Hi,
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.
Regards, Andrew.