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Title: ftplib: Add client-side SSL session resumption
Type: enhancement Stage: needs patch
Components: Library (Lib) Versions: Python 3.10
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Florian Wickert, Mark.Ribau, Ye.Wang, alex, christian.heimes, dstufft, giampaolo.rodola, janssen, larsschellhas, pitrou, python-dev, robr, xiang.zhang
Priority: low Keywords: patch

Created on 2013-11-05 04:02 by Ye.Wang, last changed 2022-04-11 14:57 by admin.

File name Uploaded Description Edit
ftplib-FTPS-bug.txt Ye.Wang, 2013-11-05 04:02 Python source and command prompt input/output
reuse_session.diff Alex Warhawk, 2015-10-03 11:35 proof-of-concept session reuse review
implement_ssl_session_reuse.patch Alex Warhawk, 2015-10-07 11:51 Patch which implements SSL session reuse and fixes corresponding FTP_TLS issue review
implement_ssl_session_reuse_3.6.patch Alex Warhawk, 2015-10-08 09:40 Patch which implements SSL session reuse and fixes corresponding FTP_TLS issue targeted at 3.6 review
SSLSession-support.patch christian.heimes, 2016-09-04 15:27 review christian.heimes, 2016-09-05 06:23
SSLSession-support-2.patch christian.heimes, 2016-09-05 20:19 review
SSL-client-side-SSL-session-resumption.patch christian.heimes, 2016-09-08 21:25 review
ftplib_session.patch robr, 2017-01-24 15:12
Messages (27)
msg202193 - (view) Author: Ye Wang (Ye.Wang) Date: 2013-11-05 04:02
According to RFC4217 (Securing FTP with TLS, aka the FTPS spec),

"  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".

Looking at the source of Python core library, 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:

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.
msg216674 - (view) Author: Giampaolo Rodola' (giampaolo.rodola) * (Python committer) Date: 2014-04-17 08:40
Interesting, I wasn't aware of this FTP(S) feature.
Unfortunately RFC-4217 really doesn't say much about how this should be done but it definitively looks like something worth having.
AFAIU this looks like something which should be implemented by servers though, not clients.
msg216679 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-17 09:42
Yuck. Is there a public FTP server available somewhere with this "feature"?
msg216680 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-17 09:57
The RFC is unhelpfully lousy. It's not enough to process a "522" error, since that can be triggered for different reasons. You also somehow have to interpret the error text to detect that session reuse is indeed mandated by the server.

Regardless, to progress with this we would first need to implement client-side SSL session reuse, which necessitates a bunch of additional APIs (since which session is to be reused is a decision made by user code), and a new opaque type to carry SSL_SESSION objects...

(see issue #8106)
msg225078 - (view) Author: Mark Ribau (Mark.Ribau) Date: 2014-08-08 16:29
Adding Python v2.7 as also exhibiting this behavior. 

Some people over on Stack Overflow have done some things to work around the issue via subclassing, but I'm not sure their solutions are "correct", so much as have useful side effects. (For example, when only the server has a key/cert and the client does not, how is that handled for reuse?)
msg252205 - (view) Author: Alex Warhawk (Alex Warhawk) Date: 2015-10-03 11:35
I encountered this problem recently and could not find a fix, so i tried fixing it myself.

Note that the patch attached is my first contribution to cpython as well as the first time I used the C extension mechanism. Therefore I do not consider the patch polished enough to be just merged upstream.

Maybe it helps in solving this issue.

The attached patch is based on:
changeset:   79113:ec373d762213
branch:      2.7
msg252467 - (view) Author: Alex Warhawk (Alex Warhawk) Date: 2015-10-07 11:51
Based on the proof-of-concept patch I submitted a few days ago I have built a more sophisticated patch. Please review it and let me know about necessary changes.
msg252468 - (view) Author: Giampaolo Rodola' (giampaolo.rodola) * (Python committer) Date: 2015-10-07 11:55
This is supposed to be a new feature hence the patch should be targeted against Python 3.6, definitively not 2.7.
msg252527 - (view) Author: Alex Warhawk (Alex Warhawk) Date: 2015-10-08 09:40
I have re-targeted the patch for 3.6. It is not a 1 to 1 port of the prior one, but quite similar.
msg252530 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2015-10-08 10:35
Thanks for your patch. There might be a simpler way. By default a SSLContext only caches server sessions. You can enable client session caching with:

  SSL_CTX_set_session_cache_mode(ctx, SSL_SESS_CACHE_CLIENT)

This may be sufficient for FTP over TLS since both sockets are created from the same context.

The new patch has a flaw. With the new SSLSession object a user could attempt to reuse a SSLSession with a different SSLContext. That's going to break OpenSSL.

From SSL_set_session(3)

       SSL_SESSION objects keep internal link information about the session cache list, when being inserted into one SSL_CTX object's session cache.  One SSL_SESSION object, regardless of its reference count, must therefore only be used with one SSL_CTX object (and the SSL objects created from this SSL_CTX object).
msg252543 - (view) Author: Alex Warhawk (Alex Warhawk) Date: 2015-10-08 14:57
Thanks for the heads up Christian I'll try enabling client session caching. If this does not work I'll try to adapt the patch to only allow session reusing within the same context.
msg252635 - (view) Author: Alex Warhawk (Alex Warhawk) Date: 2015-10-09 18:51
Even after enabling client cache one still has to call SSL_set_session. See documentation of SSL_CTX_set_session_cache_mode point SSL_SESS_CACHE_CLIENT.

I started thinking about not exposing a SSL_SESSION object to the user but rather extending wrap_socket to take an already established socket as argument and use that socket's session object. This way I can ensure that both sockets share the same SSL context

I am not really convinced by this idea myself, what do you think about this? Any better ideas?
msg274367 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-04 15:27
Here is my take on the SSLSession feature. The patch provides a SSLSession type, SSLSocket.session getter/setter and SSLSocket.session_reused getter. The setter makes sure that the session can only set for client sockets from the same SSLContext and before handshake. Tests and documentation need some improvements.
msg274391 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-05 06:23
Performance improvements with session resumption is quite noticeable. I see between 17 and 21% improvement for 10 requests GET in a simple benchmark:

session resumption: 1.264sec
no session:         1.535sec
msg274394 - (view) Author: Xiang Zhang (xiang.zhang) * (Python committer) Date: 2016-09-05 07:51
Patch LGTM. But one thing is that every time it returns a new instance of SSL.Session. That means ssl_sock.session == ssl_sock.session will always return False right now. Is it useful to make it comparable?
msg274396 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-05 10:33
Xiang, good point! I have added richcompare to SSLSession (based on session id). My branch on github implements a couple more fixes and improvements.
msg274417 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-05 18:26
Note to future me:
Don't forget to take care of X.509 client authentication. A server is allowed to bypass client cert validation when a SSL session is resumed. SSLContext.load_cert_chain() should invalidate session caches. (CVE-2016-5419
msg274893 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-07 21:03
Session resumption is currently broken in OpenSSL 1.1.0,
msg275161 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-08 21:25
This patch implements a workaround for OpenSSL 1.1.0.
msg275703 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2016-09-10 21:45
New changeset 6f2644738876 by Christian Heimes in branch 'default':
Issue #19500: Add client-side SSL session resumption to the ssl module.
msg275704 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2016-09-10 21:46
I have committed the feature with rudimentary documentation. I will provide more documentation and an example before 3.6.0b2.
msg286190 - (view) Author: Rob Reilink (robr) Date: 2017-01-24 15:12
With this code in place, ftplib should / could also be updated to support session resumption. This would fix bugs with connections to FTP servers that require session resumption [1], [2]

In ftplib.FTP_TLS.ntransfercmd, just add a reference to the current session in the wrap_socket call (maybe make this an option to do session resumption or not; I don't know if it could break something)

Proposed patch is attached.

msg312885 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2018-02-26 08:24
It's now an ftplib issue.

It's too late to land a new feature in 3.7 because we have reached feature freeze.
msg370001 - (view) Author: Florian Wickert (Florian Wickert) Date: 2020-05-26 15:21
Using vsftpd 3.0.3 with ssl_enable=YES and require_ssl_reuse=YES I get the following error when I try to upload a file with conn.storbinary():

FTP command: Client "", "STOR something.bin"
FTP response: Client "", "150 Ok to send data."
FTP response: Client "x.x.x.x", "150 Ok to send data."
DEBUG: Client "x.x.x.x", "DATA connection terminated without SSL shutdown. Buggy client! Integrity of upload cannot be asserted."
FTP response: Client "x.x.x.x", "426 Failure reading network stream."

This also happens when SSL reuse is disabled on the server side.
Uploading with FileZilla works and there is basically no difference in the log until the error happens.
msg377412 - (view) Author: Lars Schellhas (larsschellhas) Date: 2020-09-23 16:58
Excuse me, but why is this issue still open and unfixed? There are already proposed fixes and this issue has been around for nearly 7 years now. 
Filezilla has forwarded the responsibility to us for this issue (, so please, let's just fix it.
msg377413 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2020-09-23 18:23
I have provided necessary infrastructure for TLS 1.2 session resumption in the ssl module a couple of releases ago. But nobody has taken ownership of this ftplib issue and provided a full fix with regression tests and documentation update.

If you like to see ftplib enhanced to support session resumption, please provide a PR with regression tests. Demanding a fix won't accelerate work on the issue.
msg377419 - (view) Author: Lars Schellhas (larsschellhas) Date: 2020-09-23 19:34
@Christian Heimes, you are absolutely right. I'm sorry if I came off rude. 

Actually, I just had a rough day at work with the end of it being the realisation that this missing fix would solve my day-filling issue from today.

Furthermore, I've actually returned here, just to read through the whole thread again, hoping to find a way to contribute to it's resolution.
However, this is my first time contributing to Python, but I'm eager to learn and dive into the necessary resources.
Could you give me a hint about where to start?
Date User Action Args
2022-04-11 14:57:53adminsetgithub: 63699
2020-09-23 19:34:39larsschellhassetmessages: + msg377419
2020-09-23 18:23:35christian.heimessetstage: patch review -> needs patch
messages: + msg377413
versions: + Python 3.10, - Python 3.8
2020-09-23 17:26:50Stephen Ashsetnosy: - Stephen Ash
2020-09-23 16:58:06larsschellhassetnosy: + larsschellhas
messages: + msg377412
2020-05-26 15:21:56Florian Wickertsetnosy: + Florian Wickert
messages: + msg370001
2018-02-26 08:24:16christian.heimessetpriority: high -> low
title: Add client-side SSL session resumption -> ftplib: Add client-side SSL session resumption
versions: + Python 3.8, - Python 3.6
messages: + msg312885

assignee: christian.heimes ->
components: - Documentation, SSL
2017-01-24 15:12:17robrsetfiles: + ftplib_session.patch
nosy: + robr
messages: + msg286190

2016-09-15 07:49:05christian.heimessetcomponents: + SSL
2016-09-10 21:46:20christian.heimessetpriority: normal -> high
assignee: christian.heimes
messages: + msg275704

components: + Documentation
2016-09-10 21:45:02python-devsetnosy: + python-dev
messages: + msg275703
2016-09-09 06:47:38Alex Warhawksetnosy: - Alex Warhawk
2016-09-09 00:23:58Stephen Ashsetnosy: + Stephen Ash
2016-09-08 23:09:48christian.heimeslinkissue25437 superseder
2016-09-08 21:25:28christian.heimessetfiles: + SSL-client-side-SSL-session-resumption.patch

messages: + msg275161
2016-09-08 15:28:50christian.heimeslinkissue8106 superseder
2016-09-08 15:25:17christian.heimeslinkissue24542 superseder
2016-09-07 21:03:12christian.heimessetmessages: + msg274893
2016-09-05 20:19:11christian.heimessetfiles: + SSLSession-support-2.patch
2016-09-05 18:26:27christian.heimessetmessages: + msg274417
2016-09-05 10:33:08christian.heimessetmessages: + msg274396
2016-09-05 07:51:11xiang.zhangsetnosy: + xiang.zhang
messages: + msg274394
2016-09-05 06:23:07christian.heimessetfiles: +

messages: + msg274391
2016-09-04 15:29:08christian.heimessetnosy: + alex

type: behavior -> enhancement
stage: patch review
title: Error when connecting to FTPS servers not supporting SSL session resuming -> Add client-side SSL session resumption
2016-09-04 15:27:36christian.heimessetfiles: + SSLSession-support.patch

messages: + msg274367
2015-10-09 18:51:57Alex Warhawksetmessages: + msg252635
2015-10-08 14:57:57Alex Warhawksetmessages: + msg252543
2015-10-08 10:35:05christian.heimessetmessages: + msg252530
2015-10-08 09:40:46Alex Warhawksetfiles: + implement_ssl_session_reuse_3.6.patch

messages: + msg252527
2015-10-07 11:55:48giampaolo.rodolasetmessages: + msg252468
versions: + Python 3.6, - Python 2.7, Python 3.4, Python 3.5
2015-10-07 11:51:46Alex Warhawksetfiles: + implement_ssl_session_reuse.patch

messages: + msg252467
2015-10-03 11:35:55Alex Warhawksetfiles: + reuse_session.diff

nosy: + Alex Warhawk
messages: + msg252205

keywords: + patch
2014-08-10 23:41:57pitrousetversions: + Python 3.4
2014-08-08 16:29:48Mark.Ribausetnosy: + Mark.Ribau

messages: + msg225078
versions: + Python 2.7
2014-04-17 09:57:05pitrousetnosy: + janssen, christian.heimes, dstufft
messages: + msg216680
2014-04-17 09:42:46pitrousetnosy: + pitrou
messages: + msg216679
2014-04-17 08:40:06giampaolo.rodolasetnosy: + giampaolo.rodola

messages: + msg216674
versions: - Python 3.1, Python 2.7, Python 3.2, Python 3.3, Python 3.4
2013-11-05 04:02:10Ye.Wangcreate