This is an original planning guide. It is not a vendor manual, an official standard, or legal advice. Example addresses use documentation ranges. Review every command before you paste it.
What this hour is for
DNS troubleshooting starts with the name, the type, and the resolver you used. A health check and a lame-delegation check are reads of a zone you administer. They are not a scan of the internet.
Compare the answer from the authoritative server with the answer from the resolver the clients use. If they differ, the TTL and the delegation are the next sentences. Do not change both in one hour.
A lame delegation is a server listed as authoritative that does not answer as such. Write the name of that server. The fix is a zone change later, with the previous records saved.
Do not request a zone transfer. Do not point the tools at a zone you do not run.
The change you can finish
Apply nothing during the writing of the note. When the lab window opens, it is one change on the device named in the ticket. Read the draft, then the device, then decide.
Rollback is the file or the revision you saved first. You do not invent a rollback after the only copy is gone.
- Write the name, the type, and who is complaining.
- Query a resolver the client uses.
- Query the authoritative server you operate.
- Record both answers.
- If they differ, write TTL or delegation, not both fixes.
- Stop before editing the zone.
Mistakes that outlive the command
Troubleshooting DNS Issues goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- An AXFR.
- A zone you do not administer.
- Changing the record and the resolver in one hour.
- A secret from a TXT record pasted into chat.
- Random names until something answers.
Decisions to write down
One decision for Troubleshooting DNS Issues: The zone is yours. Both answers are written. One cause is named. No transfer was requested. And the zone is not edited yet.
Checks before you call it done
Confirm this before you call the change done: The name is in the ticket. Two answers were compared. The difference is one sentence. No AXFR. And the fix waits for a change window. If one of those fails, stop.
One name you operate.
No zone transfer.Before you leave the device
Troubleshooting DNS Issues ends when the hostname on the device matches the hostname in the ticket. If it does not, you are on the wrong box. Stop. A command that succeeded on the wrong box is not progress.
The draft stays a draft until a person has read the names in it. A generator does not log into the device and does not know which interface is the uplink. You delete a line that names a device, a zone, or a prefix you did not write down.
0.0.0.0/0 is not kept because a sample included it. A source you can say is the source you keep. 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. They belong in the note. They are not prefixes you type onto a production device because they appeared in a draft.
prompt-for-this is a reminder that a secret is missing. It is not a password, a key, a community string, or a pre-shared key you save in the ticket or in git. The real secret stays in the store.
One change is the window. A VLAN and a routing protocol and a VPN do not share it. If two things move and the result is wrong, you will not know which one did it. The next feature waits for its own page.
If the same step fails twice, stop for the day. The failure is the result. Adding a forum command beside a half-applied config produces a device nobody can rebuild. Restore the copy you took first.
A second person reads the device name and the change aloud. If they cannot find them in the ticket, the note is not done. Silence is not agreement. Then leave the device alone.
This page is a planning guide. It is not a vendor manual, not an official standard, and not a permission to touch a network you do not administer. The pillar guide owns the wider design. This page owns one lab proof.
Tools for this guide
These tools run in the browser on this site. They do not log into your machines.
Related pillar guides
More in this cluster
The pillar for this page, and the next published pages filed with it.
- Pillar: DNS: The Complete Guide for Admins
- Explainer: DNS Propagation Explained
- Explainer: MX Records Explained
- Explainer: TXT Records Explained
Companion: System: Linux Commands Cheat Sheet.