.INTERNAL is now reserved for private-use applications

Are there any good reasons to use a TLD like .internal for private-use applications, rather than just a regular gTLD like .com?

It's nice that this is available, but if I was building a new system today that was internal, I'd use a regular domain name as the root. There are a number of reasons, and one of them is that it's incredibly nice to have the flexibility to make a name visible on the Internet, even if it is completely private and internal.

You might want private names to be reachable that way if you're following a zero-trust security model, for example; and even if you aren't, it's helpful to have that flexibility in the future. It's undesirable for changes like these to require re-naming a system.

Using names that can't be resolved from the Internet feels like all downside. I think I'd be skeptical even if I was pretty sure that a given system would not ever need to be resolved from the Internet. [Edit:] Instead, you can use a domain name that you own publicly, like `example.com`, but only ever publish records for the domain on your private network, while retaining the option to publish them publicly later.

When I was leading Amazon's strategy for cloud-native AWS usage internally, we decided on an approach for DNS that used a .com domain as the root of everything for this reason, even for services that are only reachable from private networks. These services also employed regular public TLS certificates too (by default), for simplicity's sake. If a service needs to be reachable from a new network, or from the Internet, then it doesn't require any changes to naming or certificates, nor any messing about with CA certs on the client side. The security team was forward-thinking and was comfortable with this, though it does have tradeoffs, namely that the presence of names in CT logs can reveal information.

> Are there any good reasons to use a TLD like .internal for private-use applications, rather than just a regular gTLD like .com?

That assumes you are able to pay to rent a domain name, and keep paying for it, and that you are reasonably sure that the company you're renting it from is not going to take it away from you because of a selectively-enforced TOS, and that you are reasonably sure that both yourself and your registrar are doing anything possible to avoid getting your account compromised (resulting in your domain being transferred to someone else's and probably lost forever unless you can take legal action).

So it might depend on your threat model.

Also, a good example, and maybe the main reason for this specific name instead of other proposals, is that big corps are already using it (e.g. DNS search domains in AWS EC2 instances) and don't want someone else to register it.

If you control the DNS resolution in your company and use an internal certificate authority, technically you don't have to rent a domain name. You can control how it resolves and "hijack" whatever domain name you want. It won't be valid outside your organization/network, but if you're using it only for internal purposes then that doesn't matter.

Of course, this is a bad idea, but it does allow you to avoid the "rent".