Packet filtering on PPP over modem

[wanded_(_at_)_breathemail_(_dot_)_net]

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Is it possible to set up packet filtering on a PPP connection through a modem, via tun0 or ppp0?

We have a standalone machine connected via a 56k modem, and would like pf. 

I have seen examples where the machine has 2 network connections, one for ADSL/ethernet connection and the other for internal network connections.

The difference would seem to be that PPP does not have a static IP address.
e.g. PPP connections, the ISP allocates the IP address for you. ADSL connections, the ISP provides a static IP address.

Which PPP connection is better and why? ppp user or ppp through the kernel?

Regards
Ed