.INTERNAL is now reserved for private-use applications

My biggest frustration with .internal is that it requires a private certificate authority. Lots of organizations struggle to fully set up trust for the private CA on all internal systems. When you add BYOD or contractor systems, it's a mess.

Using a publicly valid domain offers a number of benefits, like being able to use a free public CA like Lets Encrypt. Every machine will trust your internal certificates out of the box, so there is minimal toil.

Last year I built getlocalcert [1] as a free way to automate this approach. It allows you to register a subdomain, publish TXT records for ACME DNS certificate validation, and use your own internal DNS server for all private use.

[1] https://www.getlocalcert.net/

Just be mindful that any certs you issue in this way will be public information[1] so make sure the domain names don't give away any interesting facts about your infrastructure or future product ideas. I did this at my last job as well and I can still see them renewing them, including an unfortunate wildcard cert which wasn't me.

[1] https://crt.sh/

Just use wildcard certs and internal subdomains remain internal information.

A fun tale about wildcard certificates for internal subdomains:

The browser will gladly reuse an http2 connection with a resolved IP address. If you happen to have many subdomains pointing to a single ingress / reverse proxy that returns the same certificate for different Host headers, you can very well end up in a situation where the traffic will get messed up between services. To add to that - debugging that stuff becomes kind of wild, as it will keep reusing connections between browser windows (and maybe even different Chromium browsers)

I might be messing up technical details, as it's been a long time since I've debugged some grpc Kubernetes mess. All I wanted to say is, that having an exact certificate instead of a wildcard is also a good way to ensure your traffic goes to the correct place internally.