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I've configured postfix with opendkim and opendmarc. Outgoing mail is signed correctly, and incoming mail appears to be validated (dkim, spf, dmarc) with one notable exception - anyone can connect to postfix on 25, send mail from fake@mydomain.com to me@mydomain.com, and it's delivered without question.

Logs from an example message:

postfix/smtpd[10275]: 934533DA0157: client=60-241-c-d.tpgi.com.au[60.241.c.d]
postfix/cleanup[14306]: 934533DA0157: message-id=<05846311.653A4BF8@mydomain.com>
opendkim[16729]: 934533DA0157: 60-241-c-d.tpgi.com.au [60.241.c.d] not internal
opendkim[16729]: 934533DA0157: not authenticated
opendkim[16729]: 934533DA0157: external host 60-241-c-d.tpgi.com.au attempted to send as mydomain.com
opendkim[16729]: 934533DA0157: no signature data
opendmarc[32220]: implicit authentication service: host.mydomain.com
opendmarc[32220]: 934533DA0157: mydomain.com fail
postfix/qmgr[22948]: 934533DA0157: from=<accounting@mydomain.com>, size=287090, nrcpt=2 (queue active)
postfix/smtpd[10275]: disconnect from 60-241-c-d.tpgi.com.au[60.241.c.d]
postfix/pipe[14316]: 934533DA0157: to=<techsupport@mydomain.com>, relay=spamassassin, delay=7.6, delays=4.3/0.01/0/3.2, dsn=2.0.0, status=sent (delivered via spamassassin service)

Abridged delivered headers:

Return-Path: <accounting@mydomain.com>
Delivered-To: techsupport@mydomain.com
Received: by host.mydomain.com (Postfix, from userid 987)
    id D89DD3DA023E; Thu,  8 Sep 2016 14:58:55 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mydomain.com;
    s=host; [...]
Received: from mydomain.com (60-241-c-d.tpgi.com.au [60.241.c.d])
    by host.mydomain.com (Postfix) with SMTP id 934533DA0157
    for <techsupport@mydomain.com>; Thu,  8 Sep 2016 14:58:48 +0000 (UTC)
Authentication-Results: host.mydomain.com; dmarc=fail header.from=mydomain.com
Authentication-Results: host.mydomain.com; spf=pass smtp.mailfrom=accounting@mydomain.com
Message-ID: <05846311.653A4BF8@mydomain.com>
Date: Fri, 09 Sep 2016 00:54:26 +1000
From: "accounting@mydomain.com" <accounting@mydomain.com>
To: <techsupport@mydomain.com>

SPF record - A and MX contain only IPs I control. Redundancy deliberate.

mydomain.com. 600 IN TXT "v=spf1 +a +mx +ip4:myrelay1 +ip4:myrelay2 -all"

opendkim.conf

Mode    sv
Syslog  yes
SyslogSuccess   yes
LogWhy  yes
Socket  local:/var/run/opendkim/opendkim.sock
SendReports     yes
ReportAddress   "Xyz Postmaster" <postmaster@mydomain.com>
SoftwareHeader  no
Canonicalization        relaxed/relaxed
Domain  mydomain.com
Selector        host
MinimumKeyBits  1024
KeyFile /etc/opendkim/keys/host.private
InternalHosts   refile:/etc/opendkim/TrustedHosts # contains 127.0.0.1 and ::1
OversignHeaders From
QueryCache      yes

opendmarc.conf

FailureReports true
IgnoreAuthenticatedClients true
IgnoreHosts /etc/opendmarc/ignore.hosts # contains 127.0.0.1 and ::1
Socket local:/var/run/opendmarc/opendmarc.sock
SoftwareHeader false
SPFIgnoreResults true
SPFSelfValidate true
Syslog true

postfix main.cf

mynetworks_style = host
myorigin = $mydomain
non_smtpd_milters = unix:/var/run/opendkim/opendkim.sock, unix:/var/run/opendmarc/opendmarc.sock
smtp_header_checks = regexp:/etc/postfix/submission_header_checks
smtp_tls_mandatory_ciphers = high
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3 TLSv1 TLSv1.1 TLSv1.2
smtp_tls_security_level = may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name
smtpd_milters = unix:/var/run/opendkim/opendkim.sock, unix:/var/run/opendmarc/opendmarc.sock
# rbl_override_whitelist OKs 127.0.0.1 and ::1
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination reject_unknown_sender_domain reject_unauth_pipelining check_client_access hash:/etc/postfix/rbl_override_whitelist
smtpd_sasl_auth_enable = yes
smtpd_sasl_path = private/auth
smtpd_sasl_type = dovecot
smtpd_tls_auth_only = yes
smtpd_tls_cert_file = /etc/letsencrypt/live/mail.mydomain.com/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/mail.mydomain.com/privkey.pem
smtpd_tls_mandatory_ciphers = high
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3 TLSv1 TLSv1.1 TLSv1.2
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
virtual_alias_maps = mysql:/etc/postfix/mysql-virtual-alias-maps.cf,mysql:/etc/postfix/mysql-email2email.cf
virtual_mailbox_domains = mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf
virtual_mailbox_maps = mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf
virtual_transport = dovecot

Despite the external host attempted to send as mydomain.com log entry, opendkim signed the message, then opendmarc decided spf passed.

After a day of reading the opendkim and opendmarc documentation and source I'm nearly convinced these are bugs in opendkim and opendmarc, but am holding out hope it's user error.

What do I need to do prevent opendkim from signing incoming mail marked as from mydomain.com? To troubleshoot or fix opendmarc deciding spf=pass?

  • What's in main.cf? – Ryan Babchishin Sep 9 '16 at 2:59
  • Added, @RyanBabchishin – mal Sep 9 '16 at 4:47
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Despite the fact the mail was from an external source, the accepted return path is internal. It also appears the external host used your domain in its HELO command.

Return-Path: <accounting@mydomain.com>
Received: from mydomain.com (60-241-c-d.tpgi.com.au [60.241.c.d])
    by host.mydomain.com (Postfix) with SMTP id 934533DA0157
    for <techsupport@mydomain.com>; Thu,  8 Sep 2016 14:58:48 +0000 (UTC)

The message should have been rejected based on the domain in the return path and/or the domain in the HELO command. Try to configure your server to reject mail based on these factors. I would consider accepting the message is a configuration issue for the mail server.

I believe OpenDKIM behaved correctly signing a message from a local address despite the external IP address. This would be the behavior if you had authenticated from the internet before sending.

  • Thanks for your answer. Blocking with postfix is my interim solution. OpenDKIM can tell if a user is authenticated, though, I would expect it to have something similar to OpenDMARC's IgnoreAuthenticatedClients directive, sign if localhost or auth'd else verify. There are semi-legitimate cases where my mailserver would recieve mail "from me" from eg. an echoing mailing list that doesn't rewrite the envelope address. Expected behavior would be to process normally - check for a DKIM header, fail SPF, etc. – mal Sep 9 '16 at 4:53
  • Additionally, I switched smtpd_sender_restrictions blacklisting to smtpd_helo_restrictions - OpenDKIM is still signing messages with HELO otherdomain.com MAIL FROM: fake@mydomain.com [...] From: fake@mydomain.com – mal Sep 9 '16 at 5:21
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One solution might be to use port 587 for (SASL-authenticated) message submission and to reject unauthenticated mail submitted via port 25.

See this answer for details.

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