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.\"     From: @(#)rcmd.3        8.1 (Berkeley) 6/4/93
.\"     $Id: rcmd.3,v 1.2 2001-09-27 12:01:53 chris Exp $
.\"
.Dd February 15, 1996
.Dt RCMD 3
.Os BSD 4.2
.Sh NAME
.Nm rcmd ,
.Nm rresvport ,
.Nm iruserok ,
.Nm ruserok
.Nd routines for returning a stream to a remote command
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn rcmd "char **ahost" "int inport" "const char *locuser" "const char *remuser" "const char *cmd" "int *fd2p"
.Ft int
.Fn rresvport "int *port"
.Ft int
.Fn iruserok "u_long raddr" "int superuser" "const char *ruser" "const char *luser"
.Ft int
.Fn ruserok "const char *rhost" "int superuser" "const char *ruser" "const char *luser"
.Sh DESCRIPTION
The
.Fn rcmd
function
is used by the super-user to execute a command on
a remote machine using an authentication scheme based
on reserved port numbers.
The
.Fn rresvport
function
returns a descriptor to a socket
with an address in the privileged port space.
The
.Fn ruserok
function
is used by servers
to authenticate clients requesting service with
.Fn rcmd .
All three functions are present in the same file and are used
by the
.Xr rshd 8
server (among others).
.Pp
The
.Fn rcmd
function
looks up the host
.Fa *ahost
using
.Xr gethostbyname 3 ,
returning \-1 if the host does not exist.
Otherwise
.Fa *ahost
is set to the standard name of the host
and a connection is established to a server
residing at the well-known Internet port
.Fa inport .
.Pp
If the connection succeeds,
a socket in the Internet domain of type
.Dv SOCK_STREAM
is returned to the caller, and given to the remote
command as 
.Em stdin
and
.Em stdout .
If
.Fa fd2p
is non-zero, then an auxiliary channel to a control
process will be set up, and a descriptor for it will be placed
in
.Fa *fd2p .
The control process will return diagnostic
output from the command (unit 2) on this channel, and will also
accept bytes on this channel as being
.Tn UNIX
signal numbers, to be
forwarded to the process group of the command.
If
.Fa fd2p
is 0, then the 
.Em stderr
(unit 2 of the remote
command) will be made the same as the 
.Em stdout
and no
provision is made for sending arbitrary signals to the remote process,
although you may be able to get its attention by using out-of-band data.
.Pp
The protocol is described in detail in
.Xr rshd 8 .
.Pp
The
.Fn rresvport
function is used to obtain a socket with a privileged
address bound to it.  This socket is suitable for use
by 
.Fn rcmd
and several other functions.  Privileged Internet ports are those
in the range 0 to 1023.  Only the super-user
is allowed to bind an address of this sort to a socket.
.Pp
The
.Fn iruserok
and
.Fn ruserok
functions take a remote host's IP address or name, as returned by the
.Xr gethostbyname 3
routines, two user names and a flag indicating whether the local user's
name is that of the super-user.
Then, if the user is
.Em NOT
the super-user, it checks the
.Pa /etc/hosts.equiv
file.
If that lookup is not done, or is unsuccessful, the
.Pa .rhosts
in the local user's home directory is checked to see if the request for
service is allowed.
.Pp
If this file does not exist, is not a regular file, is owned by anyone
other than the user or the super-user, or is writable by anyone other
than the owner, the check automatically fails.
Zero is returned if the machine name is listed in the
.Dq Pa hosts.equiv
file, or the host and remote user name are found in the
.Dq Pa .rhosts
file; otherwise
.Fn iruserok
and
.Fn ruserok
return \-1.
If the local domain (as obtained from
.Xr gethostname 3 )
is the same as the remote domain, only the machine name need be specified.
.Pp
The
.Fn iruserok
function is strongly preferred for security reasons.
It requires trusting the local DNS at most, while the
.Fn ruserok
function requires trusting the entire DNS, which can be spoofed.
.Sh DIAGNOSTICS
The
.Fn rcmd
function
returns a valid socket descriptor on success.
It returns \-1 on error and prints a diagnostic message on the standard error.
.Pp
The
.Fn rresvport
function
returns a valid, bound socket descriptor on success.
It returns \-1 on error with the global value
.Va errno
set according to the reason for failure.
The error code
.Dv EAGAIN
is overloaded to mean ``All network ports in use.''
.Sh SEE ALSO
.Xr rlogin 1 ,
.Xr rsh 1 ,
.Xr intro 2 ,
.Xr rexec 3 ,
.Xr rexecd 8 ,
.Xr rlogind 8 ,
.Xr rshd 8
.Sh HISTORY
These
functions appeared in 
.Bx 4.2 .