10.9. Preferring Nameservers on Certain Networks

BIND 8's topology feature is somewhat similar to sortlist, but it applies only to the process of choosing nameservers. (BIND 9 doesn't support topology as of 9.3.2.) Earlier in the book, we described how BIND chooses between a number of nameservers that are authoritative for the same zone by selecting the nameserver with the lowest round-trip time. But we lieda little. BIND 8 actually places remote nameservers in 64-millisecond bands when comparing RTT. The first band is actually only 32 milliseconds wide (there! we did it again), from 0 to 32 milliseconds. The next extends from 33 to 96 milliseconds, and so on. The bands are designed so that nameservers on different continents are always in different bands.

The idea is to favor nameservers in lower bands but to treat servers in the same band as equivalent. If a nameserver compares two remote servers' RTTs, and one is in a lower band, the nameserver chooses to query the nameserver in the lower band. But if the remote servers are in the same band, the nameserver checks to see whether one of the remote servers is topologically closer.

So topology lets you introduce an element of fudge into the process of choosing a nameserver to query. It lets you favor nameservers on certain networks over others. Topology takes as an argument an address match list, where the entries are networks, listed in the order in which the local nameserver should prefer them (highest to lowest). Therefore:

topology {
    15/8;
    172.88/16;
};

tells the local nameserver to prefer nameservers on the network 15/8 over other nameservers, and nameservers on the network 172.88/16 over nameservers on networks other than 15/8. So if the nameserver has a choice between a nameserver on network 15/8, a nameserver on 172.88/16, and a nameserver on 192.168.1/24, assuming all three have RTT values in the same band, it will choose to query the nameserver on 15/8.

You can also negate entries in the topology address match list to penalize nameservers on certain networks. The earlier in the address match list the negated entry matches, the greater the penalty. You might use this to keep your nameserver from querying remote nameservers on a network that's particularly flaky, for example.