When troubleshooting SSL certificate hostname mismatches, the most frustrating scenario occurs when a website immediately 301/302 redirects before you can inspect its certificate. Modern browsers typically follow redirects automatically, hiding the initial certificate from view. This becomes critical when debugging cases where:

• Clients report certificate warnings that don't appear in your tests
• Legacy systems fail before the redirect completes
• CDN configurations need validation

The most reliable method is using OpenSSL's s_client with the -showcerts flag while preventing redirects:

openssl s_client -connect example.com:443 -servername example.com -showcerts < /dev/null | openssl x509 -noout -text

For more control, use cURL with these options:

curl -vIk --http1.1 --resolve example.com:443:192.0.2.1 https://example.com
# -I : HEAD request
# -k : ignore certificate errors  
# --resolve : bypass DNS for testing

Chrome DevTools can capture the initial certificate if you:

1. Open DevTools (F12) and go to Network tab
2. Check "Preserve log"
3. Throttle connection to "Slow 3G"
4. Right-click the initial request → Security → View certificate

For regression testing, here's a Node.js script using the 'https' module:

const https = require('https');
const options = {
  hostname: 'problem-domain.com',
  port: 443,
  method: 'GET',
  rejectUnauthorized: false, // Bypass cert validation
  agent: new https.Agent({ maxRedirects: 0 }) // Block redirects
};

const req = https.request(options, (res) => {
  console.log('Certificate:', res.socket.getPeerCertificate());
});

req.on('error', (e) => {
  console.error('Certificate error:', e.cert ? e.cert : e);
});
req.end();

Specialized utilities provide better visualization:

• testssl.sh: Comprehensive testing suite with redirect handling

./testssl.sh --first-only example.com

• Qualys SSL Labs: Online tester with full chain analysis
• Burp Suite: Intercept the initial TLS handshake

A common production issue occurs when load balancers present a default certificate before applying host-specific certs via SNI. The diagnostic pattern:

# First connection shows default cert
openssl s_client -connect lb.example.com:443 

# Subsequent connections with SNI show correct cert
openssl s_client -connect lb.example.com:443 -servername app.example.com

Always verify both scenarios when troubleshooting certificate mismatches.

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When troubleshooting SSL/TLS issues, one particularly tricky scenario occurs when:

• A URL performs immediate redirection (HTTP 301/302) to another location
• You need to examine the certificate presented before the redirect occurs
• Standard browser inspection tools only show the final certificate

Here are reliable methods to capture the initial certificate:

Method 1: cURL with Certificate Output

curl -vkI https://example.com --output /dev/null --show-error 2>&1 | grep -A 10 "SSL certificate verify"

This command will:

• Bypass following redirects (-L not used)
• Output verbose SSL handshake info (-v)
• Show only headers (-I)

Method 2: OpenSSL Direct Connection

openssl s_client -connect example.com:443 -servername example.com -showcerts

Key parameters:

• -servername for SNI support
• -showcerts displays full certificate chain
• Add -status for OCSP verification

Method 3: Browser Developer Tools Trick

For Chrome/Edge:

1. Open Developer Tools (F12)
2. Go to Network tab
3. Check "Preserve log"
4. Refresh page
5. Right-click initial request → "Open in new tab"
6. Now inspect Security tab

For CI/CD integration, consider this Python script:

import socket
import ssl

def check_initial_cert(hostname):
    context = ssl.create_default_context()
    conn = context.wrap_socket(
        socket.socket(socket.AF_INET),
        server_hostname=hostname)
    conn.connect((hostname, 443))
    cert = conn.getpeercert()
    return cert['subject']

print(check_initial_cert("example.com"))

This technique helps diagnose:

• Expired certificates on redirect endpoints
• Certificate chain validation failures
• Hostname mismatch warnings
• HSTS preload validation

Remember that:

• Some clients may enforce different validation rules than your tools
• Mobile apps might cache certificates differently
• CDN edge nodes might present different certificates