Nginx upload limit

Today, I encountered a surprising issue with my Docker-based web application. The application has an upload limit set, but before reaching it, an unexpected error appeared:

413 Request Entity Too Large

Despite the application’s upload limit being correctly configured, the error occurred much earlier—when the file was barely over 1MB. Where does this limitation come from, and how can it be changed?

Troubleshooting

The issue occurred before the request even reached the application layer, during a critical step in request processing. The root cause was Nginx, the web server and reverse proxy used in the Docker stack.

Nginx, commonly used in modern application stacks for load balancing, caching, and HTTPS handling, acts as the gateway to the application, managing all incoming requests. However, Nginx was rejecting uploads larger than 1MB. This was due to the client_max_body_size directive, which—when unset—defaults to a relatively low limit in some configurations. As a result, Nginx blocked larger file uploads before they could reach the application.

Solution

To resolve this issue, the client_max_body_size directive in the Nginx configuration needed to be updated to allow larger file uploads.

Modify the nginx.conf file or the relevant server block configuration:

server {
    listen 80;
    server_name example.com;
    client_max_body_size 100M;  # Allow uploads up to 100MB
}

After making this change, restart Nginx to apply the new configuration:

nginx -s reload

If Nginx is running in a Docker container, you can restart the container instead:

docker restart <container_name>

With this update, the upload limit increased to 100MB, allowing the application to handle larger files without premature rejection. Once the configuration was applied, the error disappeared, and file uploads worked as expected, provided they remained within the newly defined limits.