I am going to be using a slackware 9.1 box for an internet gateway/firewall to our dial-up ISP.

So far, I have got pppd working correctly using pppsetup. I start it with ppp-start and stop it with ppp-stop. Now, I want to set it up to start at boot and then reconnect if it loses the connection or if the initial connection didn't make it for some reason (i.e. line busy etc.). I would also like to know if there is a way to make pppd get the DNS servers IPs automatically, pppsetup made me put them in, but when I use windows, it gets them automatically.

I know that I could just put ppp-start in my rc.local file to get the connection turned on at startup, but that doesn't seem like the best way of doing it.

Also, I am going to be using jay's firewall to make my IPTABLES rules. Is there anything I am going to need to consider because my external IP address will be changing or because the ppp0 interface may be appearing and disappearing as connections come up or go down? Anything else that might be different as compared to a box with two ethernet cards acting as a firewall?

Thanks in advance.

Don't know about the Slack init.d system, never seen one,
but there are scripts to bring the interface up at boot time.
They will refer to a config file which holds the dns auto, default route and other options you are looking for.

As for the firewall, you need to be using MASQUERADE instead of SNAT rules.
As I remember, Jay's script will do this automatically coz the modem is on a ppp~ interface which is usually a masqueraded interface.