diff --git a/content/blog/mail_server_alpine_postfix_dovecot_tutorial/index.md b/content/blog/mail_server_alpine_postfix_dovecot_tutorial/index.md
index cce1033..6a05ccb 100644
--- a/content/blog/mail_server_alpine_postfix_dovecot_tutorial/index.md
+++ b/content/blog/mail_server_alpine_postfix_dovecot_tutorial/index.md
@@ -94,10 +94,18 @@ The mail server will be composed of the following software:
| Mail delivery agent |
Dovecot |
+
+ | SPF authentication |
+ postfix-policyd-spf-perl |
+
| DKIM authentication and signing |
OpenDKIM |
+
+ | DMARC authentication |
+ OpenDMARC |
+
| Spam filter |
Amavis |
@@ -801,6 +809,8 @@ checks.
## Sender Policy Framework
+### Set up your DNS record
+
Add a TXT record for your root domain with the contents `v=spf1 mx ~all`, like:
```dns
@@ -863,6 +873,68 @@ Breaking down the TXT data:
+### Get Postfix to validate SPF
+
+We're going to use a Postfix SMTPd policy server called postfix-policyd-spf-perl to check SPF of incoming emails.
+postfix-policyd-spf-perl is very simple and requires almost no configuration.
+
+Install `postfix-policyd-spf-perl` and create a user, `policyd-spf` for it:
+
+ # apk add postfix-policyd-spf-perl
+ # adduser -S -s /sbin/nologin -h /dev/null -H policyd-spf
+
+Explanation of `adduser` flags:
+
+
+
+ | Option |
+ Explanation |
+
+
+ -S |
+ Create a system user |
+
+
+ -s /sbin/nologin |
+ Set shell to /sbin/nologin so the user doesn't have a shell |
+
+
+ -h /dev/null |
+ Set home directory to /dev/null |
+
+
+ -H |
+
+ Don't create a home directory (if you try to create /dev/null and assign it to
+ policyd-spf there will be all sorts of permissions issues)
+ |
+
+
+
+Now edit `/etc/postfix/master.cf` to tell Postfix to start up the postfix-policyd-spf-perl daemon:
+
+```conf
+policyd-spf unix - n n - 0 spawn
+ user=policyd-spf argv=/usr/bin/postfix-policyd-spf-perl
+```
+
+Now get Postfix to use postfix-policyd-spf-perl in `/etc/postfix/main.cf` by adding the following lines:
+
+```conf
+smtpd_recipient_restrictions =
+ permit_mynetworks,
+ reject_unauth_destination,
+ check_policy_service unix:private/policyd-spf
+policyd-spf_time_limit = 3600
+```
+
+postfix-policyd-spf-perl is now set up, and you can test it by sending yourself an email from a mainstream email
+provider (which ought to have an SPF record) and checking for the presence of this header:
+
+```
+Received-SPF: pass (protonmail.com: Sender is authorized to use 'revsuine@protonmail.com' in 'mfrom' identity (mechanism 'include:_spf.protonmail.ch' matched))
+```
+
## DomainKeys Identified Mail
### Configure OpenDKIM
@@ -1065,6 +1137,11 @@ non_smtpd_milters = $smtpd_milters
This uses the Milter extension, which is something that can be used to process mail; in this case, to add headers to
emails relating to DKIM.
+You can, again, test this on both incoming and outgoing mail. On outgoing mail, there should be a `DKIM-Signature:`
+header present. On incoming mail from domains implementing DKIM, there should be a
+`Authentication-Results: master.revsuine.xyz;` header (obviously replacing `master.revsuine.xyz` with your hostname)
+indicating whether or not the email has passed DKIM authentication.
+
## Domain-based Message Authentication, Reporting, and Conformance
### Ensure your domains are aligned in email headers
@@ -1197,6 +1274,120 @@ The `fo` tag indicates when you would like to receive reports. The options are:
+### OpenDMARC
+
+We can use software called OpenDMARC to enforce DMARC policies for incoming mail. OpenDMARC is another milter. Let's
+install it and enable its service:
+
+ # apk add opendmarc
+ # rc-update add opendmarc
+ # rc-service opendmarc start
+
+Edit the OpenDMARC config at `/etc/opendmarc/opendmarc.conf`.
+
+Change
+
+```conf
+AuthservID HOSTNAME
+```
+
+to
+
+```conf
+AuthservID OpenDMARC
+```
+
+This is so that the `Authentication-Results` header from OpenDKIM authentication. This will also make it clear which
+program adds which `Authentication-Results` header.
+
+Add the following line, replacing `mail.domain.com` with your *hostname* (so in [my
+instance](#a-note-on-my-dns-records), this is `master.revsuine.xyz`).
+
+```conf
+TrustedAuthservIDs mail.domain.com
+```
+
+Enable `RejectFailures`, which means your server will comply with `p=reject` in DMARC DNS records.
+
+```conf
+RejectFailures true
+```
+
+You also probably want to enable `RequiredHeaders`, which rejects emails that don't conform to RFC5322 standards, e.g.
+are missing a `From:` header.
+
+```conf
+RequiredHeaders true
+```
+
+In case external SPF validation fails (as in, no SPF results are placed in the message header), you probably want to
+add
+
+```conf
+SPFSelfValidate true
+```
+
+which tells OpenDMARC to perform the SPF check itself if it can't find SPF results in the message header.
+
+Now provide OpenDMARC with a socket to use for communication with sendmail. We will use a TCP socket on port 8893:
+
+```conf
+Socket inet:8893@localhost
+```
+
+For a Unix socket, you'd use the following format:
+
+```conf
+Socket local:/var/run/opendmarc/opendmarc.sock
+```
+
+By default, you will have the line
+
+```conf
+IgnoreHosts /etc/opendmarc/ignore.hosts
+```
+
+in `/etc/opendmarc/opendmarc.conf`. This tells OpenDMARC to not authenticate the list of hosts in
+`/etc/opendmarc/ignore.hosts`. An example `ignore.hosts` is
+
+ 127.0.0.1
+ 93.113.25.226
+
+Keep in mind that if you have specified `IgnoreHosts`, this file needs to exist in order for OpenDMARC to run. If you
+have the option set, make sure to `touch /etc/opendmarc/ignore.hosts` (or whatever filepath you've specified).
+Alternatively, comment out this option in order to use the default, which is to not authenticate mail coming from
+127.0.0.1.
+
+Restart OpenDMARC for these changes to take effect:
+
+ # rc-service opendmarc restart
+
+To have Postfix use the OpenDMARC milter, it's simple as adding the socket to the `smptd_milters` and
+`non_smtpd_milters` variable in `/etc/postfix/main.cf`:
+
+```conf
+milter_default_action = accept
+milter_protocol = 6
+smtpd_milters = inet:127.0.0.1:8891,inet:127.0.0.1:8893
+non_smtpd_milters = $smtpd_milters
+```
+
+Restart Postfix for the changes to take effect:
+
+ # rc-service postfix restart
+
+And when you receive emails from a legitimate source that implements DMARC, you should see the following headers in
+your emails:
+
+```
+Received-SPF: pass (protonmail.com: Sender is authorized to use 'revsuine@protonmail.com' in 'mfrom' identity (mechanism 'include:_spf.protonmail.ch' matched)) receiver=master.revsuine.xyz; identity=mailfrom; envelope-from="revsuine@protonmail.com"; helo=mail-40130.protonmail.ch; client-ip=185.70.40.130
+DMARC-Filter: OpenDMARC Filter v1.4.2 master.revsuine.xyz 88CFF1288D1
+Authentication-Results: OpenDMARC; dmarc=pass (p=quarantine dis=none) header.from=protonmail.com
+Authentication-Results: OpenDMARC; spf=pass smtp.mailfrom=protonmail.com
+Authentication-Results: master.revsuine.xyz;
+ dkim=pass (2048-bit key; secure) header.d=protonmail.com header.i=@protonmail.com header.a=rsa-sha256 header.s=protonmail3 header.b=nc4YWVM/
+```
+
### Test SPF, DKIM, and DMARC
You can use [mail-tester.com](https://www.mail-tester.com/) and send an email from your domain to check that SPF, DKIM,