DNS Resolution Failures and Timeout - How to Fix

DNS Resolution Failures and Timeout - How to Fix

DNS resolution failures and timeouts prevent applications from resolving domain names, causing connectivity issues and service failures. This troubleshooting guide helps you diagnose DNS issues, identify root causes, and fix resolution problems.

For comprehensive DNS monitoring, see DNS Performance Monitoring. For monitoring DNS query times, see Monitor DNS Query Response Times.

Symptoms of DNS Resolution Issues

• DNS query timeouts
• "Name or service not known" errors
• Slow DNS resolution
• Intermittent DNS failures
• Applications unable to resolve domains

Step 1: Test DNS Resolution

Check DNS Resolution

# Test DNS resolution
nslookup example.com

# Test with specific DNS server
nslookup example.com 8.8.8.8

# Test DNS resolution with dig
dig example.com

# Test DNS resolution with host
host example.com

Check DNS Query Time

# Measure DNS query time
time nslookup example.com

# Check query time with dig
dig example.com +stats | grep "Query time"

# Test multiple DNS servers
for dns in 8.8.8.8 1.1.1.1 9.9.9.9; do
  echo -n "$dns: "
  dig @$dns example.com +noall +stats | grep "Query time"
done

Step 2: Check DNS Configuration

Verify DNS Server Configuration

# Check DNS servers in /etc/resolv.conf
cat /etc/resolv.conf

# Check systemd-resolved configuration
systemd-resolve --status

# Check DNS configuration with nmcli
nmcli dev show | grep DNS

# Check DNS cache
systemd-resolve --statistics

Test DNS Server Connectivity

# Test DNS server connectivity
ping -c 3 8.8.8.8

# Test DNS port connectivity
nc -zv 8.8.8.8 53

# Test DNS server response
dig @8.8.8.8 example.com

Step 3: Identify Root Causes

Common Causes of DNS Failures

1. DNS Server Issues:

   • DNS server down or unreachable
   • DNS server overloaded
   • Incorrect DNS server configuration
   • DNS server timeout

2. Network Connectivity Issues:

   • Network connectivity problems
   • Firewall blocking DNS port 53
   • Routing issues
   • Network timeout

3. Configuration Issues:

   • Incorrect DNS server addresses
   • DNS cache issues
   • DNS resolver configuration errors
   • Systemd-resolved conflicts

4. DNS Record Issues:

   • DNS records not propagated
   • Incorrect DNS records
   • DNS zone configuration errors
   • TTL issues

Step 4: Diagnose Specific Issues

Check DNS Server Status

# Test primary DNS server
dig @8.8.8.8 example.com

# Test secondary DNS server
dig @1.1.1.1 example.com

# Check DNS server response time
for dns in 8.8.8.8 1.1.1.1; do
  echo -n "$dns: "
  timeout 5 dig @$dns example.com +noall +stats | grep "Query time" || echo "TIMEOUT"
done

Check Network Connectivity

# Test network connectivity to DNS server
ping -c 3 8.8.8.8

# Test DNS port connectivity
telnet 8.8.8.8 53

# Check firewall rules
sudo iptables -L | grep 53
sudo firewall-cmd --list-all | grep dns

Check DNS Cache

# Flush DNS cache
sudo systemd-resolve --flush-caches

# Check DNS cache statistics
systemd-resolve --statistics

# Clear DNS cache (alternative)
sudo /etc/init.d/nscd restart  # if nscd is used

Step 5: Fix DNS Resolution Issues

Fix DNS Server Configuration

1. Update DNS Servers:

   # Edit /etc/resolv.conf
   sudo nano /etc/resolv.conf
   
   # Add DNS servers
   nameserver 8.8.8.8
   nameserver 1.1.1.1
   

2. Update systemd-resolved:

   # Edit systemd-resolved configuration
   sudo nano /etc/systemd/resolved.conf
   
   # Set DNS servers
   DNS=8.8.8.8 1.1.1.1
   
   # Restart systemd-resolved
   sudo systemctl restart systemd-resolved
   

Fix Network Connectivity

1. Check Firewall Rules:

   # Allow DNS port
   sudo ufw allow 53/tcp
   sudo ufw allow 53/udp
   
   # Or with firewalld
   sudo firewall-cmd --add-service=dns --permanent
   sudo firewall-cmd --reload
   

2. Fix Network Routing:

   # Check routing table
   ip route show
   
   # Test network connectivity
   ping -c 3 8.8.8.8
   

Fix DNS Cache Issues

1. Flush DNS Cache:

   # Flush systemd-resolved cache
   sudo systemd-resolve --flush-caches
   
   # Restart DNS resolver
   sudo systemctl restart systemd-resolved
   

2. Clear Application DNS Cache:

   • Restart applications using DNS
   • Clear browser DNS cache
   • Restart services with DNS dependencies

Step 6: Prevent Future DNS Issues

Implement Proper Monitoring

Set up automated monitoring with Zuzia.app to track DNS resolution continuously:

1. Add DNS Resolution Monitoring:

   dig example.com +noall +stats | grep "Query time" | awk '{print $4}'
   

   • Monitor DNS query times
   • Alert when resolution fails

2. Monitor DNS Server Availability:

   timeout 5 dig @8.8.8.8 example.com && echo "OK" || echo "FAILED"
   

   • Track DNS server availability
   • Alert when servers are unreachable

Best Practices

1. Use Multiple DNS Servers:

   • Configure primary and secondary DNS servers
   • Use reliable DNS providers
   • Test DNS server performance
   • Monitor DNS server availability

2. Monitor DNS Performance:

   • Track DNS query times
   • Monitor DNS resolution success rate
   • Alert on DNS failures
   • Optimize DNS configuration

3. Implement DNS Caching:

   • Use local DNS cache
   • Configure appropriate TTL values
   • Monitor cache performance
   • Clear cache when needed

4. Test DNS Configuration:

   • Verify DNS server connectivity
   • Test DNS resolution regularly
   • Monitor DNS performance
   • Update DNS configuration

FAQ: Common Questions About DNS Issues

How do I prevent DNS resolution failures?

Prevent DNS resolution failures by using multiple DNS servers, monitoring DNS performance, implementing DNS caching, testing DNS configuration regularly, and ensuring network connectivity.

Why do I get DNS timeouts?

DNS timeouts can be caused by DNS server issues, network connectivity problems, firewall blocking DNS port, or DNS server overload. Check DNS server status and network connectivity.

How do I fix DNS cache issues?

Fix DNS cache issues by flushing DNS cache, restarting DNS resolver service, clearing application DNS cache, and ensuring DNS cache configuration is correct.

Can DNS issues impact application performance?

DNS issues can significantly impact application performance, causing slow response times, connection failures, and service unavailability. Monitor DNS performance and resolve issues promptly.

Related guides, recipes, and problems

• Related guides

  • DNS Performance Monitoring - Resolution Speed and Reliability Guide
  • Domain Monitoring Basics - HTTP Checks vs Ping vs DNS

• Related recipes

  • How to Monitor DNS Query Response Times on Linux Server
  • Quick DNS Configuration Check for Linux Server Troubleshooting

• Related problems

  • Network Connectivity Issues on Linux Server
  • Server Slow Performance Issues