client.c

#include <zmq.h>

int main(void) {

	void *context = zmq_ctx_new();

	void *sock = zmq_socket(context, ZMQ_REQ);

	zmq_connect(sock, "tcp://localhost:5999");

	while (1) {
		char response[10];
		zmq_send(sock, "hi", 2, 0);
		zmq_recv(sock, response, 10, 0);
	}

	return 0;
}

zmq-proxy.rkt

#lang racket

(require zeromq)

(define router (zmq-socket 'router #:bind "tcp://*:5999"))
(define dealer (zmq-socket 'dealer #:bind "inproc://test"))

(define (proxy-loop left right)
  (define msg (zmq-recv* left))
  (zmq-send* right msg)
  (proxy-loop left right))

(thread (λ () (proxy-loop router dealer)))
(thread (λ () (proxy-loop dealer router)))

(define worker (zmq-socket 'rep #:connect "inproc://test"))

(define (worker-loop)
  (define msg (zmq-recv* worker))
  (zmq-send* worker msg)
  (print ".")
  (worker-loop))

(worker-loop)