Understanding Name Queries

Because the dual-layer TCP/IP stack in Windows 7 means that both IPv4 and IPv6 are enabled by default, DNS name lookups by clients running Windows 7 can involve the use of both A and AAAA records. (This is true only if your name servers support IPv6, which is the case with the DNS Server role for Windows Server 2008 and Windows Server 2003.) By default, the DNS client in Windows 7 uses the following procedure when performing a name lookup using a particular interface:

1. The client computer checks to see whether it has a non-link-local IPv6 address assigned to the interface. If it has no non-link-local addresses assigned, the client sends a single name lookup to the name server to query for A records and does not query for AAAA records. If the only non-link-local address assigned to the interface is a Teredo address, the client again does not query for AAAA records. (The Teredo client in Windows Vista and later versions is explicitly built not to automatically perform AAAA lookups or register with DNS to prevent overloading of DNS servers.)
2. If the client computer has a non-link-local address assigned to the interface, the client sends a name lookup to query for A records.
   • If the client then receives a response to its query (not an error message), it follows with a second lookup to query for AAAA records.
   • If the client receives no response or receives any error message (except for Name Not Found), it does not send a second lookup to query for AAAA records.

Note Because an interface on an IPv6 host typically has multiple IPv6 addresses, the process by which source and address selection works during a name query is more complex than when DNS names are resolved by IPv4 hosts. For a detailed description of how source and address selection works for IPv6 hosts, see the Cable Guy article titled "Source and Destination Address Selection for IPv6" at http://technet.microsoft.com/en-us/library/bb877985.aspx. For additional information on DNS behavior in Windows 7 and Windows Vista, see "Domain Name System Client Behavior in Windows Vista" at http://technet.microsoft.com/en-us/library/bb727035.aspx. For information about the different types of IPv6 addresses usually assigned to an interface, see the section titled "Configuring and Troubleshooting IPv6 in Windows 7" later in this tutoriak.

Note Issues have arisen with poorly configured DNS name servers on the Internet. These issues, which are described in RFC 4074 (http://www.ietf.org/rfc/rfc4074.txt), do not cause problems on Windows Vista or later versions because Microsoft has altered the DNS client behavior specifically to compensate for them. However, administrators of DNS servers should make sure these issues are fixed, because they can cause problems with DNS name resolution for most IPv6 networking stacks, including stacks found in earlier Windows platforms such as Windows XP.