Java DNS cache viewer

Question

Is there a way to view/dump DNS cached used by java.net api?

Answer 1

The above answer does not work with Java 11. In Java 11, both positive and negative cache entries can be retrieved using the 'cache' instance variable. Here are new adaptations:

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.lang.reflect.Field;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.time.Instant;
import java.time.temporal.ChronoUnit;

public class DnsCacheFetcher {
static long startTimeinNano = System.nanoTime();

public static void main(String[] args) throws Exception {

    System.out.println("SecurityManager: " + System.getSecurityManager());

    InetAddress.getByName("stackoverflow.com");
    InetAddress.getByName("www.google.com");
    InetAddress.getByName("www.yahoo.com");
    InetAddress.getByName("www.ankit.com");

    try {
        InetAddress.getByName("nowhere.example.com");
    } catch (UnknownHostException e) {
        System.out.println("Unknown host: " + e);
    }

    String addressCache = "cache";
    System.out.println(">>>>" + addressCache);
    printDNSCache(addressCache);
    /*
     * String negativeCache = "negativeCache"; System.out.println(">>>>" +
     * negativeCache); printDNSCache(negativeCache);
     */
}

private static void printDNSCache(String cacheName) throws Exception {
    Class klass = InetAddress.class;
    Field[] fields = klass.getDeclaredFields();

    /*
     * for (Field field : fields) { System.out.println(field.getName()); }
     */

    Field acf = klass.getDeclaredField(cacheName);
    acf.setAccessible(true);
    Object addressCache = acf.get(null);
    Class cacheKlass = addressCache.getClass();

    Map cache = (Map) acf.get(addressCache);
    for (Map.Entry hi : cache.entrySet()) {
        /* System.out.println("Fetching cache for: " + hi.getKey()); */
        Object cacheEntry = hi.getValue();
        Class cacheEntryKlass = cacheEntry.getClass();
        Field expf = cacheEntryKlass.getDeclaredField("expiryTime");
        expf.setAccessible(true);
        long expires = (Long) expf.get(cacheEntry);

        Field af = cacheEntryKlass.getDeclaredField("inetAddresses");
        af.setAccessible(true);
        InetAddress[] addresses = (InetAddress[]) af.get(cacheEntry);
        List ads = null;
        if (addresses != null) {
            ads = new ArrayList(addresses.length);
            for (InetAddress address : addresses) {
                ads.add(address.getHostAddress());
            }
        }

        /*
         * System.out.println(hi.getKey() + " expires in " +
         * (Instant.now().until(Instant.ofEpochMilli(expires), ChronoUnit.SECONDS)) +
         * " seconds. inetAddresses: " + ads);
         */

        /*
         * System.nanoTime() + 1000_000_000L * cachePolicy : this how java 11 set
         * expiryTime
         */
        System.out.println(hi.getKey() + " expires in approx " + (expires - startTimeinNano) / 1000_000_000L
                + " seconds. inetAddresses: " + ads);


    }
}

}