Many cache poisoning attacks against DNS servers can be prevented by being less trusting of the information passed to them by other DNS servers, and ignoring any DNS records passed back which are not directly relevant to the query. For example, versions of BIND 9.5.0-P1[3] and above perform these checks.[4] Source port randomization for DNS requests, combined with the use of cryptographically secure random numbers for selecting both the source port and the 16-bit cryptographic nonce, can greatly reduce the probability of successful DNS race attacks.