[Q] How to change OTA server URL in AOSP?

Hello All,
I want to add my OTA server URL into AOSP for my released device and I know I can use some applications to do it but I want to modify the default URL of Google's AOSP. I had been grabed OTA URL of Nexus 5 (android.clients.google.com....) as well as got some logs (from adb logcat) then I used grep command with some keywords as "android.clients.google.com/", "SystemUpdateService", "DownloadAttemp",... to find where the URL in. But I found nothing.
Anyone can tell me where I can change default OTA server URL.
Thanks,