Debugging DNS Resolution Failures When the Server Itself Is Reachable

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A server can answer pings and still be unreachable by name because DNS resolution is failing, which is often the true cause of the problem. The author outlines a practical troubleshooting sequence, beginning with dig and nslookup to determine whether the resolver or the remote host is at fault. Common culprits include an overwritten /etc/resolv.conf pointing to a dead nameserver, an unreliable internal resolver, systemd-resolved stub confusion, stale or poisoned caches, and firewall rules that block UDP port 53. To prevent recurrence, the author recommends pinning a known-good resolver configuration in systemd-resolved and monitoring DNS query latency alongside ping checks. The full postmortem with packet captures is published on the author's ops notes site.

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ProgrammingDEV Community ·

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ProgrammingDEV Community ·

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ProgrammingDEV Community ·

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