You need authorized access to a DYNDNS2 like service that supports setting TXT records. See [1] for an example implementation as a PHP wrapper around PowerDNS API.
Copy dns_dyndns2.sh to a place where acme.sh can find it, I recommend ${ACME_HOME}/dnsapi/.
You may define an environment, if you don't then the environment name will be DEFAULT.
export DYNDNS2_ENV=DatabaseIf you deploy only one tenant (i.e. one set of credentials with which you will be able to manage all the hostnames on this particular machine) then you don't have to think about environments. Otherwise think of an environment as a way to group service endpoints and credentials in your account configuration. In this case we want this particular environment to manage certificates for our databases.
If you already issued certificates for this a particular environment and the service endpoint and credentials didn't change, then chances are that you don't have to specify all those details again.
Otherwise you'll need to make them known first.
export DYNDNS2_URL=https://mydynhost.com/dyn/update.php
export DYNDNS2_Username=dyn-db-user
export DYNDNS2_Password=super53cr3tNow you may issue the certificate:
acme.sh --issue --dns dns_dyndns2 -d example.com -d www.example.comThat's it. DYNDNS2_URL, DYNDNS2_Username and DYNDNS2_Password for the environment will be saved in ${ACME_HOME}/account.conf. The pointer to the environment will be persisted in the domain config ${ACME_HOME}/example.com/example.com.conf so it can be retrieved automatically when, e.g. renewing the certificate.