UnknowHostException cannot recover when network is back but restart JVM solved it

问题

From one timepoint, our JVM(In fact a Yarn NodeManager) start to report UnknownHostException; It is reported by JVM code

return InetAddress.getByName(host);

for the next more than 2 days, the exception always exists; During the time it is reporting this error, I do the following test:

During the error happening, ping could succeed and get the IP address(Very weird);
During the error, I write a simple test case to check the hostname resolve, it also could succeeded:
After we restarted the JVM, error is gone;

This is the code I used for test:

public class Main {
  public static void main(String[] args){
    InetSocketAddress addr = NetUtils.createSocketAddr("host-name:8020");
    System.out.println(addr.isUnresolved());
  }
}




# NetUtils is a YARN class which simply call the InetAddress.getByName()
    public static InetSocketAddress createSocketAddrForHost(String host, int port) {
        String staticHost = getStaticResolution(host);
        String resolveHost = (staticHost != null) ? staticHost : host;

        InetSocketAddress addr;
        try {
          InetAddress iaddr = SecurityUtil.getByName(resolveHost);
          // if there is a static entry for the host, make the returned
          // address look like the original given host
          if (staticHost != null) {
            iaddr = InetAddress.getByAddress(host, iaddr.getAddress());
          }
          addr = new InetSocketAddress(iaddr, port);
        } catch (UnknownHostException e) {
          addr = InetSocketAddress.createUnresolved(host, port);
        }
        return addr;
      }

We haven't change the /etc/hosts for a long time;

ENVs: JDK: java version "1.8.0_121" OS:

Distributor ID: Ubuntu
Description:    Ubuntu 14.04.5 LTS
Release:    14.04
Codename:   trusty

I believe that in the timepoint when the error start to occur, yes, the network has some problem. But what is weird is that:

why it cannot recover after the network is back(For example, when I find this error and do some test and ping). In fact the network problem happened for only 30 minutes, but the JVM still report these error;
why the problem is gone after I restart the JVM?

I checked the JVM configuration , the networkaddress.cache.ttl and networkaddress.cache.negative.ttl are all default value; So, when we find the unresolved hostname, I should retry and it should succeed after the network is back;

回答1:

What you are describing sounds like the JVM cached a hostname lookup.

From Javadoc for InetAddress:

By default, when a security manager is installed, in order to protect against DNS spoofing attacks, the result of positive host name resolutions are cached forever.

The default value for networkaddress.cache.ttl will look up a hostname one time and cache that result indefinitely for the life of the JVM. Try setting it to something non-default – for example, to cache lokoups for 10 seconds, set it "10".

Here's the entry from Networking Properties:

networkaddress.cache.ttl

Specified in java.security to indicate the caching policy for successful name lookups from the name service. The value is specified as integer to indicate the number of seconds to cache the successful lookup.

A value of -1 indicates "cache forever". The default behavior is to cache forever when a security manager is installed, and to cache for an implementation specific period of time, when a security manager is not installed.

The default for networkaddress.cache.negative.ttl is 10, but I suspect that isn't affecting your application behavior.

来源：https://stackoverflow.com/questions/59483550/unknowhostexception-cannot-recover-when-network-is-back-but-restart-jvm-solved-i

标签

jvm

inetaddress

unknown-host