RCommandClient is very similar to
RExecClient
,
from which it is derived, and implements the rcmd() facility that
first appeared in 4.2BSD Unix. rcmd() is the facility used by the rsh
(rshell) and other commands to execute a command on another machine
from a trusted host without issuing a password. The trust relationship
between two machines is established by the contents of a machine's
/etc/hosts.equiv file and a user's .rhosts file. These files specify
from which hosts and accounts on those hosts rcmd() requests will be
accepted. The only additional measure for establishing trust is that
all client connections must originate from a port between 512 and 1023.
Consequently, there is an upper limit to the number of rcmd connections
that can be running simultaneously. The required ports are reserved
ports on Unix systems, and can only be bound by a
process running with root permissions (to accomplish this rsh, rlogin,
and related commands usualy have the suid bit set). Therefore, on a
Unix system, you will only be able to successfully use the RCommandClient
class if the process runs as root. However, there is no such restriction
on Windows95 and some other systems. The security risks are obvious.
However, when carefully used, rcmd() can be very useful when used behind
a firewall.
As with virtually all of the client classes in org.apache.commons.net, this
class derives from SocketClient. But it overrides most of its connection
methods so that the local Socket will originate from an acceptable
rshell port. The way to use RCommandClient is to first connect
to the server, call the
rcommand()
method,
and then
fetch the connection's input, output, and optionally error streams.
Interaction with the remote command is controlled entirely through the
I/O streams. Once you have finished processing the streams, you should
invoke
disconnect()
to clean up properly.
By default the standard output and standard error streams of the
remote process are transmitted over the same connection, readable
from the input stream returned by
getInputStream()
. However, it is
possible to tell the rshd daemon to return the standard error
stream over a separate connection, readable from the input stream
returned by
getErrorStream()
. You
can specify that a separate connection should be created for standard
error by setting the boolean
separateErrorStream
parameter of
rcommand()
to
true
.
The standard input of the remote process can be written to through
the output stream returned by
getOutputStream()
.
connect
public void connect(InetAddress host,
int port,
InetAddress localAddr)
throws SocketException,
BindException,
IOException
Opens a Socket connected to a remote host at the specified port and
originating from the specified local address using a port in a range
acceptable to the BSD rshell daemon.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
host
- The remote host.port
- The port to connect to on the remote host.localAddr
- The local address to use.
connect
public void connect(String hostname,
int port,
InetAddress localAddr)
throws SocketException,
IOException
Opens a Socket connected to a remote host at the specified port and
originating from the specified local address using a port in a range
acceptable to the BSD rshell daemon.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
hostname
- The remote host.port
- The port to connect to on the remote host.localAddr
- The local address to use.
rcommand
public void rcommand(String localUsername,
String remoteUsername,
String command)
throws IOException
Same as
rcommand(localUsername, remoteUsername, command, false);
rcommand
public void rcommand(String localUsername,
String remoteUsername,
String command,
boolean separateErrorStream)
throws IOException
Remotely executes a command through the rshd daemon on the server
to which the RCommandClient is connected. After calling this method,
you may interact with the remote process through its standard input,
output, and error streams. You will typically be able to detect
the termination of the remote process after reaching end of file
on its standard output (accessible through
getInputStream()
. Disconnecting
from the server or closing the process streams before reaching
end of file will not necessarily terminate the remote process.
If a separate error stream is requested, the remote server will
connect to a local socket opened by RCommandClient, providing an
independent stream through which standard error will be transmitted.
The local socket must originate from a secure port (512 - 1023),
and rcommand() ensures that this will be so.
RCommandClient will also do a simple security check when it accepts a
connection for this error stream. If the connection does not originate
from the remote server, an IOException will be thrown. This serves as
a simple protection against possible hijacking of the error stream by
an attacker monitoring the rexec() negotiation. You may disable this
behavior with
setRemoteVerificationEnabled()
.
localUsername
- The user account on the local machine that is
requesting the command execution.remoteUsername
- The account name on the server through which to
execute the command.command
- The command, including any arguments, to execute.separateErrorStream
- True if you would like the standard error
to be transmitted through a different stream than standard output.
False if not.
void connect
public @Override void connect(InetAddress host,
int port)
throws SocketException,
IOException
Opens a Socket connected to a remote host at the specified port and
originating from the current host at a port in a range acceptable
to the BSD rshell daemon.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
host
- The remote host.port
- The port to connect to on the remote host.
void connect
public @Override void connect(InetAddress host,
int port,
InetAddress localAddr,
int localPort)
throws SocketException,
IOException,
IllegalArgumentException
Opens a Socket connected to a remote host at the specified port and
originating from the specified local address and port. The
local port must lie between
MIN_CLIENT_PORT
and
MAX_CLIENT_PORT
or an IllegalArgumentException will
be thrown.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
host
- The remote host.port
- The port to connect to on the remote host.localAddr
- The local address to use.localPort
- The local port to use.
void connect
public @Override void connect(String hostname,
int port)
throws SocketException,
IOException
Opens a Socket connected to a remote host at the specified port and
originating from the current host at a port in a range acceptable
to the BSD rshell daemon.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
hostname
- The name of the remote host.port
- The port to connect to on the remote host.
void connect
public @Override void connect(String hostname,
int port,
InetAddress localAddr,
int localPort)
throws SocketException,
IOException,
IllegalArgumentException
Opens a Socket connected to a remote host at the specified port and
originating from the specified local address and port. The
local port must lie between
MIN_CLIENT_PORT
and
MAX_CLIENT_PORT
or an IllegalArgumentException will
be thrown.
Before returning,
_connectAction_()
is called to perform connection initialization actions.
hostname
- The name of the remote host.port
- The port to connect to on the remote host.localAddr
- The local address to use.localPort
- The local port to use.