reverse dns lookup

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Posts: 10
Joined: Fri Feb 02, 2007 3:23 am

reverse dns lookup

Post by Burrito » Sat Feb 03, 2007 7:22 pm

I don't quite understand the whole reverse dns lookup thing, but my understanding is that some mail servers won't accept mail if the ip from which it's being sent doesn't reverse lookup to the domain of the email???

if that's the case, is there a way to specify which ip address to send emails out of if I have multiple post offices?

ie I want one post office (domain) to go out of one ip and another to go out of a different ip...

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Post by labsy » Sun Feb 04, 2007 1:15 pm

Yo do not need to bind every single postoffice to dedicated IP addres, because that's what you would need to do if you wish to send out mail with dedicated reverse DNS.

Reverse DNS is, like its name says, just reverse lookup on IP.
If you do normal lookup on domain, you would resolve domain NAME to IP of domain's machine. Let's do it for, for example:

First find MX record:

Code: Select all MX IN 3600 [Preference = 10] 
Now lt's lookup what it resolves to:

Code: Select all A IN 3600 
Now, after we have IP of mailenable MAIL server, let's REVERSE lookup, what does this IP resolve to. Reverse lookup:

Code: Select all PTR record: 
As you see, both, DNS lookup and reverse lookup are about "" domain, which means, mail server is configured well (would be weird if it wouldn't be :wink: )

about your situation:
You do not need to setup PTR records for each domain on your server, because you would need one IP per domain. Instead, you may just add your main domain MX record to all domains with some higher preference number (lower preference means, that server with lower number handles mail first, and if it does not response, higher preference number servers take care of it). For example:

Code: Select all MX IN 3600 [Preference = 10] MX IN 3600 [Preference = 20]
Then you set PTR (you would probably need to ask your ISP or company, which hosts your computer to do that) for your to point to IP of your mail server.
This way your mail will never be rejected on Reverse DNS lookup, because one of MX records will always point back to IP of your server.
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