This chapter describes a number of common configurations when working with zone files and in some cases with BIND. These solutions are presented to assist you to quickly implement some commonly used features, to recover from errors, and to illustrate some of the more subtle uses of the DNS. The following topics are covered:

• How to delegate a subdomain: This configuration allows the domain name owner to pass the responsibility to a subdomain owner, which may be another party or another part of the organization, who will be entirely responsible for the zone files describing the subdomain.
• How to delegate a virtual subdomain: This configuration uses a single zone file to provide subdomain addressing, for instance, www.us.example.com or www.uk.example.com.
• How to configure fail-over mail servers: The configuration allows backup mail servers to be provided to support a domain.
• How to reverse-map subnets: This configuration allows the delegation of reverse mapping to subnets of typically less than 256 IPv4 addresses.
• How to load balance with DNS: The configurations describe various ways in which load balancing may be implemented using DNS features. The BIND statements that control the order in which addresses are returned are also covered.
• How to define an SPF record: The Sender Policy Framework (SPF) is an antispam measure that allows an e-mail server to verify that the SMTP source is valid for the sending e-mail address. SPF records are currently implemented by Microsoft, Google, and AOL to name but three of the many hundreds of thousands of users.
• How to support http://example.com: The configuration allows both the URL www.example.com and example.com to directly address a web or other service. The required changes to the Apache server are also covered.
• How to fix an out-of-sequence SOA serial number: The process used to fix various SOA serial number errors is covered.
• How to use DNS wildcards: The DNS RRs support the use of a wildcard (*). The section on wildcards illustrates the use of this error-prone feature.

The examples shown use a number of BIND's named.conf statements, which are described in Chapter 12, and standard Resource Records, which are defined in Chapter 13. If you are running name server software other than BIND, the zone files will remain the same, but the configuration statements may differ.

In the next section, the process of delegation of a subdomain, us.example.com, is described to illustrate the general principle of delegation within an owner's domain name space. The domain owner can delegate everything to the right of the domain name in any way that makes sense-or for that matter that doesn't make sense!

Delegate a Subdomain (Subzone)

This solution configures a zone to fully delegate the responsibility for a subdomain to another name server. This is not the only possible method of defining subdomains-another solution involves configuring what this book calls a virtual or pseudo subdomain, which uses a single zone file to provide subdomain addressing structures. Assume the following addressing structure is required for the subdomain:

• Zone (domain) name: example.com
• Domain host name: bill.example.com
• Subdomain name: us.example.com
• Subdomain host or service name: ftp.us.example.com

To ease the zone administration load, this solution assumes the responsibility for the subdomain will be fully delegated to the us.example.com administrator who will be responsible for the subdomain zone files and their supporting name servers. The zone administrators of the corporate domain for example.com want nothing further to do with us.example.com other than it has generously agreed to act as the slave DNS for the subdomain name servers. When dealing with subdomains, it is important to remember that as far as the Internet registration authorities and the TLD servers are concerned, subdomains do not exist. All queries for anything that ends with example.com will be referred to the name servers for the example.com zone or domain. In turn, these name servers are responsible for referring the query to the subdomain name servers.

For the want of any better terminology, the name servers for example.com are called the domain name servers and are visible to the gTLD .com servers; the name servers for us.example.com are called the subdomain name servers and are visible only to the domain name servers (they are invisible to the gTLD servers).