add-origin-header

Add Origin header if missing

CloudFront Functions event type: viewer request

This function adds an origin header if it is not present in the incoming request. The Origin header is part of Cross-Origin Resource Sharing (CORS), a mechanism using HTTP headers to tell browsers to give a web application running at one origin access to selected resources from a different origin.

In order for your origin to receive the Origin header, you must specifically allow the Origin header (or allow all viewer headers) in a CloudFront origin request policy. Without adding this header in an origin request policy, CloudFront removes the Origin header before sending requests to your origin.

If you origin doesn't respond with CORS headers (e.g. the Access-Control-Allow-Origin header), then this function is not required.

Testing the function

To validate that the function is working as expected, you can use the JSON test objects in the test-objects directory. To test, you can use the test-function CLI command that will look something like this:

$ aws cloudfront test-function --if-match EXXXXXXXXXXXX --name add-origin-header --event-object fileb://add-origin-header/test-objects/no-origin-header.json

If the function has been correctly setup, you should see a similar result with the origin header being added in the FunctionOutput JSON object:

{
    "TestResult": {
        "FunctionSummary": {
            "Name": "add-origin-header",
            "Status": "UNPUBLISHED",
            "FunctionConfig": {
                "Comment": "",
                "Runtime": "cloudfront-js-1.0"
            },
            "FunctionMetadata": {
                "FunctionARN": "arn:aws:cloudfront::1234567890:function/add-origin-header",
                "Stage": "DEVELOPMENT",
                "CreatedTime": "2021-04-09T18:14:49.648000+00:00",
                "LastModifiedTime": "2021-04-09T18:20:08.058000+00:00"
            }
        },
        "ComputeUtilization": "15",
        "FunctionExecutionLogs": [],
        "FunctionErrorMessage": "",
        "FunctionOutput": "{\"request\":{\"headers\":{\"origin\":{\"value\":\"https://www.example.com\"},\"host\":{\"value\":\"www.example.com\"},\"accept\":{\"value\":\"text/html\"}},\"method\":\"GET\",\"querystring\":{\"test\":{\"value\":\"true\"},\"arg\":{\"value\":\"val1\"}},\"uri\":\"/index.html\",\"cookies\":{\"loggedIn\":{\"value\":\"false\"},\"id\":{\"value\":\"CookeIdValue\"}}}}"
    }
}