To save you some searching. Here's a page with a lot of good info. It's about how to do all this on postfix, so it's not a cookie cutter for doing it on our toaster, but good info nonetheless. He also uses "opendmarc" to process DMARC things, but spamasssassin also has it built in as per my previous note.


https://www.skelleton.net/2015/03/21/how-to-eliminate-spam-and-protect-your-name-with-dmarc/


Gary


On 6/3/2020 11:12 AM, Eric Broch wrote:

Thanks, Gary.

I'll have a look

On 6/3/2020 8:52 AM, Gary Bowling wrote:

 

Further to this subject. I am learning that there are more pieces that can help us out. Spamassassin gives us a way to assign a spam score to messages with various DKIM results. But it doesn't know what the original sender wanted us to do with messages that have DKIM problems, therefore we just default to giving scores with some predetermined weighting.


There are two more tools, ADSP (Author Domain Signing Practices), and DMARC (Domain based Message Authentication, Reporting and Conformance). Which are both fancy ways of saying, "I want to tell other servers that messages from MY server should have DKIM and what to do if they don't"


For outbound mail, both ADSP and DMARC simply require you to set up DNS TXT records telling remote servers how to handle messages received from your server. If you want to use either of these, do a search for them and you'll find info on how to set up the DNS records. Without explanation of all the fields, here's what I put in my bind DNS.


_adsp._domainkey.mail  IN     TXT    "dkim=all"

_demarc.mail    IN   TXT   "v=DMARC1; p=quarantine; rua=mailto:postmas...@example.com; ruf=mailto:postmas...@example.com; fo=1; adkim=r; aspf=r; pct=100; rf=afrf; ri=86400; sp=quarantine"


For inbound mail, we can set up spamassassin to query DNS records for inbound mail and score them based on info that others might have configured in DMARC. It requires a plugin called AskDNS, but that looks to already be available in our spamassassin and also in the EPEL version of spamassassin, so it should just require us to assign scores. Here's what I have configured in my /etc/spamassassin/local.cf


ifplugin Mail::SpamAssassin::Plugin::AskDNS
askdns __DMARC_POLICY_NONE _dmarc._AUTHORDOMAIN_ TXT /^v=DMARC1;.*\bp=none;/
askdns __DMARC_POLICY_QUAR _dmarc._AUTHORDOMAIN_ TXT /^v=DMARC1;.*\bp=quarantine;/
askdns __DMARC_POLICY_REJECT _dmarc._AUTHORDOMAIN_ TXT /^v=DMARC1;.*\bp=reject;/

meta DMARC_REJECT !(DKIM_VALID_AU || SPF_PASS) && __DMARC_POLICY_REJECT
score DMARC_REJECT 10
meta DMARC_QUAR !(DKIM_VALID_AU || SPF_PASS) && __DMARC_POLICY_QUAR
score DMARC_QUAR 5
meta DMARC_NONE !(DKIM_VALID_AU || SPF_PASS) && __DMARC_POLICY_NONE
score DMARC_NONE 0.1
endif # Mail::SpamAssassin::Plugin::AskDNS






On 6/2/2020 5:12 PM, Gary Bowling wrote:


Yea, I had already looked in there, they aren't there. I eventually found them in


/usr/share/perl5/vendor_perl/Mail/SpamAssassin/Plugin/DKIM.pm


Looks like the defaults are,

  score DKIM_ADSP_ALL          2.5
  score DKIM_ADSP_DISCARD     25
  score DKIM_ADSP_NXDOMAIN     3

  score DKIM_ADSP_CUSTOM_LOW   1
  score DKIM_ADSP_CUSTOM_MED   3.5
  score DKIM_ADSP_CUSTOM_HIGH  8


For right now, I'm going to adjust a few of these and also adjust some of the SPF settings. Here's what I'm trying right now in my /etc/spamassassin/local.cf


#Adjust scores for SPF FAIL
score SPF_FAIL 4.0
score SPF_HELO_FAIL 4.0
score SPF_HELO_SOFTFAIL 3.0
score SPF_SOFTFAIL 3.0
 
#adjust DKIM scores
score DKIM_ADSP_ALL 3.0
score DKIM_ADSP_DISCARD  10.0
score DKIM_ADSP_NXDOMAIN 3.0


Thanks, Gary



On 6/2/2020 12:29 PM, Eric Broch wrote:

Gary,

The stock scores for spamassassin are in /usr/share/spamassassin/*.cf.

# grep DKIM /usr/share/spamassassin/*.cf

For your local configuration you can override the scores in /etc/mail/spamassassin/local.cf on COS8 or /etc/spamassassin/local.cf on COS7. I know THAT one can manipulate scores to fit their needs with spamassassin, however, I have NEVER done it. This is me sloughing it off. ;-) The reason I like spamassassin DKIM verification is because it doesn't just reject bad DKIM which as you mentioned can have bad effects but scores it with other things for rejection.

If you find some configuration that suits you and your system I'd we willing to post in on the QMT web as a stock 'QMT' setting.

Eric

On 6/2/2020 10:11 AM, Gary Bowling wrote:


Thanks Eric. What is the config setting in local.cf to change the DKIM scoring? I don't find any setting in my /etc/spamassassin/ directories that sets that score. Is the scoring for the stock EPEL local.cf different from what we have? I assume not since you said you didn't tailor any of that in QMT.


I think that's a good move to use the stock spamassassin from EPEL.


As DKIM seems to be more pervasive these days, I might be tempted to increase the score in spamassassin if I can find the local.cf setting.


Thanks, Gary


On 6/2/2020 11:56 AM, Eric Broch wrote:

Hi Gary,

My intent, which I articulated in another email on the list and instead of reinventing the wheel, was exactly as you deduced in your email, that is, to allow spamassassin to score DKIM which it does; however, I have not done anything as far as a tailoring configuration for QMT and was content to allow users that scoring decision. My goal is to drop the specially created QMT spamassassin (and clamav) rpm, which I've done in CentOS 8, and use the stock rpm from EPEL.

I think you can override default scoring for DKIM in /etc/spamassassin/local.cf on COS7 and /etc/mail/spamassassin/local.cf on COS8.

Eric

On 6/2/2020 8:09 AM, Gary Bowling wrote:

What is everyone doing these days for DKIM verification, i.e. checking incoming mail for DKIM signatures?


Background

Many years ago, when DKIM was first introduced to the toaster (maybe it was even in the Shupp's toaster days), I installed and turned on incoming DKIM verification. Initially I set it to "reject" unsigned email and of course that was a disaster as it blocked most everything.


Back then, the choice was to have it verify emails, but not block them, or remove verification. I made the decision that checking without doing anything was a waste of resources, so I removed any DKIM verification. I don't remember how I did all this, as it was years ago.


Then at some point DKIM verification was added to spamassassin, or maybe it was always there but we didn't implement the plugin. At any rate, spamassassin DKIM verification was added to the toaster.


Which seems like a good thing as spamassassin can assign a score to DKIM verification which plays into whether a msg is marked as spam or not. The problem with it though, is the score for NOT being verified is very low, something like .01, which essentially does nothing. I can't find any "user" added parameter that would increase that score and don't really know if that's a good thing to try to do. If it were a good thing, I would think it would be a commonly used setting, which doesn't appear to be the case.


What to do in 2020?

So the question is, what to do about DKIM verification in 2020? From the way my server is configured it appears to be useless. But maybe that's because I don't know how to best configure it.


Side Note

On a side note, I do use outbound DKIM and have DNS set up, etc. I have no idea if this is useful or not, but I'll leave it, hoping that somehow this reduces my probability of being rejected by some server out there. But from what I can tell, it really does nothing. Seems to me DKIM is nothing more than an exercise in futility and extra work for postmasters :)


--
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Gary Bowling
The Moderns on Spotify
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