[issue42498] smtplib is glitchy after receive server code 500

[izomiac]

izomiac  added the comment:

I agree, it's my suspicion that the server's error message and python's 
outbound command are sent so close to each other that buffering ensures they 
disagree about the exact sequence of commands/responses.

As for my server, it was HAProxy doing the injection and I rewrote the server 
to both ignore PROXY commands and to listen on multiple ports so a proxy is not 
necessary. 

So this issue no longer exists for me, but I know that I'm not the only one who 
uses HAProxy to forward SMTP requests (residential ISPs block port 25), and 
Python was relatively unique in its intolerance of this behavior.  (Python uses 
lower case commands, which are easy to recognize on server logs to determine 
which clients are using it.)  It's certainly not a critical issue affecting 
thousands, but in my experience glitchy behavior is a good indicator that the 
code is buggy in general.  Plus, buffering errors scare me because there are so 
many potential security exploits associated with them (on top of being a pain 
to debug.)  But, yeah, this is more of a courtesy bug to let you know about a 
tricky glitch I encountered.

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[issue42498] smtplib is glitchy after receive server code 500

[Pandu E POLUAN]

Pandu E POLUAN  added the comment:

The "problem" is that smtplib.SMTP does the following, in essence:

1. Open connection to server
2. Send EHLO
3. Read server response

The PROXY injection causes a response from the server, but the response gets 
"cached" in the incoming TCP buffer. The injection script likely does not 
consume this buffer, resulting in Step 3 above reading this "cached" response. 

So from the point of view of smtplib.SMTP, the "503" message arrives _after_ 
EHLO is sent, while in actuality the "503" arrives before EHLO is sent.

I suggest rewriting the port forwarding script so that it consumes the response 
from the server first before transferring to smtplib.SMTP

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nosy: +pepoluan

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[issue42498] smtplib is glitchy after receive server code 500

[izomiac]

New submission from izomiac :

I use a custom e-mail server and noticed I wasn't getting e-mails from FreeNAS. 
 I tracked the bug down to Python's smtplib.  My home internet can't do 
outbound port 25, so I use a port forwarding script that I've since learned 
injects "PROXY TCP4\r\n".  
This isn't a basic SMTP command, so my server responds with "500 Syntax error, 
command unrecognized\r\n".  

The first glitch in smtplib is that it assumes the server has precognition and 
upon receiving the syntax error assumes that the *next* command smtplib sends 
is invalid.  This happens to be "ehlo". After assuming "ehlo" got an error it 
backs down to "helo".  My server didn't like clients to send two greetings so 
it responded with "503 Bad sequence of commands", but allowed the connection to 
proceed anyway after the mild scolding.  smtplib proceeds to issue a "mail 
FROM" command, then a "rset" command and disconnects in shame after reflecting 
upon its actions for a while.

So, I modified my server to be a bit more laid back and just accept two 
greetings.  smtplib proceeds to issue "mail FROM", "rcpt TO", and "data" 
commands before crashing (raising an "SMTPDataError" in the traceback).

I then modified my server to ignore "PROXY" commands rather than send a syntax 
error and everything worked perfectly.  I don't code in python, and obviously 
my SMTP server is a one-off creation, but I figured I ought to let you all know 
in case you want to make some improvements to smtplib to better follow the SMTP 
spec and not crash with a cryptic error message three commands later after 
receiving a non-fatal error message.

--
components: email
files: debug output.txt
messages: 382038
nosy: barry, izomiac, r.david.murray
priority: normal
severity: normal
status: open
title: smtplib is glitchy after receive server code 500
type: crash
versions: Python 3.7
Added file: https://bugs.python.org/file49634/debug output.txt

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