Troubleshooting Connection Closed Error on RHEL Machines

Troubleshooting Connection Closed Error on RHEL Machines

Have you ever encountered the frustrating issue of ‘connection closed by unknown port 65535’ when trying to SSH using AD credentials on a RHEL machine? This cryptic error message can be perplexing, but worry not as we delve into the potential causes and effective solutions to help you troubleshoot and resolve this issue. Let’s explore some common pitfalls and how to address them to ensure a seamless SSH experience on your RHEL machine.

Troubleshooting SSH Authentication Errors

The error message “Connection closed by UNKNOWN port 65535” during SSH authentication can be perplexing, but let’s break it down and explore potential solutions:

  1. Deprecated Option in SSH Configuration:

    • The error message you’re encountering might be related to a deprecated option in your SSH configuration. Specifically, the line RhostsRSAAuthentication is no longer recommended and could cause issues.
    • Consider reviewing your SSH configuration files (usually located in /etc/ssh/sshd_config) and ensure that deprecated options are removed or commented out.
    • If you’re using PAM (Pluggable Authentication Modules) for SSH authentication, ensure that it’s configured correctly and not conflicting with deprecated settings.
  2. .k5login File and User Disallowed:

    • The error message also hints at issues related to the user account. It mentions that the account checks fail for 'user@domainname' due to disallowance in the .k5login file.
    • Check if there’s a .k5login file in the user’s home directory. This file specifies which users are allowed to log in using Kerberos authentication. Ensure that the user’s UPN (User Principal Name) is correctly listed in this file.
    • If you’re using PAM, consider migrating from pam_krb5 to pam_sss for better compatibility and security.
  3. Proxy Command Issues:

    • Sometimes, the error occurs when the proxy command (if you’re using one) unexpectedly closes stdin and exits. This can lead to cryptic messages like the one you’re seeing.
    • If you’re using a proxy command, ensure that it’s configured correctly and doesn’t interfere with the SSH connection.
  4. Middleman Issues:

    • The error message can also occur when something happens “in the middle” of the SSH connection. For example, if the remote SSH daemon (sshd) cannot be reached due to network issues or other factors.
    • Investigate any network-related issues, firewalls, or other intermediaries that might affect the SSH connection.

Remember that debugging SSH issues can be challenging, especially when the error messages are vague. Consider checking logs (such as /var/log/secure) for additional clues. If all else fails, reaching out to your system administrator or IT support can provide further assistance

Diagnosing SSH Connection closed error

The “Connection closed by UNKNOWN port 65535” error can be quite cryptic and challenging to diagnose. Let’s explore some potential causes and solutions:

  1. SSH Configuration Issues:

    • Ensure that your SSH server is configured correctly. It should listen on the default port 22 (unless you’ve explicitly changed it).
    • Verify that your SSH client is using the correct port and authentication method.
    • Check your SSH configuration files (e.g., ~/.ssh/config, /etc/ssh/ssh_config) for any misconfigurations or deprecated options.
  2. Proxy and Jump Server Setup:

    • If you’re using a jump server (proxy) to connect to your target server, ensure that the proxy configuration is correct.
    • Double-check the ProxyCommand syntax in your SSH configuration. Make sure it works independently when run in the terminal.
    • Consider using verbose mode (-vvv) when connecting to get more detailed debugging information.
  3. Algorithm Compatibility:

    • Some SSH algorithms (e.g., ssh-rsa) are no longer considered secure and may be omitted by default in newer SSH versions.
    • Check the algorithms supported by your SSH client and server. You can use ssh -Q key to see the supported algorithms on the client side.
    • Verify the SSH server’s sshd_config file for any custom cipher settings. Newer SSH versions may refuse to use old algorithms.
  4. Smart Card Authentication:

    • If you’re using a PIV (smartcard) for authentication, ensure that your SSH configuration is compatible with it.
    • Remove or comment out the PKCS11Provider line in your ~/.ssh/config.
    • Run the following commands:
      ssh-add -e /usr/lib/ssh-keychain.dylib
      ssh-add -D
      killall -9 ssh-agent
      ssh-add -s /usr/lib/ssh-keychain.dylib

Remember that debugging SSH issues can be complex, and it’s essential to examine each component (client, proxy, server) to identify the root cause. If you encounter further difficulties, consider seeking assistance from IT support or your system administrator.

Troubleshooting SSH Connection Closed Error

The error message “Connection closed by UNKNOWN port 65535” can be puzzling, but let’s explore some potential solutions to diagnose and resolve the issue:

  1. Check SSH Configuration:

    • Ensure that your SSH configuration is correct. Verify the settings in /etc/ssh/sshd_config on the RHEL machine.
    • Look for any custom configurations related to Ciphers or other SSH parameters.
    • You can also run ssh -Q key (for OpenSSH 6.3+) to see which algorithms your client supports.
  2. ServerAliveInterval:

    • In your client’s ssh_config file (usually located at /etc/ssh/ssh_config), consider enabling ServerAliveInterval:
      ServerAliveInterval 30
      ServerAliveCountMax 5
      • ServerAliveInterval: Sets a timeout interval (in seconds) after which if no data is received from the server, SSH will send a message to request a response.
      • ServerAliveCountMax: Sets the number of client alive messages that can be sent without receiving any responses before disconnecting.
  3. Restart SSH Service:

    • After modifying the SSH configuration, restart the SSH service:
      sudo /etc/init.d/ssh restart


      sudo service sshd restart
  4. Verify Hostnames and IPs:

    • Check if the remote host’s IP address or hostname is correct.
    • Investigate whether connections are coming from localhost or remote hosts using tools like ss or strace.

For more detailed information, refer to the Red Hat Knowledgebase.

Troubleshooting SSH Connection Issues

Troubleshooting SSH connection issues can be a bit tricky, but I’m here to help. Let’s explore some common pitfalls and how to address them:

  1. Hostname Resolution:

    • Ensure that the hostname is spelled correctly.
    • Verify that you can resolve the hostname using the system ping command.
    • If DNS resolution is problematic, consider using the Droplet’s IP address instead of the hostname.
  2. Connection Timeout:

    • If you encounter a “Connection timed out” error, verify the following:
      • The host IP address is correct for the Droplet.
      • Your network supports connectivity over the SSH port (usually port 22).
      • Check firewall rules and ensure they allow SSH traffic.
  3. Authentication Issues:

    • Password Authentication:
      • Make sure you’re using the correct username.
      • Confirm that password authentication is allowed by the server.
    • Key-Based Authentication:
      • Ensure that the authorized_keys file and private key have correct permissions and ownership.
      • Verify that key-based authentication is allowed by the server.
  4. Firewall Rules and Ports:

    • Check Droplet firewall rules. Ensure they’re not set to a default policy of DROP, and the SSH port is allowed.
    • Some public networks may block custom SSH ports, so stick to the default port 22 if possible.

For more detailed information, you can refer to the DigitalOcean documentation on SSH connectivity issues.

Best Practices for Handling Technical Issues Beyond Expertise

As a technical support professional, encountering issues beyond your expertise is common. Here are some best practices for handling such situations effectively:

  1. Identify the Issue:

    • Clearly document the issue by gathering relevant information:
      • Symptoms
      • Steps to reproduce
      • Error messages
      • Logs
      • Screenshots
      • Hardware and software specifications
      • Impact on the user or system
    • Verify that it’s not a known problem, user error, or a simple fix you can apply.
  2. Follow the Escalation Policy:

    • Understand your organization’s escalation policy:
      • Criteria for escalation
      • Process
      • Roles and responsibilities
      • Timeframes
      • Reporting
    • Strictly adhere to the policy for consistency and quality of service.
  3. Communicate Effectively:

    • Use clear and concise language.
    • Avoid technical jargon or acronyms.
    • Provide all relevant information and documentation gathered in the first step.
    • Explain why you’re escalating the issue and what you’ve tried so far.
  4. Seek Help from the Right Source:

    • Contact colleagues, supervisors, vendors, or specialists based on the issue’s nature and complexity.
    • Follow proper escalation procedures and provide detailed information.
  5. Follow Up with the Customer:

    • Keep the customer informed about the issue’s status.
    • Provide updates on who is working on it, actions taken, and expected resolution time.
    • Apologize for any inconvenience caused.

Remember, effective communication and collaboration are key to resolving technical issues beyond your expertise. By following these steps, you can maintain customer satisfaction and trust while seeking the right solutions

In conclusion, troubleshooting the ‘connection closed by unknown port 65535’ error when attempting to SSH using AD credentials on a RHEL machine requires a systematic approach and attention to detail. By examining your SSH configuration, addressing issues with proxy and jump server setups, ensuring algorithm compatibility, and resolving authentication challenges, you can overcome this perplexing error and establish a stable SSH connection. Remember to document your steps, seek assistance as needed, and maintain effective communication throughout the troubleshooting process.

With persistence and a methodical approach, you can conquer SSH connection issues and enhance the security and efficiency of your RHEL machine.


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