DNS Diagnostics with dig and nslookup
Diagnose name-resolution problems on Linux using dig, nslookup, and host to inspect DNS records, trace resolution paths, and spot misconfigurations.
DNS Diagnostics with dig and nslookup is a free Linux Networking & TCP/IP for Developers lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Linux Networking & TCP/IP for Developers learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why DNS Diagnostics Matter
Many 'network is down' reports are really DNS failures. The host is reachable by IP, but the name will not resolve.
Linux ships tools to inspect resolution directly:
dig— flexible, scriptable lookupsnslookup— classic interactive toolhost— quick one-line answers
This lesson focuses on reading their output to find the real problem.
A Basic dig Query
Run dig followed by a hostname to fetch its A record.
The key part of the output is the ANSWER SECTION, which lists the resolved addresses and their TTL.
dig example.comReading the dig Answer Section
Each answer line shows: name TTL class type value.
- TTL — seconds the record may be cached
- type — A, AAAA, MX, CNAME, etc.
- value — the resolved data
Use +short to strip everything except the value.
dig +short example.comQuerying Specific Record Types
Append the record type to look up anything besides an A record.
dig example.com MX— mail serversdig example.com AAAA— IPv6dig example.com TXT— text records (SPF, verification)
dig example.com MXChoosing a Specific Resolver
Use @server to bypass your local resolver and ask a specific DNS server. This isolates whether your local cache is the problem.
Below queries Google Public DNS directly.
dig @8.8.8.8 example.comTracing the Resolution Path
dig +trace walks from the root servers down to the authoritative server, printing each delegation step.
This reveals broken delegations or stale glue records that a normal query hides.
dig +trace example.comReverse DNS Lookups
To map an IP back to a name, use -x. This queries the PTR record.
Missing PTR records often cause mail delivery and logging issues.
dig -x 8.8.8.8Using nslookup Interactively
nslookup can run one-shot or interactively. In interactive mode you set type= and switch servers with server.
nslookup -type=MX example.comQuick Answers with host
host gives compact output, ideal for quick checks.
host example.com— A and AAAAhost -t NS example.com— nameservers
host -t NS example.comInterpreting Status Codes
The status field in the dig header is critical:
- NOERROR — resolved fine
- NXDOMAIN — name does not exist
- SERVFAIL — server failed (often DNSSEC or upstream issues)
Checking the Local Resolver Config
Linux resolution settings live in /etc/resolv.conf and /etc/nsswitch.conf.
If dig works but applications fail, suspect nsswitch.conf ordering or a stale resolv.conf.
cat /etc/resolv.confQuick Check
Test your understanding of dig output.
Recap
You now diagnose DNS issues directly:
digfor flexible, scriptable lookups and+trace@serverto isolate resolver problems-xfor reverse lookups- Read status codes (NOERROR/NXDOMAIN/SERVFAIL)
- Check
/etc/resolv.confandnsswitch.conffor local issues
These skills complement packet capture and firewall analysis for end-to-end troubleshooting.
Frequently asked questions
Is the “DNS Diagnostics with dig and nslookup” lesson free?
Yes — the full text of “DNS Diagnostics with dig and nslookup” is free to read here on the web, and the Linux Networking & TCP/IP for Developers course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Linux Networking & TCP/IP for Developers course, upgrade to CoddyKit PRO.
What will I learn in “DNS Diagnostics with dig and nslookup”?
Diagnose name-resolution problems on Linux using dig, nslookup, and host to inspect DNS records, trace resolution paths, and spot misconfigurations. You practise Linux Networking & TCP/IP for Developers with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Linux Networking & TCP/IP for Developers?
No prior experience is required. Linux Networking & TCP/IP for Developers on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “DNS Diagnostics with dig and nslookup” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Linux Networking & TCP/IP for Developers lesson?
Yes. Every Linux Networking & TCP/IP for Developers lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Packet Capture with Wireshark/tcpdump
- Network Performance Tools
- Linux Firewall (Netfilter/iptables)
- DNS Diagnostics with dig and nslookup