/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.net.bsd;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.StandardCharsets;

import org.apache.commons.net.SocketClient;
import org.apache.commons.net.io.SocketInputStream;
import org.apache.commons.net.util.NetConstants;

/**
 * RExecClient implements the rexec() facility that first appeared in 4.2BSD Unix. This class will probably only be of use for connecting to UNIX systems and
 * only when the rexecd daemon is configured to run, which is a rarity these days because of the security risks involved. However, rexec() can be very useful
 * for performing administrative tasks on a network behind a firewall.
 * <p>
 * As with virtually all the client classes in org.apache.commons.net, this class derives from SocketClient, inheriting its connection methods. The way to
 * use RExecClient is to first connect to the server, call the {@link #rexec rexec()} 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 {@link #disconnect disconnect()} to clean up properly.
 * <p>
 * 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 {@link #getInputStream getInputStream()}. However, it is possible to tell the rexecd daemon to return the standard error stream over a separate
 * connection, readable from the input stream returned by {@link #getErrorStream getErrorStream()}. You can specify that a separate connection should be created
 * for standard error by setting the boolean <code>separateErrorStream</code> parameter of {@link #rexec rexec()} to <code>true</code>. The standard input
 * of the remote process can be written to through the output stream returned by {@link #getOutputStream getOutputSream()}.
 *
 * @see SocketClient
 * @see RCommandClient
 * @see RLoginClient
 */
public class RExecClient extends SocketClient {

    /**
     * @since 3.3
     */
    protected static final char NULL_CHAR = '\0';

    /**
     * The default rexec port. Set to 512 in BSD Unix.
     */
    public static final int DEFAULT_PORT = 512;

    private boolean remoteVerificationEnabled;

    /**
     * If a separate error stream is requested, <code>_errorStream_</code> will point to an InputStream from which the standard error of the remote process can
     * be read (after a call to rexec()). Otherwise, <code>_errorStream_</code> will be null.
     */
    protected InputStream _errorStream_;

    /**
     * The default RExecClient constructor. Initializes the default port to <code>DEFAULT_PORT</code>.
     */
    public RExecClient() {
        _errorStream_ = null;
        setDefaultPort(DEFAULT_PORT);
    }

    // This can be overridden in local package to implement port range
    // limitations of rcmd and rlogin
    InputStream createErrorStream() throws IOException {
        final Socket socket;

        try (final ServerSocket server = _serverSocketFactory_.createServerSocket(0, 1, getLocalAddress())) {
            _output_.write(Integer.toString(server.getLocalPort()).getBytes(StandardCharsets.UTF_8)); // $NON-NLS-1$
            _output_.write(NULL_CHAR);
            _output_.flush();
            socket = server.accept();
        }

        if (remoteVerificationEnabled && !verifyRemote(socket)) {
            socket.close();
            throw new IOException("Security violation: unexpected connection attempt by " + socket.getInetAddress().getHostAddress());
        }

        return new SocketInputStream(socket, socket.getInputStream());
    }

    /**
     * Disconnects from the server, closing all associated open sockets and streams.
     *
     * @throws IOException If an error occurs while disconnecting.
     */
    @Override
    public void disconnect() throws IOException {
        if (_errorStream_ != null) {
            _errorStream_.close();
        }
        _errorStream_ = null;
        super.disconnect();
    }

    /**
     * Returns the InputStream from which the standard error of the remote process can be read if a separate error stream is requested from the server.
     * Otherwise, null will be returned. The error stream will only be set after a successful rexec() invocation.
     *
     * @return The InputStream from which the standard error of the remote process can be read if a separate error stream is requested from the server.
     *         Otherwise, null will be returned.
     */
    public InputStream getErrorStream() {
        return _errorStream_;
    }

    /**
     * Returns the InputStream from which the standard output of the remote process can be read. The input stream will only be set after a successful rexec()
     * invocation.
     *
     * @return The InputStream from which the standard output of the remote process can be read.
     */
    public InputStream getInputStream() {
        return _input_;
    }

    /**
     * Returns the OutputStream through which the standard input of the remote process can be written. The output stream will only be set after a successful
     * rexec() invocation.
     *
     * @return The OutputStream through which the standard input of the remote process can be written.
     */
    public OutputStream getOutputStream() {
        return _output_;
    }

    /**
     * Return whether or not verification of the remote host providing a separate error stream is enabled. The default behavior is for verification to be
     * enabled.
     *
     * @return True if verification is enabled, false if not.
     */
    public final boolean isRemoteVerificationEnabled() {
        return remoteVerificationEnabled;
    }

    /**
     * Same as <code>rexec(user, password, command, false);</code>
     *
     * @param user the user name
     * @param password the password
     * @param command  the command to run
     * @throws IOException if an error occurs
     */
    public void rexec(final String user, final String password, final String command) throws IOException {
        rexec(user, password, command, false);
    }

    /**
     * Remotely executes a command through the rexecd daemon on the server to which the RExecClient 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 {@link #getInputStream getInputStream()}). Disconnecting from the server or closing
     * the process streams before reaching end of file will not necessarily terminate the remote process.
     * <p>
     * If a separate error stream is requested, the remote server will connect to a local socket opened by RExecClient, providing an independent stream through
     * which standard error will be transmitted. RExecClient will 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 {@link #setRemoteVerificationEnabled
     * setRemoteVerificationEnabled()} .
     *
     * @param user            The account name on the server through which to execute the command.
     * @param password            The plain text password of the user account.
     * @param command             The command, including any arguments, to execute.
     * @param separateErrorStream True if you would like the standard error to be transmitted through a different stream than standard output. False if not.
     * @throws IOException If the rexec() attempt fails. The exception will contain a message indicating the nature of the failure.
     */
    public void rexec(final String user, final String password, final String command, final boolean separateErrorStream) throws IOException {
        int ch;

        if (separateErrorStream) {
            _errorStream_ = createErrorStream();
        } else {
            _output_.write(NULL_CHAR);
        }

        _output_.write(user.getBytes(getCharset()));
        _output_.write(NULL_CHAR);
        _output_.write(password.getBytes(getCharset()));
        _output_.write(NULL_CHAR);
        _output_.write(command.getBytes(getCharset()));
        _output_.write(NULL_CHAR);
        _output_.flush();

        ch = _input_.read();
        if (ch > 0) {
            final StringBuilder buffer = new StringBuilder();

            while ((ch = _input_.read()) != NetConstants.EOS && ch != '\n') {
                buffer.append((char) ch);
            }

            throw new IOException(buffer.toString());
        }
        if (ch < 0) {
            throw new IOException("Server closed connection.");
        }
    }

    /**
     * Enable or disable verification that the remote host connecting to create a separate error stream is the same as the host to which the standard out stream
     * is connected. The default is for verification to be enabled. You may set this value at any time, whether the client is currently connected or not.
     *
     * @param enable True to enable verification, false to disable verification.
     */
    public final void setRemoteVerificationEnabled(final boolean enable) {
        remoteVerificationEnabled = enable;
    }

}