AMP CORS Middleware

The AMP CORS middleware adds CORS and AMP CORS headers to all CORS requests initiated by the AMP runtime. The middleware will only add these headers if the __amp_source_origin query parameter is present. All other requests remain unchanged.

Installation

Install via:

npm install amp-toolbox-cors --save

Usage

Here is an example using Express:

const express = require('express');
const ampCors = require('amp-toolbox-cors');

const app = express();

// That's it!
app.use(ampCors());
...

Please note that AMP CORS does not depend on Express and is based on Node's HTTP Request and Response objects.

Filtering by source origin

You can additionally filter requests by source origin. For example:

app.use(ampCors({
  sourceOriginPattern: /https:\/\/ampbyexample\.com$/
}));

This will only allow requests with https://ampbyexample.com set as the source origin. Requests from all other origins will receive a 403 response,

Origin verification

By default, the AMP CORS middleware will only allow requests from AMP Caches listed on https://cdn.ampproject.org/caches.json (with the addition of bing-amp.com). All other origins will receive a 403 response. To allow requests from all origins, disable this via the verifyOrigin option:

app.use(ampCors({
  verifyOrigin: false
}));

Allow Crendentials

By default, the AMP CORS middleware will allow crendentials mode for AMP CORS requests. To disable this, set allowCredentials to false.

app.use(ampCors({
  allowCredentials: false
}));
// => will not set "Access-Control-Allow-Credentials", "true"

Allow AMP-Redirect-To

By default, the AMP CORS middleware will allow redirects via AMP-Redirect-To. To disable this, set enableAmpRedirectTo to false.

app.use(ampCors({
  enableAmpRedirectTo: false
}));
// => Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin instead of 
// Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin, AMP-Redirect-To

Logging

For debugging requests, you can enable the verbose loggin mode via the verbose option:

app.use(ampCors({
  verbose: false
}));

Example

See express.js for a sample implementation. There are two scenarios in which the AMP CORS header will be added:

1. AMP CORS header will be set if the __amp_source_origin query parameter is set together with the AMP-SAME-ORIGIN header:

$ curl --header "AMP-SAME-ORIGIN: true" -I "http://localhost:3000/items?__amp_source_origin=https://localhost:3000"
HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: https://localhost:3000
Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin
AMP-Access-Control-Allow-Source-Origin: https://localhost:3000
Content-Type: application/json; charset=utf-8
...

2. AMP CORS header will be set if the __amp_source_origin query parameter is set together with the Origin header:

$ curl --header "Origin: https://ampbyexample-com.cdn.ampproject.org" -I "http://localhost:3000/items?__amp_source_origin=https://localhost:3000"
HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: https://ampbyexample-com.cdn.ampproject.org
Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin
AMP-Access-Control-Allow-Source-Origin: https://localhost:3000
Content-Type: application/json; charset=utf-8
...

In all other cases, no CORS header will be set.

$ curl -I localhost:3000/items
HTTP/1.1 200 OK
X-Powered-By: Express
Content-Type: application/json; charset=utf-8
...