Firewalls
Incorrectly configured firewalls are one of the most prevalent problem areas, particularly for new users. Experienced users are also apt for misunderstanding how firewall rules apply to OpenVPN routed packets. In the past, firewalls were relatively monolithic devices at or near a network edge. Today, however, firewalls exist on client devices, network devices, all along the path of a network packet. Any switch in the path can have ACLs preventing or permitting traffic. This can include both the client- and server-side LANs. The OpenVPN likely has a firewall element for translation or routing traffic, and also for preventing or permitting said traffic.
The monolithic firewall also still exists and can be a pain point when troubleshooting ...
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