Varnish Backend polling

backend default {
        .host = "127.0.0.1";
        .port = "8080";
        .probe = {
                .timeout = 100ms;
                .interval = 1s;
                .window = 5;
                .threshold = 4;
                # .url = "/test.html";
                .request =
                        "GET /test.html HTTP/1.1"
                        "Host: sys-notes.com"
                        "Connection: close"
                        "User-Agent: varnish-probe"
                        "Accept: */*";
        }
}

• .window is the number of probe results to consider
• .threshold - minimal number of good results (out of last .window results) to consider the backend healthy
• It is a good practice to set the User-Agent header as many webservers configuration filter empty useragent to defend from bots.

Nginx specific notes

• If nginx needs to proxy the request (E.g. to fastcgi) then the request will fail with error 499 because it is closed too early.