I'm building a simple server application, and I can connect to it in the Local Network. But I can't connect over the internet.
This is my Server code:
ServerSocket server;
try {
server = new ServerSocket(4000);
} catch(IOException ex) {
System.out.printf("Could not bind socket 4000\n");
System.exit(1);
}
try {
Socket socket = server.accept();
ClientThread client = new ClientThread(socket);
client.start();
} catch(IOException ex) {
}
And this is the client:
try {
System.out.printf("connecting...\n");
Socket socket = new Socket("mydomain.org", 4000);
System.out.printf("connected!\n");
} catch(UnknownHostException ex) {
ex.printStackTrace();
} catch(IOException ex) {
ex.printStackTrace();
}
I've forwarded port 4000 on my router, which should work. I've forwarded other ports before like 80 and 22.
When I run the client, I get the "connecting..." string, and it hangs there. I don't get "connected!", or a stack trace. But like I said before, it does work on the local network. It works when connecting to 127.0.0.1 and when using 192.168.1.90.
I used CanYouSeeMe.org to check if the port was open. It was successful on port 80, but it times out on 4000.
Check which ip address port 4000 is bound to. It may only be bound to the loopback address (127.0.0.1) instead of any interface.
Not sure which os you're using, but to check:
linux: netstat -ant
windows and mac: netstat -anp tcp
look for the LISTEN line on port 4000 and see whether it's bound to all interfaces (*:4000 or 0.0.0.0:4000), or to a specific interface (127.0.0.1:4000).
If it's not listening on all interfaces, that's your issue - use the constructor that lets you specify the BindAddress.
You've done the experiments to prove that it is not your java and must therefore be a firewall or firewalls issue. Even if you are properly forwarding the ports to your server, the server firewall may not be allowing inbound connections, so check that also.
You have a firewall rule that's throwing away the incoming packets. That's why your client hangs. Check the IP rules on the server computer and on any gateway system to the outside world.
Related
I'm trying to create a local server with Wi-Fi P2P between an Android phone and a Raspberry Pi, with the Android as the host. I have been able to successfully establish a P2P connection using wpa_cli on the Pi, but now I am trying to use a C client socket to connect to the phone and transfer data. However, the line Log.d("Socket waiting", serverSocket.getLocalSocketAddress().toString()); spits out D/Socket waiting: ::/:::8888. It doesn't seem to have an address at all, so how am I supposed to connect to it?
As indicated by my comment, my research told me that the correct IP should be 192.168.49.1. If the IP were any different, that would be okay, because I can just send a BLE packet to the phone, telling it the IP. My issue is that the IP is entirely blank.
My code is as follows, for a thread that waits on a connection:
public static class DataTransfer extends Thread {
#Override
public void run() {
Log.d("DataTransfer", "Start");
ServerSocket serverSocket = null;
try {
/**
* Create a server socket and wait for client connections. This
* call blocks until a connection is accepted from a client
*/
// Expects a connection at 192.168.49.1:8888
serverSocket = new ServerSocket(8888);
//serverSocket.setReuseAddress(true);
//serverSocket.toString()
Log.d("Socket waiting", serverSocket.getLocalSocketAddress().toString());
Socket client = serverSocket.accept();
InputStream inputstream = client.getInputStream();
Log.d("InputStream Available", String.valueOf(inputstream.available()));
serverSocket.close();
}
catch (IOException e) {
Log.e("Receive Error", e.getMessage());
if(serverSocket != null) {
try {
serverSocket.close();
} catch (IOException ex) {
Log.e("Failed to close socket", ex.getMessage());
}
}
return;
}
}
}
And here is the output of ip a on the Pi, once it is connected via Wi-Fi P2P
11: p2p-wlan0-8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether b2:0e:07:e6:e6:55 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.23/24 brd 192.168.1.255 scope global noprefixroute p2p-wlan0-8
valid_lft forever preferred_lft forever
inet6 fe80::e79c:33f3:6e49:b6ed/64 scope link
valid_lft forever preferred_lft forever
Final edit:
My problem was seemingly unrelated. As both comments below indicate, the IP shown off is fine, it just means it accepts connections from anything. My actual issue was that I had a static IP set up on my Pi without specifying which interface the static IP was for. The client needed to be on a 192.168.49.# address, and the static IP was preventing it.
You can specify the interface the server socket is listening on by passing an address to the constructor:
serverSocket = new ServerSocket(8888, 10, InetAddress.getByName("192.168.49.1"));
Seeing :: means your server was listening for IPv6 connections on all interfaces. That is represented by the IPv6 address of all zeros which can be written as ::. But you are trying to connect to an IPv4 address, not IPv6. Most systems I've worked with are configured so that IPv4 connections can be accepted by an IPv6 server, but I guess yours isn't. The answer to this question suggests you may be able to change your system's behavior with sysctl:
sysctl net.ipv6.bindv6only=0
:: is the IPv6 default route. It indicates that you are serving requests from all interfaces.
This is the expected behavior. Is there a problem with that?
I am using Java to do the socket programming as below.
Client program is as below:
Socket MyClient;
try {
MyClient = new Socket("Machine name", PortNumber);
}
catch (IOException e) {
System.out.println(e);
}
Server program is as below:
ServerSocket MyService;
try {
MyServerice = new ServerSocket(PortNumber);
}
catch (IOException e) {
System.out.println(e);
}
Socket clientSocket = null;
try {
clientSocket = MyService.accept();
}
catch (IOException e) {
System.out.println(e);
}
Now my question is if I run more than one thread to open several sockets in one port (as the server code above), how my client program know which socket it is connecting to?
Your client connects to the Servers port. So all clients will be having the same code
MyClient = new Socket("Machine name", <port where server is listening>);The port opened at client side is not important. The client will get a free port available in his OS.
how my client program know which socket it is connecting to?
The question doesn't make sense. It doesn't 'connect to a socket' at all, it connects to a listening port, and there is only one of those. Your server only accepts one client, so the second and subsequent threads will get an undefined behaviour ranging from a ConnectException to a ConnectionException to nothing, most probably the latter.
Your application knows it because you set it up with a specific port. There is no "auto discovery" built into TCP/IP, it's up to you to pick a server-port and make sure you set your clients up to connect to that port. Either you hard-code this into your client application or, better yet, have it in some configuration file you include with the client.
This is why you have a bunch of "known ports", like http is port 80. This means that a browser will always connect to port 80 on a web-server, unless you explicitly indicate another port in the URL.
I have a file transfer applet, and I am at the moment creating a logging system, so in the applet as it initialized, until it send file and finish will start printing a log file (originally it goes to java console on the client side) but now I'm modifying it so it will go to the server instead (create a file in the server and start writing on the file).
It working now, I'm using port 5000, but I need to open the firewall for port 5000 since opening port is dangerous, can I just use port 80 or 443 (HTTPS) since it open anyway for file transfer? how to do it?
example of the code at the moment is
in java applet there will be
handler = new SocketHandler("xxx.xxx.xxx.xx", 5000);
//on different area of the file there will be something like
log("starting the applet");
log("sending email");
...
while on the server I create a simple java server like this
public class ThreadedEchoServer {
// using port 5000
static final int PORT = 5000;
public static void main(String args[]) {
ServerSocket serverSocket = null;
Socket socket = null;
try {
serverSocket = new ServerSocket(PORT);
} catch (IOException e) {
e.printStackTrace();
}
while (true) {
try {
socket = serverSocket.accept();
} catch (IOException e) {
System.out.println("I/O error: " + e);
}
// new thread for a client
new EchoThread(socket).start();
}
}
}
You really should not think about using ports 80 and 443 - these are ports for RECEIVING connections. After a connection is picked up on one of these ports the request is actually redirected to another port for handling. Therefore, using them to write will block any incoming communications.
It working now, I'm using port 5000, but I need to open the firewall
for port 5000 since opening port is dangerous, can I just use port 80
or 443 (HTTPS)? how to do it?
Opening port 5000 is no more dangerouse than opening port 80 or 443. (Changing ports is no more difficult than modifying the numbers, and have no more effect).
I would use the 5000 port and config a chrooted environment if your server is running other important applications.
You could check this thread: What's the best way to defend against a path traversal attack?
I am trying to connect to server using a Java socket. I am trying to connect from port 80 to 90
int port;
Socket clientsocket;
String hostname = "www.google.com";
for(port = 80;port<=90; port++){
try{
clientsocket = new Socket(hostname,port);
System.out.println("Connection at port " + port + "\t" + clientsocket.isConnected());
clientsocket.close();
}
catch(Exception e){
System.out.println(e.getMessage());
}
}
When I try to connect to any website like google.com or w3schools.com my program hangs on the socket() call for port numbers except 80.
Since those websites are not serving on ports 81-90 it should raise exception but instead it gets blocked. For port 80 it works fine.
When I try to connect to the apache server installed on my machine, it doesn't block for any port number and gives me Connection refused error which is the obvious behavior.
So why is it happening. Thanks in advance.
When I try to connect to any website like google.com or w3schools.com my program hangs on the socket() call for port numbers except 80. Since those websites are not serving on ports 81-90 it should raise exception but instead it gets blocked.
This is almost certainly not Java's doing.
When you invoke the Socket(String, int) constructor, the JVM asks the OS to attempt to establish a connection to the IP address corresponding to the supplied name, using the supplied port number. Assuming that we're talking TCP/IP, the OS sends off a TCP 'SYN' message, and waits for a response:
If the response is a 'SYN-ACK', it proceeds to establish the connection as per the protocol; see http://en.wikipedia.org/wiki/Transmission_Control_Protocol#Connection_establishment.
If the response is an 'RST' (reset), the connect fails and this results in a Java "connection refused" exception. (This is typically what happens if the 'SYN' makes it to the remote server, only to discover that there is no application "listening" on the port you tried to connect on.)
If the response is an ICMP message of some kind (e.g. ICMP destination unreachable), this typically results in an immediate failure of the connection request, and a Java exception.
If there is no response, the OS tries again, and again, and again. Depending on the Java default connect timeout (or the explicit timeout), this process could continue for a long time.
So what is most likely happening is that something is filtering the 'SYN' messages on funky ports, and simply throwing them away. It could be the local firewall software on your PC, firewall software in your gateway, or your ISP's network, or software in the remote system you are attempting to talk to. Or this could be happening to the 'SYN-ACK' message coming back.
Either way, the blocking / timeout behavior is inherent to TCP/IP networking, and it is impossible to accurately diagnose at either the OS or Java levels. You simply need to adjust your expectations. (Or set a shorter connect timeout ...)
For this case, and any case:
/**
* Create end point and initialize connection with custom timeout for refused or server offline
*/
try{
InetSocketAddress endPoint = new InetSocketAddress(serverAddress, portNumber);
Socket socket = new Socket();
socket.connect(endPoint, 10000);
} catch(Exception e){
//... Handle connection error
}
I'm getting following error when my client tries to connect to my server socket:
java.net.ConnectException: Connection refused: connect
But, my server is really running, on the same machine. I try to connect to it by using the external IP of my router. But when I try to connect with "localhost", it works. And, yes I did port forwarding correcly in my router. Even canyouseeme.org can connect to my server (The site says: "success" and in my server-log appears that someone connected with the server.)
So, is it for one or another reason impossible to connect to the same machine (or to a machine in the same network) via an external IP? Or is this something typical for Windows? (Normally, I use Linux)
I also tried to completely disable Windows Firewall.
ServerSocket:
public ServerSocket ssocket;
public List<ClientHandler> handlers;
public Server(int port) { // Constructor
try {
ssocket = new ServerSocket(port);
this.handlers = new ArrayList<ClientHandler>();
IpSharingManager.uploadData(Utilities.getPublicIp(), port);
} catch (Exception e) {
e.printStackTrace();
System.exit(-1);
}
}
Client:
public InvisibleClient(String host, int port) {
try {
System.out.println("Trying to connect to " + host + ":" + port);
this.host = host;
this.socket = new Socket(host, port);
this.bis = new BufferedInputStream(this.socket.getInputStream());
this.bos = new BufferedOutputStream(this.socket.getOutputStream());
this.console = new RemoteConsole(this.socket);
initializeCommunication();
System.out.println("Successfully connected!");
new Thread(this, "Client Thread").start();
} catch (Exception e) {
e.printStackTrace();
System.out.println("No server available");
}
}
Thanks
Some routers doesn't allow the internal network to connect to the external IP address of the router.
You can try to use telnet to connect to your server socket. If telnet isn't able to establish a connection, it's likely a networking problem.
Add the java.exe process and the port to your firewall exception list?
edit: Just read you already tried that. All I can suggest is make sure the network is not blocking that port. (routers)
Have You tried running it with JVM option: java.net.preferIPv4Stack=true ?
For what I see in your code, you missed the part where you accept the conection, after instantiating the server socket you need ssocket.accept() to accept conections and then you have to start reading the outputstrem from the socket