Links

 

SSL

https://www.whynopadlock.com

https://www.ssllabs.com/ssltest/

https://www.sslshopper.com/ssl-checker.html

DNS / Domain

https://toolbox.googleapps.com/apps/dig/

https://www.whatsmydns.net/

https://intodns.com/

https://www.whois.net/

http://redirectdetective.com/

Mail

https://www.mail-tester.com/

https://getnada.com/

https://mxtoolbox.com/NetworkTools.aspx

RBL checks

https://mxtoolbox.com/blacklists.aspx

http://multirbl.valli.org/

http://www.anti-abuse.org/multi-rbl-check/

Vulnerability checker

https://www.exploit-db.com/

https://www.rapid7.com/db/search

https://wpvulndb.com/

News/Articals

http://www.linuxinsider.com/

http://www.linuxtoday.com/

http://www.linuxjournal.com/

https://www.reddit.com/r/sysadmin/

https://krebsonsecurity.com/

http://news.cpanel.com/

Tutorials

https://www.unixmen.com/category/linux-tutorials/

https://www.digitalocean.com/community/tutorials

http://www.learnitguide.net/

http://www.itzgeek.com/category/how-tos

https://www.vultr.com/docs/