| GETIPNODEBYNAME(3) | Library Functions Manual | GETIPNODEBYNAME(3) |
getipnodebyname,
getipnodebyaddr, freehostent
— nodename-to-address and address-to-nodename
translation
Standard C Library (libc, -lc)
#include
<sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
struct hostent *
getipnodebyname(const
char *name, int af,
int flags,
int *error_num);
struct hostent *
getipnodebyaddr(const
void *src, size_t
len, int af,
int *error_num);
void
freehostent(struct
hostent *ptr);
The
getipnodebyname()
and getipnodebyaddr() functions are very similar to
gethostbyname(3),
gethostbyname2(3) and
gethostbyaddr(3). The functions
cover all the functionalities provided by the older ones, and provide better
interface to programmers. The functions require additional arguments,
af, and flags, for specifying
address family and operation mode. The additional arguments allow programmer
to get address for a nodename, for specific address family (such as
AF_INET or AF_INET6). The
functions also require an additional pointer argument,
error_num to return the appropriate error code, to
support thread safe error code returns.
The type and usage of the return value, struct
hostent is described in
gethostbyname(3).
For
getipnodebyname(),
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 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);
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:
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.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.
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.AI_V4MAPPED_CFG flag is exact same as the
AI_V4MAPPED flag only if the kernel supports
IPv4-mapped IPv6 address.AI_ALL flag is used in conjunction with the
AI_V4MAPPED flag, and 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. If both
AI_ALL and AI_V4MAPPED are
specified, AI_ALL takes precedence.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_CFG | 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(3) to handle literal
address strings. When the name argument is a literal
address string, the flags argument is always
ignored.
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
AI_V4MAPPED is specified, 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.
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
the value pointed to by error_num equals
HOST_NOT_FOUND.
The
getipnodebyaddr()
function takes almost the same argument as
gethostbyaddr(3), but adds a
pointer to return an error number. Additionally it takes care of IPv4-mapped
IPv6 addresses, and IPv4-compatible IPv6 addresses.
The
getipnodebyname()
and getipnodebyaddr() functions dynamically allocate
the structure to be returned to the caller. The
freehostent()
function reclaims memory region allocated and returned by
getipnodebyname() or
getipnodebyaddr().
The getipnodebyname() and
getipnodebyaddr() functions returns
NULL on errors. The integer values pointed to by
error_num may then be checked to see whether this is a
temporary failure or an invalid or unknown host. The meanings of each error
code are described in
gethostbyname(3).
getaddrinfo(3), gethostbyaddr(3), gethostbyname(3), getnameinfo(3), hosts(5), nsswitch.conf(5), services(5), hostname(7), named(8)
R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC2553, March 1999.
The getipnodebyname() and
getipnodebyaddr() functions are documented in
“Basic Socket Interface Extensions for IPv6” (RFC2553).
The implementation first appeared in KAME advanced networking kit.
The getipnodebyname() and
getipnodebyaddr() functions do not handle scoped
IPv6 address properly. If you use these functions, your program will not be
able to handle scoped IPv6 addresses. For IPv6 address manipulation,
getaddrinfo(3) and
getnameinfo(3) are
recommended.
The text was shamelessly copied from RFC2553.
| August 6, 2004 | macOS 15.0 |