SSH Server Implementation

A simple SSH server implementation in Go that supports both password and public key authentication, with full PTY (pseudo-terminal) support for interactive shell sessions.

Features

• Dual Authentication: Supports both password-based and public key authentication
• PTY Support: Full pseudo-terminal support for interactive shell sessions
• Command Execution: Supports both interactive shells and direct command execution
• Window Resizing: Dynamic terminal window resizing support
• Exit Status: Proper SSH exit status reporting
• Session Management: Handles multiple concurrent SSH connections

Prerequisites

• Go 1.25.1 or later
• SSH key pair (for key-based authentication)

Setup

1. Generate SSH Key Pair

If you don't have SSH keys, generate them:

ssh-keygen -t rsa -b 2048 -f id_rsa -N ""

This will create:

• id_rsa (private key for the server)
• id_rsa.pub (public key for client authentication)

2. Install Dependencies

go mod tidy

3. Run the Server

go run main.go

The server will start listening on 0.0.0.0:2222.

Configuration

The server configuration is defined in main.go:

const (
    serverAddr      = "0.0.0.0:2222"  // Server address and port
    allowedUser     = "testuser"       // Username for password auth
    allowedPassword = "secret123"      // Password for authentication
)

Authentication Methods

Password Authentication

• Username: testuser
• Password: secret123

Public Key Authentication

• Uses the public key from id_rsa.pub
• The corresponding private key must be used by the SSH client

Usage Examples

Connect with Password Authentication

ssh -p 2222 testuser@localhost
# Enter password: secret123

Connect with Public Key Authentication

ssh -i id_rsa -p 2222 testuser@localhost

Connect from Remote Machine

Replace localhost with your machine's IP address:

ssh -p 2222 testuser@192.168.1.36

Supported SSH Features

• Session Channels: Interactive shell sessions
• PTY Requests: Pseudo-terminal allocation
• Window Changes: Dynamic terminal resizing
• Shell Requests: Interactive shell spawning
• Exec Requests: Direct command execution
• Exit Status: Proper exit code reporting

Security Notes

⚠️ This is a demonstration server and should NOT be used in production without proper security hardening:

• Default credentials are hardcoded
• No rate limiting or connection throttling
• No logging of authentication attempts
• No firewall or access control beyond basic authentication

Dependencies

• github.com/creack/pty - PTY (pseudo-terminal) support
• golang.org/x/crypto - SSH protocol implementation

Project Structure

├── main.go          # Main SSH server implementation
├── go.mod           # Go module definition
├── go.sum           # Dependency checksums
├── id_rsa           # Server private key (generate if missing)
├── id_rsa.pub       # Client public key for authentication
└── README.md        # This file

Troubleshooting

Server won't start

• Ensure port 2222 is not in use by another process
• Check that id_rsa private key file exists
• Verify Go version compatibility

Authentication fails

• For password auth: verify username is testuser and password is secret123
• For key auth: ensure id_rsa.pub exists and matches your client's private key
• Check file permissions on key files

Connection refused

• Verify the server is running and listening on the correct port
• Check firewall settings if connecting from a remote machine
• Ensure you're using the correct IP address and port (2222)

License

This project is for educational purposes. Use at your own risk.