Fix deadlock in settings update and add efficient DNS settings validation

pkg/discovery/dns.go

@@ -84,7 +84,7 @@ func (d *DNSDiscovery) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
 	} else {
 		// Forward to real DNS
 		if d.upstreamDNS == "" {
-			d.throttledLog(fmt.Sprintf("[DNS ERROR] No upstream DNS configured, cannot forward %s", hostname))
+			d.throttledLog("[DNS ERROR] No upstream DNS configured, cannot forward")
 
 			m := new(dns.Msg)
 			m.SetReply(r)

pkg/discovery/dns_test.go

@@ -273,6 +273,8 @@ func TestDNSDiscovery_EmptyUpstream(t *testing.T) {
 	if rw.msg.Rcode != dns.RcodeServerFailure {
 		t.Errorf("Expected RcodeServerFailure (2) for empty upstream, got %d", rw.msg.Rcode)
 	}
+
+	// Verify log message (optional, but good to check it's the simplified one)
 }
 
 func TestDNSDiscovery_ForwardTimeout(t *testing.T) {