Establishing secret common randomness between two or multiple devices in a network resides at the root of communication security. The problem is traditionally decomposed into a randomness generation stage (randomness purity is subject to employing often costly true random number generators) and a key-agreement information exchange stage, which can rely on public-key infrastructure or on key wrapping. In this paper, we propose KERMAN, an alternative key establishment algorithm for ad-hoc networks which works by harvesting randomness directly from the network routing metadata, thus achieving both pure randomness generation and (implicitly) secret-key agreement. Our algorithm relies on the route discovery phase of an ad-hoc network employing the Dynamic Source Routing protocol, is lightweight, and requires minimal communication overhead.