But then you have to redistribute the cert from that single server to all the others. Which, yes, can be done. But then you've gotta write that glue yourself. What's more, you've now chosen a special snowflake server on whom renewals depend.

In other words, no, it's not just as easy as setting up DNS-01. Different operational characteristics, and a need for bespoke glue code.

> But then you have to redistribute the cert from that single server to all the others.

Wouldn't you have to do that anyway? Or is the idea that each server requests and renews a separate cert for itself? That sounds as if you'd have to watch out for multiple servers stepping on each other's toes during the DNS-01 challenge, if there is ever a situation where two or more servers want to renew their cert at the same time.

Yup. There’s an RFC draft that addresses this dilemma.

https://datatracker.ietf.org/doc/draft-ietf-acme-dns-account...

Afaiu, that's only a problem for trying to _delegate_ to multiple clients. But routine operation with multiple clients works just fine in my experience (doing multi-region load balancing). Multiple TXT records are created, I think (speaking off the top of my head).

Ah! I stand corrected.

I wanted to quickly double-check my (albeit limited) experience against docs. The RFC[0] implies the possibility of what I described (provided a well-behaved ACME client that doesn't clobber other TXT records):

   2.  Query for TXT records for the validation domain name
   
   3.  Verify that the contents of one of the TXT records match the
       digest value

And then the certbot docs[2] show how it's a well-behaved client that wouldn't clobber TXT records from concurrent instances:

> You can have multiple TXT records in place for the same name. For instance, this might happen if you are validating a challenge for a wildcard and a non-wildcard certificate at the same time. However, you should make sure to clean up old TXT records, because if the response size gets too big Let’s Encrypt will start rejecting it. > ... > It works well even if you have multiple web servers.

That bit about "multiple webservers" is a little ambiguous, but I think the preceding line indicates clearly enough how everything is supposed to work.

[0] https://datatracker.ietf.org/doc/html/rfc8555#section-8.4

[1] https://letsencrypt.org/docs/challenge-types/#dns-01-challen...