🔴 Critical   ⏱ 5–30 min  Last verified: 19 May 2026
Affected versions: FreeBSD 13

📖 ~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

Operational users with known-good keys or passwords are denied, risking incident response delays.

Environment & Reproduction

Happens after restrictive sshd_config edits or key permission drift.

sshd -t
service sshd status
ssh -vvv admin@host

Root Cause Analysis

Conflicting auth directives, invalid Match blocks, or file permission violations break login flow.

Quick Triage

Validate daemon config syntax and check auth logs.

tail -n 80 /var/log/auth.log
grep -n 'Match|Authentication' /etc/ssh/sshd_config

Step-by-Step Diagnosis

Run sshd in debug mode on alternate port for controlled testing.

sshd -T | head -n 80
/usr/sbin/sshd -d -p 2222
ssh -p 2222 -vvv [email protected]
Illustrative mockup for freebsd-13 — terminal_or_shell
Tracing SSH authentication errors — Illustrative mockup — Progressive Robot

Solution – Primary Fix

Repair auth directives and ownership/permissions for keys and home directories.

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chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chown -R admin:admin ~/.ssh
service sshd restart
Illustrative mockup for freebsd-13 — log_or_config
Applying corrected sshd_config settings — Illustrative mockup — Progressive Robot

Solution – Alternative Approaches

Temporarily enable PasswordAuthentication with MFA controls while key issues are remediated.

Verification & Acceptance Criteria

Authorized users authenticate successfully and unauthorized attempts remain blocked.

ssh admin@host true
service sshd reload
last | head

Rollback Plan

Restore previous sshd_config and restart service if new hardening profile fails.

cp -a /etc/ssh/sshd_config /etc/ssh/sshd_config.rollback
service sshd restart

Prevention & Hardening

Use config lint in CI and keep break-glass console access for remote lockout scenarios.

Often paired with pf blocks, DNS reverse lookup delays, and expired host keys.

Related tutorial: View the step-by-step tutorial for freebsd-13.

View all freebsd-13 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

sshd_config(5), sshd(8), and FreeBSD security hardening guidelines.

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