HTTP/2 Disabled by Default in Nginx 1.19 – Need Explicit http2 Flag

The Solution

To guarantee HTTP/2 support across Nginx 1.15 and 1.19 (and subsequent versions), you must explicitly add the http2 parameter to your listen directive. While some legacy builds or specific versions may have implicitly enabled it, the explicit flag is the only way to ensure consistent behavior across different environments.

Update your server block as follows:

server {
    listen 443 ssl http2;
    server_name example.com;
    # ... other SSL configurations
}

Technical Explanation

In Nginx 1.19 and newer, the ngx_http_v2_module requires an explicit instruction to negotiate the protocol. When the http2 flag is present, Nginx uses Application-Layer Protocol Negotiation (ALPN) during the TLS handshake to agree on HTTP/2 with the client. Without this flag, the server defaults to HTTP/1.1, regardless of whether the client or the Nginx binary supports HTTP/2.

Module Availability and Build Constraints

The http2 flag only works if Nginx was compiled with the --with-http_v2_module option. If the module is missing, Nginx will fail the configuration test. Because you are seeing a performance degradation rather than a startup crash, it is highly likely the module is present but inactive.

Verification Steps

1. Syntax Check: Run the following command to ensure the http2 flag is recognized by your specific build:
   nginx -t
2. Header Verification: Use curl to confirm the protocol negotiation:
   curl -I --http2 https://your-domain.com
3. Browser Audit: Open Browser Developer Tools > Network Tab. Right-click the table header and enable the Protocol column. Look for h2 for your requests.

Environmental Assumption

This solution assumes you are using HTTPS. Most modern browsers do not support HTTP/2 over cleartext (h2c), so the ssl parameter must accompany the http2 flag for the protocol to be negotiated successfully.

Diagnostic Request: To provide a more tailored recommendation for your production environment, please provide the output of nginx -V. This will confirm if the ngx_http_v2_module is indeed compiled into your production binary.