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What is CORS, and how does it work?

CORS (Cross-Origin Resource Sharing) is a mechanism that uses the HTTP header “Access-Control-Allow-Origin” to allow or deny end-user browsers from accessing a domain other than the one from which the browser sent the request. For example, if a user visits the website http://site-a.com and requests an image, but the image is hosted on another server and has the path http://cdn.site-b.com/image.jpg, the user’s browser sends a request to the server http://cdn.site-b.com/: 
GET /image.jpg HTTP/1.1  
Host: cdn.site-b.com  
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20081130 Minefield/3.1b3pre  
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8  
Accept-Language: en-us,en;q=0.5  
Accept-Encoding: gzip,deflate  
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7  
Connection: keep-alive  
Referer: http://site-a.com/examples/access-control/test.html  
Origin: http://site-a.com
The most significant header in the response is “Origin,” which tells the server that the request was sent from http://site-a.com. The http://cdn.site-b.com server checks the “Origin” header and decides whether to accept or deny the request.
  • If the http://cdn.site-b.com server is configured for cross-platformed requests, it will add the CORS header with the allowing value (e.g., Access-Control-Allow-Origin: *) to the response. In this case, the requested image will be displayed at http://site-a.com.
  • If the http://cdn.site-b.com server isn’t configured for cross-platformed requests, the response will have no “Access-Control-Allow-Origin” header or won’t have permission for http://site-a.com, and the requested image will not be displayed.

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What is the CORS header support feature used for?

Gcore has the CORS header support feature, which allows the CDN to add the “Access-Control-Allow-Origin” header to a response to a browser. It can be used for several purposes, such as protecting content from use at third-party sites or in third-party applications and preventing errors (such as _XMLHttpRequest cannot load http://domain.ru and No “Access-Control-Allow-Origin” header is present on the requested resource that can appear when web fonts are loaded in Firefox or Internet Explorer from the CDN servers).

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Configure the CORS header support feature

There are three methods to configure CORS header supports: in the resource settings, over the rule creation, and on the origin server.

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Configure CORS in the resource settings

If you want to apply the configuration to all files sent over the CDN, use the resource settings method: 1. Go to the CDN section in the Gcore Customer Portal and click the custom domain of the resource for which you want to configure CORS to open the resource settings.
CDN section
The new page opens. Do the remaining steps on it.
new page opens
2. Go to the “HTTP headers” section and click CORS header support. 3. Enable the feature. 4. Configure one of the three available options:
5. Optionally, check the “Always add the header to the response from CDN regardless of the response code” box to add the “Access-Control-Allow-Origin” header to responses with any status code, even for those when content is not available. If you leave the box unchecked, the header will be added only to responses with specific status codes. 6. Save changes.

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Configure CORS over the rule creation

If you want to apply the configuration only to particular files, use the “Rules” method: 1. Go to the CDN section in the Gcore Customer Portal and click the custom domain of the resource for which you want to configure CORS to open the resource settings.
Configure CORS over the rule creation
2. Open the “Rules” section, click Create rule and select Create blank rule from the list.
Rules
A new page opens. Do the remaining steps in it. 3. Specify the settings (rule name and path to files you want to apply the rule) according to the “Create a rule manually or from a template to configure settings for particular files” guide. 4. Click Add option, select “CORS header support”, and close the list.
Create rule
5. Configure the option. If you enable the option, the Access-Control-Allow-Origin header will be added. If you add an option but leave it disabled, the Access-Control-Allow-Origin header will not be added. 6. Click Create rule to save the changes.

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Configure CORS on the Origin Server

We have provided an example of how to set up CORS for Apache and Nginx web servers. Apache Configuration : 
# ----------------------------------------------------------------------   
# CORS-enabled images (@crossorigin)   
# ----------------------------------------------------------------------   
# Send CORS headers if browsers request them; enabled by default for images.   
# developer.mozilla.org/en/CORS_Enabled_Image   
# blog.chromium.org/2011/07/using-cross-domain-images-in-webgl-and.html   
# hacks.mozilla.org/2011/11/using-cors-to-load-webgl-textures-from-cross-domain-images/   
# wiki.mozilla.org/Security/Reviews/crossoriginAttribute   
<IfModule mod_setenvif.c>   
<IfModule mod_headers.c>   
# mod_headers, y u no match by Content-Type?!   
<FilesMatch "\.(gif|png|jpe?g|svg|svgz|ico|webp)$">   
SetEnvIf Origin ":" IS_CORS   
Header set Access-Control-Allow-Origin "*" env=IS_CORS   
</FilesMatch>   
</IfModule>   
</IfModule>   
# ----------------------------------------------------------------------   
# Webfont access   
# ----------------------------------------------------------------------   
# Allow access from all domains for webfonts.   
# Alternatively you could only whitelist your # subdomains like "subdomain.example.com".   
<IfModule mod_headers.c>   
<FilesMatch "\.(ttf|ttc|otf|eot|woff|woff2|font.css|css|js)$">   
Header set Access-Control-Allow-Origin "*"   
</FilesMatch>   
</IfModule>
Nginx configuration : 
location ~ \.(ttf|ttc|otf|eot|woff|woff2|font.css|css|js)$ {   
add_header Access-Control-Allow-Origin "*";   
}

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Check CORS

Clear the cache of the CDN resource or files for which CORS was enabled according to the “Clear CDN resource cache by URL, pattern or all” guide, and use one of the methods below.

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Check CORS through cURL

1. Open the terminal on macOS or command prompt (cmd) on Windows. 2. Run the following command: 
curl -I http://cdn.testdomain.com/assets/image.jpg
where http://cdn.testdomain.com/assets/image.jpg is a link to your file delivered via CDN. 3. In the output, check whether the “Access-Control-Allow-Origin” header is present. If you see the Access-Control-Allow-Origin header in the response, the configuration was successful. 

HTTP/1.1 200 OK   
Server: nginx/1.13.1   
Date: Fri, 24 Feb 2023 12:54:24 GMT   
Content-Type: image/jpeg   
Content-Length: 124024   
Connection: keep-alive   
X-Image-Generated: 29   
X-Image-Meta: 1024x768   
X-Image-Read: 71   
Expires: Fr, 10 March 2023 12:51:43 GMT   
Cache-Control: max-age=15552000   
Access-Control-Allow-Origin: *   
Last-Modified: Sun, 29 Jan 2023 12:00:00 GMT   
Cache-Control: max-age=315360000, public   
Cache: HIT   
X-Cached-Since: 2022-12-09T12:51:43+00:00   
X-ID: m9-up-e245

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Check CORS with DevTools in a browser

1. Open any internet browser (for example, Google Chrome). 2. Go to your website. 3. Right-click and select Inspect to open the DevTools (Developer Toolbar). 4. Select the “Network” tab. 5. Refresh the page. 6. You will get a list of all files retrieved from your website. 7. Find any file (e.g., JPEG, PNG, IMG, CSS, etc.) integrated with the CDN and select it. 8. On the “Headers” tab on the right, you will see the headers configured on your server. If you see the Access-Control-Allow-Origin header, the configuration was successful.
Headers