Setting Up Subdomains--Different Zones

Setting up subdomains that are in different zones is more complicated than including multiple domains in a single zone, because you have to specify how clients in different zones obtain DNS information from the other zones.

To divide a network into multiple domains, create a domain hierarchy. That is, one domain becomes the top domain. Beneath the top domain, you create one or more subdomains. If you want, you can create subdomains of subdomains. But every subdomain has a set place relative to the top domain in the hierarchy of domains. When read from left to right, domain names identify the domain's place in the hierarchy. For example, the doc.com domain is above the sales.doc.com domain, while the west.sales.doc.com domain is below the sales.doc.com domain.

DNS zones acquire a hierarchy from the domains that they contain. The zone containing a network's top domain is the top zone. A zone that contains one or more subdomains below the top domain is below the top zone in the zone hierarchy. When DNS information is passed from one zone to another, it is passed up and down the zone hierarchy. This means that each zone requires records in its data files that specify how to pass information up to the zone immediately above it, and down to any zones immediately below it.

To correctly transfer DNS information from one zone to another in a multi-zone network:

• hosts.rev file. There must be a PTR record in each hosts.rev file pointing to the name of one or more master servers in the zone immediately above it. This type of PTR record is exactly the same as any other PTR record in the file, except that it identifies a server in the zone above.

• hosts file NS records. There must be a zone NS record in each hosts file identifying each name server in each zone immediately below. This type of NS record requires the name of the zone below as the first field in the NS record. (The name of the zone is specified in the SOA record of the zone's host file.)

• hosts file A records. There must be an A record in each hosts file identifying the IP address of each name server in each zone immediately below. This type of A record has to have the name of the zone below as the first field in the A record. (The name of the zone is specified in the SOA record of the zone's host file.)

The example files in Table 13-4 illustrate a network with two zones.