Troubleshooting Problems with Postfix, Dovecot, and MySQL
https://www.linode.com/docs/email/postfix/troubleshooting-problems-with-postfix-dovecot-and-mysql/
Ubuntu
How do I set up an email alert when a ssh login is successful?
Original: https://askubuntu.com/questions/179889/how-do-i-set-up-an-email-alert-when-a-ssh-login-is-successful Since the sshrc method doesn’t work if the user has their own ~/.ssh/rc file, I’ll explain how to do this with pam_exec as @adosaiguas suggested. The good thing is that this can also be easily adapted to login types other than ssh (such as local logins or even all logins) by hooking into a different file in /etc/pam.d/. First you need to … Read more
Running multiple OpenVPN instances (multi port)
https://www.hugeserver.com/kb/openvpn-multiple-ports/
An In-Depth Guide to iptables, the Linux Firewall
This is a great guide to iptables: https://www.booleanworld.com/depth-guide-iptables-linux-firewall/
IP Masquerade and Network Address Translation
http://www.oreilly.com/openbook/linag2/book/ch11.html
IP Masquerading using iptables
http://billauer.co.il/ipmasq-html.html
The Beginner’s Guide to iptables, the Linux Firewall
https://www.howtogeek.com/177621/the-beginners-guide-to-iptables-the-linux-firewall/
Tutorial: Installing OpenVPN on Ubuntu 16.04
https://blog.ssdnodes.com/blog/tutorial-installing-openvpn-on-ubuntu-16-04/ To configure iptables look here: https://www.digitalocean.com/community/tutorials/how-to-set-up-an-openvpn-server-on-ubuntu-16-04
How to Configure OpenVPN Access Server to Tunnel Traffic
https://linode.com/docs/networking/vpn/configure-openvpn-access-server-to-tunnel-traffic/
Install OpenVPN Access Server on Linux for Secure Communications
https://linode.com/docs/networking/vpn/install-openvpn-access-server-on-linux/