\"      $NetBSD: gethostbyname.3,v 1.1.1.2 2012/09/09 16:07:45 christos Exp $
\"
\" Copyright (C) 2009  Internet Systems Consortium, Inc. ("ISC")
\"
\" Permission to use, copy, modify, and/or distribute this software for any
\" purpose with or without fee is hereby granted, provided that the above
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\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
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\" Id: gethostbyname.3,v 1.3 2009/01/22 23:49:23 tbox Exp
\"
Dd June 23, 1990
Dt GETHOSTBYNAME @LIB_NETWORK_EXT_U@
Os BSD 4
Sh NAME
Nm gethostbyname ,
Nm gethostbyaddr ,
Nm gethostent ,
Nm sethostent ,
Nm endhostent ,
Nm herror
Nd get network host entry
Sh SYNOPSIS
Fd #include <netdb.h>
Ft extern int
Fa h_errno ;
Pp
Ft struct hostent *
Fn gethostbyname "char *name"
Ft struct hostent *
Fn gethostbyname2 "char *name" "int af"
Ft struct hostent *
Fn gethostbyaddr "char *addr" "int len, type"
Ft struct hostent *
Fn gethostent
Fn sethostent "int stayopen"
Fn endhostent
Fn herror "char *string"
Sh DESCRIPTION
Fn Gethostbyname ,
Fn gethostbyname2 ,
and
Fn gethostbyaddr
each return a pointer to a
Ft 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,
or broken-out fields from a line in
Pa /etc/hosts .
If the local name server is not running, these routines do a lookup in
Pa /etc/hosts .
Bd -literal -offset indent
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 h_name
Official name of the host.
It h_aliases
A zero-terminated array of alternate names for the host.
It h_addrtype
The type of address being returned; usually
Dv AF_INET .
It h_length
The length, in bytes, of the address.
It h_addr_list
A zero-terminated array of network addresses for the host.
Host addresses are returned in network byte order.
It h_addr
The first address in
Li h_addr_list ;
this is for backward compatibility.
El
Pp
When using the nameserver,
Fn gethostbyname
will search for the named host in each parent domain given in the
Dq Li search
directive of
Xr resolv.conf @FORMAT_EXT@
unless the name contains a dot
Pq Dq \&. .
If the name contains no dot, and if the environment variable
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 @DESC_EXT@
for the domain search procedure and the alias file format.
Pp
Fn Gethostbyname2
is an evolution of
Fn gethostbyname
intended to allow lookups in address families other than
Dv AF_INET ,
for example,
Dv AF_INET6 .
Currently, the
Fa af
argument must be specified as
Dv AF_INET
else the function will return
Dv NULL
after having set
Ft h_errno
to
Dv NETDB_INTERNAL .
Pp
Fn Sethostent
may be used to request the use of a connected 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 TCP
and to retain the connection after each call to
Fn gethostbyname
or
Fn gethostbyaddr .
Otherwise, queries are performed using UDP datagrams.
Pp
Fn Endhostent
closes the TCP connection.
Sh ENVIRONMENT
Bl -tag -width "HOSTALIASES  " -compact
It Ev HOSTALIASES
Name of file containing
Pq Ar host alias , full hostname
pairs.
El
Sh FILES
Bl -tag -width "HOSTALIASES  " -compact
It Pa /etc/hosts
See
Xr hosts @FORMAT_EXT@ .
El
Sh DIAGNOSTICS
Pp
Error return status from
Fn gethostbyname
and
Fn gethostbyaddr
is indicated by return of a null pointer.
The external integer
Ft 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 non-NULL, it is printed, followed by a colon and a space.
The error message is printed with a trailing newline.
Pp
Ft h_errno
can have the following values:
Bl -tag -width "HOST_NOT_FOUND  " -offset indent
It Dv NETDB_INTERNAL
This indicates an internal error in the library, unrelated to the network
or name service.
Ft errno
will be valid in this case; see
Xr perror @SYSCALL_EXT@ .
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, as one might expect.
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 hosts @FORMAT_EXT@ ,
Xr hostname @DESC_EXT@ ,
Xr resolver @LIB_NETWORK_EXT@ ,
Xr resolver @FORMAT_EXT@ .
Sh CAVEAT
Pp
Fn Gethostent
is defined, and
Fn sethostent
and
Fn endhostent
are redefined,
when
Pa libc
is built to use only the routines to lookup in
Pa /etc/hosts
and not the name server:
Bd -ragged -offset indent
Pp
Fn Gethostent
reads the next line of
Pa /etc/hosts ,
opening the file if necessary.
Pp
Fn Sethostent
is redefined to open and rewind the file.  If the
Fa stayopen
argument is non-zero,
the hosts data base will not be closed after each call to
Fn gethostbyname
or
Fn gethostbyaddr .
Pp
Fn Endhostent
is redefined to close the file.
Ed
Sh 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.

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