The original TLS handshake was too rigid for the modern web. As we moved toward hosting thousands of websites on a single IP address (Virtual Hosting) and using multiple protocols (HTTP/2, HTTP/3), we needed a way to add metadata to the handshake without breaking the core protocol.
In the early days, one IP address equaled one SSL certificate. But modern servers host many domains on one IP. Since the server needs to present the *correct* certificate, but the certificate is sent *after* the handshake starts, we have a 'chicken and egg' problem. SNI solves this by allowing the client to include the hostname (e.g., `api.vulnarex.com`) in the cleartext `ClientHello`.
💡 Without SNI, the server would just send the 'Default' certificate, which would cause a 'Hostname Mismatch' error in the browser.
The `-servername` flag tells OpenSSL to send the SNI extension. If you omit it, the server may send the wrong certificate or terminate the connection.
ALPN allows the client and server to negotiate which protocol will be used *inside* the encrypted tunnel. Instead of doing a TLS handshake and then sending an HTTP 'Upgrade' header, the client lists its preferences (e.g., `h2`, `http/1.1`) in the `ClientHello`. The server picks one, and they immediately start communicating using that protocol.
⚠️ Misconfigured ALPN can lead to 'Protocol Mismatch' errors where a browser expects HTTP/2 but the server only speaks HTTP/1.1, causing a slow or failed page load.
| Extension | Purpose | Handshake Phase | Failure Result |
|---|---|---|---|
| SNI | Specify Hostname | Client Hello | Certificate Mismatch |
| ALPN | Negotiate Protocol | Client/Server Hello | Slow Fallback / Error |
| OCSP Stapling | Cert Validity Check | Server Hello | Browser Warning |
| Session Ticket | Fast Resumption | Server Hello | Slower 2-RTT Handshake |
How does a browser know if a certificate was revoked before its expiry? Normally, it asks the CA (via OCSP). But this is slow and leaks user privacy (the CA knows who you are visiting). With OCSP Stapling, the *server* periodically asks the CA for a signed 'proof of validity' and 'staples' it to the certificate during the handshake.
Some high-security environments use 'OCSP Must-Staple', which tells the browser: 'If you don't see a stapled OCSP response, hard-fail the connection'.
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