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GETHOSTBYNAME(3) Library Functions Manual GETHOSTBYNAME(3)

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

#include <netdb.h>

extern int h_errno;

struct hostent *
gethostbyname(const char *name);

struct hostent *
gethostent(void);

void
sethostent(int stayopen);

void
endhostent(void);

void
herror(const char *string);

const char *
hstrerror(int err);

#include <sys/socket.h>
#include <netdb.h>

struct hostent *
gethostbyname2(const char *name, int af);

struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int af);

The (), gethostbyname2(), and gethostbyaddr() functions each return a pointer to an object with the following structure describing an Internet host referenced by name or addr, respectively. This structure contains either information obtained from a name server, broken-out fields from a line in /etc/hosts, or database entries supplied by the yp(8) system. resolv.conf(5) describes how the particular database is chosen. 
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 returned addresses */
};
#define	h_addr  h_addr_list[0]	/* address, for backward compat */

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.
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.

The function () will search for the named host in the current domain and its parents using the search lookup semantics detailed in resolv.conf(5) and hostname(7).

() is similar to gethostbyname() except that it supports an af of AF_INET6 in addition to AF_INET.

The () function will search for the specified address of length len in the address family af. The only address family supported is AF_INET.

The (), (), and () functions are deprecated and no longer have any effect. They could be used in the past for queries over a persistent TCP connection or to iterate entries in the hosts(5) file.

The () function prints an error message describing the failure. If its argument string is not NULL, it is prepended to the message string and separated from it by a colon (‘:’) and a space. The error message is printed with a trailing newline. The contents of the error message is the same as that returned by () with argument h_errno.

A list of options to override the resolver's internal defaults. See resolv.conf(5) for more information.

/etc/hosts
 
/etc/resolv.conf
 

Error return status from 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 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.
An internal error occurred. This may occur when an address family other than AF_INET or AF_INET6 is specified or when a resource is unable to be allocated. It is always set by gethostent().
The function completed successfully.

getaddrinfo(3), getnameinfo(3), res_init(3), hosts(5), resolv.conf(5), hostname(7)

The endhostent(), gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() functions appeared in 4.1cBSD. The function herror() was added in 4.3BSD-Tahoe, hstrerror() in 4.4BSD, and gethostbyname2() in OpenBSD 2.1.

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 formats are currently understood.

YP does not support any address families other than AF_INET and uses the traditional database format.

August 30, 2019 OpenBSD-7.0