probelabs/goreplay · error

expected %d snap length to be at least 0

Error message

expected %d snap length to be at least 0

What it means

SetSnapLen rejects a negative snaplen argument before touching the socket; snap length is a capture size bound and a negative value is invalid input (it is silently clamped to FRAMESIZE if too large, but negatives always fail).

Source

Thrown at internal/capture/sock_linux.go:189

func (sock *SockRaw) Close() (err error) {
	sock.mu.Lock()
	defer sock.mu.Unlock()
	if sock.fd != -1 {
		unix.Munmap(sock.buf)
		sock.buf = nil
		err = unix.Close(sock.fd)
		sock.fd = -1
	}
	return
}

// SetSnapLen sets the maximum capture length to the given value.
// for this to take effects on the kernel level SetBPFilter should be called too.
func (sock *SockRaw) SetSnapLen(snap int) error {
	sock.mu.Lock()
	defer sock.mu.Unlock()
	if snap < 0 {
		return fmt.Errorf("expected %d snap length to be at least 0", snap)
	}
	if snap > FRAMESIZE {
		snap = FRAMESIZE
	}
	sock.snaplen = snap
	return nil
}

// SetTimeout sets poll wait timeout for the socket.
// negative value will block forever
func (sock *SockRaw) SetTimeout(t time.Duration) error {
	sock.mu.Lock()
	defer sock.mu.Unlock()
	sock.pollTimeout = uintptr(t)
	return nil
}

// GetSnapLen returns the maximum capture length

View on GitHub (pinned to 251e45abd2)

Solutions

1. Pass a non-negative snaplen >= 0
2. Check the config computation that produced the snap value
3. Clamp caller-side values before calling SetSnapLen

When it happens

Trigger: Thrown at internal/capture/sock_linux.go:189 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of probelabs/goreplay@251e45abd2 (2026-09-02). Data as JSON: /api/errors/b46b663158ab8fa7. Report an issue: GitHub.