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Understanding forwarders

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Understanding forwarders

A forwarder is a Domain Name System (DNS) server on a network used to forward DNS queries for external DNS names to DNS servers outside of that network. You can also forward queries according to specific domain names using conditional forwarders.

A DNS server on a network is designated as a forwarder by having the other DNS servers in the network forward the queries they cannot resolve locally to that DNS server. By using a forwarder, you can manage name resolution for names outside of your network, such as names on the Internet, and improve the efficiency of name resolution for the computers in your network. For more information about forwarders and conditional forwarders, see Using forwarders.

The following figure illustrates how external name queries are directed using forwarders.

Example of a common forwarder configuration

For more information about directing external queries, see Directing queries through forwarders.

Without having a specific DNS server designated as a forwarder, all DNS servers can send queries outside of a network using their root hints. As a result, a lot of internal, and possibly critical, DNS information can be exposed on the Internet. In addition to this security and privacy issue, this method of resolution can result in a large volume of external traffic that is costly and inefficient for a network with a slow Internet connection or a company with high Internet service costs.

When you designate a DNS server as a forwarder, you make that forwarder responsible for handling external traffic, thereby limiting DNS server exposure to the Internet. A forwarder will build up a large cache of external DNS information because all of the external DNS queries in the network are resolved through it. In a small amount of time, a forwarder will resolve a good portion of external DNS queries using this cached data and thereby decrease the Internet traffic over the network and the response time for DNS clients.

A DNS server configured to use a forwarder will behave differently than a DNS server that is not configured to use a forwarder. A DNS server configured to use a forwarder behaves as follows:

  1. When the DNS server receives a query, it attempts to resolve this query using the primary and secondary zones that it hosts and its cache.

  2. If the query cannot be resolved using this local data, then it will forward the query to the DNS server designated as a forwarder.

  3. The DNS server will wait briefly for an answer from the forwarder before attempting to contact the DNS servers specified in its root hints.

When a DNS server forwards a query to a forwarder it sends a recursive query to the forwarder. This is different than the iterative query that a DNS server will send to an other DNS server during standard name resolution (name resolution that does not involve a forwarder).

Conditional forwarders

A conditional forwarder is a DNS server on a network that is used to forward DNS queries according to the DNS domain name in the query. For example, a DNS server can be configured to forward all the queries it receives for names ending with widgets.example.com to the IP address of a specific DNS server or to the IP addresses of multiple DNS servers.

Intranet name resolution

A conditional forwarder can be used to improve name resolution for domains within your intranet. Intranet name resolution can be improved by configuring DNS servers with forwarders for specific internal domain names. For example, all DNS servers in the domain widgets.example.com could be configured to forward queries for names that end with test.example.com to the authoritative DNS servers for merged.widgets.example.com, thereby removing the step of querying the root servers of example.com, or removing the step of configuring DNS servers in the widgets.example.com zone with secondary zones for test.example.com.

Internet name resolution

DNS servers can use conditional forwarders to resolve queries between the DNS domain names of companies that share information. For example, two companies, Widgets Toys and TailspinToys, want to improve how the DNS clients of Widgets Toys resolve the names of the DNS clients of Tailspin Toys. The administrators from Tailspin Toys inform the administrators of Widgets Toys about the set of DNS servers in the Tailspin Toys network where Widgets can send queries for the domain dolls.tailspintoys.com. The DNS servers within the Widgets Toys network are configured to forward all queries for names ending with dolls.tailspintoys.com to the designated DNS servers in the network for Tailspin Toys. Consequently, the DNS servers in the Widgets Toys network do not need to query their internal root servers, or the Internet root servers, to resolve queries for names ending with dolls.tailspintoys.com.

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