How can you debug a CORS request with cURL?

How can you debug CORS requests using cURL? So far I couldn't find any way to "simulate" the preflight request .

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.

Seems like just this works:

curl -I http://example.com

Look for Access-Control-Allow-Origin: * in the returned headers

The bash script "corstest" below works for me. It is based on Jun's comment above.

usage

corstest [-v] url

examples

./corstest https://api.coindesk.com/v1/bpi/currentprice.json
https://api.coindesk.com/v1/bpi/currentprice.json Access-Control-Allow-Origin: *

the positive result is displayed in green

./corstest https://github.com/IonicaBizau/jsonrequest
https://github.com/IonicaBizau/jsonrequest does not support CORS
you might want to visit https://enable-cors.org/ to find out how to enable CORS

the negative result is displayed in red and blue

the -v option will show the full curl headers

corstest

#!/bin/bash
# WF 2018-09-20
# https://stackoverflow.com/a/47609921/1497139

#ansi colors
#http://www.csc.uvic.ca/~sae/seng265/fall04/tips/s265s047-tips/bash-using-colors.html
blue='\033[0;34m'  
red='\033[0;31m'  
green='\033[0;32m' # '\e[1;32m' is too bright for white bg.
endColor='\033[0m'

#
# a colored message 
#   params:
#     1: l_color - the color of the message
#     2: l_msg - the message to display
#
color_msg() {
  local l_color="$1"
  local l_msg="$2"
  echo -e "${l_color}$l_msg${endColor}"
}


#
# show the usage
#
usage() {
  echo "usage: [-v] $0 url"
  echo "  -v |--verbose: show curl result" 
  exit 1 
}

if [ $# -lt 1 ]
then
  usage
fi

# commandline option
while [  "$1" != ""  ]
do
  url=$1
  shift

  # optionally show usage
  case $url in      
    -v|--verbose)
       verbose=true;
       ;;          
  esac
done  


if [ "$verbose" = "true" ]
then
  curl -s -X GET $url -H 'Cache-Control: no-cache' --head 
fi
origin=$(curl -s -X GET $url -H 'Cache-Control: no-cache' --head | grep -i access-control)


if [ $? -eq 0 ]
then
  color_msg $green "$url $origin"
else
  color_msg $red "$url does not support CORS"
  color_msg $blue "you might want to visit https://enable-cors.org/ to find out how to enable CORS"
fi

The preflight request is done using the OPTIONS HTTP method.

Assuming you want to test CORS on a POST request from http://mysite.example.com to https://myapi.example.com/foo, the command should be:

curl -XOPTIONS \
  -H "Access-Control-Request-Method: POST" \
  -H "Origin: http://mysite.example.com" \
  https://myapi.example.com/foo

The response is either OK or an error message like Disallowed CORS origin. You can still include the headers using -i if you’d like.

This is a lot simpler than some other responses that make either GET or HEAD requests and ask you to interprete the headers.