GETHOSTBYNAME(3)

NAME
gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror - get network host entry

SYNOPSIS
#include <netdb.h>
extern int
h_errno;

struct hostent *
gethostbyname(char *name);
struct hostent *
gethostbyname2(char *name, int af);
struct hostent *
gethostbyaddr(char *addr, int len, type);
struct hostent *
gethostent();
sethostent(int stayopen);
endhostent();
herror(char *string);

DESCRIPTION
Gethostbyname(), gethostbyname2(), and gethostbyaddr() 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, 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 zero-terminated array of alternate names for the host.

h_addrtype
    The type of address being returned; usually AF_INET.

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.

When using the nameserver, gethostbyname() 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.

Gethostbyname2() is an evolution of gethostbyname() intended to allow lookups in address families other than AF_INET, for example, AF_INET6. Currently, the af argument must be specified as AF_INET else the function will return NULL after having set h_errno to NETDB_INTERNAL.

Sethostent() 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.

Endhostent() closes the TCP connection.

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

FILES
/etc/hosts
    See hosts(5).

DIAGNOSTICS

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.

h_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(2).

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_DATA
    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.

SEE ALSO
hosts(5), hostname(7), named(8), resolver(3), resolver(5).

CAVEAT

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

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

Sethostent() is redefined to open and rewind the file. If the stayopen argument is non-zero, the hosts data base will not be closed after each call to gethostbyname() or gethostbyaddr().

Endhostent() is redefined to close the file.

BUGS
All information is contained in a static area so it must be copied if it is to be saved. Only the Internet address format is currently understood.