Diagnosing 413 Request Entity Too Large Errors in NGINX: A Step‑by‑Step Checklist

Recognizing the 413 Condition

A 413 status code is returned when the request body exceeds the size that NGINX is configured to accept. The response is almost always generated by NGINX itself, but in some setups the upstream server forwards the code. The key symptom is a client‑side error that appears immediately after the upload begins, often after the first few kilobytes are sent.

Root‑Cause Table

Possible Cause	Typical Indicator
client_max_body_size too low	NGINX error log contains "client closed request while reading request body"
Per‑location override below global value	Request hits a location block with a smaller limit
proxy_request_buffering disabled or proxy_buffer_size small	Large POST is streamed to upstream, causing upstream timeout or 413
Upstream application limit (FastCGI, uWSGI, API gateway)	Upstream logs show a size restriction error
Network device truncation	Packets are dropped or truncated before reaching NGINX

Ordered Diagnostic Checks

1. Confirm the error is generated by NGINX. Inspect the error log for the phrase client closed request while reading request body. If the log entry is absent, the error may originate upstream.
2. Dump the effective configuration. Run:

   sudo nginx -T | grep -i client_max_body_size
   

   This shows every client_max_body_size directive in the active configuration, including global and per‑location values.
3. Verify the payload size. Use curl to send a test file:

   curl -v -F "file=@/path/to/largefile" http://localhost/upload
   

   Replace /path/to/largefile with a file larger than the current limit. Observe the HTTP status.
4. Check proxy buffering settings. Search for proxy_request_buffering and proxy_buffer_size in the configuration:

   sudo nginx -T | grep -i proxy_
   

   If buffering is disabled (proxy_request_buffering off), large requests bypass NGINX buffers entirely.
5. Inspect upstream logs. If the 413 persists after increasing client_max_body_size, look at the backend logs (e.g., PHP‑FPM, uWSGI, Node.js) for their own size limits.
6. Consider network devices. Use packet capture tools (e.g., tcpdump) to confirm that the full request reaches NGINX without truncation.

Fixes Tied to Findings

• Increase global client_max_body_size. Edit /etc/nginx/nginx.conf or the relevant server block:

  http {
      client_max_body_size 100m;
    }
  

  After editing, reload NGINX:

  sudo nginx -s reload
  

• Apply per‑location override. If a specific endpoint needs a larger limit, add a location block:

  location /upload {
      client_max_body_size 50m;
    }
  

• Enable proxy buffering or increase buffer size. For upstreams that cannot handle streaming, set:

  proxy_request_buffering on;
    proxy_buffer_size 128k;
    proxy_buffers 4 256k;
  

• Adjust upstream limits. For FastCGI, edit php-fpm.conf or www.conf and set request_terminate_timeout or upload_max_filesize accordingly.
• Update network device policies. If a firewall or load balancer enforces a payload size, modify its configuration to allow larger requests or route the traffic directly to NGINX.

Escalation Criteria

If the 413 persists after applying the above fixes, consider:

• Confirm that the NGINX binary matches the version documented in your configuration (1.0+). Older releases ignore client_max_body_size.
• Verify that no limit_req_zone or limit_conn_zone directives are inadvertently throttling the request.
• Check for client_body_in_file_only settings that may force large bodies to disk, affecting the limit.
• Engage the upstream application team to audit their own request size handling.

By following this ordered checklist, you can isolate whether the limit originates in NGINX, its proxy configuration, the upstream service, or an external network element, and then apply the precise configuration change needed to eliminate the 413 error.