[gnutls-devel] Bug: SNI is ignored when resuming session from cache

Thomas Klute thomas2.klute at uni-dortmund.de
Mon Sep 18 08:28:15 CEST 2017

Am 18.09.2017 um 07:53 schrieb Nikos Mavrogiannopoulos:
> On Fri, Sep 15, 2017 at 3:41 PM, Thomas Klute
> <thomas2.klute at uni-dortmund.de> wrote:
>> Hi,
>> I have discovered that GnuTLS servers ignore the SNI extension when
>> resuming a TLS session from cache (but not when using tickets). The
>> function gnutls_server_name_get() always returns
>> GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE when called on the resumed
>> session, breaking virtual server systems (e.g. virtual hosts with Apache
>> and mod_gnutls).
>> You can reproduce the issue with gnutls-serv and gnutls-cli (paths from
>> dev build):
>> ./src/gnutls-serv --x509keyfile=doc/credentials/x509/key-rsa.pem
>> --x509certfile=doc/credentials/x509/cert-rsa.pem -p 4433 --sni-hostname
>> server.example.com --noticket
>> ./src/gnutls-cli --resume --sni server.example.com -p 4433
>> --x509cafile=doc/credentials/x509/ca.pem localhost
>> The server will warn about the missing server name during resumption:
>>   Warning: client did not include SNI extension, using default host
>> The proper fix appears to be to reclassify SNI as a "mandatory"
>> extension, because if implemented at all it must be parsed on session
>> resumption according to RFC 6066, Section 3:
>>   "A server that implements this extension MUST NOT accept the request
>>   to resume the session if the server_name extension contains a
>>   different name."
>> The attached one line patch changes the parse type of the Server Name
>> extension to GNUTLS_EXT_MANDATORY. For applications that include the
>> server name in their session cache handling this fix is sufficient, but
>> adding a check to the resumption code might still be reasonable.
> Thank you Thomas. That seems like a reasonable fix. Would you like to
> send it as a merge request?

At the moment the one-line parse type change is the only fix I have,
please just apply that so gnutls_server_name_get works with a session
cache. If I have time to add server name checks for resumption I'll look
into Gitlab's merge requests, or maybe just send a normal Git pull request.


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