CanYouSeeMe is a tool that allows you to check how certain aspects of your network appear from the outside.
The first feature (and the only one available currently) is the ability to view files in the
.well-known directory for a website.
To use this service, make an HTTP (or HTTPS) request to:
http://canyouseeme.jons.tools/well-known/{your-url}
For example, to view Gmail's STS policy from the point of view of this webserver, you would use:
http://canyouseeme.jons.tools/well-known/https://mta-sts.gmail.com/.well-known/mta-sts.txt
The response is capped at 1KB to ensure efficient usage and to avoid overloading the server.
This tool is particularly useful for pre-validation in a way that won't threaten rate limits. For example, if you are trying to issue a certificate from Let's Encrypt, you can do a pre-check with this tool to make sure your site is visible from the outside before making the real request. This helps avoid exhausting API limits.