Could a firewall be the cause of 'ogg 01223 TCP/IP error 104', and how do I check?

Yes, a firewall is a very common cause. Check the firewall on both the source (e.g., Extract) and destination (e.g., Collector) hosts. Ensure that the firewall allows TCP traffic on the port GoldenGate is configured to use. Use firewall management tools (e.g., `iptables` on Linux, Windows Firewall with Advanced Security) to verify and add rules if necessary. Temporarily disabling the firewall (for testing purposes *only*) can confirm if it's the culprit.