Name servers are adept at retrieving data from the domain namespace. They have to be, given the limited intelligence of most resolvers. Not only can they give you data about zones for which they're authoritative, they can also search through the domain namespace to find data for which they're not authoritative. This process is called name resolution or simply resolution.

Because the namespace is structured as an inverted tree, a name server needs only the domain names and addresses of the root name servers to find its way to any point in the tree. A name server can issue a query to a root name server for any domain name in the domain namespace, and the root name server will start the name server on its way.

2.6.1 Root Name Servers

The root name servers know where the authoritative name servers for each of the top-level zones are. (In fact, some of the root name servers are authoritative for some of the generic top-level zones.) Given a query about any domain name, the root name servers can at least provide the names and addresses of the name servers that are authoritative for the top-level zone the domain name ends in. In turn, the top-level name servers can provide the list of authoritative name servers for the second-level zone that the domain name ends in. Each name server queried either gives the querier information about how to get "closer" to the answer it's seeking or provides the answer itself.

The root name servers are clearly important to resolution. Because they're so important, DNS provides mechanisms—such as caching, which we'll discuss a little later—to help offload the root name servers. But in the absence of other information, resolution has to start at the root name servers. This makes the root name servers crucial to the operation of DNS; if all the Internet root name servers were unreachable for an extended period, all resolution on the Internet would fail. To protect against this, the Internet has 13 root name servers (as of this writing) spread across different parts of the network. One is on PSINet, a commercial Internet backbone; one is on the NASA Science Internet; two are in Europe; and one is in Japan.

Being the focal point for so many queries keeps the roots busy; even with 13, the traffic to each root name server is very high. A recent informal poll of root name server administrators showed each root receiving thousands of queries per second.

Despite the load placed on root name servers, resolution on the Internet works quite well. Figure 2-12 shows the resolution process for the address of a real host in a real domain, including how the process corresponds to traversing the domain namespace tree.

Figure 2-12. Resolution of on the Internet

Figure 2-12. Resolution of on the Internet

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