.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

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.\" 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

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.\" 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.\" manual page may be incorrect or out-of-date.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

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.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" 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 2017-09-15 "" "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;

.PP

.BI "struct hostent *gethostbyname(const char *" name );

.BR "#include <sys/socket.h>" "       /* for AF_INET */"

.BI "struct hostent *gethostbyaddr(const void *" addr ,

.BI "                              socklen_t " len ", int " type );

.PP

.BI "void sethostent(int " stayopen );

.PP

.B void endhostent(void);

.PP

.BI "void herror(const char *" s );

.PP

.BI "const char *hstrerror(int " err );

.PP

/* System V/POSIX extension */

.B struct hostent *gethostent(void);

.PP

/* GNU extensions */

.BI "struct hostent *gethostbyname2(const char *" name ", int " af );

.PP

.B "int gethostent_r("

.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,

.BI "        struct hostent **" result ", int *" h_errnop );

.PP

.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 );

.PP

.BI "int gethostbyname_r(const char *" name ,

.BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,

.BI "        struct hostent **" result ", int *" h_errnop );

.PP

.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

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.PD 0

.ad l

.BR gethostbyname2 (),

.BR gethostent_r (),

.BR gethostbyaddr_r (),

.BR gethostbyname_r (),

.BR gethostbyname2_r ():

.RS 4

.TP 4

Since glibc 2.19:

_DEFAULT_SOURCE

.TP 4

Glibc versions up to and including 2.19:

_BSD_SOURCE || _SVID_SOURCE

.RE

.PD

.PP

.PD 0

.BR herror (),

.BR hstrerror ():

.RS 4

.TP 4

Since glibc 2.19:

_DEFAULT_SOURCE

.TP 4

Glibc 2.8 to 2.19:

_BSD_SOURCE || _SVID_SOURCE

.TP

Before glibc 2.8:

none

.RE

.PD

.PP

.PD 0

.BR h_errno :

.RS 4

.TP 4

Since glibc 2.19

_DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L

.TP 4

Glibc 2.12 to 2.19:

_BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L

.TP

Before glibc 2.12:

none

.RE

.ad b

.PD

.SH DESCRIPTION

The

.BR gethostbyname* (),

.BR gethostbyaddr* (),

.BR herror (),

and

.BR hstrerror ()

functions are obsolete.

Applications should use

.BR getaddrinfo (3),

.BR getnameinfo (3),

and

.BR gai_strerror (3)

instead.

.PP

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)).

If

.I name

is an IPv4 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 ()

rely on the Name Service Switch

.RB ( nsswitch.conf (5))

configured sources or a local name server

.RB ( named (8)).

The default action is to query the Name Service Switch

.RB ( nsswitch.conf(5))

configured sources, failing that, a local name server

.RB ( named (8)).

.\"

.SS Historical

The

.BR nsswitch.conf (5)

file is the modern way of controlling the order of host lookups.

.PP

In glibc 2.4 and earlier, the

.I order

keyword was used to control the order of host lookups as defined in

.IR /etc/host.conf

.RB ( host.conf (5)).

.PP

.PP

The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:

.PP

.in +4n

.EX

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 */

.EE

.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_DATA

The requested name is valid but does not have an IP address.

Another type of request to the name server for this domain

may return an answer.

The constant

.BR NO_ADDRESS

is a synonym for

.BR NO_DATA .

.TP

.B NO_RECOVERY

A nonrecoverable 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 ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw18 lb lbw29

l l l.

Interface	Attribute	Value

T{

.BR gethostbyname ()

T}	Thread safety	T{

MT-Unsafe race:hostbyname env

.br

locale

T}

T{

.BR gethostbyaddr ()

T}	Thread safety	T{

MT-Unsafe race:hostbyaddr env

.br

locale

T}

T{

.BR sethostent (),

.br

.BR endhostent (),

.br

.BR gethostent_r ()

T}	Thread safety	T{

MT-Unsafe race:hostent env

.br

locale

T}

T{

.BR herror (),

.br

.BR hstrerror ()

T}	Thread safety	MT-Safe

T{

.BR gethostent ()

T}	Thread safety	T{

MT-Unsafe race:hostent

.br

race:hostentbuf env locale

T}

T{

.BR gethostbyname2 ()

T}	Thread safety	T{

MT-Unsafe race:hostbyname2

.br

env locale

T}

T{

.BR gethostbyaddr_r (),

.BR gethostbyname_r (),

.BR gethostbyname2_r ()

T}	Thread safety	MT-Safe env locale

.TE

.sp 1

In the above table,

.I hostent

in

.I race:hostent

signifies that if any of the functions

.BR sethostent (),

.BR gethostent (),

.BR gethostent_r (),

or

.BR endhostent ()

are used in parallel in different threads of a program,

then data races could occur.

.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.

.PP

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).

.PP

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, which 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.

.PP

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 nonzero error number on failure.

In addition to the errors returned by the nonreentrant

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 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.