Automatically requesting TLS/SSL certificates using the ACME protocol is quite nice. Usually to just get a regular TLS certificate, you just have to run an ACME capable tool or webserver, which will place a textfile in /.well-known/acme-challenge on your webserver, which is then used to validate control over the domain.

Alternatively you can use DNS based validation. This is even required when requesting wildcard certificates, for example, when requesting a certificate for '*.sig-io.nl'.

When validating using DNS, you need to have your ACME client create a DNS record '_acme-challenge' in your DNS domain. This can be done using various ACME clients, and I myself have written modules/plugins for Dehydrated for various DNS providers over the years.

Recently I've switched from Dehydrated as my preferred ACME client to Acme.sh. This client has a massive amount of DNS providers supported in the main codebase. So it will usually have an interface to whatever DNS system you might use.

Some of my customers however use domains hosted by (for example) TransIP.nl, which does have an API to update DNS-records, but usually takes very long (5-10 minutes) to update records, and also has no granulated access-control to their API. If you have API access, you can do everything with all domains under that account. This means you can't really leave credentials to this account in config-files somewhere.

However, I've now found a solution to these problems that I'm quite satisfied with. I'm using deSEC.io, a new non-profit, community-oriented DNS provider, with a nice API to do DNS validations. With deSEC I can create an API key that is limited in time and source-network. And since deSEC is free, I can create a seperate account just for acme-validation on an otherwise unused domain.

So to create a certificate for a customer-domain, I just have them create a CNAME entry in whatever DNS system they use, which points to my acme-validation only domain-name hosted at deSEC.io.

For example, for sig-io.nl I could create a CNAME record '_acme-challenge.sig-io.nl' pointing to '_acme-challenge.sig-io.nl.acme.example.com'. And then (given that example.com is a domain I manage with deSEC.io) use Acme.sh to create certificates using this alias domain using:

export DOMAIN="sig-io.nl" acme.sh --issue -d "*.${DOMAIN}" --challenge-alias "${DOMAIN}.acme.example.com" --server letsencrypt --dns dns_desec