Run Varnish probe on port that's different from the main backend port

Question

I'm using varnish to load balance between two servers and I have set up a director and two backends which are doing this successfully.

My problem is my two backends are running nginx on port 80 listing for my.domain.com. Nginx then reroutes my request to a process listening on port 3000. My process has a route called /healthcheck which I would like to use for the varnish backend probe.

This is currently what my .vcl file looks like:

backend b0 {
        .host = "53.243.15.198";
        .probe = { 
                .url = "/healthcheck";
                .timeout = 34 ms; 
                .interval = 1s; 
                .window = 10;
                .threshold = 8;
        }
}

So if I navigate to my.domain.com/healthcheck I get a 200 OK but when varnish initiates the request nginx doesn't pick it up since it's no longer originating from my.domain.com.

I've tried this based on this documentation but it Doesn't work:

 backend b0 {
            .host = "53.243.15.198";
            .probe = { 
                    .request =
                         "GET /healthcheck HTTP/1.1"
                         "Host: 53.243.15.198:3000"
                         "Connection: close";
                    .timeout = 34 ms; 
                    .interval = 1s; 
                    .window = 10;
                    .threshold = 8;
            }
    }

---

How can I health check my instances without running into this problem? Or in other words is there some way to query a different port then the main one for the backend?

Answer 1

You still need to send healthcheck to nginx. However, your probes must have a HTTP Host my.domain.com :

backend b0 {
        .host = "53.243.15.198";
        .host_header = "my.domain.com";
        .probe = { 
                .url = "/healthcheck";
                .timeout = 34 ms; 
                .interval = 1s; 
                .window = 10;
                .threshold = 8;
        }
}