Supporting internal resolvers that cannot handle external host names

Features available

If your organization's internal resolvers cannot handle external host names, you can use one of the following NetCache features to ensure that requests can be sent to DNS nameservers external to your organization:

Split DNS
Split DNS enables an enterprise administrator to split the DNS nameservers that the NetCache appliance queries into an internal group and an external group.

Note
The NetCache Split DNS feature is a customized feature for NetCache. It is not the same as the BIND Split DNS feature.
Per Domain mapping (Domain-to-Nameserver-Mapping option)
With this option, you configure NetCache to query a specific nameserver or set of nameservers based on the domain that is being queried.

Deciding which method is appropriate

Each organization must judge the significance of work versus runtime issues when determining the best method to use for ensuring that requests can be sent to DNS nameservers external to the organization.

Restrictions for using Split DNS

Consider using Split DNS only if all of the following are true:

Internal nameservers can resolve only internal host names.
You cannot or do not want to reconfigure your internal nameservers to resolve external host names in addition to internal host names.
Your internal resolvers handle multiple internal domains.

Comparison of configuration and runtime issues

When making your decision about whether to use Split DNS or Per Domain mapping, you should consider the amount of work required to configure each feature and the effect of each feature at runtime. The following table compares these two features.

Split DNS	Per Domain mapping
Configuration
Easier to configure because you do not have to list all internal domains that the internal nameservers resolve. If your internal nameservers handle a large number of domains, using Split DNS instead of Per Domain mapping can save you considerable configuration time.	More work to configure because you must list all internal domains that the internal nameservers resolve.
Runtime issues
With Split DNS, the appliance queries the internal nameservers first. If the queries to the internal nameservers are not resolved, the appliance queries the external nameservers. Internal nameservers, therefore, will need to handle the load of queries for both internal and external host names. Each organization must judge whether the runtime penalty is significant for the organization.	Your configuration provides a mapping of domains to external nameservers. Based on your configuration, NetCache sends queries for external host names directly to the external nameservers. Therefore, no runtime penalty exists for the internal nameservers or the appliance.

Relevant options, by feature

The options applicable to each feature discussed in this section are shown in the following table. See the Appliance Manager online Help for the DNS - General page (Setup tab) for details about configuring the relevant options.

Feature	Options to use on the DNS - General page
Split DNS	Nameservers Split DNS Enable Split DNS Nameservers
Per Domain mapping	Domain-to-Nameserver-Mapping

Split DNS example

Assume that FooBar Corporation has one DNS domain named foobar.com, and that it uses the subnet 10.0.0/24 internally. They have two sets of DNS resolvers, as follows:

Internal set of resolvers (which resolve only internal host names)
internal-ns1.foobar.com (10.0.0.1)

internal-ns2.foobar.com (10.0.0.2)

internal-ns3.foobar.com (10.0.0.3)
External set of resolvers
external-ns1 (100.0.0.1)

external-ns2 (100.0.0.2)

external-ns3 (100.0.0.3)

The previous configuration enables the NetCache appliance to resolve both the internal and external names when configured with the Split DNS feature.