Connection Closed by UNKNOWN Port 65535: SSH Troubleshooting

The frustrating “Connection closed by UNKNOWN port 65535” SSH error often arises when your SSH client can’t reach the remote SSH daemon (sshd) due to network issues. This error is particularly challenging to debug because the remote sshd might not even register the connection attempt. This article explores common causes and troubleshooting steps.

Understanding Port 65535 2

Before diving into troubleshooting, it’s helpful to understand the significance of port 65535. In networking, 65535 is the highest possible port number. This number is significant because it’s the maximum value for a 16-bit unsigned integer (2^16 – 1, or 0xFFFF in hexadecimal). When you encounter the “Connection closed by UNKNOWN port 65535” error, it usually indicates a connection failure before the SSH protocol can fully establish, often masking the true underlying issue. The “2” in “65535 2” doesn’t have a specific meaning in this context and likely represents a typo or irrelevant addition to the core error message.

Common Causes of the Error

Authentication Failure (Case A)

The remote sshd might receive the connection request but encounter an issue during authentication. This could stem from problems with Pluggable Authentication Modules (PAM), Kerberos, or other authentication mechanisms. In such cases, the sshd might reject the connection before properly logging the error, leading to the generic “65535” message.

Network Connectivity Problems (Case B)

Firewalls, network configuration errors, or routing issues can prevent your SSH client from reaching the remote server. Tools like tcping, ping, and traceroute can help diagnose network problems. Running ssh -vvv can offer additional clues, often showing connection attempts timing out after a delay. For instance:

debug1: identity file /home/ddickinson/.ssh/id_xmss-cert type -1 debug1: Local version string SSH-2.0-OpenSSH_8.6 
...
kex_exchange_identification: Connection closed by remote host Connection closed by UNKNOWN port 65535

Proxy Command Issues (Case C)

If your SSH connection utilizes a ProxyCommand, misconfigurations in the proxy setup can also result in this error. Examine your SSH configuration (ssh -G $remote_hostname) for any “proxy” or “tunnel” options and verify their correctness.

Troubleshooting Steps

1. Verify Network Connectivity: Use ping, traceroute, and tcping to confirm that you can reach the remote host.

2. Check Firewall Rules: Ensure that firewalls on both the client and server sides allow SSH traffic (port 22 by default).

3. Examine SSH Configuration: Review your SSH config file (~/.ssh/config) and the server’s sshd_config for any misconfigurations related to authentication or proxy settings. Look for “proxy*” or “tunnel*” related options in the output of:

   ssh -G $remote_hostname 

4. Inspect Authentication Mechanisms: Investigate potential issues with PAM, Kerberos, or other authentication systems on the remote server.

5. Consult Network Administrators: If network connectivity remains a problem, engage your network administrators for assistance in diagnosing and resolving underlying network issues.

Conclusion

The “Connection closed by UNKNOWN port 65535” SSH error points to a connection failure before a proper SSH handshake can occur. By systematically investigating network connectivity, firewall rules, SSH configuration, and authentication mechanisms, you can pinpoint the root cause and restore your SSH connections.