Tribblix: 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)