I have a very odd issue that started a few days back on one of our IBM MQ 9.2.0.7 queue managers that runs in a cloud provider environment on a RHEL 8.8 server. Before two days ago, we had no issues having other queue manager servers and our IBM MQ monitor server connect to this server (I will call it dest-srvr). However, a few days back, the IBM MQ monitor server and other queue manager servers can no longer connect to dest-srvr. When a TCP connection is attempted, the client side socket goes into a syn-sent state and there is no evidence (using the ss command) of a server side socket being created on the dest-srvr TCP stack. I ran a tcpdump on the dest-srvr, and I see evidence that the syn is actually making it to dest-srvr. However, there is no subsequent ack being sent back to the source server. Also, I traced the runmqlsr process, and it never receives an accept from the TCP stack on dest-srvr when the connection comes in. I am pretty baffled on what is going on. I did also validate that I can successfully run an openssl s_client directly on dest-srvr against the queue manager port, so the queue manager seems to be working fine here on dest-srvr. I was curious, has anyone else seen something like this issue happen to them, or have insight on what might be the issue here?

To close the loop, I got the issue resolved. It had to do with a firewalld change on the destination RHEL server blocking the connection attempt.