Duplex mismatch: the fault that hides in plain sight
The link is up, the speed is right, and throughput is terrible. Check the counters, not the status.
Platform
Linux
Debian, Ubuntu, RHEL and their relatives. The commands are standard unless the article says otherwise.
Asymmetric routing and the return path that betrays you
Traffic goes out one way and comes back another. Everything works until a stateful device is involved.
MTU and the packets that vanish
Small things work, large things hang, and nothing is logged. That pattern has one usual cause.
Why ping succeeds and the application still fails
A successful ping proves less than people think. Here is exactly what it proves, and what it does not.