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Title: Provide knobs to disable session ticket generation on TLS 1.3
Type: enhancement Stage: resolved
Components: SSL Versions: Python 3.9, Python 3.8
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: christian.heimes Nosy List: alex, christian.heimes, dstufft, janssen, miss-islington, njs, vstinner
Priority: normal Keywords: patch

Created on 2019-06-01 05:05 by njs, last changed 2022-04-11 14:59 by admin. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 13719 merged christian.heimes, 2019-06-01 07:33
PR 14668 merged vstinner, 2019-07-09 11:11
PR 14671 merged miss-islington, 2019-07-09 11:42
Messages (8)
msg344148 - (view) Author: Nathaniel Smith (njs) * (Python committer) Date: 2019-06-01 05:05
Maybe we should expose the SSL_CTX_set_num_tickets function:

This is a new function added in OpenSSL 1.1.1, that lets you control the number of tickets issued by a TLS 1.3 connection.

It turns out that by default, openssl 1.1.1 issues 2 tickets at the end of the server handshake. In principle this can cause deadlocks and data corruption:

And my problem specifically is that this pretty much kills all of Trio's fancy protocol testing helpers, because any protocol that's built on top of TLS is automatically broken as far as the helpers are concerned. And they're right. Right now I have to disable TLS 1.3 entirely to get Trio's test suite to avoid deadlocking.

Hopefully the openssl devs will fix this, but so far their latest comment is that they consider this a feature and so they think it has to stay broken for at least RHEL 8's lifetime.

Currently the only workaround is to either disable TLS 1.3, or disable tickets. But the 'ssl' module doesn't expose any way to control tickets. This issue proposes to add that.

Fortunately, exposing SSL_CTX_set_num_tickets should be pretty trivial, at least in theory.


Do we have a preferred convention for how to expose these kinds of settings at the Python level? Attribute on SSLContext?

There's both an SSL* and a SSL_CTX* – I guess the CTX version is sufficient, or is there another convention?

As a bonus complication, openssl sometimes ignores the configured number of tickets, and uses a completely different mechanism:

> The default number of tickets is 2; the default number of tickets sent following a resumption handshake is 1 but this cannot be changed using these functions. The number of tickets following a resumption handshake can be reduced to 0 using custom session ticket callbacks (see

Do we want to open the can-of-worms involved in wrapping this too? I think if we only wrapped SSL_CTX_set_num_tickets then that would be enough to let me kluge my tests into passing and pretend that things are just fine, so long as we don't test session resumption...
msg344156 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2019-06-01 07:44
+1 for the idea

Yes, for simple flags and settings, an attribute on the SSLContext is prefer. The SSLContext object is the configuration space for its connections. I would prefer to keep the setting only on the context and not clutter SSLSocket and SSLObject with additional attributes. PR 13719 goes one step further and restricts the setter to a PROTOCOL_TLS_SERVER, too.

Let's look in the callback another time. I won't be able to come up with a wrapper for that in the next three days.
msg344463 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2019-06-03 19:00
New changeset 78c7d527799dacca91b9ed67057cb996efe526b0 by Christian Heimes in branch 'master':
bpo-37120: Add SSLContext.num_tickets (GH-13719)
msg344466 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2019-06-03 19:02
I have merged the PR to land the feature in time for the feature freeze.

Regarding your comment on client_context.num_ticket getter: IMHO it's not a good idea to raise an exception on attribute access. It may break introspection.
msg344677 - (view) Author: Nathaniel Smith (njs) * (Python committer) Date: 2019-06-05 06:33
> Regarding your comment on client_context.num_ticket getter: IMHO it's not a good idea to raise an exception on attribute access. It may break introspection.

Hmm, I see what you mean.

Basically my intuition is: it's a bit weird to make the attribute's existence "sort of" depend on whether it's a client or server context. It would make sense to me to have it entirely disappear on client contexts (from __dir__, read-access, and write-access), and it would make sense to me to have it always be present, just a no-op. But having it present, and almost the same as on server contexts, except that assigning to it fails... that feels a little weird.
msg345914 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2019-06-17 21:29
New changeset 78c7d527799dacca91b9ed67057cb996efe526b0 by Christian Heimes in branch 'master':
bpo-37120: Add SSLContext.num_tickets (GH-13719)

This change introduced this warning on Windows:

c:\vstinner\python\master\modules\_ssl.c(3624): warning C4267: 'function': conv 
ersion from 'size_t' to 'long', possible loss of data [C:\vstinner\python\maste 

SSL_CTX_get_num_tickets() return type is size_t. I suggest this change:

diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 2331c58ad7..3ffb6380d3 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -3621,7 +3621,7 @@ set_maximum_version(PySSLContext *self, PyObject *arg, void *c)
 static PyObject *
 get_num_tickets(PySSLContext *self, void *c)
-    return PyLong_FromLong(SSL_CTX_get_num_tickets(self->ctx));
+    return PyLong_FromSize_t(SSL_CTX_get_num_tickets(self->ctx));
 static int
msg347548 - (view) Author: miss-islington (miss-islington) Date: 2019-07-09 11:30
New changeset 76611c7c0af6b2f4d0d98a5db827d34cff54ce25 by Miss Islington (bot) (Victor Stinner) in branch 'master':
bpo-37120: Fix _ssl get_num_tickets() (GH-14668)
msg347557 - (view) Author: miss-islington (miss-islington) Date: 2019-07-09 12:43
New changeset bbad695e7890513be7a9bc662e2d8ae13bfcd313 by Miss Islington (bot) in branch '3.8':
bpo-37120: Fix _ssl get_num_tickets() (GH-14668)
Date User Action Args
2022-04-11 14:59:16adminsetgithub: 81301
2021-04-19 19:59:46christian.heimessetstatus: open -> closed
stage: patch review -> resolved
2019-07-09 12:43:03miss-islingtonsetmessages: + msg347557
2019-07-09 11:42:16miss-islingtonsetpull_requests: + pull_request14479
2019-07-09 11:30:55miss-islingtonsetnosy: + miss-islington
messages: + msg347548
2019-07-09 11:11:51vstinnersetstage: commit review -> patch review
pull_requests: + pull_request14476
2019-06-17 21:29:28vstinnersetnosy: + vstinner
messages: + msg345914
2019-06-05 06:33:51njssetstatus: pending -> open

messages: + msg344677
2019-06-03 19:03:03christian.heimessetstatus: open -> pending
resolution: fixed
stage: patch review -> commit review
2019-06-03 19:02:50christian.heimessetmessages: + msg344466
2019-06-03 19:00:13christian.heimessetmessages: + msg344463
2019-06-01 07:44:03christian.heimessetmessages: + msg344156
2019-06-01 07:33:22christian.heimessetkeywords: + patch
stage: patch review
pull_requests: + pull_request13607
2019-06-01 05:05:29njscreate