Planning errors often turn out to be particularly serious because they can easily create extensive vulnerabilities. If the use of DNS is insufficiently planned, this may result in problems and vulnerabilities during live operations. If, for example, the firewall rules that control DNS traffic are defined in too lax a manner, this might result in an attack under certain circumstances. On the other hand, overly restrictive rules may prevent legitimate clients from sending requests to DNS servers and hinder them when using services such as e-mail or FTP.