GETHOSTBYNAME(3) BSD Programmer's 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 *
gethostbyname2(const char *name, int af);
struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int af);
struct hostent *
gethostent(void);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
The gethostbyname(), gethostbyname2(), and gethostbyaddr() functions each
return a pointer to an object with the following structure describing an
Internet host referenced by name or by address, respectively. This struc-
ture contains either information obtained from the name server (i.e.,
resolver(3) and named(8)), 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 com-
patibility.
The function gethostbyname() 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).
gethostbyname2() is an advanced form of gethostbyname() which allows
lookups in address families other than AF_INET. Currently, the only sup-
ported address family besides AF_INET is AF_INET6.
The gethostbyaddr() function will search for the specified address of
length len in the address family af. The only address family currently
supported is AF_INET.
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() or gethostbyaddr(). Other-
wise, queries are performed using UDP datagrams.
The endhostent() function closes the TCP connection.
The herror() function prints an error message describing the failure. If
its argument string is non-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 hstrerror() with argument h_errno.
HOSTALIASES A file containing local host aliases. See hostname(7) for
more information.
RES_OPTIONS A list of options to override the resolver's internal de-
faults. See resolver(3) for more information.
/etc/hosts
/etc/resolv.conf
Error return status from gethostbyname(), gethostbyname2(), and gethost-
byaddr() 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:
HOST_NOT_FOUND No such host is known.
TRY_AGAIN This is usually a temporary error and means that the lo-
cal server did not receive a response from an authorita-
tive 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 ad-
dress; 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.
NETDB_INTERNAL An internal error occurred. This may occur when an ad-
dress family other than AF_INET or AF_INET6 is specified
or when a resource is unable to be allocated.
NETDB_SUCCESS The function completed successfully.
getaddrinfo(3), getnameinfo(3), resolver(3), hosts(5), resolv.conf(5),
hostname(7), named(8)
The herror() function appeared in 4.3BSD. The endhostent(), gethost-
byaddr(), gethostbyname(), gethostent(), and sethostent() functions ap-
peared in 4.2BSD.
If the search routines in resolv.conf(5) decide to read the /etc/hosts
file, gethostent() and other functions 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(), or gethostbyaddr().
The endhostent() function closes the file.
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.
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