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GETHOSTBYNAME(3) Library Functions Manual GETHOSTBYNAME(3)

gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror — get network host entry

#include <netdb.h>
extern int h_errno;

struct hostent *
gethostbyname(char *name);

struct hostent *
gethostbyaddr(char *addr, int len, int type);

struct hostent *
gethostent(void);

sethostent(int stayopen);

endhostent(void);

herror(char *string);

The () and () functions each return a pointer to an object with the following structure describing an internet host referenced by name or by address, respectively. This structure contains either the information obtained from the name server, named(8), 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 NULL-terminated array of alternate names for the host.
h_addrtype
The type of address being returned; currently always AF_INET.
h_length
The length, in bytes, of the address.
h_addr_list
A NULL-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.

When using the nameserver, () will search for the named host in the current domain and its parents unless the name ends in 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.

The () function may be used to request the use of a connected TCP socket for queries. If the stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to gethostbyname() or gethostbyaddr(). Otherwise, queries are performed using UDP datagrams.

The () function closes the TCP connection.

/etc/hosts
 

Error return status from gethostbyname() and gethostbyaddr() is indicated by return of a null pointer. The external integer h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine herror() can be used to print an error message describing the failure. If its argument string is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.

The variable h_errno can have the following values:

No such host is known.
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.
Some unexpected server failure was encountered. This is a non-recoverable error.
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.

resolver(3), hosts(5), hostname(7), named(8)

The () function is defined, and sethostent() and endhostent() are redefined, when libc(3) is built to use only the routines to lookup in /etc/hosts and not the name server.

The () function reads the next line of /etc/hosts, opening the file if necessary.

The () function opens and/or rewinds the file /etc/hosts. If the stayopen argument is non-zero, the file will not be closed after each call to gethostbyname() or gethostbyaddr().

The () function closes the file.

The herror() function appeared in 4.3BSD. The endhostent(), gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() functions appeared in 4.2BSD.

These functions use static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. Only the Internet address format is currently understood.

BSD 4.2 May 25, 1995 GETHOSTBYNAME(3)