Require STARTTLS for incoming port 25 connections

We already require that outgoing connections
use STARTTLS so other servers need a valid TLS
certificate to accept messages from us.
It is then very unlikely that they cannot use TLS
to send messages to us.

Conversely, if they only can send messages to use without TLS,
it likely does not have STARTLS on its port 25
and then we don't want to accept messages from them
because we will likely not be able to reply.

Author: link2xt

CHANGELOG.md

@@ -5,6 +5,9 @@
 - Require TLS 1.2 for outgoing SMTP connections
   ([#685](https://github.com/chatmail/relay/pull/685))
 
+- require STARTTLS for incoming port 25 connections
+  ([#684](https://github.com/chatmail/relay/pull/684))
+
 - filtermail: run CPU-intensive handle_DATA in a thread pool executor
   ([#676](https://github.com/chatmail/relay/pull/676))
 

cmdeploy/src/cmdeploy/postfix/master.cf.j2

@@ -14,6 +14,7 @@ smtp      inet  n       -       y       -       -       smtpd -v
 {%- else %}
 smtp      inet  n       -       y       -       -       smtpd 
 {%- endif %}
+  -o smtpd_tls_security_level=encrypt
   -o smtpd_proxy_filter=127.0.0.1:{{ config.filtermail_smtp_port_incoming }}
 submission inet n       -       y       -       5000    smtpd
   -o syslog_name=postfix/submission

cmdeploy/src/cmdeploy/tests/online/test_0_login.py

@@ -1,5 +1,5 @@
 import queue
-import socket
+import smtplib
 import threading
 
 import pytest
@@ -91,25 +91,23 @@ def login_smtp_imap(smtp, imap):
 
 def test_no_vrfy(chatmail_config):
     domain = chatmail_config.mail_domain
-    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-    sock.settimeout(10)
-    try:
-        sock.connect((domain, 25))
-    except socket.timeout:
-        pytest.skip(f"port 25 not reachable for {domain}")
-    banner = sock.recv(1024)
-    print(banner)
-    sock.send(b"VRFY wrongaddress@%s\r\n" % (chatmail_config.mail_domain.encode(),))
-    result = sock.recv(1024)
+
+    s = smtplib.SMTP(domain)
+    s.starttls()
+
+    s.putcmd("vrfy", f"wrongaddress@{chatmail_config.mail_domain}")
+    result = s.getreply()
     print(result)
-    sock.send(b"VRFY echo@%s\r\n" % (chatmail_config.mail_domain.encode(),))
-    result2 = sock.recv(1024)
+    s.putcmd("vrfy", f"echo@{chatmail_config.mail_domain}")
+    result2 = s.getreply()
     print(result2)
-    assert result[0:10] == result2[0:10]
-    sock.send(b"VRFY wrongaddress\r\n")
-    result = sock.recv(1024)
+    assert result[0] == result2[0] == 252
+    assert result[1][0:6] == result2[1][0:6] == b"2.0.0 "
+    s.putcmd("vrfy", "wrongaddress")
+    result = s.getreply()
     print(result)
-    sock.send(b"VRFY echo\r\n")
-    result2 = sock.recv(1024)
+    s.putcmd("vrfy", "echo")
+    result2 = s.getreply()
     print(result2)
-    assert result[0:10] == result2[0:10] == b"252 2.0.0 "
+    assert result[0] == result2[0] == 252
+    assert result[1][0:6] == result2[1][0:6] == b"2.0.0 "