FTPS with Python ftplib - Session reuse required

Question

So, I am trying to connect to an FTP server to get directory listings and download files. But the first command after the prot_p() function is raising an exception - Producing these errors from the log:

*get* '150 Here comes the directory listing.\r\n'
*resp* '150 Here comes the directory listing.'
*get* '522 SSL connection failed; session reuse required: see require_ssl_reuse
option in vsftpd.conf man page\r\n'
*resp* '522 SSL connection failed; session reuse required: see require_ssl_reuse
 option in vsftpd.conf man page'
Traceback (most recent call last):
  File "C:\temp\download.py", line 29, in <module>
    files = ftps.dir()
  File "C:\Python27\lib\ftplib.py", line 522, in dir
    self.retrlines(cmd, func)
  File "C:\Python27\lib\ftplib.py", line 725, in retrlines
    return self.voidresp()
  File "C:\Python27\lib\ftplib.py", line 224, in voidresp
    resp = self.getresp()
  File "C:\Python27\lib\ftplib.py", line 219, in getresp
    raise error_perm, resp
ftplib.error_perm: 522 SSL connection failed; session reuse required: see requir
e_ssl_reuse option in vsftpd.conf man page

Here is the code:

from ftplib import FTP_TLS
import os
import socket

host = 'example.com'
port = 34567
user = 'user1'
passwd = 'pass123'
acct = 'Normal'

ftps = FTP_TLS()

ftps.set_debuglevel(2)

ftps.connect(host, port)

print(ftps.getwelcome())
print(ftps.sock)

ftps.auth()

ftps.login(user, passwd, acct)

ftps.set_pasv(True)
ftps.prot_p()

print('Current directory:')
print(ftps.pwd())
files = ftps.dir()

ftps.quit()

I want to do this securely, hence using FTP over TLS Explicit. I have the idea that I may need to manipulate some settings in the Socket class referenced by ftplib. Changing the settings on the server is not a possibility. I have tested the server successfully with FileZilla client, an older version of WinSCP was raising the same error - although an upgrade to the newest version fixed it.

Any ideas?

Answer 1

It can be now easily fixed for Python 3.6+ by this class (descendant of FTP_TLS):

class MyFTP_TLS(ftplib.FTP_TLS):
    """Explicit FTPS, with shared TLS session"""
    def ntransfercmd(self, cmd, rest=None):
        conn, size = ftplib.FTP.ntransfercmd(self, cmd, rest)
        if self._prot_p:
            conn = self.context.wrap_socket(conn,
                                            server_hostname=self.host,
                                            session=self.sock.session)  # this is the fix
        return conn, size

Answer 2

It looks more likely a vsftpd issue than ftplib as you mention an upgrade to the newest version fixed the problem.

Provided that you cannot touch server's settings, sub-classing the FTP_TLS may help resolve your issue, although it is quite a HACK in my opinion, referenced to this SO question & answers Python FTP TLS connection issue. You can also take a look from this python bug issue 19500:

" It is reasonable for the server to insist that the data connection uses a TLS cached session. This might be a cache of a previous data
connection or of a cleared control connection. If this is the reason for the refusal to allow the data transfer, then the '522' reply
should indicate this.

Note: This has an important impact on client design, but allows
servers to minimize the cycles used during TLS negotiation by
refusing to perform a full negotiation with a previously
authenticated client."

It appears that vsftpd server implemented exactly that by enforcing the "SSL session reuse between the control and data connection".

http://scarybeastsecurity.blogspot.com/2009/02/vsftpd-210-released.html

Looking at the source of Python core library ftplib.py, there isn't any regard to the idea of SSL session reuse between data connection vs. control connection (correct me if I am wrong here. I've tried FTP_TLS.transfercmd(cmd[, rest])¶, didn't work).

This issue is well documented on other FTP clients that supports FTPS, I.E. WinSCP: https://winscp.net/tracker/668

See test log file attached. A vsftpd server with "require_ssl_reuse" set to true in vsftpd.conf would do the trick and can be reproduced.

Hope this helps.