I'm trying to connect a socket to an non-existent server, and I really don't understand why an exception is not being raised.
Here is my code:
public class TestSocket extends Activity {
private static final String TAG = "TestSocket";
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
BasicThread t = new BasicThread();
t.start();
}
class BasicThread extends Thread {
#Override
public void run() {
Log.d(TAG, "Before");
try {
new Socket("42.42.42.42", 12345);
Log.d(TAG, "Connected");
} catch (Exception e) {
Log.d(TAG, "Exception");
}
Log.d(TAG, "After");
}
}
}
I also tried with my own IP address while running Wireshark, and I first get [SYN] from Android to my computer and then [RST, ACK] from my computer to Android (because nothing is listening at this port), but I still do not get an exception on Android.
Also I’m testing on a physical phone (Nexus S), and I do have the internet permission in my Manifest.
Why aren't I getting an Exception?
Edit:
More precisely, the output I get is
D/TestSocket(17745): Before
D/TestSocket(17745): Connected
D/TestSocket(17745): After
(and not Exception)
In the Socket constructor, it's thrown when the IP address of the host can't be determined, so I assume that because you aren't passing a hostname which needs resolution, a different exception would be getting thrown instead. I believe the exception actually comes from the URL class or such which does the resolution, and from nowhere else.
The connect(..) method should throw an exception but doesn't appear to, as you say.
Edit: apparently Android (some versions) doesn't work properly here, so it's probably a bug: http://code.google.com/p/android/issues/detail?id=6144. It doesn't look like the link refers to the emulator as I had thought.
There are a variety of things that can cause a socket connect to fail with an exception.
However, if the SYN message that the TCP protocol sends to start the connection process is
blocked by a firewall,
routed through a borked network, or
routed to some endpoint that doesn't respond,
then TCP stack on the initiating machine will just retry, and retry. If you have a connect timeout set, you will eventually get an exception, but it could take a long time.
The fact that it works on the Android emulator and not on the real device simply means that they are implemented or configured differently. (For instance, they may have different default connect timeouts ... of the emulator may be designed to give connection refused in that scenario.)
The bottom line is that you need to make your code work on the real device. Figure out the best way to make your device set a connect timeout, and check that that works when talking to the non-existent server.
Your try catch code doesn't catch IO Exceptions. Try something like this
try
{
// to get the ip address of the server by the name<br>
InetAddress ip =InetAddress.getByName("example.com");
sock= new Socket(ip,Server.PORT);
ps= new PrintStream(sock.getOutputStream());
ps.println(" Hi from client");
DataInputStream is = new
DataInputStream(sock.getInputStream());
System.out.println(is.readLine());
}catch(SocketException e){
System.out.println("SocketException " + e);
}catch(IOException e){
System.out.println("IOException " + e);
}
Related
I'm trying to build a project where I must pilot an IoT device from smartphone via Wifi.
This device has the SPWF01 Wifi Module integrated, and is configured as an access point (that has no Internet access) with security type WEP. On this access point configuration we also have a TCP Socket Server that intercepts smartphone communications.
On the smartphone side, we have the part which scans and connects to our device's access point(which works, although i get the esclamation point on the wifi icon since it has no Internet access). After we've connected, we start the Client Socket which connects to the server on our IoT device(the ip address of the server socket is actually the gateway of the access point). And here is where the trouble starts, because the client socket won't start. Here is the code:
public void SocketInit(String ip, int port) throws IOException {
InetAddress addr = InetAddress.getByName(ip);
SocketAddress sockaddr = new InetSocketAddress(addr, port);
nsocket = new Socket();
nsocket.setReuseAddress(true);
nsocket.setTcpNoDelay(false);
nsocket.setReceiveBufferSize(700); //Must be less than 730byte witch is the module buffer
nsocket.setSendBufferSize(700);
nsocket.connect(sockaddr, 5000); //5 second connection timeout
}
And here is the exception i get:
java.net.SocketException: socket failed: ENONET (Machine is not on the network)
And I get that error even before reaching nsocket.connect(), precisely on setReuseAddress.
Since the exception I get is ENONET, I thought that it must be because the access point has no internet access so I used the solution proposed here for testing purpose:
adb shell settings put global captive_portal_detection_enabled 0
This is a solution that can't be done programmatically without having root access, but I wanted to test if that was the problem. But although the exclamation mark on the wifi icon had disappeared, the client socket still gave me the same exception error.
Does anybody have a solution for this behaviour? Thank you in advance!
Sometimes the client socket manages to open, with a success rate of 1 out of 20 times. But when it does, i usually get another exception after a couple of messages sent:
java.net.SocketException: recvfrom failed: ECONNRESET (Connection reset by peer)
Here is the code I used to connect to the access point from the smartphone:
WifiConfiguration wc=new WifiConfiguration();
wc.SSID= host;
wc.status = WifiConfiguration.Status.ENABLED;
wc.priority = 40;
wc.allowedKeyManagement.set(WifiConfiguration.KeyMgmt.NONE);
wc.allowedProtocols.set(WifiConfiguration.Protocol.RSN);
wc.allowedProtocols.set(WifiConfiguration.Protocol.WPA);
wc.allowedAuthAlgorithms.set(WifiConfiguration.AuthAlgorithm.OPEN);
wc.allowedAuthAlgorithms.set(WifiConfiguration.AuthAlgorithm.SHARED);
wc.allowedGroupCiphers.clear();
wc.allowedPairwiseCiphers.set(WifiConfiguration.PairwiseCipher.CCMP);
wc.allowedPairwiseCiphers.set(WifiConfiguration.PairwiseCipher.TKIP);
wc.allowedGroupCiphers.set(WifiConfiguration.GroupCipher.WEP40);
wc.allowedGroupCiphers.set(WifiConfiguration.GroupCipher.WEP104);
wc.wepKeys[0] = password;
wc.wepTxKeyIndex = 0;
int netId = mainWifi.addNetwork(wc);
try {
//mainWifi.setWifiEnabled(true);
mainWifi.disconnect();
mainWifi.enableNetwork(netId, true);
mainWifi.reconnect();
startConnectionCheck = true;
System.out.println("enabled network");
} catch (Exception e) {
e.printStackTrace();
System.out.println(e.getMessage());
}
The access point's security type is WEP. That is because the Wifi module can't implement WPA.
Tests done on Marshmallow.
I am not 100% certain if this issue is the same.
I had to do a project a whileback and used Java sockets.
When doing intial testing, I used the local loopback and used ports on the same computer and multiple threads. Eventually it worked well enough to test between two computers. I found it didn't work between two computers. After turning off all my firewalls and protections etc on the network and getting desperate enough to use direct connection ethernet cables, I figured out the issue.
Sockets cares which gateway you use. The solution was for me to use the gateway rather than the loopback... It is obvious looking back now...
Anyway, it is likely that your mobile gateway, wifi gateway, and local loopback are all different.
Here is an ugly code blurbe that I hope gives direction with very little inspiration...
Socket socket = null;
try {
socket = new Socket(ip, port, InetAddress.getLoopbackAddress(), localServerPort);
}
catch (Exception e) {
}
if (socket == null) {
try {
socket = new Socket(ip, port, InetAddress.getLocalHost(), localServerPort);
}
catch (Exception e) {
}
}
if(socket == null) {
throw new Exception("Neither the loop back nor the host could find this sucker.");
}
I am using java.net.DatagramSocket to send UDP packets to a statsd server from a Google App Engine servlet. This generally works; however, we periodically see the following exception:
IOException - Socket is closed: Unknown socket_descriptor..
When these IOExceptions occur, calling DatagramSocket.isClosed() returns false.
This issue happens frequently enough that it is concerning, and although I've put in place some workarounds (allocate a new socket and use a DeferredTask queue to retry), it would be good to understand the underlaying reason for these errors.
The Google docs mention, "Sockets may be reclaimed after 2 minutes of inactivity; any socket operation keeps the socket alive for a further 2 minutes." It is unclear to me how this would play into UDP datagrams; however, one suspicion I have is that this is related to GAE instance lifecycle in some way.
My code (sanitized and extracted) looks like:
DatagramSocket _socket;
void init() {
_socket = new DatagramSocket();
}
void send() {
DatagramPacket packet = new DatagramPacket(<BYTES>, <LENGTH>, <HOST>, <PORT>);
_socket.send(packet);
}
Appreciate any feedback on this!
The approach taken to workaround this issue was simply to manage a single static DatagramSocket instance with a couple of helper methods, getSocket() and releaseSocket() to release sockets throwing IOExceptions through the release method, and then allocate upon next access through the get method. Not shown in this code is retry logic to retry the failed socket.send(). Under load testing, this seems to work reliably.
try {
DatagramPacket packet = new DatagramPacket(<BYTES>, <LENGTH>, <HOST>, <PORT>);
getSocket().send(packet);
} catch (IOException ioe) {
releaseSocket();
}
this is my first post in stackoverflow, i'm trying to implement a chat using RMI but i'm stuck, i am avoiding the use of rmiregistry, everything was binding well on localhost, but i need it to be accesible from the internet, the ports are forwarded, the firewall down, and actually i can retrieve through the internet an object, i've achived this using
System.setProperty("java.rmi.server.hostname","ruso.89.x.ignorelist.com");
So then my NameServer.java binds to that IP, but i lose all access through localhost, now let me explain this. This is the code for NameServer.java
public class NameServer
{
public static void main (String args [] )
{
String direscucha = "ruso.89.x.ignorelist.com";
int puertonombres = 5000;
try{
System.setProperty("java.rmi.server.hostname", direscucha);
System.setProperty("java.security.policy", "C:\\Users\\Antonio\\Desktop\\DistributedChat\\myServer.policy");
System.setSecurityManager(new RMISecurityManager());
NameService ns = new NameService();
Registry reg=LocateRegistry.createRegistry(puertonombres);
reg.rebind("ns",ns);
while(true){}
}catch(Exception e){
System.out.println(e);
}
}
}
This is my policy file:
grant {
permission java.security.AllPermission;
permission java.net.SocketPermission "ruso.89.x.ignorelist.com:*", "connect,resolve,accept";
permission java.net.SocketPermission "localhost:*", "connect,resolve,accept";
permission java.net.SocketPermission "127.0.0.1:*", "connect,resolve,accept";
};
Then I need the ChatServer.java to run, this takes ns and binds himself to it, so the ChatServer can be found, the problem is when i use it from outside my LAN I can get the server running! but when i use localhost i got an exception
ChatServer.java (Snippet)
public void work ()
{
String direscucha = "ruso.89.x.ignorelist.com";
int puertonombres = 5000;
System.setProperty("java.rmi.server.hostname", direscucha); //Binds to the Public IP
System.setProperty("java.security.policy", "C:\\Users\\Antonio\\Desktop\\DistributedChat\\myClient.policy"); //marca la politica
System.setSecurityManager(new RMISecurityManager()); //Ejecuta el RMISecurityManager
try
{
String[] list = Naming.list("//localhost:5000/"); //Debugging
System.out.println("Tamanyo: "+list.length);
for(int i=0; i<list.length; i++)
System.out.println(list[i]);
NameService_Stub ns = (NameService_Stub) Naming.lookup("rmi://localhost:5000/ns");
ns.rebind (conf.getServerName(), this);
System.out.println("Servidor registrado::");
}
catch (java.rmi.ConnectException e)
{
System.out.println (e);
System.exit(-1);
}
catch (Exception e)
{
System.out.println(e);
System.exit(-1);
}
So when the ServerChat is registered now any user can execute ChatClient.java to retrieve from the NameService the name of the ChatServer and then connect, but didn't got even there yet because i can't manage to the ChatServer to work. My first though was that when seting the hostname property the localhost get's blocked but nowhere is mentioned, afterwards i've though it may be a problem with the router NAT, could it be?
Exception:
java.rmi.ConnectException: Connection refused to host: ruso.89.x.ignorelist.com; nested exception is:
java.net.ConnectException: Connection refused: connect
Client and server policy files are identical.
Used WireShark to see what is happening, a SYN is send and a ACK,RST is recieved.
I've burnt all my ideas, any suggestions?
i've achived this using
System.setProperty("java.rmi.server.hostname","ruso.89.x.ignorelist.com");
So then my NameServer.java binds to that IP
No it doesn't. It's a common misperception, but incorrect. RMI always binds its ServerSockets to 0.0.0.0 unless you provide an RMIServerSocketFactory that does otherwise. The effect of java.rmi.server.hostname is to embed its value into the stub (essentially instead of the value returned by InetAddress.getLocalHost()).
while(true){}
You don't need to smoke your CPU by doing that, but you do need to make the Registry reference a static variable. RMI starts a thread when you export a remote object that will prevent the JVM from exiting until you unexport it. Unexporting happens automatically when the remote object is garbage-collected, and the static reference will prevent that.
Connection refused to host: ruso.89.x.ignorelist.com
That may mean that your internal firewall won't let you connect outwards to that host even though it's your own. Or that ruso.89.x.ignorelist.com has a different, incorrect DNS mapping inside your network.
Background: I'm writing a simple UDP application to ping a beta server I manage every minute or so to tell me it is still up and running (I can't enable ping on the server for those that are wondering). I plan to run this on my phone to warn me when the server is no longer responding.
I'm trying to use the seemingly simple java.net.DatagramSocket as such:
try
{
socket = new DatagramSocket();
socket.bind(null);
}
catch (SocketException e)
{
System.out.println(e.toString());
throw e;
}
Let me also say that I have enabled the Internet permissions through the android manifest and if I remove the uses clause to do so, I get a permissions error so I'm sure that is working OK. When I download this code to an Android Virtual Device (AVD) and execute it, on the call to bind() I am presented with this exception:
03-17 19:07:39.401: INFO/System.out(338): java.net.BindException: Invalid argument
According to this documentation, the null argument is correct:
public void bind (SocketAddress localAddr)
Since: API Level 1
Binds this socket to the local address and port specified by localAddr. If this value is null any free port on a valid local address is used.
But not trusting documentation, I decided to enumerate the IP addresses on my device like this:
ArrayList<NetworkInterface> allInterfaces = Collections.list(NetworkInterface.getNetworkInterfaces());
NetworkInterface eth = allInterfaces.get(0);
InetSocketAddress addr = new InetSocketAddress(eth.getInetAddresses().nextElement(), port);
try
{
socket = new DatagramSocket();
socket.bind(addr);
}
catch (SocketException e)
{
System.out.println(e.toString());
throw e;
}
When I step through the code, it works great and I can see the two IP address on the AVD but I get the exact same exception on the bind() call. Does anybody out there see what i might be missing? I will continue to research and hopefully post a solution to my own problem, but I am hoping somebody out there will be able to shortcut this for me.
[Edited: if you saw my previous response I made the classic debugging mistake of changing two variable in one test and it was the other one that solved my problem.]
I found the problem. It is the way I'm declaring the DatagramSocket that appears to cause problems. If I use a DatagramChannel to open the DatagramSocket in the following way then the bind() call is successful.
DatagramChannel channel = DatagramChannel.open();
DatagramSocket socket = channel.socket();
I've stumbled across this problem either and have found the cause: if you call the parameterless constructor new DatagramSocket(), this creates "a UDP datagram socket which is bound to any available port on the local host using a wildcard address" (as per the API docs). So this actually means, the Socket is already bound. My "fix" for this is as follows:
SocketAddress socketAddress = new SocketAddress(yourInetAddress, yourPort);
DatagramSocket serverSocket = new DatagramSocket(null);
serverSocket.bind(socketAddress);
This explicitly creates an unbound Socket (through the DatagramSocket (SocketAddress localAddr) constructor), making it possible to bind the Socket in turn.
This is probably the more elegant solution than creating an unnecessary channel.
P.S.: Strangely enough, this is where DatagramSocket differs from a TCP ServerSocket: the parameterless constructor of the latter will create an unbound ServerSocket, not triggering this problem.
Because I didn't want to implement a communication protocol for my client-server based application, I implemented a RMI client and a RMI server on both sides for the information exchange between the two components.
If I try to use my application by starting the two components on the same machine, everything is working fine. But if I split the components to two different computers (Kubuntu 9.04 within as a virtual machine within an Windows 7 RC environment with disabled firewall and a native Ubuntu 9.04 environment), it seems like the RMI client is not able to execute the methods which are defined on the server side. (Every functions call leads to a RMI exception.)
Currently I only set the system property "java.rmi.server.hostname" on both sides to the network interface which should be used for the data exchange and registered the default port for the communication with rmi daemon (?) rmid.
Does somebody has an idea what might be going wrong? Do I have to set some other parameters like "java.rmi.server.codebase" (http://java.sun.com/j2se/1.4.2/docs/guide/rmi/javarmiproperties.html) to be able to use the RMI functionality within my application?
Edit: Okay, here is some additional information for you:
In the initialization phase my client tries to establish a connection to the RMI server of server component, which was initialized using the following two methods:
private void initialize()
{
// set ip address of rmi server
System.setProperty("java.rmi.server.hostname", ipAddress);
// try to register rmi server
try
{
LocateRegistry.createRegistry(Registry.REGISTRY_PORT);
}
catch (Exception e)
{
// ignore
}
}
public void start()
{
System.out.print("starting master control RMI server ...");
try
{
Naming.rebind("MasterControl", this);
}
catch (Exception e)
{
System.out.println("error: could not initialize master control RMI server");
System.exit(1);
}
// set running flag
isRunning = true;
System.out.println(" done");
}
"ipAddress" is here the ip address of the network interface of the server component.
The method which is used by the client component to establish the connection looks like this:
public void connect()
{
// build connection url
String url = "rmi://" + masterControlIpAddress + "/MasterControl";
System.out.println(url);
System.out.print("connecting to master control ...");
// try to connect to master control server
while (connection == null)
{
try
{
connection = (MasterControlInterface) Naming.lookup(url);
id = connection.register(localIpAddress);
}
catch (Exception e)
{
// ignore
}
if (connection == null)
{
try
{
Thread.sleep(100);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
System.out.println(" done");
}
As you can see my client calls a function to register the connection at the server:
#Override
public int register(String ipAddress) throws RemoteException
{
// add connection to registrationHandler
masterControl.registrationHandler.addConnection(ipAddress);
// log
int connectionCount = masterControl.registrationHandler.getConnectionCount();
System.out.println("slave control (" + ipAddress + ") instance has been registered at the master control server under the following id: " + connectionCount);
return connectionCount;
}
If I run my program using a real network connection, the text "slave control ..." is not displayed on the server side. Therefore I'm not sure, if the function is really called by the client component.
After the client component is intialized it tries to notify the server component by calling the following method using it's RMI connection to the server:
public void sendInitializationDone()
{
try
{
connection.initializationDone();
}
catch (RemoteException e)
{
System.out.println("error: could not send 'initializationDone' message to master control");
System.out.println(e);
System.exit(1);
}
}
to set a flag on the server side.
The error occures inside this function on the client side:
java.rmi.ConnectException: Connection refused to host 127.0.1.1; nested exception is: java.net.ConnectException: Connection refused.
I have no idea why the host is here 127.0.1.1 ...
#nos
Of course, I disabled the windows firewall and the protection mechanismn of Kaspersky Internet Security. I don't think that there is a running firewall in my Kubuntu. In generell it is possible to establish a connection, because I already used scp to copy my program to the other machine.
Edit2:
Mhhh, after setting the entry in /etc/hosts which refers to the machine to the ip address of the machine it seems to work, but don't really understand why it does ...
BR,
Markus
You need to add an entry to the hosts file of the machines containing an entry of the form
machinename privateip
e.g.
virtualmachine 192.168.1.16
This will prevent RMI from sending the localhost host name as a 'call me back' address.
To test this approach, run the following code before and after performing the change.
System.out.println(java.net.InetAddress.getLocalHost());
It should output a local address before the changes and a non-local address after the changes.
Using different versions of the JDK on each server could cause this problem.
Use the
java -version
command to make sure that you are using the same version of the jre.