Possible Alternative to disable LE check for natted servers.
As an ISPConfig user that is behind a nat router (I have not yet figured nat hairpinning in cisco routers) I propose the following as an alternative to just disabling the LE check.
Instead, it would be possible to request an external service verify the host/domain is indeed accessible.
How I see this in practice:
Ispconfig > system > server config > ssl > NAT Router (checkbox) (as oppose to disable LE check)
When performing the check, if the NAT box is checked, Call out to verification server.
[It could be a service hosted by ISPConfig but could just as easily be any of the "is this site up" services that has a free user api. (with a quick google, I see that: check-host.net for example has an array of check types that could be used for this.)
Get the result and proceed with cert creation or report back an issue.
I believe this approach would be more effective than just disabling the check because it will mean misconfigured hostnames/domains, missing dns or websites, wrong server used for a site, firewall woes and the rest of the usual suspects will not result in a failed cert request to LE.
One or two fails may not be an issue but we know there is a rate limit so whatever we can do to keep the failures from occurring in the first place would be a bonus.
Essentially this will allow ISPConfig to still pre-empt failures and would only affect those that have the NAT configuration set in server configs. For everybody else you can just perform the normal check.
An option in the installer that allows for enabling the option from the outset would be preferable although that would just be a small bonus addition to the overall feature.