I know that the Socket class can be instantiated in this way (as an example):
new Socket("taranis", 7);
where "taranis" is the name of the server in a local network. In this respect I have two questions:
1. If my computers do not form a local network but are connected to the Internet, can I use the IP addresses of computers to instantiate a socket?
2. How my computers get names in the local network? Do I give names to the computers when I create the network.
P.S. I herd that computers can establish a network by themself (using zeroconf). Who then gives names to the computers and how can I know these names in advance (I need them to write my code).
Yes, you can create a socket using the ip address, you can do so like this: new Socket("192.168.1.00",8888)
When you install an OS on your computer, usually one of the steps that the OS makes you go through is giving your machine a name. Each OS also has a way of changing these names after installation. So, everyone of your computers probably does have a name. However, the tricky part is getting one machine to know the name of the other. This can be done in a few ways. One is by using a DNS server. This kind of like a middle man which will translate the name (i.e Computer1) to its IP address (192.1.168.1.100). You can also keep these mapping locally, you can put this in the hosts file. This is a mapping between names and ip addresses, and if you use this method, you need to make sure that these hosts files line up across the machines.
Zeroconf is an interesting protocol. The way it works is one computer creates a named service and the second computer just looks up the service by name, and once it finds the service it can connect to it. When the service is discovered, the connecting client and can query for the ip and port to connect to.
Before socket programming, you need some background on networking. Unluckily the questions you ask are not simple to answer as it depends on the specific network configuration you have. Here are some short answers, but may be incorrect due to the dependency on the particular configuration. You will do better to read up on TCP/IP for example here.
Yes. But I doubt that that your computers do not form a local network (LAN). In case they do, you can use their LAN IP address. To find the ipaddress, you can use "ipconfig" command on the "Command Prompt" on Windows, and "ifconfig" on unix. The output is the configuration of each network interface in the computer.
Yes, you can configure the name of each computer on the computer.
For programming, usually you would use ipaddress (use name when the name can be dynamically assigned to one of the computers (using a Dynamic DNS)). IP Address may also be (and is quite commonly) dynamically assigned using a DHCP server.
Related
I made a networked game in Java using Sockets. It works great, except... only through my LAN. :/ What I want to be able to do is set it up so that I can start the server running on my home computer and send the client code, in an executable jar, to someone else, and have them be able to launch it and connect through the internet. But like... I have no idea how to set up my modem & router for that. :( I have a wireless Clear modem, for which the configuration page looks like this:
http://imageshack.us/photo/my-images/254/modem.jpg
And a Netgear router, whose page looks like this:
http://imageshack.us/photo/my-images/443/routerq.jpg
Right now, in the client and server runner classes, I pass my PC's private IP address along with my chosen port number to the ServerSocket and Socket objects. I hear that Sockets are able to be either UDP or TCP, depending on how you set them up, or something...? I don't do anything like that though, I just instantiate them and pass them the 2 values, and off they go...
In the code for the server:
ServerSocket sock = new ServerSocket();
sock.bind(new InetSocketAddress(IP, 9001));
Code for the client:
Socket sock = new Socket(IP, 9001);
So also, I would need to know how to change the code accordingly with the changes to my router/modem settings.
"IP" is a String representing my computer's private IP, 192.168.1.10, and the program only works if that is the string I pass to the Socket & ServerSocket. Like I said, I have a modem which is connected to a router which is connected to my computer. So, can someone please explain to me (in detail, because I'm a noob) what exactly I would have to do to configure both of them, AND the changes I would have to make to my code?
You'll have to manually port forward, or look at a solution like UPnP or NAT-PMP for automatic port forwarding. Unfortunately, NAT is a pain in the ass to configure and most routers don't properly support it. It's also not a simple task, as you have to support a whole range of different implementations and mechanisms.
It shouldn't be too difficult to set up your router to forward one port to your machine, though. See http://portforward.com/ for an awesome list of how to do it on pretty much any router.
It sounds like you're asking a number of questions here... I'm no network/socket programming specialist, but here's some ideas.
For distribution, you may wish to look into Java Web Start. It'll make it easy for your clients to obtain the app as well as automating updates.
If you're using the Socket constructor like that, the actual implementation will be a system default. You could override this by calling this method with a suitable SocketImplFactory implementation.
As for the IP address and port... Using a hard-coded value for the port should be okay. You'll need to decide on a port or at least some default anyway for clients to connect to. Even so, it might be useful to have the value read from some external configuration file. This'll make it easier should you ever decide to distribute the server app to let other people run servers. They might want to use a different port.
Having the server's IP address (yours) hard-coded in code is definitely a no-no, though. Clients should connect using either an IP address or host name that they need to provide or set in some configuration file. A host name would be required if you don't have a static IP address.
Unless this is something for you and some friends to enjoy, where you can always just initiate a game by providing IP and port in a chat session or something, you'd be better off finding some external hosting solution at a company that takes care of the DNS resolution and network setup for you. These days you'll find stuff like that pretty cheap if you don't have requirements like a Java EE server or database.
EDIT: thought of something else. TCP might be okay for your use-case, but if this is a game that requires minimal lag and quick input synchronization (like a shooter or fighting game) then it's not the best choice. In that case UDP would be better. It would induce the requirement of adding some sort of detection for desync or a mechanism that makes up for any dropped packets.
I'm writing an application that has 10 (as an Example) threads running. each thread creates a connection using HttpURLConnection to external host. I need to use a different IP address for each connection in different threads. can I do it considering the fact that there are about 10 valid static IP addresses assigned to my server?
note that I have seen something like same question here and here but in first one though it has an answer checked as correct by asker, but I see too many comments telling that Its not working and in second one is talking about How to make the JVM use a given source IP by default? which does not help me I guess.
beside I don't want to risk and pay for the server and then find out I cant do such a thing!
I was wondering if someone could give me a clue. The situation is we have one computer that has a server application, the rest have a client application. Now my question is how would i keep track of the server ip address? What I want is have the clients automatically hunt the server application, regardless of the ip address and find it, because using a wireless network computers are using DHCP. So I was wondering is there anything that can be buried in a computer using Java with which other computers can identify the application. Thanks.
The "good" way is to use DNS (imho the ONLY solution in a productive environment), as #Peter Lawrey already mentioned, and i would prefer that solution.
You have two other possibilities:
Let the server broadcast his address on the network.
Let the clients scan the network.
Both this solutions are not ideal, because you create a lot of noise on the network, may be blocked by a firewall, etc. Therefore, i really suggest you to use DNS.
If you can not use the DNS solution i would go with a solution that lets the clients search for the server.
Take a look at this SO question, might get you started.
If they're on the same physical network, your server can broadcast its IP address via UDP to the broadcast address for that sub net.
You could even follow the uPnP standard.
Alternatively, you could host the location of the server from a known other location, like a web server. For example, call an action on the known web server to retrieve the IP address of the server. This allows you to utilize DNS to look up the first host, then go from there.
If you know the DNS-Name of the server you can use that name instead of the IP. Java takes care of the DNS lookup.
Otherwise, I guess the simplest solution is just to scan the IP range of your network and just try to connect to each IP and test if it works.
Other solutions would include broadcasts by the server and would be much more complicated and maybe overkill for a application hosted in a small network.
I want to find all systems that are on the same local network i.e. that have the same subnet mask.
I think I must work with InetAddress Class in Java but I don't know what should I do.
My algorithm is:
Find all available system in a local network
Check if they run on a my requested port
Keep in mind the IP of system which runs on this port
InetAddress localHost = Inet4Address.getLocalHost();
NetworkInterface networkInterface = NetworkInterface.getByInetAddress(localHost);
System.out.println(networkInterface);
networkInterface.getByInetAddress(localHost);
for (){
networkInterface.getByInetAddress(localHost)
}
I think that the networkInterface.getByInetAddress(localHost) can give me all available IP addresses in a subnet, but I don't how I can do this for all available systems, and what is the condition for the for clause.
Finding all of the possible IP addresses in an IP subnet is simple. Just write a loop that enumerates all possible byte[] representations in the subnet and constructs InetAddress instances for them.
That's the easy bit, the hard bit is solving these two problems:
how do you find all if the live IP addresses on a subnet, and
for a given IP address, how do you figure out if it is using a particular port.
The first problem can be solved in two ways:
You could use the InetAddress.isAlive() method to see if the host responds to a ping. The problem is that some hosts may be configured to not respond to pings, or the pings may be blocked by an internal firewall. This also doesn't work if the subnet is large. The problem is that you need to ping a large number of hosts, and that will take a long time and generate a lot of network traffic ... especially if there are multiple application instances doing this.
The smarter alternative is to examine your local machine's ARP cache, and extract all IP addresses that it knows about. The problem is that 1) your application may not have the access rights to access the ARP cache, 2) some IP addresses may not be in the cache, and 3) there is no way to access the ARP cache in pure Java.
The second problem depends on the nature of the service on the port:
If it is a TCP-based service, then you can attempt to connect to an IP/port using a plain socket. If the connection is accepted, then the port is in use by something. It may or may not be the service you expect, but in general the only way to tell that is to try to use the service.
If it is a UDP-based service, then there's now way in general to know if something is using the port.
I'm going to hope that you mean the following:
"I wish to discover all systems on the same local network as (the computer running my program)'s primary IP address which are listening to a certain TCP port."
With UDP, this is a slightly harder problem - you could look for bounces, but your packet may be eaten or lost (as could the reply).
Now, assuming TCP, you have three steps.
Get the IP address of the local interface you believe is the "primary" one.
Enumerate hosts on the same subnet as this address.
For each host, determine if the appropriate port is listening.
For step 1, use InetAddress.getLocalHost().
For step 2, get the subnet portion of the retrieved address, and then try every possible host within that subnet. Let's hope it's a Class C and there are only 254 hosts. There is no 100% sure way to get all the hosts without trying each one. A non-100%-certain (and more complex) way is to use ARP tables to get a router's view of the network, to send a broadcast ping to the local net, or to fall down to layer 2 and send a broadcast Ethernet packet.
For step 3, open a connection to each host on the appropriate port and then close it. If the connection times out or is denied, scratch the host. Otherwise, add it to the "listening" list. You probably want to try more than one connection at a time and set the timeout interval to something small, since most people won't be listening.
Step 4: congratulations! You've just made a very feature-poor version of nmap in Java.
I have been struggling with this for the entire day now, I hope somebody can help me with this.
My problem is fairly simple: I wish to transfer data (mostly simple commands) from one PC to another over the internet.
I have been able to achieve this using sockets in Java when both computers are connected to my home router. I then connected both computers to the internet using two different mobile phones and attempted to transmit the data again. I used the mobile phones as this provides a direct route to the internet and if I use my router I have to set up port forwarding, at least, that is how I understand it.
I think the problem lies in the method that I set up the client socket. I used:
Socket kkSocket = new Socket(ipAddress, 3333);
where ipAddress is the IP address of the computer running the server. I got the IP address by right-clicking on the connection, status, support. Is that the correct IP address to use or where can I obtain the address of the server? Also, is it possible to get a fixed name for my computer that I can use instead of entering the IP address, as this changes every time I connect to the internet using my mobile phone?
Alternatively, are there better methods to solving my problem such as using http, and if so, where can I find more information about this?
EDIT:
Would it be possible to have the server program running on a server on the internet somewhere. My original server would then be a client that send information to this server. This server would then pass this information to my original client and vice versa. That way, the IP address of my computer won't matter, as I only need to know the address of the server hosted somewhere on the web. Does this seem like a better solution? Where do I begin implementing such a solution?
Thanks!
When you connected to the server that serves StackOverflow, did you type in the IP address? It's 64.34.119.12, if that jogs your memory.
You probably didn't. You probably typed "stackoverflow.com". There's a huge, complex, clever, and in some ways, poorly implemented system called DNS that translates sensible and human-readable names into IP addresses.
One problem with DNS, though, is you need a "static IP", which is exactly what it sounds like: an IP address that doesn't change, which is exactly what you don't have.
So, what can you do?
You can buy a static IP account from your ISP (pretty expensive)
You can use some proxy out in the Internet (a machine that does have a static IP and is willing to bounce your packets back and forth -- I'm not aware of any service that does this for you; you could write one and put it up on Amazon Web Services or Google App Engine, both of which would be free at your level of usage, but they'd be slow, since every packet trying to cross your living room would have have to go via some data-center in Virginia).
You can keep doing what you're doing, looking in the net-configuration of your machine.
You could speed (3) up a little by having your server program look up its own IP address and print it out where you could see it and type it into the server by hand.
You can use DynDNS, as Sergey mentioned (this is the "right" solution, in that it's very general, it just might be a little complicated to set up)
You can use multi-casting.
Multi-casting is an interesting solution, and it may work for you. The idea is, when your server starts up, it announces to the net, "Here I am, I'm providing X server, here's my IP address, talk to me." The problem is, a multi-cast won't leave your living room. Obviously, if every multi-cast were spread to every computer on the Internet, the whole thing would collapse, so your router will ignore, and not route, multi-cast packets. That may or may not be a deal-breaker for you. EDIT Re-reading your question, I see it is a deal-breaker for you. I'd go with #5, but be aware there may be routing issues (address translations that prevent a server from knowing the address that other computers can find it at) or fire-wall issues (that is, your ISP may prevent your server from receiving incoming packets even if the address is correct).
using a direct socket connection with a port like 3333 is usually complicated because different network configurations.
firewalls will make a pleasure preventing the connection, or killing it from time to time.
maintaining a 2-way connection can be a nighmare. the SIP protocol is struggling with this kind of problems.
For a simple application, i suggest you look into the comet technology, where your clients can establish an http connection with a shared server. The server can then bridge commands between them.
html5 will also bring the websocket protocol to the table.
I got the IP address by right-clicking
on the connection, status, support.
Not sure about the "support" part, and I'm not on a Windows machine right now, but I think that the most easy and reliable way to figure out the IP address on Windows is to run "ipconfig" from the command line (Win+R, type "cmd", then "ipconfig" in the opened window). This, of course, should be done on the server side.
However, the problem is that depending on the ISP your IP address may be not within the Internet, but within a local ISP network (so-called NAT). In this case, you'll need to use some sort of black magic called TCP hole punching, which is very complicated and not guaranteed to work. You can figure out if your address is local or not by looking at it. For IPv4 local addresses are almost always like 10.x.x.x or 172.16-31.x.x, or 192.168.x.x. Don't know about IPv6.
You can also check your IP by visiting one of the special sites like www.whatismyip.com. If the address they tell you is different from the one you see by running "ipconfig" or looking at the connection properties, then you're almost certainly behind a NAT (or your ISP is using a transparent proxy, but that's rare).
If you are directly connected to Internet (no local addresses and NAT), then you should also check if you have any firewall software and either to configure it to allow connections to the port you use, or make sure it's in "ask the user" (and not "silently reject") mode, or just disable it completely (this may put your computer at risk, especially if there is no anti-virus software or the system isn't up-to-date).
Also, is it possible to get a fixed
name for my computer that I can use
instead of entering the IP address, as
this changes every time I connect to
the internet using my mobile phone?
Yes, it's possible. There is the thing called DynDNS, and there are DynDNS providers like DynDNS.com, where you can get a third-level domain name for free (like mycoolpc.dyndns.org). You'll have to install and configure some DynDNS client on your PC that will tell the DynDNS server its new IP each time each changed. I don't know anything about particular clients to use because I'm using the one built-in in my home router.
No need to write networking code for this, unless it really floats your boat. Take a look at SCP. http://amath.colorado.edu/computing/software/man/scp.html. There is a windows implementation where you can download putty (windows ssh client), and it is on most linux distributions. Alternatively, you could set up an FTP or SSH server on one or both of the machines.
"a fixed name for my computer that I can use instead of entering the IP address" would be a domain name, these are purchasable online for a few bucks.