DNS Resolution Failure

# Check if your domain exists in the global DNS hierarchy
dig yourdomain.com +trace

# Verify nameserver delegation from the parent zone
dig yourdomain.com NS +short

# Check nameserver records from the root perspective
dig @a.root-servers.net yourdomain.com NS

# Verify domain registration status (look for registrar info)
whois yourdomain.com | grep -i "expiry|expiration|expires"

# Get list of all your nameservers
NAMESERVERS=$(dig +short yourdomain.com NS)
echo "Your nameservers: $NAMESERVERS"

# Test each nameserver individually with detailed output
for ns in $NAMESERVERS; do
    echo "======== Testing $ns ========"
    
    # Basic connectivity test
    dig @$ns yourdomain.com SOA +timeout=10 +retry=2
done

    # Check if server responds to different record types
    dig @$ns yourdomain.com A +short
    dig @$ns yourdomain.com NS +short
    dig @$ns yourdomain.com MX +short

    # Test query processing time
    time dig @$ns yourdomain.com A > /dev/null
    
    echo ""
done

# Test basic network connectivity to each nameserver
for ns in $(dig +short yourdomain.com NS); do
    echo "Testing connectivity to $ns:"
    
    # Test ICMP reachability (may be blocked by firewalls)
    ping -c 3 $ns
done

    # Test DNS port specifically (UDP 53)
    nc -u -z -v $ns 53
    
    # Test DNS over TCP (port 53) - used for large responses
    nc -z -v $ns 53

    # Show network path to the server
    traceroute $ns | head -10
    
    echo "---"
done

# Test from different network perspectives
echo "Testing via different public DNS resolvers:"
for resolver in 8.8.8.8 1.1.1.1 208.67.222.222; do
    echo "Via $resolver:"
    dig @$resolver yourdomain.com A +short +timeout=5
done

# Get detailed DNS response information
dig @ns1.yourdomain.com yourdomain.com ANY +noall +answer +authority +additional +stats

# Check response codes and flags
dig @ns1.yourdomain.com yourdomain.com A +noall +answer +stats +comments

# Test DNSSEC validation if enabled
dig @ns1.yourdomain.com yourdomain.com A +dnssec +noall +answer +authority

# Check for proper SOA record configuration
dig @ns1.yourdomain.com yourdomain.com SOA +noall +answer +authority

# Test zone transfer capability (should usually be restricted)
dig @ns1.yourdomain.com yourdomain.com AXFR

# Check DNS server logs for errors (BIND example)
tail -50 /var/log/named/named.log | grep -i error
tail -50 /var/log/named/named.log | grep yourdomain.com

# Validate zone file syntax
named-checkzone yourdomain.com /etc/bind/zones/db.yourdomain.com

# Check overall DNS server configuration
named-checkconf /etc/bind/named.conf

# Look for security-related blocks or rate limiting
grep -i "denied|blocked|rate.limit" /var/log/named/security.log

# Check server resource usage and performance
systemctl status named
journalctl -u named -n 50

# For other DNS servers (examples):
# PowerDNS: pdns_control show * | grep yourdomain
# Unbound: unbound-control status

# Test from different geographic DNS resolvers
echo "Testing from various global locations:"

# United States
dig @4.2.2.1 yourdomain.com A +short   # Level3/Lumen (US)
dig @8.8.8.8 yourdomain.com A +short   # Google (Global, US-based)

# Europe  
dig @213.73.91.35 yourdomain.com A +short   # UltraDNS (Europe)

# Asia Pacific
dig @202.12.27.33 yourdomain.com A +short   # University of Tokyo

# Check with your ISP's DNS
dig yourdomain.com A +short

# Use online tools for broader geographic testing
echo "Additional testing via online tools:"
echo "- https://www.whatsmydns.net/#A/yourdomain.com"
echo "- https://dnschecker.org/"  
echo "- https://www.dnswatch.info/"

# Test IPv6 resolution if applicable
dig yourdomain.com AAAA +short

# Test all nameservers respond correctly
for ns in $(dig +short yourdomain.com NS); do
    echo "Verifying $ns:"
    dig @$ns yourdomain.com SOA +short
    dig @$ns yourdomain.com A +short
done

# Clear local DNS caches to ensure fresh results
sudo systemctl flush-dns                    # Linux
sudo dscacheutil -flushcache               # macOS  
ipconfig /flushdns                          # Windows

# Test from multiple external resolvers
for resolver in 8.8.8.8 1.1.1.1 208.67.222.222 9.9.9.9; do
    echo "Testing via $resolver:"
    dig @$resolver yourdomain.com A +short +timeout=10
done

# Monitor resolution consistency over time
for i in {1..10}; do
    dig yourdomain.com A +short
    sleep 30
done

# Document what was fixed for future reference
echo "Resolution fixed on $(date)" >> /var/log/dns-resolution-fixes.log
echo "Issue: [describe the problem]" >> /var/log/dns-resolution-fixes.log
echo "Solution: [describe what was changed]" >> /var/log/dns-resolution-fixes.log