Request certificate with underscore character

I am trying to create a new certificate for one our server. It keeps failing due to FQDN having invalid character.

Does let's encrypt allow special character?

While DNS may allow underscores in DNS labels, they are not allowed in "hostnames". See e.g. RFC 1123 - Requirements for Internet Hosts - Application and Support and e.g. also Hostname - Wikipedia.

"Special characters" as in, non-ASCII characters are allowed in so-called " Internationalized Domain Name (IDN)", see Introducing Internationalized Domain Name (IDN) Support - Let's Encrypt.

Note that a simple hyphen (-) is just fine in a certificates hostname.


If you must have that name and you must have an LE cert....
Then, you could use an LE wildcard certificate that could cover that name.

Of course, the easy fix is to replace the underscore.


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