.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.

There's a larger risk that if someone breaches a system with a wildcard cert, then you can end up with them being able to impersonate _every_ part of your domain, not just the one application.

Can't you have a limited wildcard?

Something like *.for-testing-only.company.com?

Yes, but then you are putting more information into the publically logged certificate. So it is a tradeoff between scope of certificate and data leak.

I guess you can use a pattern like {human name}.{random}.internal but then you lose memoribility.