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DNS Client (System Services for the Windows Server 2003 Family and Windows XP Operating Systems)

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

Service Name: Dnscache

Executable Name: svchost.exe -k NetworkService

Log On As: NetworkService

Description: The Domain Name System (DNS) client service resolves and caches DNS names. The DNS client service must be running on every computer that will perform DNS name resolution. The ability to resolve DNS names is crucial for locating domain controllers in Active Directory domains. The DNS client service is also critical for locating devices identified using DNS name resolution.

The DNS Client service running on Windows implements the following features:

  • System-wide caching

    Resource records (RRs) from query responses are added to the client cache as applications query DNS servers. This information is then cached for a set Time to Live (TTL) and can be used again to answer subsequent queries.

  • RFC-compliant negative caching support

    In addition to caching positive query responses from DNS servers (which contain resource record information in the answered reply), the DNS Client service also caches negative query responses. A negative response results when a resource record for the queried name does not exist.

    Negative caching prevents the repeating of additional queries for names that do not exist, which can adversely affect client system performance. Any query information negatively cached is kept for a shorter period of time than is used for positive query responses; by default, no more than 5 minutes. This avoids continued negative caching of stale information if the records later become available.

    Negative caching is a new DNS standard specification defined in RFC 2308. For more information, refer to this RFC. For more information on obtaining RFCs, see TCP/IP RFCs.

  • Avoidance of unresponsive DNS servers

    The DNS Client service uses a server search list, ordered by preference. This list includes all preferred and alternate DNS servers configured for each of the active network connections on the system. Windows rearranges these lists based on the following criteria:

    • Preferred DNS servers are given first priority.

    • If no preferred DNS servers are available, then alternate DNS servers are used.

    • Unresponsive servers are removed temporarily from these lists.

If the DNS client service is stopped or disabled on your computer, you may not be able to locate the domain controllers of the Active Directory domains and will no longer be able to locate the devices identified using DNS names.

See also DNS Server.

Available on: Windows XP Home, Windows XP Professional; Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; Windows Server 2003, Datacenter Edition and Windows Server 2003, Web Edition.

Installed through: Default operating system installation

Startup type: Automatic

Service status: Started

This service depends on the following system components:

TCP/IP Protocol Driver

IPSEC Driver

The following system components depend on this service: None

IP Port Numbers used:

TCP: 53

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