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GETHOSTBYNAME(3)
Contents
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gethostbyname, gethostbyname2, gethostbyaddr, sethostent, endhostent,
herror, hstrerror - get network host entry
Standard C Library (libc, -lc)
#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, socklen_t len, int type);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
The gethostbyname(), gethostbyname2() 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 the information obtained from the name server,
named(8), broken-out fields from a line in /etc/hosts, or database
entries supplied by the yp(8) system. The order of the lookups is controlled
by the `hosts' entry in nsswitch.conf(5).
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 NULL-terminated array of alternative 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 NULL-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 compatibility.
When using the nameserver, gethostbyname() and gethostbyname2() 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.
The gethostbyname2() function is an evolution of gethostbyname() which is
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
or AF_INET6, else the fuction will return NULL after having set h_errno
to NETDB_INTERNAL.
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(), gethostbyname2() or
gethostbyaddr(). Otherwise, queries are performed using UDP datagrams.
The endhostent() function closes the TCP connection.
The herror() function writes a message to the diagnostic output consisting
of the string parameter s, the constant string ": ", and a message
corresponding to the value of h_errno.
The hstrerror() function returns a string which is the message text corresponding
to the value of the err parameter.
/etc/hosts
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 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.
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 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.
resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
If the search routines specified in nsswitch.conf(5) decide to read the
/etc/hosts file, gethostbyname() 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.
The herror() function appeared in 4.3BSD. The endhostent(),
gethostbyaddr(), gethostbyname(), and sethostent() functions appeared in
4.2BSD. The gethostbyname2() function first appeared in bind-4.9.4.
IPv6 support was implemented in WIDE Hydrangea IPv6 protocol stack kit.
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 format is currently understood.
BSD April 26, 2001 BSD
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