diff --git a/client/src/actions/index.js b/client/src/actions/index.js
index 5d96b045..d403a77e 100644
--- a/client/src/actions/index.js
+++ b/client/src/actions/index.js
@@ -343,7 +343,12 @@ export const testUpstreamFailure = createAction('TEST_UPSTREAM_FAILURE');
 export const testUpstreamSuccess = createAction('TEST_UPSTREAM_SUCCESS');
 
 export const testUpstream = (
-    { bootstrap_dns, upstream_dns, local_ptr_upstreams }, upstream_dns_file,
+    {
+        bootstrap_dns,
+        upstream_dns,
+        local_ptr_upstreams,
+        fallback_dns,
+    }, upstream_dns_file,
 ) => async (dispatch) => {
     dispatch(testUpstreamRequest());
     try {
@@ -352,6 +357,7 @@ export const testUpstream = (
         const config = {
             bootstrap_dns: splitByNewLine(bootstrap_dns),
             private_upstream: splitByNewLine(local_ptr_upstreams),
+            fallback_dns: splitByNewLine(fallback_dns),
             ...(upstream_dns_file ? null : {
                 upstream_dns: removeComments(upstream_dns),
             }),
@@ -386,12 +392,14 @@ export const testUpstreamWithFormValues = () => async (dispatch, getState) => {
         bootstrap_dns,
         upstream_dns,
         local_ptr_upstreams,
+        fallback_dns,
     } = getState().form[FORM_NAME.UPSTREAM].values;
 
     return dispatch(testUpstream({
         bootstrap_dns,
         upstream_dns,
         local_ptr_upstreams,
+        fallback_dns,
     }, upstream_dns_file));
 };
 
diff --git a/internal/dnsforward/http.go b/internal/dnsforward/http.go
index 95e7f407..3827e101 100644
--- a/internal/dnsforward/http.go
+++ b/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{
diff --git a/internal/dnsforward/http_test.go b/internal/dnsforward/http_test.go
index 8eda01c4..7987d94e 100644
--- a/internal/dnsforward/http_test.go
+++ b/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 {
diff --git a/openapi/CHANGELOG.md b/openapi/CHANGELOG.md
index 9ae20255..3ea61c33 100644
--- a/openapi/CHANGELOG.md
+++ b/openapi/CHANGELOG.md
@@ -6,6 +6,11 @@
 
 ## v0.107.37: API changes
 
+### The new field `"fallback_dns"` in `UpstreamsConfig` object
+
+* The new field `"fallback_dns"` in `POST /control/test_upstream_dns` is the
+  list of fallback DNS servers to test.
+
 ### The new field `"fallback_dns"` in `DNSConfig` object
 
 * The new field `"fallback_dns"` in `GET /control/dns_info` and `POST
diff --git a/openapi/openapi.yaml b/openapi/openapi.yaml
index 497cb210..52331c7f 100644
--- a/openapi/openapi.yaml
+++ b/openapi/openapi.yaml
@@ -1503,7 +1503,7 @@
           - 'tls://1.0.0.1'
     'UpstreamsConfig':
       'type': 'object'
-      'description': 'Upstreams configuration'
+      'description': 'Upstream configuration to be tested'
       'required':
       - 'bootstrap_dns'
       - 'upstream_dns'
@@ -1511,8 +1511,7 @@
         'bootstrap_dns':
           'type': 'array'
           'description': >
-            Bootstrap servers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Bootstrap DNS servers, port is optional after colon.
           'items':
             'type': 'string'
           'example':
@@ -1521,18 +1520,25 @@
         'upstream_dns':
           'type': 'array'
           'description': >
-            Upstream servers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Upstream DNS servers, port is optional after colon.
           'items':
             'type': 'string'
           'example':
           - 'tls://1.1.1.1'
           - 'tls://1.0.0.1'
+        'fallback_dns':
+          'type': 'array'
+          'description': >
+            Fallback DNS servers, port is optional after colon.
+          'items':
+            'type': 'string'
+          'example':
+          - '8.8.8.8'
+          - '1.1.1.1:53'
         'private_upstream':
           'type': 'array'
           'description': >
-            Local PTR resolvers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Local PTR resolvers, port is optional after colon.
           'items':
             'type': 'string'
           'example':