GETHOSTBYNAME(3)

NAME
gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent, endhostent, herror, hstrerror - get network host entry

LIBRARY
libc

SYNOPSIS
#include <netdb.h>
extern int h_errno;
struct hostent *
gethostbyname(const char *name);
struct hostent *
gethostbyname2(const char *name, int af);
struct hostent *
gethostbyaddr(const char *addr, socklen_t len, int type);
struct hostent *
gethostent(void);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);

DESCRIPTION
The gethostbyname(), gethostbyname2() and gethostbyaddr() functions each return a pointer to an object with the following structure describing an internet host.

    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 alternative 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.

In the case of gethostbyname() and gethostbyname2(), the host is specified by name, or using a string representation of a numeric address. In the case of gethostbyaddr(), the host is specified using a binary representation of an address.

The returned struct hostent structure may contain the result of a simple string to binary conversion, information obtained from the domain name resolver (see resolver(3)), broken-out fields from a line in /etc/hosts, or database entries supplied by the yp(8) system. The order of the lookups is controlled by the ‘hosts’ entry in nsswitch.conf(5).

When using the domain name resolver, gethostbyname() and gethostbyname2() 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 gethostbyname2() function is an evolution of gethostbyname() which is 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 or AF_INET6, else the function will return NULL after having set h_errno to NETDB_INTERNAL.

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

The sethostent() 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(), gethostbyname2(), or gethostbyaddr(). Otherwise, queries are performed using UDP datagrams.

The endhostent() function closes the TCP connection.

The herror() function writes a message to the diagnostic output consisting of the string parameter s, the constant string ": ", and a message corresponding to the value of h_errno.

The hstrerror() function returns a string which is the message text corresponding to the value of the err parameter.

FILES
/etc/hosts

DIAGNOSTICS
Error return status from gethostbyent(), gethostbyname(), gethostbyname2(), 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:

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.

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
resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

HISTORY
The herror() function appeared in 4.3BSD. The endhostent(), gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() functions appeared in 4.2BSD. The gethostbyname2() function first appeared in bind-4.9.4. IPv6 support was implemented in WIDE Hydrangea IPv6 protocol stack kit.

CAVEATS
If the search routines specified in nsswitch.conf(5) decide to read the /etc/hosts file, gethostbyname(), gethostbyname2(), and gethostbyaddr() will read the next line of the file, re-opening the file if necessary.

The sethostent() 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(), gethostbyname2(), gethostbyaddr(), or gethostent().

The endhostent() function closes the file.

BUGS
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.

The gethostent() does not currently follow the search order specified in nsswitch.conf(5) and only reads the /etc/hosts file.