A DESCRIPTION OF THE PROBLEM :
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Host

HTTP/1.1 requires the Host header be sent - this includes proxies - when using apache with apache_mod_proxy, it returns 400, dutifully honoring the spec

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Watch the following java code in your favorite tcpdump on port 8080; you'll see that only the request line is sent; no Host header

        var httpResponse = HttpClient.newBuilder()
                .proxy(ProxySelector.of(new InetSocketAddress("localhost", 8080)))
                .build()
                .send(HttpRequest.newBuilder(URI.create("https://www.google.com/"))
                        .build(), HttpResponse.BodyHandlers.ofString());
        assert httpResponse.statusCode() == HttpURLConnection.HTTP_OK;


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
CONNECT www.google.com:443 HTTP/1.1\r\n
Host: www.google.com\r\n
\r\n

ACTUAL -
CONNECT www.google.com:443 HTTP/1.1\r\n
\r\n


---------- BEGIN SOURCE ----------
        var httpResponse = HttpClient.newBuilder()
                .proxy(ProxySelector.of(new InetSocketAddress("localhost", 8080)))
                .build()
                .send(HttpRequest.newBuilder(URI.create("https://www.google.com/"))
                        .build(), HttpResponse.BodyHandlers.ofString());
        assert httpResponse.statusCode() == HttpURLConnection.HTTP_OK;

---------- END SOURCE ----------

FREQUENCY : always