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.\"     @(#)gethostbyname.3   8.4 (Berkeley) 5/25/95
.\"
.Dd August 19, 2013
.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
.Fd 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 void *addr" "socklen_t 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.
.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 alternative names for the host.
.It Fa h_addrtype
The type of address being returned; currently always
.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
In the case of
.Fn gethostbyname
and
.Fn gethostbyname2 ,
the host is specified by name,
or using a string representation of a numeric address.
In the case of
.Fn gethostbyaddr ,
the host is specified using a binary representation of an address.
.Pp
The returned
.Ft "struct hostent"
structure may contain
the result of a simple string to binary conversion,
information obtained from the domain name resolver (see
.Xr resolver 3 ) ,
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 .
.Pp
When using the domain name resolver,
.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 .
Currently the
.Fa af
argument must be specified as
.Dv AF_INET
or
.Dv AF_INET6 ,
else the function will return
.Dv NULL
after having set
.Va h_errno
to
.Dv NETDB_INTERNAL .
.Pp
The
.Fn gethostent
function
reads the next line of the
.Pa /etc/hosts
file, opening the file if necessary.
.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/hosts -compact
.It Pa /etc/hosts
.El
.Sh RETURN VALUES
Error return status from
.Fn gethostbyent ,
.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 nsswitch.conf 5 ,
.Xr hostname 7 ,
.Xr named 8
.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.
IPv6 support was implemented in WIDE Hydrangea IPv6 protocol stack kit.
.Sh CAVEATS
If the search routines specified in
.Xr nsswitch.conf 5
decide to read the
.Pa /etc/hosts
file,
.Fn gethostbyname ,
.Fn gethostbyname2 ,
and
.Fn gethostbyaddr
will
read the next line of the file,
re-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 ,
.Fn gethostbyaddr ,
or
.Fn gethostent .
.Pp
The
.Fn endhostent
function closes the file.
.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
The
.Fn gethostent
does not currently follow the search order specified in
.Xr nsswitch.conf 5
and only reads the
.Pa /etc/hosts
file.