Another issue that is only somewhat related to firewalls (but that the security folks putting up firewalls are often asked to address) is where to locate the terminal servers and modem pools within a site's network. You definitely need to pay as much attention to the security of your dial-up access ports as you do to the security of your Internet connection. However, dial-up security (authentication systems, callback systems, etc.) is a whole topic of its own, separate from firewalls. We'll therefore restrict our comments to those related to firewalls.
The big firewall question concerning terminal servers and modem pools is where to put them: do you put them inside your security perimeter, or outside? Our advice is to put them on the inside and to protect them carefully. You'll not only be doing yourself a favor, you'll also be a good neighbor. Putting open terminal servers on the Internet is a risk to other people's sites as well as your own.
If the modem ports are going to be used primarily to access internal systems and data (that is, employees working from home or on the road), then it makes sense to put them on the inside. If you put them on the outside, you'd have to open holes in your perimeter to allow them access to the internal systems and data - holes that an attacker might be able to take advantage of. Also, if you put them on the outside, then an attacker who has compromised your perimeter (broken into your bastion host, for example) could potentially monitor the work your users do, essentially looking over their shoulders as they access private, sensitive data. If you do put the modems on the inside, you'll have to protect them very carefully, so they don't become an easier break-in target than your firewall. It doesn't do any good to build a first-class firewall if someone can bypass it by dialing into an unprotected modem connected to the internal network.
On the other hand, if the modem ports are going to be used primarily to access external systems (that is, by employees or guests who mainly use your site as an access point for the Internet), then it makes more sense to put them on the outside. There's no sense in giving someone access to your internal systems if they don't need it. This external modem pool should be treated just as suspiciously as the bastion host and the other components of your firewall.
If you find that you need both types of access, then you might want to consider two modem pools: one on the inside, carefully protected, to access internal systems and another on the outside to access the Internet.
If your terminal servers and modem pools are being used to support dial-up network connections from homes or other sites, you should make sure you enforce any implicit assumptions you have about that usage. For instance, people setting up PPP accounts on terminal servers generally assume that the PPP account is going to be used by a single remote machine running standalone. More and more machines, however, are part of LANs, even at home (Dad's PC is in the den, Mom's in the living room). That PPP connection could be used not just by the machine you set it up for, but by anything that machine is connected to, and anything those machines are connected to, and so forth. The machine that uses the PPP account might be connected to a LAN, with any number of other machines on it; any of them might be connected (via other PPP connections, for example) to another site or an Internet service provider. If you don't do anything to prevent it, traffic could flow from the Internet, to the second PC, to the "legitimate" PC, and finally into your own net, completely bypassing your firewall.
You can prevent this problem by simply enabling packet filtering on the PPP connection that limits what it can do to what you expect it to do (i.e., that limits packets on the connection to only packets to or from the machine you expect to be at the other end of the connection).
Some sites with significant dial-up networking activity take the approach of building a separate firewall just for that activity. See the discussion of multiple perimeter networks in Chapter 4, Firewall Design.