wgengine/router: flushdns in windows when router config changes

Fixes: https://github.com/tailscale/tailscale/issues/1430

Signed-off-by: Aleksandar Pesic <peske.nis@gmail.com>

wgengine/router/router_windows.go

@@ -111,6 +111,14 @@ func (r *winRouter) Set(cfg *Config) error {
 		return fmt.Errorf("dns set: %w", err)
 	}
 
+	// Flush DNS on router config change to clear cached DNS entries (solves #1430)
+	out, err := exec.Command("ipconfig", "/flushdns").CombinedOutput()
+	if err != nil {
+		r.logf("flushdns error: %v; output: %s", err, out)
+	} else {
+		r.logf("flushdns successful")
+	}
+
 	return nil
 }