ping网络上的计算机

Question

I am a researcher at University and I'm trying my hand at a bit of amateur java2 programming.

I've been working through the KnockKnockServer/Client Java2 tutorial to send strings from one program to another using Java Sockets.

http://download.oracle.com/javase/tutorial/networking/sockets/clientServer.html

Full source can be downloaded there and it compiles nicely and is a great example of using IP's to get two programs to talk to each other in Java.

This works fine when I use both the server and the client running on the same machine and the machine name as "localhost"

kkSocket = new Socket("localhost", 4444);

And it also works at home when I use it on my home network with a InetAddress:

InetAddress myaddress = InetAddress.getByName("129.11.138.64");
kkSocket = new Socket(myaddress, 4444);

However when I try to do the same on the university machines it doesn't work because they are behind some kind of proxy despite the fact that the two machines are next to each other and plugged into adjacent ethernet ports.

Question:

How to I work out how to get the two machines to speak to each other, when I don't have full control of the network they are on? I can't even work out how to get them to ping each other. I have visited "www.whatismyip.com" to get the external ip's of each machine but that doesn't work, and I've looked at the next work adapters ip and the gateway they are on but still I can't get them to talk to each other.

Any ideas.

Thanks!

Answer 1

在我看来，您的网络正在阻止端口4444上的流量。请尝试使用其他端口，或确认是否允许端口4444。

Answer 2

Even i cannot ping two machines at my work place.

you need to change the network preferences or try the following:
(instructions for IE) Go to tools-> internet options -> lan settings -> advanced.
Add exceptions to the field: "Do not use proxy server for addresses beginning with:"
enter the networks ip starting numbers, for eg if your network has ip adds starting with 10.--.--.-- write 10.* in the field.

Answer 3

You need to make sure that each machine knows about the other's private IP address on the LAN (see the ipconfig or ifconfig command). It changes (nearly) each time the devices are connected on the LAN, and sometimes even while they are connected on the LAN.

www.whatismyip.com won't help you, because it will only give you the public IP address of the router/NAT which is closest to that server. There is no guarantee it is also the closest one to your devices. Even if it were, this information would be useless, because you (most often) can't predict how the NAT will translate your private IP address into public IP address.

Even if you could 'guess' that information, there is a possibility that the router/NAT might not let you use this translation behind it (ie, on the LAN).

To summarize it, make sure you use the proper private IP addresses assigned to your devices and make sure that no router or firewall is blocking traffic over the considered port.

Answer 4

There are several different parts to this problem:

1) Can the two machines connect to each other?

You can test this from the command line with the "ping" command which is available in both Windows and *nix. Ping tests for the existence and reliability of a connection to the named machine.

2) Could the route be filtered?

Just because you can ping a machine, doesn't mean there is no firewall or proxy between them. You can find the route taken between the two machines with the "tracert" command. The more steps in the route, the more things that might stop your signal

3) Is Java firewalled?

Just because it is installed, does not mean it has open network access. Try fetching a few URLs or similar to see if Java has network connectivity.

4) Is the port blocked?

A port could be blocked in two ways: some other application is using it, or a firewall is limiting listening ports. If the former, Java will not be able to attach a Socket to the port. If the latter, try a different port. There is nothing to stop you running 10000 clients simultaneously and seeing if you can connect to any of them.

5) How do you do this in Java?

Ping can be implemented by InetAddress.isReachable().

6) What should I say to the angry Network Admin guy who has just turned up?

"I'm not port and network scanning! Honest!" Hunting for usable ports is a basic hacking operation. Some workplaces have automatic tools to check for it and will view it as inappropriate.