GETHOSTBYNAME(3) BSD Library Functions Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
endhostent, hstrerror, herror - get network host entry
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, int len, int af);
struct hostent *
gethostent(void);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
DESCRIPTION
The gethostbyname() 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 structure 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 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 zero-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 zero-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 cur-
rent 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, for example 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.
FILES
/etc/hosts
/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 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 ALSO
resolver(3), getaddrinfo(3), getnameinfo(3), hosts(5), resolv.conf(5),
hostname(7), named(8)
CAVEAT
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.
HISTORY
The herror() function appeared in 4.3BSD. The endhostent(),
gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() func-
tions appeared in 4.2BSD.
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 formats are currently understood.
YP does not support any address families other than AF_INET and uses the
traditional database format.
BSD March 13, 1997 BSD
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