Causes and Configuration of CORS Errors in Django

Set up

Have you ever encountered a “CORS policy error” when trying to connect your frontend to a Django backend?

This issue is particularly common when running frontend and backend on separate instances in cloud environments like AWS or GCP. By default, browsers block cross-origin requests, leading to CORS errors that prevent frontend-backend communication.

Fortunately, this problem can be resolved by properly configuring Django and, if necessary, adjusting Nginx settings. This article explains the causes of CORS errors and provides a step-by-step guide on how to configure Django to handle them.

If you’ve set up CORS but still encounter errors, or if you’re unsure how to configure cross-server communication, this guide is for you.

Contents
  1. Overview of CORS Errors
  2. Solutions to CORS Errors
  3. Resolving CORS Errors in Django
  4. Configuring CORS in Nginx
  5. Conclusion

Overview of CORS Errors

CORS (Cross-Origin Resource Sharing) is a mechanism that uses additional HTTP headers to allow a web application running on one origin (domain) to access resources from a different origin. (developer.mozilla.org)

By default, browsers enforce the Same-Origin Policy, restricting web pages from making requests to a different origin than the one that served the page. (developer.mozilla.org)

Without proper CORS configuration, browsers block cross-origin requests, leading to “CORS policy errors.”

For example, if a frontend is hosted at https://frontend.example.com and the backend API is hosted at https://api.example.com, CORS errors will occur unless CORS is properly configured.

Solutions to CORS Errors

CORS errors can be resolved through proper server-side configuration. In a Django application, two main configurations may be needed:

  • Django’s CORS settings
  • (If applicable) Nginx configuration

Below, we explain each solution.

Resolving CORS Errors in Django

Django does not have built-in CORS support, so the recommended approach is to use the external package django-cors-headers. (pypi.org)

Installing django-cors-headers

Run the following command to install the package:

pip install django-cors-headers

Configuring settings.py

Add corsheaders to INSTALLED_APPS

INSTALLED_APPS = [...,
                 'corsheaders',
                 ...]

Add CorsMiddleware to MIDDLEWARE

CorsMiddleware must be placed before django.middleware.common.CommonMiddleware.

MIDDLEWARE = ['corsheaders.middleware.CorsMiddleware',
              'django.middleware.common.CommonMiddleware',
               ...]

Allowing All Origins (For Development Only)

CORS_ALLOW_ALL_ORIGINS = True  # Allows all origins (not recommended for production)

⚠️ Warning: Setting CORS_ALLOW_ALL_ORIGINS = True in production is a security risk and should be avoided.

Allowing Specific Origins (Recommended for Production)

CORS_ALLOWED_ORIGINS = ["https://example.com",
                        "https://sub.example.com",
                        "http://localhost:8080",
                        "http://127.0.0.1:9000"]

Key Points:

  • Include the protocol (http/https) when specifying origins.
  • localhost and 127.0.0.1 are treated as different origins, so include both if necessary.

Using Regular Expressions for Allowed Origins

CORS_ALLOWED_ORIGIN_REGEXES = [r"^https://\w+\.example\.com$",  # Allow all subdomains of example.com
                              ]

Allowing IP Addresses in Development Environments

In development environments, when using AWS or GCP with separate instances, accessing the backend via an IP address might cause CORS settings not to be applied correctly. If this happens, using regular expressions may resolve the issue.

CORS_ALLOWED_ORIGIN_REGEXES = [
    r"^http://(localhost|127\.0\.0\.1):\d+$",  # Allow all ports for localhost
    r"^http://192\.168\.1\.\d+:3000$",  # Allow local network (e.g., Docker environment)
]

After making these changes, restart Django:

python manage.py runserver

Configuring CORS in Nginx

If Django runs behind an Nginx reverse proxy, CORS headers can also be configured in Nginx.

Editing Nginx Configuration

Modify /etc/nginx/nginx.conf or /etc/nginx/sites-available/default and add the following settings in the location block:

server {
    ...
    location / {
        add_header 'Access-Control-Allow-Origin' 'https://example.com' always;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range' always;
    }
}

Applying the Configuration

Restart Nginx to apply the new settings:

sudo systemctl restart nginx

Conclusion

CORS errors are detected on the client-side (browser), but correct server-side configuration is essential for resolution.

  • Use django-cors-headers to configure CORS in Django.
  • Nginx can supplement CORS headers if necessary.
  • In development, consider using regex to allow IP addresses.

By following these steps, you can resolve CORS errors and enable seamless frontend-backend communication! 🚀