gethostbyaddr
Hurricane Electric Internet Services
NAME
gethostbyname, gethostbyaddr, gethostent, sethostent, end-
hostent, herror - get network host entry
SYNOPSIS
#include <netdb.h>
extern int h_errno;
struct hostent *gethostbyname(name)
char *name;
struct hostent *gethostbyname2(name, af)
char *name; int af;
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, gethostbyname2, 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
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_addrtype The type of address being returned; usually
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 ``HOSTALAIASES'' con-
tains 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.
Gethostbyname2 is an evolution of gethostbyname 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 else the function will return NULL
after having set h_errno to NETDB_INTERNAL.
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 connection 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:
NETDB_INTERNAL This indicates an internal error in
the library, unrelated to the net-
work or name service. errno will
be valid in this case; see per-
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 ALSO
resolver(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 necessary.
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.
Hurricane Electric Internet Services
Copyright (C) 1998
Hurricane Electric.
All Rights Reserved.