Network Fault Diagnosis

You will be able to

• Walk a failing connection through link, address, route, name, port and firewall layers, and stop at the first failing one.
• Repair address, gateway, DNS and NetworkManager profile faults persistently.
• Use nc, curl, ss, tcpdump and firewall tools to tell refused, filtered, unreachable and unresolved apart.

Lessons

1. 01Diagnosing a network fault layer by layerTest link, address, route, name, port and application in order, and read refused, timed out and unreachable correctly.ip linkip -br addrip routeip neighpingtracepath0/3 exercises
2. 02Addresses, routes and name resolutionRepair NetworkManager profiles, routes and DNS persistently, and understand which source answers a name.nmcli connection shownmcli connection modifynmcli device statusip route getip route addip addr0/3 exercises
3. 03Firewalls, SELinux ports and proving it with tcpdumpTell a drop from a reject, read firewalld and nftables state, and use tcpdump to show where packets stop.tcpdumptcpdump -nn -ifirewall-cmd --list-allfirewall-cmd --get-active-zonesfirewall-cmd --permanentnft list ruleset0/3 exercises