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I run into a similar situation testing OPNsense running in a Hyper-V VM. Running an iperf3 client with multiple parallel streams (-P option) will cause traffic to stop flowing through OPNSense VM. Trying to ping any IP address though the outside interface returns ping: sendto: No buffer space available. Bringing the interface down and up restores connectivity. I was running into the same issue when running an earlier version of OPNSense on a physical box (Intel N3150 CPU, Dual RTL8111/8168/8411, 4GB RAM)
