Four echo requests reached the Cisco router ingress, the server saw none and one ACL deny counter rose by four. Captures at three points and the counter located the drop; removing the ACL attachment fixed it.
Reordering produced 378 TCP retransmissions with zero queue drops, and deleting one frame from a healthy capture made Wireshark flag an unseen segment. Real loss showed only when a segment was missing from the receiver trace.
A 10 Mbit/s path delivered 9.56 Mbit/s while loaded ping RTT rose to a 325 ms median from 0.071 ms idle. Read throughput, latency and queue backlog on one timeline, then verify RTT drops back when the load stops.
50 ms of netem delay added about 50 ms of RTT, not 100, because only one direction crossed it. Ten percent loss dropped 12 probes, and reordering returned all 40 replies with zero drops. Read the queue counters with each result.
Build a bridged LAN and routed server path with Linux namespaces and veth pairs. Follow ARP across the bridge and a packet through the router, then delete one default route: the client fails at its own route lookup while its LAN peer still answers.
The trunk was up and allowed VLANs 10 and 20, yet the Linux host could not ping its gateway. Its ARP request carried a VLAN 20 tag and the switch learned its MAC in VLAN 20, while the gateway was on VLAN 10.
The three-way handshake is the first thing anyone learns about TCP and the last thing most people actually look at. SYN, SYN-ACK, ACK is easy to draw. What is harder, and far more useful, is reading a real
Discover, Offer, Request, Ack. You can recite DORA in your sleep, and it still will not help you the day a client comes up with a 169.254 address and the server insists the pool is fine. What helps is
Every OSPF adjacency you have ever built started with a Hello packet, and almost every adjacency that refuses to build is a Hello packet being quietly thrown away. The trouble is that show ip ospf neighbor only tells you the
Two monitoring platforms, one Ubuntu box, zero port collisions. Plus the built-in host that cannot reach its own agent, and why apt was not an option.
From an empty Ubuntu box to a Grafana login screen: APT repo, systemd, and the 20-second gotcha that makes people think the install failed.
Ten printable pages of Linux networking commands, flags and real lab output, including a ten-symptom troubleshooting decision tree. Free for members.