π ~1 min read
Table of contents
Symptom & Impact
SSH authentication succeeds but shell prompt appears after long delay.
Environment & Reproduction
Seen after changes to SSH banners, PAM modules, or reverse DNS behavior.
Root Cause Analysis
Blocking name lookups or slow PAM scripts delay post-auth session creation.
Quick Triage
Test SSH with verbose mode and compare behavior by source IP.
Step-by-Step Diagnosis
Profile sshd logs, PAM stack order, and UseDNS setting impact.
Solution – Primary Fix
Disable expensive checks, tune UseDNS, and simplify PAM execution path.
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Solution – Alternative Approaches
Use bastion access while investigating directory or DNS dependencies.
Verification & Acceptance Criteria
Interactive shell appears quickly and consistently for privileged and normal users.
Rollback Plan
Restore prior sshd_config and PAM files from versioned backups.
Prevention & Hardening
Stage SSH policy changes and monitor authentication latency trends.
Related Errors & Cross-Refs
Related with GSSAPI timeouts, DNS PTR failures, and PAM LDAP stalls.
Related tutorial: View the step-by-step tutorial for Ubuntu 14.04 LTS.
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Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Consult OpenSSH server and PAM administration manuals.
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