illumos: manual page: getipnodebyname.3socket

GETIPNODEBYNAME(3SOCKET) Sockets Library Functions GETIPNODEBYNAME(3SOCKET)

NAME

getipnodebyname, getipnodebyaddr, freehostent - get IP node entry

SYNOPSIS

cc [ flag... ] file... -lsocket -lnsl [ library... ]
#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);

PARAMETERS

af
Address family

flags
Various flags

name
Name of host

error_num
Error storage

src
Address for lookup

len
Length of address

ptr
Pointer to hostent structure

DESCRIPTION

The getipnodebyname() function searches the ipnodes database from the
beginning. The function finds the first h_name member that matches the
hostname specified by name. The function takes an af argument that
specifies the address family. The address family can be AF_INET for IPv4
addresses or AF_INET6 for IPv6 addresses. The flags argument determines
what results are returned based on the value of flags. If the flags
argument is set to 0 (zero), the default operation of the function is
specified as follows:

o If the af argument is AF_INET, a query is made for an IPv4
address. If successful, IPv4 addresses are returned and the
h_length member of the hostent structure is 4. Otherwise, the
function returns a NULL pointer.

o If the af argument is AF_INET6, a query is made for an IPv6
address. If successful, IPv6 addresses are returned and the
h_length member of the hostent structure is 16. Otherwise, the
function returns a NULL pointer.

The flags argument changes the default actions of the function. Set the
flags argument with a logical OR operation on any of combination of the
following values:

o AI_V4MAPPED

o AI_ALL

o AI_ADDRCONFIG

The special flags value, AI_DEFAULT, should handle most applications.
Porting simple applications to use IPv6 replaces the call

hptr = gethostbyname(name);

with

hptr = getipnodebyname(name, AF_INET6, AI_DEFAULT, &error_num);

The flags value 0 (zero) implies a strict interpretation of the af
argument:

o If flags is 0 and af is AF_INET, the caller wants only IPv4
addresses. A query is made for A records. If successful, IPv4
addresses are returned and the h_length member of the hostent
structure is 4. Otherwise, the function returns a NULL
pointer.

o If flags is 0 and af is AF_INET6, the caller wants only IPv6
addresses. A query is made for AAAA records. If successful,
IPv6 addresses are returned and the h_length member of the
hostent structure is 16. Otherwise, the function returns a
NULL pointer.

Logically OR other constants into the flags argument to modify the
behavior of the getipnodebyname() function.

o If the AI_V4MAPPED flag is specified with af set to AF_INET6,
the caller can accept IPv4-mapped IPv6 addresses. If no AAAA
records are found, a query is made for A records. Any A
records found are returned as IPv4-mapped IPv6 addresses and
the h_length is 16. The AI_V4MAPPED flag is ignored unless af
equals AF_INET6.

o The AI_ALL flag is used in conjunction with the AI_V4MAPPED
flag, exclusively with the IPv6 address family. When AI_ALL is
logically ORed with AI_V4MAPPED flag, the caller wants all
addresses: IPv6 and IPv4-mapped IPv6 addresses. A query is
first made for AAAA records and, if successful, IPv6 addresses
are returned. Another query is then made for A records. Any A
records found are returned as IPv4-mapped IPv6 addresses and
the h_length is 16. Only when both queries fail does the
function return a NULL pointer. The AI_ALL flag is ignored
unless af is set to AF_INET6.

o The AI_ADDRCONFIG flag specifies that a query for AAAA records
should occur only when the node is configured with at least
one IPv6 source address. A query for A records should occur
only when the node is configured with at least one IPv4 source
address. For example, if a node is configured with no IPv6
source addresses, af equals AF_INET6, and the node name
queried has both AAAA and A records, then:

o A NULL pointer is returned when only the AI_ADDRCONFIG
value is specified.

o The A records are returned as IPv4-mapped IPv6 addresses
when the AI_ADDRCONFIG and AI_V4MAPPED values are
specified.

The special flags value, AI_DEFAULT, is defined as

#define AI_DEFAULT (AI_V4MAPPED | AI_ADDRCONFIG)

The getipnodebyname() function allows the name argument to be a node name
or a literal address string: a dotted-decimal IPv4 address or an IPv6 hex
address. Applications do not have to call inet_pton(3C) to handle literal
address strings.

Four scenarios arise based on the type of literal address string and the
value of the af argument. The two simple cases occur when name is a
dotted-decimal IPv4 address and af equals AF_INET and when name is an
IPv6 hex address and af equals AF_INET6. The members of the returned
hostent structure are:

h_name
Pointer to a copy of the name argument

h_aliases
NULL pointer.

h_addrtype
Copy of the af argument.

h_length
4 for AF_INET or 16 for AF_INET6.

h_addr_list
Array of pointers to 4-byte or 16-byte binary addresses.
The array is terminated by a NULL pointer.

RETURN VALUES

Upon successful completion, getipnodebyname() and getipnodebyaddr()
return a hostent structure. Otherwise they return NULL.

The hostent structure does not change from the existing definition when
used with gethostbyname(3NSL). For example, host entries are represented
by the struct hostent structure defined in <netdb.h>:

struct hostent {
char *h_name; /* canonical name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses */
};

An error occurs when name is an IPv6 hex address and af equals AF_INET.
The return value of the function is a NULL pointer and error_num equals
HOST_NOT_FOUND.

The getipnodebyaddr() function has the same arguments as the existing
gethostbyaddr(3NSL) function, but adds an error number. As with
getipnodebyname(), getipnodebyaddr() is 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 can be
returned for error_num:

HOST_NOT_FOUND
Host is unknown.

NO_DATA
No address is available for the name specified in the
server request. This error is not a soft error.
Another type of name server request might be
successful.

NO_RECOVERY
An unexpected server failure occurred, which is a non-
recoverable error.

TRY_AGAIN
This error is a soft error that indicates that the
local server did not receive a response from an
authoritative server. A retry at some later time might
be successful.

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.

3. If af is AF_INET6, lookup the name for the given IPv6 address.

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.

This structure, and the information pointed to by this structure, are
dynamically allocated by getipnodebyname() and getipnodebyaddr(). The
freehostent() function frees this memory.

EXAMPLES

Example 1: Getting the Canonical Name, Aliases, and Internet IP Addresses

for a Given Hostname

The following is a sample program that retrieves the canonical name,
aliases, and all Internet IP addresses, both version 6 and version 4, for
a given hostname.

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

main(int argc, const char **argv)
{
char abuf[INET6_ADDRSTRLEN];
int error_num;
struct hostent *hp;
char **p;

if (argc != 2) {
(void) printf("usage: %s hostname\n", argv[0]);
exit (1);
}

/* argv[1] can be a pointer to a hostname or literal IP address */
hp = getipnodebyname(argv[1], AF_INET6, AI_ALL | AI_ADDRCONFIG |
AI_V4MAPPED, &error_num);
if (hp == NULL) {
if (error_num == TRY_AGAIN) {
printf("%s: unknown host or invalid literal address "
"(try again later)\n", argv[1]);
} else {
printf("%s: unknown host or invalid literal address\n",
argv[1]);
}
exit (1);
}
for (p = hp->h_addr_list; *p != 0; p++) {
struct in6_addr in6;
char **q;

bcopy(*p, (caddr_t)&in6, hp->h_length);
(void) printf("%s\t%s", inet_ntop(AF_INET6, (void *)&in6,
abuf, sizeof(abuf)), hp->h_name);
for (q = hp->h_aliases; *q != 0; q++)
(void) printf(" %s", *q);
(void) putchar('\n');
}
freehostent(hp);
exit (0);
}

ATTRIBUTES

NOTES

No enumeration functions are provided for IPv6. Existing enumeration
functions such as sethostent(3NSL) do not work in combination with the
getipnodebyname() and getipnodebyaddr() functions.

All the functions that return a struct hostent must always return the
canonical in the h_name field. This name, by definition, is the well-
known and official hostname shared between all aliases and all addresses.
The underlying source that satisfies the request determines the mapping
of the input name or address into the set of names and addresses in
hostent. Different sources might make such as determination in different
ways. If more than one alias and more than one address in hostent exist,
no pairing is implied between the alias and address.

The current implementations of these functions return or accept only
addresses for the Internet address family (type AF_INET) or the Internet
address family Version 6 (type AF_INET6).

IPv4-mapped addresses are not recommended. The getaddrinfo(3SOCKET)
function is preferred over getipnodebyaddr() because it allows
applications to lookup IPv4 and IPv6 addresses without relying on
IPv4-mapped addresses.

The form for an address of type AF_INET is a struct in_addr defined in
<netinet/in.h>. The form for an address of type AF_INET6 is a struct
in6_addr, also defined in <netinet/in.h>. The functions described in
inet_ntop(3C) and inet_pton(3C) that are illustrated in the EXAMPLES
section are helpful in constructing and manipulating addresses in either
of these forms.

March 30, 2022 GETIPNODEBYNAME(3SOCKET)