How to enable (disable) PPTP multi login (of a same account) on Linux

The availability of clients for OpenVPN is more wide than that of SSTP (at least, right now). I can buy an IP phone with an embedded OpenVPN client, for example. AFAIK, Microsoft didn't back-port the SSTP client to Windows XP (which, initially, they said they would), so that cuts off a large client-base. In contrast, though, SSTP doesn't require the installation of third-party software on supported client operating systems.

There are no per-client license fees with OpenVPN as there are with Microsoft's offering. (I won't offer my opinion on which specific usages need a Windows CAL and which doesn't... In some documentation Microsoft claims that a DHCP client needs a CAL, so I tend to give them a wide berth. If my janitor dusts around my Windows Server machine I probably need a CAL for them. The right place to find out about licensing is the software "manufacturer" anyway...)

The functionality built-in to the OpenVPN client to receive "pushed" routes is more flexible than Microsoft's VPN client (unless you use the CMAK, and that hasn't been reliable for me in practice).

The SecurID tokens aren't supported because the validation software is non-free, as far as I know. I've had a lot of fun with the yubikey hardware OTP generator for better-than-username-and-password authentication via PAM. The yubikeys are also noticeably cheaper than SecurID tokens, and don't seem to have a limited lifespan.

Specifically, I've setup ssh using the yubikey for authentication, which opens up the possibility of using ssh-based VPNs. My writeup's at http://www.teaparty.net/technotes/yubikey.html if it's of any use to you. Everything involved is GPLed or better.

I've also seen people using PAM-based authentication steps with OpenVPN, which opens the possibility of getting OpenVPN to work with the yubikey. The guys at Securix Live say they're working on a fully two-factor PAM module for the yubikey, and while I haven't been able to get it to work yet, that would give you the final piece of what you asked for.

If you do get OpenVPN working with a yubikey, do let us know - and write about it!