freebsd-skq/contrib/bind/doc/man/gethostbyname.3

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.\"	@(#)gethostbyname.3	6.12 (Berkeley) 6/23/90
.\"
.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,
.Xr @INDOT@named @SYS_OPS_EXT@ ,
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 @INDOT@named @SYS_OPS_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.