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Re: Tweaking DNS timeouts

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  • /dev/rob0
    ... snip ... I understand all this and agree. I m not advocating a 30+ second greet pause. My original goal was to reduce delays. Most of those who manage
    Message 1 of 25 , May 17, 2013
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      On Fri, May 17, 2013 at 05:53:47PM +0000, Viktor Dukhovni wrote:
      > On Fri, May 17, 2013 at 12:26:13PM -0500, /dev/rob0 wrote:
      > Wietse:
      > > > Increasing the greet-wait to 10+ seconds could result in
      > > > legitimate clients hanging up, so I would not recommend that.
      > >
      > > Do we have any testing to validate this? I'm pretty sure I
      > > recall from a few years back on the old original SPAM-L list
      > > that some Sendmail people[1] were saying they used greet
      > > pauses in excess of 30 seconds.
      >
      > It creates a lot of needless congestion on legitimate sending
      > systems even if they don't hang up.
      >
      snip
      >
      > Much of the damage to the SMTP infrastructure is done by
      > well-meaning anti-spam measures. Let's not take it too far.

      I understand all this and agree. I'm not advocating a 30+ second
      greet pause. My original goal was to reduce delays.

      Most of those who manage really busy outbounds will have gone to the
      trouble of getting listed on DNS whitelists. And for these outbounds,
      an occasional 10-second greet pause is better than "Service currently
      unavailable" and PASS NEW.

      But I think this is all moot, and my quick fix, to stop querying
      psbl.surriel.com, was the best. The moral of the story being, use
      DNSBL sites with adequate response times and five nines. It's
      probably also moot if the postscreen_dnsbl_threshold score is only
      calculated when in excess thereof in case of DNS timeouts.
      --
      http://rob0.nodns4.us/ -- system administration and consulting
      Offlist GMX mail is seen only if "/dev/rob0" is in the Subject:
    • Wietse Venema
      ... [begin background material] I mis-understood how postscreen works (I do not constantly stare at Postfix source code, having other things to work on that
      Message 2 of 25 , May 17, 2013
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        /dev/rob0:
        >
        > I guess this says that postscreen_dnsbl_action fires at the end of
        > the greet pause when postscreen_dnsbl_threshold is met, but
        > postscreen_dnsbl_whitelist_threshold is not calculated. Here's the

        [begin background material]

        I mis-understood how postscreen works (I do not constantly stare
        at Postfix source code, having other things to work on that pay the
        bills).

        I thought that the whitelist will be applied only when DNS lookups
        complete *before* the pregreet timer expires. That is,

        - When some DNS lookup is taking too long, no DNS score is available.

        This is consistent with how postscreen whitelisting works for non-DNS
        tests. It applies the whitelist threshold only when DNS lookup
        completes before the pregreet timer expires.

        However, the bullet above is incorrect. When soe DNS lookup takes
        too long, a DNS score is available, and the postscreen DNS blocking
        code uses that partial score.

        This is safe when there are only positive scores (if the partial
        client is already over the threshold then the client should be
        blocked even if some DNS results are not yet in).

        This is less safe when there may also be exculpatory evidence (in
        the form of DNSWL lookups). But, sites are usually not listed in
        both white and block lists.

        [end background material]

        I can change postscreen to also use partial scores for whitelisting
        of non-DNS tests, and thereby make whitelisting of non-DNS tests
        consistent with DNS-based blocking (that's one less WTF factor).
        This requires minor code duplication.

        Wietse
      • Wietse Venema
        ... Released as snapshot 20130517. Wietse
        Message 3 of 25 , May 17, 2013
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          Wietse Venema:
          > I can change postscreen to also use partial scores for whitelisting
          > of non-DNS tests, and thereby make whitelisting of non-DNS tests
          > consistent with DNS-based blocking (that's one less WTF factor).
          > This requires minor code duplication.

          Released as snapshot 20130517.

          Wietse
        • /dev/rob0
          ... For testing I reenabled PSBL, and I ll see what comes in overnight. I thought I could make my own pseudo-DNSBL on a random IP address with blocked ports
          Message 4 of 25 , May 17, 2013
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            On Fri, May 17, 2013 at 10:06:38PM -0400, Wietse Venema wrote:
            > Wietse Venema:
            > > I can change postscreen to also use partial scores for
            > > whitelisting of non-DNS tests, and thereby make whitelisting
            > > of non-DNS tests consistent with DNS-based blocking (that's one
            > > less WTF factor). This requires minor code duplication.
            >
            > Released as snapshot 20130517.

            For testing I reenabled PSBL, and I'll see what comes in overnight.
            I thought I could make my own pseudo-DNSBL on a random IP address
            with blocked ports 53, but I need to set up an NS record to point to
            that. I'll do that tomorrow if results tonight are inconclusive.
            --
            http://rob0.nodns4.us/ -- system administration and consulting
            Offlist GMX mail is seen only if "/dev/rob0" is in the Subject:
          • Wietse Venema
            ... For whitelisting I used a wild-card A record, and for timeout testing I used an NS record that resolves to a firewalled port (a black hole). This
            Message 5 of 25 , May 18, 2013
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              /dev/rob0:
              > On Fri, May 17, 2013 at 10:06:38PM -0400, Wietse Venema wrote:
              > > Wietse Venema:
              > > > I can change postscreen to also use partial scores for
              > > > whitelisting of non-DNS tests, and thereby make whitelisting
              > > > of non-DNS tests consistent with DNS-based blocking (that's one
              > > > less WTF factor). This requires minor code duplication.
              > >
              > > Released as snapshot 20130517.
              >
              > For testing I reenabled PSBL, and I'll see what comes in overnight.
              > I thought I could make my own pseudo-DNSBL on a random IP address
              > with blocked ports 53, but I need to set up an NS record to point to
              > that. I'll do that tomorrow if results tonight are inconclusive.

              For whitelisting I used a wild-card "A" record, and for timeout
              testing I used an NS record that resolves to a firewalled port (a
              black hole).

              This confirmed that postscreen will now use partial scores to
              whitelist pending non-dnbsbl tests.

              I can make those domain names available for general testing (but
              not now as I am in the middle of a copper-to-fiber conversion).

              Wietse
            • /dev/rob0
              Still watching logs, this one just passed by. Probably unrelated to the changes in 20130517, but I was curious about it: May 19 13:24:20 harrier
              Message 6 of 25 , May 19, 2013
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                Still watching logs, this one just passed by. Probably unrelated to
                the changes in 20130517, but I was curious about it:

                May 19 13:24:20 harrier postfix/postscreen[3533]: CONNECT from [188.42.15.19]:48706 to [207.223.116.211]:25
                May 19 13:24:26 harrier postfix/postscreen[3533]: NOQUEUE: reject: RCPT from [188.42.15.19]:48706: 450 4.3.2 Service currently unavailable; from=<bounce@...>, to=<munged@...>, proto=ESMTP, helo=<mail18.consumer-news123.com>
                May 19 13:24:26 harrier postfix/postscreen[3533]: PASS NEW [188.42.15.19]:48706
                May 19 13:24:26 harrier postfix/postscreen[3533]: DISCONNECT [188.42.15.19]:48706

                All is well and good for a non-whitelisted host, but apparently it
                was too quick in coming back to the secondary MX IP address ...

                May 19 13:24:26 harrier postfix/postscreen[3533]: CONNECT from [188.42.15.9]:33610 to [207.223.116.214]:25
                May 19 13:24:26 harrier postfix/postscreen[3533]: WHITELIST VETO [188.42.15.9]:33610

                ... all in the same second, but according to syslog, sequentially
                after having earned whitelist status.

                May 19 13:24:32 harrier postfix/postscreen[3533]: NOQUEUE: reject: RCPT from [188.42.15.9]:33610: 450 4.3.2 Service currently unavailable; from=<bounce@...>, to=<munged@...>, proto=ESMTP, helo=<mail8.consumer-news123.com>
                May 19 13:24:32 harrier postfix/postscreen[3533]: DISCONNECT [188.42.15.9]:33610

                Another six seconds pass before this one is turned away, which
                suggests that the greet pause was repeated. Makes sense, because
                "WHITELIST VETO" means it was not seen before.
                --
                http://rob0.nodns4.us/ -- system administration and consulting
                Offlist GMX mail is seen only if "/dev/rob0" is in the Subject:
              • Wietse Venema
                ... postscreen does not find the client IP address in the permanent postscreen_access_list, does not find client the IP address in the temporary
                Message 7 of 25 , May 19, 2013
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                  /dev/rob0:
                  > Still watching logs, this one just passed by. Probably unrelated to
                  > the changes in 20130517, but I was curious about it:
                  >
                  > May 19 13:24:20 harrier postfix/postscreen[3533]: CONNECT from [188.42.15.19]:48706 to [207.223.116.211]:25
                  > May 19 13:24:26 harrier postfix/postscreen[3533]: NOQUEUE: reject: RCPT from [188.42.15.19]:48706: 450 4.3.2 Service currently unavailable; from=<bounce@...>, to=<munged@...>, proto=ESMTP, helo=<mail18.consumer-news123.com>
                  > May 19 13:24:26 harrier postfix/postscreen[3533]: PASS NEW [188.42.15.19]:48706
                  > May 19 13:24:26 harrier postfix/postscreen[3533]: DISCONNECT [188.42.15.19]:48706

                  postscreen does not find the client IP address in the permanent
                  postscreen_access_list, does not find client the IP address in the
                  temporary postscreen_cache_map, logs the "all tests passed" status,
                  updates the temporary postscreen_cache_map with the expiration time
                  for each test, and forgets the test results.

                  > All is well and good for a non-whitelisted host, but apparently it
                  > was too quick in coming back to the secondary MX IP address ...
                  >
                  > May 19 13:24:26 harrier postfix/postscreen[3533]: CONNECT from [188.42.15.9]:33610 to [207.223.116.214]:25
                  > May 19 13:24:26 harrier postfix/postscreen[3533]: WHITELIST VETO [188.42.15.9]:33610
                  >
                  > ... all in the same second, but according to syslog, sequentially
                  > after having earned whitelist status.

                  postscreen logs "CONNECT from", does not find the client IP address
                  in the permanent postscreen_access_list, and does not find the
                  client IP address in the temporary postscreen_cache_map. Therefore
                  this is handled as a non-whitelisted client that connects to the
                  "wrong" IP address.

                  Why wasn't the client IP address found in the temporary
                  postscreen_cache_map? Maybe silent corruption of the cache database.

                  Wietse
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