gethostbyname.3

用于嵌入式Linux系统的标准C的库函数

.\" Copyright (c) 1983, 1987, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     From: @(#)gethostbyname.3	8.4 (Berkeley) 5/25/95
.\" $FreeBSD: src/lib/libc/net/gethostbyname.3,v 1.26 2002/01/14 16:59:00 ru Exp $
.\"
.Dd May 25, 1995
.Dt GETHOSTBYNAME 3
.Os
.Sh NAME
.Nm gethostbyname ,
.Nm gethostbyname2 ,
.Nm gethostbyaddr ,
.Nm gethostent ,
.Nm sethostent ,
.Nm endhostent ,
.Nm herror ,
.Nm hstrerror
.Nd get network host entry
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In netdb.h
.Vt extern int h_errno ;
.Ft struct hostent *
.Fn gethostbyname "const char *name"
.Ft struct hostent *
.Fn gethostbyname2 "const char *name" "int af"
.Ft struct hostent *
.Fn gethostbyaddr "const char *addr" "int len" "int type"
.Ft struct hostent *
.Fn gethostent void
.Ft void
.Fn sethostent "int stayopen"
.Ft void
.Fn endhostent void
.Ft void
.Fn herror "const char *string"
.Ft const char *
.Fn hstrerror "int err"
.Sh DESCRIPTION
.Bf -symbolic
The
.Xr getaddrinfo 3
and
.Xr getnameinfo 3
functions are preferred over the
.Fn gethostbyname ,
.Fn gethostbyname2 ,
and
.Fn gethostbyaddr
functions.
.Ef
.Pp
The
.Fn gethostbyname ,
.Fn gethostbyname2
and
.Fn gethostbyaddr
functions
each return a pointer to an object with the
following structure describing an internet host
referenced by name or by address, respectively.
.Pp
The
.Fa name
parameter passed to
.Fn gethostbyname
or
.Fn gethostbyname2
should point to a
.Dv NUL Ns -terminated
hostname.
The
.Fa addr
parameter passed to
.Fn gethostbyaddr
should point to an address which is
.Fa len
bytes long,
in binary form
(i.e. not an IP address in human readable
.Tn ASCII
form).
The
.Fa type
parameter specifies the address family
(e.g.\&
.Dv AF_INET , AF_INET6 ,
etc.) of this address.
.Pp
The structure returned contains either the information obtained from the name
server,
.Xr named 8 ,
broken-out fields from a line in
.Pa /etc/hosts ,
or database entries supplied by the
.Xr yp 8
system.
The order of the lookups is controlled by the
.Sq hosts
entry in
.Xr nsswitch.conf 5 .
.Bd -literal
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 */
.Ed
.Pp
The members of this structure are:
.Bl -tag -width h_addr_list
.It Va h_name
Official name of the host.
.It Va h_aliases
A
.Dv NULL Ns -terminated
array of alternate names for the host.
.It Va h_addrtype
The type of address being returned; usually
.Dv AF_INET .
.It Va h_length
The length, in bytes, of the address.
.It Va h_addr_list
A
.Dv NULL Ns -terminated
array of network addresses for the host.
Host addresses are returned in network byte order.
.It Va h_addr
The first address in
.Va h_addr_list ;
this is for backward compatibility.
.El
.Pp
When using the nameserver,
.Fn gethostbyname
and
.Fn 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
.Dq Ev HOSTALIASES
contains the name of an alias file, the alias file will first be searched
for an alias matching the input name.
See
.Xr hostname 7
for the domain search procedure and the alias file format.
.Pp
The
.Fn gethostbyname2
function is an evolution of
.Fn gethostbyname
which is intended to allow lookups in address families other than
.Dv AF_INET ,
for example
.Dv AF_INET6 .
.Pp
The
.Fn sethostent
function
may be used to request the use of a connected
.Tn TCP
socket for queries.
If the
.Fa stayopen
flag is non-zero,
this sets the option to send all queries to the name server using
.Tn TCP
and to retain the connection after each call to
.Fn gethostbyname ,
.Fn gethostbyname2
or
.Fn gethostbyaddr .
Otherwise, queries are performed using
.Tn UDP
datagrams.
.Pp
The
.Fn endhostent
function
closes the
.Tn TCP
connection.
.Pp
The
.Fn herror
function writes a message to the diagnostic output consisting of the
string parameter
.Fa s ,
the constant string
.Qq Li ":\ " ,
and a message corresponding to the value of
.Va h_errno .
.Pp
The
.Fn hstrerror
function returns a string which is the message text corresponding to the
value of the
.Fa err
parameter.
.Sh FILES
.Bl -tag -width /etc/nsswitch.conf -compact
.It Pa /etc/hosts
.It Pa /etc/nsswitch.conf
.It Pa /etc/resolv.conf
.El
.Sh EXAMPLES
Print out the hostname associated with a specific IP address:
.Bd -literal -offset indent
const char *ipstr = "127.0.0.1";
struct in_addr ip;
struct hostent *hp;

if (!inet_aton(ipstr, &ip))
	errx(1, "can't parse IP address %s", ipstr);

if ((hp = gethostbyaddr((const char *)&ip,
    sizeof ip, AF_INET)) == NULL)
	errx(1, "no name associated with %s", ipstr);

printf("name associated with %s is %s\en", ipstr, hp->h_name);
.Ed
.Sh DIAGNOSTICS
Error return status from
.Fn gethostbyname ,
.Fn gethostbyname2
and
.Fn gethostbyaddr
is indicated by return of a
.Dv NULL
pointer.
The external integer
.Va h_errno
may then be checked to see whether this is a temporary failure
or an invalid or unknown host.
The routine
.Fn herror
can be used to print an error message describing the failure.
If its argument
.Fa string
is
.Pf non- Dv NULL ,
it is printed, followed by a colon and a space.
The error message is printed with a trailing newline.
.Pp
The variable
.Va h_errno
can have the following values:
.Bl -tag -width HOST_NOT_FOUND
.It Dv HOST_NOT_FOUND
No such host is known.
.It Dv 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.
.It Dv NO_RECOVERY
Some unexpected server failure was encountered.
This is a non-recoverable error.
.It Dv 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.
.El
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr getnameinfo 3 ,
.Xr inet_aton 3 ,
.Xr resolver 3 ,
.Xr hosts 5 ,
.Xr hostname 7 ,
.Xr named 8
.Sh CAVEAT
The
.Fn gethostent
function
is defined, and
.Fn sethostent
and
.Fn endhostent
are redefined,
when
.Xr libc 3
is built to use only the routines to lookup in
.Pa /etc/hosts
and not the name server.
.Pp
The
.Fn gethostent
function
reads the next line of
.Pa /etc/hosts ,
opening the file if necessary.
.Pp
The
.Fn sethostent
function
opens and/or rewinds the file
.Pa /etc/hosts .
If the
.Fa stayopen
argument is non-zero,
the file will not be closed after each call to
.Fn gethostbyname ,
.Fn gethostbyname2
or
.Fn gethostbyaddr .
.Pp
The
.Fn endhostent
function
closes the file.
.Sh HISTORY
The
.Fn herror
function appeared in
.Bx 4.3 .
The
.Fn endhostent ,
.Fn gethostbyaddr ,
.Fn gethostbyname ,
.Fn gethostent ,
and
.Fn sethostent
functions appeared in
.Bx 4.2 .
The
.Fn gethostbyname2
function first appeared in
.Tn BIND
version 4.9.4.
.Sh BUGS
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.
.Pp
.Fn gethostbyname2
cannot perform
.Dv AF_INET6
lookups over NIS.
.Xr getaddrinfo 3
must be used instead.