This is a good point, I'm not sure about how others do it, but in Office 365 we compare the 5322.From domain against the domain that was used to authenticate SPF. That's the 5321.MailFrom unless it is <>, in which case we use the HELO/EHLO domain. That would allow a DMARC pass in the absence of a DKIM signature.
-- Terry -----Original Message----- From: dmarc-discuss [mailto:dmarc-discuss-boun...@dmarc.org] On Behalf Of Sistemisti Posta via dmarc-discuss Sent: Monday, May 9, 2016 3:38 AM To: dmarc-discuss@dmarc.org Subject: [dmarc-discuss] DMARC and null path Hello DMARC users, because I'm new in DMARC world I'm trying to understand some details before to start with policy implementation. A detail I would understand is related to mails with null path, or null sender address, typically sent by Delivery Status Notifications. It seems that the only way to pass DMARC with null path is to DKIM sign the mails. Is it true? I ask this because RFC7489 says that exists a condition when DMARC considers the null path: "Note that the RFC5321.HELO identity is not typically used in the context of DMARC (except when required to "fake" an otherwise null reverse-path)" And: "DMARC uses the result of SPF authentication of the MAIL FROM identity. Section 2.4 of [SPF] describes MAIL FROM processing for cases in which the MAIL command has a null path." RFC4408 says accordingly: 'When the reverse-path is null, this document defines the "MAIL FROM" identity to be the mailbox composed of the localpart "postmaster" and the "HELO" identity (which may or may not have been checked separately before).' So if a mail with null path and HELO domain equal to RFC5322.From passes the SPF check, why should DMARC fail? See at this live example: libero.it descriptive text "v=spf1 ip4:212.48.25.128/25 ip4:212.48.14.160/27 include:srs.bis.na.blackberry.com include:srs.bis.eu.blackberry.com include:srs.bis.ap.blackberry.com include:mail.zendesk.com -all" _dmarc.libero.it descriptive text "v=DMARC1\; p=quarantine\; ... If 212.48.14.166 sends a mail with null path, RFC5322.From=<local>@libero.it and *helo=libero.it*, then SPF thinks to have a 'MAIL FROM' like "postmas...@libero.it", passing this result to DMARC for alignment with RFC5322.From. (If it passes the helo domain is the same) The result I see: ~~~ 2016-05-09T09:47:06.481095+02:00 postfix/smtpd[14063]: 3r3Dx63PshzFpVy: client=smtp-32-i6.italiaonline.it[212.48.14.166] 2016-05-09T09:47:06.636894+02:00 postfix/qmgr[17134]: 3r3Dx63PshzFpVy: from=<>, size=308079, nrcpt=x (queue active) 2016-05-09T09:47:06.551037+02:00 opendkim[6782]: 3r3Dx63PshzFpVy: smtp-32-i6.italiaonline.it [212.48.14.166] not internal 2016-05-09T09:47:06.551173+02:00 opendkim[6782]: 3r3Dx63PshzFpVy: not authenticated 2016-05-09T09:47:06.551960+02:00 opendkim[6782]: 3r3Dx63PshzFpVy: no signature data 2016-05-09T09:47:06.594831+02:00 opendmarc[9812]: SPF: 3r3Dx63PshzFpVy: libero.it pass 2016-05-09T09:47:06.595936+02:00 opendmarc[9812]: 3r3Dx63PshzFpVy: libero.it pass ~~~ The mail with null path and no DKIM signs passes DMARC. For me this is a correct result; isn't it? In this particular case we could complain that the client doesn't send an helo equal to his hostname, but this is not DMARC related. I would implement DMARC. For DSN sent to Internet by any authorized MTA I would declare an SPF record as: mta.example.com IN TXT "v=spf1 a -all" _dmarc.example.com descriptive text "v=DMARC1\; p=reject\;" Let suppose the above host sends a DSN with null path, helo="mta.example.com" and RFC5322.From=postmas...@mta.example.com. I expect DMARC passes (in relaxed mode) because SPF passes. Could you explain me where I'm wrong? Thank you very much for your help. Best Regards Marco _______________________________________________ dmarc-discuss mailing list dmarc-discuss@dmarc.org http://www.dmarc.org/mailman/listinfo/dmarc-discuss NOTE: Participating in this list means you agree to the DMARC Note Well terms (http://www.dmarc.org/note_well.html) _______________________________________________ dmarc-discuss mailing list dmarc-discuss@dmarc.org http://www.dmarc.org/mailman/listinfo/dmarc-discuss NOTE: Participating in this list means you agree to the DMARC Note Well terms (http://www.dmarc.org/note_well.html)
