Docker: Let's Encrypt Error "Domain Name Contains an Invalid Character"

This documentation is part of the HTTPS Routing guide. You can view the complete guide here: How to manage HTTP routing and TLS encryption (HTTPS) in your Docker containers.

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When creating a subdomain, Let's Encrypt follows RFC 952 and 1123, which allow only characters in the set [a-zA-Z0-9-].

Although underscores ('_') are permitted in DNS record names, they are not acceptable in hostnames. As a result, Let's Encrypt rejects subdomains such as "my_subdomain.example.com" and displays the error "Domain name contains an invalid character".

To resolve this issue, simply remove any underscores from your subdomains.