gofiber/fiber · critical

boundary generation: %w

Error message

boundary generation: %w

What it means

Returned in parserRequestHeader (client/hooks.go:153) when unsafeRandString fails while generating the random suffix appended to the default multipart boundary for a filesBody request. It is a thin wrapper around the underlying rand.Read failure (errors 84/85). The boundary is only auto-generated when the request still uses the default boundary constant 'FiberFormBoundary'.

Source

Thrown at client/hooks.go:153

	// Set Content-Type and Accept headers based on the request body type.
	switch req.bodyType {
	case jsonBody:
		req.RawRequest.Header.SetContentType(applicationJSON)
		req.RawRequest.Header.Set(headerAccept, applicationJSON)
	case xmlBody:
		req.RawRequest.Header.SetContentType(applicationXML)
	case cborBody:
		req.RawRequest.Header.SetContentType(applicationCBOR)
	case formBody:
		req.RawRequest.Header.SetContentType(applicationForm)
	case filesBody:
		req.RawRequest.Header.SetContentType(multipartFormData)
		// If boundary is default, append a random string to it.
		if req.boundary == boundary {
			randStr, err := unsafeRandString(16)
			if err != nil {
				return fmt.Errorf("boundary generation: %w", err)
			}
			req.boundary += randStr
		}
		req.RawRequest.Header.SetMultipartFormBoundary(req.boundary)
	default:
		// noBody or rawBody do not require special handling here.
	}

	// Set User-Agent header.
	req.RawRequest.Header.SetUserAgent(defaultUserAgent)
	if c.userAgent != "" {
		req.RawRequest.Header.SetUserAgent(c.userAgent)
	}
	if req.userAgent != "" {
		req.RawRequest.Header.SetUserAgent(req.userAgent)
	}

	// Set Referer header.

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Fix the environment so crypto/rand works (expose /dev/urandom, allow getrandom).
2. As a workaround, set an explicit boundary via Request.SetBoundary so unsafeRandString is skipped.
3. Treat the error as fatal/infra-level — retrying on the same host will keep failing.

Example fix

// before — relies on auto-generated boundary, fails without /dev/urandom
client.R().SetFiles("./a.txt").Get(url)

// after — set a boundary explicitly to bypass unsafeRandString
client.R().SetBoundary("MyAppBoundary123").SetFiles("./a.txt").Get(url)

// Detect CSPRNG unavailability before attempting uploads.
func canGenerateBoundary() bool {
    return rand.Read(make([]byte, 1)) == nil
}

resp, err := client.R().SetFiles(f).Get(url)
if err != nil && strings.Contains(err.Error(), "boundary generation") {
    // rand unavailable — retry with an explicit boundary
    resp, err = client.R().SetBoundary("ManualBoundary").SetFiles(f).Get(url)
}

When it happens

Trigger: Sending a multipart file upload (Request.SetFiles) without a custom boundary, on a host where crypto/rand.Read fails. The header-preparation hook tries to build boundary = 'FiberFormBoundary' + randomString(16) and returns this wrapped error.

Common situations: Same CSPRNG-unavailability contexts as errors 84/85 (locked-down container, missing /dev/urandom, blocked getrandom syscall), specifically manifesting during the header-building phase of a file upload.

Related errors

• rand.Read failed: %w
• set boundary error: %w
• failed to close multipart writer: %w
• write formdata error: %w
• create file error: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/a2f6e24f7a5f60cc.json. Report an issue: GitHub.