📖 ~1 min read
Table of contents
Symptom & Impact
Apt update reports repository signature failures and metadata is rejected.
Environment & Reproduction
Debian 9 systems include third-party repositories with aging signing keys.
Root Cause Analysis
Expired, rotated, or missing keys break Secure APT verification checks.
Quick Triage
Capture failing key IDs and map them to source list entries quickly.
Step-by-Step Diagnosis
Inspect keyrings, expiry dates, and signed-by mappings for each repository.
Solution – Primary Fix
Install current vendor keys, update trust configuration, and refresh repository metadata.
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Solution – Alternative Approaches
Temporarily disable affected external repositories until key rotation is complete.
Verification & Acceptance Criteria
Apt update finishes without NO_PUBKEY, EXPKEYSIG, or signature mismatch errors.
Rollback Plan
Restore previous source list files if package availability is impacted.
Prevention & Hardening
Track key expiry in monitoring and document ownership for key rotations.
Related Errors & Cross-Refs
Related to stale Release files and mirror sync inconsistencies.
Related tutorial: View the step-by-step tutorial for debian-9.
View all debian-9 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Debian Secure APT guidance and repository key management references.
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