What Phoenix is
Phoenix is a web framework for Elixir, running on the Erlang virtual machine. It is usually served by Cowboy, the Erlang HTTP server, which names itself in the server header. The detection is therefore an inference from the server rather than a direct statement by the framework.
How DNSTrace.dev detects it
Detection reads the final HTTPS response and the first bytes of its markup. One matching signal is reported as strong evidence; two or more independent signals are reported as confirmed.
server header naming Cowboy— The Erlang HTTP server that Phoenix runs on by default.
What a detection proves
The response came from an Erlang HTTP server, which in practice usually means Phoenix.
What it does not prove
That Phoenix is present. Cowboy can serve an Elixir or Erlang application that does not use Phoenix at all, so this is the weakest inference DNSTrace.dev reports in this group.
Common questions
Why report it if it is an inference?
Because the signal is genuine and the limit is stated. The evidence is a Cowboy server header and nothing more.
Is LiveView detectable?
Not from the home document alone.
Compare with
GunicornOpen source
Application runtime
Java servlet containerVarious, Jakarta EE standard
Application framework
ExpressOpen source, OpenJS Foundation
Check a domain
Run a full lookup to see the delivery layer, the detected application stack, live TLS and the announcing network side by side.