The "Bad Request 400" error in Nginx when running Django with Gunicorn usually indicates an issue with the HTTP request that Nginx is forwarding to Gunicorn. Here's a breakdown of common causes and how to troubleshoot them:

1. ALLOWED_HOSTS Setting in Django:

• Problem: Django's ALLOWED_HOSTS setting is a security measure that prevents requests with unknown host headers from being processed. If Nginx forwards a request with a host that's not listed in ALLOWED_HOSTS, Django will return a 400 error.

• Solution:

  • Open your Django project's settings.py file.
  • Add your domain name or IP address to the ALLOWED_HOSTS list. For example:
  Python

   

  ALLOWED_HOSTS = ['yourdomain.com', '127.0.0.1', 'localhost']
  

  • If you're unsure of the host, you can temporarily use ALLOWED_HOSTS = ['*'] for testing, but do not use this in production.
  • Restart Gunicorn and Nginx after making changes.

2. Nginx Configuration Issues:

• Problem: Nginx might not be correctly passing the Host header to Gunicorn, or it might be passing incorrect headers.

• Solution:

  • Check your Nginx configuration file (usually located at /etc/nginx/sites-available/your_site_config).
  • Ensure that you have the following lines within your server block and location block (where you define the proxy_pass to Gunicorn):
  Nginx

   

  proxy_set_header Host $http_host;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header X-Forwarded-Proto $scheme;
  

  • These lines ensure that the correct Host, client IP, and protocol information are passed to Gunicorn.
  • Restart Nginx after making changes.

3. Underscores in Hostnames:

• Problem: Hostnames with underscores are not always supported by Django.
• Solution:
  • If your domain name or hostname contains underscores, consider changing it to a valid format without underscores.

4. Request Size Limits:

• Problem: If the request size exceeds the limits set in Nginx or Gunicorn, a 400 error might occur.¹

• Solution:

  • Check the client_max_body_size directive in your Nginx configuration. Increase it if necessary:
  Nginx

   

  client_max_body_size 10M; # Example: 10 megabytes
  

  • Check Gunicorn's --limit-request-line and --limit-request-fields options if you suspect request line or header size issues.

5. Other Potential Issues:

• Incorrect Gunicorn Binding: Ensure that Gunicorn is binding to the correct address and port that Nginx is proxying to.
• Firewall Issues: Check if your firewall is blocking communication between Nginx and Gunicorn.
• Gunicorn Worker Issues: If Gunicorn workers are crashing or not responding, it can lead to 400 errors. Check Gunicorn logs for any errors.

Debugging Steps:

1. Check Nginx Error Logs: Look for specific error messages in your Nginx error logs (usually at /var/log/nginx/error.log).
2. Check Gunicorn Logs: Examine Gunicorn logs for any errors or warnings.
3. Simplify: Try temporarily disabling parts of your Nginx configuration or Django middleware to isolate the issue.
4. Test with curl: Use the curl command to send requests directly to Gunicorn (bypassing Nginx) to see if the issue is with Gunicorn or Nginx.

If you've checked these common causes and are still facing the issue, check the following information for further debug:

• Your Nginx configuration file.
• Relevant parts of your Django settings.py file (especially ALLOWED_HOSTS).
• Any error messages from Nginx or Gunicorn logs.
• Details about the specific requests that are causing the 400 error.