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Round Robin priority vs. MX preference.

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  • Round Robin priority vs. MX preference.

    Hello Everyone

    I have noticed that Exchange 2003 (SP2) whenever has to query MX records for a particular domain, always receive Round Robin between ALL MX records even though MX preference is different among all of them.

    The strangest thing is that it seems to ignore MX preference and give higher priority to Round Robin sequence, by sending mail to multiple @repubblica.it recipients by using different MX for every mail sent.; I can see it from SMTP Logs.

    This is a try with "repubblica.it" MX queries from my Exchange, just to show you the answers of DNS

    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3

    Non-authoritative answer:
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it

    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it

    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it

    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it

    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it

    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it

    mr_trend5.repubblica.it internet address = 213.92.86.55
    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57



    C:\>nslookup -q=MX repubblica.it
    Server: atlas.mmi.ita
    Address: 192.168.80.3



    Non-authoritative answer:
    repubblica.it MX preference = 40, mail exchanger = mr_trend4.repubblica.it
    repubblica.it MX preference = 50, mail exchanger = mr_trend3.repubblica.it
    repubblica.it MX preference = 60, mail exchanger = mr_trend2.repubblica.it
    repubblica.it MX preference = 250, mail exchanger = imail.repubblica.it
    repubblica.it MX preference = 300, mail exchanger = relay1.inet.it
    repubblica.it MX preference = 350, mail exchanger = relay2.inet.it
    repubblica.it MX preference = 10, mail exchanger = mr_trend6.repubblica.it
    repubblica.it MX preference = 20, mail exchanger = mr_trend7.repubblica.it
    repubblica.it MX preference = 30, mail exchanger = mr_trend5.repubblica.it

    mr_trend4.repubblica.it internet address = 213.92.86.54
    mr_trend3.repubblica.it internet address = 213.92.86.53
    mr_trend2.repubblica.it internet address = 213.92.86.52
    imail.repubblica.it internet address = 213.92.86.95
    relay1.inet.it internet address = 213.92.5.56
    relay2.inet.it internet address = 213.92.5.56
    mr_trend6.repubblica.it internet address = 213.92.86.56
    mr_trend7.repubblica.it internet address = 213.92.86.57
    mr_trend5.repubblica.it internet address = 213.92.86.55

    and so on
    Last edited by lukeandmax; 31st May 2006, 15:29.
    Luke and Max Hit the Road

  • #2
    Re: Round Robin priority vs. MX preference.

    It is not Exchange only. To my understanding this is a normal DNS behavior, as the priority is the DATA part of the RR returned, while all the RRs are round-robined.
    I believe that it is actually up to the SMTP to look at the query results and select the destination based on Priority and not the order in which the records were returned.

    Example from Linux box:
    Code:
    [[email protected] antid0t]$ dig repubblica.it -t MX +short
    10 mr_trend6.repubblica.it.
    20 mr_trend7.repubblica.it.
    30 mr_trend5.repubblica.it.
    40 mr_trend4.repubblica.it.
    50 mr_trend3.repubblica.it.
    60 mr_trend2.repubblica.it.
    250 imail.repubblica.it.
    300 relay1.inet.it.
    350 relay2.inet.it.
    
    [[email protected] antid0t]$ dig repubblica.it -t MX +short
    20 mr_trend7.repubblica.it.
    30 mr_trend5.repubblica.it.
    40 mr_trend4.repubblica.it.
    50 mr_trend3.repubblica.it.
    60 mr_trend2.repubblica.it.
    250 imail.repubblica.it.
    300 relay1.inet.it.
    350 relay2.inet.it.
    10 mr_trend6.repubblica.it.
    
    [[email protected] antid0t]$ dig repubblica.it -t MX +short
    60 mr_trend2.repubblica.it.
    250 imail.repubblica.it.
    300 relay1.inet.it.
    350 relay2.inet.it.
    10 mr_trend6.repubblica.it.
    20 mr_trend7.repubblica.it.
    30 mr_trend5.repubblica.it.
    40 mr_trend4.repubblica.it.
    50 mr_trend3.repubblica.it.
    Are you looking at the SMTP logs on the sending or receiving server ? Any mail-relays in transparent-bridge mode on the way ?
    Guy Teverovsky
    "Smith & Wesson - the original point and click interface"

    Comment


    • #3
      Re: Round Robin priority vs. MX preference.

      Originally posted by guyt
      It is not Exchange only. To my understanding this is a normal DNS behavior, as the priority is the DATA part of the RR returned, while all the RRs are round-robined.
      I believe that it is actually up to the SMTP to look at the query results and select the destination based on Priority and not the order in which the records were returned.
      Yes, this is what Exchange is expected to do, I'm sorry to realize that it doesn't!
      It seems that Exchange's SMTP is not RFC Compliant

      Are you looking at the SMTP logs on the sending or receiving server ? Any mail-relays in transparent-bridge mode on the way ?
      SMPT Logs on the sender.
      Sending multple messages to different @repubblica.it recipients (in some cases also sending multiple mail to the same recpient) results in every single mail sent to a different MX record, regardless of MX preference.
      BTW, follow-up to http://forums.dnsstuff.com/tool/post...ost?id=1156796 where I started the same thread (I was using a different nickname but it's always me )
      Luke and Max Hit the Road

      Comment

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