การแชร์ทรัพยากรข้ามต้นทาง (CORS)

Imagine you're building a web application. Your frontend (like a React app) runs on app.example.com, but it needs to fetch data from your API running on api.example.com.

This is where Cross-Origin Resource Sharing (CORS) comes in. It's a security feature implemented by web browsers to control how web pages from one origin can request resources from another origin.

CORS is a relaxation of the browser's Same-Origin Policy. This policy is a critical security mechanism that prevents a malicious website from reading sensitive data from another site.

• Origin is defined by the protocol, host, and port.
• https://app.example.com:443 is different from http://app.example.com:80 or https://api.example.com:443.

Without CORS, browsers would block your frontend from talking to your API because they have different origins.

When your browser detects a cross-origin request, it adds an Origin header to the request. The server then needs to respond with specific CORS headers to tell the browser it's allowed.

The most important header is Access-Control-Allow-Origin. If this header is present in the server's response and its value matches the client's origin (or is *), the browser allows the request.

CORS requests can be categorized into two types:

• Simple Requests: These are direct GET, HEAD, or POST requests with specific content types (like text/plain). The browser sends them immediately, expecting CORS headers in the response.
• Preflight Requests: For more complex requests (e.g., PUT, DELETE, custom headers, or specific content types), the browser first sends an OPTIONS request. This 'preflight' checks with the server if the actual request is safe to send.

Since Nginx often acts as a reverse proxy or API gateway, it's the perfect place to manage CORS headers for your backend services. We can use Nginx directives to add the necessary Access-Control-* headers to responses.

The primary directive for this is add_header, which allows us to inject custom HTTP headers into Nginx responses.

Let's configure Nginx to allow requests from a specific origin, https://app.example.com, to your API.

We'll add the Access-Control-Allow-Origin header within a location block that handles your API requests.

What if you have multiple frontends that need to access your API? You can't list multiple origins in Access-Control-Allow-Origin directly. Instead, you can use Nginx's map directive to dynamically set the header based on the incoming Origin header.

For preflight (OPTIONS) requests, the browser expects specific headers in response to its OPTIONS call. Nginx needs to intercept these requests and respond with the appropriate CORS headers, often without proxying to the backend.

Beyond Access-Control-Allow-Origin, these headers are crucial for full CORS support:

• Access-Control-Allow-Methods: Specifies allowed HTTP methods (e.g., GET, POST, PUT).
• Access-Control-Allow-Headers: Lists headers the client is allowed to send (e.g., Content-Type, Authorization).
• Access-Control-Allow-Credentials: Set to true if the client can send cookies or HTTP authentication.
• Access-Control-Max-Age: How long the preflight response can be cached by the browser (in seconds).

Here's a more complete Nginx configuration snippet that handles both simple and preflight CORS requests, allowing a specific origin to interact with your API, including sending credentials.

In this lesson, you learned about Cross-Origin Resource Sharing (CORS) and why it's vital for secure web applications. We covered:

• The Same-Origin Policy and why CORS exists.
• The difference between simple and preflight requests.
• How to configure Nginx using add_header and map directives to manage CORS.
• Key CORS headers like Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Max-Age.

Properly configuring CORS in Nginx ensures your frontend applications can securely communicate with your backend APIs across different domains.