Network routers occupy a key role in modern data transport and consequently are attractive targets for attackers. By manipulating, diverting or dropping packets arriving at a compromised router, an attacker can trivially mount denial-of-service, surveillance or man-in-the-middle attacks on end host systems. In this paper, we specify the problem of detecting routers with incorrect packet forwarding behavior and we explore the design space of protocols that implement such a detector. We further present a concrete protocol that is inexpensive enough for practical implementation at scale.