Develop Reference

TCP socket connection over internet

I am doing a project for which connection between server and client is required.
I did it by adding TCP sockets.
Here is the code fraction :
Server:
ServerSocket welcomeSocket = new ServerSocket(80);
while(true)
{
Socket connectionSocket = welcomeSocket.accept();
WorkerThread wt = new WorkerThread(connectionSocket, id);
Thread t = new Thread(wt);
t.start();
workerThreadCount++;
}
Client :
Socket skt = new Socket("192.168.0.108", 80); // The IP address is from cmd->ipconfig/all-> IPv4 Address
outToServer = new PrintWriter(skt.getOutputStream(), true);
inFromServer = new BufferedReader(new InputStreamReader(skt.getInputStream()));
It all works when both ends are in same device/under same WiFi.But I don't understand what to do for creating connection over internet.
Please help with clear steps.

Here:
Socket skt = new Socket("192.168.0.108", 80);
That is local address. If you want to have a server that is reachable on the internet, then that server needs to have its global public IP address!
In other words: you have to make sure that the server can be reached from the internet somehow. For example by turning to some service provider that hosts servers that you can then equip with your code!
The whole purpose of 192.168 addresses is to be defined only in a local subnet.
Alternatively, you have to check if your ISP has a service where the ISP assigns an IP address to your connection, and that allows calls from the internet to go to your "place".
Meaning: when you want to receive phone calls, you need a phone that is connected to the phone net!

In order to connect to a socket over WAN, you must port forward that port to your local device. This can be done in your routers' settings.
192.168.0.108 --> That's your local IP-address.
This can be used on your local network without any requirements for port forwarding whatsoever. However, to use it over WAN, execute the following steps:
Step 1: Search for your routers' model number and port forwarding on Google on how-to forward port 80 to your local IP-address. Warning: use a static IP-address on your local device to prevent your IP from changing after a reboot.
Step 2: Go to a website like IP Chicken and find your external IP-address.
You can then connect to your socket using:
Socket skt = new Socket("[EXTERNALIP]", 80);
Please be noticed: unless you have a business network, your external IP-address will probably change from time to time.

Android dealing with IoT devices with NO Internet connection

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.");
}

How to overcome Unknown Host Exception with no access to system32\drivers\etc\hosts

I tried to use the following connection
URL oracle = new URL("http://www.oracle.com/");
URLConnection yc = oracle.openConnection();
yc.setConnectTimeout(100000);
BufferedReader in = new BufferedReader(new InputStreamReader(
yc.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
System.out.println(inputLine);
in.close();
It throws java.net.UnknownHostException
I cannot set mapping in system32\drivers\etc\hosts . And I cannot make any changes there . Is there any way to overcome this ? It works on my other computer, but does not on the one want to use it on
i tried using
searchUrl="http://96.7.228.140/";
And got the following error
java.net.ConnectException: Connection timed out: connect

You already tried substituting the domain name with the IP in the URL and it didn't work.
That indicates there's a different problem. Most likely not a java problem.
You shouldn't try to fix it using the hosts file.
If you can ping oracle.com then Java should be able to connect to it.
The only issue could be on the length of the connection, as your connection might be timing out if your network is too slow.
Do a ping on oracle.com and check what is the time it takes for them to respond. Adjust your connection timeout accordingly.
If that doesn't work, you might have a firewall problem in the other computer. Either that or your network adapter is not properly installed.
Can you open an internet browser on the machine and navigate to http://www.oracle.com?
If you can, then try your program. If it doesn't work then check the connection time.

Java.net.SocketException: Permission denied: connect

I have two PCs in one network that I want to connect. One of them should send a notification to the other via TCP. One the one PC I have a "server" (Python script) socket which waits for the "client"(Jar file) to send a specific String and then it gives me a notification. This works perfectly fine when I'm trying it out one one PC. But when I want to do the intended action the "client" PC's .jar gives me an error that the connection is refused. Do I have to open a specific port on the other PC or what else could cause trouble? One PC runs Fedora the other Windows 8
"Server Code"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(("", 5005))
s.listen(1)
try:
while True:
komm, addr = s.accept()
while True:
data = komm.recv(1024)
if data == "$":
noty()
if not data:
komm.close()
break
finally:
s.close()
"Client" Code
public static void main(String[] args) throws Exception {
Socket socket = new Socket("192.168.178.25", 5005);
OutputStream out = socket.getOutputStream();
String dat = "$";
out.write(dat.getBytes());
socket.close();
}

Your server is probably binding to the wrong interface,
calling
s.bind(("", 5005))
Without setting an interface will allow the program to pick what ip address / interface it will connect to.
Since your client is trying to connect to ("192.168.178.25", 5005); you may want to put an IP address into the bind call to prevent the server picking the wrong ip interface.
Example:
s.bind(("192.168.178.25", 5005))

if its permission denied then something is blocking your connection with the computer. i would try to open a port and see if that works. if you want an example of java sockets you can take a look at my SUPER Tic-Tac-Toe Multiplayer it uses java sockets to send strings to the clients as a way to represent what actions the clients should take.

a bizarre Connection refused: connect error in Java client

I have encountered the following error:
java.net.ConnectException: Connection refused: connect
I'll show some code below but first, a description because it baffles me. I wrote a very basic chat program that can act as either a server or client depending on the initial button you press (host or connect). Tested on both localhost and over the internet with friends, works perfectly, as intended. I've started writing a second software and using almost the same code, but having more classes, I get this error when trying to connect to a host of the same program. So, prog A can connect to prog A fine, but B cannot connect to B. One might assume firewall issues, but here's the plot twist: B can connect to A and A can connect to B again perfectly. But B cannot connect to another copy of itself. I used the official oracle tutorials for TCP and worked perfectly on the chat program referred to as A. Depending on the press of a button, A will run one of its main class' methods (to start a server or client). B on the other hand, will create a new object of a class, either a Host
public class Host
{
static ServerSocket serverSocket = null;
static Socket clientSocket = null;
static PrintWriter out = null;
static BufferedReader in = null;
static String inputLine;
public Host() throws IOException
{
serverSocket = new ServerSocket(4444);
System.out.println("Server created, waiting for guest.");
clientSocket = serverSocket.accept();
System.out.println("Guest connected.");
out = new PrintWriter(clientSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
while ((inputLine = in.readLine()) != null)
{
//if (inputLine.equals("q")) {break;}
System.out.println("Message recieved:" + inputLine);
}
}
}
or a Guest
public class Guest
{
static Socket socket = null;
static PrintWriter out;
static BufferedReader in = null;
static String inputLine;
public Guest() throws IOException
{
System.out.println("Connecting to host");
try
{socket = new Socket("localhost", 4444);
out = new PrintWriter(socket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
}
catch (UnknownHostException err) {System.out.println("unknown host"); System.exit(3);}
catch (IOException err) {System.out.println(err); /*System.exit(4);*/}
System.out.println("Connected to host");
while ((inputLine = in.readLine()) != null)
{
//if (inputLine.equals("q")) {break;}
System.out.println("Message recieved:" + inputLine);
}
}
}
I have the proper imports, I just didn't paste them here. Both programs try to connect to localhost and only A works, this other one does not. Again, creating a Guest, I can connect to the chatprogram's server. A chatprogram client can connect to this server. But this client cannot connect to this server.
Has anyone experienced anything similar? Is there an obvious solution, something I'm missing? I'm really clueless here, I literally copy/pasted server/client codes.
Edit: stack trace.
java.net.ConnectException: Connection refused: connect
at java.net.DualStackPlainSocketImpl.connect0(Native Method)
java.net.ConnectException: Connection refused: connect
at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:69)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:157)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:391)
at java.net.Socket.connect(Socket.java:579)
at java.net.Socket.connect(Socket.java:528)
at java.net.Socket.<init>(Socket.java:425)
at java.net.Socket.<init>(Socket.java:208)
at mainPack.Guest.<init>(Guest.java:21)
at mainPack.GameStarter.grandMain(GameStarter.java:101)
at mainPack.GameStarter.<init>(GameStarter.java:27)
at mainPack.GameStarter.main(GameStarter.java:34)

In your Host, you only accept one connection, and then the server stops listening. If something is tying up that connection, nothing else would be able to connect to the Host. An accepted connection should immediately be delegated to another thread for processing so that the Host can loop and listen on the server socket again. See "Supporting Multiple Clients" in the Socket tutorial.

Remove your static declarations from the variables in Guest. Same for Host (although if you have only one host it should be ok).
Update
e.g. pseudo-code
create server socket
while true (or quit received) {
accept connection
spawn thread to handle communication
}

Need more details as to how this is being run and what the error is exactly (I suspect client returns a network-level connect error when attempting to reach the host) but in general there is only one port being used: 4444. I presume the error (not posted) is that this port is in use by the first programs that are communicating on that port on the same machine.
Further, the static references only allow one instance of the various sockets to exist in a given JVM at a time. These cannot be concurrently used by multiple threads possibly implemented by other client classes using these same (one) static resources.