OSError during authenticating to an ftps server with ftplib.FTP_TLS

Question

I'm trying to connect to an ftps (IIS) server with ftplib with the following code:

>>> ftps=FTP_TLS()
>>> ftps.set_debuglevel(2)
>>> ftps.connect(host, port)
*get* '220 Microsoft FTP Service\n'
*resp* '220 Microsoft FTP Service'
'220 Microsoft FTP Service'
>>> ftps.login(user, pw)
*cmd* 'AUTH TLS'
*put* 'AUTH TLS\r\n'
*get* '234 AUTH command ok. Expecting TLS Negotiation.\n'
*resp* '234 AUTH command ok. Expecting TLS Negotiation.'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/ftplib.py", line 749, in login
    self.auth()
  File "/usr/local/lib/python3.7/ftplib.py", line 761, in auth
    server_hostname=self.host)
  File "/usr/local/lib/python3.7/ssl.py", line 423, in wrap_socket
    session=session
  File "/usr/local/lib/python3.7/ssl.py", line 870, in _create
    self.do_handshake()
  File "/usr/local/lib/python3.7/ssl.py", line 1139, in do_handshake
    self._sslobj.do_handshake()
OSError: [Errno 0] Error

Why do I get the - not too descreptive - exception above?
I'm using Python 3.7.5

I have tried to login with lftp and I could connect to it. The only thing I had to do is disable ssl certificate verification. Can this be the source of the problem for ftplib?

output of lftp:

lftp :~> set ssl:verify-certificate false
lftp :~> open hapuser@192.168.140.225:2121
notice: cannot open /home/jenkins/.netrc: No such file or directory
Password:
---- dns cache hit
lftp hapuser@192.168.140.225:~> ls
---- dns cache hit
---- attempt number 1 (max_retries=1000)
---- Connecting to 192.168.140.225 (192.168.140.225) port 2121
<--- 220 Microsoft FTP Service
---> FEAT
<--- 211-Extended features supported:
<---  LANG EN*
<---  UTF8
<---  AUTH TLS;TLS-C;SSL;TLS-P;
<---  PBSZ
<---  PROT C;P;
<---  CCC
<---  HOST
<---  SIZE
<---  MDTM
<---  REST STREAM
<--- 211 END
---> AUTH TLS
<--- 234 AUTH command ok. Expecting TLS Negotiation.
---> LANG
Certificate: C=HU,ST=.,L=.,O=GDF,OU=.,CN=APP-FS-FTP
 Issued by: DC=hu,DC=egaz-degaz,CN=egaz-degaz-MASTER-DC-CA
WARNING: Certificate verification: Not trusted (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
WARNING: Certificate verification: Expired (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
WARNING: Certificate verification: certificate common name doesn't match requested host name ‘192.168.140.225’ (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
<--- 200 Language is now English, UTF-8 encoding.
---> OPTS UTF8 ON
<--- 200 OPTS UTF8 command successful - UTF8 encoding now ON.
---> HOST 192.168.140.225
<--- 504 Server cannot accept argument.
---> USER hapuser
<--- 331 Password required
---> PASS XXXX
<--- 230-Directory has 111,319,724,032 bytes of disk space available.
<--- 230 User logged in.
---> PWD
<--- 257 "/" is current directory.
---> PBSZ 0
<--- 200 PBSZ command successful.
---> PROT P
<--- 200 PROT command successful.
---> PASV
<--- 227 Entering Passive Mode (192,168,140,225,22,108).
---- Connecting data socket to (192.168.140.225) port 5740
---- Data connection established
0:0 translated to pair 0:0 (0,0)
0 translated to pair 0:0 (0,0)
0:0 translated to pair 0:0 (0,0)
0 translated to pair 0:0 (0,0)
0:0 translated to pair 0:0 (0,0)
0 translated to pair 0:0 (0,0)
---> LIST
<--- 125 Data connection already open; Transfer starting.
Certificate: C=HU,ST=.,L=.,O=GDF,OU=.,CN=APP-FS-FTP
 Issued by: DC=hu,DC=egaz-degaz,CN=egaz-degaz-MASTER-DC-CA
WARNING: Certificate verification: Not trusted (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
WARNING: Certificate verification: Expired (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
WARNING: Certificate verification: certificate common name doesn't match requested host name ‘192.168.140.225’ (AF:7B:B6:5F:D1:EF:C9:CC:AA:18:EF:3E:94:15:EF:DB:77:F5:3D:4D)
<--- 226-Directory has 111,319,670,784 bytes of disk space available.
<--- 226 Transfer complete.
drwxrwxrwx   1 owner    group               0 Jul 14 15:07 docuscan
drwxrwxrwx   1 owner    group             112 Jul 14 15:07 Leolvasas
drwxrwxrwx   1 owner    group          662540 Jul 14 15:07 ContactExport
drwxrwxrwx   1 owner    group         1099644 Jul 14 15:07 HAP
drwxrwxrwx   1 owner    group               0 Sep 25  2015 aspnet_client
drwxrwxrwx   1 owner    group               0 May 15  2017 Dgaaa
drwxrwxrwx   1 owner    group               0 Sep 15  2016 EFMH
drwxrwxrwx   1 owner    group               0 Dec  4  2014 EFMH_TESZT
drwxrwxrwx   1 owner    group               0 Mar  6  2015 Flowlogic
drwxrwxrwx   1 owner    group               0 Jul  6  2016 Kimenő_levelek
drwxrwxrwx   1 owner    group               0 Aug 16  2018 Leolvasas_arch
drwxrwxrwx   1 owner    group               0 Jun 12  2017 Logs
drwxrwxrwx   1 owner    group               0 Aug 16  2019 SDszámla
drwxrwxrwx   1 owner    group               0 Jan  3  2017 SDszámla-teszt
drwxrwxrwx   1 owner    group               0 Dec  4  2014 SzlaHitelesites
---- Got EOF on data connection
---- Closing data socket
copy: get hit eof
copy: waiting for put confirmation
copy: put confirmed store
copy: get is finished - all done
---- Closing idle connection
---> QUIT
<--- 221 Goodbye.
---- Closing control socket```

Answer 1

I tried lots of different things, including using different ssl protocols by doing:

ftps.ssl_version = ssl.PROTOCOL_TLSv1_2

Turns out this line won't have any effect, because it is overwritten by the library default.

Details: Connect to FTP TLS 1.2 Server with ftplib

The solution is to give the SSL version wrapped in a context while constructing FTP_TLS like this:

ctx = ssl._create_stdlib_context(ssl.PROTOCOL_TLSv1_2)
ftps = FTP_TLS(context=ctx)
...

This solved my problem.

Answer 2

This is a OS related error

OSError: [Errno 0] Error

meaning it's arising from your system.

It's probably to do with some firewall/NAT/Proxy interference between the client and the server.

You can try to disable your firewall and see if it solves it. You should also look into your Antivirus and see if that's blocking anything.