Analysis and Solutions for SSH Exchange Identification Connection Closed by Remote Host

Problem Background and Phenomenon Analysis

In a Windows 7 operating system environment, users set up a Git server based on SSH and successfully completed repository initialization and cloning operations in the Cygwin environment. However, when attempting to execute the same clone command in the Git Bash environment, the system returned the ssh_exchange_identification: Connection closed by remote host error message. This phenomenon indicates that the SSH connection was actively terminated by the remote host during the authentication phase.

Detailed Error Diagnosis Process

By executing the ssh -vvv git@localhost command to obtain detailed debugging information, several key technical details can be observed:

debug2: ssh_connect: needpriv 0
debug1: Connecting to localhost [127.0.0.1] port 22.
debug1: Connection established.
debug1: identity file /c/Users/MoreFreeze/.ssh/identity type -1
debug3: Not a RSA1 key file /c/Users/MoreFreeze/.ssh/id_rsa.
debug2: key_type_from_name: unknown key type '-----BEGIN'
debug3: key_read: missing keytype
debug3: key_read: missing whitespace
// The above pattern repeats 24 times
debug2: key_type_from_name: unknown key type '-----END'
debug3: key_read: missing keytype
debug1: identity file /c/Users/MoreFreeze/.ssh/id_rsa type 1
debug1: identity file /c/Users/MoreFreeze/.ssh/id_dsa type -1
ssh_exchange_identification: Connection closed by remote host

The debug output shows that the SSH client successfully established a connection to localhost port 22, but encountered problems during the key parsing phase. The system reports Not a RSA1 key file, along with multiple unknown key type and missing keytype warnings, indicating potential abnormalities in the private key file format.

Core Problem Root Cause Investigation

Based on the analysis from the best answer, the fundamental cause of the problem may involve multiple technical aspects:

Network Adapter Conflict: When a computer is simultaneously connected to both wired (LAN) and wireless (WLAN) networks, SSH keys may be bound to the MAC address of a specific network adapter, leading to authentication failures. Disconnecting one of the network connections can verify this hypothesis.

SSH Key Format Anomalies: The key parsing errors shown in the debug information suggest potential format issues with the private key file. Standard RSA private keys should contain clear -----BEGIN RSA PRIVATE KEY----- and -----END RSA PRIVATE KEY----- identifiers, while repeated parsing errors indicate that the file structure does not meet expectations.

Environmental Difference Impact: Although both Cygwin and Git Bash provide Unix-like environments, there are subtle differences in file path handling, SSH configuration loading, and other aspects that may cause the same SSH key to behave differently across environments.

Solution Implementation Steps

Network Configuration Optimization: First verify the network connection status. If multiple network adapters are active, try disconnecting one (particularly the Ethernet adapter) and then retest the SSH connection. This method can quickly eliminate network-level conflict issues.

SSH Key Verification and Regeneration: Use the ssh-keygen -l -f ~/.ssh/id_rsa command to check the fingerprint information of the private key file and confirm the key format is correct. If issues are found, regenerate the key pair using ssh-keygen -t rsa -b 4096 and ensure the public key is properly deployed to the Git server.

SSH Configuration Check: Examine the host configuration in the ~/.ssh/config file to ensure there are no conflicting configuration items. Referring to suggestions from other answers, explicit Host configuration can be added:

Host localhost
    Hostname localhost
    Port 22
    User git

In-depth Technical Principle Analysis

The SSH protocol authentication process involves complex key exchange mechanisms. When a client initiates a connection, protocol version negotiation occurs first, followed by the key exchange phase. The ssh_exchange_identification error typically occurs during this early stage, indicating that the server terminated the connection immediately after receiving the client's initial information.

As can be seen from the reference article, similar problems may be caused by various factors:

Firewall Restrictions: Next-generation firewalls (NGFW) in some network environments can identify and block SSH traffic regardless of the port used. This deep packet inspection technology can filter based on protocol characteristics rather than port numbers.

Server Resource Limitations: If the server is under high load or has insufficient resources, it may reject new SSH sessions during the connection establishment phase to protect system stability.

Configuration Inconsistencies: Different SSH client implementations may have subtle differences in key parsing, protocol support, and other aspects, causing the same key file to behave differently across environments.

Prevention and Best Practices

To avoid similar problems, developers are advised to follow these best practices:

Unified Development Environment: Maintain environmental consistency during project development, avoiding frequent switching between different shell environments.

Regular Key Maintenance: Periodically check the validity and format correctness of SSH keys, using standard tools to generate and manage keys.

Detailed Logging: When encountering connection problems, make full use of SSH's verbose debug mode (-vvv parameter) to obtain complete connection process information, providing sufficient basis for problem diagnosis.

Alternative Solution Preparation: As mentioned in the reference article, when SSH connections consistently fail, consider using the HTTPS protocol as an alternative for Git operations. While slightly less secure, it typically offers better network compatibility.

Conclusion

SSH connection issues often involve interactions across multiple technical levels, from network configuration to key management, from client implementation to server status. Through systematic problem investigation methods, combined with detailed debugging information and practical experience, developers can effectively identify and resolve complex technical issues such as ssh_exchange_identification: Connection closed by remote host. The analytical framework and solutions provided in this article are not only applicable to the current specific case but also offer general methodological guidance for handling similar SSH connection problems.