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Thread: Reverse DNS Lookup

  1. #21
    AO Ancient: Team Leader
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    Rock Bottom????? That doesn't have a "ring" to it...<LOL>

    I can't get on with most of those micro-brews... They seem to think that the funkier the taste the better.... Yeah, I really love that Blackcurrant Amber Ale....

    Give me a nice cold Fosters.... or a nice Labatts Blue... I can _live_ with that... and do!!!
    Don\'t SYN us.... We\'ll SYN you.....
    \"A nation that draws too broad a difference between its scholars and its warriors will have its thinking done by cowards, and its fighting done by fools.\" - Thucydides

  2. #22
    Senior Member RoadClosed's Avatar
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    MMM Fosters... 2 for 1, Outback 4 to 6 daily.
    West of House
    You are standing in an open field west of a white house, with a boarded front door.
    There is a small mailbox here.

  3. #23
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    rDNS to beer... love it!
    Fosters??? Yuck! LOL Even Australians don't drink Fosters!
    It was created and marketed by Americans! LOL
    GUINNESS ROCKS Warm or Cold ----> A meal in itself!
    On a lighter note, YuengLing beer! Americans FIRST brewery! (Good Stuff)
    %42%75%75%75%75%72%70%21%00

  4. #24
    AO Ancient: Team Leader
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    Seriously... If you want to talk _good_ beer you use the word Grolsch... Alas, so difficult to find in quantity on a regular basis....
    Don\'t SYN us.... We\'ll SYN you.....
    \"A nation that draws too broad a difference between its scholars and its warriors will have its thinking done by cowards, and its fighting done by fools.\" - Thucydides

  5. #25
    Senior Member RoadClosed's Avatar
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    I kind of live in a beer town. We love our beer. Grolsch is easy to find and you can get the bottles with the old style caps. They are plastic now instead of ceramic but still cool.

    rDNS to beer... love it!
    It's easy to go off topic when talking beer. Beer and IT go hand in hand. Just look at any 'Con out there. Take a security gathering like ... DefCon. 80 percent of the activities involves the scientific study of the chemical reaction that takes place when water, simple sugars, grains and yeast react to one another and the resultant product of nature slides over the tongue.
    West of House
    You are standing in an open field west of a white house, with a boarded front door.
    There is a small mailbox here.

  6. #26
    AO Ancient: Team Leader
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    the scientific study of the chemical reaction that takes place when water, simple sugars, grains and yeast react to one another and the resultant product of nature slides over the tongue.
    Dammit Road.... You just gave me a woody.... Don't ever do that again!!!!!!!!
    Don\'t SYN us.... We\'ll SYN you.....
    \"A nation that draws too broad a difference between its scholars and its warriors will have its thinking done by cowards, and its fighting done by fools.\" - Thucydides

  7. #27
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    LOL... and he is in school... teacher just asked him to go to the Chalk Board.
    %42%75%75%75%75%72%70%21%00

  8. #28
    The ******* Shadow dalek's Avatar
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    rDNS, yes let me see......hhmmmmm Keith's India Pale ale.....nuff said.
    PC Registered user # 2,336,789,457...

    "When the water reaches the upper level, follow the rats."
    Claude Swanson

  9. #29
    Senior Member Maestr0's Avatar
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    Short Answer:
    A reverse DNS lookup should only resolve to one host.

    Long Answer:

    A reverse lookup is just another zone. In a forward lookup the record that relates an IP address to a hostname is a 'A' or address record. A record that resolves a hostname to IP is a 'PTR' or pointer record. Older DNS servers may not handle multiple 'A' or 'PTR' records, but this is not against RFC and you can do it, but this can cause undesired results if you don't know what your doing. Here's why:
    Many newer DNS servers will return the whole list of records, but they also return the records in round-robin order. This means that if you have multiple 'A' records, the whole list may be returned, but the order will differ everytime. Depending on how the OS implements gethostbyaddr() and gethostbyip() you may only recieve the first entry, or you may actually get them all but only use the first. This is useful for load balancing services like a webserver, so clients are spread accross diffrent machine.Like 'A' records, 'PTR' records are also treated round-robin if there are multiples. This means any thing trying to use reverse lookup to verify source on such a host will have a chance of failure that grows with each additional record. (aka 2 records gives you a 50/50 shot). What most people do, is stick to the one 'A' record and one 'PTR' record per host, using a 'CNAME' or cannonical name record which acts as an alias to the true 'A' record. In this way forward and reverse will only return one record yet multiple names can be aliased to the 'A' record and not confuse anything.

    -Maestr0
    \"If computers are to become smart enough to design their own successors, initiating a process that will lead to God-like omniscience after a number of ever swifter passages from one generation of computers to the next, someone is going to have to write the software that gets the process going, and humans have given absolutely no evidence of being able to write such software.\" -Jaron Lanier

  10. #30

    Re: Reverse DNS Lookup

    Originally posted here by XTC46
    If I understand this correctly (and please correct me if I am wrong) reverse DNS works by trying to match an IP address to a domain name, rather than the domain name to the IP address. But how does this work if a single IP address has multiple Domain names associated with it? for instance we host a mail server which has about 35 different domains associated with it. Does a reverse DNS entry have to be made for each domain all associating it with that servers IP address? and if so will the reverse DNS then just pull a list of associated domain names and then match the sending domain to that?
    PTR records must be created from the ISP who assigned you the IP. You can have 35 domains for 1 IP and PTR records that reflects that, but generally, you only need to have a single PTR exist for that IP and the domain associated with it does not matter, what matters is that PTR record exists. This will prevent your mail server from being blacklisted.

    And in fact having multiple PTR records defined can break some buggy apps.

    If you're worried about mail server stuff, the usual check is:
    * Does the connecting IP address have a PTR record? Call that name x.
    * If I do a DNS A record lookup on x, do I get the connecting IP address?

    If you have both of those right, you should be fine -- the exact name doesn't matter. With my mail server the name in the PTR record is of the form pppnnn-nnn.static.internode.on.net, which has nothing to do with the email domains it handles.

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