Java Networking - Connecting two computers

Question

I'm trying to write a simply program client-server program that would connect a Client machine to a Server machine.

My code so far works well on the localhost, but when I replace the ip address in the Client code to the local ip address of the Server machine no connection is done. I think my understanding of InetAddress is off.

Socket connect code: Client.java

InetAddress ip = InetAddress.getByName("my_ip_address");
Socket s = new Socket(ip, 9876); //<- where the connection timeout happens

@ScaryWombat yes the ports or initialised as 9876 on both the client and server — AlexT
– AlexT, Commented Mar 19, 2019 at 1:16
@Joni I gwt an error at Socket s = new Socket(ip, 9876); : java.net.ConnectException: Connection timed out: connect — AlexT
– AlexT, Commented Mar 19, 2019 at 1:27
are you trying in an enterprise network? The firewall may block for non-standard port access. — javapedia.net
– javapedia.net, Commented Mar 19, 2019 at 1:53
@javapedia.net I don't have the knowledge to tell you that, is there any way I can find out — AlexT
– AlexT, Commented Mar 19, 2019 at 1:55
Is it linux client? Try nc command like this "ncat IP_address port_number". Try for stardard ports like 80 and then non-standard port. If standard ports work then possibly firewall issue for non-standard port. if this doesn't help, use machine-name instead. Did "ping ip" work? NCAT command reference: linuxtechi.com/nc-ncat-command-examples-linux-systems — javapedia.net
– javapedia.net, Commented Mar 19, 2019 at 2:01

Elliott Frisch · Accepted Answer · 2019-03-19 01:16:25Z

5

You don't call getBytes() from a String to get your ip address like that; option 1: call getByName(String) like

InetAddress ip = InetAddress.getByName("127.0.0.1");

Option 2: construct the proper byte[]. Like,

InetAddress ip = InetAddress.getByAddress(new byte[] { 127, 0, 0, 1 });

answered Mar 19, 2019 at 1:16

Elliott Frisch

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8 Comments

AlexT Over a year ago

Thanks for your response, I found this out from another question. Now I'm getting a connection timeout

Elliott Frisch Over a year ago

@AlexT General debugging techniques - Test locally with telnet; check firewall settings on machine acting as server; test with telnet from proposed client. You should be able to telnet myserver 9876 and get a connection.

nortontgueno Over a year ago

Debugging techniques worth more than gold these days :)

Elliott Frisch Over a year ago

@AlexT Not really. Ping can only test icmp. Not your socket. If you don't have telnet, you can also debug with netcat (which is more sophisticated).

AlexT Over a year ago

@ElliottFrisch well I tried running telnet server_ip 9876 on the client computer and I get: Could not open connection to the host, on port 9876: Connect failed

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