DNS Records: Mail Cannot Find Its Server
Web by name works, but mail for the domain bounces. Walk the MX, A, AAAA, CNAME, PTR and NS records with dig and name the broken one. Work through it yourself in the live host terminal below.
Symptom
Web by name works, but mail for the domain bounces. Walk the MX, A, AAAA, CNAME, PTR and NS records with dig and name the broken one.
Your mission
Inspect, recover, then verify
Format
Linux shell
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: Linux
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
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Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
External senders report that mail to @lab.example bounces with a host-not-found error. www.lab.example works normally. The mail server itself is up at 203.0.113.25.
What you'll practice
- Read an MX record and its preference value
- Recognise NXDOMAIN and the SOA in the authority section
- Follow a CNAME to its A record
- Use a PTR lookup to identify a host by address
- Find the authoritative servers with an NS query
The topology
One Ubuntu admin host uses DNS server 10.20.30.53, which is authoritative for lab.example. The mail server's real name is mx1.lab.example at 203.0.113.25.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
dig lab.example MX +shortdig mail.lab.example Adig www.lab.example +shortTopology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Why does +short print nothing for a missing name?
+short prints only the answer section, and an NXDOMAIN response has no answers. Run the same query without +short to see status: NXDOMAIN and the zone's SOA record in the authority section.
What does the 10 in the MX record mean?
It is the preference. When a domain has several MX records, senders try the lowest number first.
Why can't I fix this from the admin host?
Records live on the authoritative servers for the zone. The NS query tells you which servers those are, so the change can be made there.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.