XFOR: Internet Mail Service Caches Domain Name System (DNS) Lookups (198021)
The information in this article applies to:
- Microsoft Exchange Server 4.0
- Microsoft Exchange Server 5.0
- Microsoft Exchange Server 5.5
This article was previously published under Q198021 IMPORTANT: This article contains information about editing the registry.
Before you edit the registry, make sure you understand how to restore it if a
problem occurs. For information about how to do this, view the "Restoring the
Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help
topic in Regedt32.exe. SYMPTOMS When you use the load-balancing features inherent in the
later builds of Domain Name System (DNS), you may notice that the DNS expected
performance is not achieved. Further investigation using a network trace will
show that the Exchange server is not sending DNS requests to the DNS server.
CAUSE By default, the Internet Mail Service caches all successful
DNS lookups to a maximum of 1,000 entries. Subsequent lookups to the same
domain will be retrieved from the Internet Mail Service's cache rather than
being submitted to the DNS for name resolution. The intent of this caching is
to offer a performance gain. A side-effect of this caching is that Domain Name
System's inherent load-balancing cannot be used when caching is turned on.
WORKAROUND To work around this problem, modify the behavior by
performing the following steps: WARNING: Using Registry Editor incorrectly can cause serious problems
that may require you to reinstall your operating system. Microsoft cannot
guarantee that problems resulting from the incorrect use of Registry Editor can
be solved. Use Registry Editor at your own risk. For information
about how to edit the registry, view the "Changing Keys And Values" Help topic
in Registry Editor (Regedit.exe) or the "Add and Delete Information in the
Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you
should back up the registry before you edit it. If you are running Windows NT,
you should also update your Emergency Repair Disk (ERD). - Start Registry Editor (Regedt32.exe or Regedit.exe).
- Create the DestinationDomainThreshold value under the
following key in the registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIMC\Parameters - For Regedt32.exe, select Edit:Add Value. Enter the value
name from above and select REG_DWORD, click OK and then the desired value. For
regedit.exe, select the Edit:New:DWORD value, type the value name and then hit
enter.
The default for this value is 1000 (decimal). Setting the default
to zero will disable the caching (not recommended due to the amount of network
traffic this will generate). - Quit Registry Editor.
MORE INFORMATION If it is necessary in your environment to set this to zero
to force the IMS to not cache at all, you can avoid the network traffic
overhead by installing NT DNS on the same machine, pointing the TCP/IP stack to
itself for DNS name resolution and have the DNS service run as a caching-only
server doing lookups to your regular DNS server. The NT DNS service respects
TTL and will do a record shuffle back to the IMS thus achieving the goals of
disabling the cache to force round-robin behavior.
| Modification Type: | Minor | Last Reviewed: | 4/28/2005 |
|---|
| Keywords: | kbprb KB198021 |
|---|
|