lib/httpserver: add handler to serve /robots.txt and deny search in…

…dexing (#4143)

This handler will instruct search engines that indexing is not allowed for the content exposed to the internet. This should help to address issues like #4128 when instances are exposed to the internet without authentication.

docs/CHANGELOG.md

@@ -15,6 +15,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 ## v1.87.x long-time support release (LTS)
 
+* SECURITY: serve `/robots.txt` content to disallow indexing of the exposed instances by search engines. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4128) for details.
+
 * BUGFIX: reduce the probability of sudden increase in the number of small parts on systems with small number of CPU cores.
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): fix a panic when the duration in the query contains uppercase `M` suffix. Such a suffix isn't allowed to use in durations, since it clashes with `a million` suffix, e.g. it isn't clear whether `rate(metric[5M])` means rate over 5 minutes, 5 months or 5 million seconds. See [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3589) and [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4120) issues.
 * BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html): prevent from possible panic when the number of vmstorage nodes increases when [automatic vmstorage discovery](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#automatic-vmstorage-discovery) is enabled.

lib/httpserver/httpserver.go

@@ -320,6 +320,11 @@ func handlerWrapper(s *server, w http.ResponseWriter, r *http.Request, rh Reques
 		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1833
 		fmt.Fprintf(w, "VictoriaMetrics is Ready.\n")
 		return
+	case "/robots.txt":
+		// This prevents search engines from indexing contents
+		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4128
+		fmt.Fprintf(w, "User-agent: *\nDisallow: /\n")
+		return
 	default:
 		if strings.HasPrefix(r.URL.Path, "/debug/pprof/") {
 			pprofRequests.Inc()