Java Socket, binding to local port

Question

I am trying to bind the Socket on the client side to any particular local port, in this code I used 20000.

Normal connections such as below works just fine. But does not allow for me to choose the local port.

hostSocket = new Socket(host,80);

So I tried this:

hostSocket = new Socket(host, 80, InetAddress.getLocalHost(), 20000);

and this:

hostSocket = new Socket();
hostSocket.bind(new InetSocketAddress("localhost", 20000));
hostSocket.connect(new InetSocketAddress(host,80));

But they both leave me with this exception... in the second case the exception occurred on the connect call. I'm not really sure what I am missing and I would love some pointers.

java.net.SocketException: Invalid argument or cannot assign requested address
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 com.mb.proxy.netflix.NetflixPrefetchingAgent.connect(NetflixPrefetchingAgent.java:98)
at com.mb.proxy.netflix.NetflixPrefetchingAgent.run(NetflixPrefetchingAgent.java:164)
at java.lang.Thread.run(Thread.java:679)
Exception in thread "Thread-19" java.lang.NullPointerException
at com.mb.proxy.netflix.NetflixPrefetchingAgent.prefetchChunk(NetflixPrefetchingAgent.java:272)
at com.mb.proxy.netflix.NetflixPrefetchingAgent.run(NetflixPrefetchingAgent.java:176)
at java.lang.Thread.run(Thread.java:679)

Answer 1

You have to bind to the external (outbound) IP address of your machine, not localhost (127.0.0.1).

The following works on my box without issue:

Socket s = new Socket();
s.bind(new InetSocketAddress("172.16.1.102", 5000));
s.connect(new InetSocketAddress("google.com", 80));

Where 172.16.1.102 is the NAT'd private network address assigned to this box via DHCP from my router.