fix(plugin): prevent panic when ListenHosts is empty (#7565)

plugin/loop/setup.go

@@ -31,7 +31,10 @@ func setup(c *caddy.Controller) error {
 		go func() {
 			deadline := time.Now().Add(30 * time.Second)
 			conf := dnsserver.GetConfig(c)
-			lh := conf.ListenHosts[0]
+			lh := ""
+			if len(conf.ListenHosts) > 0 {
+				lh = conf.ListenHosts[0]
+			}
 			addr := net.JoinHostPort(lh, conf.Port)
 
 			for time.Now().Before(deadline) {

plugin/trace/setup.go

@@ -37,7 +37,7 @@ func traceParse(c *caddy.Controller) (*trace, error) {
 	)
 
 	cfg := dnsserver.GetConfig(c)
-	if cfg.ListenHosts[0] != "" {
+	if len(cfg.ListenHosts) > 0 && cfg.ListenHosts[0] != "" {
 		tr.serviceEndpoint = cfg.ListenHosts[0] + ":" + cfg.Port
 	}
 

test/bind_test.go

@@ -0,0 +1,24 @@
+package test
+
+import "testing"
+
+func TestBind_FilterAll(t *testing.T) {
+	t.Parallel()
+	corefile := `.:0 {
+        bind 127.0.0.1 {
+            except 127.0.0.1
+        }
+        trace
+        loop
+        whoami
+    }`
+	inst, err := CoreDNSServer(corefile)
+	if inst != nil {
+		CoreDNSServerStop(inst)
+	}
+	if err == nil {
+		t.Log("server started; stopping immediately")
+	} else {
+		t.Logf("server failed to start as expected without listeners: %v", err)
+	}
+}