How to specify ip and domain with a http request?

I have a apache server and configure some vhosts.

Apache server with ip 172.20.20.20.

A domain url is 'http://www.atest.com/' and another domain url is 'http://www.btest.com/'

In test enviroment, I use firefox with 'Switchhost' plug-in to access Domain A and Domain B.

It works well.

The problem is how can I make a http request by node.js?

my code is here:

var options = {
    host: 'http://www.atest.com/',
    port: 80,
    path: '/msg/putMsg',
    method: 'POST'
  };

  var req = http.request(options, function(res){
    var data = '';
    res.setEncoding('utf8');

    res.on('data', function(chunk){
      data += chunk;
    });

    res.on('end', function(){

    });
  });

  req.on('error', function(err){
    console.log('problem with request: ' + e.message);
  });

  req.write(msg);
  req.end();
}

this code snippet works to post some message to Domain A, but dns doesn't work.