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:

• 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.

• 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:

• AI_V4MAPPED

• AI_ALL

• 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:

• 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.

• 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.

• 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.

• 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.

• 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:

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

  • 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(3SOCKET) 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

See attributes(5) for descriptions of the following attributes:

See Also

getaddrinfo(3SOCKET), gethostbyname(3NSL), htonl(3SOCKET), inet(3SOCKET), netdb.h(3HEAD), hosts(4), nsswitch.conf(4), attributes(5)

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(3SOCKET) and inet_pton(3SOCKET) that are illustrated in the EXAMPLES section are helpful in constructing and manipulating addresses in either of these forms.