Tracing HTTP Requests with Go's net/http/httptrace

net/http/httptrace has been in the standard library since Go 1.7 and most Go developers I talk to have never used it. It exposes hooks for the points in an outgoing HTTP request that you usually cannot see from outside the transport: DNS resolution, connection acquisition, TLS handshake, the moment bytes go on the wire, the moment the first response byte comes back. The interesting part is how it plugs in. There is no Tracer interface on http.Client, no middleware to register. You attach a ClientTrace to a context.Context and the transport pulls it back out via httptrace.ContextClientTrace at the points where it matters. I want to walk through that design choice first because it explains how the package composes with the rest of the stdlib, then build two things with it: a curl --trace-style CLI and a reusable http.RoundTripper that logs timings for every request. Why Context, Not an Interface The obvious design for request tracing would be to define a Tracer interface, add a Tracer…