🟠 High   ⏱ 5–30 min  Last verified: 19 May 2026
Affected versions: Debian 11

πŸ“– ~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

Hostname lookups fail, breaking package updates and external service calls.

Environment & Reproduction

Observed after resolver edits, VPN profile changes, or DHCP updates.

resolvectl status
getent hosts deb.debian.org
cat /etc/resolv.conf

Root Cause Analysis

Incorrect nameserver entries, stale stub resolver links, or per-link DNS overrides.

Quick Triage

Check active DNS servers and resolver symlink target.

ls -l /etc/resolv.conf
resolvectl dns
resolvectl query deb.debian.org

Step-by-Step Diagnosis

Inspect resolver logs and network link-specific DNS state.

journalctl -u systemd-resolved -n 120 --no-pager
networkctl status
resolvectl domain
Illustrative mockup for debian-11 β€” terminal_or_shell
Testing DNS queries and resolver status β€” Illustrative mockup β€” Progressive Robot

Solution – Primary Fix

Set valid DNS servers, flush cache, and restart resolver stack.

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sudo resolvectl dns eth0 1.1.1.1 8.8.8.8
sudo resolvectl flush-caches
sudo systemctl restart systemd-resolved
resolvectl query deb.debian.org
Illustrative mockup for debian-11 β€” log_or_config
Applying corrected DNS settings β€” Illustrative mockup β€” Progressive Robot

Solution – Alternative Approaches

Set static DNS in network configuration for persistence.

sudoedit /etc/systemd/resolved.conf
sudo systemctl restart systemd-resolved

Verification & Acceptance Criteria

DNS queries and apt update complete successfully.

getent hosts security.debian.org
resolvectl statistics
sudo apt update

Rollback Plan

Restore prior resolver settings from backup.

sudo cp -a /etc/systemd/resolved.conf /var/tmp/resolved.conf.bak.$(date +%s)
sudo systemctl restart systemd-resolved

Prevention & Hardening

Define primary and fallback resolvers and monitor lookup failures.

resolvectl status
journalctl -u systemd-resolved --since -1h --no-pager

May coincide with default route or firewall egress failures.

Related tutorial: View the step-by-step tutorial for Debian 11.

View all Debian 11 tutorials on the Tutorials Hub β†’

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Debian systemd-resolved and DNS operations documentation.

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