[vlovich123]

You can restrict outbound network to HTTP using the outbound worker mentioned elsewhere in the thread and filter the domain name of the outbound request against a whitelist of domains you control. The DNS resolution of the domain happens within the CF network stack that you have no control over and that can’t be overwritten in anyway meaning if you restrict outbound to Google.com, there’s no way for that request to end up anywhere else. The whitelist filter you put in place would disallow raw IP addresses and DoH isn’t relevant because again your whitelist of servers you control can just not expose DoH.

[nevon]

When you say that the filter would disallow connecting directly to IP addresses, how would that work? When I open a tcp connection, there's no reference to any domain name. Do you think CF would proactively resolve all the domain names in my whitelist (repeatedly, in case the IPs change) and check the IP I'm connecting to against the list of IPs those domains would resolve to? That sounds like a very brittle solution.

[vlovich123]

It sounds like you haven’t done the requisite research and are asking me to do it for you. That’s not very nice. The TLDR is that the outbound request doesn’t go directly to the internet. It first goes through your interposer worker where you can sent direct TCP requests and only allow HTTP requests through after filtering for domain.

[stavros]

Can I send a UDP packet to a server on port 53 and receive a packet back?

[vlovich123]

You choose. But you can also choose to block that.