The Domain Name System (DNS) is a critical component of the internet that translates human-friendly domain names (like example.com) into IP addresses that computers use to identify each other on the network. When DNS issues arise, they can disrupt your ability to access websites, send emails, or use online services. This guide provides a comprehensive approach to troubleshooting common DNS issues, helping you identify and resolve problems efficiently.

Understanding DNS and Common DNS Issues

Before diving into troubleshooting steps, it’s important to understand how DNS works and some common issues that might occur:

How DNS Works

When you type a domain name into your browser, your computer sends a DNS query to a DNS server to find out the IP address associated with that domain. If the DNS server has the information cached, it returns the IP address to your computer, allowing the connection to proceed. If the information isn’t cached, the DNS server queries other DNS servers until it finds the correct IP address.

Common DNS Issues

1. DNS Server Not Responding: This occurs when your computer cannot communicate with the DNS server, often resulting in a “DNS server not responding” error.
2. DNS Cache Poisoning: This happens when the DNS cache is corrupted with incorrect information, redirecting you to the wrong website or causing connection failures.
3. Incorrect DNS Configuration: Misconfigured DNS settings can prevent your computer from resolving domain names correctly.
4. ISP DNS Problems: Sometimes, the DNS servers provided by your ISP may experience outages or performance issues.
5. Network Connectivity Issues: Problems with your internet connection can also manifest as DNS errors.
6. Blocked DNS Traffic: DNS works using UDP Port 53. If this port is blocked by any upstream firewalls, gateways, or ISP modems, DNS resolution will fail.

DNS Troubleshooting Steps

Step 1: Verify Network Connectivity

Before troubleshooting DNS, ensure that your network connection is working properly:

• Check Your Connection: Make sure your computer is connected to the internet, either via Wi-Fi or Ethernet.
• Ping a Known IP Address: Use the ping command to check connectivity by pinging a known IP address, such as 8.8.8.8 (Google DNS). If the ping fails, the issue might be with your network connection, not DNS.

Step 2: Clear DNS Cache

Clearing your DNS cache can resolve issues related to cache poisoning or outdated information:

• Windows: Open Command Prompt and run ipconfig /flushdns.
• macOS: Open Terminal and run sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder.
• Linux: The command varies depending on the distribution, but sudo systemctl restart nscd often works.

Step 3: Check Firewalls & ISP Restrictions

DNS works using UDP Port 53. Ensure that this port is not being blocked by any upstream firewalls, gateways, or ISP modems. If this port is blocked, DNS resolution will fail. If necessary, adjust your firewall settings or consult with your ISP to ensure DNS traffic is not being restricted.

Step 4: Change DNS Servers

If your ISP’s DNS servers are slow or unreliable, switching to a public DNS service can improve performance and reliability. It is also recommended to ping multiple DNS servers from different providers to see if one is more responsive than others. Here is a table of common DNS providers and their server IP addresses:

To change DNS servers on:

• Windows: Go to Network and Sharing Center > Change adapter settings > Right-click your network connection > Properties > IPv4 > Properties > Use the following DNS server addresses.
• macOS: Go to System Preferences > Network > Select your network connection > Advanced > DNS > Click the + button to add a new DNS server.
• Router: Log into your router’s web interface, find the DNS settings (often under WAN or Internet settings), and replace the DNS servers with your preferred ones.

Step 5: Test DNS Configuration

Test your DNS configuration to ensure it’s working correctly:

• Use nslookup or dig: These command-line tools help you check DNS resolution.
  • Windows/macOS: Open Command Prompt or Terminal and run nslookup example.com or dig example.com.
• Check Multiple Domains: If only one domain is failing, the issue might be with that domain’s DNS settings rather than your own.

Step 6: Restart Network Equipment

Sometimes, a simple restart of your network equipment can resolve DNS issues:

• Restart Your Router/Modem: Power off your router and modem for about 30 seconds, then power them back on.
• Restart Your Computer: Reboot your computer to reset the network settings.

Step 7: Inspect Hosts File

The hosts file on your computer can override DNS settings and direct certain domain names to specific IP addresses. If this file is misconfigured, it can cause DNS resolution issues:

• Windows: The hosts file is located at C:\Windows\System32\drivers\etc\hosts.
• macOS/Linux: The hosts file is located at /etc/hosts.
• Check for Incorrect Entries: Open the hosts file with a text editor and look for any incorrect or suspicious entries. Remove or correct these entries as needed.

Step 8: Reconfigure DNS Settings in Network Devices

For network devices, ensuring correct DNS configuration is critical for services like updating and remote management. Depending on your setup:

• UniFi Cloud Gateways: Navigate to UniFi Network > Settings > Internet > DNS Server and enter the new DNS Server.
• CloudKeys, Network Video Recorders & Other Non-Gateway Consoles: Navigate to UniFi OS > Console Settings and check if the IP Configuration is set to DHCP or Static.
  • If Static, enter the new DNS Server and select Apply Changes.
  • If DHCP, modify the DNS Server directly from your DHCP server (found in UniFi Network > Settings > Networks > [Network Name] > DHCP Service Management > DNS Server).

Step 9: Disable VPN or Proxy

Well…. This should’ve been an earlier step but it won’t apply to most users. If you’re using a VPN or proxy service, it might be affecting your DNS resolution:

• Disable VPN/Proxy: Temporarily disable your VPN or proxy service and test your DNS resolution. If the problem is resolved, the issue may lie with the VPN/proxy configuration.

Step 10: Test on a Different Network

To isolate the issue, try connecting your device to a different network (e.g., a mobile hotspot or a neighbor’s Wi-Fi):

• Different Network Test: If DNS works on a different network, the issue might be with your home network or ISP.

Step 11: Advanced DNS Troubleshooting

Analyze DNS Traffic with Wireshark

For advanced users, analyzing DNS traffic using a tool like Wireshark can help identify the root cause of DNS issues:

• Capture DNS Traffic: Use Wireshark to capture and analyze DNS traffic on your network. Look for any failed DNS queries or unusual activity.

Configure a Local DNS Server

In cases where you need more control over DNS resolution, setting up a local DNS server might be beneficial:

• Local DNS Server: Tools like BIND or dnsmasq can be configured to serve as a local DNS server, offering greater control over DNS queries and caching.

Step 12: Contact Your ISP

If all else fails, the issue might be with your ISP:

• ISP Support: Contact your ISP to see if there are any known DNS issues in your area. They may also offer further troubleshooting or guidance.

Final Thoughts

DNS issues can be frustrating, but by following these troubleshooting steps, you can usually identify and resolve the problem. Whether it’s a simple configuration error or a more complex network issue, taking a systematic approach will help you restore DNS functionality and ensure smooth internet access.

If DNS problems persist despite troubleshooting, consider consulting with a network professional or your ISP for further assistance.

Need more help with DNS issues? Contact our support team or explore our advanced DNS solutions.

This article is part of our series on network troubleshooting. Stay tuned for more tips and best practices.