📖 ~1 min read
Table of contents
Symptom & Impact
Docker daemon fails to start or containers lose outbound connectivity, affecting deployments.
Environment & Reproduction
Ubuntu 20.04 host with docker-ce, ufw enabled, and custom bridge or iptables/nft settings.
Root Cause Analysis
Firewall backend mismatch, bridge subnet conflicts, or daemon.json misconfiguration blocks networking.
Quick Triage
Run ‘systemctl status docker’, inspect ‘/etc/docker/daemon.json’, and review journalctl daemon errors.
Step-by-Step Diagnosis
Check bridge interfaces, iptables rules, and overlap between Docker subnets and host/VPN routes.
Solution – Primary Fix
Adjust daemon bridge CIDR, align firewall rules, restart docker, and validate container DNS/egress.
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Solution – Alternative Approaches
Use host networking selectively or migrate to user-defined bridge networks with explicit policies.
Verification & Acceptance Criteria
Docker service stays active and test container can resolve DNS and reach required endpoints.
Rollback Plan
Restore previous daemon.json and firewall state if new network plan disrupts production traffic.
Prevention & Hardening
Reserve non-overlapping subnets, version-control daemon config, and monitor container network errors.
Related Errors & Cross-Refs
failed to start daemon, iptables rule programming failed, and bridge address conflicts.
Related tutorial: View the step-by-step tutorial for Ubuntu 20.04 LTS.
View all Ubuntu 20.04 LTS tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Docker on Ubuntu docs, ufw with Docker caveats, and systemd service debugging references.
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