I created a small chat program, that works flawlessly when client & server are run on the same computer (and probably network, too).
However, as soon as I try to connect to another computer over the internet, the socket connection simply times out.
Is this because of firewalls / routers, etc?
And how can I connect a ServerSocket & Socket over the internet?
However, as soon as I try to connect to another computer over the internet, the socket connection simply times out. Is this because of firewalls / routers, etc?
Yes, most likely. You're running into the NAT problem: essentially, the same externally visible IP address maps to many internally visible endpoints, and external endpoint doesn't know which internal endpoint to give your socket request to.
The easiest way around this is to have both your clients connect to a third party which both of them can see, and then have the third party mediate the communication. This is how most instant-messaging protocols work, for example.
If you have no way to control a third-party entity like that, an alternative to directly connect two clients is to have both clients open up an agreed-upon port, and then map communications on that port to their own internal endpoint. This provides the missing link that the externally visible endpoint (e.g. your home router) needs to deliver the communication to its intended destination.
If your server is behind a NAT router box (and most home computers are, especially if you use WiFi), then it won't be reachable from the outside unless you set up your router to port forward to that server.
What's the IP of your server computer? If it's 192.168.x.x or 10.x.x.x, then it's a non-routable address and can't be reached from outside.
Assuming with running on the same computer you mean that you tell the client the server is at 127.0.0.1 / localhost, it shouldn't be a problem in your code but because of firewalls or routers. If your server is behind a router performing masquerading (i.e., the server doesn't have a public but private IP address like 192.168.x.y for instance), you have to configure the router to pass a connection from the internet to the computer running the server.
Another reason why it doesn't work might be the way you bind your server to the interface. If you specify 127.0.0.1 there, the server will only listen for requests coming from the same system. If you don't specify an address, it will listen on all interfaces.
Edit Your comment indicates that you indeed have the NAT problem like others said. Configuring your router accordingly is probably the easiest solution.
First, test to see if it really works on a LAN; it sounds like you're just assuming it does.
If it works on your local network, perhaps it's failing because the server lacks a public IP, and is running behind a NAT'ing router.
Related
I'm working on a server-client project. I hosted server on Google app engine so there is no problem with IP there, all the clients can connect to the server easily. Yet the problem occurs when I try to connect to a client, which is quite complex because I don't have static IP for the clients. Can anyone suggest me a good way for server-client coomucication in this case, without requiring that clients must have static IP address?
Thank you very much.
Well, obviously the client should register itself with the server and update it's IP when it changes.
There is, for example, a program which does exactly that and then publishes the IP with a DNS.
But you should be aware that the IPv4 address space is not that big and a lot of internet clients do not own an IP (and work thru the ISP's NAT). If you have clients that do not own an IP then you might want to stick to the usual Pull: the clients should periodically issue a request to the server to check if there are new messages for them. With a Keep-Alive connection and an efficient server implementation the price of such checks might actually be low, although that kind of communication might not work very well with the GAE pricing.
I just made a little chat programm with a server and a client in java. The client needs to connect to the server with the ip of the server. All my testing I did with the localhost (127.0.0.1) and everything went fine, but when I tested it later on with a friend of mine, I had to notice that if I put my ip address (I run the server, he runs the client) that it doesn't work.
Is there a way to set up a private little server on my pc to run my server on, or maybe another way to get it working?
EDIT:
Just found a tool called "Apache" to set up your own server, could that might be helpful?
EDIT: When I say it doesnt work I mean I get an IOException, because this fails:
public void connectToServer() throws IOException{
showMessage("Attemption to connect...");
connection = new Socket(InetAddress.getByName(serverIP), 6789);
showMessage("\nConnected to: " + connection.getInetAddress().getHostName());
}
There is a whole host of things that you need to look at before your application will work.
Firewalls on both ends (and anti-virus applications) need to allow
the programs to communicate
Your ISP needs to allow messages to be sent via your designated ports
Your router (and the clients) need to not-filter these messages.
As a start, see if you can ping each others IP addresses and take it from there.
I recommend you first try and disable your firewall. If you are using windows, here is the instruction for turning off windows firewall: Turn Windows Firewall on or off
If you are testing with someone outside of your local network, you may need to setup NAT on the router of the person hosting the server. You can access the router by typing in it's local IP address in the web browser. This is usually something like 192.168.1.1 or 192.168.1.254 but it will depend on the model and network setup.
Once you have connected you should find an option (usually under advanced) for "NAT" or "Port Forwarding". I suggest you do a google search with the router model and how to setup port forwarding.
You also need to be aware that some ISPs will block certain ports. I suggest testing on a common port such as port 80 (HTTP) since it's unlikely an ISP will block this (be aware that you will need to disable Skype or any local web servers to test this)
I created a Battleship game in Java to work with Sockets and ServerSockets. The game works fine on LAN, but I would like to be able to play against someone on a completely different network. I understand port forwarding would be necessary, but would I have to forward the port on every client that is playing the game? Or would it only be necessary to forward the port on the router the server is using? Thanks.
Port forwarding is only required on the server. Any packets bound for the client will be auto-forwarded from the initial request thanks to the action on many NATs.
So, you only need to forward the port on the server. If using Socket and ServerSocket you need to forward TCP if your router gives you the choice. If using DatagramSockets, forward UDP at either of the endpoints.
(if you had to forward on clients, you most likely wouldn't have been reading this page without forwarding port 80)
To elaborate on what #hexafraction is saying, here's what you can do:
Build a server that any client can connect to. Clients can generally connect to anything they want. Firewalls usually place restrictions on information that can flow in, but not flow out. So if the clients know the server to connect to, they can connect to it without modifying their firewall and then the server will coordinate transferring the data between the clients.
I did this and all I had to do was forward the port I was using in my router to my local I.P. address. If you don't know your local ip address run a command prompt (assuming your using windows ) and run ipconfig to figure it out. If your at home using a wireless router access it by browser to 192.168.1.1 Hope this helps!
I'm developing a game that can be played with computer or versus other players. The GUI is Swing but irrelevant for the matter. My question is : how do I connect with other players in a network?
I am familiar with terms like client / server, sockets etc, and i can write a basic client/server program, but it can only be run from ONE computer. I don't know how to connect to this network from a computer, say, half way across the country.
You are probably setting behind a router which hides your local IP address from the rest of the world (look for NAT for more info on this). Basically, the world only sees your YOUR_ROUTER_IP, your router takes care of carrying all IP packets from the outside world to you and vice versa. You will need to change your router settings so that your computer/server gets the router ip address. This way you can access your server application from anywhere in the world by using simple socket operations.
I hope this helps.
I like to use Kryonet for network connections, it works very well and has really good documentation.
You indicated you know how to use client and server sockets, so I'll just throw out there that you ought to try connecting with "real" ip address instead of localhost (127.0.0.1). Take two machines on your local network, get the IP address of the "server" machine and use that address to connect from the client.
This will work all the way around the world, except for the fact that you are likely behind some Network Address Translation (NAT) firewalls and will likely need to "open" or "forward" the ports you need. If you need more information on NAT, google and serverfault will be pretty helpful.
If your client and server is located on the same machine, then you can use loopback address(ie 127.0.0.1), in LAN you can do with private ip addresses
Private ip address for LAN
CLASS A - 10.x.x.x
CLASS B - 172.16.0.0 - 172.31.255.255
CLASS C - 192.168.0.0 - 192.168.255.255
you can use this for LAN environment
For connecting someone over the internet, you will need Public ip addresses.
Address apart from the private ip in every range are public address.
Now if you have a server which is having a static ip then it wont be a problem for the client to access it anywhere from the world over internet.
But if its over a LAN , and accessing the internet from a gateway, then there will be a NAT, then you will need to set the inbound and outbound traffic rules at the gateway, for letting the client access the server.
I'm writing a Java application like AIM where I want a lookup server to help two clients connect to each other with the ServerSocket class. Unfortunately a serversocket needs an open port or it will be blocked by your firewall. Without having the clients manually change their firewall settings is there a way I can either find an open port, open a port (it's okay if it asks for permission) I just don't want every client to manually have to change firewall settings. Thanks!
Creating a ServerSocket is what opens the port on the server end (assuming the port is NOT also blocked on the firewall). On many systems, port numbers <1024 are not blocked by the firewall, and therefore often don't require custom configuration. However, since many ISPs have been more and more stringent with what ports they block within their network, many pieces of software have moved to operating over port 80. Why port 80? It's the port used for HTTP, and therefore pretty much open (at firewalls) 100% of the time for any service that operates on the web.
To answer the question of how to find an open port, port scanners perform this function. Basically all they do is try to establish a connection on a range of ports (say port 1-1024). When a connection is successful (in Java you would see this as a good connection over a Socket), you can consider that port "open". You don't need to exchange any data in order to make the connection, you just need to establish the connection, and then close it.
Also, if there is no server software of any kind running on the server, it won't appear "open", even if it isn't blocked. Without a successful Socket (TCP) connection, you won't know what ports are not blocked by user's firewall.
To reiterate, I'm switching back and forth between two related, but separate concepts. Firewalls can block/open ports, or have port forwarding. That's not the same as a port being "open" for connections on a given machine. In order for a machine to accept connections on a given port, there must be a piece of software listening for a connection on that port, otherwise no connection can be made.
I would suggest to look at a couple of alternative solutions that are less cumbersome - scanning for open ports can take quite some time, can cause panicky reactions from firewalls that feel attacked, and so on. There are a couple of techniques that are in active use and have been developed exactly to solve the problem of servers behind firewalls.
UDP Hole Punching, zero user side configuration needed. Simple explanation on how Skype uses this technique can be found here
UPnP / IGD could be used as an alternative, though less devices support it out of the box nowadays due to security problems.
STUN with a Java implementation of client and serverside called JSTUN
Whatever solution you choose, test it thoroughly with different internet service providers, there's a plethora of limitations you can expect wrt blocked ports.