Links
SSL
https://www.ssllabs.com/ssltest/
https://www.sslshopper.com/ssl-checker.html
DNS
https://toolbox.googleapps.com/apps/dig/
https://mxtoolbox.com/NetworkTools.aspx
RBL checks
https://mxtoolbox.com/blacklists.aspx
http://www.anti-abuse.org/multi-rbl-check/
Vulnerability checker
https://www.rapid7.com/db/search
News/Articals
https://www.reddit.com/r/sysadmin/
Tutorials
https://www.unixmen.com/category/linux-tutorials/
https://www.digitalocean.com/community/tutorials