Re: reject_unknown_reverse_client_hostname safe?
- On 8 May 2013 at 3:03, Stan Hoeppner wrote:
> On 5/7/2013 5:36 PM, /dev/rob0 wrote:Your initial understanding is correct.
> > 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.
FCrDNS is commonly associated with reverse and forward lookup results
that are "in agreement", as described in RFC 5451 for the "iprev" message
header field (see section 2.4.3. "iprev" Results). At least one of the returned
names from the reverse lookup must resolve back to the IP:
188.8.131.52 -> host.example.com [ host2.example.com, host.other.co.uk... ]
host.example.com [ || host2.example.com || host.other.co.uk... ] -> 184.108.40.206
However, RFC 5451 can be paraphrased thus:
"iprev" is a nice idea in theory, but not recommended as a practical global
For public facing MXs that expect to receive emails from almost anywhere:
Regional and corporate variations in rDNS implementation currently render
FCrDNS impractical as a primary client rejection method.
- On 5/14/2013 11:45 AM, Steve Jenkins wrote:
> On Tue, May 14, 2013 at 8:33 AM, /dev/rob0 <rob0@...> wrote:"permits" always come before "rejects". Thus whitelist type entries
>> On Tue, May 14, 2013 at 07:49:50AM -0700, Steve Jenkins wrote:
>>> smtpd_recipient_restrictions =
>>> warn_if_reject reject_non_fqdn_helo_hostname,
>>> warn_if_reject reject_unknown_helo_hostname,
>>> 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],
>> 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.
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.
This shows you are explicitly permitting anything/everything listed in
the dnswl. Are you sure that is what you want? I use...
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