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Re: reject_unknown_reverse_client_hostname safe?

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  • Peter
    ... No, they have to match, I ll go into detail so that hopefully you understand... Your host claims to be greer.hardwarefreak.com and connects from the IP ...
    Message 1 of 67 , May 8, 2013
      On 05/08/2013 08:03 PM, Stan Hoeppner wrote:
      > On 5/7/2013 5:36 PM, /dev/rob0 wrote:
      > ...
      >> Peter has explained this: you indeed seem to have FCrDNS, just not
      >
      > Maybe my understanding of the definition of Forward Confirmed reverse
      > DNS is incorrect. I thought the definition of FCrDNS is that that the
      > forward and reverse names not only exist but also match. Apparently
      > they both must simply exist.

      No, they have to match, I'll go into detail so that hopefully you
      understand...

      Your host claims to be greer.hardwarefreak.com and connects from the IP
      65.41.216.221, so first we do a lookup on your hostname:

      > greer.hardwarefreak.com. 3448 IN A 65.41.216.221

      ...it matches the IP of your connection ... This is good.

      Next we do a reverse lookup on the IP:

      > 221.216.41.65.in-addr.arpa. 86176 IN PTR mo-65-41-216-221.sta.embarqhsd.net.

      This doesn't return your initial hostname, but that's ok because that is
      not specifically required for FCRDNS. What is required at this stage is
      that the PTR lookup return a valid FQDN and it does indeed.

      Note that the FQDN that is returned "looks" like a generic ISP-assigned
      hostname (because of the IP address in the name). This isn't a good
      thing and there are some mail hosts that will reject based on this, but
      very few in my experience. That generic-looking hostname does not mean
      that it won't pass FCRDNS, though.

      Ok, next we do a lookup on the hostname we got back from the previous step:

      > mo-65-41-216-221.sta.embarqhsd.net. 86166 IN A 65.41.216.221

      ...this returns your IP address as well, and that is good because this
      is required for FCRDNS.


      Peter
    • Stan Hoeppner
      ... permits always come before rejects . Thus whitelist type entries should always be at the top of the restrictions list. As you are using
      Message 67 of 67 , May 14, 2013
        On 5/14/2013 11:45 AM, Steve Jenkins wrote:
        > On Tue, May 14, 2013 at 8:33 AM, /dev/rob0 <rob0@...> wrote:
        >
        >> On Tue, May 14, 2013 at 07:49:50AM -0700, Steve Jenkins wrote:
        >>> smtpd_recipient_restrictions =
        >>> reject_invalid_helo_hostname,
        >>> warn_if_reject reject_non_fqdn_helo_hostname,
        >>> reject_unknown_reverse_client_hostname,
        >>> warn_if_reject reject_unknown_helo_hostname,
        >>> check_reverse_client_hostname_access
        >> pcre:/etc/postfix/fqrdns.pcre,
        >>> check_helo_access hash:/etc/postfix/helo_access,
        >>> check_sender_access hash:/etc/postfix/sender_access,
        >>> reject_rbl_client zen.spamhaus.org,
        >>> reject_rhsbl_client dbl.spamhaus.org,
        >>> reject_rhsbl_sender dbl.spamhaus.org,
        >>> reject_rhsbl_helo dbl.spamhaus.org,
        >>> permit_dnswl_client list.dnswl.org=127.0.[0..255].[1..3],
        >>> permit
        >>
        >> The last two lines are no-op. If you have anything you want to be
        >> subjected to the list.dnswl.org whitelist, put it after the
        >> permit_dnswl_client. If not, there is no point in querying it.
        >
        >
        > Excellent point. If the next step is going to "permit" anyway, then no use
        > in the extra query. I've moved the dnswl.org line up so that it's just
        > above the three "local" check_* lines.

        "permits" always come before "rejects". Thus whitelist type entries
        should always be at the top of the restrictions list. As you are using
        (client|helo|sender|recipient) sections any whitelisting in
        smtpd_recipient_restrictions should typically be at the very top.

        permit_dnswl_client list.dnswl.org=127.0.[0..255].[1..3]
        ^^^^^^ ^^^^

        This shows you are explicitly permitting anything/everything listed in
        the dnswl. Are you sure that is what you want? I use...

        permit_dnswl_client list.dnswl.org=127.0.[2..14].[2..3]

        which does not explicitly permit email marketing providers nor any IP
        with trustworthiness score of 1. A score of 1 is equivalent to a
        SpamAssassin score of -1, which does not merit a direct shot to the
        queue. That would typically require an SA score of -5. I want these
        clients to go through all of my other restrictions before allowing their
        payload into my queue.

        Also worth noting, there are currently only 14 categories (3rd octet of
        a reply), so specifying 255 is not necessary, and possibly problematic.
        Hypothetically, if dnswl decided one day to create categories 16,
        political campaigns, and 17, religious newsletters, you are currently
        setup to automatically permit such clients.

        Remember, the sole purpose of whitelisting is to bypass all of your
        other spam checks and get the mail into your queue unmolested. IMO, not
        every IP listed by dnswl is deserving of this honor, not even close to
        all of them.

        See section "Return codes" at: http://www.dnswl.org/tech

        --
        Stan
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