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SYNOPSIS
cc ... -lsocket
#include <netdb.h>
extern int h_errno;
struct hostent *
gethostbyname(char *name);
struct hostent *
gethostbyname2(char *name, int af);
struct hostent *
gethostbyaddr(char *addr, int len, type);
struct hostent *
gethostent();
sethostent(int stayopen);
endhostent();
herror(char *string);
DESCRIPTION
Gethostbyname(), gethostbyname2(), and gethostbyaddr() each return a
pointer to a hostent structure (see below) describing an internet host
referenced by name or by address, as the function names indicate. This
structure contains either the information obtained from the name server,
named(ADMN), 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; 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 com-
patibility.
When using the nameserver, gethostbyname() will search for the named host
in each parent domain given in the ``search'' directive of resolv.conf(SFF)
unless the name contains 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 in-
put name. See hostname(ADMN) 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 af-
ter each call to gethostbyname() or gethostbyaddr(). Otherwise, queries
are performed using UDP datagrams.
Endhostent() closes the TCP connection.
ENVIRONMENT
HOSTALIASES Name of file containing (host alias, full hostname) pairs.
FILES
/etc/hosts See hosts(SFF).
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 un-
known host. The routine herror() can be used to print an error message
describing the failure. If its argument string is non-NULL, it is print-
ed, 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 network or name service. errno
will be valid in this case; see perror(2).
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, as one might ex-
pect.
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
hosts(SFF), hostname(ADMN), named(ADMN), resolver(SFF), resolver(SLIB).
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 under-
stood.
4th Berkeley Distribution June 23, 1990