Pull request 1992: 3701-fallback-dns-check

Squashed commit of the following:

commit a5191def0b950cb7ab1ea737e9c8c6e8b65ec720
Merge: 7d127bfca 5bd452516
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Sep 6 12:21:08 2023 +0300

    Merge branch 'master' into 3701-fallback-dns-check

commit 7d127bfca54ac940c611681e6f48b89881491602
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Sep 5 13:21:35 2023 +0300

    client: add fallback_dns to the upstream test

commit 970da7215503b5f05c53fcad47c715a96d9fc3c4
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Sep 4 19:23:35 2023 +0300

    all: add fallback dns check

internal/dnsforward/http.go

@@ -392,6 +392,7 @@ func (s *Server) setConfigRestartable(dc *jsonDNSConfig) (shouldRestart bool) {
 type upstreamJSON struct {
 	Upstreams        []string `json:"upstream_dns"`
 	BootstrapDNS     []string `json:"bootstrap_dns"`
+	FallbackDNS      []string `json:"fallback_dns"`
 	PrivateUpstreams []string `json:"private_upstream"`
 }
 
@@ -750,11 +751,10 @@ func (s *Server) handleTestUpstreamDNS(w http.ResponseWriter, r *http.Request) {
 	req.Upstreams = stringutil.FilterOut(req.Upstreams, IsCommentOrEmpty)
 	req.PrivateUpstreams = stringutil.FilterOut(req.PrivateUpstreams, IsCommentOrEmpty)
 
-	upsNum := len(req.Upstreams) + len(req.PrivateUpstreams)
+	upsNum := len(req.Upstreams) + len(req.FallbackDNS) + len(req.PrivateUpstreams)
 	result := make(map[string]string, upsNum)
 	resCh := make(chan upsCheckResult, upsNum)
 
-	// TODO(s.chzhen):  Check fallback DNS servers.
 	for _, ups := range req.Upstreams {
 		go func(ups string) {
 			resCh <- upsCheckResult{
@@ -763,6 +763,14 @@ func (s *Server) handleTestUpstreamDNS(w http.ResponseWriter, r *http.Request) {
 			}
 		}(ups)
 	}
+	for _, ups := range req.FallbackDNS {
+		go func(ups string) {
+			resCh <- upsCheckResult{
+				host: ups,
+				err:  s.checkDNS(ups, opts, checkDNSUpstreamExc),
+			}
+		}(ups)
+	}
 	for _, ups := range req.PrivateUpstreams {
 		go func(ups string) {
 			resCh <- upsCheckResult{

internal/dnsforward/http_test.go

@@ -560,6 +560,23 @@ func TestServer_HandleTestUpstreamDNS(t *testing.T) {
 			hostsUps: "OK",
 		},
 		name: "etc_hosts",
+	}, {
+		body: map[string]any{
+			"fallback_dns": []string{goodUps},
+		},
+		wantResp: map[string]any{
+			goodUps: "OK",
+		},
+		name: "fallback_success",
+	}, {
+		body: map[string]any{
+			"fallback_dns": []string{badUps},
+		},
+		wantResp: map[string]any{
+			badUps: `couldn't communicate with upstream: exchanging with ` +
+				badUps + ` over tcp: dns: id mismatch`,
+		},
+		name: "fallback_broken",
 	}}
 
 	for _, tc := range testCases {