Why is root required for listening on the loopback in Java?

Question

How come when I try listening to a loopback host in Java (using a plain old socket), it requires root access, yet if I use nio, it doesn't? If I tried listening on any other language on the loopback, it doesn't require any elevated privileges too.

This is my code right now:

import java.net.*;

class Test
{
    public static void main( String[] args )
        throws Exception
    {
        ServerSocket server = new ServerSocket( 80 );
        while( true )
        {
            Socket socket = server.accept();
            System.out.print( "." );
        }
    }
}

When I run this as a normal user, this exception is thrown: Exception in thread "main" java.net.BindException: Permission denied. Yet when I run this as root, it works fine as expected.

It is not about Java, it's about OS permissions. See http://stackoverflow.com/questions/413807/is-there-a-way-for-non-root-processes-to-bind-to-privileged-ports-1024-on-li — adrian.tarau, Oct 02 '11 at 01:55
Argh, didn't realize that I was listening on 8080 on the other one. — , Oct 02 '11 at 01:59

Oscar Korz · Accepted Answer · 2011-10-02T01:59:20.740

You need root permissions to bind a socket in the port range 0-1023. These ports are used for common services like HTTP and SSH and it would be dangerous to allow random users to bind their arbitrary applications to these ports.

See wikipedia: http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers#Well-known_ports

Edit: Keep in mind that arbitrary applications can connect to well-known ports, just not bind to them. Otherwise users would not be able to run honest programs ssh or mozilla. This is likely why you are not able to reproduce this error in other languages.

Why is root required for listening on the loopback in Java?

1 Answers1