pfSense

In some countries the connection to ISP works like this: A computer(pfsense in this case) takes an IP from the modem via DHCP. This IP is from private network (i.e. 172.x.x.x), then on top of it the "dialer" brings up the PPTP/L2TP interface and gets the public IP.

Now, basically, with the 1.2.3 release I was able to get it working. This is how I did it:
1. Assuming WAN is the interface to connect to the internet - set it to DHCP, write down the IP, netmask and gateway that came from the modem.
2. On some other PC do a nslookup for the ISP's PPTP server name, which is supposed to be a pool of IPs. Choose one. Write it down.
3. Set the WAN interface to PPTP and besides the user/pass, in the local IP address type the IP/netmask from the DHCP we got earlier. In the remote IP address type the PPTP server of choice from step 2.
4. Set up a static route:
interface: WAN
network: PTPP server from step 2 (/32)
gateway: the one that we got from DHCP.

That's it. But there's a problem with such setup: if the ISP decides to change the PPTP server or DHCP pool of modems - we have to manually change these settings again.

Another thing is that it would be great if you could implement the same thing for L2TP.
By the way, all of these things are already implemented in m0n0wall-mod distro, so maybe it will be possible to port it to pfsense.