SSH Key Authentication Issue
SSH key authentication issues prevent passwordless login for distributed training and remote execution.
SSH key authentication issues prevent passwordless login for distributed training and remote execution.
What this failure is
SSH Key Authentication Issue is a Environment failure seen during ML training runs. SSH key authentication issues prevent passwordless login for distributed training and remote execution. Common tags: Ssh, Authentication, Key, Environment.
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Why it happens (the mechanism)
SSH key not in authorized_keys. Wrong SSH key permissions (must be 600). Host key not in known_hosts. SSH agent not running or key not loaded. Different SSH key for different hosts. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- Distributed training fails at SSH step
- ssh: Permission denied (publickey)
- mpirun or torchrun can't connect to nodes
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Permission denied (publickey) | SSH key not in authorized_keys |
| Host key verification failed | Wrong SSH key permissions (must be 600) |
| Connection closed by remote host | Host key not in known_hosts |
Which systems are affected
- Multi-node training with SSH
- Remote development with VSCode
- CI/CD with remote runners
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: Permission denied (publickey)
- ✓Verified signal present: Host key verification failed
- ✓Verified signal present: Connection closed by remote host
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Diagnose this failure in VS Code
Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.
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Root cause
- SSH key not in authorized_keys
- Wrong SSH key permissions (must be 600)
- Host key not in known_hosts
- SSH agent not running or key not loaded
- Different SSH key for different hosts
The fix and how to prevent it
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Don't just read the fix, diagnose your run
The encyclopedia tells you what went wrong. Denpex tells you what went wrong in YOUR training run. With your logs, your config, and your stack.