Regarding the error occurred in my test case in nodeload.js script

I have designed an app using elastic search. And when Iam trying to write the test case using node load.js. I have got a problem that when I increase the number users I was getting the warning that "WARN: Error during HTTP request: Error: ECONNREFUSED, Could not contact DNS servers" and Iam unable rectify the problem so please help me in solving this error.

nl.run({
name: "test",
host: 'localhost',
port: 9200,
//path: '/my_river/page/_search?q=sweden',
numUsers: 2000,   //Increased my num of user**
timeLimit: 180,
targetRps: 500,
stats: [
    'result-codes', 
    { name: 'latency', percentiles: [0.9, 0.99] },
    'concurrency',
    'rps',
    'uniques',
    { name: 'http-errors', successCodes: [200,404], log: 'http-errors.log' }
],