Happy Eyeballs Webtester

The Happy Eyeballs version 1 defines the racing of IPv4 vs IPv6. The term Connection Attempt Delay (CAD) describes the delay a client waits for the IPv6 response before issuing an IPv4 connection. We test this behavior of your client by artificially adding latency to IPv6 packets. Depending on your clients settings it will wait for the IPv6 connection attempt.

The Happy Eyeballs version 2 defines the racing of A vs AAAA DNS queries and uses the term Resolution Delay (RD) to describe the configured accepted delay between A and AAAA responses. We test this behavior of your client by artificially adding latency to A or AAAA queries. In this test not only your client but also the configured resolver and any forwarder in the resolution chain can impact the result.

This test does not test your device, OS, and browser but targets your recursive resolver. It uses a more complex DNS setup to infer if your resolver performs any type of Happy Eyeballs. While the Happy Eyeballs algorithm and suggested configuration parameters are not adjusted to the iterative resolution process of a resolver, we are still interested to see if and how resolvers prefer IPv6.

Only our authoritative name server observe the requests and analyze the results. As we did not yet implement a way to live feed the results back to your browser session, we cannot currently show these.

This is a tool to generate a specific domain for a selected test configuration. The domain can then be used with any client to test its HE behavior. The server always returns the IP address at which it was contacted. This enables you to easily know if your client used IPv4 or IPv6.

@inproceedings{sattler2025happyeyeballs,
    title = {{Lazy Eye Inspection: Capturing the State of Happy Eyeballs Implementations}},
    author = {Sattler, Patrick and Kirstein, Matthias and Wüstrich, Lars and Zirngibl, Johannes and Carle, Georg},
    booktitle = {Proceedings of the 2025 Internet Measurement Conference},
    year = {2025},
    location = {Madison, WI, USA},
    abbreviation = {IMC'25},
    publisher = {ACM},
    month = oct,
    homepage = {https://www.happy-eyeballs.net/},
    month_numeric = {10}
}
                                

View our source code for the tests and the local testbed setup on GitHub