SP4 changes DNS name resolution

After applying Windows NT 4.0 Service Pack 4 (SP4), the DNS resolver (DNR) changes how it uses DNS servers to resolve names.

With previous versions of Windows NT 4.0, the DNR sends a DNS request to the first DNS server address listed in the DNS server search order in the TCP/IP DNS Properties dialog box. If no response is received within 5 seconds, it then sends a DNS request to the second DNS server address configured. The total delay for DNS name resolution to timeout is typically 45 seconds or more.

After Windows NT 4.0 SP4 is installed, the DNR sends a DNS query to the first DNS server(s) address configured in all of three possible lists. If no response is received within 1 second, the DNR requeries the first DNS server(s) and sends the same query to all other DNS server addresses configured on the client. If the DNS resolver does not get a response from any DNS server within 2 seconds, it resends the query to all DNS servers configured and waits for 4 seconds. If no response is received, the DNR resends the request again to all DNS servers configured and waits for 8 seconds. The total delay for DNS name resolution timeout is now 15 seconds.

Depending on the name being resolved, it may repeat the process adding the domain name configured on the client and devolving the specified domain name two labels deep. Also if DNS fails and NetBIOS name resolution is used an additional 3 seconds can be used.

The DNS server(s) queried from these 3 possible DNS Server lists:

• Static (configured through Control Panel)
• DHCP (obtained through all DHCP-enabled Adapters)
• RAS (obtained through dial-up networking)

If you have multiple adapters of the same type, the above order is further controlled by the adapter binding order, controllable through the Bindings tab.

A new (post SP4) version of the client DNR, which adds a 0.5 second delay between querying different DNS servers, is available. This allows strict adherence to the DNS search order, but adds (N-1)*.5 seconds to the overall timeout delay, where N is total number of DNS name servers configured on the client. With this fix the total time spent on DNS name timeout useS the following algorythm:

Depending on the configuration the total timeouts for a particular query can vary from (1+2+4+8=15) seconds to at worst (0.5+0.5+1+0.5+0.5+2+0.5+0.5+4+0.5+0.5+8=19) seconds. For additional information, please see the following article in the Microsoft Knowledge Base: This fix also helpS in instances of nonconvergent namespace; that is, some name servers authoritative for the zone do not contain zone information and are replying with NX domain.