GETHOSTBYNAME(3) BSD Library Functions Manual GETHOSTBYNAME(3)
NAME
endhostent, gethostbyaddr, gethostbyname, gethostbyname2, gethostent, herror, hstrerror, sethostent --get -get
get network host entry
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <netdb.h>
extern int h_errno;
void
endhostent(void);
struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int type);
struct hostent *
gethostbyname(const char *name);
struct hostent *
gethostbyname2(const char *name, int af);
struct hostent *
gethostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
void
sethostent(int stayopen);
DESCRIPTION
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, respec-tively. respectively.
tively. 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; usually 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, 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. Both of these address families are sup-ported supported
ported in the Mac OS X implemention.
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(). Other-wise, Otherwise,
wise, 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
/etc/host.conf
/etc/resolv.conf
DIAGNOSTICS
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 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.
LEGACY SYNOPSIS
#include <netdb.h>
extern int h_errno;
struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int type);
The type of addr has changed slightly.
SEE ALSO
getaddrinfo(3), resolver(3), hosts(5), hostname(7), named(8)
CAVEAT
The gethostent() function 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.
The gethostent() function reads the next line of /etc/hosts, opening the file if necessary.
The sethostent() function opens and/or rewinds the file /etc/hosts. If the stayopen argument is non-zero, nonzero,
zero, the file will not be closed after each call to gethostbyname(), gethostbyname2() or
gethostbyaddr().
The endhostent() function closes the file.
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 version 4.9.4.
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.
BSD May 25, 1995 BSD
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