[conspire] (forw) [BALUG-Admin] So, DMARC. A week ago.
Rick Moen
rick at linuxmafia.com
Wed Feb 7 21:15:32 PST 2024
And, BTW, for the last n years, I _have_ had a DMARC record in my DNS,
but one that was deliberately non-compliant. Until about half an hour
ago, if you asked for the TXT record contents for FQDN
_dmarc.linuxmafia.com, you got this DNS answer:
"DMARC: tragically misdesigned since 2012. Check our SPF RR, instead."
----- Forwarded message from Rick Moen <rick at linuxmafia.com> -----
Date: Wed, 7 Feb 2024 21:11:24 -0800
From: Rick Moen <rick at linuxmafia.com>
To: balug-admin at lists.balug.org
Subject: [BALUG-Admin] So, DMARC. A week ago.
1. New Problem - Whee!
At the beginning of this month, Yahoo (or whatever they're called)
and Google tightened down the screws on requiring DMARC and encouraging
DKIM (and/or SPF).
https://blog.google/products/gmail/gmail-security-authentication-spam-protection/
https://support.google.com/a/answer/81126?visit_id=638429646339446077-2384736010&rd=1
See links for GMail's alleged implementation, which has numerous parts.
The page _claims_ the Feb. 1 enforcement is just for "bulk senders —
those who send more than 5,000 messages to Gmail addresses in one day",
but word around the Net is that they started spamboxing mail from even
_tiny_ mail sending domains not in compliance.
2. Parts of the Problem
(Gods, I hope my MTA uses TLS for transmitting mail. It's been so long
since I looked at that, I'm not sure. Anyway, that was a hard
requirement for reaching GMail starting 2023-12-31, per above pages.)
Here, verbatim, are the requirements Gmail says it henceforth requires
of all sending domains (with some others for "bulk senders"):
o Set up SPF or DKIM email authentication for your domain.
o Ensure that sending domains or IPs have valid forward and reverse DNS
records, also referred to as PTR records. Learn more
o Use a TLS connection for transmitting email. For steps to set up TLS in
Google Workspace, visit Require a secure connection for email.
o Keep spam rates reported in Postmaster Tools below 0.10% and avoid ever
reaching a spam rate of 0.30% or higher. Learn more about spam rates.
o Format messages according to the Internet Message Format standard (RFC
5322).
o Don’t impersonate Gmail From: headers. Gmail will begin using a DMARC
quarantine enforcement policy, and impersonating Gmail From: headers
might impact your email delivery.
o If you regularly forward email, including using mailing lists or inbound
gateways, add ARC headers to outgoing email. ARC headers indicate the
message was forwarded and identify you as the forwarder. Mailing list
senders should also add a List-id: header, which specifies the mailing
list, to outgoing messages.
(I'm blearily looking up what is meant by "ARC headers", and I have
never previously had cause or interest to look at Google's Postmaster
Tools.)
3. DKIM Considered Harmful
Long rant not indulged here: DKIM is mailing list-hostile (though
mitigations are possible). I'm sitting out DKIM mail-signing, but also
my present host can't do it as equipped anyway. DMARC is a metastandard
atop either or both of SPF and DKIM. By the definition of DMARC, if you
have both (a) a valid DMARC DNS record and (b) _either_ DKIM or SPF (or
both), then you are DMARC-compliant.
4. SPF Is My Jedi Master
linuxmafia.com has had bullet-proof, simple SPF for ages. I like SPF
and consider it well-scoped, well-implemented for preventing others to
believably forge your domain for mail. It's good here, it works. Here:
:r! dig -t txt @ns1.linuxmafia.com linuxmafia.com +short
"v=spf1 ip4:96.95.217.99 -all"
Dirt-simple: It says "If mail arrives from any IP other than
96.95.217.99 purporting to be from linuxmafia.com, please treat it as
forged."
5. Let's Look at a Good DMARC RR
So, obviously I need a value DMARC record as of a week ago. A couple
of years ago, I tried one, and could swear I remember it impairing my
deliverability of mailing list mail (but can't remember details), not
to mention the avalanche of DMARC "report" mails that are the hallmark
of that botched (IMO) Yahoo anti-forgery metastandard.
But, let's look at the RR for mxtoolbox.com. The RR is always hostname
_dmarc , and is a TXT record. In mxtoolbox.com's case, that's a CNAME
pointing to mxtoolbox.com.hosted.dmarc-report.com . Here 'tis:
:r! dig -t txt mxtoolbox.com.hosted.dmarc-report.com +short
"v=DMARC1; p=reject; rua=mailto:634990a7 at mxtoolbox.dmarc-report.com,mailto:634990a7 at mxtoolbox.dmarc-report.net; ruf=mailto:634990a7 at forensics.dmarc-report.com; fo=1; pct=100"
Breaking that down:
FQDN: mxtoolbox.com.hosted.dmarc-report.com.
TTL: 300
class: IN
RR type: TXT
TXT record breakdown (from within doublequotes):
Version: v=DMARC1;
Action to take if the mail fails authentication: p=reject
Report URIs for Aggregate data to (optional): rua=mailto:634990a7 at mxtoolbox.dmarc-report.com,mailto:634990a7 at mxtoolbox.dmarc-report.net;
Report URIs for Forensics/Failure data to (optional): ruf=mailto:634990a7 at forensics.dmarc-report.com;
Forensic Options: fo=1;
Percentage of mail subjected to filtering: pct=100
6. Let's build a _dmarc.linuxmafia.com DMARC RR
So, in consultation with the pages mentioned below, I constructed one
(and these were my resulting notes):
Include v=DMARC1; because DMARCv1 is the only game in town. Note that
this tag must appear first, or the RR is not a valid DMARC record.
Start policy adoption with p=none; (mostly because of mailing list
forwarding).
https://dmarc.org/2017/03/can-i-use-dmarc-if-i-have-only-deployed-spf/
Include sp=none; This is the "p" policy setting, except as applied to
subdomains.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-sp
Omit aspf=$THING option for now; defaults to "relaxed" SPF alignment
(though I think all mail from linuxmafia.com will originate from that
FQDN as MailFROM and Header From domains).
https://mxtoolbox.com/dmarc/details/dmarc-tags/aspf
Include fo=s; This generates an SPF failure report if the message
failed SPF evaluation, regardless of its alignment. This setting is
used to decide what to send to addresses specified in the RUF tag.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-failure-reporting-options
Include rua=mailto:hostmaster at linuxmafia.com; This says where to send
DMARC Aggregate Feedback reports to. Be warned that this can be a lot
of... stuff.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-rua
Include ruf=mailto:hostmaster at linuxmafia.com; This says where to send
failure/forensic reports, which are much more detailed than are DMARC
Aggregate reports. Only a few mail senders issue those. Prepare to
revise/lose this if I get swamped with reports.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-ruf
Omit rf=$THING (report format), because there's only one format
(Authentication Failure Reporting Format = AFRF) so far, and it's
default, making this tag so far pointless.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-report-format
Include ri=604800; report interval (default 86400 = 1 day), so that
reporting sites send me aggregate reports every 7 days rather than
daily.
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-report-interval
Include pct=100; Means apply the declared policy to mail purporting to
be mine 100% of the time. Since I'm saying "none" for now, this will
matter really only when I shift to p=quarantine; or p=reject; .
https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-percentage
Results in:
_dmarc.linuxmafia.com 300 IN TXT "v=DMARC1; p=none; sp=none; rua=mailto:hostmaster at linuxmafia.com; ruf=mailto:hostmaster at linuxmafia.com; fo=s; ri=604800; pct=100"
7. How about Y'all? What are you guys doing about the problem?
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