Develop Reference

Port-forwarding a Multicast IP

I've spent some time learning about UDP connections, particularly with Multicast Sockets in Java.
I was able to make a simple Multicast Socket "group chat" on my local network, but I've since been trying to expand this to work beyond my local network.
In attempts to achieve this, I port-forwarded a Class D IP address on my router in order to allow other people to access my Multicast group from outside my network.
However, when trying to connect to my "group chat" via my public IP and specified port (during the port-forwarding), I would receive the following error message...
Exception in thread "main" java.net.SocketException: Not a multicast address
at java.net.MulticastSocket.joinGroup(MulticastSocket.java:310)
...
This error makes some sense, given that my public IP isn't a class D address. But since I port-forwarded a multicast address to the specified port on my router, shouldn't this problem not occur?
Here's the relevant part of my code...
InetAddress group = InetAddress.getByName("192.___.___.___"); // my public IP
MulticastSocket socket = new MulticastSocket(1234); // the port-forwarded port
socket.joinGroup(group);
Where had I gone wrong here, and how could I get to fixing this issue?

A multicast address is between 224.0.0.0 - 239.255.255.255 with different sub-ranges within for different scenarios. More here: https://en.wikipedia.org/wiki/Multicast_address
So by attempting to join a group at 192.x.y.z, that's an invalid multicast address. That's why you get the exception thrown.
I could be mistaken, I doubt most consumer/home NAT, much less ISPs support multicast traffic. (Begs the questions - whatever happened to the MBONE - I thought that would have taken off and been the solution for everything.)
It sounds like what you need is a proxy program that intercepts multicast traffic and tunnels it to a proxy on a different network running the same code. The proxy in turn, takes the tunnelled packet and redirects back to a multicast\broadcast group.
You might have better luck with broadcast sockets instead of multicast.

Java TCP Get Domain Name?

How would you get the domain name of from a TCP connection?
As I'm trying to make a proxy type software but it must detect what the domain is and then go where it needs to. However I'm unsure how to get the domain name from the client.

There is no general way to get the target domain or host name from of the TCP connection, because a connection is only defined by its target IP address and not the host name and there might be several names for a single target IP address. But while there is no general way to get the target name from all TCP connections it is possible with some protocols on top of HTTP:
In case of HTTP you might look at the HTTP Host header which contains the target host name and is set by nearly all HTTP stacks (required with HTTP/1.1).
With SSL you might try to extract the host name from the initial ClientHello message in the SSL handshake, in case the client uses SNI (server name indication). All modern browsers use SNI, but older browser like IE8 not and also not older Java, Python, Perl, Ruby ... applications.

You may use the following code snippet which will give local domain name -
try {
InetAddress me = InetAddress.getLocalHost();
String dottedQuad = me.getHostAddress();
System.out.println("My address is " + dottedQuad);
} catch (UnknownHostException e) {
System.out.println("I'm sorry. I don't know my own address.");
}

How to establish connections to a ServerSocket from internet?

My ServerSocket listens to LAN Connections and accepts them well, but when I try to connect to the same through my Phone - using the 3G connection - it doesn't seem to connect.
I tried using getMyIP site to get the IP and try to connect to it, it does get the right IP (checked with my router) but then no connections are accepted at all.
I tried opening the port on windows 7 and on my router altogether.
I put those lines in my Server constructor:
ss = new ServerSocket(port);
host=ss.getInetAddress().getHostAddress();
and I get the ip on host to 0.0.0.0
Thanks for your help.

- While you are at LAN, you can use the Private IP as well as Public IP ranges
- But when you are using the Internet to access the Server which is at your place, then you need to have a static Public IP address.
- You can ask for a static Public IP address from your ISP at some extra cost, there are also some site over net that some how provides a static IP on the basis of your Dynamic IP.
Private IP ranges Can't be used over the Internet.
Class A - 10.0.0.0 - 10.255.255.255
Class B - 172.16.0.0 - 172.31.255.255
Class C - 192.168.0.0 - 192.168.255.255

You need to have a public IP address. If you have a router it must pass traffic for the port you want to expose to the internet to your machine. If you have a firewall, it must allow external connections to this port.
All the changes you do are the same regardless of language you use and there is nothing you can do from Java to work around needing to do these things.

Check your firewall if it allows incoming connection. You need to make and exception there.
you need to bind explicitly the IP address on your machine which is allocated for that instance of time by your ISP.
You can get the IP address allocated to you by running ipconfig command on windows command prompt.
Use the following code to bind to a specific IP address
InetSocketAddress insa = new InetSocketAddress("22.23.23.111", 9090);
ServerSocket ss = new ServerSocket();
ss.bind(insa);
String host=ss.getInetAddress().getHostAddress();
System.out.println(host);
This prints the IP address allocated to you.

Remotely connecting two non-local computers with sockets

This question seems like something very obvious to ask, and yet I spent more than an hour trying to find an answer.
First I host and wait for someone to connect. Then, from another instance of the application, I try to connect with a socket - for the constructor, I use InetAddress, port. The port is always right, and everything works if I use "localhost" for the address. However, if I type my IP (the one I got from Googling "what is my ip"), I get an IOException. I even sent the application to someone else, gave him my IP, and it didn't work.
The aim of the application is to connect two computers. It's in Java. Here is the relevant code.
Server:
ServerSocket serverSocket = new ServerSocket(port);
Socket clientSocket = serverSocket.accept();
Client:
InetAddress a = InetAddress.getByName(ip);
Socket s = new Socket(a, port);
I don't get past that. Obviously, the values of int port and String ip are taken from text fields.
Edit: the purpose of my application is to connect two non-local computers.

As mentionned by Greg Hewgill, if you are behind a NAT Device (Router, etc...) you will have to do some Port Forwarding.
Basically, your public IP Address that you get from using "What is my IP" from google is your public IP Address, but since you are using a router with multiple computers connected to it, there is a protocol that maps multiple computers to a single public address called NAT.
What you'll need to do is tell your router to forward the incoming packets on a certain port to a certain computer.
The way to do this is highlighted in this article http://www.wikihow.com/Port-Forward

Java Socket on Different Machine Does Not Work

I've tried many examples on web and one of them is this:
http://zerioh.tripod.com/ressources/sockets.html
All of the server-client socket examples work fine when they are tested with 127.0.0.1
BUT it never ever EVAR works on two different computers with actual raw real IP address ("could not connect to host" on telnet and "connection timed out" when tested on java client - the server program just waits for connection)
Note:
Firewall is turned off for sure
IP address from ipconfig didn't work
IP address from myipaddress.com (which is totally different for no reason than that from ipconfig) didn't work
What is it that I'm missing?
If I can only figure this out...

Try binding on 0.0.0.0. This tells your socket to accept connections on every IP your local can accept upon.

Based on the comment where the the following snippet of code is mentioned:
requestSocket = new Socket("10.0.0.5", 2004); // ip from ipconfig
it would be better to use the hostname instead of the IP address in the constructor, as the two-parameter Socket constructor with a String argument expects the hostname as the String, and not an IP address. A lookup of the IP address is then performed on the provided hostname.
If you need to pass in an IP address, use the two-parameter constructor that accepts the InetAddress as an argument. You can then provide a raw IP address to the InetAddress.getByAddress method, as shown in the following snippet:
InetAddress addr = InetAddress.getByAddress(new byte[]{10,0,0,5});
You'll need to be careful when specifying arguments via the byte array, as bytes are signed in Java (-127 through +128), and numbers beyond this range (but valid octets of IP addresses) may have to be specified using Integer.byteValue.
Finally, it should be noted that it is important to specify the IP address of the remote machine, as visible to the client. The IP address listed at myipaddress.com may be the address of a proxy, as that is the public IP of your entire network as visible to the host server at myipaddress.com. Therefore, you ought to be specify the IP address of the remote machine that is visible to your machine and not myipaddress.com.