It depends what machine you are running the ACME client on
From Boulder
In order to talk to a letsencrypt client running on the host, the fake DNS client used in Boulder's start.py needs to know what the host's IP is from the perspective of the container. The default value is 127.0.0.1. If you'd like your Boulder instance to always talk to some other host, you can set FAKE_DNS to that host's IP address.
From another thread -