Allow setting the Host header in a httpGet probe

Fixes #24288

pkg/probe/http/http.go

@@ -64,6 +64,9 @@ func DoHTTPProbe(url *url.URL, headers http.Header, client HTTPGetInterface) (pr
 		return probe.Failure, err.Error(), nil
 	}
 	req.Header = headers
+	if headers.Get("Host") != "" {
+		req.Host = headers.Get("Host")
+	}
 	res, err := client.Do(req)
 	if err != nil {
 		// Convert errors into failures to catch timeouts.

pkg/probe/http/http_test.go

@@ -85,6 +85,20 @@ func TestHTTPProbeChecker(t *testing.T) {
 				"X-Muffins-Or-Cupcakes: muffins",
 			},
 		},
+		{
+			// Echo handler that returns the contents of Host in the body
+			func(w http.ResponseWriter, r *http.Request) {
+				w.WriteHeader(200)
+				w.Write([]byte(r.Host))
+			},
+			http.Header{
+				"Host": {"muffins.cupcakes.org"},
+			},
+			probe.Success,
+			[]string{
+				"muffins.cupcakes.org",
+			},
+		},
 		{
 			handleReq(FailureCode, "fail body"),
 			nil,