Network incident basics: 10 problems
DNS, firewall rules, proxy 502/504, packet captures, and routing — where to look when connections fail.
0/10
1A DNS server is reachable and one zone still failsBeginner6 minFree2Resolver search suffix turns short name into wrong destinationBeginner13 minFree3DNS resolves on the host but not inside the containerBeginner16 minFree4Firewall allow rule exists but interface match is wrongBeginner14 minFree5502 Bad Gateway: Nginx cannot connect to its upstream after an app upgradeBeginner14 minFree6504 Gateway Timeout: a slow export endpoint behind a 60-second proxy timeoutIntermediate16 minFree7Capturing HTTP traffic with tcpdump and reading the request flowBeginner17 minFree8ACL shadow rule blocks load balancer health checks from the SNAT poolIntermediate18 minFree9Only the load balancer health check is blocked by the firewall, dropping the whole serviceIntermediate20 minFree10OSPF neighbors stay in EXSTARTIntermediate21 minFree
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