This is a great guide to iptables:
An In-Depth Guide to iptables, the Linux Firewall
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Category Archives: Ubuntu
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/
Install Roundcube on Ubuntu 16.04
https://linode.com/docs/email/clients/install-roundcube-on-ubuntu/
Site-To-Site Routed VPN Between Two or More Routers using dd-wrt/openvpn
http://wadihzaatar.com/?p=11
Install the Windows Subsystem for Linux
https://docs.microsoft.com/en-us/windows/wsl/install-win10