review fixes 1

Signed-off-by: anemyte <anemyte@gmail.com>

main.go

@@ -120,20 +120,12 @@ func probeHandler(w http.ResponseWriter, r *http.Request, c *config.Config, logg
 		return
 	}
 
-	if module.Prober == "http" {
-		paramHost := params.Get("hostname")
-		if paramHost != "" {
-			if module.HTTP.Headers == nil {
-				module.HTTP.Headers = make(map[string]string)
-			} else {
-				for name, value := range module.HTTP.Headers {
-					if strings.Title(name) == "Host" && value != paramHost {
-						http.Error(w, fmt.Sprintf("Host header defined both in module configuration (%s) and with parameter 'hostname' (%s)", value, paramHost), http.StatusBadRequest)
-						return
-					}
-				}
-			}
-			module.HTTP.Headers["Host"] = paramHost
+	hostname := params.Get("hostname")
+	if module.Prober == "http" && hostname != "" {
+		err = setHttpHost(hostname, &module)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
 		}
 	}
 
@@ -167,6 +159,23 @@ func probeHandler(w http.ResponseWriter, r *http.Request, c *config.Config, logg
 	h.ServeHTTP(w, r)
 }
 
+func setHttpHost(hostname string, module *config.Module) error {
+	// By creating a new hashmap and copying values there we
+	// ensure that the initial configuration remain intact.
+	headers := make(map[string]string)
+	if module.HTTP.Headers != nil {
+		for name, value := range module.HTTP.Headers {
+			if strings.Title(name) == "Host" && value != hostname {
+				return fmt.Errorf("host header defined both in module configuration (%s) and with URL-parameter 'hostname' (%s)", value, hostname)
+			}
+			headers[name] = value
+		}
+	}
+	headers["Host"] = hostname
+	module.HTTP.Headers = headers
+	return nil
+}
+
 type scrapeLogger struct {
 	next         log.Logger
 	buffer       bytes.Buffer

prober/http.go

@@ -341,20 +341,18 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 
 	httpClientConfig := module.HTTP.HTTPClientConfig
 	if len(httpClientConfig.TLSConfig.ServerName) == 0 {
-		// If there is no `server_name` in tls_config it makes
-		// sense to use the host header value to avoid possible
-		// TLS handshake problems.
-		changed := false
+		// If there is no `server_name` in tls_config, use
+		// the hostname of the target.
+		httpClientConfig.TLSConfig.ServerName = targetHost
+
+		// However, if there is a Host header it is better to use
+		// its value instead. This helps avoid TLS handshake error
+		// if targetHost is an IP address.
 		for name, value := range httpConfig.Headers {
 			if strings.Title(name) == "Host" {
 				httpClientConfig.TLSConfig.ServerName = value
-				changed = true
 			}
 		}
-		if !changed {
-			// Otherwise use the hostname of the target.
-			httpClientConfig.TLSConfig.ServerName = targetHost
-		}
 	}
 	client, err := pconfig.NewClientFromConfig(httpClientConfig, "http_probe", pconfig.WithKeepAlivesDisabled())
 	if err != nil {