How would you fix intermittent connectivity and DNS issues in a mixed Windows and Linux server network, ensuring security?

Outline the steps for diagnosing the problem. List the tools you would use (e.g., network monitoring, DNS troubleshooting). Explain DNS configuration management between different systems. Suggest improvements to prevent future issues, addressing security and reliability. This question requires a detailed approach that demonstrates advanced knowledge in troubleshooting network and DNS issues across mixed operating systems and applying best practices for network management.