Is Active Directory required for a VPN

I looked into various Linux-based VPN solutions for a project and found a lot of pros and cons with each. I ended up with OpenVPN because it's fast, flexible and -- using the built-in scripts and examples in /usr/share -- easy to setup. Not a huge fan of the Windows client for OpenVPN, but it's getting better.

There's also OpenSwan/strongSwan (successors to FreeSwan) if you want to go mobile IPSec, but this isn't going to be any easier than OpenVPN. However, you do get to use the very nice (and open/free) Shrewsoft VPN client that runs on all major OSes.

There's no stopping you from using poptop though; it's still widely-used and supported in the Ubuntu repository as far as I know.

PPTP is the 'least' secure of the bunch; but it's still secure enough for all but FIPS-140 data when you use the EAP-* auth protocols. MS-CHAPv2 is also very secure. But it relies on passwords, which is almost always the weak point (that and social engineering, which is amazingly effective).

L2TP is more secure, and you should deploy it with certificates, not a PSK. This requires a PKI, which isn't too hard to setup. It's more secure than PPTP, but only on account of requiring the certificate or PSK.

SSTP is basically as secure as L2TP, it uses certificates again, still need a PKI. It's main advantage is that it works with cheap firewalls (or tightly configured ones) that block GRE and UDP traffic (for PPTP and L2TP respectively).

It's best practice to only have AD installed by itself (or with DNS) on a server; but people frequently install other services without significant problems. Bear in mind if this Internet connected server is compromised your AD DB is automatically compromised too.