📖 ~1 min read
Table of contents
Symptom & Impact
Interfaces flap or lose routes because two network management frameworks compete on Debian 11.
Environment & Reproduction
IP addresses disappear, default route changes unexpectedly, and connectivity is intermittent.
Root Cause Analysis
Use `ip a`, `ip r`, `systemctl status NetworkManager`, and review `/etc/network/interfaces` for overlapping ownership.
Quick Triage
Both ifupdown and NetworkManager attempt to configure the same device definitions.
Step-by-Step Diagnosis
Standardize on one network stack, mark unmanaged interfaces explicitly, and restart only the chosen manager.
Solution – Primary Fix
Confirm stable addressing with repeated `ip a` and persistent gateway via `ip route show default`.
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Solution – Alternative Approaches
Document interface ownership and enforce baseline network templates per host role.
Verification & Acceptance Criteria
Reapply previous interfaces file and service enablement states from configuration backup.
Rollback Plan
Run periodic route consistency checks and alert when default route or DNS settings drift.
Prevention & Hardening
`ip a`; `ip r`; `systemctl status NetworkManager`; `grep -v ‘^#’ /etc/network/interfaces`
Related Errors & Cross-Refs
Share interface configs, service states, and timing of route changes for netops troubleshooting.
Related tutorial: View the step-by-step tutorial for debian-11.
View all debian-11 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Cloud-init can also overwrite network settings if not coordinated with local management tools.
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