Ubuntu 18.04 LTS Docker containers cannot reach the internet

📖 ~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

Containerized apps cannot download dependencies or reach APIs.

Environment & Reproduction

Ubuntu host running Docker with custom iptables and UFW rules.

Root Cause Analysis

NAT forwarding chain conflicts block outbound packets from docker0.

Quick Triage

Test host egress, container DNS, and bridge interface state.

Step-by-Step Diagnosis

Inspect iptables nat table and routing policy for container CIDRs.

Solution – Primary Fix

Recreate bridge networking and allow forwarding/NAT rules.

Solution – Alternative Approaches

Use host networking for affected workloads as temporary workaround.

Verification & Acceptance Criteria

Containers resolve DNS and complete outbound HTTP requests.

Rollback Plan

Restore saved firewall config and docker daemon settings.

Prevention & Hardening

Document firewall ownership and persist rule ordering.

Related Errors & Cross-Refs

Differentiate MTU mismatch symptoms from pure firewall blocks.

Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.

View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Docker networking docs and Ubuntu firewall integration notes.