feat: Add SSH port forwarding support (-L, -R, -D options)

[inureyes]

Overview

Implement comprehensive SSH port forwarding capabilities in bssh to support local (-L), remote (-R), and dynamic (-D) port forwarding, enabling full SSH tunneling functionality compatible with both single-host and multi-host cluster modes.

Technical Approach

The implementation uses russh's channel capabilities with a custom async forwarding manager built on Tokio. Key architectural decisions include:

• Async-first design for handling thousands of concurrent connections efficiently
• Centralized ForwardingManager for lifecycle management and monitoring
• Multiplexed channels over single SSH connections for resource efficiency
• Automatic reconnection with exponential backoff for resilience
• Graceful shutdown with proper resource cleanup on termination

Implementation Tasks

Phase 1: Foundation and Local Forwarding

• Create port forwarding module structure (src/forwarding/mod.rs)
• Implement forwarding specification parser for -L/-R/-D formats
• Create ForwardingManager with lifecycle management
• Implement local TCP listener with async accept loop
• Add channel multiplexing support to SSH client wrapper
• Create bidirectional data pump for channel I/O
• Implement local port forwarding (-L) with error handling
• Add connection retry logic with exponential backoff

Phase 2: Remote and Dynamic Forwarding

• Implement SSH tcpip-forward request protocol
• Handle forwarded-tcpip channel requests from server
• Create remote port forwarding (-R) implementation
• Implement SOCKS v4/v5 protocol parser
• Create SOCKS server with authentication support
• Implement dynamic port forwarding (-D) with SOCKS proxy
• Add DNS resolution through remote connection

Phase 3: Multi-Host Support

• Extend ForwardingManager for multi-host cluster mode
• Implement forwarding persistence across reconnections
• Add bandwidth throttling and rate limiting
• Create forwarding status monitoring interface

Success Criteria

• Local port forwarding works identically to OpenSSH -L
• Remote port forwarding works identically to OpenSSH -R
• Dynamic forwarding provides full SOCKS v5 compatibility
• All forwarding types work in single-host and cluster modes
• Performance within 10% of OpenSSH for throughput

Labels

enhancement, help wanted, ssh-compat