Linux DNS Troubleshooting — systemd-resolved, /etc/resolv.conf, and DNS Debugging

Fix Linux DNS resolution issues including systemd-resolved conflicts, /etc/resolv.conf management, DNS-over-HTTPS setup, and debugging DNS with dig and nslookup. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Modern Linux uses systemd-resolved for DNS management — /etc/resolv.conf is often a symlink. Multiple DNS resolution paths can conflict: systemd-resolved, NetworkManager, manual /etc/resolv.conf. systemd-resolved listens on 127.0.0.53 which can confuse applications expecting a real DNS server. dig and nslookup bypass systemd-resolved and query DNS servers directly. resolvectl (or systemd-resolve) shows the current DNS configuration per network interface. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: /etc/resolv.conf overwritten by NetworkManager, VPN, or DHCP client. systemd-resolved not running or misconfigured. VPN client replacing DNS settings, breaking resolution after disconnection. Multiple DNS management systems conflicting (resolved + NetworkManager + manual). DNS server unreachable or returning SERVFAIL for specific domains. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Check current DNS: resolvectl status to see DNS servers per interface. Test resolution: dig example.com (uses system DNS) and dig @8.8.8.8 example.com (bypass local). Check resolv.conf: ls -la /etc/resolv.conf to see if it is a symlink and where it points. Set DNS manually: resolvectl dns eth0 8.8.8.8 1.1.1.1 for systemd-resolved. Fix for containers/Docker: add DNS=8.8.8.8 to /etc/systemd/resolved.conf and restart systemd-resolved. Flush DNS cache: resolvectl flush-caches. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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