cdn-cgi/trace
Cloudflare's diagnostic endpoint, used by IPCombo to get per-target real exit IPs.
Cloudflare operates a free, unauthenticated diagnostic endpoint at https://<any-cloudflare-site>/cdn-cgi/trace. It returns plain-text key=value pairs: the source IP of the request as seen by Cloudflare's edge, the country, the colo (Cloudflare's data center code), the TLS version, and a few other fields. The endpoint is rate-limited per IP, not per domain, so it's stable under load.
IPCombo uses cdn-cgi/trace to get a per-target real exit IP. Because each probe goes to a different Cloudflare site, we get a different source IP per target — which means we can see exactly which proxy rule fired for each destination, and we can verify that the source IP is what we expected.