RTFM · Troubleshooting

DNS problems

Names fail while IPs work: work through clients, caches, recursion and delegation, each with a command that isolates it.

Isolate the failing layer

DNS failure lives at exactly one of four layers; query each explicitly.

Drill through the layers

# Direct authoritative answer?
drill @198.51.100.53 example.net SOA

# Our configured resolver answers?
drill example.net

# Full chain visible?
drill -T example.net

cat /etc/resolv.conf

resolv.conf is wrong or rewritten

Hand-edited resolv.conf overwritten at boot means a DHCP client or provisioning script owns it. Fix the source, not the file.

Some client configurations rewrite resolv.conf on every lease renewal. Configure search domains and nameservers centrally, otherwise edits evaporate silently.

Authority vs cache confusion

A cached NXDOMAIN outlives the reason it was created. Distinguish stale cache from genuine absence by querying the authoritative server directly.

Cache versus authority

drill example.net MX                      # our answer + TTLs
drill @<authoritative-ns> example.net MX  # origin truth

Prove it works: resolution recovered

responses match the authoritative source around any recent record changes, and both A and AAAA return expected values.

Running our own DNS?

dnsDragon deployments add their own failure modes: zone transfer failures, DNSSEC signature expiry, and firewall rules blocking UDP/53 inbound. The dedicated chapters cover verification drills for each.

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