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292785Re: [feature request] Subzero postscreen/dnsblog score to bypass after-220 tests?

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  • /dev/rob0
    Apr 23, 2013
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      On Tue, Apr 23, 2013 at 08:05:34PM -0400, Wietse Venema wrote:
      > On Fri, Apr 12, 2013 at 06:34:24AM -0400, Wietse Venema wrote:
      > > /dev/rob0:
      > > > I finally got around to my upgrade to 2.11-20130405 and was
      > > > watching logs. A gmail message fell afoul of the after-220
      > > > tests; each time it came from a different host. Each one got
      > > > a "PASS NEW" and of course the "450 4.3.2 Service currently
      > > > unavailable" rejection.
      > > >
      > > > These gmail outbounds are all listed in list.dnswl.org as
      > > >, and I give that a negative score in my
      > > > postscreen_dnsbl_sites. So with no offsetting DNSBL scores,
      > > > these hosts all got a subzero score. It would be nice if we
      > > > could put those whitelist scores to work, and not have to
      > > > maintain so big of a postscreen_access_list whitelist.
      > >
      > > Disabling tests based on DNSWL score would make sense (currently
      > > they "disable" DNSBL tests only). Perhaps this needs a "disable"
      > > flag in the postscreen cache.
      > On second consideration, this can be done as follows:
      > - One parameter with the (negative) postscreen_dnsbl_sites score
      > that is needed to allow the client to skip tests.

      "postscreen_skip_tests_threshold", or should there be a _dnsbl or
      _dnswl in there? "postscreen_dnsbl_skip_tests_threshold" is good
      because it lumps it in with the other postscreen_dnsbl_* settings
      and makes it clear that this is associated with the DNSBL lookups.
      OTOH _dnswl almost does this too, and it's more accurate. To see how
      it looks: "postscreen_dnswl_skip_tests_threshold".

      > - One parameter with the names of tests that are skipped (using
      > !name to exclude a name, and static:all to match everything).
      > This may include "greet" to cancel a "greet wait" in progress.

      As for "type:name", like postscreen_access_list, you're going to want
      to discourage lookups which might slow this down. I guess only pcre,
      regexp, static, and texthash would be suitable for this? I did not
      include such a warning in the proposed documentation below.

      I think you might want to consider an "all", because that's used in
      numerous other places and meets the "minimum surprise" ideal.

      * Postscreen Skippable Test Names *

      before-220 after-220
      ========== =========

      I don't think there's much point in skipping the blacklist test. It
      would be very strange if the postscreen_access_list lookup came in
      after the DNSBL lookups. Furthermore, if I listed something there
      that I don't want to see, it should never pass. But maybe someone
      would want this? A test can be "skipped" even if already completed.
      (But what if the postscreen_access_list result is reject? Shouldn't
      that be done immediately, before the DNSBL lookups are in?)

      Likewise, I think the mx_policy (postscreen_whitelist_interfaces)
      test should also be absolute. If a client is not connecting on the
      proper IP address, this should be cause for at least having it talk
      to postscreen and try again later. But again, maybe someone would
      trust the DNS whitelists' judgment?

      I like the idea of two umbrella categories, before-220 and after-220,
      in the spirit of inet_interfaces' "all" and "loopback-only". But
      there's only three per category, so this is not major at this point.
      (I suppose in the future more tests could be added.)

      I think the default should be either "after-220" or "greet,
      after-220". Typically the result would come in during a greet pause,
      and even though it's only a few seconds, it can add up in the Big
      Scheme of Things.

      > The procedure is: postscreen does a postscreen_dnsbl_sites query
      > for the client IP address. If the score satifies the threshold
      > in the first parameter, then all tests with a name that matches
      > the second parameter will be skipped until the next
      > postscreen_dnsbl_sites query for that client IP address (i.e.
      > after postscreen_dnsbl_ttl).

      postscreen_skip_tests (default: greet, after-220)

      Allow a remote SMTP client with a score less than or equal to
      postscreen_skip_tests_threshold based on its combined DNSBL
      score as defined with the postscreen_dnsbl_sites parameter,
      to skip the listed tests, if enabled. Specify zero or more of
      blacklist, greet, mx_policy (these three collectively can be
      "before-220"), bare_newline, non_smtp_command, pipelining
      (these three collectively can be "after-220"), or
      "static:all" to skip all postscreen tests except for the
      DNSBL test itself. Specify "!pattern" to exclude a test from
      the list.


      postscreen_dnsbl_sites = dnsbl.example.org,
      postscreen_skip_tests = !blacklist, !mx_policy,

      This feature is available in Postfix 2.11.

      postscreen_skip_tests_threshold (default: -1)

      The inclusive upper bound for allowing a remote SMTP client,
      based on its combined DNSBL score as defined with the
      postscreen_dnsbl_sites parameter, to bypass the tests listed
      in the postscreen_skip_tests parameter.

      Note: this typically would be a negative number, and it only
      makes sense when using DNS whitelists with negative weights
      in the postscreen_dnsbl_sites list. See the example at

      This feature is available in Postfix 2.11.

      I hope this is getting closer? The only point of confusion about it
      in my mind is whether/how to skip the blacklist test. Should
      postscreen, knowing "blacklist" is in the postscreen_skip_tests list,
      await the dnsblog results for a blacklisted client? Why? It's
      certainly not going to hold up a postscreen_access_list "permit"

      Thanks again for considering this.
      http://rob0.nodns4.us/ -- system administration and consulting
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