The DNS Client (Dnscache) service resolves and caches Domain Name System (DNS) names for the computer. The DNS Client service must run on every computer that performs DNS name resolution. DNS name resolution is needed to locate domain controllers in AD DS domains. The DNS Client service is also needed to enable the location of the devices that are identified through DNS name resolution. Windows 7 includes both Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6) protocol stacks that are installed and enabled by default. DNS name queries and registrations can involve IPv4 address records (A records) and IPv6 address records (AAAA records).
If the DNS Client service stops, the computer cannot resolve DNS names or locate Active Directory domain controllers, and users cannot log on to the computer.
Computers must perform both A and AAAA queries to determine the best method of connectivity to the endpoint that is wanted. By obtaining IPv4 and IPv6 addresses, there is an increased chance of access to the wanted endpoint if one of the addresses is unreachable. The DNS Client service minimizes the impact on DNS servers when performing DNS name queries through the following behavior:
- If the host has only link-local or Teredo IPv6 addresses assigned, the DNS Client service sends a single query for A records. For more information, see the Teredo Overview on Microsoft TechNet.
- If the host has at least one IPv6 address assigned that is not a link-local or Teredo address, the DNS Client service sends a DNS query for A records and then a separate DNS query to the same DNS server for AAAA records. If an A record query times out or has an error (other than name not found), the corresponding AAAA record query is not sent.
This service is installed by default and it startup type is Automatic. When the DNS Service is started in its default configuration, it logs on by using the Network Service account.
The DNS Client service is dependent upon the following system components:
- NetIO Legacy TDI Support Driver
- TCP/IP Protocol Driver
- Network Store Interface Service
- NSI proxy service driver