The definition of "internal" is not clear.
- Your request stay in the Google Network. Is it internal or external?
- To resolve the Custom Domain, a DNS resolution request (port 53) is performed on the public network, but the content of the request stays in the Google Network and forwarded after the resolution. Is it internal or external?
So, as long as you use Google Services (in premium network option), you don't go out of the Google Network and thus you can consider this as highly secured.
I assume, my answer isn't very clear, in fact all depend if you trust or not the Google Cloud network.
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