Ideally, a geolocation system should also work in an adversarial setting, when somebody is trying to spoof their geolocation. We recently learned that some VPN providers do that, to appear to have proxies in more countries: https://research.owlfolio.org/pubs/2018-catch-proxies-lie.pd...

Obviously, people can lie in their reverse DNS record but active measurements from distributed vantage points (e.g., by using the RIPE Atlas network) could expose that.