SSL Certificate Checker Guide: TLS Explained & Expiration Monitoring

Updated March 2026 · 7 min read

SSL/TLS certificates are the backbone of web security. They encrypt data between browsers and servers, authenticate website identity, and are now required for SEO ranking. An expired or misconfigured certificate breaks trust, triggers browser warnings, and can cost you traffic and revenue. This guide covers how SSL/TLS works, what certificate grades mean, and how to automate renewal.

What Is SSL/TLS?

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) are cryptographic protocols that secure communications over networks. When you see the padlock icon and https:// in your browser's address bar, TLS is protecting the connection.

• SSL 3.0 — deprecated in 2015, known vulnerabilities (POODLE).
• TLS 1.0/1.1 — deprecated in 2020, disabled in modern browsers.
• TLS 1.2 — widely supported, still considered secure.
• TLS 1.3 — current standard (2018), faster handshake, stronger security.

Even though we call them "SSL certificates," modern certificates actually use TLS. The term "SSL" persists for historical reasons.

How the TLS Handshake Works

1. Client Hello — browser sends supported TLS versions and cipher suites.
2. Server Hello — server selects TLS version and cipher suite, sends its certificate.
3. Certificate Verification — browser verifies the certificate chain against trusted CAs.
4. Key Exchange — client and server establish a shared session key (using ECDHE in TLS 1.3).
5. Encrypted Communication — all subsequent data is encrypted with the session key.

Certificate Types

Understanding Certificate Grades

SSL testing tools grade your server configuration from A+ to F:

Check your grade with the SSL/TLS Checker and review your HTTP security headers to reach A+.

Let's Encrypt: Free SSL Certificates

Let's Encrypt is a free, automated Certificate Authority that has issued billions of certificates. It provides DV certificates that auto-renew every 90 days.

Setting Up Let's Encrypt with Certbot

# Install certbot (Ubuntu/Debian)
sudo apt install certbot python3-certbot-nginx

# Obtain certificate for Nginx
sudo certbot --nginx -d example.com -d www.example.com

# Obtain certificate for Apache
sudo certbot --apache -d example.com

# Standalone mode (no web server running)
sudo certbot certonly --standalone -d example.com

# DNS challenge (for wildcard certificates)
sudo certbot certonly --manual --preferred-challenges dns -d "*.example.com"

Setting Up Let's Encrypt with Caddy

Caddy automatically obtains and renews Let's Encrypt certificates with zero configuration:

# Caddyfile - automatic HTTPS
devtoolkit.ink {
    reverse_proxy frontend:80
}

# That's it. Caddy handles SSL automatically.

Auto-Renewal

# Certbot auto-renewal (usually set up automatically)
sudo certbot renew --dry-run

# Check the renewal cron/timer
systemctl list-timers | grep certbot

# Manual cron for renewal (if needed)
0 0,12 * * * certbot renew --quiet

Use cron expressions to schedule certificate renewal checks.

Checking Certificate Details from the Command Line

# View certificate details
openssl s_client -connect devtoolkit.ink:443 -servername devtoolkit.ink < /dev/null 2>/dev/null | openssl x509 -text -noout

# Check expiration date only
echo | openssl s_client -connect devtoolkit.ink:443 -servername devtoolkit.ink 2>/dev/null | openssl x509 -noout -dates

# Output:
# notBefore=Jan  1 00:00:00 2026 GMT
# notAfter=Apr  1 00:00:00 2026 GMT

# Check certificate chain
openssl s_client -connect devtoolkit.ink:443 -servername devtoolkit.ink -showcerts

Achieving an A+ Grade

To get the highest SSL grade, configure these settings on your web server:

# Nginx example for A+ configuration
ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384;
ssl_prefer_server_ciphers off;
ssl_session_timeout 1d;
ssl_session_cache shared:SSL:10m;
ssl_session_tickets off;

# HSTS (HTTP Strict Transport Security)
add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;

# OCSP Stapling
ssl_stapling on;
ssl_stapling_verify on;

Common SSL Issues

• Certificate expired — the most common problem. Set up auto-renewal and monitoring.
• Name mismatch — the certificate does not cover the requested domain. Check SANs and the common name.
• Incomplete chain — the server does not send intermediate certificates. Browsers may still work, but API clients and mobile apps will fail.
• Mixed content — HTTPS page loads HTTP resources. Browser blocks them or shows a warning.
• Wrong redirect order — make sure HTTP redirects to HTTPS, and non-www redirects are consistent. Check DNS records match your redirect configuration.

FAQ

Is Let's Encrypt as secure as paid certificates?

Yes. The encryption provided by Let's Encrypt DV certificates is identical to paid certificates. The only difference is the level of identity validation (DV vs OV/EV). For most websites, DV is perfectly sufficient.

How often should I check my SSL certificate?

Set up automated monitoring that checks at least weekly. Many services (Uptime Robot, Pingdom) can alert you when a certificate is approaching expiration. DevToolKit's SSL Checker gives you an instant snapshot.

Do I need SSL for a development/staging server?

Yes. Many modern browser APIs (Service Workers, Geolocation, Web Crypto) require HTTPS. Use Let's Encrypt for staging, or self-signed certificates with proper trust configuration for local development.

DevToolKit — 42+ Free Online Developer Tools