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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 

.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 .