logf := func(format string, args ...any) {
		mu.Lock()
		defer mu.Unlock()
		fmt.Fprintf(&buf, format, args...)
		buf.WriteByte('\n')
	}

	addrStr := cst.ts.Listener.Addr().String()
	ip, port, err := net.SplitHostPort(addrStr)
	if err != nil {
		t.Fatal(err)
	}

	// Install a fake DNS server.