Postfix: pin key material to our MX:es for fripost.org and its subdomains.

This solves an issue where an attacker would strip the STARTTLS keyword
from the EHLO response, thereby preventing connection upgrade; or spoof
DNS responses to route outgoing messages to an attacker-controlled
SMTPd, thereby allowing message MiTM'ing.  With key material pinning in
place, smtp(8postfix) immediately aborts the connection (before the MAIL
command) and places the message into the deferred queue instead:

    postfix-out/smtp[NNN]: … dsn=4.7.5, status=undeliverable (Server certificate not verified)

This applies to the smarthost as well as for verification probes on the
Mail Submission Agent.  Placing message into the deferred queue might
yield denial of service, but we argue that it's better than a privacy
leak.

This only covers *internal messages* (from Fripost to Fripost) though:
only messages with ‘fripost.org’ (or a subdomain of such) as recipient
domain.  Other domains, even those using mx[12].fripost.org as MX, are
not covered.  A scalable solution for arbitrary domains would involve
either DANE and TLSA records, or MTA-STS [RFC8461].  Regardless, there
is some merit in hardcoding our internal policy (when the client and
server are both under our control) in the configuration.  It for
instance enables us to harden TLS ciphers and protocols, and makes the
verification logic independent of DNS.

1 files changed, 13 insertions, 0 deletions

diff --git a/roles/out/tasks/main.yml b/roles/out/tasks/main.yml
index 48c162a..7a297f1 100644
--- a/roles/out/tasks/main.yml
+++ b/roles/out/tasks/main.yml
@@ -11,40 +11,53 @@
             owner=root group=root
             mode=0644
   with_items:
     - main.cf
     - master.cf
   notify:
     - Reload Postfix
 
 - name: Copy the canonical maps
   template: src=etc/postfix/canonical.j2
             dest=/etc/postfix-{{ postfix_instance[inst].name }}/canonical
             owner=root group=root
             mode=0644
 
 - name: Compile the canonical maps
   # no need to reload upon change, as cleanup(8) is short-running
   postmap: cmd=postmap src=/etc/postfix-{{ postfix_instance[inst].name }}/canonical db=lmdb
            owner=root group=root
            mode=0644
 
+- name: Copy the SMTP TLS policy maps
+  template: src=etc/postfix/smtp_tls_policy.j2
+            dest=/etc/postfix-{{ postfix_instance[inst].name }}/smtp_tls_policy
+            owner=root group=root
+            mode=0644
+
+- name: Compile the SMTP TLS policy maps
+  postmap: cmd=postmap src=/etc/postfix-{{ postfix_instance[inst].name }}/smtp_tls_policy db=lmdb
+           owner=root group=root
+           mode=0644
+  notify:
+    - Reload Postfix
+
 - meta: flush_handlers
 
 - name: Start Postfix
   service: name=postfix state=started
 
 
 - name: Install 'postfix_mailqueue_' Munin wildcard plugin
   file: src=/usr/local/share/munin/plugins/postfix_mailqueue_
         dest=/etc/munin/plugins/postfix_mailqueue_postfix-{{ postfix_instance[inst].name }}
         owner=root group=root
         state=link force=yes
   tags:
     - munin
     - munin-node
   notify:
     - Restart munin-node
 
 - name: Install 'postfix_stats_' Munin wildcard plugin
   file: src=/usr/local/share/munin/plugins/postfix_stats_
         dest=/etc/munin/plugins/postfix_stats_{{ item }}_postfix-{{ postfix_instance[inst].name }}