kex_exchange_identification: Connection closed by remote host

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Demystifying kex_exchange_identification: Connection closed by remote host in SSH

Demystifying kex_exchange_identification: Connection closed by remote host in SSH

Unraveling the common SSH error kex_exchange_identification: Connection closed by remote host, exploring its root causes, and providing practical solutions for shared hosting environments.

The error message kex_exchange_identification: Connection closed by remote host is a frequent and frustrating hurdle for users attempting to establish an SSH connection. This cryptic message often indicates a problem during the initial key exchange (KEX) phase, where the client and server negotiate cryptographic parameters. While it can stem from various issues, in shared hosting environments, it commonly points to server-side restrictions or misconfigurations. This article will delve into the common causes of this error, particularly within shared hosting contexts, and provide actionable steps to diagnose and resolve it.

Understanding the SSH Handshake and KEX Phase

Before diving into solutions, it's crucial to understand what happens during an SSH connection attempt. The process begins with a TCP handshake, followed by the SSH protocol's key exchange (KEX) phase. During KEX, the client and server agree on algorithms for encryption, integrity checking, and authentication. If either side encounters an issue with the proposed algorithms, or if a security policy is violated, the connection can be abruptly terminated. This termination often manifests as the kex_exchange_identification error. Common triggers include unsupported ciphers, outdated SSH clients/servers, or network-level interference.

A flowchart diagram illustrating the SSH connection handshake process. It starts with 'Client Initiates TCP Connection' -> 'Server Acknowledges TCP' -> 'SSH Protocol Version Exchange' -> 'Key Exchange (KEX) Initiation' -> 'KEX Algorithm Negotiation (Client & Server)' -> 'Key Generation & Exchange' -> 'Server Authentication (Host Key Verification)' -> 'Client Authentication' -> 'Secure Channel Established'. Use light blue boxes for client actions, dark blue for server actions, and green for successful outcomes. Arrows indicate the flow direction.

Simplified SSH Handshake Flow

Common Causes in Shared Hosting Environments

Shared hosting providers often implement strict security policies and resource limits, which can lead to SSH connection failures. Several factors specific to these environments can trigger the kex_exchange_identification error:

  1. Outdated SSH Client/Server Versions: Shared hosting providers might run older versions of OpenSSH for stability, or your local client might be too old/new, leading to incompatible KEX algorithms.
  2. Unsupported KEX Algorithms or Ciphers: Modern SSH clients might propose KEX algorithms or ciphers that are explicitly disabled or not supported by the shared hosting server due to security policies or an older sshd_config.
  3. Firewall or Network Restrictions: The hosting provider's firewall (e.g., Mod_Security, CSF/LFD) might be blocking your IP address or specific SSH traffic patterns, especially if multiple failed login attempts occurred.
  4. Resource Limits: Less common but possible, if the server is under heavy load or resource constraints, it might prematurely close connections.
  5. Incorrect SSH Port: While Connection refused is more common for incorrect ports, some firewalls might drop packets for non-standard ports, leading to a closed connection.

Diagnosing and Resolving the Issue

Resolving this error typically involves a process of elimination, starting from the client side and moving to server-side configurations. Here's a structured approach:

1. Client-Side Diagnostics: Verbose Output

The first step is to get more information from your SSH client. Use the -v, -vv, or -vvv flags for verbose output, which can often pinpoint where the handshake fails.

ssh -vvv user@your_host.com

Using verbose flags to get detailed debug output from the SSH client.

Look for lines indicating kex_exchange_identification or no matching key exchange method found. This output will often show the client's proposed algorithms and the server's response, or lack thereof.

2. Specifying KEX Algorithms and Ciphers

If the verbose output suggests incompatible algorithms, you can try specifying older, more commonly supported KEX algorithms and ciphers. This is a common workaround for older shared hosting servers. Consult the man ssh_config page for a full list of options, but here are some common ones to try:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 -oHostKeyAlgorithms=+ssh-dss -c aes128-cbc user@your_host.com

Attempting connection with explicitly defined older KEX algorithms and ciphers. Use with caution as these are less secure.

You can also add these options to your ~/.ssh/config file to avoid typing them repeatedly:

Host your_host.com
  KexAlgorithms +diffie-hellman-group1-sha1
  Ciphers aes128-cbc
  HostKeyAlgorithms +ssh-dss

Configuration for specific KEX algorithms in ~/.ssh/config.

Server-Side (Shared Hosting) Solutions

Access to the server's sshd_config is often limited in shared hosting. However, you can still investigate and act:

1. Check IP Address Blocking

Contact your hosting provider's support. Ask them to check if your IP address has been blocked by their firewall (e.g., CSF/LFD, Fail2Ban) due to too many failed login attempts or other security triggers. Providing your public IP address will expedite this.

2. SSH Port Check

Confirm you are using the correct SSH port. While 22 is standard, many shared hosts use a non-standard port for security. Your hosting provider's documentation or support will confirm this.

3. Server-Side SSH Configuration (If you have access)

If you have root or sudo access (unlikely in typical shared hosting, but possible in VPS/dedicated), you can inspect /etc/ssh/sshd_config.

Look for directives like KexAlgorithms, Ciphers, HostKeyAlgorithms, and MACs. Ensure they include algorithms supported by your client. If the server is too restrictive, you might need to add more algorithms. After modifying, always restart the SSH service.

sudo systemctl restart sshd
# Or for older systems
sudo service sshd restart

Commands to restart the SSH daemon after configuration changes.

Summary of Troubleshooting Steps

Here's a condensed list of steps to take when encountering kex_exchange_identification:

1. Step 1

Check Verbose Output: Connect with ssh -vvv user@your_host.com to gather detailed error information.

2. Step 2

Identify Algorithm Mismatch: Look for messages about no matching key exchange method found or similar in the verbose output.

3. Step 3

Try Older Algorithms (Client-Side): Use -oKexAlgorithms, -oCiphers, and -oHostKeyAlgorithms flags, or add them to ~/.ssh/config.

4. Step 4

Contact Hosting Support: Ask if your IP is blocked or if they have specific SSH port/configuration requirements.

5. Step 5

Update SSH Client: Ensure your local SSH client is up-to-date.

6. Step 6

Review Server Configuration (if applicable): If you have access, check /etc/ssh/sshd_config for restrictive KEX, Ciphers, or HostKeyAlgorithms.

By systematically working through these steps, you should be able to diagnose and resolve the kex_exchange_identification: Connection closed by remote host error, restoring your ability to connect via SSH to your shared hosting environment.