Switch to smtplib.SMTP for more compliant SMTP handling

The current "client" did not wait for the greeting to finish before
sending commands, and took some other liberties with the SMTP standard.
This causes issues with some servers.

sslyze/connection_helpers/opportunistic_tls_helpers.py

@@ -2,6 +2,7 @@
 import struct
 from abc import abstractmethod, ABC
 from enum import Enum
+from smtplib import SMTP, SMTPException
 from typing import ClassVar, Optional
 
 
@@ -65,19 +66,37 @@ def __init__(self, smtp_ehlo_hostname: str):
         self._smtp_ehlo_hostname = smtp_ehlo_hostname
 
     def prepare_socket_for_tls_handshake(self, sock: socket.socket) -> None:
-        # Get the SMTP banner
-        sock.recv(2048)
+        # SMTP parsing has some complicated areas and some unusual but legal
+        # server behavior - this code uses Python's smtplib to handle the protocol.
+        smtp = SMTP(local_hostname=self._smtp_ehlo_hostname)
+        smtp.sock = sock
 
-        # Send a EHLO and wait for the 250 status
-        sock.send(f"EHLO {self._smtp_ehlo_hostname}\r\n".encode("ascii"))
-        data = sock.recv(2048)
-        if b"250 " not in data:
-            raise OpportunisticTlsError(f"SMTP EHLO was rejected: {repr(data)}")
+        try:
+            code, server_reply = smtp.getreply()
+            message = server_reply.decode()
+        except SMTPException as exc:
+            code, message = -1, str(exc)
+        if code != 220:
+            raise OpportunisticTlsError(f"Unable to find 220 service ready response: {message}")
+
+        try:
+            code, server_reply = smtp.getreply()
+            message = server_reply.decode()
+        except SMTPException as exc:
+            code, message = -1, str(exc)
+        if code != 250:
+            raise OpportunisticTlsError(f"SMTP EHLO was rejected: {message}")
+
+        if not smtp.has_extn("starttls"):
+            raise OpportunisticTlsError(f"Server does not support STARTTLS: {message}")
 
-        # Send a STARTTLS
-        sock.send(b"STARTTLS\r\n")
-        if b"220" not in sock.recv(2048):
-            raise OpportunisticTlsError("SMTP STARTTLS not supported")
+        try:
+            code, server_reply = smtp.getreply()
+            message = server_reply.decode()
+        except SMTPException as exc:
+            code, message = -1, str(exc)
+        if code != 220:
+            raise OpportunisticTlsError(f"SMTP STARTTLS rejected: {message}")
 
 
 class _XmppHelper(_OpportunisticTlsHelper):