GETHOSTBYNAME(3)GETHOSTBYNAME(3)NAME
gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, her‐
ror - get network host entry
SYNOPSIS
#include <netdb.h>
extern int h_errno;
struct hostent *gethostbyname(name)
char *name;
struct hostent *gethostbyaddr(addr, len, type)
char *addr; int len, type;
struct hostent *gethostent()sethostent(stayopen)
int stayopen;
endhostent()herror(string)
char *string;
DESCRIPTION
Gethostbyname and gethostbyaddr 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 the infor‐
mation 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 zero terminated array of alternate 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 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
compatiblity.
When using the nameserver, gethostbyname 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 ``HOSTALA‐
IASES'' 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.
Sethostent 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 connec‐
tion after each call to gethostbyname or gethostbyaddr. Otherwise,
queries are performed using UDP datagrams.
Endhostent closes the TCP connection.
DIAGNOSTICS
Error return status from gethostbyname 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.
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.
FILES
/etc/hosts
SEE ALSOresolver(3), hosts(5), hostname(7), named(8)CAVEAT
Gethostent 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.
Gethostent reads the next line of /etc/hosts, opening the file if nec‐
essary.
Sethostent is redefined to open and rewind the file. If the stayopen
argument is non-zero, the hosts data base will not be closed after each
call to gethostbyname or gethostbyaddr. Endhostent is redefined to
close the file.
BUGS
All information is contained in a static area so it must be copied if
it is to be saved. Only the Internet address format is currently
understood.
4.2 Berkeley Distribution June 23, 1990 GETHOSTBYNAME(3)