Linux – Losing DNS when connecting via pptp vpn from a Windows client to a Linux server

Basically no, you can't change it. The TCP port is only used to setup the initial connection. All traffic is sent over GRE, not TCP. I highly recommend requiring client certificates. PPTP as a protocol is plenty secure when you pair it with client certs, no need to upgrade to something like SSL VPN.

See this question from the other day for links on how to setup this up.

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!