cloudflare/cloudflared · error

error piping traceroute's output: %w

Error message

error piping traceroute's output: %w

What it means

decodeNetworkOutputToFile attaches a stdout pipe to the traceroute command before starting it. If exec.Cmd.StdoutPipe fails (typically because the pipe could not be created, or was already set), this wrapped error is returned and the traceroute is never started. It indicates the transport for reading traceroute output could not be established.

Source

Thrown at diagnostic/network/collector_utils.go:16

package diagnostic

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"os/exec"
)

type DecodeLineFunc func(text string) (*Hop, error)

func decodeNetworkOutputToFile(command *exec.Cmd, decodeLine DecodeLineFunc) ([]*Hop, string, error) {
	stdout, err := command.StdoutPipe()
	if err != nil {
		return nil, "", fmt.Errorf("error piping traceroute's output: %w", err)
	}

	if err := command.Start(); err != nil {
		return nil, "", fmt.Errorf("error starting traceroute: %w", err)
	}

	// Tee the output to a string to have the raw information
	// in case the decode call fails
	// This error is handled only after the Wait call below returns
	// otherwise the process can become a zombie
	buf := bytes.NewBuffer([]byte{})
	tee := io.TeeReader(stdout, buf)
	hops, err := Decode(tee, decodeLine)
	// regardless of success of the decoding
	// consume all output to have available in buf
	_, _ = io.ReadAll(tee)

	if werr := command.Wait(); werr != nil {

View on GitHub (pinned to 2253eeeb25)

Solutions

1. Check open file descriptor limits (ulimit -n) and raise them if exhausted.
2. Ensure nothing sets command.Stdout before StdoutPipe is called (library-internal; check for forks/modifications).
3. Close other resources to free fds; look for fd leaks in the process (lsof).
4. Retry the diagnostic collection once fds are available.

// check fd headroom before collecting
var lim syscall.Rlimit
syscall.Getrlimit(syscall.RLIMIT_NOFILE, &lim)
// if usage near lim.Cur, expect StdoutPipe failures

hops, _, err := network.Collect(ctx, cfg)
if err != nil {
	var oe *os.PathError
	if errors.As(err, &oe) && errors.Is(oe.Err, syscall.EMFILE) {
		// too many open files: raise ulimit or free fds, then retry
	}
}

When it happens

Trigger: Calling decodeNetworkOutputToFile (via network Collect) when StdoutPipe fails — e.g. the command's Stdout field was already set before StdoutPipe was called, or OS-level pipe/fd exhaustion (too many open files).

Common situations: Process fd limit (ulimit -n) exhausted on hosts with many concurrent connections; accidental reconfiguration of exec.Cmd causing 'exec: Stdout already set'; highly constrained containers.

Related errors

• error starting traceroute: %w
• error finishing traceroute: %w
• error getting stderr pipe: %v
• error waiting from command '%s': %w
• couldn't parse index from timeout hop: %w

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/a3bc7d33d9f10a68. Report an issue: GitHub.