Java apps can't connect to IP, but I can ping it

Question

I have a play application connecting to a database at 1.2.3.4 but for some reason Play! (and other java apps too) cannot connect to it.

I did try editing the hosts file and setting a name for the IP like app01 1.2.3.4 however it only worked on the box where the DB was hosted and it didn't work for external machines. I can confirm that the right port is open in iptables and I've tried restarting it, too.

If I ping postgres at port 4567 at 1.2.3.4 from the same box, it works. But setting the same IP address in my java apps fail. What's wrong? Is it permissions or iptables issue? Thanks.

Stack trace:

Caused by: java.net.NoRouteToHostException: No route to host
        at java.net.PlainSocketImpl.socketConnect(Native Method)
        at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:327)
        at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:193)
        at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:180)
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:384)
        at java.net.Socket.connect(Socket.java:546)
        at java.net.Socket.connect(Socket.java:495)
        at java.net.Socket.<init>(Socket.java:392)
        at java.net.Socket.<init>(Socket.java:206)
        at org.postgresql.core.PGStream.<init>(PGStream.java:62)
        at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:76)
        ... 35 more

Answer 1

Can you connect to your DB via pgadmin (http://www.pgadmin.org/)? Can you telnet from your machine on to the DB Server on the given port (telnet 1.2.3.4 4567)? If you cannot telnet maybe the DB Server is blocking your connections? Have you set any proxy server either only for java or only for rest of the system? From the exception it seems more of a networking issue than anything with java...

Answer 2

Can you check firewall settings on the db server. You can try by telnet to check whether you are able to connect or not.

Answer 3

If you're behind a router you WILL BE ABLE to ping it from another local machine (usin the local IP), but you might not be able to ping from the outside if you are not port-forwarding correctly (You can configure this from your router).

Another thing to take in mind is that you CAN'T ping your own external IP from within the network, that means that you can ping 192.168.1.100 (Server) from 192.168.1.xx (Client), but you can't ping 72.15.xx.xx (port-forwarded to 192.168.1.100) from 192.168.1.xx (Client).

In order to do this you have to use an external tool, there are many webs that can do this for you, here's the one that I use:

http://www.yougetsignal.com/tools/open-ports/

If it says the port is open then it is open and it should work now, if it shows closed then you should double check that your firewall is allowing the data, maybe shut it down temporary to see it thats the prob.

If you need help adding a port exception to your firewall or forwarding a port from your router just tell me which distro/routerModel you're using and I'll try to dig some more info for you.