What causes "error reading %s: %s" in tsenart/vegeta?

Check the wrapped underlying error for the real cause (I/O error, permission, etc.).

error reading %s: %s

Error message

error reading %s: %s

What it means

After successfully opening the body file, attack() reads it fully with io.ReadAll; if reading fails (I/O error rather than open error) the filename and underlying error are wrapped. This indicates the file exists but its contents could not be read.

Source

Thrown at attack.go:148

	net.DefaultResolver.PreferGo = true

	files := map[string]io.Reader{}
	for _, filename := range []string{opts.targetsf, opts.bodyf} {
		if filename == "" {
			continue
		}
		f, err := file(filename, false)
		if err != nil {
			return fmt.Errorf("error opening %s: %s", filename, err)
		}
		defer f.Close()
		files[filename] = f
	}

	var body []byte
	if bodyf, ok := files[opts.bodyf]; ok {
		if body, err = io.ReadAll(bodyf); err != nil {
			return fmt.Errorf("error reading %s: %s", opts.bodyf, err)
		}
	}

	var (
		tr       vegeta.Targeter
		src      = files[opts.targetsf]
		hdr      = opts.headers.Header
		proxyHdr = opts.proxyHeaders.Header
	)

	switch opts.format {
	case vegeta.JSONTargetFormat:
		tr = vegeta.NewJSONTargeter(src, body, hdr)
	case vegeta.HTTPTargetFormat:
		tr = vegeta.NewHTTPTargeter(src, body, hdr)
	default:
		return fmt.Errorf("format %q isn't one of [%s]",
			opts.format, strings.Join(vegeta.TargetFormats, ", "))

View on GitHub (pinned to cf58112690)

Solutions

Check the wrapped underlying error for the real cause (I/O error, permission, etc.).
Copy the body to a regular local file and pass that path instead of a device/mounted path.
Re-run with a smaller/known-good body file to isolate the failing input.

Defensive patterns

Strategy: try-catch

Validate before calling

if b, err := os.ReadFile(bodyPath); err != nil {
    return fmt.Errorf("body file unreadable: %w", err)
}

Try / catch

if _, err := os.ReadFile(bodyPath); err != nil {
    log.Fatalf("pre-check read of body %s failed: %v", bodyPath, err)
}

Prevention

Keep body files on local, reliable storage rather than network mounts or device nodes.
Pre-read the body file in a health check before long-running attacks.
Avoid passing special files (FIFOs, /dev entries) as -body.

When it happens

Trigger: `-body` file that becomes unreadable mid-read, a special/character device or FIFO that errors during read, or hardware/permission errors surfacing during ReadAll.

Common situations: Body file on a flaky network mount; passing a device path like /dev/stdin in an environment where stdin cannot be read; disk I/O errors.

Related errors

AI-assisted analysis of tsenart/vegeta@cf58112690 (2026-08-31). Data as JSON: /api/errors/355216985706f18a. Report an issue: GitHub.