Re: Cannot query the MX record for cn.ibm.com


For refrence... you can turn on edns with:

dnscmd <servername> /Config /EnableEDNSProbes 1

DNSCmd is included with the Windows Support Tools.

We should also note that the Firewall must support UDP Packets greater than
512, an old known issue with Cisco PIX (pre-6.3(2) iirc). Above that
version the UDP limit can be increased with:

fixup protocol dns maximum-length 4096

Where 4096 is the maximum packet length you wish to support. Probably
doesn't need to be greater than the MTU, but I'm not entirely clear on that.

--
Chris Dent
MVP Directory Services


"Mr_Huang" <mr.huang.hk@xxxxxxxxx> wrote in message
news:6085dfb9-9af4-4db4-806e-bfd1d7238b29@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Thank you for all the inputs,

It is strange that my Linux and Windows DNS is on the same subnet and
same router, where the Linux rocks but not MS. I have temporary
forwarded ibm.com to my Linux box in the beginning.

Finally, I found the solution to tackle this problem.

Because the DNS-UDP response is larger than 512 bytes, so EDNS needs
to be turned on in my MS DNS server.

* Search the Technet for "Modify EDNS0 configuration"
* Add those registry
* restart DNS server
thanks again,
huang


On Jun 4, 9:07 pm, "Chris Dent [MVP]" <ch...@xxxxxxxxxxxxxxx> wrote:
I'm inclined to think it's down to Firewall rules.

The response to the request for that particular MX record is huge, it
overflows the UDP limit which will force the requesting DNS server to
switch
to TCP. If outbound TCP on Port 53 is blocked you won't get the response.

If you happen to have dig on your *nix system you can run "dig cn.ibm.com
mx
+trace", you should see that it's truncated and forces retry on TCP.

--
Chris Dent
MVP Directory Services

"Mr_Huang" <mr.huang...@xxxxxxxxx> wrote in message

news:0efb30a6-3984-4e3a-9518-0d42e760619d@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

dear guru,
Why I cannot query the MX record for cn.ibm.com from my Windows DNS?
I can query the MX for au.ibm.com from my Windows server.
and I can query the MX for cn.ibm.com from another linux DNS.
Is there any different in the DNS implementation for Windows and
Linux?
any ideas would be appreciated.
thanks
huang



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