How to Clear DNS Cache on a VPS? Print

DNS cache stores resolved domain name mappings to IP addresses, speeding up subsequent requests. On a Virtual Private Server (VPS), clearing the DNS cache is necessary when domains resolve to outdated IPs, causing connectivity issues, or after DNS configuration changes (e.g., updating records for a website). This guide provides practical steps to clear DNS cache on a Linux VPS (e.g., Ubuntu or CentOS) for common DNS services like systemd-resolved, dnsmasq, or bind.

Why Clear DNS Cache?

• Resolve Connectivity Issues: Fix incorrect domain resolutions due to stale cache.
• Apply DNS Changes: Ensure updated DNS records take effect immediately.
• Troubleshoot Errors: Address issues with websites or services hosted on the VPS.
• Improve Security: Remove potentially corrupted or malicious cache entries.

Prerequisites

• Root Access: Administrative access via SSH or console.
• Linux OS: Ubuntu, CentOS, or another distribution with DNS tools.
• DNS Service Knowledge: Identify if your VPS runs systemd-resolved, dnsmasq, bind, or another DNS resolver.
• Backup: Save DNS configurations before changes to avoid misconfigurations.

Steps to Clear DNS Cache on a Linux VPS

This guide uses Ubuntu 20.04, with notes for CentOS where applicable, and assumes the VPS hosts a web server or application requiring DNS resolution.

Step 1: Access the VPS

• Connect via SSH:

  ssh user@your-vps-ip
  

• If SSH is unavailable, use the provider’s console. For example, VPS.DO’s SolusVM control panel offers VNC console access for direct management.

Step 2: Identify the DNS Resolver

• Check for common DNS services:

  sudo systemctl status systemd-resolved  # Ubuntu
  sudo systemctl status dnsmasq  # Common on lightweight setups
  sudo systemctl status named  # For BIND
  

• If no service is running, the VPS may rely on external resolvers (e.g., /etc/resolv.conf), which typically don’t cache locally.

Step 3: Clear Cache for systemd-resolved (Ubuntu)

• Check if systemd-resolved is active:

  sudo systemctl status systemd-resolved
  

• Clear the cache:

  sudo systemd-resolve --flush-caches
  

• Verify cache is cleared:

  sudo systemd-resolve --statistics
  

  Look for Cache Size: 0 or reduced entries.
• Restart the service if needed:

  sudo systemctl restart systemd-resolved
  

Step 4: Clear Cache for dnsmasq

• Check if dnsmasq is running:

  sudo systemctl status dnsmasq
  

• Clear the cache by restarting the service:

  sudo systemctl restart dnsmasq
  

• Alternatively, send a signal to clear cache without restarting:

  sudo killall -HUP dnsmasq
  

Step 5: Clear Cache for bind (Named)

• Check if BIND is running:

  sudo systemctl status named
  

• Clear the cache:

  sudo rndc flush
  

• Clear cache for a specific domain (optional):

  sudo rndc flushname your-domain.com
  

• Restart BIND if necessary:

  sudo systemctl restart named
  

Step 6: Flush Local Resolver Cache (Fallback)

• If no DNS service is running, clear the local resolver cache (if applicable):

  sudo resolvectl flush-caches  # Modern systems with resolvectl
  

• Check /etc/resolv.conf for external resolvers:

  cat /etc/resolv.conf
  

  If it points to external DNS (e.g., 8.8.8.8), no local cache exists, and clearing is unnecessary.

Step 7: Test DNS Resolution

• Test resolution to verify cache clearing:

  nslookup your-domain.com
  

  Or:

  dig your-domain.com
  

• Ensure the returned IP matches the updated DNS record.
• Monitor connectivity to hosted services (e.g., web server):

  curl http://your-domain.com
  

• Use VPS.DO’s SolusVM dashboard to check bandwidth usage, ensuring DNS tests don’t exceed plan limits (e.g., 1–10 TB for USA plans).

Step 8: Verify with Logs (Optional)

• Check logs for DNS activity:

  sudo journalctl -u systemd-resolved  # Ubuntu
  sudo tail -f /var/log/messages  # CentOS (for BIND or dnsmasq)
  

• Look for cache flush or service restart entries.

Troubleshooting

• Persistent Old IPs: Verify the DNS service was restarted or flushed correctly. Check /etc/resolv.conf for stale nameservers.
• Service Not Found: If no DNS service is running, focus on external resolvers or client-side caches.
• Resolution Failures: Ensure nameservers in /etc/resolv.conf are valid (e.g., 8.8.8.8 or 1.1.1.1).
• Provider Restrictions: Some providers may filter DNS traffic; contact support if resolution issues persist.

Best Practices

• Verify DNS Changes: Confirm updated records with tools like dig or nslookup before clearing cache.
• Monitor Regularly: Schedule periodic cache clearing for dynamic environments:

  echo "0 0 * * * systemd-resolve --flush-caches" | sudo crontab -
  

• Backup Configurations: Save /etc/dnsmasq.conf or /etc/named.conf before changes.
• Use Reliable Nameservers: Set trusted DNS servers (e.g., Google or Cloudflare) in /etc/resolv.conf.
• Secure DNS: Consider enabling DNSSEC for added security if using BIND.

When to Seek Help

If DNS resolution issues persist after clearing the cache, review logs (/var/log/syslog or /var/log/messages) and verify configurations. Contact your VPS provider with details, including test results. Providers typically offer 24/7 support for network-related issues.

Clearing the DNS cache on a VPS ensures accurate domain resolution, maintaining connectivity and reliability for hosted services.