NAME

getipnodebyname, getipnodebyaddr — get network host entry
freehostent — free network host entry

SYNOPSIS

#include <netdb.h>

struct hostent *
getipnodebyname(const char *name, int af, int flags, int *error);

struct hostent *
getipnodebyaddr(const void *addr, size_t len, int af, int *error);

void
freehostent(struct hostent *he);

DESCRIPTION

Getipnodebyname(), and getipnodebyaddr() each return a pointer to a hostent structure (see below) describing an internet host referenced by name or by address, as the function names indicate. This structure contains either the information obtained from the name server, or broken-out fields from a line in /etc/hosts. If the local name server is not running, these routines do a lookup in /etc/hosts.

struct	hostent {
	char	*h_name;	/* official 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 from name server */
};

#define	h_addr  h_addr_list[0]	/* address, for backward compatibility */

The members of this structure are:

h_name: Official name of the host.
h_aliases: A zero-terminated array of alternate names for the host.
h_addrtype: The type of address being returned.
h_length: The length, in bytes, of the address.
h_addr_list: A zero-terminated array of network addresses for the host. Host addresses are returned in network byte order.
h_addr: The first address in h_addr_list; this is for backward compatibility.

This structure should be freed after use by calling freehostent().

When using the nameserver, getiphostbyaddr() will search for the named host in each parent domain given in the “search” directive of resolv.conf(5) unless the name contains a dot (“.”). If the name contains no dot, and if the environment variable HOSTALIASES contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname(7) for the domain search procedure and the alias file format.

Getiphostbyaddr() can be told to look for IPv4 addresses, IPv6 addresses or both IPv4 and IPv6. If IPv4 addresses only are to be looked up then af should be set to AF_INET, otherwise it should be set to AF_INET6.

There are three flags that can be set

AI_V4MAPPED: Return IPv4 addresses if no IPv6 addresses are found. This flag is ignored unless af is AF_INET6.
AI_ALL: Return IPv4 addresses as well IPv6 addresses if AI_V4MAPPED is set. This flag is ignored unless af is AF_INET6.
AI_ADDRCONFIG: Only return addresses of a given type if the system has an active interface with that type.

Also AI_DEFAULT is defined to be (AI_V4MAPPED|AI_ADDRCONFIG).

Getipnodebyaddr() will lookup IPv4 mapped and compatible addresses in the IPv4 name space and IPv6 name space

Freehostent() frees the hostent structure allocated be getipnodebyname() and getipnodebyaddr(). The structures returned by gethostbyname(), gethostbyname2(), gethostbyaddr() and gethostent() should not be passed to freehostent() as they are pointers to static areas.

ENVIRONMENT

HOSTALIASES: Name of file containing (host alias, full hostname) pairs.

FILES

/etc/hosts: See hosts(5).

DIAGNOSTICS

Error return status from getipnodebyname() and getipnodebyaddr() is indicated by return of a null pointer. In this case error may then be checked to see whether this is a temporary failure or an invalid or unknown host. errno can have the following values:

NETDB_INTERNAL: This indicates an internal error in the library, unrelated to the network or name service. errno will be valid in this case; see perror.
HOST_NOT_FOUND: No such host is known.
TRY_AGAIN: This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
NO_RECOVERY: Some unexpected server failure was encountered. This is a non-recoverable error, as one might expect.
NO_ADDRESS: The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain.