Not Receiving Messages in ZeroMQ Output Plugin

I have a simple client subscribing to messages on port 2120:

zmq = require('zmq');

port = 2120;
sock = zmq.socket('sub');
sock.connect("tcp://" + port);

sock.on('message', function (topic, msg) {
  console.log('received message');

console.log("zeromq listening to port" + port)

Logstash is outputting to the zeromq plugin like this:

output {
    zeromq {
      topology => "pushpull"
      topic => "foo"

When I execute run logstash ./bin/logstash -f site.conf and subscribe with my node.js client node index, I never receive a message. Why is that?

Are messages being pushed through the LS pipeline? Are you sure of that?