feat: Add SSH configuration file parsing (-F option)

[inureyes]

Overview

Implement SSH configuration file parsing to support standard OpenSSH config format (~/.ssh/config), enabling bssh to read and apply SSH settings from config files with the -F option.

Technical Approach

Build a custom SSH config parser that:

• Parses standard SSH config format with pattern matching
• Integrates seamlessly with existing bssh configuration
• Maintains precedence (CLI > SSH config > bssh config > defaults)
• Supports wildcards and Include directives
• Incremental implementation starting with core options

Implementation Tasks

Phase 1: Parser Foundation

• Create SSH config parser module (src/ssh_config/mod.rs)
• Implement tokenizer for SSH config format
• Create data structures for config representation
• Add basic parsing for Host blocks
• Implement error handling with line numbers

Phase 2: Core SSH Options

• Parse and apply Host/HostName options
• Support Port, User, IdentityFile options
• Implement StrictHostKeyChecking parsing
• Add UserKnownHostsFile support
• Handle IdentitiesOnly option

Phase 3: Pattern Matching

• Implement wildcard pattern matching (* and ?)
• Support Include directive for modular configs
• Add first-match-wins semantics
• Handle Host block inheritance

Phase 4: Advanced Options

• Parse ProxyJump/ProxyCommand options
• Support ConnectTimeout and timeouts
• Implement ServerAliveInterval/CountMax
• Add Compression and cipher preferences
• Parse PreferredAuthentications

Phase 5: Integration

• Integrate parser with CLI argument processing
• Merge SSH config with bssh configuration
• Apply configuration precedence rules
• Add config validation and warnings

Priority Options to Support

Priority 1 (Must Have):

• Host, HostName, Port, User
• IdentityFile, StrictHostKeyChecking
• UserKnownHostsFile

Priority 2 (Should Have):

• ProxyJump, ProxyCommand
• ConnectTimeout, ServerAliveInterval
• Compression, PreferredAuthentications

Priority 3 (Nice to Have):

• Match blocks, CanonicalizeHostname
• LocalCommand, SendEnv

Success Criteria

• Parse standard SSH configs without errors
• Correctly resolve host patterns with wildcards
• Apply options with proper precedence
• Parse 1000-line config in < 10ms
• Clear warnings for unsupported options
• Full backward compatibility maintained

Examples

# Use specific SSH config
bssh -F ~/.ssh/config myhost

# Config example that should work:
Host bastion
    HostName bastion.example.com
    User admin
    Port 2222

Host internal-*
    ProxyJump bastion
    User deploy
    IdentityFile ~/.ssh/internal_key

# Usage:
bssh -F ~/.ssh/config internal-server

Labels

enhancement, help wanted, ssh-compat