GETHOSTBYNAME(3) BSD Library Functions Manual GETHOSTBYNAME(3)NAME
gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
endhostent, herror, hstrerror — get network host entry
LIBRARY
Standard C Library (libc, -lc)
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 com‐
patibility.
In the case of gethostbyname() and gethostbyname2(), the host is speci‐
fied by name, or using a string representation of a numeric address. In
the case of gethostbyaddr(), the host is specified using a binary repre‐
sentation 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 consist‐
ing 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 cor‐
responding 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 authori‐
tative 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 ALSOresolver(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() func‐
tions appeared in 4.2BSD. The gethostbyname2() function first appeared
in bind-4.9.4. IPv6 support was implemented in WIDE Hydrangea IPv6 pro‐
tocol 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.
BSD October 7, 2006 BSD
