DNS Troubleshooting Guide
Step-by-step solutions for common DNS problems and diagnostic techniques
5-Step DNS Diagnostic Process
1
Identify the Problem
Gather specific error messages, symptoms, and affected services
- Document exact error messages
- Note when the issue started
- Identify affected domains/subdomains
2
Check DNS Records
Verify that all necessary DNS records are properly configured
- Use dig or nslookup to check records
- Verify record syntax and values
- Check from multiple DNS servers
3
Test Propagation
Ensure DNS changes have propagated globally
- Use online propagation checkers
- Test from different geographic locations
- Check with multiple DNS resolvers
4
Validate Configuration
Ensure nameservers and zone files are correctly configured
- Verify nameserver delegation
- Check zone file syntax
- Confirm SOA record parameters
5
Monitor and Test
Implement monitoring and perform comprehensive testing
- Set up DNS monitoring alerts
- Test from end-user perspective
- Document resolution for future reference
Common DNS Issues & Solutions
DNS Propagation Delays
Symptoms
- • Changes not visible globally
- • Inconsistent results from different locations
- • Old records still resolving
Common Causes
- • High TTL values
- • ISP DNS caching
- • CDN edge caching
Solutions
- • Wait for TTL expiration (check original TTL)
- • Use different DNS resolvers for testing
- • Clear local DNS cache
- • Use DNS propagation checker tools
Diagnostic Commands
nslookup domain.com 8.8.8.8dig @1.1.1.1 domain.comipconfig /flushdns (Windows)sudo dscacheutil -flushcache (macOS)Website Not Loading
Symptoms
- • Domain not found errors
- • Connection timeouts
- • Intermittent loading issues
Common Causes
- • Missing A/AAAA records
- • Incorrect DNS configuration
- • Server downtime
Solutions
- • Verify A and AAAA records exist
- • Check nameserver configuration
- • Test with direct IP access
- • Verify server is responding
Diagnostic Commands
nslookup domain.comdig domain.com Aping domain.comtraceroute domain.comEmail Delivery Problems
Symptoms
- • Emails bouncing back
- • Emails going to spam
- • Cannot send emails
Common Causes
- • Missing/incorrect MX records
- • No SPF record
- • Missing DKIM/DMARC
Solutions
- • Verify MX records are configured
- • Add SPF record for sender validation
- • Configure DKIM and DMARC
- • Check reverse DNS (PTR) records
Diagnostic Commands
dig domain.com MXdig domain.com TXT (check SPF)nslookup -type=mx domain.comdig 1.2.3.4.in-addr.arpa PTRSubdomain Resolution Issues
Symptoms
- • Subdomain not found
- • www not working
- • Wildcard issues
Common Causes
- • Missing CNAME/A records
- • Incorrect delegation
- • Wildcard misconfiguration
Solutions
- • Add appropriate CNAME or A records
- • Verify subdomain delegation
- • Check wildcard record syntax
- • Test with different record types
Diagnostic Commands
dig subdomain.domain.comnslookup www.domain.comdig *.domain.comdig subdomain.domain.com NSDNSSEC Validation Failures
Symptoms
- • DNSSEC validation errors
- • SERVFAIL responses
- • Security warnings
Common Causes
- • Broken DNSSEC chain
- • Expired signatures
- • Clock synchronization
Solutions
- • Check DNSSEC chain integrity
- • Verify signature expiration dates
- • Ensure proper key rollover
- • Check server time synchronization
Diagnostic Commands
dig domain.com +dnssecdig domain.com DNSKEYdig domain.com DSdelv domain.comSlow DNS Resolution
Symptoms
- • Long page load times
- • DNS lookup timeouts
- • High latency
Common Causes
- • Slow DNS servers
- • Network issues
- • Heavy DNS load
Solutions
- • Switch to faster DNS resolvers
- • Use DNS performance monitoring
- • Implement DNS caching
- • Optimize TTL values
Diagnostic Commands
dig domain.com +tracenslookup -debug domain.comtime dig domain.comping nameserver.comEssential DNS Diagnostic Tools
dig
Powerful DNS lookup tool with detailed output
Example Commands:
dig domain.comdig @8.8.8.8 domain.com MXdig +trace domain.comnslookup
Interactive DNS lookup tool available on most systems
Example Commands:
nslookup domain.comnslookup -type=mx domain.comnslookup domain.com 1.1.1.1host
Simple DNS lookup utility (Linux/macOS)
Example Commands:
host domain.comhost -t mx domain.comhost -a domain.comping
Test basic connectivity and DNS resolution
Example Commands:
ping domain.comping -c 4 domain.comping6 domain.comtraceroute
Trace network path to destination
Example Commands:
traceroute domain.comtracepath domain.commtr domain.comwhois
Check domain registration and nameserver information
Example Commands:
whois domain.comwhois -h whois.iana.org domain.comQuick Reference Commands
DNS Cache Management
Windows:
ipconfig /flushdnsmacOS:
sudo dscacheutil -flushcacheLinux:
sudo systemctl restart systemd-resolvedTesting Different Resolvers
Cloudflare:
dig @1.1.1.1 domain.comGoogle:
dig @8.8.8.8 domain.comQuad9:
dig @9.9.9.9 domain.comTest Your DNS Configuration
Use our DNS diagnostic tools to identify and resolve DNS issues across your domains and services.