.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-05-22, David Metcalfe
.\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
.\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
.\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2002-08-05, Michael Kerrisk
.\" Modified 2004-10-31, Andries Brouwer
.\"
.TH GETHOSTBYNAME 3 2009-03-15 "" "Linux Programmer's Manual"
.SH NAME
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
h_errno,
herror, hstrerror,
gethostbyaddr_r,
gethostbyname2, gethostbyname2_r, gethostbyname_r,
gethostent_r \- get network host entry
.SH SYNOPSIS
.nf
.B #include <netdb.h>
.B extern int h_errno;
.sp
.BI "struct hostent *gethostbyname(const char *" name );
.sp
.BR "#include <sys/socket.h>" "       /* for AF_INET */"
.BI "struct hostent *gethostbyaddr(const void *" addr ,
.BI "                              socklen_t " len ", int " type );
.sp
.BI "void sethostent(int " stayopen );
.sp
.B void endhostent(void);
.sp
.BI "void herror(const char *" s );
.sp
.BI "const char *hstrerror(int " err );
.sp
/* System V/POSIX extension */
.br
.B struct hostent *gethostent(void);
.sp
/* GNU extensions */
.br
.BI "struct hostent *gethostbyname2(const char *" name ", int " af );
.sp
.B "int gethostent_r("
.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
.BI "        struct hostent **" result ", int *" h_errnop );
.sp
.BI "int gethostbyaddr_r(const void *" addr ", socklen_t " len ", int " type ,
.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
.BI "        struct hostent **" result ", int *" h_errnop );
.sp
.BI "int gethostbyname_r(const char *" name ,
.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
.BI "        struct hostent **" result ", int *" h_errnop );
.sp
.BI "int gethostbyname2_r(const char *" name ", int " af,
.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
.BI "        struct hostent **" result ", int *" h_errnop );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR gethostbyname2 (),
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
.BR gethostbyname2_r ():
_BSD_SOURCE || _SVID_SOURCE
.ad b
.SH DESCRIPTION
The
.BR gethostbyname* ()
and
.BR gethostbyaddr* ()
functions are obsolete.
Applications should use
.BR getaddrinfo (3)
and
.BR getnameinfo (3)
instead.

The
.BR gethostbyname ()
function returns a structure of type
.I hostent
for the given host
.IR name .
Here
.I name
is either a hostname, or an IPv4 address in standard dot notation (as for
.BR inet_addr (3)),
or an IPv6 address in colon (and possibly dot) notation.
(See RFC\ 1884 for the description of IPv6 addresses.)
If
.I name
is an IPv4 or IPv6 address, no lookup is performed and
.BR gethostbyname ()
simply copies
.I name
into the
.I h_name
field and its
.I struct in_addr
equivalent into the
.I h_addr_list[0]
field of the returned
.I hostent
structure.
If
.I name
doesn't end in a dot and the environment variable
.B HOSTALIASES
is set, the alias file pointed to by
.B HOSTALIASES
will first be searched for
.I name
(see
.BR hostname (7)
for the file format).
The current domain and its parents are searched unless \fIname\fP
ends in a dot.
.PP
The
.BR gethostbyaddr ()
function returns a structure of type \fIhostent\fP
for the given host address \fIaddr\fP of length \fIlen\fP and address type
\fItype\fP.
Valid address types are
.B AF_INET
and
.BR AF_INET6 .
The host address argument is a pointer to a struct of a type depending
on the address type, for example a \fIstruct in_addr *\fP (probably
obtained via a call to
.BR inet_addr (3))
for address type
.BR AF_INET .
.PP
The
.BR sethostent ()
function specifies, if \fIstayopen\fP is true (1),
that a connected TCP socket should be used for the name server queries and
that the connection should remain open during successive queries.
Otherwise, name server queries will use UDP datagrams.
.PP
The
.BR endhostent ()
function ends the use of a TCP connection for name
server queries.
.PP
The (obsolete)
.BR herror ()
function prints the error message associated
with the current value of \fIh_errno\fP on \fIstderr\fP.
.PP
The (obsolete)
.BR hstrerror ()
function takes an error number
(typically \fIh_errno\fP) and returns the corresponding message string.
.PP
The domain name queries carried out by
.BR gethostbyname ()
and
.BR gethostbyaddr ()
use a combination of any or all of the name server
.BR named (8),
a broken out line from \fI/etc/hosts\fP, and the Network
Information Service (NIS or YP), depending upon the contents of the
\fIorder\fP line in
.IR /etc/host.conf .
.\" (See
.\" .BR resolv+ (8)).
The default action is to query
.BR named (8),
followed by
.IR /etc/hosts .
.PP
The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
.sp
.in +4n
.nf
.ne 7
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 */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
.fi
.in
.PP
The members of the \fIhostent\fP structure are:
.TP
.I h_name
The official name of the host.
.TP
.I h_aliases
An array of alternative names for the host, terminated by a NULL pointer.
.TP
.I h_addrtype
The type of address; always
.B AF_INET
or
.B AF_INET6
at present.
.TP
.I h_length
The length of the address in bytes.
.TP
.I h_addr_list
An array of pointers to network addresses for the host (in network byte
order), terminated by a NULL pointer.
.TP
.I h_addr
The first address in \fIh_addr_list\fP for backward compatibility.
.SH "RETURN VALUE"
The
.BR gethostbyname ()
and
.BR gethostbyaddr ()
functions return the
.I hostent
structure or a NULL pointer if an error occurs.
On error, the
.I h_errno
variable holds an error number.
When non-NULL, the return value may point at static data, see the notes below.
.SH ERRORS
The variable \fIh_errno\fP can have the following values:
.TP
.B HOST_NOT_FOUND
The specified host is unknown.
.TP
.BR NO_ADDRESS " or " NO_DATA
The requested name is valid but does not have an IP address.
.TP
.B NO_RECOVERY
A non-recoverable name server error occurred.
.TP
.B TRY_AGAIN
A temporary error occurred on an authoritative name server.
Try again later.
.SH FILES
.TP
.I /etc/host.conf
resolver configuration file
.TP
.I /etc/hosts
host database file
.TP
.I /etc/nsswitch.conf
name service switch configuration
.SH "CONFORMING TO"
POSIX.1-2001 specifies
.BR gethostbyname (),
.BR gethostbyaddr (),
.BR sethostent (),
.BR endhostent (),
.BR gethostent (),
and
.IR h_errno ;
.BR gethostbyname (),
.BR gethostbyaddr (),
and
.IR h_errno
are marked obsolescent in that standard.
POSIX.1-2008 removes the specifications of
.BR gethostbyname (),
.BR gethostbyaddr (),
and
.IR h_errno ,
recommending the use of
.BR getaddrinfo (3)
and
.BR getnameinfo (3)
instead.
.SH NOTES
The functions
.BR gethostbyname ()
and
.BR gethostbyaddr ()
may return pointers to static data, which may be overwritten by
later calls.
Copying the
.I struct hostent
does not suffice, since it contains pointers; a deep copy is required.
.LP
In the original BSD implementation the
.I len
argument
of
.BR gethostbyname ()
was an
.IR int .
The SUSv2 standard is buggy and declares the
.I len
argument of
.BR gethostbyaddr ()
to be of type
.IR size_t .
(That is wrong, because it has to be
.IR int ,
and
.I size_t
is not.
POSIX.1-2001 makes it
.IR socklen_t ,
which is OK.)
See also
.BR accept (2).
.LP
The BSD prototype for
.BR gethostbyaddr ()
uses
.I const char *
for the first argument.
.SS "System V/POSIX Extension"
POSIX requires the
.BR gethostent ()
call, that should return the next entry in the host data base.
When using DNS/BIND this does not make much sense, but it may
be reasonable if the host data base is a file that can be read
line by line.
On many systems a routine of this name reads
from the file
.IR /etc/hosts .
.\" e.g., Linux, FreeBSD, Unixware, HP-UX
It may be available only when the library was built without DNS support.
.\" e.g., FreeBSD, AIX
The glibc version will ignore ipv6 entries.
This function is not reentrant,
and glibc adds a reentrant version
.BR gethostent_r ().
.SS "GNU Extensions"
Glibc2 also has a
.BR gethostbyname2 ()
that works like
.BR gethostbyname (),
but permits to specify the address family to which the address must belong.
.LP
Glibc2 also has reentrant versions
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r ()
and
.BR gethostbyname2_r ().
The caller supplies a
.I hostent
structure
.I ret
which will be filled in on success, and a temporary work buffer
.I buf
of size
.IR buflen .
After the call,
.I result
will point to the result on success.
In case of an error
or if no entry is found
.I result
will be NULL.
The functions return 0 on success and a non-zero error number on failure.
In addition to the errors returned by the non-reentrant
versions of these functions, if
.I buf
is too small, the functions will return
.BR ERANGE ,
and the call should be retried with a larger buffer.
The global variable
.I h_errno
is not modified, but the address of a variable in which to store error numbers
is passed in
.IR h_errnop .
.SH BUGS
.BR gethostbyname ()
does not recognize components of a dotted IPv4 address string
that are expressed in hexadecimal.
.\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
.SH "SEE ALSO"
.BR getaddrinfo (3),
.\" .BR getipnodebyaddr (3),
.\" .BR getipnodebyname (3),
.BR getnameinfo (3),
.BR inet (3),
.BR inet_ntop (3),
.BR inet_pton (3),
.BR resolver (3),
.BR hosts (5),
.BR nsswitch.conf (5),
.BR hostname (7),
.BR named (8)
.\" .BR resolv+ (8)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
