How can you debug a CORS request with cURL?

Here's how you can debug CORS requests using curl.

Sending a regular CORS request using cUrl:

curl -H "Origin: http://example.com" --verbose \
  https://www.googleapis.com/discovery/v1/apis?fields=

The -H "Origin: http://example.com" flag is the third party domain making the request. Substitute in whatever your domain is.

The --verbose flag prints out the entire response so you can see the request and response headers.

The url I'm using above is a sample request to a Google API that supports CORS, but you can substitute in whatever url you are testing.

The response should include the Access-Control-Allow-Origin header.

Sending a preflight request using cUrl:

curl -H "Origin: http://example.com" \
  -H "Access-Control-Request-Method: POST" \
  -H "Access-Control-Request-Headers: X-Requested-With" \
  -X OPTIONS --verbose \
  https://www.googleapis.com/discovery/v1/apis?fields=

This looks similar to the regular CORS request with a few additions:

The -H flags send additional preflight request headers to the server

The -X OPTIONS flag indicates that this is an HTTP OPTIONS request.

If the preflight request is successful, the response should include the Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers response headers. If the preflight request was not successful, these headers shouldn't appear, or the HTTP response won't be 200.

You can also specify additional headers, such as User-Agent, by using the -H flag.

Updated answer that covers most cases

curl -H "Access-Control-Request-Method: GET" -H "Origin: http://localhost" --head http://www.example.com/

1. Replace http://www.example.com/ with URL you want to test.
2. If response includes Access-Control-Allow-* then your resource supports CORS.

Rationale for alternative answer

I google this question every now and then and the accepted answer is never what I need. First it prints response body which is a lot of text. Adding --head outputs only headers. Second when testing S3 URLs we need to provide additional header -H "Access-Control-Request-Method: GET".

Hope this will save time.