< draft-ietf-ipngwg-bsd-api-05.txt   draft-ietf-ipngwg-bsd-api-06.txt >
Internet Engineering Task Force R. E. Gilligan (Sun) Internet Engineering Task Force R. E. Gilligan (Freegate)
INTERNET-DRAFT S. Thomson (Bellcore) INTERNET-DRAFT S. Thomson (Bellcore)
J. Bound (Digital) J. Bound (Digital)
April 18, 1996 W. R. Stevens (Consultant)
November 23, 1996
Basic Socket Interface Extensions for IPv6 Basic Socket Interface Extensions for IPv6
<draft-ietf-ipngwg-bsd-api-05.txt> <draft-ietf-ipngwg-bsd-api-06.txt>
Abstract Abstract
In order to implement the version 6 Internet Protocol (IPv6) [1] in The de facto standard application program interface (API) for TCP/IP
an operating system based on Berkeley Unix (4.x BSD), changes must be applications is the "sockets" interface. Although this API was
made to the application program interface (API). TCP/IP applications developed for Unix in the early 1980s it has also been implemented on
written for BSD-based operating systems have in the past enjoyed a a wide variety of non-Unix systems. TCP/IP applications written
high degree of portability because most of the systems derived from using the sockets API have in the past enjoyed a high degree of
BSD provide the same API, known informally as "the socket interface". portability and we would like the same portability with IPv6
We would like the same portability with IPv6. This memo presents a applications. But changes are required to the sockets API to support
basic set of extensions to the BSD socket API to support IPv6. The IPv6 and this memo describes these changes. These include a new
changes include a new data structure to carry IPv6 addresses, new socket address structure to carry IPv6 addresses, new address
address conversion functions, and some new setsockopt() options. The conversion functions, and some new socket options. These extensions
extensions are designed to provide access to IPv6 features, while are designed to provide access to the basic IPv6 features required by
introducing a minimum of change into the system and providing TCP and UDP applications, including multicasting, while introducing a
complete compatibility for existing IPv4 applications. Additional minimum of change into the system and providing complete
extensions for new IPv6 features may be added at a later time. 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 Status of this Memo
This document is an Internet Draft. Internet Drafts are working This document is an Internet Draft. Internet Drafts are working
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Internet Drafts are draft documents valid for a maximum of six Internet Drafts are draft documents valid for a maximum of six
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Distribution of this memo is unlimited. Distribution of this memo is unlimited.
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
1. Introduction 1. Introduction
While IPv4 addresses are 32-bits long, IPv6 nodes are identified by
While IPv4 addresses are 32 bits long, IPv6 nodes are identified by
128-bit addresses. The socket interface make the size of an IP 128-bit addresses. The socket interface make the size of an IP
address quite visible to an application; virtually all TCP/IP address quite visible to an application; virtually all TCP/IP
applications for BSD-based systems have knowledge of the size of an applications for BSD-based systems have knowledge of the size of an
IP address. Those parts of the API that expose the addresses need to IP address. Those parts of the API that expose the addresses must be
be extended to accommodate the larger IPv6 address size. IPv6 also changed to accommodate the larger IPv6 address size. IPv6 also
introduces new features, some of which must be made visible to introduces new features (e.g., flow label and priority), some of
applications via the API. This paper defines a set of extensions to which must be made visible to applications via the API. This memo
the socket interface to support the larger address size and new defines a set of extensions to the socket interface to support the
features of IPv6. larger address size and new features of IPv6.
This specification is preliminary. These API extensions are expected
to evolve as we gain more implementation experience.
2. Design Considerations 2. Design Considerations
There are a number of important considerations in designing changes There are a number of important considerations in designing changes
to this well-worn API: to this well-worn API:
- The extended API should provide both source and binary - The API changes should provide both source and binary
compatibility for programs written to the original API. That is, compatibility for programs written to the original API. That is,
existing program binaries should continue to operate when run on existing program binaries should continue to operate when run on
a system supporting the new API. In addition, existing a system supporting the new API. In addition, existing
applications that are re-compiled and run on a system supporting applications that are re-compiled and run on a system supporting
the new API should continue to operate. Simply put, the API the new API should continue to operate. Simply put, the API
changes for IPv6 should not break existing programs. changes for IPv6 should not break existing programs.
- The changes to the API should be as small as possible in order to - The changes to the API should be as small as possible in order to
simplify the task of converting existing IPv4 applications to simplify the task of converting existing IPv4 applications to
IPv6. IPv6.
- Where possible, applications should be able to use the extended - Where possible, applications should be able to use this API to
API to interoperate with both IPv6 and IPv4 hosts. Applications interoperate with both IPv6 and IPv4 hosts. Applications should
should not need to know which type of host they are communicating not need to know which type of host they are communicating with.
with.
- IPv6 addresses carried in data structures should be 64-bit - IPv6 addresses carried in data structures should be 64-bit
aligned. This is necessary in order to obtain optimum aligned. This is necessary in order to obtain optimum
performance on 64-bit machine architectures. performance on 64-bit machine architectures.
Because of the importance of providing IPv4 compatibility in the API, Because of the importance of providing IPv4 compatibility in the API,
these extensions are explicitly designed to operate on machines that these extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this provide complete support for both IPv4 and IPv6. A subset of this
API could probably be designed for operation on systems that support API could probably be designed for operation on systems that support
only IPv6. However, this is not addressed in this document. only IPv6. However, this is not addressed in this memo.
2.1. What Needs to be Changed 2.1. What Needs to be Changed
The socket interface API consists of a few distinct components: The socket interface API consists of a few distinct components:
- Core socket functions. - Core socket functions.
- Address data structures. - Address data structures.
- Name-to-address translation functions. - Name-to-address translation functions.
- Address conversion functions. - Address conversion functions.
The core socket functions -- those functions that deal with such The core socket functions -- those functions that deal with such
things as setting up and tearing down TCP connections, and sending things as setting up and tearing down TCP connections, and sending
and receiving UDP packets -- were designed to be transport and receiving UDP packets -- were designed to be transport
independent. Where protocol addresses are passed as function independent. Where protocol addresses are passed as function
arguments, they are carried via opaque pointers. A protocol specific arguments, they are carried via opaque pointers. A protocol-specific
address data structure is defined for each protocol that the socket address data structure is defined for each protocol that the socket
functions support. Applications must cast these protocol specific functions support. Applications must cast pointers to these
address structures into the generic "sockaddr" data type when using protocol-specific address structures into pointers to the generic
the socket functions. These functions need not change for IPv6, but "sockaddr" address structure when using the socket functions. These
a new IPv6 specific address data structure is needed. 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 The "sockaddr_in" structure is the protocol-specific data structure
for IPv4. This data structure actually includes 8-octets of unused 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 space, and it is tempting to try to use this space to adapt the
sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in
structure is not large enough to hold the 16-octet IPv6 address as structure is not large enough to hold the 16-octet IPv6 address as
well as the other information (2-octet address family and 2-octet well as the other information (address family and port number) that
port number) that is needed. So a new address data structure must be is needed. So a new address data structure must be defined for IPv6.
defined for IPv6.
The name-to-address translation functions in the socket interface are The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr(). Gethostbyname() does not gethostbyname() and gethostbyaddr(). These must be modified to
provide enough flexibility to accommodate protocols other than IPv4. support IPv6 and the semantics defined must provide 100% backward
POSIX, in its 1003.g draft specification, has proposed a new hostname compatibility for all existing IPv4 applications, along with IPv6
to address translation function which is protocol independent. This support for new applications. Additionally, the POSIX 1003.g draft
function can be used with IPv6, so no new function is defined here. [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() -- The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form. These convert IPv4 addresses between binary and printable form. These
functions are quite specific to 32-bit IPv4 addresses. We have functions are quite specific to 32-bit IPv4 addresses. We have
designed two analogous functions which convert both IPv4 and IPv6 designed two analogous functions that convert both IPv4 and IPv6
addresses, and carry an address type parameter so that they can be addresses, and carry an address type parameter so that they can be
extended to other protocol families as well. extended to other protocol families as well.
Finally, a few miscellaneous features are needed to support IPv6. A Finally, a few miscellaneous features are needed to support IPv6.
new interface is needed in order to support the IPv6 flow label and New interfaces are needed to support the IPv6 flow label, priority,
priority header fields. New interfaces are needed in order to and hop limit header fields. New socket options are needed to
receive IPv6 multicast packets and control the sending of multicast control the sending and receiving of IPv6 multicast packets.
packets.
The socket interface may be further extended in the future to provide The socket interface may be enhanced in the future to provide access
access to other IPv6 features. These extensions will be made in to other IPv6 features. These extensions are described in [5].
separate documents.
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 3. Socket Interface
This section specifies the socket interface changes for IPv6. This section specifies the socket interface changes for IPv6.
The data types of the structure elements given in the following 3.1. IPv6 Address Family and Protocol Family
section are intended to be examples, not absolute requirements.
System implementations may use other types if they are appropriate.
In some cases, such as when a field of a data structure holds a
protocol value, the structure field must be of some minimum size.
These size requirements are noted in the text. For example, since
the UDP and TCP port values are 16-bit quantities, the sin6_port
field must be at least a 16-bit data types. The sin6_port field is
specified as a u_int16m_t type, but an implementation may use any
data type that is at least 16-bits long.
3.1. New Address Family
A new address family macro, named AF_INET6, is defined in A new address family name, AF_INET6, is defined in <sys/socket.h>.
<sys/socket.h>. The AF_INET6 definition is used to distinguish The AF_INET6 definition distinguishes between the original
between the original sockaddr_in address data structure, and the new sockaddr_in address data structure, and the new sockaddr_in6 data
sockaddr_in6 data structure. structure.
A new protocol family macro, named PF_INET6, is defined in A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
<sys/socket.h>. Like most of the other protocol family macros, this Like most of the other protocol family names, this will usually be
will usually be defined to have the same value as the corresponding defined to have the same value as the corresponding address family
address family macro: name:
#define PF_INET6 AF_INET6 #define PF_INET6 AF_INET6
The PF_INET6 is used in the first argument to the socket() function The PF_INET6 is used in the first argument to the socket() function
to indicate that an IPv6 socket is being created. to indicate that an IPv6 socket is being created.
3.2. IPv6 Address Data Structure 3.2. IPv6 Address Structure
A new data structure to hold a single IPv6 address is defined as A new data structure to hold a single IPv6 address is defined as
follows: follows:
struct in6_addr { struct in6_addr {
u_char s6_addr[16]; /* IPv6 address */ u_char s6_addr[16]; /* IPv6 address */
} }
This data structure contains an array of sixteen 8-bit elements, This data structure contains an array of sixteen 8-bit elements,
which make up one 128-bit IPv6 address. The IPv6 address is stored which make up one 128-bit IPv6 address. The IPv6 address is stored
in network byte order. in network byte order.
Applications obtain the declaration for this structure by including Applications obtain the declaration for this structure by including
the system header file <netinet/in.h>. the header <netinet/in.h>.
3.3. Socket Address Structure for 4.3 BSD-Based Systems 3.3. Socket Address Structure for 4.3BSD-Based Systems
In the socket interface, a different protocol-specific data structure In the socket interface, a different protocol-specific data structure
is defined to carry the addresses for each of the protocol suite. is defined to carry the addresses for each protocol suite. Each
Each protocol-specific data structure is designed so it can be cast protocol-specific data structure is designed so it can be cast into a
into a protocol-independent data structure -- the "sockaddr" protocol-independent data structure -- the "sockaddr" structure.
structure. Each has a "family" field which overlays the "sa_family" Each has a "family" field that overlays the "sa_family" of the
of the sockaddr data structure. This field can be used to identify sockaddr data structure. This field identifies the type of the data
the type of the data structure. structure.
The sockaddr_in structure is the protocol-specific address data The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between structure for IPv4. It is used to pass addresses between
applications and the system in the socket functions. The following applications and the system in the socket functions. The following
structure is defined to carry IPv6 addresses: structure is defined to carry IPv6 addresses:
struct sockaddr_in6 { struct sockaddr_in6 {
u_int16m_t sin6_family; /* AF_INET6 */ u_int16m_t sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* Transport layer port # */ u_int16m_t sin6_port; /* transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */ u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */ struct in6_addr sin6_addr; /* IPv6 address */
}; };
This structure is designed to be compatible with the sockaddr data This structure is designed to be compatible with the sockaddr data
structure used in the 4.3 BSD release. structure used in the 4.3BSD release.
The sin6_family field is used to identify this as a sockaddr_in6 The sin6_family field identifies this as a sockaddr_in6 structure.
structure. This field is designed to overlay the sa_family field This field overlays the sa_family field when the buffer is cast to a
when the buffer is cast to a sockaddr data structure. The value of sockaddr data structure. The value of this field must be AF_INET6.
this field must be AF_INET6.
The sin6_port field is used to store the 16-bit UDP or TCP port The sin6_port field contains the 16-bit UDP or TCP port number. This
number. This field is used in the same way as the sin_port field of field is used in the same way as the sin_port field of the
the sockaddr_in structure. The port number is stored in network byte sockaddr_in structure. The port number is stored in network byte
order. order.
The sin6_flowinfo field is a 32-bit field that is used to store two The sin6_flowinfo field is a 32-bit field that contains two pieces of
pieces of information: the 24-bit IPv6 flow label and the 4-bit information: the 24-bit IPv6 flow label and the 4-bit priority field.
priority field. The IPv6 flow label is represented as the low-order The IPv6 flow label is represented as the low-order 24 bits of the
24-bits of the 32-bit field. The priority is represented in the next 32-bit field. The priority is represented in the next 4 bits above
4-bits above this. The high-order 4 bits of this field are reserved. this. The high-order 4 bits of this field are reserved. The
The sin6_flowinfo field is stored in network byte order. The use of sin6_flowinfo field is stored in network byte order. The use of the
the flow label and priority fields are explained in sec 4.9. flow label and priority fields are explained in Section 3.8.
The sin6_addr field is a single in6_addr structure (defined in the The sin6_addr field is a single in6_addr structure (defined in the
previous section). This field holds one 128-bit IPv6 address. The previous section). This field holds one 128-bit IPv6 address. The
address is stored in network byte order. address is stored in network byte order.
The ordering of elements in this structure is specifically designed The ordering of elements in this structure is specifically designed
so that the sin6_addr field will be aligned on a 64-bit boundary. so that the sin6_addr field will be aligned on a 64-bit boundary.
This is done for optimum performance on 64-bit architectures. This is done for optimum performance on 64-bit architectures.
Applications obtain the declaration of the sockaddr_in6 structure by Applications obtain the declaration of the sockaddr_in6 structure by
including the system header file <netinet/in.h>. including the header <netinet/in.h>.
3.4. Socket Address Structure for 4.4 BSD-Based Systems 3.4. Socket Address Structure for 4.4BSD-Based Systems
The 4.4 BSD release includes a small, but incompatible change to the The 4.4BSD release includes a small, but incompatible change to the
socket interface. The "sa_family" field of the sockaddr data 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 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 space saved used to hold a length field, named "sa_len". The
sockaddr_in6 data structure given in the previous section can not be sockaddr_in6 data structure given in the previous section cannot be
correctly cast into the newer sockaddr data structure. For this correctly cast into the newer sockaddr data structure. For this
reason, following alternative IPv6 address data structure is provided reason, the following alternative IPv6 address data structure is
to be used on systems based on 4.4 BSD: provided to be used on systems based on 4.4BSD:
#define SIN6_LEN #define SIN6_LEN
struct sockaddr_in6 { struct sockaddr_in6 {
u_char sin6_len; /* length of this struct */ u_char sin6_len; /* length of this struct */
u_char sin6_family; /* AF_INET6 */ u_char sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* Transport layer port # */ u_int16m_t sin6_port; /* Transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */ u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */ struct in6_addr sin6_addr; /* IPv6 address */
}; };
The only differences between this data structure and the 4.3 BSD The only differences between this data structure and the 4.3BSD
variant are the inclusion of the length field, and the change of the 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 family field to a 8-bit data type. The definitions of all the other
fields are identical to the 4.3 BSD variant defined in the previous fields are identical to the structure defined in the previous
section. section.
Systems that provide this version of the sockaddr_in6 data structure Systems that provide this version of the sockaddr_in6 data structure
must also declare the SIN6_LEN as a result of including the must also declare SIN6_LEN as a result of including the
<netinet/in.h> header file. This macro allows applications to <netinet/in.h> header. This macro allows applications to determine
determine whether they are being built on a system that supports the whether they are being built on a system that supports the 4.3BSD or
4.3 BSD or 4.4 BSD variants of the data structure. Applications can 4.4BSD variants of the data structure.
be written to run on both systems by simply making their assignments
and use of the sin6_len field conditional on the SIN6_LEN field. For
example, to fill in an IPv6 address structure in an application, one
might write:
struct sockaddr_in6 sin6;
bzero((char *) &sin6, sizeof(struct sockaddr_in6));
#ifdef SIN6_LEN
sin6.sin6_len = sizeof(struct sockaddr_in6);
#endif
sin6.sin6_family = AF_INET6;
sin6.sin6_port = htons(23);
Note that the size of the sockaddr_in6 structure is larger than the Note that the size of the sockaddr_in6 structure is larger than the
size of the sockaddr structure. Applications that use the size of the sockaddr structure. Applications that use the
sockaddr_in6 structure need to be aware that they can not use sockaddr_in6 structure need to be aware that they cannot use
sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6 sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6
structure. They should use sizeof(sockaddr_in6) instead. structure. They should use sizeof(sockaddr_in6) instead.
3.5. The Socket Functions 3.5. The Socket Functions
Applications use the socket() function to create a socket descriptor Applications call the socket() function to create a socket descriptor
that represents a communication endpoint. The arguments to the that represents a communication endpoint. The arguments to the
socket() function tell the system which protocol to use, and what socket() function tell the system which protocol to use, and what
format address structure will be used in subsequent functions. For format address structure will be used in subsequent functions. For
example, to create an IPv4/TCP socket, applications make the call: example, to create an IPv4/TCP socket, applications make the call:
s = socket (PF_INET, SOCK_STREAM, 0); s = socket(PF_INET, SOCK_STREAM, 0);
To create an IPv4/UDP socket, applications make the call: To create an IPv4/UDP socket, applications make the call:
s = socket (PF_INET, SOCK_DGRAM, 0); s = socket(PF_INET, SOCK_DGRAM, 0);
Applications may create IPv6/TCP and IPv6/UDP sockets by simply using 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 the constant PF_INET6 instead of PF_INET in the first argument. For
example, to create an IPv6/TCP socket, applications make the call: example, to create an IPv6/TCP socket, applications make the call:
s = socket (PF_INET6, SOCK_STREAM, 0); s = socket(PF_INET6, SOCK_STREAM, 0);
To create an IPv6/UDP socket, applications make the call: To create an IPv6/UDP socket, applications make the call:
s = socket (PF_INET6, SOCK_DGRAM, 0); s = socket(PF_INET6, SOCK_DGRAM, 0);
Once the application has created a PF_INET6 socket, it must use the Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the sockaddr_in6 address structure when passing addresses in to the
system. The functions which the application uses to pass addresses system. The functions that the application uses to pass addresses
into the system are: into the system are:
bind() bind()
connect() connect()
sendmsg() sendmsg()
sendto() sendto()
The system will use the sockaddr_in6 address structure to return The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets. The addresses to applications that are using PF_INET6 sockets. The
functions that return an address from the system to an application functions that return an address from the system to an application
skipping to change at page 8, line 31 skipping to change at page 9, line 22
No changes to the syntax of the socket functions are needed to No changes to the syntax of the socket functions are needed to
support IPv6, since the all of the "address carrying" functions use support IPv6, since the all of the "address carrying" functions use
an opaque address pointer, and carry an address length as a function an opaque address pointer, and carry an address length as a function
argument. argument.
3.6. Compatibility with IPv4 Applications 3.6. Compatibility with IPv4 Applications
In order to support the large base of applications using the original In order to support the large base of applications using the original
API, system implementations must provide complete source and binary API, system implementations must provide complete source and binary
compatibility with the original API. This means that systems must compatibility with the original API. This means that systems must
continue to support PF_INET sockets and the sockaddr_in addresses continue to support PF_INET sockets and the sockaddr_in address
structure. Applications must be able to create IPv4/TCP and IPv4/UDP structure. Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the PF_INET constant in the socket() function, as sockets using the PF_INET constant in the socket() function, as
described in the previous section. Applications should be able to described in the previous section. Applications should be able to
hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
sockets simultaneously within the same process. sockets simultaneously within the same process.
Applications using the original API should continue to operate as Applications using the original API should continue to operate as
they did on systems supporting only IPv4. That is, they should they did on systems supporting only IPv4. That is, they should
continue to interoperate with IPv4 nodes. It is not clear, though, continue to interoperate with IPv4 nodes.
how, or even if, those IPv4 applications should interoperate with
IPv6 nodes. The open issues section (section 9) discusses some of
the alternatives.
3.7. Compatibility with IPv4 Nodes 3.7. Compatibility with IPv4 Nodes
The API also provides a different type of compatibility: the ability The API also provides a different type of compatibility: the ability
for applications using the extended API to interoperate with IPv4 for IPv6 applications to interoperate with IPv4 applications. This
nodes. This feature uses the IPv4-mapped IPv6 address format defined feature uses the IPv4-mapped IPv6 address format defined in the IPv6
in the IPv6 addressing architecture specification [2]. This address addressing architecture specification [2]. This address format
format allows the IPv4 address of an IPv4 node to be represented as allows the IPv4 address of an IPv4 node to be represented as an IPv6
an IPv6 address. The IPv4 address is encoded into the low-order 32- address. The IPv4 address is encoded into the low-order 32 bits of
bits of the IPv6 address, and the high-order 96-bits hold the fixed the IPv6 address, and the high-order 96 bits hold the fixed prefix
prefix 0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows: 0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows:
::FFFF:<IPv4-address> ::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).
Applications may use PF_INET6 sockets to open TCP connections to IPv4 Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and destination's IPv4 address as an IPv4-mapped IPv6 address, and
passing that address, within a sockaddr_in6 structure, in the passing that address, within a sockaddr_in6 structure, in the
connect() or sendto() call. When applications use PF_INET6 sockets connect() or sendto() call. When applications use PF_INET6 sockets
to accept TCP connections from IPv4 nodes, or receive UDP packets to accept TCP connections from IPv4 nodes, or receive UDP packets
from IPv4 nodes, the system returns the peer's address to the from IPv4 nodes, the system returns the peer's address to the
application in the accept(), recvfrom(), or getpeername() call using application in the accept(), recvfrom(), or getpeername() call using
a sockaddr_in6 structure encoded this way. a sockaddr_in6 structure encoded this way.
Few applications will likely need to know which type of node they are Few applications will likely need to know which type of node they are
interoperating with. However, for those applications that do need to interoperating with. However, for those applications that do need to
know, the inet6_isipv4addr() function, defined in section 6.3, is know, the inet6_isipv4mapped() function, defined in Section 6.6, is
provided. provided.
3.8. Flow Information 3.8. Flow Information
The IPv6 header has a 24-bit field to hold a "flow label", and a 4- The IPv6 header has a 24-bit field to hold a "flow label", and a 4-
bit field to hold a "priority" value. Applications have control over bit field to hold a "priority" value. Applications must have control
what values for these fields are used in packets that they originate, over what values for these fields are used in packets that they
and have access to the field values of packets that they receive. 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 The sin6_flowinfo field of the sockaddr_in6 structure encodes two
pieces of information: IPv6 flow label and IPv6 priority. pieces of information: IPv6 flow label and IPv6 priority.
Applications use this field to set the flow label and priority in Applications use this field to set the flow label and priority in
IPv6 headers of packets they generate, and to retrieve the flow label IPv6 headers of packets they generate, and to retrieve the flow label
and priority from the packets they receive. The header fields of an and priority from the packets they receive. The header fields of an
actively opened TCP connection are set by assigning in the actively opened TCP connection are set by assigning in the
sin6_flowinfo field of the destination address sockaddr_in6 structure sin6_flowinfo field of the destination address sockaddr_in6 structure
passed in the connect() function. The same technique can be used passed in the connect() function. The same technique can be used
with the sockaddr_in6 structure passed in to the sendto() or with the sockaddr_in6 structure passed to the sendto() or sendmsg()
sendmsg() function to set the flow label and priority fields of UDP function to set the flow label and priority fields of UDP packets.
packets. Similarly, the flow label and priority values of received Similarly, the flow label and priority values of received UDP packets
UDP packets and accepted TCP connections are reflected in the and accepted TCP connections are reflected in the sin6_flowinfo field
sin6_flowinfo field of the sockaddr_in6 structure returned to the of the sockaddr_in6 structure returned to the application by the
application by the recvfrom(), recvmsg(), and accept() functions. recvfrom(), recvmsg(), and accept() functions. An application may
And an application may specify the flow label and priority to use in specify the flow label and priority to use in transmitted packets of
transmitted packets of a passively accepted TCP connection, by a passively accepted TCP connection, by setting the sin6_flowinfo
setting the sin6_flowinfo field of the address passed in the bind() field of the address passed to the bind() function.
function.
Implementations provide two bitmask constant declarations to help Implementations provide two bitmask constant declarations to help
applications select out the flow label and priority fields. These applications select out the flow label and priority fields. These
constants are: constants are:
IPV6_FLOWINFO_FLOWLABEL IPV6_FLOWINFO_FLOWLABEL
IPV6_FLOWINFO_PRIORITY IPV6_FLOWINFO_PRIORITY
These constants can be applied to the sin6_flowinfo field of These constants can be applied to the sin6_flowinfo field of
addresses returned to the application, for example: addresses returned to the application, for example:
struct sockaddr_in6 sin6; 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); recvfrom(s, buf, buflen, flags, (struct sockaddr *) &sin6, &fromlen);
. . . . . .
received_flowlabel = sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL; recv_flow = ntohl(sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL);
received_priority = sin6.sin6_flowinfo & IPV6_FLOWINFO_PRIORITY; 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 On the sending side, applications are responsible for selecting the
flow label value. The system provides constant declarations for the flow label value and specifying a priority. The headers provide
IPv6 priority values defined in the IPv6 specification [1]. These constant declarations for the 16 IPv6 priority values defined in the
constants are: IPv6 specification [1]. These constants are:
IPV6_PRIORITY_UNCHARACTERIZED IPV6_PRIORITY_UNCHARACTERIZED
IPV6_PRIORITY_FILLER IPV6_PRIORITY_FILLER
IPV6_PRIORITY_UNATTENDED IPV6_PRIORITY_UNATTENDED
IPV6_PRIORITY_RESERVED1 IPV6_PRIORITY_RESERVED1
IPV6_PRIORITY_BULK IPV6_PRIORITY_BULK
IPV6_PRIORITY_RESERVED2 IPV6_PRIORITY_RESERVED2
IPV6_PRIORITY_INTERACTIVE IPV6_PRIORITY_INTERACTIVE
IPV6_PRIORITY_CONTROL IPV6_PRIORITY_CONTROL
IPV6_PRIORITY_8 IPV6_PRIORITY_8
IPV6_PRIORITY_9 IPV6_PRIORITY_9
IPV6_PRIORITY_10 IPV6_PRIORITY_10
IPV6_PRIORITY_11 IPV6_PRIORITY_11
IPV6_PRIORITY_12 IPV6_PRIORITY_12
IPV6_PRIORITY_13 IPV6_PRIORITY_13
IPV6_PRIORITY_14 IPV6_PRIORITY_14
IPV6_PRIORITY_15 IPV6_PRIORITY_15
Applications can use these constants along with the flow label they Most applications will use these constants (e.g.,
selected to assign the sin6_flowinfo field, for example: IPV6_PRIORITY_INTERACTIVE can be built into Telnet clients and
servers). Since these constants are defined in network byte order an
example is:
struct sockaddr_in6 sin6; 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;
. . . . . .
send_flowlabel = . . . ; 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;
. . . . . .
sin6.sin6_flowinfo = IPV6_PRIORITY_UNATTENDED | sendto( ... )
(IPV6_FLOWINFO_FLOWLABEL & send_flowlabel);
The macro declarations for these constants are obtained by including The declarations for these constants are obtained by including the
the header file <netinet/in.h>. header <netinet/in.h>.
3.9. Binding to System-Selected Address 3.9. IPv6 Wildcard Address
While the bind() function allows applications to select the source IP While the bind() function allows applications to select the source IP
address of UDP packets and TCP connections, applications often wish address of UDP packets and TCP connections, applications often want
to let the system select the source address for them. In IPv4, this the system select the source address for them. With IPv4, one
is done by specifying the IPv4 address represented by the symbolic specifies the address as the symbolic constant INADDR_ANY (called the
constant INADDR_ANY in the bind() call, or by simply by skipping the "wildcard" address) in the bind() call, or simply omits the bind()
bind() entirely. entirely.
Since the IPv6 address type is a structure (struct in6_addr), a Since the IPv6 address type is a structure (struct in6_addr), a
symbolic constant can be used to initialize an IPv6 address variable, symbolic constant can be used to initialize an IPv6 address variable,
but can not be used in an assignment. Therefore systems provide the but cannot be used in an assignment. Therefore systems provide the
IPv6 address value that can be used to instruct the system to select IPv6 wildcard address in two forms.
the source IPv6 address in two forms.
The first version is a global variable named "in6addr_any" which is The first version is a global variable named "in6addr_any" that is an
an in6_addr type structure. The extern declaration for this variable in6_addr structure. The extern declaration for this variable is:
is:
extern const struct in6_addr in6addr_any; extern const struct in6_addr in6addr_any;
Applications use in6addr_any similarly to the way they use INADDR_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 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 the system select the source address, an application could use the
following code: following code:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_family = AF_INET6; sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0; sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23); sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_any; sin6.sin6_addr = in6addr_any; /* structure assignment */
. . . . . .
if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1) if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . . . . .
The other version is a symbolic constant named IN6ADDR_ANY_INIT. The other version is a symbolic constant named IN6ADDR_ANY_INIT.
This constant can be used to initialize an in6_addr structure: This constant can be used to initialize an in6_addr structure:
struct in6_addr anyaddr = IN6ADDR_ANY_INIT; struct in6_addr anyaddr = IN6ADDR_ANY_INIT;
Note that this constant can be used ONLY at declaration type. It can Note that this constant can be used ONLY at declaration type. It can
not be used assign a previously declared in6_addr structure. For not be used to assign a previously declared in6_addr structure. For
example, the following code will not work: example, the following code will not work:
/* This is the WRONG way to assign an unspecified address */ /* This is the WRONG way to assign an unspecified address */
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_addr = IN6ADDR_ANY_INIT; /* Will NOT compile */ sin6.sin6_addr = IN6ADDR_ANY_INIT; /* Will NOT compile */
The extern declaration for in6addr_any and the macro declaration for The extern declaration for in6addr_any and the declaration for
IN6ADDR_ANY_INIT are obtained by including <netinet/in.h>. IN6ADDR_ANY_INIT are obtained by including the header <netinet/in.h>.
3.10. Communicating with Local Services 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 Applications may need to send UDP packets to, or originate TCP
connections to, services residing on the local node. In IPv4, they connections to, services residing on the local node. In IPv4, they
can do this by using the constant IPv4 address INADDR_LOOPBACK in can do this by using the constant IPv4 address INADDR_LOOPBACK in
their connect(), sendto(), or sendmsg() call. their connect(), sendto(), or sendmsg() call.
IPv6 also provides a loopback address which can be used to contact IPv6 also provides a loopback address to contact local TCP and UDP
local TCP and UDP services. Like the unspecified address, the IPv6 services. Like the unspecified address, the IPv6 loopback address is
loopback address is provided in two forms -- a global variable and a provided in two forms -- a global variable and a symbolic constant.
symbolic constant.
The global variable is an in6_addr type structure named The global variable is an in6_addr structure named
"in6addr_loopback." The extern declaration for this variable is: "in6addr_loopback." The extern declaration for this variable is:
extern const struct in6_addr in6addr_loopback; extern const struct in6_addr in6addr_loopback;
Applications use in6addr_loopback as they would use INADDR_LOOPBACK Applications use in6addr_loopback as they would use INADDR_LOOPBACK
in IPv4 applications. For example, to open a TCP connection to the in IPv4 applications (but beware of the byte ordering difference
local telnet server, an application could use the following code: 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; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_family = AF_INET6; sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0; sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23); sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_loopback; sin6.sin6_addr = in6addr_loopback; /* structure assignment */
. . . . . .
if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1) if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . . . . .
The symbolic constant is named IN6ADDR_LOOPBACK_INIT. It can be used The symbolic constant is named IN6ADDR_LOOPBACK_INIT. It can be used
at declaration time ONLY; for example: at declaration time ONLY; for example:
struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT; struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
Like IN6ADDR_ANY_INIT, this constant can not be used in an assignment Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
to a previously declared IPv6 address variable. to a previously declared IPv6 address variable.
The extern declaration for in6addr_loopback and the macro declaration The extern declaration for in6addr_loopback and the declaration for
for IN6ADDR_LOOPBACK_INIT are obtained by including <netinet/in.h>. IN6ADDR_LOOPBACK_INIT are obtained by including the header
<netinet/in.h>.
4. Socket Options 4. Interface Identification
This API uses an interface index (a small positive integer) to
identify the local interface on which a multicast group is joined
(Section 5.3). Additionally, the advanced API [5] uses these same
interface indexes to identify the interface on which a datagram is
received, or to specify the interface on which a datagram is to be
sent.
Interfaces are normally known by names such as "le0", "sl1", "ppp2",
and the like. On Berkeley-derived implementations, when an interface
is made known to the system, the kernel assigns a unique positive
integer value (called the interface index) to that interface. These
are small positive integers that start at 1. (Note that 0 is never
used for an interface index.) There may be gaps so that there is no
current interface for a particular positive interface index.
This API defines two functions that map between an interface name and
index, 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.
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 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" new options are at the IPPROTO_IPV6 level. That is, the "level"
parameter in the getsockopt() and setsockopt() call is IPPROTO_IPV6 parameter in the getsockopt() and setsockopt() calls is IPPROTO_IPV6
when using these options. The constant name prefix IPV6_ is used in when using these options. The constant name prefix IPV6_ is used in
all of the new socket options. This serves to clearly identify these all of the new socket options. This serves to clearly identify these
options as applying to IPv6. options as applying to IPv6.
The macro declaration for IPPROTO_IPV6, the new IPv6 socket options, The declaration for IPPROTO_IPV6, the new IPv6 socket options, and
and related constants defined in this section are obtained by related constants defined in this section are obtained by including
including the header file <netinet/in.h> the header <netinet/in.h>.
4.1 Changing Socket Type 5.1. Changing Socket Type
Unix allows open sockets to be passed between processes via the Unix allows open sockets to be passed between processes via the
exec() call and other means. It is a relatively common application exec() call and other means. It is a relatively common application
practice to pass open sockets across exec() calls. Thus it is practice to pass open sockets across exec() calls. Thus it is
possible for an application using the original API to pass an open 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_INET socket to an application that is expecting to receive a
PF_INET6 socket. Similarly, it is possible for an application using PF_INET6 socket. Similarly, it is possible for an application using
the extended API to pass an open PF_INET6 socket to an application 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 using the original API, which would be equipped only to deal with
PF_INET sockets. Either of these cases could cause problems, because PF_INET sockets. Either of these cases could cause problems, because
the application which is passed the open socket might not know how to the application that is passed the open socket might not know how to
decode the address structures returned in subsequent socket decode the address structures returned in subsequent socket
functions. functions.
To remedy this problem, a new setsockopt() option is defined that To remedy this problem, a new setsockopt() option is defined that
allows an application to "transform" a PF_INET6 socket into a PF_INET allows an application to "convert" a PF_INET6 socket into a PF_INET
socket and vice-versa. socket and vice versa.
An IPv6 application that is passed an open socket from an unknown An IPv6 application that is passed an open socket from an unknown
process may use the IPV6_ADDRFORM setsockopt() option to "convert" process may use the IPV6_ADDRFORM setsockopt() option to "convert"
the socket to PF_INET6. Once that has been done, the system will the socket to PF_INET6. Once that has been done, the system will
return sockaddr_in6 address structures in subsequent socket return sockaddr_in6 address structures in subsequent socket
functions. Similarly, an IPv6 application that is about to pass an functions.
open PF_INET6 socket to a program that may not be IPv6 capable may
"downgrade" the socket to PF_INET before calling exec(). After that, An IPv6 application that is about to pass an open PF_INET6 socket to
the system will return sockaddr_in address structures to the a program that is not be IPv6 capable can "downgrade" the socket to
application that was exec()'ed. 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 The IPV6_ADDRFORM option is valid at both the IPPROTO_IP and
IPPROTO_IPV6 levels. The only valid option values are PF_INET6 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 PF_INET. For example, to convert a PF_INET6 socket to PF_INET, a
program would call: program would call:
int addrform = PF_INET; int addrform = PF_INET;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform, if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
sizeof(addrform)) == -1) (char *) &addrform, sizeof(addrform)) == -1)
perror("setsockopt IPV6_ADDRFORM"); perror("setsockopt IPV6_ADDRFORM");
An application may use IPV6_ADDRFORM in the getsockopt() function to An application may use IPV6_ADDRFORM with getsockopt() to learn
learn whether an open socket is a PF_INET of PF_INET6 socket. For whether an open socket is a PF_INET of PF_INET6 socket. For example:
example:
int addrform; int addrform;
size_t len = sizeof(int); size_t len = sizeof(addrform);
if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform, if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
&len) == -1) (char *) &addrform, &len) == -1)
perror("getsockopt IPV6_ADDRFORM"); perror("getsockopt IPV6_ADDRFORM");
if (addrform == PF_INET) else if (addrform == PF_INET)
printf("This is an IPv4 socket.\n"); printf("This is an IPv4 socket.\n");
else if (addrform == PF_INET6) else if (addrform == PF_INET6)
printf("This is an IPv6 socket.\n"); printf("This is an IPv6 socket.\n");
else else
printf("This system is broken.\n"); printf("This system is broken.\n");
4.2. Unicast Hop Limit 5.2. Unicast Hop Limit
A new setsockopt() option is used to control the hop limit used in A new setsockopt() option controls the hop limit used in outgoing
outgoing unicast IPv6 packets. The name of this option is unicast IPv6 packets. The name of this option is IPV6_UNICAST_HOPS,
IPV6_UNICAST_HOPS, and it is used at the IPPROTO_IPV6 layer. The and it is used at the IPPROTO_IPV6 layer. The following example
following example illustrates how it is used: illustrates how it is used:
int hoplimit = 10; int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
sizeof(hoplimit)) == -1) (char *) &hoplimit, sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS"); perror("setsockopt IPV6_UNICAST_HOPS");
When the IPV6_UNICAST_HOPS option is set with setsockopt(), the When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
option value given is used as the hop limit for all subsequent 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 unicast packets sent via that socket. If the option is not set, the
system selects a default value. system selects a default value.
The IPV6_UNICAST_HOPS option may be used in the getsockopt() function The IPV6_UNICAST_HOPS option may be used with getsockopt() to
to determine the hop limit value that the system will use for determine the hop limit value that the system will use for subsequent
subsequent unicast packets sent via that socket. For example: unicast packets sent via that socket. For example:
int hoplimit; int hoplimit;
int len = sizeof(hoplimit); size_t len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
&len) == -1) (char *) &hoplimit, &len) == -1)
perror("getsockopt IPV6_UNICAST_HOPS"); perror("getsockopt IPV6_UNICAST_HOPS");
else else
printf("Using %d for hop limit.\n", hoplimit); printf("Using %d for hop limit.\n", hoplimit);
4.3. Sending and Receiving Multicast Packets 5.3. Sending and Receiving Multicast Packets
IPv6 applications may send UDP multicast packets by simply specifying IPv6 applications may send UDP multicast packets by simply specifying
an IPv6 multicast address in the address argument of the sendto() an IPv6 multicast address in the address argument of the sendto()
function. function.
A few setsockopt options at the IPPROTO_IPV6 layer are used to Three socket options at the IPPROTO_IPV6 layer control some of the
control some of the parameters of sending multicast packets. These parameters for sending multicast packets. Setting these options is
options are optional: applications may send multicast packets without not required: applications may send multicast packets without using
using these options. The setsockopt() options for controlling the these options. The setsockopt() options for controlling the sending
sending of multicast packets are summarized below: of multicast packets are summarized below:
IPV6_MULTICAST_IF IPV6_MULTICAST_IF
Set the interface to use for outgoing multicast packets. The Set the interface to use for outgoing multicast packets. The
argument is an IPv6 address of the interface to use. argument is the index of the interface to use.
Argument type: struct in6_addr Argument type: unsigned int
IPV6_MULTICAST_HOPS IPV6_MULTICAST_HOPS
Set the hop limit to use for outgoing multicast packets. Set the hop limit to use for outgoing multicast packets.
(Note a separate option - IPV6_UNICAST_HOPS - is provided to (Note a separate option - IPV6_UNICAST_HOPS - is provided to
set the hop limit to use for outgoing unicast packets.) set the hop limit to use for outgoing unicast packets.)
Argument type: unsigned int Argument type: unsigned int
IPV6_MULTICAST_LOOP IPV6_MULTICAST_LOOP
skipping to change at page 16, line 7 skipping to change at page 19, line 34
option is set to 1, multicast packets are looped back. If it option is set to 1, multicast packets are looped back. If it
is set to 0, they are not. is set to 0, they are not.
Argument type: unsigned int Argument type: unsigned int
The reception of multicast packets is controlled by the two The reception of multicast packets is controlled by the two
setsockopt() options summarized below: setsockopt() options summarized below:
IPV6_ADD_MEMBERSHIP IPV6_ADD_MEMBERSHIP
Join a multicast group. Requests that multicast packets sent Join a multicast group on a specified local interface. If
to a particular multicast address be delivered to this the interface index is specified as 0, the kernel chooses the
socket. The argument is the IPv6 multicast address of the local interface by looking up the multicast group in the
group to join. normal IPv6 routing table and using the resulting interface.
Argument type: struct ipv6_mreq Argument type: struct ipv6_mreq
IPV6_DROP_MEMBERSHIP IPV6_DROP_MEMBERSHIP
Leave a multicast group. Requests that multicast packets Leave a multicast group on a specified interface.
sent to a particular multicast address no longer be delivered
to this socket. The argument is the IPv6 multicast address
of the group to join.
Argument type: struct ipv6_mreq Argument type: struct ipv6_mreq
The argument type of both of these options is the ipv6_mreq The argument type of both of these options is the ipv6_mreq
structure, which is defined as follows: structure, defined as follows:
struct ipv6_mreq { struct ipv6_mreq {
/* IPv6 multicast address of group */ struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
struct in6_addr ipv6mr_multiaddr; unsigned int ipv6mr_interface; /* interface index */
/* local IPv6 address of interface */
struct in6_addr ipv6mr_interface;
}; };
5. Library Functions 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 New library functions are needed to perform a variety of operations
with IPv6 addresses. Functions are needed to lookup IPv6 addresses with IPv6 addresses. Functions are needed to lookup IPv6 addresses
in the Domain Name System (DNS). Both forward lookup (hostname to in the Domain Name System (DNS). Both forward lookup (hostname-to-
address translation) and reverse lookup (address to hostname address translation) and reverse lookup (address-to-hostname
translation) need to be supported. Functions are also needed to translation) need to be supported. Functions are also needed to
convert IPv6 addresses between their binary and textual form. convert IPv6 addresses between their binary and textual form.
5.1. Hostname to Address Translation 6.1. Hostname-to-Address Translation
A new hostname to address translation function is being defined by The commonly used function gethostbyname() remains unchanged as does
the Institute of Electrical and Electronic Engineers (IEEE) as part the hostent structure to which it returns a pointer. Existing
of the POSIX 1003.1g (Protocol Independent Interfaces) draft applications that call this function continue to receive only IPv4
specification [4]. This function, named getaddrinfo(), has been addresses that are the result of a query in the DNS for A records.
designed to be protocol independent, so it can be used without change (We assume the DNS is being used; some environments may be using a
to lookup IPv6 addresses. hosts file or some other name resolution system, either of which may
impede renumbering.)
As discussed in the "Transition Mechanisms for IPv6 Hosts and Two new changes are made to support IPv6 addresses. First the
Routers" specification [5], systems may provide the ability to following function is new:
transparently query for IPv4 address records when the application
requests an IPv6 lookup. The getaddrinfo() function can implement
this by automatically performing a query for IPv4 records if its
initial query for IPv6 records finds none. Or it may elect to always
query for both IPv6 and IPv4 records on all lookups. (Many DNS
implementations do not support querying for multiple record types in
a single request, so the IPv6 and IPv4 lookups can not be performed
simultaneously.) If IPv4 records are found, the addresses can be
returned to the application as IPv4-mapped IPv6 addresses. Systems
that support transparent querying for IPv4 address records should
provide a system-wide configuration switch to allow the system
administrator to enable or disable that feature.
5.2. Address to Hostname Translation struct hostent *gethostbyname2(const char *name, int af);
The POSIX 1003.1g specification includes no function to perform a The af argument specifies the address family. The default operation
reverse DNS lookup (query based on IPv6 address). Therefore, we have of this function is simple:
defined the following function:
int getnameinfo( - If the af argument is AF_INET, then a query is made for A
const struct sockaddr *sa, records. If successful, IPv4 addresses are returned and the
size_t addrlen, h_length member of the hostent structure will be 4, else the
char *host, function returns a NULL pointer.
size_t hostlen,
char *serv, - If the af argument is AF_INET6, then a query is made for AAAA
size_t servlen); 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.
+------------------+---------------------------------------------------+
| | 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);
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:
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
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:
#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 This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text caller in the DNS and system-specific database, and returns text
strings for both in buffers provided by the caller. The first strings for both in buffers provided by the caller. The first
argument, sa, points to either a sockaddr_in structure (for IPv4) or argument, sa, points to either a sockaddr_in structure (for IPv4) or
a sockaddr_in6 structure (for IPv6) which holds the IP address and a sockaddr_in6 structure (for IPv6) that holds the IP address and
port number. The addrlen argument gives the length of the port number. The salen argument gives the length of the sockaddr_in
sockaddr_in or sockaddr_in6 structure. The function returns the or sockaddr_in6 structure. The function returns the hostname
hostname associated with the IP address in the buffer pointed to by associated with the IP address in the buffer pointed to by the host
the host argument. The caller provides the size of this buffer via argument. The caller provides the size of this buffer via the
the hostlen argument. The service name associated with the port hostlen argument. The service name associated with the port number
number is returned in the buffer pointed to by serv, and the servlen is returned in the buffer pointed to by serv, and the servlen
argument gives the length of this buffer. The caller may instruct argument gives the length of this buffer. The caller specifies not
the function not to return either string by providing a zero value to return either string by providing a zero value for the hostlen or
for the hostlen or servlen arguments. Otherwise, the caller must servlen arguments. Otherwise, the caller must provide buffers large
provide buffers large enough to hold the fully qualified domain enough to hold the fully qualified domain hostname, and the full
hostname, and the full service name, including the terminating null service name, including the terminating null character. The function
character. The function indicates successful completion by a zero indicates successful completion by a zero return value; a non-zero
return value; a non-zero return value indicates failure. return value indicates failure.
Applications obtain the function prototype declaration for Note that this function does not know the protocol of the socket
getnameinfo() by including the header file <netdb.h>. 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,
513, and 514).
5.3. Address Conversion Functions 6.5. Address Conversion Functions
BSD Unix provides two functions, inet_addr() and inet_ntoa(), to The two functions inet_addr() and inet_ntoa() convert an IPv4 address
convert an IPv4 address between binary and text form. IPv6 between binary and text form. IPv6 applications need similar
applications need similar functions. The following two functions functions. The following two functions convert both IPv6 and IPv4
convert both IPv6 and IPv4 addresses: addresses:
ssize_t inet_pton( int inet_pton(int af, const char *src, void *dst);
int af,
const char *cp,
void *ap);
and and
char *inet_ntop( const char *inet_ntop(int af, const void *src,
int af, char *dst, size_t size);
const void *ap,
size_t len,
char *cp);
The first function converts an address in its standard text The inet_pton() function converts an address in its standard text
presentation form into its numeric binary form. The af argument presentation form into its numeric binary form. The af argument
specifies the family of the address. Currently AF_INET and AF_INET6 specifies the family of the address. Currently the AF_INET and
address families are supported. The cp argument points to the string AF_INET6 address families are supported. The src argument points to
being passed in. The ap argument points to a buffer into which the the string being passed in. The dst argument points to a buffer into
function stores the numeric address. The address is returned in which the function stores the numeric address. The address is
network byte order. Inet_pton() returns the length of the address in returned in network byte order. Inet_pton() returns 1 if the
octets if the conversion succeeds, and -1 otherwise. The function conversion succeeds, 0 if the input is not a valid IPv4 dotted-
does not modify the buffer pointed to by ap if the conversion fails. decimal string or a valid IPv6 address string, or -1 with errno set
The calling application must ensure that the buffer referred to by ap to EAFNOSUPPORT if the af argument is unknown. The function does not
is large enough to hold the converted address. 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 If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted decimal form: standard IPv4 dotted-decimal form:
ddd.ddd.ddd.ddd ddd.ddd.ddd.ddd
where ddd is a one to three digit decimal number between 0 and 255. 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 If the af argument is AF_INET6, then the function accepts a string in
one of the standard IPv6 text forms defined in the addressing one of the standard IPv6 text forms defined in Section 2.2 of the
architecture specification [2]. addressing architecture specification [2].
The second function converts a numeric address into a text string The inet_ntop() function converts a numeric address into a text
suitable for presentation. The af argument specifies the family of string suitable for presentation. The af argument specifies the
the address. This can be AF_INET or AF_INET6. The ap argument family of the address. This can be AF_INET or AF_INET6. The src
points to a buffer holding an IPv4 address if the af argument is argument points to a buffer holding an IPv4 address if the af
AF_INET, or an IPv6 address if the af argument is AF_INET6. The len argument is AF_INET, or an IPv6 address if the af argument is
field specifies the length in octets of the address pointed to by ap. AF_INET6. The dst argument points to a buffer where the function
This must be 4 if af is AF_INET, or 16 if af is AF_INET6. The cp will store the resulting text string. The size argument specifies
argument points to a buffer that the function can use to store the the size of this buffer. The application must specify a non-NULL dst
text string. If the cp argument is NULL, the function uses its own argument. For IPv6 addresses, the buffer must be at least 46-octets.
private static buffer. If the application specifies a non-NULL cp For IPv4 addresses, the buffer must be at least 16-octets. In order
argument, the buffer must be large enough to hold the text to allow applications to easily declare buffers of the proper size to
representation of the address, including the terminating null octet. store IPv4 and IPv6 addresses in string form, implementations should
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 provide the following constants, made available to applications that
include <netinet/in.h>: include <netinet/in.h>:
#define INET_ADDRSTRLEN 16 #define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46 #define INET6_ADDRSTRLEN 46
The inet_ntop() function returns a pointer to the buffer containing The inet_ntop() function returns a pointer to the buffer containing
the text string if the conversion succeeds, and NULL otherwise. The the text string if the conversion succeeds, and NULL otherwise. Upon
function does not modify the storage pointed to by cp if the 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. conversion fails.
Applications obtain the prototype declarations for inet_ntop() and Applications obtain the prototype declarations for inet_ntop() and
inet_pton() by including the header file <arpa/inet.h>. inet_pton() by including the header <arpa/inet.h>.
5.4. Embedded IPv4 Addresses 6.6. IPv4-Mapped Addresses
The IPv4-mapped IPv6 address format is used to represent IPv4 The IPv4-mapped IPv6 address format represents IPv4 addresses as IPv6
addresses as IPv6 addresses. Most applications should be able to to addresses. Most applications should be able to manipulate IPv6
manipulate IPv6 addresses as opaque 16-octet quantities, without addresses as opaque 16-octet quantities, without needing to know
needing to know whether they represent IPv4 addresses. However, a whether they represent IPv4 addresses. However, a few applications
few applications may need to determine whether an IPv6 address is an may need to determine whether an IPv6 address is an IPv4-mapped
IPv4-mapped address or not. The following function is provided for address or not. The following function is provided for those
those applications: applications:
int inet6_isipv4addr (const struct in6_addr *addr); int inet6_isipv4mapped(const struct in6_addr *addr);
The "addr" argument to this function points to a buffer holding an The "addr" argument to this function points to a buffer holding an
IPv6 address in network byte order. The function returns true (non- IPv6 address in network byte order. The function returns non-zero if
zero) if that address is an IPv4-mapped address, and returns 0 that address is an IPv4-mapped address, and returns 0 otherwise.
otherwise.
This function could be used by server applications to determine This function could be used by server applications to determine
whether the peer is an IPv4 node or an IPv6 node. After accepting a whether the peer is an IPv4 node or an IPv6 node. After accepting a
TCP connection via accept(), or receiving a UDP packet via TCP connection via accept(), or receiving a UDP packet via
recvfrom(), the application can apply the inet6_isipv4addr() function recvfrom(), the application can apply the inet6_isipv4mapped()
to the returned address. function to the returned address.
Applications obtain the prototype for this function by including the Applications obtain the prototype for this function by including the
header file <arpa/inet.h>. header <arpa/inet.h>.
6. Security Considerations 7. Security Considerations
IPv6 provides a number of new security mechanisms, many of which need IPv6 provides a number of new security mechanisms, many of which need
to be accessible to applications. A companion document detailing the to be accessible to applications. A companion memo detailing the
extensions to the socket interfaces to support IPv6 security is being extensions to the socket interfaces to support IPv6 security is being
written [3]. At some point in the future, that document and this one written [3].
may be merged into a single API specification.
7. Change History 8. Change History
Changes from the January 1996 Edition Changes from the April 1996 Edition (-05 draft)
- Eliminated source routing and interface identification features - Rewrote Abstract.
in order to simplify the spec. API features to provide this
functionallity can be defined at a later time.
- Eliminated definitions of hostname2addr() and addr2hostname(). - Added Table of Contents.
Added reference to POSIX getaddrinfo() function to provide
functionallity previously provided by hostname2addr(). Added
definition of getnameinfo() function to provide functionallity of
addr2hostname().
- Changed name of addr2ascii() and ascii2addr() functions to - New Section 2.2 (Data Types).
inet_ntop() and inet_pton() to be more consistent with BSD
function naming conventions.
- Changed some type definitions to align with POSIX. - 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().
Changes from the November 1995 Edition - 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.
- Added the symbolic constants IPV6ADDR_ANY_INIT and - Warning placed at the end of Section 3.9 concerning the byte
IPV6ADDR_LOOPBACK_INIT for applications to use for 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.
- 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. initializations.
- Eliminated restrictions on the value of ipv6addr_any. Systems - Eliminated restrictions on the value of ipv6addr_any. Systems
may now choose any value, including all-zeros. may now choose any value, including all-zeros.
- Added a mechanism for returning time to live with the address in - Added a mechanism for returning time to live with the address in
the name-to-address translation functions. the name-to-address translation functions.
- Added a mechanism for applications to specify the interface in - Added a mechanism for applications to specify the interface in
the setsockopt() options to join and leave a multicast group. the setsockopt() options to join and leave a multicast group.
skipping to change at page 21, line 26 skipping to change at page 31, line 19
- Defined a set of constants for subfields of sin6_flowid and for - Defined a set of constants for subfields of sin6_flowid and for
priority values. priority values.
- Defined constants for getting and setting the source route flag. - Defined constants for getting and setting the source route flag.
- Define where ansi prototypes for hostname2addr(), - Define where ansi prototypes for hostname2addr(),
addr2hostname(), addr2ascii(), ascii2addr(), and addr2hostname(), addr2ascii(), ascii2addr(), and
ipv6_isipv4addr() reside. ipv6_isipv4addr() reside.
- Clarified the include file requirements. Say that the structure - Clarified the include file requirements. Say that the structure
definitions are defined as a result of including the header file definitions are defined as a result of including the header
<netinet/in.h>, not that the structures are necessarily defined <netinet/in.h>, not that the structures are necessarily defined
there. there.
- Removed underscore chars from is_ipv4_addr() function name for - Removed underscore chars from is_ipv4_addr() function name for
BSD compatibility. BSD compatibility.
- Added inet6_ prefix to is_ipv4_addr() function name to avoid name - Added inet6_ prefix to is_ipv4_addr() function name to avoid name
space conflicts. space conflicts.
- Changes setsockopt option naming convention to use IPV6_ prefix - Changes setsockopt option naming convention to use IPV6_ prefix
skipping to change at page 22, line 26 skipping to change at page 32, line 19
Changes from the June 1995 Edition Changes from the June 1995 Edition
- Added capability for application to select loose or strict source - Added capability for application to select loose or strict source
routing. routing.
Changes from the March 1995 Edition Changes from the March 1995 Edition
- Changed the definition of the ipv6_addr structure to be an array - Changed the definition of the ipv6_addr structure to be an array
of sixteen chars instead of four longs. This change is necessary of sixteen chars instead of four longs. This change is necessary
to support machines which implement the socket interface, but do to support machines that implement the socket interface, but do
not have a 32-bit addressable word. Virtually all machines which not have a 32-bit addressable word. Virtually all machines that
provide the socket interface do support an 8-bit addressable data provide the socket interface do support an 8-bit addressable data
type. type.
- Added a more detailed explanation that the data types defined in - Added a more detailed explanation that the data types defined in
this documented are not intended to be hard and fast this documented are not intended to be hard and fast
requirements. Systems may use other data types if they wish. requirements. Systems may use other data types if they wish.
- Added a note flagging the fact that the sockaddr_in6 structure is - Added a note flagging the fact that the sockaddr_in6 structure is
not the same size as the sockaddr structure. not the same size as the sockaddr structure.
- Changed the sin6_flowlabel field to sin6_flowinfo to accommodate - Changed the sin6_flowlabel field to sin6_flowinfo to accommodate
the addition of the priority field to the IPv6 header. the addition of the priority field to the IPv6 header.
Changes from the October 1994 Edition Changes from the October 1994 Edition
- Added variant of sockaddr_in6 for 4.4 BSD-based systems (sa_len - Added variant of sockaddr_in6 for 4.4BSD-based systems (sa_len
compatibility). compatibility).
- Removed references to SIT transition specification, and added - Removed references to SIT transition specification, and added
reference to addressing architecture document, for definition of reference to addressing architecture document, for definition of
IPv4-mapped addresses. IPv4-mapped addresses.
- Added a solution to the problem of the application not providing - Added a solution to the problem of the application not providing
enough buffer space to hold a received source route. enough buffer space to hold a received source route.
- Moved discussion of IPv4 applications interoperating with IPv6 - Moved discussion of IPv4 applications interoperating with IPv6
skipping to change at page 23, line 23 skipping to change at page 33, line 15
IP_MULTICAST_HOPS. IP_MULTICAST_HOPS.
- Removed specification of numeric values for AF_INET6, - Removed specification of numeric values for AF_INET6,
IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on
different implementations. different implementations.
- Added a definition for the in_addr6 IPv6 address data structure. - Added a definition for the in_addr6 IPv6 address data structure.
Added this so that applications could use sizeof(struct in_addr6) 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 to get the size of an IPv6 address, and so that a structured type
could be used in the is_ipv4_addr(). could be used in the is_ipv4_addr().
8. Open Issues
A few open issues for IPv6 socket interface API specification remain,
including:
- An API should be provided to allocate and free a flow label that
meets the uniqueness and randomness requirements outlined in the
IPv6 protocol spec.
- Should we add a timeout parameter to the hostname/address
translation functions? DNS lookups need to be given some finite
timeout interval, so it might be nice to let the application
specify that interval.
- Can the IPV6_ADDRFORM option really be implemented?
- Can existing IPv4 applications interoperate with IPv6 nodes? 9. Acknowledgments
This issue is discussed in more detail in the following section.
8.1. IPv4 Applications Interoperating with IPv6 Nodes
This problem primarily has to do with the how IPv4 applications
represent addresses of IPv6 nodes. What address should be returned
to the application when an IPv6/UDP packet is received, or an
IPv6/TCP connection is accepted? The peer's address could be any
arbitrary 128-bit IPv6 address. But the application is only equipped
to deal with 32-bit IPv4 addresses encoded in sockaddr_in data
structures.
We have not discovered any solution that provides complete
transparent interoperability with IPv6 nodes for applications using
the original IPv4 API. However, two techniques that partially solve
the problem are:
1) Prohibit communication between IPv4 applications and IPv6 nodes.
Only UDP packets received from IPv4 nodes would be passed up to
the application, and only TCP connections received from IPv4
nodes would be accepted. UDP packets from IPv6 nodes would be
dropped, and TCP connections from IPv6 nodes would be refused.
2) The system could generate a local 32-bit cookie to represent the
full 128-bit IPv6 address, and pass this value to the
application. The system would maintain a mapping from cookie
value into the 128-bit IPv6 address that it represents. When the
application passed a cookie back into the system (for example, in
a sendto() or connect() call) the system would use the 128-bit
IPv6 address that the cookie represents.
The cookie would have to be chosen so as to be an invalid IPv4
address (e.g. an address on net 127.0.0.0), and the system would
have to make sure that these cookie values did not escape into
the Internet as the source or destination addresses of IPv4
packets.
Both of these techniques have drawbacks. This is an area for further
study. System implementors may use one of these techniques or
implement another solution.
Acknowledgments
Thanks to the many people who made suggestions and provided feedback Thanks to the many people who made suggestions and provided feedback
to to the numerous revisions of this document, including: Werner to to the numerous revisions of this document, including: Werner
Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson, Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson,
Alex Conta, Alan Cox, Steve Deering, Francis Dupont, Robert Elz, Marc Alex Conta, Alan Cox, Steve Deering, Francis Dupont, Robert Elz, Marc
Hasson, Tom Herbert, Christian Huitema, Wan-Yen Hsu, Alan Lloyd, Hasson, Tom Herbert, Christian Huitema, Wan-Yen Hsu, Alan Lloyd,
Charles Lynn, Dan McDonald, Craig Metz, Erik Nordmark, Josh Osborne, Charles Lynn, Dan McDonald, Craig Metz, Erik Nordmark, Josh Osborne,
Craig Partridge, Richard Stevens, Matt Thomas, Dean D. Throop, Glenn Craig Partridge, Matt Thomas, Dean D. Throop, Glenn Trewitt, Paul
Trewitt, Paul Vixie, David Waitzman, and Carl Williams. The Vixie, David Waitzman, and Carl Williams.
getnameinfo() function was based on the getinfobysockaddr() function
defined by Keith Sklower. 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 Ramesh Govindan made a number of contributions and co-authored an
earlier version of this paper. earlier version of this memo.
References 10. References
[1] S. Deering, R. Hinden. "Internet Protocol, Version 6 (IPv6) [1] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification". RFC 1883. December 1995. Specification", RFC 1883, December 1995.
[2] R. Hinden, S. Deering. "IP Version 6 Addressing Architecture". [2] R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
RFC 1884. December 1995. RFC 1884, December 1995.
[3] D. McDonald. "IPv6 Security API for BSD Sockets". Internet [3] D. McDonald, "A Simple IP Security API Extension to BSD Sockets",
Draft. January 1995. Internet-Draft, <draft-mcdonald-simple-ipsec-api-00.txt>,
November 1996.
[4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT [4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
6.3. November 1995. 6.3, November 1995.
[5] R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts [5] W. R. Stevens, M. Thomas, "Advanced Sockets API for IPv6",
and Routers". RFC 1933. April 1996. Internet-Draft, <draft-stevens-advanced-api-00.txt>, October
1996.
Authors' Address 11. Authors' Addresses
Jim Bound Robert E. Gilligan
Digital Equipment Corporation Freegate Corporation
110 Spitbrook Road ZK3-3/U14 710 Lakeway Dr. STE 230
Nashua, NH 03062-2698 Sunnyvale, CA 94086
Phone: +1 603 881 0400 Phone: +1 408 524 4804
Email: bound@zk3.dec.com Email: gilligan@freegate.net
Susan Thomson Susan Thomson
Bell Communications Research Bell Communications Research
MRE 2P-343, 445 South Street MRE 2P-343, 445 South Street
Morristown, NJ 07960 Morristown, NJ 07960
Telephone: +1 201 829 4514 Telephone: +1 201 829 4514
Email: set@thumper.bellcore.com Email: set@thumper.bellcore.com
Robert E. Gilligan Jim Bound
Mailstop MPK 17-202 Digital Equipment Corporation
Sun Microsystems, Inc. 110 Spitbrook Road ZK3-3/U14
2550 Garcia Avenue Nashua, NH 03062-2698
Mountain View, CA 94043-1100 Phone: +1 603 881 0400
Phone: +1 415 786 5151 Email: bound@zk3.dec.com
Email: gilligan@eng.sun.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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