My company uses stunnel on a telecom product and we have run into a strange problem. One of our customers sees an issue every couple weeks where the stunnel process becomes busted. Attempts to establish a connection through stunnel to any host hang indefinitely. Interface stats indicate that stunnel sends and receives a flurry of packets--as if a TLS connection was successfully established--but the local plaintext socket connection to stunnel never passes data. For example, if attempting to run telnet over stunnel, the telnet client just sits there and doesn't write anything to the terminal. During this time the network is otherwise functioning fine.
Unfortunately nothing is logged by stunnel when this happens and we haven't been able to reproduce the problem in a controlled environment. Does anyone have any ideas what might be happening?
We currently use stunnel 4.35 with debug = warning, sslVersion = TLSv1, and verify = 2.