slimtoolkit/slim · error

cannot detect host port

Error message

cannot detect host port

What it means

After starting `kubectl port-forward`, this code scans its stdout for the actual locally-bound host port. If the process is running but no line matching the expected output pattern was parsed, it returns this error while still handing back the running command.

Source

Thrown at pkg/app/master/kubernetes/kubectl.go:176

	if err != nil {
		return cmd, "", err
	}

	if err := cmd.Start(); err != nil {
		return cmd, "", err
	}

	var actualHostPort int
	pattern := fmt.Sprintf("Forwarding from %s:%s -> %s", address, "%d", podPort)
	scanner := bufio.NewScanner(out)
	for scanner.Scan() {
		n, err := fmt.Sscanf(scanner.Text(), pattern, &actualHostPort)
		if err == nil && n == 1 {
			return cmd, fmt.Sprintf("%d", actualHostPort), nil
		}
	}

	return cmd, "", errors.New("cannot detect host port")
}

View on GitHub (pinned to 81940d17fa)

Solutions

1. Check kubectl stdout/stderr for connection errors (bad pod, port closed)
2. Verify the kubectl version's port-forward output format matches what is parsed
3. Increase the wait timeout and confirm the pod/port are valid before forwarding

Example fix

// ensure the forwarded port is valid before calling
if _, err := strconv.Atoi(podPort); err != nil { return fmt.Errorf("bad podPort %q", podPort) }

// sanity-check the target before forwarding
_, err := clientset.CoreV1().Pods(ns).Get(ctx, pod, metav1.GetOptions{})
if err != nil { return err } // pod must exist and be running

cmd, hostPort, err := k.PortForward(ctx, pod, addr, "", podPort)
if err != nil && strings.Contains(err.Error(), "cannot detect host port") {
  // capture cmd stdout/stderr, kill cmd, retry once with verbose kubectl
}

When it happens

Trigger: kubectl emits output not matching the expected "Forwarding from ... -> ..." pattern (locale/CLI version differences, warnings interleaved, stderr vs stdout) so Sscanf never yields n==1.

Common situations: Newer kubectl versions changing the message format; kubectl writing forwarding info to stderr; immediate connection failures producing error text instead of the forwarding line.

Related errors

• malformed Kubernetes workload name
• podPort cannot be empty
• Pod terminated
• Pod not running
• Container terminated

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/a9d4da556bee5f0f. Report an issue: GitHub.