kex_exchange_identification: connection closed by remote host

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04-04-2025

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The dreaded "kex_exchange_identification: connection closed by remote host" error message often pops up when attempting SSH connections, leaving users stranded and frustrated. This article delves into the causes and solutions for this common SSH problem, drawing upon insights from Stack Overflow and offering practical advice.

Understanding the Error

This error signifies that the SSH connection attempt failed during the key exchange (kex) phase. The "kex_exchange_identification" part indicates the initial handshake where the client and server authenticate each other. The "connection closed by remote host" portion means the remote server abruptly terminated the connection before successful authentication could occur. This isn't necessarily a problem with your client; the issue often lies on the server-side.

Common Causes and Solutions (Based on Stack Overflow Insights)

1. Incorrect Hostname or IP Address:

• Problem: A frequent cause is specifying a wrong hostname or IP address in your SSH command. The server simply doesn't recognize the request.

• Stack Overflow Relevance: Numerous Stack Overflow questions address typos in the hostname or using an incorrect IP address. (While specific examples are hard to cite directly without violating SO's license, this is a widely reported issue).

• Solution: Double-check your SSH command for any typos in the hostname or IP address. Use ping <hostname/ip> to verify network connectivity before attempting the SSH connection.

• Example: ssh user@wrong_hostname.com should be corrected to ssh user@correct_hostname.com

2. Network Connectivity Issues:

• Problem: Firewalls, network outages, or routing problems can prevent the connection from reaching the server.

• Stack Overflow Relevance: Questions frequently relate to firewall configurations (e.g., blocking SSH port 22), network issues behind NAT, or DNS resolution failures.

• Solution: Check your network connection. Try ping <hostname/ip> and traceroute <hostname/ip> to identify potential network bottlenecks or connectivity issues. Confirm that port 22 (the default SSH port) is open on both the client and server firewalls. Contact your network administrator if you suspect a network problem.

3. Server-Side Problems:

• Problem: The remote server may be down, overloaded, experiencing software issues, or have SSH disabled. Sometimes, resource limitations on the server (memory, CPU) can also lead to unexpected connection drops.

• Stack Overflow Relevance: Stack Overflow threads often discuss server-side issues like full disk space, overloaded CPU, or problems with the SSH daemon (sshd).

• Solution: Check the server's status. If you have access to the server, examine server logs (e.g., /var/log/auth.log on Linux systems) for clues about the connection failure. Restart the SSH service (sudo systemctl restart ssh on most Linux distributions). If the problem persists, it might require server maintenance or contacting the server administrator.

4. SSH Server Configuration Issues:

• Problem: Incorrect SSH server configuration (e.g., incorrect sshd_config settings, disabled password authentication, restrictive firewall rules on the server) can lead to connection failures.

• Stack Overflow Relevance: Many posts tackle issues with sshd_config settings like AllowUsers, PermitRootLogin, and PasswordAuthentication.

• Solution: If you have server administrative access, carefully review your /etc/ssh/sshd_config file. Ensure that password authentication is enabled (if needed), and that any firewall rules on the server aren't blocking incoming SSH connections.

5. Client-Side SSH Configuration:

• Problem: Though less common, problems within your local SSH client configuration (like incorrect key permissions or missing keys) can prevent authentication.

• Stack Overflow Relevance: Users occasionally post about issues with their SSH key files, agent forwarding, or client-side configuration.

• Solution: Verify your SSH keys are properly configured and have the correct permissions. Ensure your SSH agent is running if you're using key-based authentication.

Advanced Troubleshooting

If the problem persists after addressing the common causes, consider these advanced steps:

• SSH debugging: Enable verbose SSH logging (ssh -vvv user@hostname) to gain more detailed information about the connection attempt.
• Network sniffing: Using tools like tcpdump or Wireshark can help identify network-related issues affecting the SSH connection.
• Server logs: Thoroughly analyze the server's SSH logs to pinpoint the exact cause of the failure.

By systematically investigating these potential causes and leveraging the collective wisdom from the Stack Overflow community, you can effectively diagnose and resolve the "kex_exchange_identification: connection closed by remote host" error. Remember that thorough investigation and careful attention to detail are key to successfully troubleshooting this common SSH problem.