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Ask Slashdot: How Useful Are DMARC and DKIM?
whoever57 writes How widely are DKIM and DMARC being implemented? Some time ago, Yahoo implemented strict checks on DKIM before accepting email, breaking many mailing lists. However, Spamassassin actually assigns a positive score (more likely to be spam) to DKIM-signed emails, unless the signer domain matches the from domain. Some email marketing companies don't provide a way for emails to be signed with the sender's domain — instead, using their own domain to sign emails. DMARC doesn't seem to have a delegation mechanism, by which a domain owner could delegate other domains as acceptable signatures for emails their emails. All of these issues suggest that the value of DKIM and DMARC is quite low, both as a mechanism to identify valid emails and as a mechanism to identify spam. In fact, spam is often dkim-signed.
Are Slashdot users who manage email delivery actually using DKIM and DMARC?
I do technical support for an industry leading antispam email appliance. Very, very few of the admins I speak with every day utilize DKIM.
I send bulk email for an opt-in list with mailman (opt in as in you have to walk in the store and physically write your email on our sign up sheet).
We have Google host the email for the business and use self hosted for the important stuff.
To get SPF and DKIM working for the business I determined that I could not do this through google. The bounces get redirected to the wrong place and the sender auth fails. I needed bounces to come to me, not Google, so mailman could do the bounce processing. So I had to set up a separate self hosted mail machine with a separate domain, so that the sending domain could match the sender and the bounces could come back to the same place and get bounce processed.
Email sucks and SPF, SKIM and probably DMARC suck.
I should use this sig to advertise my book ISBN-13 : 978-1501515132.
Default scores in SpamAssassin have been assigned based on tests against a large corpus of both emails to obtain a statistical likelihood that a given email will be spam or not for ages, so I take the positive score (more likely to be spam) as a pretty solid indication that its use doesn't provide a good indicator of legitimate mail. Ironically, the biggest culprit for that is probably one of DKIM's biggest proponents, the sheer volume of spam from compromised Yahoo accounts and signed by Yahoo's outbound mail relays is largely responsible for that positive score in my experience - if only they'd do better spam filtering of their outbound email... Not that they are the only ESP with that failing, of course.
UNIX? They're not even circumcised! Savages!
Why the mailing lists broke... They didn't follow RFC 2476 with regard to RFC2822 headers and what can and can not be rewritten, and then they failed to sign the messages with their own mail server signatures.
If you are going to send messages, the policies and protocols force you to take responsibility for the fact that you've sent them, and if you're unwilling to do that, then you don't get to send mail to people who don't like you not taking responsibility.
Too bad, so sad, fix your configuration or you lose.
Former technical support rep for an email marketing company, here.
You only need DKIM if you send a massive amount of mail to users at Yahoo or Microsoft (outlook.com, hotmail) domains.
The purpose of DKIM is to verify the mail you're sending is actually coming from your domain and not someone who is spoofing your domain.
Nobody cares about DMARK.
Yahoo and Microsoft throttle email based on whether or not your domain has proper DKIM keys setup.
If you don't have them set up you can only spam about a thousand messages before you get blocked.
However if you set up DKIM you can spam Yahoo and Microsoft mail (hotmail, outlook.com, etc) users all day long and those mail providers will turn a blind eye.
These mechanisms are only valid for "transactional" business email, where business correspondents need the email credibly labelled by the sending company. It's OK for stuff where you establish who to talk to by mail, telephone or wild-ass-guess, and make deals based on that lebel of security.
It's utterly inappropriate for mailing lists, remailers, discussion groups or material gatewayted between email and usenet or web services. The workaround are lies, told to convince the anti-spam functions of DKIM et all to let it through.
About a week after DKIM broke all the IETF and ISOC lists, the spammers were signing their spam so as to be deliverable once more. I was on the ISOC list at the time, and some unkind words got said about Yahoos.
davecb@spamcop.net
I implemented the strictest controls possible for a site that was being heavily phished and it worked very well. Here's the things you have to understand about DMARC, DKIM, and SPF (since SPF matters to DMARC too).
As a basic overview, here's what these do.
SPF = Only allow emails from specific domains / ip addresses
DKIM = When an email arrives, verify the signature with the domain it claims to be from to ensure it actually came from there
DMARC = How strict should we be with SPF and DKIM?
DMARC in itself isn't an actual verification system. What DMARC does is allows you to tell mail servers exactly how to handle emails that do not pass SPF and/or DKIM checks. Without DMARC, mail servers have to guess and basically follow their own rules. If you've taken the time to document where email from a particular domain comes from (including 3rd party services), ensured that your SPF includes everything, and have verified that all emails are signed with DKIM then eventually you can be strict enough with your DMARC settings to say that anything not passing both SPF and DKIM can simply be trashed. That's what the strictest setting looks like. You can also tell mail servers to send it to the spam folder, just in case you missed something. You can tell it to treat SPF strictly and ignore DKIM or vice versa. You can tell it to apply your DMARC rules to a percentage of your emails (to make it easier to transition into to using it with a small group of messages). You can also have providers send you an XML based email of the days activity to see how messages were handled from different services and where those messages originated. The reports can be a pain to make sense of but once you have everything setup properly you tend to stop looking at them.
It's important to remember, because SPF if easier to implement since it's just a DNS rule. For DKIM you have to actually sign the email before it's sent which may or may not be possible from all of your various points of email origin. DKIM is better, but that makes it more complicated. And that's why you have to have something like DMARC so that you can tell mail servers just how thoroughly those rules have been implemented.
The site that I implemented it for was a very old site where people managed high dollar transactions over email. Phishing was RAMPANT but even more so because there was a good chance a phisher could pass off an email as actually coming from our domain. The combination of 3 protocols in strict mode stopped that completely. It didn't stop PHISHING, but it did secure our domain against it. After that phishers had to use other domains, leaving off a middle letter, trying spelling variations, etc. This gave us the ability to work with registrars to either buy the domains or report the domains for abuse.
As an early poster said, you can't completely stop phishing but there are preventative measures you can take to protect compromised accounts.
After that we took additional steps to secure users accounts. We started recording ip addresses with all logins or return visits along with geographic data from MaxMind. Once we had enough sample data to create a general point of origin, we started locking accounts if they were accessed more than 200 miles from their normal center point and always if they logged in from a different country. As soon as the account was locked for a geographic reason, we sent users an email notifying them that their account had been accessed from another country or outside of their area and that if this lock was in error, they could click a link to disable that function for 2 weeks while they were traveling. Otherwise they should change their password. Users really appreciated it. We expected some usability frustration, but overall these users were very happy to know we were watching out for them.
People also tried to create fake accounts on the system to initiate transactions. For that, we took a page out of Fark's playbook. On Fark, when you get blocked / banned you don't KNOW you've been banned.
"Don't teach a man to fish, feed yourself. He's a grown man. Fishing's not that hard." - Ron Swanson
It breaks a few mailing (discussion, not advertising) list programs (such as my uni's one) if you send from a SPF protected address because the list server forwards it with you address in the from boxs. Other then that it works well.
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