SSH command failed with exit code: 255

[Rami-0]

The issue started while deploying a new feature. The last successful deployment was about two weeks ago.

During deployment, we began getting:

Could not detect BuildKit capabilities on deployment server
SSH command failed with exit code: 255

The failure happens at the SSH stage before the build starts. Coolify cannot execute remote commands, so the deployment stops immediately.

At the same time we noticed:

• Automatic restart of the PostgreSQL TCP proxy
• One failed backup
• Some database timeout errors (P1002)
• Intermittent SSH connection issues on the server

From server logs, it appears SSH was dropping connections under load, and Coolify’s SSH session was getting reset. This explains the exit code 255 and failed deployments.

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Temporary Fix Applied

We adjusted the SSH configuration to reduce connection drops and throttling:

• Disabled password authentication
• Ensured only key-based login is allowed
• Increased SSH connection limits (MaxStartups, MaxSessions)
• Restarted the SSH service after validating config

After these changes:

• SSH became stable
• Deployments started working again
• No further BuildKit detection failures

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Current Status

Deployment is working again after tuning SSH.

We would like to know:

• Is adjusting SSH limits considered a valid long-term solution?
• Does Coolify open multiple parallel SSH sessions during deployment?
• Is there a recommended SSH configuration for production deployments?

At the moment, the issue appears resolved, but we want to confirm whether this is the correct and final approach.

[Rami-0]

Had the same issue again on another server that barely has traffic or connections, and it's totally on a different organization.

The backup failed due to the same error, what's going on?

[Cinzya]

A different user noticed that their server was under a SSH Brute Force Attack. Maybe check your servers for similar activity.