafts as reference
material or to cite them other than as a "working draft" or "work in
progress."
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
munnari.oz.au.
Distribution of this memo is unlimited.
draft-ietf-ipngwg-bsd-api-06.txt [Page 1]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
Table of Contents
1. Introduction ..................................................... 3
2. Design Considerations ............................................ 3
2.1. What Needs to be Changed ....................................... 3
2.2. Data Types ..................................................... 5
3. Socket Interface ................................................. 5
3.1. IPv6 Address Family and Protocol Family ........................ 5
3.2. IPv6 Address Structure ......................................... 5
3.3. Socket Address Structure for 4.3BSD-Based Systems .............. 6
3.4. Socket Address Structure for 4.4BSD-Based Systems .............. 7
3.5. The Socket Functions ........................................... 8
3.6. Compatibility with IPv4 Applications ........................... 9
3.7. Compatibility with IPv4 Nodes .................................. 9
3.8. Flow Information ............................................... 10
3.9. IPv6 Wildcard Address .......................................... 12
3.10. IPv6 Loopback Address ......................................... 13
4. Interface Identification ......................................... 14
4.1. Name-to-Index .................................................. 15
4.2. Index-to-Name .................................................. 15
4.3. Return All Interface Names and Indexes ......................... 15
5. Socket Options ................................................... 16
5.1. Changing Socket Type ........................................... 16
5.2. Unicast Hop Limit .............................................. 17
5.3. Sending and Receiving Multicast Packets ........................ 18
6. Library Functions ................................................ 20
6.1. Hostname-to-Address Translation ................................ 20
6.2. Address To Hostname Translation ................................ 22
6.3. Protocol-Independent Hostname and Service Name Translation ..... 23
6.4. Socket Address Structure to Hostname and Service Name .......... 26
6.5. Address Conversion Functions ................................... 27
6.6. IPv4-Mapped Addresses .......................................... 28
7. Security Considerations .......................................... 29
8. Change History ................................................... 29
9. Acknowledgments .................................................. 33
10. References ...................................................... 33
11. Authors' Addresses .............................................. 34
draft-ietf-ipngwg-bsd-api-06.txt [Page 2]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
1. Introduction
While IPv4 addresses are 32 bits long, IPv6 nodes are identified by
128-bit addresses. The socket interface make the size of an IP
address quite visible to an application; virtually all TCP/IP
applications for BSD-based systems have knowledge of the size of an
IP address. Those parts of the API that expose the addresses must be
changed to accommodate the larger IPv6 address size. IPv6 also
introduces new features (e.g., flow label and priority), some of
which must be made visible to applications via the API. This memo
defines a set of extensions to the socket interface to support the
larger address size and new features of IPv6.
2. Design Considerations
There are a number of important considerations in designing changes
to this well-worn API:
- The API changes should provide both source and binary
compatibility for programs written to the original API. That is,
existing program binaries should continue to operate when run on
a system supporting the new API. In addition, existing
applications that are re-compiled and run on a system supporting
the new API should continue to operate. Simply put, the API
changes for IPv6 should not break existing programs.
- The changes to the API should be as small as possible in order to
simplify the task of converting existing IPv4 applications to
IPv6.
- Where possible, applications should be able to use this API to
interoperate with both IPv6 and IPv4 hosts. Applications should
not need to know which type of host they are communicating with.
- IPv6 addresses carried in data structures should be 64-bit
aligned. This is necessary in order to obtain optimum
performance on 64-bit machine architectures.
Because of the importance of providing IPv4 compatibility in the API,
these extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this
API could probably be designed for operation on systems that support
only IPv6. However, this is not addressed in this memo.
2.1. What Needs to be Changed
draft-ietf-ipngwg-bsd-api-06.txt [Page 3]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
The socket interface API consists of a few distinct components:
- Core socket functions.
- Address data structures.
- Name-to-address translation functions.
- Address conversion functions.
The core socket functions -- those functions that deal with such
things as setting up and tearing down TCP connections, and sending
and receiving UDP packets -- were designed to be transport
independent. Where protocol addresses are passed as function
arguments, they are carried via opaque pointers. A protocol-specific
address data structure is defined for each protocol that the socket
functions support. Applications must cast pointers to these
protocol-specific address structures into pointers to the generic
"sockaddr" address structure when using the socket functions. These
functions need not change for IPv6, but a new IPv6-specific address
data structure is needed.
The "sockaddr_in" structure is the protocol-specific data structure
for IPv4. This data structure actually includes 8-octets of unused
space, and it is tempting to try to use this space to adapt the
sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in
structure is not large enough to hold the 16-octet IPv6 address as
well as the other information (address family and port number) that
is needed. So a new address data structure must be defined for IPv6.
The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr(). These must be modified to
support IPv6 and the semantics defined must provide 100% backward
compatibility for all existing IPv4 applications, along with IPv6
support for new applications. Additionally, the POSIX 1003.g draft
[4] specifies a new hostname-to-address translation function which is
protocol independent. This function can also be used with IPv6.
The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form. These
functions are quite specific to 32-bit IPv4 addresses. We have
designed two analogous functions that convert both IPv4 and IPv6
addresses, and carry an address type parameter so that they can be
extended to other protocol families as well.
Finally, a few miscellaneous features are needed to support IPv6.
New interfaces are needed to support the IPv6 flow label, priority,
and hop limit header fields. New socket options are needed to
draft-ietf-ipngwg-bsd-api-06.txt [Page 4]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
control the sending and receiving of IPv6 multicast packets.
The socket interface may be enhanced in the future to provide access
to other IPv6 features. These extensions are described in [5].
2.2. Data Types
The data types of the structure elements given in this memo are
intended to be examples, not absolute requirements. Whenever
possible, POSIX 1003.1g data types are used: u_intN_t means an
unsigned integer of exactly N bits (e.g., u_int16_t) and u_intNm_t
means an unsigned integer of at least N bits (e.g., u_int32m_t). We
also assume the argument data types from 1003.1g when possible (e.g.,
the final argument to setsockopt() is a size_t value). Whenever
buffer sizes are specified, the POSIX 1003.1 size_t data type is used
(e.g., the two length arguments to getnameinfo()).
3. Socket Interface
This section specifies the socket interface changes for IPv6.
3.1. IPv6 Address Family and Protocol Family
A new address family name, AF_INET6, is defined in <sys/socket.h>.
The AF_INET6 definition distinguishes between the original
sockaddr_in address data structure, and the new sockaddr_in6 data
structure.
A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
Like most of the other protocol family names, this will usually be
defined to have the same value as the corresponding address family
name:
#define PF_INET6 AF_INET6
The PF_INET6 is used in the first argument to the socket() function
to indicate that an IPv6 socket is being created.
3.2. IPv6 Address Structure
A new data structure to hold a single IPv6 address is defined as
follows:
draft-ietf-ipngwg-bsd-api-06.txt [Page 5]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
struct in6_addr {
u_char s6_addr[16]; /* IPv6 address */
}
This data structure contains an array of sixteen 8-bit elements,
which make up one 128-bit IPv6 address. The IPv6 address is stored
in network byte order.
Applications obtain the declaration for this structure by including
the header <netinet/in.h>.
3.3. Socket Address Structure for 4.3BSD-Based Systems
In the socket interface, a different protocol-specific data structure
is defined to carry the addresses for each protocol suite. Each
protocol-specific data structure is designed so it can be cast into a
protocol-independent data structure -- the "sockaddr" structure.
Each has a "family" field that overlays the "sa_family" of the
sockaddr data structure. This field identifies the type of the data
structure.
The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between
applications and the system in the socket functions. The following
structure is defined to carry IPv6 addresses:
struct sockaddr_in6 {
u_int16m_t sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
};
This structure is designed to be compatible with the sockaddr data
structure used in the 4.3BSD release.
The sin6_family field identifies this as a sockaddr_in6 structure.
This field overlays the sa_family field when the buffer is cast to a
sockaddr data structure. The value of this field must be AF_INET6.
The sin6_port field contains the 16-bit UDP or TCP port number. This
field is used in the same way as the sin_port field of the
sockaddr_in structure. The port number is stored in network byte
order.
The sin6_flowinfo field is a 32-bit field that contains two pieces of
draft-ietf-ipngwg-bsd-api-06.txt [Page 6]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
information: the 24-bit IPv6 flow label and the 4-bit priority field.
The IPv6 flow label is represented as the low-order 24 bits of the
32-bit field. The priority is represented in the next 4 bits above
this. The high-order 4 bits of this field are reserved. The
sin6_flowinfo field is stored in network byte order. The use of the
flow label and priority fields are explained in Section 3.8.
The sin6_addr field is a single in6_addr structure (defined in the
previous section). This field holds one 128-bit IPv6 address. The
address is stored in network byte order.
The ordering of elements in this structure is specifically designed
so that the sin6_addr field will be aligned on a 64-bit boundary.
This is done for optimum performance on 64-bit architectures.
Applications obtain the declaration of the sockaddr_in6 structure by
including the header <netinet/in.h>.
3.4. Socket Address Structure for 4.4BSD-Based Systems
The 4.4BSD release includes a small, but incompatible change to the
socket interface. The "sa_family" field of the sockaddr data
structure was changed from a 16-bit value to an 8-bit value, and the
space saved used to hold a length field, named "sa_len". The
sockaddr_in6 data structure given in the previous section cannot be
correctly cast into the newer sockaddr data structure. For this
reason, the following alternative IPv6 address data structure is
provided to be used on systems based on 4.4BSD:
#define SIN6_LEN
struct sockaddr_in6 {
u_char sin6_len; /* length of this struct */
u_char sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* Transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
};
The only differences between this data structure and the 4.3BSD
variant are the inclusion of the length field, and the change of the
family field to a 8-bit data type. The definitions of all the other
fields are identical to the structure defined in the previous
section.
Systems that provide this version of the sockaddr_in6 data structure
must also declare SIN6_LEN as a result of including the
draft-ietf-ipngwg-bsd-api-06.txt [Page 7]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
<netinet/in.h> header. This macro allows applications to determine
whether they are being built on a system that supports the 4.3BSD or
4.4BSD variants of the data structure.
Note that the size of the sockaddr_in6 structure is larger than the
size of the sockaddr structure. Applications that use the
sockaddr_in6 structure need to be aware that they cannot use
sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6
structure. They should use sizeof(sockaddr_in6) instead.
3.5. The Socket Functions
Applications call the socket() function to create a socket descriptor
that represents a communication endpoint. The arguments to the
socket() function tell the system which protocol to use, and what
format address structure will be used in subsequent functions. For
example, to create an IPv4/TCP socket, applications make the call:
s = socket(PF_INET, SOCK_STREAM, 0);
To create an IPv4/UDP socket, applications make the call:
s = socket(PF_INET, SOCK_DGRAM, 0);
Applications may create IPv6/TCP and IPv6/UDP sockets by simply using
the constant PF_INET6 instead of PF_INET in the first argument. For
example, to create an IPv6/TCP socket, applications make the call:
s = socket(PF_INET6, SOCK_STREAM, 0);
To create an IPv6/UDP socket, applications make the call:
s = socket(PF_INET6, SOCK_DGRAM, 0);
Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the
system. The functions that the application uses to pass addresses
into the system are:
bind()
connect()
sendmsg()
sendto()
The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets. The
functions that return an address from the system to an application
draft-ietf-ipngwg-bsd-api-06.txt [Page 8]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
are:
accept()
recvfrom()
recvmsg()
getpeername()
getsockname()
No changes to the syntax of the socket functions are needed to
support IPv6, since the all of the "address carrying" functions use
an opaque address pointer, and carry an address length as a function
argument.
3.6. Compatibility with IPv4 Applications
In order to support the large base of applications using the original
API, system implementations must provide complete source and binary
compatibility with the original API. This means that systems must
continue to support PF_INET sockets and the sockaddr_in address
structure. Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the PF_INET constant in the socket() function, as
described in the previous section. Applications should be able to
hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
sockets simultaneously within the same process.
Applications using the original API should continue to operate as
they did on systems supporting only IPv4. That is, they should
continue to interoperate with IPv4 nodes.
3.7. Compatibility with IPv4 Nodes
The API also provides a different type of compatibility: the ability
for IPv6 applications to interoperate with IPv4 applications. This
feature uses the IPv4-mapped IPv6 address format defined in the IPv6
addressing architecture specification [2]. This address format
allows the IPv4 address of an IPv4 node to be represented as an IPv6
address. The IPv4 address is encoded into the low-order 32 bits of
the IPv6 address, and the high-order 96 bits hold the fixed prefix
0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows:
::FFFF:<IPv4-address>
These addresses are often generated automatically by the
gethostbyname() function when the specified host has only IPv4
addresses (as described in Section 6.1).
draft-ietf-ipngwg-bsd-api-06.txt [Page 9]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and
passing that address, within a sockaddr_in6 structure, in the
connect() or sendto() call. When applications use PF_INET6 sockets
to accept TCP connections from IPv4 nodes, or receive UDP packets
from IPv4 nodes, the system returns the peer's address to the
application in the accept(), recvfrom(), or getpeername() call using
a sockaddr_in6 structure encoded this way.
Few applications will likely need to know which type of node they are
interoperating with. However, for those applications that do need to
know, the inet6_isipv4mapped() function, defined in Section 6.6, is
provided.
3.8. Flow Information
The IPv6 header has a 24-bit field to hold a "flow label", and a 4-
bit field to hold a "priority" value. Applications must have control
over what values for these fields are used in packets that they
originate, and must have access to the field values of packets that
they receive.
The sin6_flowinfo field of the sockaddr_in6 structure encodes two
pieces of information: IPv6 flow label and IPv6 priority.
Applications use this field to set the flow label and priority in
IPv6 headers of packets they generate, and to retrieve the flow label
and priority from the packets they receive. The header fields of an
actively opened TCP connection are set by assigning in the
sin6_flowinfo field of the destination address sockaddr_in6 structure
passed in the connect() function. The same technique can be used
with the sockaddr_in6 structure passed to the sendto() or sendmsg()
function to set the flow label and priority fields of UDP packets.
Similarly, the flow label and priority values of received UDP packets
and accepted TCP connections are reflected in the sin6_flowinfo field
of the sockaddr_in6 structure returned to the application by the
recvfrom(), recvmsg(), and accept() functions. An application may
specify the flow label and priority to use in transmitted packets of
a passively accepted TCP connection, by setting the sin6_flowinfo
field of the address passed to the bind() function.
Implementations provide two bitmask constant declarations to help
applications select out the flow label and priority fields. These
constants are:
IPV6_FLOWINFO_FLOWLABEL
IPV6_FLOWINFO_PRIORITY
draft-ietf-ipngwg-bsd-api-06.txt [Page 10]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
These constants can be applied to the sin6_flowinfo field of
addresses returned to the application, for example:
int recv_flow; /* host byte ordered, 0-0x00ffffff */
int recv_prio; /* host byte ordered, 0-15 */
struct sockaddr_in6 sin6;
. . .
recvfrom(s, buf, buflen, flags, (struct sockaddr *) &sin6, &fromlen);
. . .
recv_flow = ntohl(sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL);
recv_prio = ntohl(sin6.sin6_flowinfo & IPV6_FLOWINFO_PRIORITY) >> 24;
printf("flow = %d, prio = %d\n", recv_flow, recv_prio);
Recall that sin6_flowinfo is network byte ordered, as are the two
IPV6_FLOWINFO_xxx constants.
On the sending side, applications are responsible for selecting the
flow label value and specifying a priority. The headers provide
constant declarations for the 16 IPv6 priority values defined in the
IPv6 specification [1]. These constants are:
IPV6_PRIORITY_UNCHARACTERIZED
IPV6_PRIORITY_FILLER
IPV6_PRIORITY_UNATTENDED
IPV6_PRIORITY_RESERVED1
IPV6_PRIORITY_BULK
IPV6_PRIORITY_RESERVED2
IPV6_PRIORITY_INTERACTIVE
IPV6_PRIORITY_CONTROL
IPV6_PRIORITY_8
IPV6_PRIORITY_9
IPV6_PRIORITY_10
IPV6_PRIORITY_11
IPV6_PRIORITY_12
IPV6_PRIORITY_13
IPV6_PRIORITY_14
IPV6_PRIORITY_15
Most applications will use these constants (e.g.,
IPV6_PRIORITY_INTERACTIVE can be built into Telnet clients and
servers). Since these constants are defined in network byte order an
example is:
draft-ietf-ipngwg-bsd-api-06.txt [Page 11]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
int send_flow; /* host byte ordered, 0-0x00ffffff */
struct sockaddr_in6 sin6;
send_flow = /* undefined at this time; perhaps a system call */
sin6.sin6_flowinfo = htonl(send_flow) & IPV6_FLOWINFO_FLOWLABEL |
IPV6_PRIORITY_INTERACTIVE;
. . .
connect( ... )
Some applications may specify the priority as a value between 0 and
15 (perhaps a command-line argument) and the following example shows
the required byte ordering and shifting:
int send_flow; /* host byte ordered, 0-0x00ffffff */
int send_prio; /* host byte ordered, 0-15 */
struct sockaddr_in6 sin6;
send_flow = /* undefined at this time; perhaps a system call */
send_prio = 12; /* or some other host byte ordered value, 0-15 */
sin6.sin6_flowinfo = htonl(send_flow) & IPV6_FLOWINFO_FLOWLABEL |
htonl(send_prio << 24) & IPV6_FLOWINFO_PRIORITY;
. . .
sendto( ... )
The declarations for these constants are obtained by including the
header <netinet/in.h>.
3.9. IPv6 Wildcard Address
While the bind() function allows applications to select the source IP
address of UDP packets and TCP connections, applications often want
the system select the source address for them. With IPv4, one
specifies the address as the symbolic constant INADDR_ANY (called the
"wildcard" address) in the bind() call, or simply omits the bind()
entirely.
Since the IPv6 address type is a structure (struct in6_addr), a
symbolic constant can be used to initialize an IPv6 address variable,
but cannot be used in an assignment. Therefore systems provide the
IPv6 wildcard address in two forms.
The first version is a global variable named "in6addr_any" that is an
in6_addr structure. The extern declaration for this variable is:
extern const struct in6_addr in6addr_any;
draft-ietf-ipngwg-bsd-api-06.txt [Page 12]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
Applications use in6addr_any similarly to the way they use INADDR_ANY
in IPv4. For example, to bind a socket to port number 23, but let
the system select the source address, an application could use the
following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_any; /* structure assignment */
. . .
if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The other version is a symbolic constant named IN6ADDR_ANY_INIT.
This constant can be used to initialize an in6_addr structure:
struct in6_addr anyaddr = IN6ADDR_ANY_INIT;
Note that this constant can be used ONLY at declaration type. It can
not be used to assign a previously declared in6_addr structure. For
example, the following code will not work:
/* This is the WRONG way to assign an unspecified address */
struct sockaddr_in6 sin6;
. . .
sin6.sin6_addr = IN6ADDR_ANY_INIT; /* Will NOT compile */
The extern declaration for in6addr_any and the declaration for
IN6ADDR_ANY_INIT are obtained by including the header <netinet/in.h>.
Be aware that the IPv4 INADDR_xxx constants are all defined in host
byte order but the IPv6 IN6ADDR_xxx constants and the IPv6
in6addr_xxx externals are defined in network byte order.
3.10. IPv6 Loopback Address
Applications may need to send UDP packets to, or originate TCP
connections to, services residing on the local node. In IPv4, they
can do this by using the constant IPv4 address INADDR_LOOPBACK in
their connect(), sendto(), or sendmsg() call.
IPv6 also provides a loopback address to contact local TCP and UDP
services. Like the unspecified address, the IPv6 loopback address is
provided in two forms -- a global variable and a symbolic constant.
draft-ietf-ipngwg-bsd-api-06.txt [Page 13]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
The global variable is an in6_addr structure named
"in6addr_loopback." The extern declaration for this variable is:
extern const struct in6_addr in6addr_loopback;
Applications use in6addr_loopback as they would use INADDR_LOOPBACK
in IPv4 applications (but beware of the byte ordering difference
mentioned at the end of the previous section). For example, to open
a TCP connection to the local telnet server, an application could use
the following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_loopback; /* structure assignment */
. . .
if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The symbolic constant is named IN6ADDR_LOOPBACK_INIT. It can be used
at declaration time ONLY; for example:
struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
to a previously declared IPv6 address variable.
The extern declaration for in6addr_loopback and the declaration for
IN6ADDR_LOOPBACK_INIT are obtained by including the header
<netinet/in.h>.
4. Interface Identification
This API uses an interface index (a small positive integer) to
identify the local interface on which a multicast group is joined
(Section 5.3). Additionally, the advanced API [5] uses these same
interface indexes to identify the interface on which a datagram is
received, or to specify the interface on which a datagram is to be
sent.
Interfaces are normally known by names such as "le0", "sl1", "ppp2",
and the like. On Berkeley-derived implementations, when an interface
is made known to the system, the kernel assigns a unique positive
integer value (called the interface index) to that interface. These
are small positive integers that start at 1. (Note that 0 is never
draft-ietf-ipngwg-bsd-api-06.txt [Page 14]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
used for an interface index.) There may be gaps so that there is no
current interface for a particular positive interface index.
This API defines two functions that map between an interface name and
index, and a third function that returns all the interface names and
indexes. How these three functions are implemented is left up to the
implementation. 4.4BSD implementations can implement all three
functions using the existing sysctl() function with the NET_RT_LIST
command. Other implementations may wish to use ioctl() for this
purpose. The function prototypes for these three functions, the
constant IF_MAXNAME, and the if_nameindex structure are defined as a
result of including the <sys/socket.h> header.
4.1. Name-to-Index
The first function maps an interface names into its corresponding
index.
unsigned int if_nametoindex(const char *ifname);
If the specified interface does not exist, the return value is 0.
4.2. Index-to-Name
The second function maps an interface index into its corresponding
name.
char *if_indextoname(unsigned int ifindex, char *ifname);
The ifname argument must point to a buffer of at least IF_MAXNAME
bytes into which the interface name corresponding to the specified
index is returned. This pointer is also the return value of the
function. If there is no interface corresponding to the specified
index, NULL is returned and the buffer pointed to by ifname is not
modified.
4.3. Return All Interface Names and Indexes
The final function returns an array of if_nameindex structures, one
structure per interface.
draft-ietf-ipngwg-bsd-api-06.txt [Page 15]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
struct if_nameindex {
unsigned int if_index; /* 1, 2, ... */
char *if_name; /* null terminated name: "le0", ... */
};
struct if_nameindex *if_nameindex(void);
The end of the array of structures is indicated by a structure with
an if_index of 0 and an if_name of NULL. The memory used for this
array of structures along with the interface names pointed to by the
if_name members is obtained using one call to malloc() and can be
returned by calling free() with an argument that is the pointer
returned by if_nameindex().
5. Socket Options
A number of new socket options are defined for IPv6. All of these
new options are at the IPPROTO_IPV6 level. That is, the "level"
parameter in the getsockopt() and setsockopt() calls is IPPROTO_IPV6
when using these options. The constant name prefix IPV6_ is used in
all of the new socket options. This serves to clearly identify these
options as applying to IPv6.
The declaration for IPPROTO_IPV6, the new IPv6 socket options, and
related constants defined in this section are obtained by including
the header <netinet/in.h>.
5.1. Changing Socket Type
Unix allows open sockets to be passed between processes via the
exec() call and other means. It is a relatively common application
practice to pass open sockets across exec() calls. Thus it is
possible for an application using the original API to pass an open
PF_INET socket to an application that is expecting to receive a
PF_INET6 socket. Similarly, it is possible for an application using
the extended API to pass an open PF_INET6 socket to an application
using the original API, which would be equipped only to deal with
PF_INET sockets. Either of these cases could cause problems, because
the application that is passed the open socket might not know how to
decode the address structures returned in subsequent socket
functions.
To remedy this problem, a new setsockopt() option is defined that
allows an application to "convert" a PF_INET6 socket into a PF_INET
socket and vice versa.
draft-ietf-ipngwg-bsd-api-06.txt [Page 16]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
An IPv6 application that is passed an open socket from an unknown
process may use the IPV6_ADDRFORM setsockopt() option to "convert"
the socket to PF_INET6. Once that has been done, the system will
return sockaddr_in6 address structures in subsequent socket
functions.
An IPv6 application that is about to pass an open PF_INET6 socket to
a program that is not be IPv6 capable can "downgrade" the socket to
PF_INET before calling exec(). After that, the system will return
sockaddr_in address structures to the application that was exec()'ed.
Be aware that you cannot downgrade an IPv6 socket to an IPv4 socket
unless all nonwildcard addresses already associated with the IPv6
socket are IPv4-mapped IPv6 addresses.
The IPV6_ADDRFORM option is valid at both the IPPROTO_IP and
IPPROTO_IPV6 levels. The only valid option values are PF_INET6 and
PF_INET. For example, to convert a PF_INET6 socket to PF_INET, a
program would call:
int addrform = PF_INET;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
(char *) &addrform, sizeof(addrform)) == -1)
perror("setsockopt IPV6_ADDRFORM");
An application may use IPV6_ADDRFORM with getsockopt() to learn
whether an open socket is a PF_INET of PF_INET6 socket. For example:
int addrform;
size_t len = sizeof(addrform);
if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
(char *) &addrform, &len) == -1)
perror("getsockopt IPV6_ADDRFORM");
else if (addrform == PF_INET)
printf("This is an IPv4 socket.\n");
else if (addrform == PF_INET6)
printf("This is an IPv6 socket.\n");
else
printf("This system is broken.\n");
5.2. Unicast Hop Limit
A new setsockopt() option controls the hop limit used in outgoing
unicast IPv6 packets. The name of this option is IPV6_UNICAST_HOPS,
and it is used at the IPPROTO_IPV6 layer. The following example
draft-ietf-ipngwg-bsd-api-06.txt [Page 17]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
illustrates how it is used:
int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS");
When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
option value given is used as the hop limit for all subsequent
unicast packets sent via that socket. If the option is not set, the
system selects a default value.
The IPV6_UNICAST_HOPS option may be used with getsockopt() to
determine the hop limit value that the system will use for subsequent
unicast packets sent via that socket. For example:
int hoplimit;
size_t len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, &len) == -1)
perror("getsockopt IPV6_UNICAST_HOPS");
else
printf("Using %d for hop limit.\n", hoplimit);
5.3. Sending and Receiving Multicast Packets
IPv6 applications may send UDP multicast packets by simply specifying
an IPv6 multicast address in the address argument of the sendto()
function.
Three socket options at the IPPROTO_IPV6 layer control some of the
parameters for sending multicast packets. Setting these options is
not required: applications may send multicast packets without using
these options. The setsockopt() options for controlling the sending
of multicast packets are summarized below:
draft-ietf-ipngwg-bsd-api-06.txt [Page 18]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
IPV6_MULTICAST_IF
Set the interface to use for outgoing multicast packets. The
argument is the index of the interface to use.
Argument type: unsigned int
IPV6_MULTICAST_HOPS
Set the hop limit to use for outgoing multicast packets.
(Note a separate option - IPV6_UNICAST_HOPS - is provided to
set the hop limit to use for outgoing unicast packets.)
Argument type: unsigned int
IPV6_MULTICAST_LOOP
Controls whether outgoing multicast packets sent should be
delivered back to the local application. A toggle. If the
option is set to 1, multicast packets are looped back. If it
is set to 0, they are not.
Argument type: unsigned int
The reception of multicast packets is controlled by the two
setsockopt() options summarized below:
IPV6_ADD_MEMBERSHIP
Join a multicast group on a specified local interface. If
the interface index is specified as 0, the kernel chooses the
local interface by looking up the multicast group in the
normal IPv6 routing table and using the resulting interface.
Argument type: struct ipv6_mreq
IPV6_DROP_MEMBERSHIP
Leave a multicast group on a specified interface.
Argument type: struct ipv6_mreq
The argument type of both of these options is the ipv6_mreq
structure, defined as follows:
draft-ietf-ipngwg-bsd-api-06.txt [Page 19]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
unsigned int ipv6mr_interface; /* interface index */
};
Note that to receive multicast datagrams a process must join the
multicast group and bind the UDP port to which datagrams will be
sent. Some processes also bind the multicast group address to the
socket, in addition to the port, to prevent other datagrams destined
to that same port from being delivered to the socket.
6. Library Functions
New library functions are needed to perform a variety of operations
with IPv6 addresses. Functions are needed to lookup IPv6 addresses
in the Domain Name System (DNS). Both forward lookup (hostname-to-
address translation) and reverse lookup (address-to-hostname
translation) need to be supported. Functions are also needed to
convert IPv6 addresses between their binary and textual form.
6.1. Hostname-to-Address Translation
The commonly used function gethostbyname() remains unchanged as does
the hostent structure to which it returns a pointer. Existing
applications that call this function continue to receive only IPv4
addresses that are the result of a query in the DNS for A records.
(We assume the DNS is being used; some environments may be using a
hosts file or some other name resolution system, either of which may
impede renumbering.)
Two new changes are made to support IPv6 addresses. First the
following function is new:
struct hostent *gethostbyname2(const char *name, int af);
The af argument specifies the address family. The default operation
of this function is simple:
- If the af argument is AF_INET, then a query is made for A
records. If successful, IPv4 addresses are returned and the
h_length member of the hostent structure will be 4, else the
function returns a NULL pointer.
- If the af argument is AF_INET6, then a query is made for AAAA
draft-ietf-ipngwg-bsd-api-06.txt [Page 20]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
records. If successful, IPv6 addresses are returned and the
h_length member of the hostent structure will be 16, else the
function returns a NULL pointer.
The second change, that provides additional functionality, is a new
resolver option RES_USE_INET6, which is defined as a result of
including the <resolv.h> header. (This option is provided starting
with the BIND 4.9.4 release.) There are three ways to set this
option.
- The first way is
res_init();
_res.options |= RES_USE_INET6;
and then call either gethostbyname() or gethostbyname2(). This
option then affects only the process that is calling the
resolver.
- The second way to set this option is to set the environment
variable RES_OPTIONS, as in RES_OPTIONS=inet6. This method
affects any processes that see this environment variable.
- The third way is to set this option in the resolver configuration
file (normally /etc/resolv.conf) and the option then affects all
applications on the host. This final method should not be done
until all applications on the host are capable of dealing with
IPv6 addresses.
When the RES_USE_INET6 option is set, two changes occur:
- gethostbyname(host) first calls gethostbyname2(host, AF_INET6)
looking for AAAA records, and if this fails it then calls
gethostbyname2(host, AF_INET) looking for A records.
- gethostbyname2(host, AF_INET) always returns IPv4-mapped IPv6
addresses with the h_length member of the hostent structure set
to 16.
An application must not enable the RES_USE_INET6 option until it is
prepared to deal with 16-byte addresses in the returned hostent
structure.
The following table summarizes the operation of the existing
gethostbyname() function, the new function gethostbyname2(), along
with the new resolver option RES_USE_INET6.
draft-ietf-ipngwg-bsd-api-06.txt [Page 21]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
+------------------+---------------------------------------------------+
| | RES_USE_INET6 option |
| +-------------------------+-------------------------+
| | off | on |
+------------------+-------------------------+-------------------------+
| |Search for A records. |Search for AAAA records. |
| gethostbyname | If found, return IPv4 | If found, return IPv6 |
| (host) | addresses (h_length=4). | addresses (h_length=16).|
| | Else error. | Else search for A |
| | | records. If found, |
| |Provides backward | return IPv4-mapped IPv6 |
| | compatibility with all | addresses (h_length=16).|
| | existing IPv4 appls. | Else error. |
+------------------+-------------------------+-------------------------+
| |Search for A records. |Search for A records. |
| gethostbyname2 | If found, return IPv4 | If found, return |
| (host, AF_INET) | addresses (h_length=4). | IPv4-mapped IPv6 |
| | Else error. | addresses (h_length=16).|
| | | Else error. |
+------------------+-------------------------+-------------------------+
| |Search for AAAA records. |Search for AAAA records. |
| gethostbyname2 | If found, return IPv6 | If found, return IPv6 |
| (host, AF_INET6) | addresses (h_length=16).| addresses (h_length=16).|
| | Else error. | Else error. |
+------------------+-------------------------+-------------------------+
It is expected that when a typical naive application that calls
gethostbyname() today is modified to use IPv6, it simply changes the
program to use IPv6 sockets and then enables the RES_USE_INET6
resolver option before calling gethostbyname(). This application
will then work with either IPv4 or IPv6 peers.
Note that gethostbyname() and gethostbyname2() are not thread-safe,
since both return a pointer to a static hostent structure. But
several vendors have defined a thread-safe gethostbyname_r() function
that requires four additional arguments. We expect these vendors to
also define a gethostbyname2_r() function.
6.2. Address To Hostname Translation
The existing gethostbyaddr() function already requires an address
family argument and can therefore work with IPv6 addresses:
struct hostent *gethostbyaddr(const char *src, int len, int af);
draft-ietf-ipngwg-bsd-api-06.txt [Page 22]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
One possible source of confusion is the handling of IPv4-mapped IPv6
addresses and IPv4-compatible IPv6 addresses. Current thinking
involves the following logic:
- If af is AF_INET6, and if len equals 16, and if the IPv6 address
is an IPv4-mapped IPv6 address or an IPv4-compatible IPv6
address, then skip over the first 12 bytes of the IPv6 address,
set af to AF_INET, and set len to 4.
- If af is AF_INET, then query for a PTR record in the in-addr.arpa
domain.
- If af is AF_INET6, then query for a PTR record in the ip6.int
domain.
- If the function is returning success, and if af equals AF_INET,
and if the RES_USE_INET6 option was set, then the single address
that is returned in the hostent structure (a copy of the first
argument to the function) is returned as an IPv4-mapped IPv6
address and the h_length member is set to 16.
The same caveats regarding a thread-safe version of gethostbyname()
that were made at the end of the previous section apply here as well.
6.3. Protocol-Independent Hostname and Service Name Translation
Hostname-to-address translation is done in a protocol-independent
fashion using the getaddrinfo() function that is taken from the
Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) draft specification [4].
The official specification for this function will be the final POSIX
standard. We are providing this independent description of the
function because POSIX standards are not freely available (as are
IETF documents). Should there be any discrepancies between this
description and the POSIX description, the POSIX description takes
precedence.
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *hostname, const char *servname,
const struct addrinfo *hints,
struct addrinfo **res);
The addrinfo structure is defined as:
draft-ietf-ipngwg-bsd-api-06.txt [Page 23]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
struct addrinfo {
int ai_flags; /* AI_PASSIVE, AI_CANONNAME */
int ai_family; /* PF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
size_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for hostname */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
The return value from the function is 0 upon success or a nonzero
error code. The following names are the nonzero error codes from
getaddrinfo():
EAI_ADDRFAMILY address family for hostname not supported
EAI_AGAIN temporary failure in name resolution
EAI_BADFLAGS invalid value for ai_flags
EAI_FAIL non-recoverable failure in name resolution
EAI_FAMILY ai_family not supported
EAI_MEMORY memory allocation failure
EAI_NODATA no address associated with hostname
EAI_NONAME hostname nor servname provided, or not known
EAI_SERVICE servname not supported for ai_socktype
EAI_SOCKTYPE ai_socktype not supported
EAI_SYSTEM system error returned in errno
The hostname and servname arguments are pointers to null-terminated
strings or NULL. One or both of these two arguments must be a non-
NULL pointer. In the normal client scenario, both the hostname and
servname are specified. In the normal server scenario, only the
servname is specified. A non-NULL hostname string can be either a
host name or a numeric host address string (i.e., a dotted-decimal
IPv4 address or an IPv6 hex address). A non-NULL servname string can
be either a service name or a decimal port number.
The caller can optionally pass an addrinfo structure, pointed to by
the third argument, to provide hints concerning the type of socket
that the caller supports. In this hints structure all members other
than ai_flags, ai_family, ai_socktype, and ai_protocol must be zero
or a NULL pointer. A value of PF_UNSPEC for ai_family means the
caller will accept any protocol family. A value of 0 for ai_socktype
means the caller will accept any socket type. A value of 0 for
ai_protocol means the caller will accept any protocol. For example,
if the caller handles only TCP and not UDP, then the ai_socktype
member of the hints structure should be set to SOCK_STREAM when
getaddrinfo() is called. If the caller handles only IPv4 and not
draft-ietf-ipngwg-bsd-api-06.txt [Page 24]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
IPv6, then the ai_family member of the hints structure should be set
to PF_INET when getaddrinfo() is called. If the third argument to
getaddrinfo() is a NULL pointer, this is the same as if the caller
had filled in an addrinfo structure initialized to zero with
ai_family set to PF_UNSPEC.
Upon successful return a pointer to a linked list of one or more
addrinfo structures is returned through the final argument. The
caller can process each addrinfo structure in this list by following
the ai_next pointer, until a NULL pointer is encountered. In each
returned addrinfo structure the three members ai_family, ai_socktype,
and ai_protocol are the corresponding arguments for a call to the
socket() function. In each addrinfo structure the ai_addr member
points to a filled-in socket address structure whose length is
specified by the ai_addrlen member.
If the AI_PASSIVE bit is set in the ai_flags member of the hints
structure, then the caller plans to use the returned socket address
structure in a call to bind(). In this case, if the hostname
argument is a NULL pointer, then the IP address portion of the socket
address structure will be set to INADDR_ANY for an IPv4 address or
IN6ADDR_ANY_INIT for an IPv6 address. Notice that if the AI_PASSIVE
bit is set and the hostname argument is a NULL pointer then the
caller must also specify a nonzero ai_family, otherwise getaddrinfo()
is unable to allocate and initialize a socket address structure of
the correct type.
If the AI_PASSIVE bit is not set in the ai_flags member of the hints
structure, then the returned socket address structure will be ready
for a call to connect() (for a connection-oriented protocol) or
either connect(), sendto(), or sendmsg() (for a connectionless
protocol). In this case, if the hostname argument is a NULL pointer,
then the IP address portion of the socket address structure will be
set to the loopback address.
If the AI_CANONNAME bit is set in the ai_flags member of the hints
structure, then upon successful return the ai_canonname member of the
first addrinfo structure in the linked list will point to a null-
terminated string containing the canonical name of the specified
hostname.
All of the information returned by getaddrinfo() is dynamically
allocated: the addrinfo structures, and the socket address structures
and canonical host name strings pointed to by the addrinfo
structures. To return this information to the system the function
freeaddrinfo() is called:
draft-ietf-ipngwg-bsd-api-06.txt [Page 25]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
#include <sys/socket.h>
#include <netdb.h>
void freeaddrinfo(struct addrinfo *ai);
The addrinfo structure pointed to by the ai argument is freed, along
with any dynamic storage pointed to by the structure. This operation
is repeated until a NULL ai_next pointer is encountered.
6.4. Socket Address Structure to Hostname and Service Name
The POSIX 1003.1g specification includes no function to perform the
reverse conversion from getaddrinfo(): to look up a hostname and
service name, given the binary address and port. Therefore, we
define the following function:
#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(const struct sockaddr *sa, size_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen);
This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text
strings for both in buffers provided by the caller. The first
argument, sa, points to either a sockaddr_in structure (for IPv4) or
a sockaddr_in6 structure (for IPv6) that holds the IP address and
port number. The salen argument gives the length of the sockaddr_in
or sockaddr_in6 structure. The function returns the hostname
associated with the IP address in the buffer pointed to by the host
argument. The caller provides the size of this buffer via the
hostlen argument. The service name associated with the port number
is returned in the buffer pointed to by serv, and the servlen
argument gives the length of this buffer. The caller specifies not
to return either string by providing a zero value for the hostlen or
servlen arguments. Otherwise, the caller must provide buffers large
enough to hold the fully qualified domain hostname, and the full
service name, including the terminating null character. The function
indicates successful completion by a zero return value; a non-zero
return value indicates failure.
Note that this function does not know the protocol of the socket
address structure. Normally this is not a problem because the same
port is assigned to a given service for both TCP and UDP. But there
exist historical artifacts that violate this rule (e.g., ports 512,
draft-ietf-ipngwg-bsd-api-06.txt [Page 26]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
513, and 514).
6.5. Address Conversion Functions
The two functions inet_addr() and inet_ntoa() convert an IPv4 address
between binary and text form. IPv6 applications need similar
functions. The following two functions convert both IPv6 and IPv4
addresses:
int inet_pton(int af, const char *src, void *dst);
and
const char *inet_ntop(int af, const void *src,
char *dst, size_t size);
The inet_pton() function converts an address in its standard text
presentation form into its numeric binary form. The af argument
specifies the family of the address. Currently the AF_INET and
AF_INET6 address families are supported. The src argument points to
the string being passed in. The dst argument points to a buffer into
which the function stores the numeric address. The address is
returned in network byte order. Inet_pton() returns 1 if the
conversion succeeds, 0 if the input is not a valid IPv4 dotted-
decimal string or a valid IPv6 address string, or -1 with errno set
to EAFNOSUPPORT if the af argument is unknown. The function does not
modify the buffer pointed to by dst if the conversion fails. The
calling application must ensure that the buffer referred to by dst is
large enough to hold the numeric address (e.g., 4 bytes for AF_INET
or 16 bytes for AF_INET6).
If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted-decimal form:
ddd.ddd.ddd.ddd
where ddd is a one to three digit decimal number between 0 and 255.
If the af argument is AF_INET6, then the function accepts a string in
one of the standard IPv6 text forms defined in Section 2.2 of the
addressing architecture specification [2].
The inet_ntop() function converts a numeric address into a text
string suitable for presentation. The af argument specifies the
family of the address. This can be AF_INET or AF_INET6. The src
draft-ietf-ipngwg-bsd-api-06.txt [Page 27]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
argument points to a buffer holding an IPv4 address if the af
argument is AF_INET, or an IPv6 address if the af argument is
AF_INET6. The dst argument points to a buffer where the function
will store the resulting text string. The size argument specifies
the size of this buffer. The application must specify a non-NULL dst
argument. For IPv6 addresses, the buffer must be at least 46-octets.
For IPv4 addresses, the buffer must be at least 16-octets. In order
to allow applications to easily declare buffers of the proper size to
store IPv4 and IPv6 addresses in string form, implementations should
provide the following constants, made available to applications that
include <netinet/in.h>:
#define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46
The inet_ntop() function returns a pointer to the buffer containing
the text string if the conversion succeeds, and NULL otherwise. Upon
failure, errno is set to EAFNOSUPPORT if the af argument is invalid
or ENOSPC if the size of the result buffer is inadequate. The
function does not modify the storage pointed to by dst if the
conversion fails.
Applications obtain the prototype declarations for inet_ntop() and
inet_pton() by including the header <arpa/inet.h>.
6.6. IPv4-Mapped Addresses
The IPv4-mapped IPv6 address format represents IPv4 addresses as IPv6
addresses. Most applications should be able to manipulate IPv6
addresses as opaque 16-octet quantities, without needing to know
whether they represent IPv4 addresses. However, a few applications
may need to determine whether an IPv6 address is an IPv4-mapped
address or not. The following function is provided for those
applications:
int inet6_isipv4mapped(const struct in6_addr *addr);
The "addr" argument to this function points to a buffer holding an
IPv6 address in network byte order. The function returns non-zero if
that address is an IPv4-mapped address, and returns 0 otherwise.
This function could be used by server applications to determine
whether the peer is an IPv4 node or an IPv6 node. After accepting a
TCP connection via accept(), or receiving a UDP packet via
recvfrom(), the application can apply the inet6_isipv4mapped()
function to the returned address.
draft-ietf-ipngwg-bsd-api-06.txt [Page 28]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
Applications obtain the prototype for this function by including the
header <arpa/inet.h>.
7. Security Considerations
IPv6 provides a number of new security mechanisms, many of which need
to be accessible to applications. A companion memo detailing the
extensions to the socket interfaces to support IPv6 security is being
written [3].
8. Change History
Changes from the April 1996 Edition (-05 draft)
- Rewrote Abstract.
- Added Table of Contents.
- New Section 2.2 (Data Types).
- Removed the example from Section 3.4 (Socket Address Structure
for 4.4BSD-Based Systems) implying that the process must set the
sin6_len field. This field need not be set by the process before
passing a socket address structure to the kernel: bind(),
connect(), sendto(), and sendmsg().
- The examples in Section 3.8 (Flow Information) on setting and
fetching the flow label and priority have been expanded, since
the byte ordering and shifting required to set and fetch these
fields can be confusing. It is also explicitly stated that the
two IPV6_FLOWLABEL_xxx constants and the 16 IPV6_PRIORITY_xxx
constants are all network byte ordered.
- Warning placed at the end of Section 3.9 concerning the byte
ordering of the IPv4 INADDR_xxx constants versus the IPv6
IN6ADDR_xxx constants and in6addr_xxx externals.
- Added a new Section 4 (Interface Identification). This provides
functions to map between an interface name and an interface
index.
- In Section 5.1 (Changing Socket Type) the qualifier was added
that you cannot downgrade an IPv6 socket to an IPv4 socket unless
all nonwildcard addresses already associated with the IPv6 socket
are IPv4-mapped IPv6 addresses.
draft-ietf-ipngwg-bsd-api-06.txt [Page 29]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
- In Section 5.3 (Sending and Receiving Multicast Packets) the
method of specifying the local interface was changed from using a
local IPv6 address to using the interface index. This changes
the argument type for IPV6_MULTICAST_IF and the second member of
the ipv6_mreq structure.
- In Section 5.3 (Sending and Receiving Multicast Packets) the
IPV6_ADD_MEMBERSHIP socket option description was corrected. A
note was also added at the end of this section concerning joining
the group versus binding the group address to the socket.
- The old Sections 5.1, 5.2, and 5.3 are gone, and new Sections
6.1, 6.2, 6.3, 6.4, and 6.5 are provided. The new sections
describe the BIND 4.9.4 implementation of the name-to-address
functions (which support IPv6), a POSIX-free description of the
getaddrinfo() function, a description of the new getnameinfo()
function, and the inet_ntop() and inet_pton() functions. The old
Section 5.4 (Embedded IPv4 addresses) is now Section 6.6 (IPv4-
Mapped Addresses).
- Renamed inet6_isipv4addr() to inet6_isipv4mapped() so the name
better describes the function.
- Section 8 (Open Issues) was removed.
Changes from the January 1996 Edition (-04 draft)
- Re-arranged the ipv6_hostent_addr structure, placing the IPv6
address element first.
Changes from the November 1995 Edition (-03 draft)
- Added the symbolic constants IN6ADDR_ANY_INIT and
IN6ADDR_LOOPBACK_INIT for applications to use for
initializations.
- Eliminated restrictions on the value of ipv6addr_any. Systems
may now choose any value, including all-zeros.
- Added a mechanism for returning time to live with the address in
the name-to-address translation functions.
- Added a mechanism for applications to specify the interface in
the setsockopt() options to join and leave a multicast group.
Changes from the July 1995 Edition
- Changed u_long and u_short types in structures to u_int32_t and
draft-ietf-ipngwg-bsd-api-06.txt [Page 30]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
u_int16_t for consistency and clarity.
- Added implementation-provided constants for IPv4 and IPv6 text
address buffer length.
- Defined a set of constants for subfields of sin6_flowid and for
priority values.
- Defined constants for getting and setting the source route flag.
- Define where ansi prototypes for hostname2addr(),
addr2hostname(), addr2ascii(), ascii2addr(), and
ipv6_isipv4addr() reside.
- Clarified the include file requirements. Say that the structure
definitions are defined as a result of including the header
<netinet/in.h>, not that the structures are necessarily defined
there.
- Removed underscore chars from is_ipv4_addr() function name for
BSD compatibility.
- Added inet6_ prefix to is_ipv4_addr() function name to avoid name
space conflicts.
- Changes setsockopt option naming convention to use IPV6_ prefix
instead of IP_ so that there is clearly no ambiguity with IPv4
options. Also, use level IPPROTO_IPV6 for these options.
- Made hostname2addr() and addr2hostname() functions thread-safe.
- Added support for sendmsg() and recvmsg() in source routing
section.
- Changed in_addr6 to in6_addr for consistency.
- Re-structured document into sub-sections.
- Deleted the implementation experience section. It was too wordy.
- Added argument types to multicast socket options.
- Added constant for largest source route array buffer.
- Added the freehostent() function.
- Added receiving interface determination and sending interface
selection options.
draft-ietf-ipngwg-bsd-api-06.txt [Page 31]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
- Added definitions of ipv6addr_any and ipv6addr_loopback.
- Added text making the lookup of IPv4 addresses by hostname2addr()
optional.
Changes from the June 1995 Edition
- Added capability for application to select loose or strict source
routing.
Changes from the March 1995 Edition
- Changed the definition of the ipv6_addr structure to be an array
of sixteen chars instead of four longs. This change is necessary
to support machines that implement the socket interface, but do
not have a 32-bit addressable word. Virtually all machines that
provide the socket interface do support an 8-bit addressable data
type.
- Added a more detailed explanation that the data types defined in
this documented are not intended to be hard and fast
requirements. Systems may use other data types if they wish.
- Added a note flagging the fact that the sockaddr_in6 structure is
not the same size as the sockaddr structure.
- Changed the sin6_flowlabel field to sin6_flowinfo to accommodate
the addition of the priority field to the IPv6 header.
Changes from the October 1994 Edition
- Added variant of sockaddr_in6 for 4.4BSD-based systems (sa_len
compatibility).
- Removed references to SIT transition specification, and added
reference to addressing architecture document, for definition of
IPv4-mapped addresses.
- Added a solution to the problem of the application not providing
enough buffer space to hold a received source route.
- Moved discussion of IPv4 applications interoperating with IPv6
nodes to open issues section.
- Added length parameter to addr2ascii() function to be consistent
with addr2hostname().
- Changed IP_MULTICAST_TTL to IP_MULTICAST_HOPS to match IPv6
draft-ietf-ipngwg-bsd-api-06.txt [Page 32]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
terminology, and added IP_UNICAST_HOPS option to match
IP_MULTICAST_HOPS.
- Removed specification of numeric values for AF_INET6,
IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on
different implementations.
- Added a definition for the in_addr6 IPv6 address data structure.
Added this so that applications could use sizeof(struct in_addr6)
to get the size of an IPv6 address, and so that a structured type
could be used in the is_ipv4_addr().
9. Acknowledgments
Thanks to the many people who made suggestions and provided feedback
to to the numerous revisions of this document, including: Werner
Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson,
Alex Conta, Alan Cox, Steve Deering, Francis Dupont, Robert Elz, Marc
Hasson, Tom Herbert, Christian Huitema, Wan-Yen Hsu, Alan Lloyd,
Charles Lynn, Dan McDonald, Craig Metz, Erik Nordmark, Josh Osborne,
Craig Partridge, Matt Thomas, Dean D. Throop, Glenn Trewitt, Paul
Vixie, David Waitzman, and Carl Williams.
The getaddrinfo() and getnameinfo() functions are taken from an
earlier Internet Draft by Keith Sklower. As noted in that draft,
William Durst, Steven Wise, Michael Karels, and Eric Allman provided
many useful discussions on the subject of protocol-independent name-
to-address translation, and reviewed early versions of Keith
Sklower's original proposal. Eric Allman implemented the first
prototype of getaddrinfo(). The observation that specifying the pair
of name and service would suffice for connecting to a service
independent of protocol details was made by Marshall Rose in a
proposal to X/Open for a "Uniform Network Interface".
Ramesh Govindan made a number of contributions and co-authored an
earlier version of this memo.
10. References
[1] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 1883, December 1995.
[2] R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
RFC 1884, December 1995.
draft-ietf-ipngwg-bsd-api-06.txt [Page 33]
INTERNET-DRAFT IPv6 Socket Interface Extensions November 23, 1996
[3] D. McDonald, "A Simple IP Security API Extension to BSD Sockets",
Internet-Draft, <draft-mcdonald-simple-ipsec-api-00.txt>,
November 1996.
[4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
6.3, November 1995.
[5] W. R. Stevens, M. Thomas, "Advanced Sockets API for IPv6",
Internet-Draft, <draft-stevens-advanced-api-00.txt>, October
1996.
11. Authors' Addresses
Robert E. Gilligan
Freegate Corporation
710 Lakeway Dr. STE 230
Sunnyvale, CA 94086
Phone: +1 408 524 4804
Email: gilligan@freegate.net
Susan Thomson
Bell Communications Research
MRE 2P-343, 445 South Street
Morristown, NJ 07960
Telephone: +1 201 829 4514
Email: set@thumper.bellcore.com
Jim Bound
Digital Equipment Corporation
110 Spitbrook Road ZK3-3/U14
Nashua, NH 03062-2698
Phone: +1 603 881 0400
Email: bound@zk3.dec.com
W. Richard Stevens
1202 E. Paseo del Zorro
Tucson, AZ 85718-2826
Phone: +1 520 297 9416
Email: rstevens@kohala.com
draft-ietf-ipngwg-bsd-api-06.txt [Page 34]
