What causes "scanner reported an error: %w" in cloudflare/cloudflared?

Check the raw traceroute output (the collector tees it) for truncation or garbage.

scanner reported an error: %w

Error message

scanner reported an error: %w

What it means

Decode reads traceroute output line by line via a bufio.Scanner and returns the parsed hops. If the underlying reader produced an I/O error during scanning (scanner.Err() != nil), Decode wraps it with this message. It indicates the traceroute output stream was interrupted, not that a line was unparseable.

Source

Thrown at diagnostic/network/collector_utils.go:70

		text := scanner.Text()
		if text == "" {
			continue
		}

		hop, err := decodeLine(text)
		if err != nil {
			// This continue is here on the error case because there are lines at the start and end
			// that may not be parsable. (check windows tracert output)
			// The skip is here because aside from the start and end lines the other lines should
			// always be parsable without errors.
			continue
		}

		hops = append(hops, hop)
	}

	if scanner.Err() != nil {
		return nil, fmt.Errorf("scanner reported an error: %w", scanner.Err())
	}

	return hops, nil
}

View on GitHub (pinned to 2253eeeb25)

Solutions

Check the raw traceroute output (the collector tees it) for truncation or garbage.
If lines are unusually long, raise the scanner buffer in a fork/patch: scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024).
Verify traceroute completes normally; a crashed process breaks the pipe mid-read.
Retry the diagnostic collection.

Defensive patterns

Strategy: fallback

Validate before calling

// ensure traceroute terminates normally first
cmd := exec.Command("traceroute", "-n", "-m", "15", host)
if err := cmd.Run(); err != nil {
	// traceroute itself fails; decoding will see pipe errors
}

Try / catch

hops, raw, err := network.Collect(ctx, cfg)
if err != nil {
	// raw output tee is still available for manual inspection
	log.Warn().Err(err).Str("raw", raw).Msg("decode failed")
}

Prevention

Let traceroute finish; don't kill it mid-run
Keep hop output lines small (use -n to skip DNS) to avoid scanner buffer limits
Use the raw tee output preserved by the collector for debugging
Retry collection on transient pipe errors

When it happens

Trigger: The stdout pipe from the traceroute process returns an I/O error while Decode reads it — process killed abruptly breaking the pipe, or read errors on the pipe; also reachable if a scanner buffer limit is exceeded, though that surfaces as bufio.ErrTooLong through Err().

Common situations: Traceroute process crashing mid-output; extremely long output lines exceeding bufio.Scanner's default 64KB token limit; pipe errors under fd pressure.

Related errors

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/f819234dca39159d. Report an issue: GitHub.