📖 ~1 min read
Table of contents
Symptom & Impact
Dual-stack applications exhibit intermittent failures, often preferring broken IPv6 paths over IPv4.
Environment & Reproduction
Seen after partial IPv6 enablement where routing, DNS AAAA, or firewall policy is incomplete.
Root Cause Analysis
Host advertises IPv6 readiness but cannot consistently route or accept return traffic.
Quick Triage
Test connectivity with ping -6 and compare behavior to IPv4 path to isolate protocol-specific issues.
Step-by-Step Diagnosis
Inspect ip -6 addr, ip -6 route, sysctl net.ipv6.conf.all.disable_ipv6, and firewall rules for ICMPv6 allowances.
Solution – Primary Fix
Correct IPv6 addressing and routes, permit required ICMPv6/firewall traffic, and align DNS records with actual reachability.
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Solution – Alternative Approaches
Temporarily disable IPv6 on affected interface until upstream network and policy readiness is complete.
Verification & Acceptance Criteria
Dual-stack tests pass for DNS, TCP handshakes, and application flows without fallback instability.
Rollback Plan
Restore previous network profile and disable broken IPv6 path if remediation introduces broader connectivity risk.
Prevention & Hardening
Adopt staged IPv6 rollout with explicit validation gates and monitoring by protocol family.
Related Errors & Cross-Refs
Related logs may show “Network is unreachable” for IPv6 despite healthy IPv4 connectivity.
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References & Further Reading
Review Debian IPv6 networking docs and RFC operational guidance for dual-stack deployments.
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