GETHOSTBYNAME(3) Library Functions Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror - 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 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_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 ``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.
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 mes-
sage 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 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.
4.2 Berkeley Distribution October 30, 1996 GETHOSTBYNAME(3)