GETIPNODEBYNAME(3)GETIPNODEBYNAME(3)NAME
getipnodebyname, getipnodebyaddr, freehostent
- nodename-to-address and address-to-nodename translation
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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, socklen_t len,
int af, int *error_num);
void freehostent(struct hostent *ptr);
DESCRIPTIONgetipnodebyname and getipnodebyaddr functions are very similar to
gethostbyname(3N) and gethostbyaddr(3N). 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(3N).
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.
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GETIPNODEBYNAME(3)GETIPNODEBYNAME(3)
A flags of 0 implies a strict interpretation of the af argument:
* If flags is 0 and af is AF_INET, then a query is made for IPv4
addresses. 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 a query is made for IPv6
addresses. 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 a query is made for IPv6 addresses. If successful, the IPv6
addresses are returned, and no query is made for IPv4 addresses.
Else, a query is made for IPv4 addresses and any found are returned
as IPv4-mapped IPv6 addresses. h_length will be 16 in either case of
addresses being returned. The AI_V4MAPPED flag is ignored unless af
equals AF_INET6.
* If the AI_ALL and AI_V4MAPPED flags are both set and af is AF_INET6,
then a query is made for IPv6 addresses, and any found are returned.
Another query is made for IPv4 addresses, and any found are returned
as IPv4-mapped IPv6 addresses, and h_length is set to 16. Only if
both queries fail the function returns a NULL pointer. This flag is
ignored unless af is AF_INET6.
* The AI_ADDRCONFIG flag specifies that a query for IPv6 addresses
should be made only if the node has at least one IPv6 source address
configured, and that a query for IPv4 addresses should be made only
if the node has at least one IPv4 source address configured.
* If the AI_V4MAPPED and AI_ADDRCONFIG flags are both set and af is
AF_INET6, then
-- If the node has at least one IPv6 source address configured,
a query is made for IPv6 addresses.
-- If it is successful, the IPv6 addresses are returned and no query
is made for IPv4 addresses.
-- If the node has no IPv6 source address configured, or if the
query for IPv6 addresses is not successful, then if the node has at
least one IPv4 source address configured, a query is made for IPv4
addresses and any found are returned as IPv4-mapped IPv6 addresses.
h_length will be 16 in either case of addresses being returned.
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GETIPNODEBYNAME(3)GETIPNODEBYNAME(3)
* Macro AI_DEFAULT is defined as the logical OR of AI_V4MAPPED and
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.
getipnodebyaddr 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.
getipnodebyname and getipnodebyaddr dynamically allocate the structure to
be returned to the caller. freehostent reclaims memory region allocated
and returned by getipnodebyname or getipnodebyaddr
FILES
/etc/hosts
/etc/ipnodes
/etc/nsswitch.conf
/etc/resolv.conf
DIAGNOSTICSgetipnodebyname and getipnodebyaddr 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).
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GETIPNODEBYNAME(3)GETIPNODEBYNAME(3)SEE ALSOgetaddrinfo(3), getnameinfo(3), gethostbyaddr(3), gethostbyname(3),
hosts(5), nsswitch.conf(5), services(5), hostname(7), named(8).
STANDARDSgetipnodebyname and getipnodebyaddr are documented in "Basic Socket
Interface Extensions for IPv6" (RFC2553).
BUGSgetipnodebyname and getipnodebyaddr 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 current implementation is not thread-safe.
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