With the wide adoption, Linux-based IoT devices have emerged as one primary target of today's cyber attacks. Traditional malware-based attacks can quickly spread across these devices, but they are well-understood threats with effective defense techniques such as malware fingerprinting and community-based fingerprint sharing. Recently, fileless attacks—attacks that do not rely on malware files—have been increasing on Linux-based IoT devices, and posing significant threats to the security and privacy of IoT systems. Little has been known in terms of their characteristics and attack vectors, which hinders research and development efforts to defend against them. In this paper, we present our endeavor in understanding fileless attacks on Linux-based IoT devices in the wild. Over a span of twelve months, we deploy 4 hardware IoT honeypots and 108 specially designed software IoT honeypots, and successfully attract a wide variety of real-world IoT attacks. We present our measurement study on these attacks, with a focus on fileless attacks, including the prevalence, exploits, environments, and impacts. Our study further leads to multi-fold insights towards actionable defense strategies that can be adopted by IoT vendors and end users.