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.\"     From: @(#)gethostbyname.3       8.4 (Berkeley) 5/25/95

.\"     $Id: gethostbyname.3,v 1.2 2001-09-27 12:01:53 chris Exp $

.\"

.Dd May 25, 1995

.Dt GETHOSTBYNAME 3

.Os BSD 4.2

.Sh NAME

.Nm gethostbyname ,

.Nm gethostbyname2 ,

.Nm gethostbyaddr ,

.Nm gethostent ,

.Nm sethostent ,

.Nm endhostent ,

.Nm herror ,

.Nm hstrerror

.Nd get network host entry

.Sh SYNOPSIS

.Fd #include 

.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

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.

This structure contains either the information obtained from the name server,

.Xr named 8 ,

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

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 Fa h_name

Official name of the host.

.It Fa h_aliases

A NULL-terminated array of alternate names for the host.

.It Fa h_addrtype

The type of address being returned; usually

.Dv AF_INET .

.It Fa h_length

The length, in bytes, of the address.

.It Fa h_addr_list

A NULL-terminated array of network addresses for the host.

Host addresses are returned in network byte order.

.It Fa h_addr

The first address in

.Fa h_addr_list ;

this is for backward compatibility.

.El

.Pp

When using the nameserver,

.Fn gethostbyname

and

.Fn gethostbyname

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 .

Currently the

.Fa af

argument must be specified as

.Dv AF_INET

else the function will return

.Dv NULL

after having set

.Va h_errno

to

.Dv NETDB_INTERNAL

.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 ": ", 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/resolv.conf -compact

.It Pa /etc/hosts

.It Pa /etc/host.conf

.It Pa /etc/resolv.conf

.El

.Sh DIAGNOSTICS

Error return status from

.Fn gethostbyname ,

.Fn gethostbyname2

and

.Fn gethostbyaddr

is indicated by return of a 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 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 bind-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.