Comparing DNS servers?

Discussion in 'DNS Server' started by Fredxx, Aug 18, 2009.

  1. Fredxx

    Fredxx Guest

    I have no idea if this is the best group to post this question, apologies if
    it's not.

    Are there any website which test alternative DNS servers resolving accuracy?
    It would be useful to give an indication of how a change of name servers
    propagates through the network.

    Many thanks.
     
    Fredxx, Aug 18, 2009
    #1
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  2. Resolving accuracy? They either resolve what's in their database, or if it
    doesn't exist, they provide a NULL response (they don't have the answer). So
    I'm not sure what you mean in this respect.

    As far as a change of nameservers, do you mean for your public record or for
    AD? If public records, that take a little time to propagate due to the TTL
    time on the records. If AD, if you add a DNS server, assuming the zone is AD
    integrated, it depends on the network's topology and replication schedule.


    --
    Ace

    This posting is provided "AS-IS" with no warranties or guarantees and
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    Please reply back to the newsgroup or forum to benefit from collaboration
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    Ace Fekay, MCT, MCTS Exchange, MCSE, MCSA 2003 & 2000, MCSA Messaging
    Microsoft Certified Trainer

    For urgent issues, please contact Microsoft PSS directly. Please check
    http://support.microsoft.com for regional support phone numbers.
     
    Ace Fekay [MCT], Aug 19, 2009
    #2
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  3. Fredxx

    Fredxx Guest

    You hit the nail on the head when you said what's in their database. Are
    there any websites which compare the results?

    I thought most TTLs are in the order of a couple of hours, but it seems to
    take far longer for any changes to propagate across the internet, making
    email unreliable for our customers for a couple of days.

    Many thanks.
     
    Fredxx, Aug 19, 2009
    #3
  4. Fredxx

    Chris Dent Guest

    If a DNS server doesn't have a Cached response to a record it will seek
    an answer via whatever means it has been told to. Typically that either
    means via a Forwarder or via Root Hints. The latter will eventually lead
    back to the DNS server that can provide an up to date answer where a
    Forwarder may still provide a cached response.

    The TTL itself is entirely arbitrary, it can be set to almost anything.
    Most tend to be in the 24 to 48 hour range, but it can go either way
    (shorter or longer).

    It's one of those things you must account for if you're making changes
    to records. In the case of an MX that can include inserting a new MX
    record prior to the change (while the new system is offline), or
    reducing the TTL.

    Chris
     
    Chris Dent, Aug 19, 2009
    #4
  5. Fredxx

    Fredxx Guest

    Not sure what you mean about "inserting a new MX" record?

    Both new and old mail servers were operating, the problem was the old server
    one knew the old IP address was wrong and returned a failure notice.

    As you say 48 hours later, all's well. Bit not sure how to ensure future
    moves won't have the same problem.

    How would I change the TTL? DNS servers aren't under my control?
    Many thanks for your reply.
     
    Fredxx, Aug 19, 2009
    #5
  6. Fredxx

    Chris Dent Guest

    It is a useful work-around if you can't change the TTL for the existing
    record.

    Say you had an MX record like this before moving over:

    domain.com. IN MX 10 mail.domain.com.

    That's all fine until you want to change things. However, you could make
    the MX record into this prior to the change:

    domain.com. IN MX 10 mail.domain.com. <-- The existing mail server
    domain.com. IN MX 20 newmail.domain.com. <-- The new mail server

    Now if mail.domain.com is offline systems sending mail will attempt to
    deliver mail to newmail.domain.com.

    If newmail.domain.com is offline as well then delivery will be deferred
    in the usual manner.

    If we fast-forward to the change-over, mail.domain.com won't be online
    anymore, but that's okay because the secondary MX, newmail, will be and
    messages will be delivered there.

    The old, and now redundant, MX record would be removed post-change.

    If you opt for this kind of change over it must be made far enough in
    advance that anyone who might have a cached / remembered version of your
    MX will have time to figure it out. So typically you'd make this kind of
    change between 2 days to a week in advance of the change-over.
    For that you would need to be able to have access to the DNS service
    (some ISPs permit TTL changes via a web-page interface to DNS). If you
    don't have access at all you may be able to ask for it to be changed on
    your behalf, otherwise the method above works well.

    Chris
     
    Chris Dent, Aug 19, 2009
    #6
  7. Fredxx

    Fredxx Guest

    Many thanks for that, I'm not sure if I have that level of control, but will
    be much wiser next time!
     
    Fredxx, Aug 19, 2009
    #7

  8. As stated, this is normally something you have to plan. I make my changes on
    a Friday afternoon, for the most part, some of it's ready by the next day,
    but it will all have propogated by Monday morning's start of business.

    Ace
     
    Ace Fekay [MCT], Aug 19, 2009
    #8
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