Saturday, January 31, 2009

AddressAnswerLimit Registry Key

This registry key determines the limit on maximum number of Type A resource record responses can be sent in with a response packet from name resolver to the calling dns server /dns client. This key value also influences the setting of the truncation bit in query response packet.

Key Name: AddressAnswerLimit
Type: DWORD Default: NoKey (No limit)
Functionality: Limits number of A records put in answer to query.
Default: No limit on no response records.
Range: 5<= x<=28
Location: "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters"

By default, Type A record responses are not limited. Name resolver sends as many records as possible in it's UDP response packet. If the no of response records exceeds the space in the response packet, then truncation bit is set in the packet and sent to the client. In this case,
DNS servers often use TCP to retry queries when they receive a response with the truncation bit set.

However, If this registry is set, then the truncation bit is not set in a response packet, even if the packet space is exhausted. In this case, quering dns server retry query if the server receives packet with truction bit is set.

If the AddressAnswerLimit registry key is zero or not exist, Then Type A record responses are not limited and A responses records are written in DNS UDP packet. If all the records do not fit in a UDP DNS packet, then the truncation bit is set.

AddressAnswerLimit value is limited to range 5<= x <=28. If AddressAnswerLimit value x> 28, then it will limit to 28, and if it 0 < style="font-weight: bold;"> The following setting the reg key through reg add command.

reg add "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters" /v AddressAnswerLimit /t reg_dwrod /d

Note: 1. AddressAnswerLimit is restricted to only for type A record queries. It doesn't have any effect on other dns queries.
2. Changes to registry key through regedit.exe will be effective only after restarting dns server.


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