DNS as a Tree

With what you know now, it should be easier to see the tree data structure in DNS. Figure 1.3 shows the few names you have seen in this chapter in a series of zones that are nodes in the tree connected by edges in the form of NS and A records.

Figure 1.3. Nodes and edges in a DNS tree.

When a DNS server must find an RR that goes with a domain name, it first checks its own cache to see whether it has a match or how good a match it has. It might already know the nameserver for the zone or a zone closer to the root even if it doesn't have the answer. If nothing is found then a root server is queried. Each NS record returned leads you toward the server that knows the answer, and when queried, that server will in turn either answer the query or give you another NS record to pursue. The important linkage between zones, domains, and subdomains is the delegations, in the form of NS records. If they are not in place for a domain, the resolution of that domain will simply not work.

In some cases, such as www.math.uio.no, a CNAME record is returned instead of what you asked for, causing a detour to the name to which the CNAME points to ultimately find the information you asked for.