We have several internal servers that are not accessible via the Internet. We also have a Ubuntu nginx reverse proxy server that handles the ssl certificates for the Internet facing servers.
Is it possible to use LE on the non internet facing servers by utilizing the reverse proxy server somehow? If so any guidance on that?
You can create one single wildcard-certificate *.example.com and use this internal. There is a dns-entry required. Or you use some different internal1.example.com, internal2.example.com etc. with dns-01 - validation.
http-01 - validation needs a public server and port 80 / http.