Ubuntu 26.04 LTS – netplan apply does not update DNS settings – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Custom DNS entries in netplan are ignored, causing failed name resolution and service outages.

Environment & Reproduction

Appears after editing YAML with invalid indentation or conflicting renderer settings.

ls /etc/netplan
cat /etc/netplan/*.yaml
networkctl status
resolvectl status

Root Cause Analysis

Netplan validation passes partially, but DNS is overridden by NetworkManager, systemd-resolved, or cloud-init.

Quick Triage

Confirm renderer and effective resolver source before reapplying.

sudo netplan get
cat /etc/systemd/resolved.conf
ls -l /etc/resolv.conf
resolvectl dns

Step-by-Step Diagnosis

Generate debug output and inspect whether final config includes intended nameservers.

sudo netplan generate --debug
sudo netplan try
resolvectl status
journalctl -u systemd-resolved -n 60 --no-pager

Solution – Primary Fix

Correct netplan YAML, apply, and restart resolver services in controlled order.

sudoedit /etc/netplan/01-netcfg.yaml
sudo netplan generate
sudo netplan apply
sudo systemctl restart systemd-resolved
resolvectl flush-caches

Solution – Alternative Approaches

If cloud-init keeps overriding network config, disable its network rendering.

printf 'network: {config: disabled}
' | sudo tee /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg
sudo netplan apply
sudo reboot

Verification & Acceptance Criteria

Accepted when nameserver list and DNS query results match intended configuration.

resolvectl dns
resolvectl query archive.ubuntu.com
ping -c 2 security.ubuntu.com

Rollback Plan

Restore previous YAML and reapply if new config causes packet loss or DNS outage.

sudo cp -a /etc/netplan /root/netplan.rollback.$(date +%s)
sudo cp /root/netplan.backup.yaml /etc/netplan/01-netcfg.yaml
sudo netplan apply

Prevention & Hardening

Version-control netplan files and validate syntax in CI before production rollout.

sudo apt install yamllint -y
yamllint /etc/netplan/*.yaml
sudo netplan generate --debug

Related Errors & Cross-Refs

Related errors include YAML parser exceptions, temporary failure in name resolution, and fallback DNS server usage.

Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.

View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Netplan documentation, systemd-resolved manual, and Ubuntu cloud-init networking notes.