Even Palo Alto notes that they can only effectively block DoH if you’re MITMing all https traffic already (e.g. using a root certificate on corporate-managed devices). If not able to MITM the connection, it will still try to block popular DoH providers, though.
For rather cheap I can see what traffic is suspicious. If you throw more resources at the problem and scale up it becomes simple to see traffic that looks like dns over https without having to decrypt it. Indicators such as size, frequency, consistent traffic going from your host to your DoH provider and then traffic going to other parts of the internet….these patterns become easy to establish. Once you have a good idea that a host on the internet is a DoH provider you can drop it into that category and block it.
Even Palo Alto notes that they can only effectively block DoH if you’re MITMing all https traffic already (e.g. using a root certificate on corporate-managed devices). If not able to MITM the connection, it will still try to block popular DoH providers, though.
https://live.paloaltonetworks.com/t5/blogs/protecting-organizations-in-a-world-of-doh-and-dot/ba-p/313171
For rather cheap I can see what traffic is suspicious. If you throw more resources at the problem and scale up it becomes simple to see traffic that looks like dns over https without having to decrypt it. Indicators such as size, frequency, consistent traffic going from your host to your DoH provider and then traffic going to other parts of the internet….these patterns become easy to establish. Once you have a good idea that a host on the internet is a DoH provider you can drop it into that category and block it.