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MX Toobox reports Reverse DNS Fail but it appears correct

Posted on 2011-09-22
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I have been having DNS issues since getting a different IP address about 5 days ago.  MX Toolbox give a Reverse DNS fail while dnsstuff.com gives it a Reverse DNS authenticity:       [Verified]

Now I know the real test is to wait for the client to send an email successfully that failed, but I am a little worried that it is an issue on my side.  However, everything I read is that it is a ISP issue.  All the DNS setting appear correct through ATT and I am getting a success from one site.  Is the mxtoolbox wrong, or is there something I can check on my side?

I have a watchguard x750e firewall and an Exchange 2003 mail server.
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Question by:ctrisdale
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Papertrip earned 1000 total points
ID: 36581635
Go to dnswatch.info and put your IP into the IP Lookup box and click resolve.  If the IP resolves to what you expect, then you should be fine.
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by:DrDave242
DrDave242 earned 1000 total points
ID: 36581683
You can use nslookup to query the public DNS to see if the PTR record of your mail server really does match its host record, using the following sequence of commands (in a Windows environment):

nslookup  [Runs nslookup in interactive mode]
set q=a  [Sets query type to A (host)]
<FQDN of your mail server>  [The full public DNS name of your mail server - e. g., mail.mydomain.com]

The above query returns the public IP address of your mail server.  You probably already know this address, but it can't hurt to be sure.  This address will be used below.

set q=ptr  [Sets query type to PTR (reverse DNS)]
<PTR record name of your mail server>  [Take your server's public IP address, reverse the octets, and append .in-addr.arpa to it.  If your mail server's public IP address is 1.2.3.4, this will be 4.3.2.1.in-addr.arpa]

Make sure the name returned by the PTR query matches the FQDN of your server.
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by:Papertrip
ID: 36581717
What DrDave said is all correct, but unless you tell nslookup to use an external server, those commands could just be seeing an internal view.  For situations like this it's best to test from an external source.
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Expert Comment

by:DrDave242
ID: 36581747
If your internal and external domain names are identical, then yes, you will need to tell nslookup to use an external server (using the server <IP address> command).  Otherwise, your server will send the query out to the public DNS anyway via forwarders or root hints.
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Author Closing Comment

by:ctrisdale
ID: 36589662
Thanks guys.  You are both correct.  I got the correct record with my ISP and all is good to go.  Thank you for your input
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