<p>Determines how Route 53 Resolver handles queries during failures, for example when all traffic that is sent to DNS Firewall fails to receive a reply. </p> <ul> <li> <p>By default, fail open is disabled, which means the failure mode is closed. This approach favors security over availability. DNS Firewall blocks queries that it is unable to evaluate properly. </p> </li> <li> <p>If you enable this option, the failure mode is open. This approach favors availability over security. DNS Firewall allows queries to proceed if it is unable to properly evaluate them. </p> </li> </ul> <p>This behavior is only enforced for VPCs that have at least one DNS Firewall rule group association. </p>