Hash functions are often used to consistently assign objects to particular resources, for example to load balancing in networks. These functions can be randomly selected from a family, to prevent attackers generating many colliding objects, which usually results in poor performance. We describe a number of attacks allowing us to identify which hash function from a family is being used by observing a relatively small number of collisions. This knowledge can then be used to generate a large number of colliding inputs. In particular we detail attacks against small families of hashes, Pearson-like hash functions and linear hashes, such as the Toeplitz hash used in Microsoft's Receive Side Scaling. Â© 2012 IEEE.