RFC2553 - Basic Socket Interface Extensions for IPv6

Network Working Group R. Gilligan
Request for Comments: 2553 FreeGate
Obsoletes: 2133 S. Thomson
Category: Informational Bellcore
J. Bound
Compaq
W. Stevens
Consultant
March 1999
Basic Socket Interface Extensions for IPv6
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
The de facto standard application program interface (API) for TCP/IP
applications is the "sockets" interface. Although this API was
developed for Unix in the early 1980s it has also been implemented on
a wide variety of non-Unix systems. TCP/IP applications written
using the sockets API have in the past enjoyed a high degree of
portability and we would like the same portability with IPv6
applications. But changes are required to the sockets API to support
IPv6 and this memo describes these changes. These include a new
socket address strUCture to carry IPv6 addresses, new address
conversion functions, and some new socket options. These extensions
are designed to provide Access to the basic IPv6 features required by
TCP and UDP applications, including multicasting, while introducing a
minimum of change into the system and providing complete
compatibility for existing IPv4 applications. Additional extensions
for advanced IPv6 features (raw sockets and access to the IPv6
extension headers) are defined in another document [4].
Table of Contents
1. Introduction.................................................3
2. Design Considerations........................................3
2.1 What Needs to be Changed....................................4
2.2 Data Types..................................................5
2.3 Headers.....................................................5
2.4 Structures..................................................5
3. Socket Interface.............................................6
3.1 IPv6 Address Family and Protocol Family.....................6
3.2 IPv6 Address Structure......................................6
3.3 Socket Address Structure for 4.3BSD-Based Systems...........7
3.4 Socket Address Structure for 4.4BSD-Based Systems...........8
3.5 The Socket Functions........................................9
3.6 Compatibility with IPv4 Applications.......................10
3.7 Compatibility with IPv4 Nodes..............................10
3.8 IPv6 Wildcard Address......................................11
3.9 IPv6 Loopback Address......................................12
3.10 Portability Additions.....................................13
4. Interface Identification....................................16
4.1 Name-to-Index..............................................16
4.2 Index-to-Name..............................................17
4.3 Return All Interface Names and Indexes.....................17
4.4 Free Memory................................................18
5. Socket Options..............................................18
5.1 Unicast Hop Limit..........................................18
5.2 Sending and Receiving Multicast Packets....................19
6. Library Functions...........................................21
6.1 Nodename-to-Address Translation............................21
6.2 Address-To-Nodename Translation............................24
6.3 Freeing memory for getipnodebyname and getipnodebyaddr.....26
6.4 Protocol-Independent Nodename and Service Name Translation.26
6.5 Socket Address Structure to Nodename and Service Name......29
6.6 Address Conversion Functions...............................31
6.7 Address Testing Macros.....................................32
7. Summary of New Definitions..................................33
8. Security Considerations.....................................35
9. Year 2000 Considerations....................................35
Changes From RFC2133..........................................35
Acknowledgments................................................38
References.....................................................39
Authors" Addresses.............................................40
Full Copyright Statement.......................................41
1. Introduction
While IPv4 addresses are 32 bits long, IPv6 interfaces are identified
by 128-bit addresses. The socket interface makes the size of an IP
address quite visible to an application; virtually all TCP/IP
applications for BSD-based systems have knowledge of the size of an
IP address. Those parts of the API that eXPose the addresses must be
changed to accommodate the larger IPv6 address size. IPv6 also
introduces new features (e.g., traffic class and flowlabel), some of
which must be made visible to applications via the API. This memo
defines a set of extensions to the socket interface to support the
larger address size and new features of IPv6.
2. Design Considerations
There are a number of important considerations in designing changes
to this well-worn API:
- The API changes should provide both source and binary
compatibility for programs written to the original API. That
is, existing program binaries should continue to operate when
run on a system supporting the new API. In addition, existing
applications that are re-compiled and run on a system supporting
the new API should continue to operate. Simply put, the API
changes for IPv6 should not break existing programs. An
additonal mechanism for implementations to verify this is to
verify the new symbols are protected by Feature Test Macros as
described in IEEE Std 1003.1. (Such Feature Test Macros are not
defined by this RFC.)
- The changes to the API should be as small as possible in order
to simplify the task of converting existing IPv4 applications to
IPv6.
- Where possible, applications should be able to use this API to
interoperate with both IPv6 and IPv4 hosts. Applications should
not need to know which type of host they are communicating with.
- IPv6 addresses carried in data structures should be 64-bit
aligned. This is necessary in order to oBTain optimum
performance on 64-bit machine architectures.
Because of the importance of providing IPv4 compatibility in the API,
these extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this
API could probably be designed for operation on systems that support
only IPv6. However, this is not addressed in this memo.
2.1 What Needs to be Changed
The socket interface API consists of a few distinct components:
- Core socket functions.
- Address data structures.
- Name-to-address translation functions.
- Address conversion functions.
The core socket functions -- those functions that deal with such
things as setting up and tearing down TCP connections, and sending
and receiving UDP packets -- were designed to be transport
independent. Where protocol addresses are passed as function
arguments, they are carried via opaque pointers. A protocol-specific
address data structure is defined for each protocol that the socket
functions support. Applications must cast pointers to these
protocol-specific address structures into pointers to the generic
"sockaddr" address structure when using the socket functions. These
functions need not change for IPv6, but a new IPv6-specific address
data structure is needed.
The "sockaddr_in" structure is the protocol-specific data structure
for IPv4. This data structure actually includes 8-octets of unused
space, and it is tempting to try to use this space to adapt the
sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in
structure is not large enough to hold the 16-octet IPv6 address as
well as the other information (address family and port number) that
is needed. So a new address data structure must be defined for IPv6.
IPv6 addresses are scoped [2] so they could be link-local, site,
organization, global, or other scopes at this time undefined. To
support applications that want to be able to identify a set of
interfaces for a specific scope, the IPv6 sockaddr_in structure must
support a field that can be used by an implementation to identify a
set of interfaces identifying the scope for an IPv6 address.
The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr(). These are left as is and new
functions are defined to support IPv4 and IPv6. Additionally, the
POSIX 1003.g draft [3] specifies a new nodename-to-address
translation function which is protocol independent. This function
can also be used with IPv4 and IPv6.
The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form. These
functions are quite specific to 32-bit IPv4 addresses. We have
designed two analogous functions that convert both IPv4 and IPv6
addresses, and carry an address type parameter so that they can be
extended to other protocol families as well.
Finally, a few miscellaneous features are needed to support IPv6.
New interfaces are needed to support the IPv6 traffic class, flow
label, and hop limit header fields. New socket options are needed to
control the sending and receiving of IPv6 multicast packets.
The socket interface will be enhanced in the future to provide access
to other IPv6 features. These extensions are described in [4].
2.2 Data Types
The data types of the structure elements given in this memo are
intended to be examples, not absolute requirements. Whenever
possible, data types from Draft 6.6 (March 1997) of POSIX 1003.1g are
used: uintN_t means an unsigned integer of exactly N bits (e.g.,
uint16_t). We also assume the argument data types from 1003.1g when
possible (e.g., the final argument to setsockopt() is a size_t
value). Whenever buffer sizes are specified, the POSIX 1003.1 size_t
data type is used (e.g., the two length arguments to getnameinfo()).
2.3 Headers
When function prototypes and structures are shown we show the headers
that must be #included to cause that item to be defined.
2.4 Structures
When structures are described the members shown are the ones that
must appear in an implementation. Additional, nonstandard members
may also be defined by an implementation. As an additional
precaution nonstandard members could be verified by Feature Test
Macros as described in IEEE Std 1003.1. (Such Feature Test Macros
are not defined by this RFC.)
The ordering shown for the members of a structure is the recommended
ordering, given alignment considerations of multibyte members, but an
implementation may order the members differently.
3. Socket Interface
This section specifies the socket interface changes for IPv6.
3.1 IPv6 Address Family and Protocol Family
A new address family name, AF_INET6, is defined in <sys/socket.h>.
The AF_INET6 definition distinguishes between the original
sockaddr_in address data structure, and the new sockaddr_in6 data
structure.
A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
Like most of the other protocol family names, this will usually be
defined to have the same value as the corresponding address family
name:
#define PF_INET6 AF_INET6
The PF_INET6 is used in the first argument to the socket() function
to indicate that an IPv6 socket is being created.
3.2 IPv6 Address Structure
A new in6_addr structure holds a single IPv6 address and is defined
as a result of including <netinet/in.h>:
struct in6_addr {
uint8_t s6_addr[16]; /* IPv6 address */
};
This data structure contains an array of sixteen 8-bit elements,
which make up one 128-bit IPv6 address. The IPv6 address is stored
in network byte order.
The structure in6_addr above is usually implemented with an embedded
union with extra fields that force the desired alignment level in a
manner similar to BSD implementations of "struct in_addr". Those
additional implementation details are omitted here for simplicity.
An example is as follows:
struct in6_addr {
union {
uint8_t _S6_u8[16];
uint32_t _S6_u32[4];
uint64_t _S6_u64[2];
} _S6_un;
};
#define s6_addr _S6_un._S6_u8
3.3 Socket Address Structure for 4.3BSD-Based Systems
In the socket interface, a different protocol-specific data structure
is defined to carry the addresses for each protocol suite. Each
protocol- specific data structure is designed so it can be cast into a
protocol- independent data structure -- the "sockaddr" structure.
Each has a "family" field that overlays the "sa_family" of the
sockaddr data structure. This field identifies the type of the data
structure.
The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between applications
and the system in the socket functions. The following sockaddr_in6
structure holds IPv6 addresses and is defined as a result of including
the <netinet/in.h> header:
struct sockaddr_in6 {
sa_family_t sin6_family; /* AF_INET6 */
in_port_t sin6_port; /* transport layer port # */
uint32_t sin6_flowinfo; /* IPv6 traffic class & flow info */
struct in6_addr sin6_addr; /* IPv6 address */
uint32_t sin6_scope_id; /* set of interfaces for a scope */
};
This structure is designed to be compatible with the sockaddr data
structure used in the 4.3BSD release.
The sin6_family field identifies this as a sockaddr_in6 structure.
This field overlays the sa_family field when the buffer is cast to a
sockaddr data structure. The value of this field must be AF_INET6.
The sin6_port field contains the 16-bit UDP or TCP port number. This
field is used in the same way as the sin_port field of the
sockaddr_in structure. The port number is stored in network byte
order.
The sin6_flowinfo field is a 32-bit field that contains two pieces of
information: the traffic class and the flow label. The contents and
interpretation of this member is specified in [1]. The sin6_flowinfo
field SHOULD be set to zero by an implementation prior to using the
sockaddr_in6 structure by an application on receive operations.
The sin6_addr field is a single in6_addr structure (defined in the
previous section). This field holds one 128-bit IPv6 address. The
address is stored in network byte order.
The ordering of elements in this structure is specifically designed
so that when sin6_addr field is aligned on a 64-bit boundary, the
start of the structure will also be aligned on a 64-bit boundary.
This is done for optimum performance on 64-bit architectures.
The sin6_scope_id field is a 32-bit integer that identifies a set of
interfaces as appropriate for the scope of the address carried in the
sin6_addr field. For a link scope sin6_addr sin6_scope_id would be
an interface index. For a site scope sin6_addr, sin6_scope_id would
be a site identifier. The mapping of sin6_scope_id to an interface
or set of interfaces is left to implementation and future
specifications on the subject of site identifiers.
Notice that the sockaddr_in6 structure will normally be larger than
the generic sockaddr structure. On many existing implementations the
sizeof(struct sockaddr_in) equals sizeof(struct sockaddr), with both
being 16 bytes. Any existing code that makes this assumption needs
to be examined carefully when converting to IPv6.
3.4 Socket Address Structure for 4.4BSD-Based Systems
The 4.4BSD release includes a small, but incompatible change to the
socket interface. The "sa_family" field of the sockaddr data
structure was changed from a 16-bit value to an 8-bit value, and the
space saved used to hold a length field, named "sa_len". The
sockaddr_in6 data structure given in the previous section cannot be
correctly cast into the newer sockaddr data structure. For this
reason, the following alternative IPv6 address data structure is
provided to be used on systems based on 4.4BSD. It is defined as a
result of including the <netinet/in.h> header.
struct sockaddr_in6 {
uint8_t sin6_len; /* length of this struct */
sa_family_t sin6_family; /* AF_INET6 */
in_port_t sin6_port; /* transport layer port # */
uint32_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
uint32_t sin6_scope_id; /* set of interfaces for a scope */
};
The only differences between this data structure and the 4.3BSD
variant are the inclusion of the length field, and the change of the
family field to a 8-bit data type. The definitions of all the other
fields are identical to the structure defined in the previous
section.
Systems that provide this version of the sockaddr_in6 data structure
must also declare SIN6_LEN as a result of including the
<netinet/in.h> header. This macro allows applications to determine
whether they are being built on a system that supports the 4.3BSD or
4.4BSD variants of the data structure.
3.5 The Socket Functions
Applications call the socket() function to create a socket descriptor
that represents a communication endpoint. The arguments to the
socket() function tell the system which protocol to use, and what
format address structure will be used in subsequent functions. For
example, to create an IPv4/TCP socket, applications make the call:
s = socket(PF_INET, SOCK_STREAM, 0);
To create an IPv4/UDP socket, applications make the call:
s = socket(PF_INET, SOCK_DGRAM, 0);
Applications may create IPv6/TCP and IPv6/UDP sockets by simply using
the constant PF_INET6 instead of PF_INET in the first argument. For
example, to create an IPv6/TCP socket, applications make the call:
s = socket(PF_INET6, SOCK_STREAM, 0);
To create an IPv6/UDP socket, applications make the call:
s = socket(PF_INET6, SOCK_DGRAM, 0);
Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the
system. The functions that the application uses to pass addresses
into the system are:
bind()
connect()
sendmsg()
sendto()
The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets. The
functions that return an address from the system to an application
are:
accept()
recvfrom()
recvmsg()
getpeername()
getsockname()
No changes to the syntax of the socket functions are needed to
support IPv6, since all of the "address carrying" functions use an
opaque address pointer, and carry an address length as a function
argument.
3.6 Compatibility with IPv4 Applications
In order to support the large base of applications using the original
API, system implementations must provide complete source and binary
compatibility with the original API. This means that systems must
continue to support PF_INET sockets and the sockaddr_in address
structure. Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the PF_INET constant in the socket() function, as
described in the previous section. Applications should be able to
hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
sockets simultaneously within the same process.
Applications using the original API should continue to operate as
they did on systems supporting only IPv4. That is, they should
continue to interoperate with IPv4 nodes.
3.7 Compatibility with IPv4 Nodes
The API also provides a different type of compatibility: the ability
for IPv6 applications to interoperate with IPv4 applications. This
feature uses the IPv4-mapped IPv6 address format defined in the IPv6
addressing architecture specification [2]. This address format
allows the IPv4 address of an IPv4 node to be represented as an IPv6
address. The IPv4 address is encoded into the low-order 32 bits of
the IPv6 address, and the high-order 96 bits hold the fixed prefix
0:0:0:0:0:FFFF. IPv4- mapped addresses are written as follows:
::FFFF:<IPv4-address>
These addresses can be generated automatically by the
getipnodebyname() function when the specified host has only IPv4
addresses (as described in Section 6.1).
Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination"s IPv4 address as an IPv4-mapped IPv6 address, and
passing that address, within a sockaddr_in6 structure, in the
connect() or sendto() call. When applications use PF_INET6 sockets
to accept TCP connections from IPv4 nodes, or receive UDP packets
from IPv4 nodes, the system returns the peer"s address to the
application in the accept(), recvfrom(), or getpeername() call using
a sockaddr_in6 structure encoded this way.
Few applications will likely need to know which type of node they are
interoperating with. However, for those applications that do need to
know, the IN6_IS_ADDR_V4MAPPED() macro, defined in Section 6.7, is
provided.
3.8 IPv6 Wildcard Address
While the bind() function allows applications to select the source IP
address of UDP packets and TCP connections, applications often want
the system to select the source address for them. With IPv4, one
specifies the address as the symbolic constant INADDR_ANY (called the
"wildcard" address) in the bind() call, or simply omits the bind()
entirely.
Since the IPv6 address type is a structure (struct in6_addr), a
symbolic constant can be used to initialize an IPv6 address variable,
but cannot be used in an assignment. Therefore systems provide the
IPv6 wildcard address in two forms.
The first version is a global variable named "in6addr_any" that is an
in6_addr structure. The extern declaration for this variable is
defined in <netinet/in.h>:
extern const struct in6_addr in6addr_any;
Applications use in6addr_any similarly to the way they use INADDR_ANY
in IPv4. For example, to bind a socket to port number 23, but let
the system select the source address, an application could use the
following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_any; /* structure assignment */
. . .
if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The other version is a symbolic constant named IN6ADDR_ANY_INIT and
is defined in <netinet/in.h>. This constant can be used to
initialize an in6_addr structure:
struct in6_addr anyaddr = IN6ADDR_ANY_INIT;
Note that this constant can be used ONLY at declaration time. It can
not be used to assign a previously declared in6_addr structure. For
example, the following code will not work:
/* This is the WRONG way to assign an unspecified address */
struct sockaddr_in6 sin6;
. . .
sin6.sin6_addr = IN6ADDR_ANY_INIT; /* will NOT compile */
Be aware that the IPv4 INADDR_xxx constants are all defined in host
byte order but the IPv6 IN6ADDR_xxx constants and the IPv6
in6addr_xxx externals are defined in network byte order.
3.9 IPv6 Loopback Address
Applications may need to send UDP packets to, or originate TCP
connections to, services residing on the local node. In IPv4, they
can do this by using the constant IPv4 address INADDR_LOOPBACK in
their connect(), sendto(), or sendmsg() call.
IPv6 also provides a loopback address to contact local TCP and UDP
services. Like the unspecified address, the IPv6 loopback address is
provided in two forms -- a global variable and a symbolic constant.
The global variable is an in6_addr structure named
"in6addr_loopback." The extern declaration for this variable is
defined in <netinet/in.h>:
extern const struct in6_addr in6addr_loopback;
Applications use in6addr_loopback as they would use INADDR_LOOPBACK
in IPv4 applications (but beware of the byte ordering difference
mentioned at the end of the previous section). For example, to open
a TCP connection to the local telnet server, an application could use
the following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_loopback; /* structure assignment */
. . .
if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined
in <netinet/in.h>. It can be used at declaration time ONLY; for
example:
struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
to a previously declared IPv6 address variable.
3.10 Portability Additions
One simple addition to the sockets API that can help application
writers is the "struct sockaddr_storage". This data structure can
simplify writing code portable across multiple address families and
platforms. This data structure is designed with the following goals.
- It has a large enough implementation specific maximum size to
store the desired set of protocol specific socket address data
structures. Specifically, it is at least large enough to
accommodate sockaddr_in and sockaddr_in6 and possibly other
protocol specific socket addresses too.
- It is aligned at an appropriate boundary so protocol specific
socket address data structure pointers can be cast to it and
access their fields without alignment problems. (e.g. pointers
to sockaddr_in6 and/or sockaddr_in can be cast to it and access
fields without alignment problems).
- It has the initial field(s) isomorphic to the fields of the
"struct sockaddr" data structure on that implementation which
can be used as a discriminants for deriving the protocol in use.
These initial field(s) would on most implementations either be a
single field of type "sa_family_t" (isomorphic to sa_family
field, 16 bits) or two fields of type uint8_t and sa_family_t
respectively, (isomorphic to sa_len and sa_family_t, 8 bits
each).
An example implementation design of such a data structure would be as
follows.
/*
* Desired design of maximum size and alignment
*/
#define _SS_MAXSIZE 128 /* Implementation specific max size */
#define _SS_ALIGNSIZE (sizeof (int64_t))
/* Implementation specific desired alignment */
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE - sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t)+
_SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
sa_family_t __ss_family; /* address family */
/* Following fields are implementation specific */
char __ss_pad1[_SS_PAD1SIZE];
/* 6 byte pad, this is to make implementation
/* specific pad up to alignment field that */
/* follows explicit in the data structure */
int64_t __ss_align; /* field to force desired structure */
/* storage alignment */
char __ss_pad2[_SS_PAD2SIZE];
/* 112 byte pad to achieve desired size, */
/* _SS_MAXSIZE value minus size of ss_family */
/* __ss_pad1, __ss_align fields is 112 */
};
On implementations where sockaddr data structure includes a "sa_len",
field this data structure would look like this:
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE -
(sizeof (uint8_t) + sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t)+
_SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
uint8_t __ss_len; /* address length */
sa_family_t __ss_family; /* address family */
/* Following fields are implementation specific */
char __ss_pad1[_SS_PAD1SIZE];
/* 6 byte pad, this is to make implementation
/* specific pad up to alignment field that */
/* follows explicit in the data structure */
int64_t __ss_align; /* field to force desired structure */
/* storage alignment */
char __ss_pad2[_SS_PAD2SIZE];
/* 112 byte pad to achieve desired size, */
/* _SS_MAXSIZE value minus size of ss_len, */
/* __ss_family, __ss_pad1, __ss_align fields is 112 */
};
The above example implementation illustrates a data structure which
will align on a 64 bit boundary. An implementation specific field
"__ss_align" along "__ss_pad1" is used to force a 64-bit alignment
which covers proper alignment good enough for needs of sockaddr_in6
(IPv6), sockaddr_in (IPv4) address data structures. The size of
padding fields __ss_pad1 depends on the chosen alignment boundary.
The size of padding field __ss_pad2 depends on the value of overall
size chosen for the total size of the structure. This size and
alignment are represented in the above example by implementation
specific (not required) constants _SS_MAXSIZE (chosen value 128) and
_SS_ALIGNMENT (with chosen value 8). Constants _SS_PAD1SIZE (derived
value 6) and _SS_PAD2SIZE (derived value 112) are also for
illustration and not required. The implementation specific
definitions and structure field names above start with an underscore
to denote implementation private namespace. Portable code is not
expected to access or reference those fields or constants.
The sockaddr_storage structure solves the problem of declaring
storage for automatic variables which is large enough and aligned
enough for storing socket address data structure of any family. For
example, code with a file descriptor and without the context of the
address family can pass a pointer to a variable of this type where a
pointer to a socket address structure is expected in calls such as
getpeername() and determine the address family by accessing the
received content after the call.
The sockaddr_storage structure may also be useful and applied to
certain other interfaces where a generic socket address large enough
and aligned for use with multiple address families may be needed. A
discussion of those interfaces is outside the scope of this document.
Also, much existing code assumes that any socket address structure
can fit in a generic sockaddr structure. While this has been true
for IPv4 socket address structures, it has always been false for Unix
domain socket address structures (but in practice this has not been a
problem) and it is also false for IPv6 socket address structures
(which can be a problem).
So now an application can do the following:
struct sockaddr_storage __ss;
struct sockaddr_in6 *sin6;
sin6 = (struct sockaddr_in6 *) &__ss;
4. Interface Identification
This API uses an interface index (a small positive integer) to
identify the local interface on which a multicast group is joined
(Section 5.3). Additionally, the advanced API [4] uses these same
interface indexes to identify the interface on which a datagram is
received, or to specify the interface on which a datagram is to be
sent.
Interfaces are normally known by names such as "le0", "sl1", "ppp2",
and the like. On Berkeley-derived implementations, when an interface
is made known to the system, the kernel assigns a unique positive
integer value (called the interface index) to that interface. These
are small positive integers that start at 1. (Note that 0 is never
used for an interface index.) There may be gaps so that there is no
current interface for a particular positive interface index.
This API defines two functions that map between an interface name and
index, a third function that returns all the interface names and
indexes, and a fourth function to return the dynamic memory allocated
by the previous function. How these functions are implemented is
left up to the implementation. 4.4BSD implementations can implement
these functions using the existing sysctl() function with the
NET_RT_IFLIST command. Other implementations may wish to use ioctl()
for this purpose.
4.1 Name-to-Index
The first function maps an interface name into its corresponding
index.
#include <net/if.h>
unsigned int if_nametoindex(const char *ifname);
If the specified interface name does not exist, the return value is
0, and errno is set to ENXIO. If there was a system error (such as
running out of memory), the return value is 0 and errno is set to the
proper value (e.g., ENOMEM).
4.2 Index-to-Name
The second function maps an interface index into its corresponding
name.
#include <net/if.h>
char *if_indextoname(unsigned int ifindex, char *ifname);
The ifname argument must point to a buffer of at least IF_NAMESIZE
bytes into which the interface name corresponding to the specified
index is returned. (IF_NAMESIZE is also defined in <net/if.h> and
its value includes a terminating null byte at the end of the
interface name.) This pointer is also the return value of the
function. If there is no interface corresponding to the specified
index, NULL is returned, and errno is set to ENXIO, if there was a
system error (such as running out of memory), if_indextoname returns
NULL and errno would be set to the proper value (e.g., ENOMEM).
4.3 Return All Interface Names and Indexes
The if_nameindex structure holds the information about a single
interface and is defined as a result of including the <net/if.h>
header.
struct if_nameindex {
unsigned int if_index; /* 1, 2, ... */
char *if_name; /* null terminated name: "le0", ... */
};
The final function returns an array of if_nameindex structures, one
structure per interface.
struct if_nameindex *if_nameindex(void);
The end of the array of structures is indicated by a structure with
an if_index of 0 and an if_name of NULL. The function returns a NULL
pointer upon an error, and would set errno to the appropriate value.
The memory used for this array of structures along with the interface
names pointed to by the if_name members is obtained dynamically.
This memory is freed by the next function.
4.4 Free Memory
The following function frees the dynamic memory that was allocated by
if_nameindex().
#include <net/if.h>
void if_freenameindex(struct if_nameindex *ptr);
The argument to this function must be a pointer that was returned by
if_nameindex().
Currently net/if.h doesn"t have prototype definitions for functions
and it is recommended that these definitions be defined in net/if.h
as well and the struct if_nameindex{}.
5. Socket Options
A number of new socket options are defined for IPv6. All of these
new options are at the IPPROTO_IPV6 level. That is, the "level"
parameter in the getsockopt() and setsockopt() calls is IPPROTO_IPV6
when using these options. The constant name prefix IPV6_ is used in
all of the new socket options. This serves to clearly identify these
options as applying to IPv6.
The declaration for IPPROTO_IPV6, the new IPv6 socket options, and
related constants defined in this section are obtained by including
the header <netinet/in.h>.
5.1 Unicast Hop Limit
A new setsockopt() option controls the hop limit used in outgoing
unicast IPv6 packets. The name of this option is IPV6_UNICAST_HOPS,
and it is used at the IPPROTO_IPV6 layer. The following example
illustrates how it is used:
int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS");
When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
option value given is used as the hop limit for all subsequent
unicast packets sent via that socket. If the option is not set, the
system selects a default value. The integer hop limit value (called
x) is interpreted as follows:
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
The IPV6_UNICAST_HOPS option may be used with getsockopt() to
determine the hop limit value that the system will use for subsequent
unicast packets sent via that socket. For example:
int hoplimit;
size_t len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, &len) == -1)
perror("getsockopt IPV6_UNICAST_HOPS");
else
printf("Using %d for hop limit.n", hoplimit);
5.2 Sending and Receiving Multicast Packets
IPv6 applications may send UDP multicast packets by simply specifying
an IPv6 multicast address in the address argument of the sendto()
function.
Three socket options at the IPPROTO_IPV6 layer control some of the
parameters for sending multicast packets. Setting these options is
not required: applications may send multicast packets without using
these options. The setsockopt() options for controlling the sending
of multicast packets are summarized below. These three options can
also be used with getsockopt().
IPV6_MULTICAST_IF
Set the interface to use for outgoing multicast packets. The
argument is the index of the interface to use.
Argument type: unsigned int
IPV6_MULTICAST_HOPS
Set the hop limit to use for outgoing multicast packets. (Note
a separate option - IPV6_UNICAST_HOPS - is provided to set the
hop limit to use for outgoing unicast packets.)
The interpretation of the argument is the same as for the
IPV6_UNICAST_HOPS option:
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
If IPV6_MULTICAST_HOPS is not set, the default is 1
(same as IPv4 today)
Argument type: int
IPV6_MULTICAST_LOOP
If a multicast datagram is sent to a group to which the sending
host itself belongs (on the outgoing interface), a copy of the
datagram is looped back by the IP layer for local delivery if
this option is set to 1. If this option is set to 0 a copy
is not looped back. Other option values return an error of
EINVAL.
If IPV6_MULTICAST_LOOP is not set, the default is 1 (loopback;
same as IPv4 today).
Argument type: unsigned int
The reception of multicast packets is controlled by the two
setsockopt() options summarized below. An error of EOPNOTSUPP is
returned if these two options are used with getsockopt().
IPV6_JOIN_GROUP
Join a multicast group on a specified local interface. If the
interface index is specified as 0, the kernel chooses the local
interface. For example, some kernels look up the multicast
group in the normal IPv6 routing table and using the resulting
interface.
Argument type: struct ipv6_mreq
IPV6_LEAVE_GROUP
Leave a multicast group on a specified interface.
Argument type: struct ipv6_mreq
The argument type of both of these options is the ipv6_mreq structure,
defined as a result of including the <netinet/in.h> header;
struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
unsigned int ipv6mr_interface; /* interface index */
};
Note that to receive multicast datagrams a process must join the
multicast group and bind the UDP port to which datagrams will be
sent. Some processes also bind the multicast group address to the
socket, in addition to the port, to prevent other datagrams destined
to that same port from being delivered to the socket.
6. Library Functions
New library functions are needed to perform a variety of operations
with IPv6 addresses. Functions are needed to lookup IPv6 addresses
in the Domain Name System (DNS). Both forward lookup (nodename-to-
address translation) and reverse lookup (address-to-nodename
translation) need to be supported. Functions are also needed to
convert IPv6 addresses between their binary and textual form.
We note that the two existing functions, gethostbyname() and
gethostbyaddr(), are left as-is. New functions are defined to handle
both IPv4 and IPv6 addresses.
6.1 Nodename-to-Address Translation
The commonly used function gethostbyname() is inadequate for many
applications, first because it provides no way for the caller to
specify anything about the types of addresses desired (IPv4 only,
IPv6 only, IPv4-mapped IPv6 are OK, etc.), and second because many
implementations of this function are not thread safe. RFC2133
defined a function named gethostbyname2() but this function was also
inadequate, first because its use required setting a global option
(RES_USE_INET6) when IPv6 addresses were required, and second because
a flag argument is needed to provide the caller with additional
control over the types of addresses required.
The following function is new and must be thread safe:
#include <sys/socket.h>
#include <netdb.h>
struct hostent *getipnodebyname(const char *name, int af, int flags
int *error_num);
The name argument can be either a node name or a numeric address
string (i.e., a dotted-decimal IPv4 address or an IPv6 hex address).
The af argument specifies the address family, either AF_INET or
AF_INET6. The error_num value is returned to the caller, via a
pointer, with the appropriate error code in error_num, to support
thread safe error code returns. error_num will be set to one of the
following values:
HOST_NOT_FOUND
No such host is known.
NO_ADDRESS
The server recognised the request and the name but no address is
available. Another type of request to the name server for the
domain might return an answer.
NO_RECOVERY
An unexpected server failure occurred which cannot be recovered.
TRY_AGAIN
A temporary and possibly transient error occurred, such as a
failure of a server to respond.
The flags argument specifies the types of addresses that are searched
for, and the types of addresses that are returned. We note that a
special flags value of AI_DEFAULT (defined below) should handle most
applications.
That is, porting simple applications to use IPv6 replaces the call
hptr = gethostbyname(name);
with
hptr = getipnodebyname(name, AF_INET6, AI_DEFAULT, &error_num);
and changes any subsequent error diagnosis code to use error_num
instead of externally declared variables, such as h_errno.
Applications desiring finer control over the types of addresses
searched for and returned, can specify other combinations of the
flags argument.
A flags of 0 implies a strict interpretation of the af argument:
- If flags is 0 and af is AF_INET, then the caller wants only
IPv4 addresses. A query is made for A records. If successful,
the IPv4 addresses are returned and the h_length member of the
hostent structure will be 4, else the function returns a NULL
pointer.
- If flags is 0 and if af is AF_INET6, then the caller wants only
IPv6 addresses. A query is made for AAAA records. If
successful, the IPv6 addresses are returned and the h_length
member of the hostent structure will be 16, else the function
returns a NULL pointer.
Other constants can be logically-ORed into the flags argument, to
modify the behavior of the function.
- If the AI_V4MAPPED flag is specified along with an af of
AF_INET6, then the caller will accept IPv4-mapped IPv6
addresses. That is, if no AAAA records are found then a query
is made for A records and any found are returned as IPv4-mapped
IPv6 addresses (h_length will be 16). The AI_V4MAPPED flag is
ignored unless af equals AF_INET6.
- The AI_ALL flag is used in conjunction with the AI_V4MAPPED
flag, and is only used with the IPv6 address family. When AI_ALL
is logically or"d with AI_V4MAPPED flag then the caller wants
all addresses: IPv6 and IPv4-mapped IPv6. A query is first made
for AAAA records and if successful, the IPv6 addresses are
returned. Another query is then made for A records and any found
are returned as IPv4-mapped IPv6 addresses. h_length will be 16.
Only if both queries fail does the function return a NULL pointer.
This flag is ignored unless af equals AF_INET6.
- The AI_ADDRCONFIG flag specifies that a query for AAAA records
should occur only if the node has at least one IPv6 source
address configured and a query for A records should occur only
if the node has at least one IPv4 source address configured.
For example, if the node has no IPv6 source addresses
configured, and af equals AF_INET6, and the node name being
looked up has both AAAA and A records, then:
(a) if only AI_ADDRCONFIG is specified, the function
returns a NULL pointer;
(b) if AI_ADDRCONFIG AI_V4MAPPED is specified, the A
records are returned as IPv4-mapped IPv6 addresses;
The special flags value of AI_DEFAULT is defined as
#define AI_DEFAULT (AI_V4MAPPED AI_ADDRCONFIG)
We noted that the getipnodebyname() function must allow the name
argument to be either a node name or a literal address string (i.e.,
a dotted-decimal IPv4 address or an IPv6 hex address). This saves
applications from having to call inet_pton() to handle literal
address strings.
There are four scenarios based on the type of literal address string
and the value of the af argument.
The two simple cases are:
When name is a dotted-decimal IPv4 address and af equals AF_INET, or
when name is an IPv6 hex address and af equals AF_INET6. The members
of the returned hostent structure are: h_name points to a copy of the
name argument, h_aliases is a NULL pointer, h_addrtype is a copy of
the af argument, h_length is either 4 (for AF_INET) or 16 (for
AF_INET6), h_addr_list[0] is a pointer to the 4-byte or 16-byte
binary address, and h_addr_list[1] is a NULL pointer.
When name is a dotted-decimal IPv4 address and af equals AF_INET6,
and flags equals AI_V4MAPPED, an IPv4-mapped IPv6 address is
returned: h_name points to an IPv6 hex address containing the IPv4-
mapped IPv6 address, h_aliases is a NULL pointer, h_addrtype is
AF_INET6, h_length is 16, h_addr_list[0] is a pointer to the 16-byte
binary address, and h_addr_list[1] is a NULL pointer. If AI_V4MAPPED
is set (with or without AI_ALL) return IPv4-mapped otherwise return
NULL.
It is an error when name is an IPv6 hex address and af equals
AF_INET. The function"s return value is a NULL pointer and error_num
equals HOST_NOT_FOUND.
6.2 Address-To-Nodename Translation
The following function has the same arguments as the existing
gethostbyaddr() function, but adds an error number.
#include <sys/socket.h> #include <netdb.h>
struct hostent *getipnodebyaddr(const void *src, size_t len,
int af, int *error_num);
As with getipnodebyname(), getipnodebyaddr() must be thread safe.
The error_num value is returned to the caller with the appropriate
error code, to support thread safe error code returns. The following
error conditions may be returned for error_num:
HOST_NOT_FOUND
No such host is known.
NO_ADDRESS
The server recognized the request and the name but no address
is available. Another type of request to the name server for
the domain might return an answer.
NO_RECOVERY
An unexpected server failure occurred which cannot be
recovered.
TRY_AGAIN
A temporary and possibly transient error occurred, such as a
failure of a server to respond.
One possible source of confusion is the handling of IPv4-mapped IPv6
addresses and IPv4-compatible IPv6 addresses, but the following logic
should apply.
1. If af is AF_INET6, and if len equals 16, and if the IPv6
address is an IPv4-mapped IPv6 address or an IPv4-compatible
IPv6 address, then skip over the first 12 bytes of the IPv6
address, set af to AF_INET, and set len to 4.
2. If af is AF_INET, lookup the name for the given IPv4 address
(e.g., query for a PTR record in the in-addr.arpa domain).
3. If af is AF_INET6, lookup the name for the given IPv6 address
(e.g., query for a PTR record in the ip6.int domain).
4. If the function is returning success, then the single address
that is returned in the hostent structure is a copy of the
first argument to the function with the same address family
that was passed as an argument to this function.
All four steps listed are performed, in order. Also note that the
IPv6 hex addresses "::" and "::1" MUST NOT be treated as IPv4-
compatible addresses, and if the address is "::", HOST_NOT_FOUND MUST
be returned and a query of the address not performed.
Also for the macro in section 6.7 IN6_IS_ADDR_V4COMPAT MUST return
false for "::" and "::1".
6.3 Freeing memory for getipnodebyname and getipnodebyaddr
The hostent structure does not change from its existing definition.
This structure, and the information pointed to by this structure, are
dynamically allocated by getipnodebyname and getipnodebyaddr. The
following function frees this memory:
#include <netdb.h>
void freehostent(struct hostent *ptr);
6.4 Protocol-Independent Nodename and Service Name Translation
Nodename-to-address translation is done in a protocol-independent
fashion using the getaddrinfo() function that is taken from the
Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) draft specification [3].
The official specification for this function will be the final POSIX
standard, with the following additional requirements:
- getaddrinfo() (along with the getnameinfo() function described
in the next section) must be thread safe.
- The AI_NUMERICHOST is new with this document.
- All fields in socket address structures returned by
getaddrinfo() that are not filled in through an explicit
argument (e.g., sin6_flowinfo and sin_zero) must be set to 0.
(This makes it easier to compare socket address structures.)
- getaddrinfo() must fill in the length field of a socket address
structure (e.g., sin6_len) on systems that support this field.
We are providing this independent description of the function because
POSIX standards are not freely available (as are IETF documents).
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *nodename, const char *servname,
const struct addrinfo *hints,
struct addrinfo **res);
The addrinfo structure is defined as a result of including the
<netdb.h> header.
struct addrinfo {
int ai_flags; /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
int ai_family; /* PF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
size_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for nodename */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
The return value from the function is 0 upon success or a nonzero
error code. The following names are the nonzero error codes from
getaddrinfo(), and are defined in <netdb.h>:
EAI_ADDRFAMILY address family for nodename not supported
EAI_AGAIN temporary failure in name resolution
EAI_BADFLAGS invalid value for ai_flags
EAI_FAIL non-recoverable failure in name resolution
EAI_FAMILY ai_family not supported
EAI_MEMORY memory allocation failure
EAI_NODATA no address associated with nodename
EAI_NONAME nodename nor servname provided, or not known
EAI_SERVICE servname not supported for ai_socktype
EAI_SOCKTYPE ai_socktype not supported
EAI_SYSTEM system error returned in errno
The nodename and servname arguments are pointers to null-terminated
strings or NULL. One or both of these two arguments must be a non-
NULL pointer. In the normal client scenario, both the nodename and
servname are specified. In the normal server scenario, only the
servname is specified. A non-NULL nodename string can be either a
node name or a numeric host address string (i.e., a dotted-decimal
IPv4 address or an IPv6 hex address). A non-NULL servname string can
be either a service name or a decimal port number.
The caller can optionally pass an addrinfo structure, pointed to by
the third argument, to provide hints concerning the type of socket
that the caller supports. In this hints structure all members other
than ai_flags, ai_family, ai_socktype, and ai_protocol must be zero
or a NULL pointer. A value of PF_UNSPEC for ai_family means the
caller will accept any protocol family. A value of 0 for ai_socktype
means the caller will accept any socket type. A value of 0 for
ai_protocol means the caller will accept any protocol. For example,
if the caller handles only TCP and not UDP, then the ai_socktype
member of the hints structure should be set to SOCK_STREAM when
getaddrinfo() is called. If the caller handles only IPv4 and not
IPv6, then the ai_family member of the hints structure should be set
to PF_INET when getaddrinfo() is called. If the third argument to
getaddrinfo() is a NULL pointer, this is the same as if the caller
had filled in an addrinfo structure initialized to zero with
ai_family set to PF_UNSPEC.
Upon successful return a pointer to a linked list of one or more
addrinfo structures is returned through the final argument. The
caller can process each addrinfo structure in this list by following
the ai_next pointer, until a NULL pointer is encountered. In each
returned addrinfo structure the three members ai_family, ai_socktype,
and ai_protocol are the corresponding arguments for a call to the
socket() function. In each addrinfo structure the ai_addr member
points to a filled-in socket address structure whose length is
specified by the ai_addrlen member.
If the AI_PASSIVE bit is set in the ai_flags member of the hints
structure, then the caller plans to use the returned socket address
structure in a call to bind(). In this case, if the nodename
argument is a NULL pointer, then the IP address portion of the socket
address structure will be set to INADDR_ANY for an IPv4 address or
IN6ADDR_ANY_INIT for an IPv6 address.
If the AI_PASSIVE bit is not set in the ai_flags member of the hints
structure, then the returned socket address structure will be ready
for a call to connect() (for a connection-oriented protocol) or
either connect(), sendto(), or sendmsg() (for a connectionless
protocol). In this case, if the nodename argument is a NULL pointer,
then the IP address portion of the socket address structure will be
set to the loopback address.
If the AI_CANONNAME bit is set in the ai_flags member of the hints
structure, then upon successful return the ai_canonname member of the
first addrinfo structure in the linked list will point to a null-
terminated string containing the canonical name of the specified
nodename.
If the AI_NUMERICHOST bit is set in the ai_flags member of the hints
structure, then a non-NULL nodename string must be a numeric host
address string. Otherwise an error of EAI_NONAME is returned. This
flag prevents any type of name resolution service (e.g., the DNS)
from being called.
All of the information returned by getaddrinfo() is dynamically
allocated: the addrinfo structures, and the socket address structures
and canonical node name strings pointed to by the addrinfo
structures. To return this information to the system the function
freeaddrinfo() is called:
#include <sys/socket.h> #include <netdb.h>
void freeaddrinfo(struct addrinfo *ai);
The addrinfo structure pointed to by the ai argument is freed, along
with any dynamic storage pointed to by the structure. This operation
is repeated until a NULL ai_next pointer is encountered.
To aid applications in printing error messages based on the EAI_xxx
codes returned by getaddrinfo(), the following function is defined.
#include <sys/socket.h> #include <netdb.h>
char *gai_strerror(int ecode);
The argument is one of the EAI_xxx values defined earlier and the
return value points to a string describing the error. If the
argument is not one of the EAI_xxx values, the function still returns
a pointer to a string whose contents indicate an unknown error.
6.5 Socket Address Structure to Nodename and Service Name
The POSIX 1003.1g specification includes no function to perform the
reverse conversion from getaddrinfo(): to look up a nodename and
service name, given the binary address and port. Therefore, we
define the following function:
#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen,
int flags);
This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text
strings for both in buffers provided by the caller. The function
indicates successful completion by a zero return value; a non-zero
return value indicates failure.
The first argument, sa, points to either a sockaddr_in structure (for
IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP
address and port number. The salen argument gives the length of the
sockaddr_in or sockaddr_in6 structure.
The function returns the nodename associated with the IP address in
the buffer pointed to by the host argument. The caller provides the
size of this buffer via the hostlen argument. The service name
associated with the port number is returned in the buffer pointed to
by serv, and the servlen argument gives the length of this buffer.
The caller specifies not to return either string by providing a zero
value for the hostlen or servlen arguments. Otherwise, the caller
must provide buffers large enough to hold the nodename and the
service name, including the terminating null characters.
Unfortunately most systems do not provide constants that specify the
maximum size of either a fully-qualified domain name or a service
name. Therefore to aid the application in allocating buffers for
these two returned strings the following constants are defined in
<netdb.h>:
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
The first value is actually defined as the constant MAXDNAME in recent
versions of BIND"s <arpa/nameser.h> header (older versions of BIND
define this constant to be 256) and the second is a guess based on the
services listed in the current Assigned Numbers RFC.
The final argument is a flag that changes the default actions of this
function. By default the fully-qualified domain name (FQDN) for the
host is looked up in the DNS and returned. If the flag bit NI_NOFQDN
is set, only the nodename portion of the FQDN is returned for local
hosts.
If the flag bit NI_NUMERICHOST is set, or if the host"s name cannot be
located in the DNS, the numeric form of the host"s address is returned
instead of its name (e.g., by calling inet_ntop() instead of
getipnodebyaddr()). If the flag bit NI_NAMEREQD is set, an error is
returned if the host"s name cannot be located in the DNS.
If the flag bit NI_NUMERICSERV is set, the numeric form of the service
address is returned (e.g., its port number) instead of its name. The
two NI_NUMERICxxx flags are required to support the "-n" flag that
many commands provide.
A fifth flag bit, NI_DGRAM, specifies that the service is a datagram
service, and causes getservbyport() to be called with a second
argument of "udp" instead of its default of "tcp". This is required
for the few ports (e.g. 512-514) that have different services for UDP
and TCP.
These NI_xxx flags are defined in <netdb.h> along with the AI_xxx
flags already defined for getaddrinfo().
6.6 Address Conversion Functions
The two functions inet_addr() and inet_ntoa() convert an IPv4 address
between binary and text form. IPv6 applications need similar
functions. The following two functions convert both IPv6 and IPv4
addresses:
#include <sys/socket.h>
#include <arpa/inet.h>
int inet_pton(int af, const char *src, void *dst);
const char *inet_ntop(int af, const void *src,
char *dst, size_t size);
The inet_pton() function converts an address in its standard text
presentation form into its numeric binary form. The af argument
specifies the family of the address. Currently the AF_INET and
AF_INET6 address families are supported. The src argument points to
the string being passed in. The dst argument points to a buffer into
which the function stores the numeric address. The address is
returned in network byte order. Inet_pton() returns 1 if the
conversion succeeds, 0 if the input is not a valid IPv4 dotted-
decimal string or a valid IPv6 address string, or -1 with errno set
to EAFNOSUPPORT if the af argument is unknown. The calling
application must ensure that the buffer referred to by dst is large
enough to hold the numeric address (e.g., 4 bytes for AF_INET or 16