I am not a linux person. I know enough to break things.
I have a pi, MQTT is working perfectly on that I set up all though I do not remember the exact steps (probably a lot of unneeded ones included).
Problem is a serial program I need reboots the pi then it locks it up before it finishes rebooting. I thought it was just my program, but then another (supposedly working) program did the same thing so I started suspecting linux.
I upgraded linux. It never booted again.
So I started fresh. Latest linux (rasbian in all cases) locks up, never finished booting. I dropped back a release. Ok finally, a working Pi again.
Same MQTT program refuses to run and I have tried everything I can think of. Restored an old back up image. Yes, my MQTT clients are fine, but the serial port program is critical.
So it could be many things and I have no idea where to start. What is preventing a working MQTT client from connecting mosquitto mqtt broker? And assuming I am willing to try to same thing I had to do on the OS, drop back a version (or 2 or 3) what command even does that? I looked into apt-get, but apparently old versions are not kept track of on the Pi.
Update: thanks Joan, not quite what I am looking for. I am looking for someone who knows the protocols and what changes have been made in the last 3-4 months that would break a working system. Initial digging looks like a message from the client is now expecting a response, and never receives it. Apparently the same message did not expect a response before (or something like that).