disabling Java DNS cache

Question

I'm using Java 8 (OpenJDK). It seems like the best way to disable dns cache in Java is to set networkaddress.cache.ttl=0 in $JAVA_HOME/jre/lib/security/java.security.

I've tried that, but it seems to have no effect, no matter what value I put there, DNS entries are cached for ~5 seconds. I know that the value is set properly because I check it with java.security.Security.getProperty("networkaddress.cache.ttl").

I'm testing the java cache with a HttpURLConnection client, and comparing it to a raw curl client, curl doesn't cache at all. My server is a load balancer that returns one of 3 different ips for each request in round robin. Those ips each host a web service that returns the hostname it's running on.

This is how the my java client looks like:

    public static void main(String[] args) throws Exception {
        System.out.println("networkaddress.cache.ttl=" + java.security.Security.getProperty("networkaddress.cache.ttl"));
        Thread.sleep(1000);
        printServerHost();
        Thread.sleep(2000);
        printServerHost();
        Thread.sleep(4000);
        printServerHost();
        Thread.sleep(6000);
        printServerHost();
        Thread.sleep(11000);
        printServerHost();
    }

    public static void printServerHost() throws Exception{
        URL url = new URL("http://server/");
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod("GET");
        con.setRequestProperty("Content-Type", "application/json");
        con.setRequestProperty("Cache-Control", "no-cache");
        con.setRequestProperty("accept", "application/json");

        BufferedReader reader = new BufferedReader(new InputStreamReader(con.getInputStream()));
        String response = reader.readLine();
        reader.close();
        con.disconnect();

        System.out.println(response);
    }

Which outputs:

networkaddress.cache.ttl=0
ip-172-31-12-68.ec2.internal
ip-172-31-12-68.ec2.internal
ip-172-31-12-68.ec2.internal
ip-172-31-41-56.ec2.internal
ip-172-31-16-34.ec2.internal

And this is how my curl client looks like:

curl server
sleep 2s
curl server
sleep 4s
curl server
sleep 6s
curl server
sleep 11s
curl server

Which outputs:

ip-172-31-41-56.ec2.internal
ip-172-31-16-34.ec2.internal
ip-172-31-12-68.ec2.internal
ip-172-31-41-56.ec2.internal
ip-172-31-16-34.ec2.internal

What am I missing? Should I also configure something in HttpURLConnection? Another jvm wide config? Maybe it's another cache, not a dns one, a connection pool or a socket pool?

score -1 · Answer 1 · answered Mar 15 '19 at 18:06

Issue was another cache, not related to dns, probably in connection or socket, as using a ForkJoinPool shows different hosts:

ForkJoinPool forkJoinPool = new ForkJoinPool(3);        
forkJoinPool.submit(() -> IntStream.range(0, 10).parallel().forEach(i -> printServerHost())).get();

disabling Java DNS cache

1 Answers1