Common reasons and solutions for free SSL certificate generation failure

　　With HTTPS becoming standard on websites, SSL certificates are no longer optional but a key factor affecting website security, SEO ranking, and user trust. Currently, the most commonly used free SSL certificates come from organizations like Let’s Encrypt, ZeroSSL, and Google Trust Services. While the application process seems simple, many website owners encounter a common problem: SSL certificate generation fails, sometimes repeatedly, and they don't know why. This article systematically outlines the reasons for free SSL certificate generation failures and provides corresponding solutions. Even if you're building a website for the first time, you can follow the steps to troubleshoot.

　　I. What is a free SSL certificate? Why does it fail to generate?

　　Before we delve into the troubleshooting, let's briefly understand how free SSL certificates work.

　　The core logic of free SSL certificates is that the certificate authority needs to confirm that "this domain name truly belongs to you." Only after successful confirmation will a certificate be issued.

　　Therefore, the fundamental reason for generation failure is singular: domain ownership verification failed.

　　Verification failure can be broken down into several specific situations, which we will explain one by one below.

　　II. Domain Name Resolution Failure (One of the Most Common Reasons)

　　1. Problem Symptoms: The domain name has been resolved, but the process still fails. The SSL application interface displays messages such as "Domain validation failed," "Unable to reach domain," and "Timeout during validation."

　　2. Cause Analysis: Free SSL certificates require public access to your domain when verifying it. Possible causes include: the domain name has just been resolved, DNS is not yet fully effective; the A record/AAAA record points to the wrong IP; the domain name is resolving to an old server; or the origin server is misconfigured when using a CDN.

　　3. Solution: Confirm that the domain's A record points to the current server IP. Use `ping domain` or `nslookup domain` to check the resolution results. For newly resolved domains, it is recommended to wait 10-60 minutes before applying for a certificate. If using a CDN, ensure the origin server address is correct.

　　III. Port 80 Not Open or Blocked by Firewall

　　1. Problem Symptom: DNS is correct, but SSL verification still fails; the failure rate is extremely high when using HTTP verification.

　　2. Cause Analysis: Most free SSL certificates use HTTP-01 verification by default, which requires port 80 of the domain to be open. The certificate authority can access http://yourdomain/.well-known/. If port 80 is not open on the server, not allowed by the cloud firewall/security group, or blocked by the local firewall (iptables/firewalld), failure will occur.

　　3. Solution: Allow TCP 80 in the server security group, check the server firewall rules, temporarily disable the firewall for testing (re-enable it after testing), and confirm that the web service (Nginx/Apache) is running.

　　IV. Website Forced HTTPS Redirect Causes Verification Failure

　　1. Problem Symptom: Even though port 80 is open, SSL still prompts HTTP verification failure.

　　2. Cause Analysis: Many website owners set up automatic HTTP redirection to HTTPS before installing SSL. The problem is that you don't have SSL yet, so HTTPS is inaccessible. The verification request is forcibly redirected to HTTPS, causing the certificate authority access to fail.

　　3. Solution: Before applying for SSL, temporarily disable HTTP → HTTPS redirection. After the certificate is successfully issued, enable forced redirection again. Pay special attention to: Nginx rewrite rules and CDN forced HTTPS settings.

　　V. Domain blocked by CDN or DDoS protection products

　　1. Problem symptoms: Local access is normal, but certificate authority verification fails.

　　2. Cause analysis: Some CDN/DDoS protection products block the .well-known directory, returning a verification code page and restricting overseas IPs. Most verification IPs for free SSL certificates come from overseas.

　　3. Solution: Before applying for the certificate, pause CDN or switch the domain to "origin-only mode" to allow the .well-known directory. After verification, restore CDN.

　　VI. Too Many Applications for the Same Domain in a Short Period

　　1. Problem Symptom: An application frequency limit is displayed, showing "rate limit exceeded".

　　2. Cause Analysis: Taking Let’s Encrypt as an example: the same domain can only be issued a maximum of 5 times per week, and there are also limits on the same IP/account. Frequent failures and repeated attempts will directly trigger the limit.

　　3. Solution: Wait 1-7 days before reapplying. Use a testing environment (Staging) to debug and ensure the configuration is correct before officially applying.

　　VII. Server Time Error Causing Certificate Verification Failure

　　1. Problem Symptom: Certificate generation fails or the browser displays an invalid certificate message after generation.

　　2. Cause Analysis: SSL certificates are extremely sensitive to server time. If the system time is inaccurate or the timezone is incorrect, verification will fail.

　　3. Solution: Synchronize server time, set the correct timezone (e.g., Asia/Shanghai), and restart the web service.

　　VIII. Outdated Server Environment or Client Version

　　1. Problem Symptom: ACME client reports an error and cannot complete certificate issuance.

　　2. Cause Analysis: OpenSSL version is too low, Certbot/acme.sh version is too old, or the system is too old (e.g., CentOS 6).

　　3. Solution: Upgrade the ACME client, upgrade OpenSSL, and use acme.sh (better compatibility). For older systems, it is recommended to change the server environment.

　　How to Avoid Repeated SSL Certificate Generation Failures?

　　Several practical suggestions: First, ensure the domain name can be accessed normally via HTTP. Do not enable HTTPS for forced redirection in advance. Pause CDN/anti-DDoS protection before applying. Confirm that port 80 is open to the outside world. Do not frequently apply repeatedly. Prioritize using mature one-click scripts or panels.

　　Many novice website owners fall into a common misconception after SSL certificate generation fails: "Is the server faulty? Is there a problem with the certificate?" In reality, 90% of SSL failures stem from fundamental configuration issues.

　　Just remember this: the certificate authority must be accessible from the public internet via HTTP. Troubleshooting along these lines will often resolve the problem quickly.