How to enable free HTTPS cert from Let’s Encrypt on a Server without a Domain Name

Unfortunately, it is not possible to obtain a free HTTPS certificate from Let’s Encrypt using Certbot for a server with a static public IP without a domain name. Let’s Encrypt issues Domain Validation (DV) certificates, which require a fully qualified domain name (FQDN) to verify ownership through challenges like HTTP-01 or DNS-01. Their policy explicitly does not support issuing certificates for bare IP addresses, as certificates must be tied to a domain name to comply with Certificate Authority/Browser Forum (CA/B Forum) standards and ensure proper validation. Here are the key points:

1. Let’s Encrypt Requirements: Let’s Encrypt validates domain ownership, not IP addresses. Certbot, as an ACME client, is designed to work with domain names, and its commands (e.g., certbot certonly --standalone) prompt for a domain name, not an IP address. Even with a static public IP, you need a domain name to complete the validation process.
2. IP-Based Certificates: While some certificate authorities (e.g., Sectigo or DigiCert) offer SSL certificates for public IP addresses, Let’s Encrypt does not. IP-based certificates are technically possible but rare, and Let’s Encrypt’s infrastructure does not support them due to security and policy reasons, such as the lack of a reliable way to verify IP ownership.
3. Workarounds:
   • Obtain a Domain Name: The simplest solution is to register a domain name (even a free or low-cost one from providers like DuckDNS or afraid.org) and point it to your static public IP via a DNS A record. You can then use Certbot to obtain a Let’s Encrypt certificate for that domain. This domain does not need to be publicly accessible for the DNS-01 challenge, but the DNS records must be publicly resolvable.
   • Self-Signed Certificates: If you don’t need a publicly trusted certificate (e.g., for internal use or testing), you can generate a self-signed certificate using tools like OpenSSL. However, browsers will display a warning since it’s not trusted by a public CA. This is not a Let’s Encrypt solution but avoids the domain requirement.
   • Alternative CAs: For IP-based certificates, you could explore paid options from other CAs like Sectigo, which offer certificates for public IPs. However, these are not free and require proof of IP control.
4. Certbot Limitations: Certbot does not support IP addresses in its configuration. Commands like certbot certonly --standalone -d <IP> will fail because Let’s Encrypt rejects IP addresses as valid identifiers. A pull request to Certbot discussed IP support, but it has not changed Let’s Encrypt’s policy against issuing such certificates.
5. Practical Example with Domain: If you acquire a domain (e.g., example.com), you can set up a DNS A record pointing to your static public IP and run:

#  sudo certbot certonly --standalone -d example.com

This will issue a certificate if the HTTP-01 challenge succeeds (requires port 80 open). Alternatively, use the DNS-01 challenge for internal servers:

#   sudo certbot certonly --manual --preferred-challenges dns -d example.com

Follow the prompts to add a TXT record to your DNS provider. Recommendation: For a free and straightforward solution, register a cheap or free domain name and use Certbot with Let’s Encrypt. This aligns with Let’s Encrypt’s automated, domain-based validation process and avoids the limitations of IP-based certificates. If you absolutely cannot use a domain, consider a self-signed certificate for non-public use or explore paid CAs for IP-based certificates.