Slashdot Mirror
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?
Your post advocates a
(X ) technical ( ) legislative ( ) market-based ( ) vigilante
approach to fighting spam. Your idea will not work. Here is why it won't work. (One or more of the following may apply to your particular idea, and it may have other flaws which used to vary from state to state before a bad federal law was passed.)
( ) Spammers can easily use it to harvest email addresses
(X ) Mailing lists and other legitimate email uses would be affected
( ) No one will be able to find the guy or collect the money
( ) It is defenseless against brute force attacks
(X ) It will stop spam for two weeks and then we'll be stuck with it
( ) Users of email will not put up with it
( ) Microsoft will not put up with it
( ) The police will not put up with it
( ) Requires too much cooperation from spammers
(X ) Requires immediate total cooperation from everybody at once
(X ) Many email users cannot afford to lose business or alienate potential employers
( ) Spammers don't care about invalid addresses in their lists
( ) Anyone could anonymously destroy anyone else's career or business
Specifically, your plan fails to account for
( ) Laws expressly prohibiting it
(X ) Lack of centrally controlling authority for email
( ) Open relays in foreign countries
( ) Ease of searching tiny alphanumeric address space of all email addresses
(X ) Asshats
( ) Jurisdictional problems
( ) Unpopularity of weird new taxes
( ) Public reluctance to accept weird new forms of money
(X ) Huge existing software investment in SMTP
( ) Susceptibility of protocols other than SMTP to attack
( ) Willingness of users to install OS patches received by email
(X ) Armies of worm riddled broadband-connected Windows boxes
(X ) Eternal arms race involved in all filtering approaches
( ) Extreme profitability of spam
( ) Joe jobs and/or identity theft
( ) Technically illiterate politicians
( ) Extreme stupidity on the part of people who do business with spammers
( ) Dishonesty on the part of spammers themselves
( ) Bandwidth costs that are unaffected by client filtering
( ) Outlook
and the following philosophical objections may also apply:
(X ) Ideas similar to yours are easy to come up with, yet none have ever
been shown practical
( ) Any scheme based on opt-out is unacceptable
( ) SMTP headers should not be the subject of legislation
( ) Blacklists suck
(X ) Whitelists suck
( ) We should be able to talk about Viagra without being censored
( ) Countermeasures should not involve wire fraud or credit card fraud
(X ) Countermeasures should not involve sabotage of public networks
( ) Countermeasures must work if phased in gradually
( ) Sending email should be free
(X ) Why should we have to trust you and your servers?
( ) Incompatiblity with open source or open source licenses
( ) Feel-good measures do nothing to solve the problem
( ) Temporary/one-time email addresses are cumbersome
( ) I don't want the government reading my email
(X ) Killing them that way is not slow and painful enough
Furthermore, this is what I think about you:
(X ) Sorry dude, but I don't think it would work.
( ) This is a stupid idea, and you're a stupid person for suggesting it.
( ) Nice try, assh0le! I'm going to find out where you live and burn your
house down!
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.
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