Flash Socket Code and crossdomain Policy Serving

I’ve just spent the past day trying to get my flash app talking to another device on my network via socket 23. I found some sample telnet code (which operates on port 23) and allowed me to “talk” to the RFID reader. It worked fine as a new project in Flex Builder and being served from a local file. The moment I served the application from a web server (tomcat) on my laptop, I get crossdomain issues. Flash won’t open a socket that is different from the one that served your application unless that socket authorizes it. I will spare you the details that took many hours of my day. If you’re trying to talk to another web server on a different port, no problem.. just put the crossdomain.xml file on that server that authorizes the connection. In this case, I was trying to connect to another host and another port (which runs telnet, not http). The RFID reader can’t be modified to serve up a crossdomain.xml file, so I had to get creative.

My solution was to run a TCP proxy on my web server machine that proxied requests to the RFID reader. I made it listen on port 8023 and forward requests to 23 on the RFID reader. This was the start because I still got errors about that localhost:8023  not being authorized. It turns out that when you try the connection, flash connects to the socket and sends 23 bytes which contain “<policy-file-request/>”. Flash expects whatever is running at that port to respond with the policy string (that would have been in the crossdomain.xml file). So, I modified this little proxy class I got off the internet to recognize the proxy request and respond with a proxy string (null terminated.. that is very important!). Once I had this set up right, I was able to communicate from my flash app to my RFID reader. Not the most elegant solution, but this is something temporary for a demo.


To run the code below, compile with javac and invoke “java -classpath <class.file.location> ProxyThread 8023 23”. Those options are what I used to talk to my RFID reader, but you’ll likely use different values.

import java.net.*;
import java.io.*;
  Java Transparent Proxy
  Copyright (C) 1999 by Didier Frick (http://www.dfr.ch/)
  This software is provided under the GNU general public license (http://www.gnu.org/copyleft/gpl.html).

public class ProxyThread extends Thread {
     protected class StreamCopyThread extends Thread {
	private Socket inSock;
	private Socket outSock;
	private boolean done=false;
	private StreamCopyThread peer;
	private boolean inFromLocal;	// in from local port
	private OutputStream out;
	private String policy = "<cross-domain-policy>\n<allow-access-from domain=\"*\" to-ports=\"8023\"/>\n</cross-domain-policy>";
	public StreamCopyThread(Socket inSock, Socket outSock, boolean in) {
	    this.inFromLocal = in;
	public void sendPolicy() {
		try {
			System.err.println("Sent policy");
		} catch (IOException ex) {
			System.err.println("Error sending policy file");
	public void run() {
	    byte[] buf=new byte[bufSize];
	    int count=-1;
	    try {
		InputStream in=inSock.getInputStream();
		try {
		    while(((count=in.read(buf))>0)&&!isInterrupted()) {
		    	if (inFromLocal && count==23 && new String(buf).startsWith("<policy-file-request/>")) {
				// send policy file back.. don't forward this to other port
				System.err.println("Got policy request");
			else {
				//System.err.println(count+" bytes "+(inFromLocal?"sent":"received"));
		} catch(Exception xc) {
		    if(debug) {
			// FIXME
			// It's very difficult to sort out between "normal"
			// exceptions (occuring when one end closes the connection
			// normally), and "exceptional" exceptions (when something
			// really goes wrong)
			// Therefore we only log exceptions occuring here if the debug flag
			// is true, in order to avoid cluttering up the log.
		} finally {
		    // The input and output streams will be closed when the sockets themselves
		    // are closed.
	    } catch(Exception xc) {
	    synchronized(lock) {
		try {
		    if((peer==null)||peer.isDone()) {
			// Cleanup if there is only one peer OR
			// if _both_ peers are done
			// Signal the peer (if any) that we're done on this side of the connection
		} catch(Exception xc) {
		} finally {
	public boolean isDone() {
	    return done;
	public void setPeer(StreamCopyThread peer) {

    // Holds all the currently active StreamCopyThreads
    private java.util.Vector connections=new java.util.Vector();
    // Used to synchronize the connection-handling threads with this thread
    private Object lock=new Object();
    // The address to forward connections to
    private InetAddress dstAddr;
    // The port to forward connections to
    private int dstPort;
    // Backlog parameter used when creating the ServerSocket
    protected static final int backLog=100;
    // Timeout waiting for a StreamCopyThread to finish
    public static final int threadTimeout=2000; //ms
    // Linger time
    public static final int lingerTime=180; //seconds (?)
    // Size of receive buffer
    public static final int bufSize=2048;
    // Header to prepend to log messages
    private String header;
    // This proxy's server socket
    private ServerSocket srvSock;
    // Debug flag
    private boolean debug=false;
    // Log streams for output and error messages
    private PrintStream out;
    private PrintStream err;
    private static final String 
	argsMessage="Arguments: ( [source_address] source_port dest_address dest_port ) | config_file";
    private static final String 

    public ProxyThread(InetAddress srcAddr,int srcPort,
		       InetAddress dstAddr,int dstPort, PrintStream out, PrintStream err) 
	throws IOException {
	this.srvSock=(srcAddr==null) ? new ServerSocket(srcPort,backLog) :  
	    new ServerSocket(srcPort,backLog,srcAddr);
	this.header=(srcAddr==null ? "" : srcAddr.toString())+":"+srcPort+" <-> "+dstAddr+":"+dstPort;
    public void run() {
	out.println(header+" : starting");
	try {
	    while(!isInterrupted()) {
		Socket serverSocket=srvSock.accept();
		try {
		    Socket clientSocket=new Socket(dstAddr,dstPort);
		    StreamCopyThread sToC=new StreamCopyThread(serverSocket,clientSocket, true);
		    StreamCopyThread cToS=new StreamCopyThread(clientSocket,serverSocket, false);
		    synchronized(lock) {
		} catch(Exception xc) {
	} catch(IOException xc) {
	} finally {
	    out.println(header+" : stopped");
     private void cleanup() {
	synchronized(lock) {
	    try {
		while(connections.size()>0) {
		    StreamCopyThread sct=(StreamCopyThread)connections.elementAt(0);
	    } catch(InterruptedException xc) {
    private static ProxyThread addProxy(String src,String srcPort, String dst, String dstPort,
					PrintStream out, PrintStream err) throws
					UnknownHostException, IOException
	InetAddress srcAddr=(src==null) ? null : InetAddress.getByName(src);
	return new ProxyThread(srcAddr,Integer.parseInt(srcPort),
    private static java.util.Vector parseConfigFile(String fileName,PrintStream out,PrintStream err) throws 
        FileNotFoundException, IOException, UnknownHostException
	java.util.Vector result=new java.util.Vector();
	FileInputStream in=new FileInputStream(fileName);
	java.util.Properties props= new java.util.Properties();
	for(int i=0;;i++) {
	    String srcAddr=props.getProperty(propertyPrefix+"."+i+".sourceAddr");
	    String srcPort=props.getProperty(propertyPrefix+"."+i+".sourcePort");
	    String dstAddr=props.getProperty(propertyPrefix+"."+i+".destAddr");
	    String dstPort=props.getProperty(propertyPrefix+"."+i+".destPort");
	    if(dstAddr==null) {
		throw new IllegalArgumentException("Missing destination address for proxy "+i);
	    if(dstPort==null) {
		throw new IllegalArgumentException("Missing destination port for proxy "+i);
	return result;
    static java.util.Vector parseArguments(String[] argv,PrintStream out,PrintStream err) throws
        FileNotFoundException, IOException, UnknownHostException
	java.util.Vector result=null;
	int argBase=0;
	String src=null;
	if(argv.length>1) {
	    if(argv.length>3) {
	    result=new java.util.Vector();
	} else if(argv.length==1) {
	} else {
	    throw new IllegalArgumentException(argsMessage);
	return result;
    public static void main(String[] argv) throws Exception {
	System.out.println("Java Transparent Proxy");
	System.out.println("Copyright (C) 1999 by Didier Frick (http://www.dfr.ch/)");
	System.out.println("This software is provided under the GNU general public license"+
			   " (http://www.gnu.org/copyleft/gpl.html)");
	try {
	} catch(IllegalArgumentException xc) {
The initial ProxyThread code came from here: http://www.dfr.ch/en/proxy.html

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s