Comment by thedougd
10 hours ago
The primary purpose of DMARC is to prevent impersonation not to prevent spam. I own a domain, I implement DMARC to make sure others know when email from my domain is legitimately from my domain.
10 hours ago
The primary purpose of DMARC is to prevent impersonation not to prevent spam. I own a domain, I implement DMARC to make sure others know when email from my domain is legitimately from my domain.
Then you don't deal with email in the real world. DMARC prevents legitimate emails from people in the real world from being delivered because people make mistakes with their email systems. Passing DMARC is not a signal that the email isn't legitimate; it's merely a sign that someone correctly set up their mail server to modern standards. The email might be an impersonation, or not. There's no way to know.
DMARC does absolutely nothing to prevent the kind of impersonation that occurs in the real world. It doesn't block homoglyphs or or typo-squatting or all the other forms impersonation that matter. It has failed at preventing impersonation.
Just a few days ago I had a phishing email from an elderly woman I had previously done some work for. The message passed DMARC and everything else, and the domain it was sent from was valid. But it was not legitimate; it was an impersonation of her which became obvious once the content was read.
DMARC is noise, not signal. It has to be ignored in the real world as it provides virtually no value beyond blocking emails pretty randomly because someone made a mistake when rotating their DKIM keys or one of a million other mistakes that do happen.
Okay but that doesn’t detract from the intended purpose of DMARC.
Perfect example of "The Purpose Of A System Is What It Does."[1]
1: https://en.wikipedia.org/wiki/The_purpose_of_a_system_is_wha...
2 replies →
Isn't that the purpose of DKIM and SPF already?
DKIM and SPF validate a message. DMARC sets a policy as to what to do with it (quarantine/reject.)
ELI5: https://www.reddit.com/r/sysadmin/comments/16gvtdj/comment/k...
Wish I didn't have to log in to reddit to read that post. RIP useful reddit links.
edit: looks like I had an extension that was redirecting to old.reddit.com, and it was old reddit that required login. Though when I turned that extension off, I got a "blocked by reddit security" error. ugggh.
Yes and no. DKIM signs part of the envelope to help recipients detect alteration (by verifying authenticity), SPF locks down the permissible origins for the sender. SPF is in itself imperfect and can in some situations be exploited on open-access shared systems. If the two are used in concert they offer decent protection.
Using both has to be done very carefully, because a positive result from the weaker one (SPF) will override a negative result from the stronger one (DKIM). You should maximally use DKIM and minimally use SPF. Ideally, you should not use SPF at all, but there are some senders that still don't support DKIM.
3 replies →
This is true, and yet DMARC v1 does not require you to use them in concert. Either one (a valid DKIM-signed message with sender alignment or a message that passes SPF checks with sender alignment) is enough to pass DMARC.