Chapter 10. DNS and Dial-Up Connections

BIND has a tendency to want to talk to nameservers every now and then. Quite often in fact. If you use a dial-up connection to the Internet and pay by the minute for your connection time, you're wasting money. Many companies connect to the Internet by auto-dialing ISDN or analog modems. These ISDN and analog modems can be connected to ISDN network routers, Linux machines, or other inexpensive UNIX machines used as firewalls or ISDN routers. The goal, then, is to minimize the connection time (at least before and after working hours) but still allow BIND, and people, to work.

Keeping your dial-up connection time as low as possible is important, but it also is important to note that if your server is master for any zones, the slaves must be capable of connecting to the master. To ensure this, your server should be online for at least two times the retry interval in the SOA record daily. Recall the pattern in which a slave tries to transfer zones from masters. This ensures that the slaves can check for new versions of the zone(s) and retrieve it if necessary, and that the zone does not expire and become unavailable.

In addition, if your server is a master, it should be an unlisted server, as explained in Chapter 2, "DNS in Practice." If your master is unavailable most of the time, DNS resolvers on the outside just waste time and network resources when they try to get answers from it.

In any native auto-dial setup, the default behavior will take up the Internet connection and then keep it up until the disconnect timeout occurs. Because getting ISDN connections up is a quick process, a simple fix is to set a short timeout, such as 120 seconds without traffic, before disconnecting. Working with analog connections is a bit harder because they can take as long as 30 seconds to establish, and therefore are difficult to take up and down. As a result, most people want a long timeout. If the timeout is long enough, BIND can manage to keep the connection up all the time, all by itself. However, this can become quite expensive.