One of the most deceptive network faults: ping works, DNS resolves, but SSH hangs after the banner and large downloads stall at a few kilobytes. The culprit is almost always MTU — small packets pass, large ones get silently dropped, and Path MTU Discovery can't recover because the ICMP "fragmentation needed" messages are being filtered (a PMTUD black hole).
Prove it with packet sizes
ping -c1 -s 64 -M do host # small: succeeds ping -c1 -s 1472 -M do host # ~1500B with DF set: "Frag needed" or silence ip link show eth0 | grep -o 'mtu [0-9]*' # e.g. mtu 9000
-M do sets the Don't-Fragment bit, so an oversized packet is dropped rather than fragmented — that's how you force the failure into the open. If the large ping fails while the small one works, you've found your black hole.
Fix the MTU
ip link set eth0 mtu 1500 # back to a path-safe value # make it persist in your network config, and only use jumbo (9000) # when every device on the path is configured for it end-to-end
Practise it
MTU black holes are one of the simulator's trickier networking incidents. Drill them in Linux troubleshooting practice.