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Thread: Reverse DNS

  1. #1
    Forum Administrator Power Poster Lyle@Spry's Avatar
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    May 2005
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    Default Reverse DNS

    Reverse DNS works like forward dns but in reverse, hence the name.

    Forward DNS takes a hostname like 'www.example.com' and returns the IP address '192.0.34.166'.

    Reverse DNS takes an IP address and returns the hostname assigned to that IP. i.e. '192.0.34.166' = 'www.example.com'.

    Unlike forward DNS, only one name can be assigned to an IP address. Meaning www.example.com and example.net may resolve to the same IP address, but asking '192.0.34.166' for its name will only return 'www.example.com'. A somewhat bad analogy for this would be, "Bob and Sue Doe live at 1800 W. Somewhere St. Bob's address is the same as Sue's, but "1800 W. Somewhere St." is the Doe's address. Not Bob or Sue individually.

    Reverse DNS is most commonly used on the Internet to cut down on spam. AOL's mail servers will not accept any email coming from a server without a reverse DNS entry. It's a little like SPF (which I discuss in this thread.) i.e. Mail server receives connection from server with no reverse DNS entry. Mail server attempts to match name sent by remote mail server to reverse dns entry. If no reverse dns entry exists, or the existing reverse dns entry says a completely different name, the receiving mail server will reject the message.

    A common question that comes up is, "I'm hosting multiple domain names/sites on this server. If I'm only allowed one reverse DNS entry, what name am I supposed to use?"

    Answer: Use the hostname of your server.
    Reason: When your server tries to send a message for one of your domains/sites to another mail server, it announces its hostname to the remote mail server for identification. It does NOT announce the domain name/site that generated the email. This is the name the remote mail server compares to the reverse dns entry.

    P.S. Reverse DNS entries are technically referred to as "PTR" records or "Pointer" records.

    Also, Spry will NOT create reverse DNS records for your servers IP addresses without your specific request. Log in at Support Login to request reverse DNS entries.
    Last edited by Lyle@Spry; 12-27-2005 at 02:39 PM.

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