Many application-layer protocols contain a variety of security issues due to the complexity and lack of security in their design. Some of these protocols (e.g., FTP and DNS) have been around for decades, and therefore have few security capabilities; however, they are still widely used on the Internet. Application-layer software, which is frequently exposed to network traffic from untrusted sources, is another common source of security vulnerabilities.To protect against these threats, it is necessary to utilize secure protocols, secure software, and secure configurations.Tools such as Nessus should be periodically run against systems to ensure that they do not contain any security issues.