Ubuntu 26.04 LTS – cloud-init rewrites netplan on reboot unexpectedly – 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

Manual network changes disappear after reboot, causing interface drift and connectivity loss.

Environment & Reproduction

Common on cloud images where cloud-init continues to manage networking.

cloud-init status
ls /etc/cloud/cloud.cfg.d
ls /etc/netplan

Root Cause Analysis

Datasource-provided network config regenerates netplan files, replacing manual edits.

Quick Triage

Confirm which component last modified netplan files.

sudo ls -ltr /etc/netplan
sudo grep -RIn 'network:' /etc/cloud/cloud.cfg.d /etc/cloud/cloud.cfg 2>/dev/null
journalctl -u cloud-init -n 120 --no-pager

Step-by-Step Diagnosis

Trace cloud-init stages and datasource metadata influencing network output.

cloud-init analyze show 2>/dev/null || true
cloud-init query ds 2>/dev/null || true
cat /run/cloud-init/network-config.json 2>/dev/null || true

Solution – Primary Fix

Disable cloud-init network management and persist custom netplan configuration.

sudo tee /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg >/dev/null <<'EOF'
network: {config: disabled}
EOF
sudo netplan generate
sudo netplan apply

Solution – Alternative Approaches

Keep cloud-init enabled but supply stable network-data in metadata for immutable provisioning.

sudo cloud-init clean
sudo cloud-init init

Verification & Acceptance Criteria

Netplan files remain unchanged across reboot and interfaces keep expected settings.

sudo sha256sum /etc/netplan/*.yaml
sudo reboot
sudo sha256sum /etc/netplan/*.yaml

Rollback Plan

Remove disable file if platform requires datasource-managed networking.

sudo rm -f /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg
sudo cloud-init clean
sudo reboot

Prevention & Hardening

Decide ownership model for networking during image build and enforce it consistently.

cloud-init status
grep -RIn 'config: disabled' /etc/cloud/cloud.cfg.d

Related Errors & Cross-Refs

Tied to DHCP lease churn, wrong interface names, and boot-time renderer conflicts.

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

Ubuntu cloud-init network configuration references and operational best practices.