SSH-based checks (see Monitoring Linux Servers) fail for a narrower set of reasons than SNMP, which usually makes them faster to diagnose once you know what to check.
Checklist
- Network connectivity — confirm TCP 22 (or your configured SSH port) is reachable from the iPM server to the target host, and isn't blocked by a firewall. This is the same kind of check as any other reachability issue — see Device Not Reachable if basic ping is also failing.
- Key mismatch — confirm the SSH key selected for this host under Administration, then SSH Keys, is the current one. A key that was rotated (see SSH Keys) but not updated on this specific host's configuration will cause authentication to fail even though the key itself is valid elsewhere.
- Account status — confirm the monitoring account on the target server hasn't been disabled, locked, or had its password/key expired independently of iPM's own key management.
- Host key changes — if the target server was reinstalled, rebuilt, or had its SSH host key regenerated, the server's identity as far as SSH is concerned has changed. Depending on how strict the configuration is, this can block the connection until the change is acknowledged.
- Permissions — confirm the monitoring account actually has permission to run whatever commands the specific check needs. A successful login with checks still failing usually points here rather than at connectivity or authentication.
Isolating the problem
If you have terminal access, manually SSH to the target host using the same key and account iPM is configured with. If that manual connection also fails, the problem is on the server or network side; if it succeeds while iPM still reports a failure, double-check that iPM has the correct key, account name, and port configured for that specific host.
Once SSH connectivity itself is confirmed working, a check that's still failing or returning no data is more likely a Monitoring Data Missing issue than an SSH problem specifically.