Proxying API Requests in Development

Note: this feature is available with [email protected] and higher.

People often serve the front-end React app from the same host and port as their backend implementation.

For example, a production setup might look like this after the app is deployed:

Such setup is not required. However, if you do have a setup like this, it is convenient to write requests like fetch('/api/todos') without worrying about redirecting them to another host or port during development.

To tell the development server to proxy any unknown requests to your API server in development, add a proxy field to your package.json, for example:

This way, when you fetch('/api/todos') in development, the development server will recognize that it’s not a static asset, and will proxy your request to http://localhost:4000/api/todos as a fallback. The development server will only attempt to send requests without text/html in its Accept header to the proxy.

Conveniently, this avoids CORS issues and error messages like this in development:

Keep in mind that proxy only has effect in development (with npm start), and it is up to you to ensure that URLs like /api/todos point to the right thing in production. You don’t have to use the /api prefix. Any unrecognized request without a text/html accept header will be redirected to the specified proxy.

The proxy option supports HTTP, HTTPS and WebSocket connections.

If the proxy option is not flexible enough for you, alternatively you can:

• Configure the proxy yourself
• Enable CORS on your server (here’s how to do it for Express).
• Use environment variables to inject the right server host and port into your app.

"Invalid Host Header" Errors After Configuring Proxy#

When you enable the proxy option, you opt into a more strict set of host checks. This is necessary because leaving the backend open to remote hosts makes your computer vulnerable to DNS rebinding attacks. The issue is explained in this article and this issue.

This shouldn’t affect you when developing on localhost, but if you develop remotely like described here, you will see this error in the browser after enabling the proxy option:

Invalid Host header

To work around it, you can specify your public development host in a file called .env.development in the root of your project:

If you restart the development server now and load the app from the specified host, it should work.

If you are still having issues or if you’re using a more exotic environment like a cloud editor, you can bypass the host check completely by adding a line to .env.development.local. Note that this is dangerous and exposes your machine to remote code execution from malicious websites:

We don’t recommend this approach.

Configuring the Proxy Manually#

Note: this feature is available with [email protected] and higher.

If the proxy option is not flexible enough for you, you can get direct access to the Express app instance and hook up your own proxy middleware.

You can use this feature in conjunction with the proxy property in package.json, but it is recommended you consolidate all of your logic into src/setupProxy.js.

First, install http-proxy-middleware using npm or Yarn:

Next, create src/setupProxy.js and place the following contents in it:

You can now register proxies as you wish! Here's an example using the above http-proxy-middleware:

Note: You do not need to import this file anywhere. It is automatically registered when you start the development server.

Note: This file only supports Node's JavaScript syntax. Be sure to only use supported language features (i.e. no support for Flow, ES Modules, etc).

Note: Passing the path to the proxy function allows you to use globbing and/or pattern matching on the path, which is more flexible than the express route matching.