Utility class for doing DNS reverse lookups, without the fluff.

InetAddressUtils.java

import sun.net.spi.nameservice.NameService;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;
import java.net.InetAddress;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.List;

/**
 * Utility class for doing DNS reverse lookups, without the fluff.
 */
public class InetAddressUtils {

    public static List<NameService> nameServices;

    static {
        // load name services once on initialisation...
        nameServices = getNameServices();
    }

    @SuppressWarnings("unchecked")
    public static List<NameService> getNameServices() {
        try {
            // do naughty things...
            Field staticField = InetAddress.class.getDeclaredField("nameServices");
            staticField.setAccessible(true);
            return (List<NameService>) staticField.get(null);
        } catch (Throwable t) {
            throw new RuntimeException("Got caught doing naughty things.", t);
        }
    }

    /**
     * <p>
     * {@link InetAddress#getHostAddress()}'s implementation does a few additional checks which I don't want (or which
     * may not be supported in my system e.g. forward DNS lookups). So here's an implementation that JUST get's the
     * hostname (i.e. performs a reverse DNS lookup).
     * </p>
     * <p>
     * Unfortunately, everything in `java.net` is locked down like a mofo, so we have to do some "naughty" things,
     * to go places that we're not supposed to go.
     * </p>
     */
    public static String getHostName(InetAddress addr) {
        String host = null;
        for (NameService nameService : nameServices) {
            try {
                host = nameService.getHostByAddr(addr.getAddress());
            } catch (Throwable t) {
                // NOOP: problem getting hostname from this name service, continue looping...
            }
        }
        return host != null ? host : addr.getHostAddress();
    }

    /**
     * Set's the accessiblity on an object.
     */
    static void setAccessible(final AccessibleObject ao,
                              final boolean accessible) {
        if (System.getSecurityManager() == null)
            ao.setAccessible(accessible);
        else {
            AccessController.doPrivileged(new PrivilegedAction() {
                public Object run() {
                    ao.setAccessible(accessible);
                    return null;
                }
            });
        }
    }

}