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Re: Possible MX Lookup/Ordering Issue

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  • Victor Duchovni
    ... Your anecdotal observations are flawed. Postfix makes new connections to the MX hosts in the order supplied by DNS. Claims to the contrary need to be
    Message 1 of 44 , Nov 1, 2007
      On Thu, Nov 01, 2007 at 01:35:46PM +0000, gordan@... wrote:

      > Hi,
      >
      > I'm seeing some strange behaviour with my Postfix setup. It occasionally
      > doesn't try the MX records in the correct order. It will try a low
      > priority one, and if the machine is on a local subnet but not running,
      > it'll get back a "no route to host", and get stuck. It'll keep retrying
      > this one quite a few times, before it gives up and rolls over to the high
      > priority MX-es which it should have tried first.
      >
      > Is this a known bug?

      Your anecdotal observations are flawed. Postfix makes new connections to
      the MX hosts in the order supplied by DNS. Claims to the contrary need
      to be backed up with detailed logging for the transactions in question,
      including not only the final delivery success/fail indication, but also
      any prior connection attempts or partial deliveries.

      > Does Postfix try to cut a corner here and use the MX with the IP returned
      > in the additional section, instead of going purely based on the MX list
      > and trying the records in the priority order as it should?

      It is pointless to speculate without evidence.

      --
      Viktor.

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    • gordan@bobich.net
      ... Sure - but I ve tested this across different networks and different domains. There is always the dominant shape of the curve: disproportionate number of
      Message 44 of 44 , Nov 1, 2007
        On Thu, 1 Nov 2007, mouss wrote:

        > gordan@... wrote:
        >> On Thu, 1 Nov 2007, mouss wrote:
        >>> this does not prove that using 10 records significantly reduces the spam
        >>> received on the real MXes. This only shows the dsitribution of spam
        >>> attempts when using 10 records.
        >>
        >> Sure - but unless spam that went to MX10 then went and tried MX2, the
        >> spam wasn't delivered to MX2.
        >>
        >
        > As Jorey said, it's not like there is a finite quantity of spam to be
        > distributed among MXes. I have domains that receive 0 spam (and they
        > have an MX). BTW. I also see smtp attempts to machines that are not
        > listed as MX for any domain.

        Sure - but I've tested this across different networks and different
        domains. There is always the dominant shape of the curve: disproportionate
        number of connections on the 1st nth, n-1 and n-2 MX records (where n is
        the number of MX-es).

        >>> the experiment would be:
        >>>
        >>> test 1: with only 2 records, what amount of spam is targetting the real
        >>> MX. do this for some period of time (so that there are actually many bot
        >>> runs).
        >>>
        >>> test 2: do the same test with 10 records.
        >>>
        >>> if the amount of spam (on the "real" MX) in test 2 is significantly
        >>> lower than in test 1, then 10 records would be useful. otherwise, you
        >>> are just putting more honey for the flies.
        >>
        >> The difference is extremely signifficant. It is also signifficant
        >> between 3 and 5 MX-es, although it gets less measurable when going from
        >> 10 upward.
        >
        > you did not show actual numbers for this.

        It worked so well that I never bothered gathering any stats. But I guess I
        could go through my spam folder and put some numbers to it when I have a
        moment.

        >>> No. see above. you are comparing numbers in a single setup. you are not
        >>> comparing different setups (different number of records).
        >>
        >> Yes I was. I tested with increasing numbers of MX records and the amount
        >> of spam reduced. You do get into diminishing returns (statistically, 10
        >> gets around 90% of it away, going from 10 to 100 only reduces it by
        >> another 9%), so usually I don't bother with more than about 15. The
        >> drop-off is actually better than linear because spammers seem to target
        >> the 1st highest and 3 lowest MX-es, so adding more in the middle just
        >> dilutes the ones that target a random MX.
        >>
        >
        > If they target 1st and last 3, then why 10 instead of 5?

        Because there is still a measurable drop, and it isn't exactly an
        expensive solution.

        >> You could, of course, just try it yourself for some figures you can
        >> trust. :-)
        >
        > I suspect there may be broken MTAs out there, so I keep myself under the
        > 2 MX limit to avoid any risk on "real" domains. but I may test this on
        > domains unused in email.

        You'll need some quite spam-heavy unused domains to gather the statistics
        quickly enough.

        Gordan
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