Tools:

Mxtool box: https://mxtoolbox.com/

DMARC training: https://www.learndmarc.com/

Technitium DNS Server: https://technitium.com/dns/

Sectools: https://sectools.org

Systemd: https://systemd.io/

LinuxJourney: https://linuxjourney.com/lesson/debian

RedHat Lab: https://lab.redhat.com

KodeKloud: https://kodekloud.com/

AWS Training: https://aws.amazon.com/training

IP Subnet Calculator: https://www.calculator.net/ip-subnet-calculator.html

News & Research:

CVEtrends: https://cvetrends.com/

Linux Foundation: https://www.linuxfoundation.org/

CVE: https://www.cve.org/

NIST: https://nvd.nist.gov/

AWS Blog: https://aws.amazon.com/blogs/aws/

Home Lab: https://williamlam.com/home-lab

AWS Whitepapers: https://aws.amazon.com/whitepapers

OWASP Vulnerabilities: https://owasp.org/www-community/vulnerabilities/

OWASP Top10: https://owasp.org/Top10/

Python: https://pythonbooks.org/free-books/

Sans: https://www.sans.org/uk/

Sans Top 25 Software Errors: https://www.sans.org/top25-software-errors/