DNS Troubleshooting — dig, nslookup, and Common Failures

In Lessons 19 and 20 you learned how DNS works and how SIP relies on NAPTR/SRV records to locate services. Now it's time to get your hands dirty. When a customer's calls aren't connecting, DNS is one of the first things you check. This lesson arms you with the commands, patterns, and mental models to diagnose DNS failures fast.

dig — Your Primary DNS Weapon

dig (Domain Information Groper) is the NOC engineer's go-to tool. Unlike nslookup, it shows the full DNS response including flags, authority, and additional sections.

Basic Queries

# A record lookup
dig sip.telnyx.com A

# SRV record for SIP
dig _sip._udp.customer-domain.com SRV

# NAPTR record
dig customer-domain.com NAPTR

# Query a specific nameserver
dig @8.8.8.8 sip.telnyx.com A

# Short output (just the answer)
dig +short sip.telnyx.com A

dig +trace — Follow the Entire Resolution Path

When you suspect a delegation or caching issue, +trace walks the entire resolution chain from the root servers down:

dig +trace sip.telnyx.com A

This outputs each step:

Root servers (.) → refer to .com TLD servers
.com TLD servers → refer to Telnyx's authoritative NS
Telnyx authoritative NS → return the final A record

💡 NOC Tip: +trace stalls at a particular delegation level, you've found your problem. A broken delegation between the registrar and the authoritative nameserver is a common cause of "DNS works from some places but not others."

nslookup — Quick and Cross-Platform

🔒 Full Course Content Available

This is a preview of the Telephony Mastery Course. The complete lesson includes:

✅ Detailed technical explanations
✅ Advanced configuration examples
✅ Hands-on lab exercises
✅ Real-world troubleshooting scenarios
✅ Practice quizzes and assessments

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