[stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections?

Lynch, Andrew andrew.lynch at atos.net
Fri Sep 27 09:23:29 CEST 2019

Hi Eric,

Thank you for your suggestion.  It is certainly worth looking into, although I suspect it may be impractical in our environment.

Now we have a single stunnel.conf with 70 services which would translate into 70 additional entries in inetd.conf.  Each of those entries then needs to have its own reduced stunnel.conf so that it will only load the cert and key for its endpoint.

The stunnel HOWTO recommends daemon mode over inetd mode due to the overhead for forking and SSL initialization.  We would have additional overhead for engine init and HSM connection.


-----Original Message-----
From: Eric Eberhard [mailto:flash at vicsmba.com] 
Sent: Thursday, September 26, 2019 7:20 PM
To: Lynch, Andrew; stunnel-users at stunnel.org
Subject: RE: [stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections?

Try running in inetd mode -- even if you don't like this you will learn something.  Inetd will close connections as needed.  E

-----Original Message-----
From: stunnel-users [mailto:stunnel-users-bounces at stunnel.org] On Behalf Of Lynch, Andrew
Sent: Thursday, September 26, 2019 5:56 AM
To: stunnel-users at stunnel.org
Subject: [stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections?


We are using stunnel as a server to terminate incoming TLS connections.  The config has around 70 services with certificates whose EC private keys are stored in an HSM and accessed using an OpenSSL engine.

Over time, after numerous reloads of stunnel (kill -HUP) the HSM reports that its connection table is full.  Logging from the engine shows that stunnel is never freeing the keys and therefore the engine is not closing the associated sessions with the HSM.  Each stunnel reload opens 70 new sessions until eventually the HSM's configured limit is exceeded.

This behaviour has been observed on Suse Enterprise Linux 12.3 with the system-provided stunnel-5.00-4.3.4, but I can reproduce it with my own build of the current version 5.55.

Is this a known issue?  It appears that other (ephemeral) keys are being freed, just not those associated with the service certificates.

Currently our workaround is to perform a full restart instead of a reload.
This closes all HSM sessions when the process terminates, but of course it also kills any open client connections so it can only be done during the scheduled maintenance windows.


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