Common SSH Issues


SSH is a reliable and secure method for remote access, but like any network service, it can encounter issues. Some of the most common problems involve connection failures and permission denied errors. This guide outlines how to address these issues.

## Debugging Connection Failures

SSH connection failures can arise from several factors, including network issues, incorrect configuration, or server misconfiguration. Common causes include:

- **Network or Firewall Issues**: Ensure the SSH port (typically 22) is open on both the client and the server. Firewalls or network rules might block the SSH connection.

- **Incorrect Hostname or IP**: Verify the hostname or IP address by pinging the remote host or using `nslookup` to check for proper resolution.

- **Wrong Port**: While SSH defaults to port 22, some servers use custom ports for security reasons. In such cases, specify the correct port with the `-p` option:

  ```bash
  ssh -p 2222 user@hostname

To diagnose these issues more effectively, use SSH’s verbose mode to output additional connection details:

ssh -v user@hostname

This can help pinpoint issues like “connection refused” or “no route to host.”

Fixing Permission Denied Errors

Permission denied errors typically occur when authentication fails. These can result from various factors:

Incorrect Username: Ensure the correct username is being used. Use whoami on the remote server to confirm the right user account.
Incorrect Key Permissions: SSH requires that the private key file has proper permissions. If the file is too open, SSH will refuse to use it. Correct permissions with:
```
chmod 600 ~/.ssh/id_rsa
```
Missing Public Key on the Server: For key-based authentication, ensure that the public key is properly added to the ~/.ssh/authorized_keys file on the server.

Other possible causes include:

Password Authentication Disabled: If password login is disabled (PasswordAuthentication no in sshd_config), key-based authentication must be configured.
Account Restrictions: Servers may impose restrictions like time-based access or IP filtering. Checking the server logs can reveal if such restrictions are applied.

By verifying key permissions, configuration, and using SSH’s verbose mode, most issues can be quickly resolved. For persistent problems, checking the server-side logs often provides further clues.