I have a complex Tailscale-based network setup that includes blocking all Google hostnames. Unfortunately RCS on iOS doesn’t work when sending photos.
I’ve scoured AT&T’s website and App Privacy Report on iOS (which doesn’t show DNS names for Messages, Phone apps) but I do know they switched to Google as their RCS provider at one point.
I’d like to set up a Tailscale App Connector using hostnames, but if they’re using IP addresses I can work with those as well (subnet routing).
Update: Resolved by allowlisting rcs-copper-us.googleapis.com specifically, but I also added all of telephony.goog to unblock rcs.telephony.goog. A simply tcpdump got me the DNS requests for those domains.
This article has helped me tremendously over the last couple years resolving host and port issues. Unsure if RCS relations are in here but still could be a useful resource for you, especially after checking your logs. Apple Enterprise Networks
I resolved the issue and edited the post.