You can use Lambda@Edge function (https://docs.aws.amazon.com/lambda/latest/dg/lambda-edge.html) assigned to your CloudFront distribution. You would need two functions:
- Viewer-Request event handler, that will read
User-Agent header and copy it to e.g. X-My-User-Agent. Viewer-Request handler is invoked before the request from the client reaches your Cloudfront Distribution.
- Origin-Request event handler, that will read
X-My-User-Agent and replace User-Agent. Origin-Request handler is invoked when Cloudfront did not find requested page in its cache and sends the request to the origin.
Please note that you should NOT add User-Agent to Cloudfront whitelist:
You can configure CloudFront to cache objects based on values in the
Date and User-Agent headers, but we don't recommend it. These headers
have a lot of possible values, and caching based on their values would
cause CloudFront to forward significantly more requests to your
origin.
Ref: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/RequestAndResponseBehaviorCustomOrigin.html#request-custom-headers-behavior
Example of Viewer-Request handler (Lambda@Edge can be written only in NodeJS or Python, Ref: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-requirements-limits.html#lambda-requirements-lambda-function-configuration):
'use strict';
exports.handler = (event, context, callback) => {
const request = event.Records[0].cf.request;
const headers = request.headers;
const customUserAgentHeaderName = 'X-My-User-Agent';
const userAgent = headers['user-agent'][0].value;
headers[customUserAgentHeaderName.toLowerCase()] = [
{
key: customUserAgentHeaderName,
value: userAgent
}
];
callback(null, request);
};
Example of Origin-Request handler:
'use strict';
exports.handler = (event, context, callback) => {
const request = event.Records[0].cf.request;
const headers = request.headers;
const customUserAgentHeaderName = 'X-My-User-Agent';
const realUserAgent = headers[customUserAgentHeaderName.toLowerCase()][0].value;
headers['user-agent'] = [
{
key: 'User-Agent',
value: realUserAgent
}
];
callback(null, request);
};
