ssh-remote

What this skill does

# SSH Remote Access

## Overview

SSH (Secure Shell) provides encrypted remote access, file transfer, and tunneling over untrusted networks. OpenSSH is the standard implementation on Linux, macOS, and Windows (via built-in client). The client configuration lives at `~/.ssh/config` and supports per-host settings, identity management, and connection reuse.

**When to use:** Remote server management, secure file transfers, port forwarding, jump host traversal, automated deployments, SOCKS proxying.

**When NOT to use:** High-throughput bulk data transfer across WANs (use Globus or similar), GUI-heavy remote desktop (use VNC/RDP), container orchestration (use kubectl/docker CLI).

## Quick Reference

| Pattern           | Command / Directive                      | Key Points                             |
| ----------------- | ---------------------------------------- | -------------------------------------- |
| Basic connect     | `ssh user@host`                          | Add `-p PORT` for non-default port     |
| Identity file     | `ssh -i ~/.ssh/key user@host`            | Specify private key explicitly         |
| Remote command    | `ssh user@host "command"`                | Add `-t` for interactive commands      |
| SSH config alias  | `Host myserver` block in `~/.ssh/config` | Simplifies repeated connections        |
| File copy (rsync) | `rsync -avzP src user@host:dest`         | Preferred over scp for all transfers   |
| File copy (scp)   | `scp file user@host:path`                | Legacy protocol; uses SFTP internally  |
| Local tunnel      | `ssh -L local:remote_host:remote_port`   | Access remote services locally         |
| Remote tunnel     | `ssh -R remote:localhost:local_port`     | Expose local services to remote        |
| SOCKS proxy       | `ssh -D 1080 user@host`                  | Dynamic port forwarding                |
| Jump host         | `ssh -J jump user@target`                | ProxyJump, available since OpenSSH 7.3 |
| Key generation    | `ssh-keygen -t ed25519`                  | Ed25519 recommended for all new keys   |
| FIDO2 key         | `ssh-keygen -t ed25519-sk`               | Hardware-backed, requires OpenSSH 8.2+ |
| Agent             | `ssh-add ~/.ssh/key`                     | Cache key passphrase for session       |
| Multiplexing      | `ControlMaster auto` in config           | Reuse TCP connections across sessions  |
| Debug             | `ssh -v user@host`                       | Up to `-vvv` for maximum verbosity     |

## Key Type Recommendations

| Algorithm          | Recommendation                     | Notes                                                     |
| ------------------ | ---------------------------------- | --------------------------------------------------------- |
| Ed25519            | Default for all new keys           | 256-bit, fast, secure, supported on OpenSSH 6.5+          |
| Ed25519-SK (FIDO2) | Strongest option with hardware key | Requires physical security key, OpenSSH 8.2+              |
| RSA 4096           | Legacy compatibility only          | Use only when Ed25519 is unsupported by the remote system |
| ECDSA              | Avoid                              | Implementation concerns; prefer Ed25519                   |

## File Transfer Decision Guide

| Scenario                             | Tool                       | Why                                       |
| ------------------------------------ | -------------------------- | ----------------------------------------- |
| Recurring syncs or large directories | `rsync -avzP`              | Delta sync, compression, resume, progress |
| Quick one-off file copy              | `scp` or `rsync`           | scp is simpler; rsync is more capable     |
| Interactive file browsing            | `sftp`                     | Tab completion, directory navigation      |
| High-bandwidth WAN transfers         | Specialized tools (Globus) | SSH buffer limits reduce WAN throughput   |

## Common Mistakes

| Mistake                                          | Correct Pattern                                                             |
| ------------------------------------------------ | --------------------------------------------------------------------------- |
| Using RSA keys for new setups                    | Generate Ed25519 keys -- faster, smaller, and equally secure                |
| Using `scp` for large or recurring transfers     | Use `rsync -avzP` for compression, progress, and resumable delta sync       |
| Typing passphrase repeatedly during sessions     | Use `ssh-agent` and `ssh-add` to cache keys for the session                 |
| Connecting through multiple hops with nested SSH | Use `-J` (ProxyJump) for clean bastion/jump host traversal                  |
| Running interactive commands without `-t` flag   | Use `ssh -t user@host "htop"` to allocate a pseudo-terminal                 |
| Using `ForwardAgent yes` through untrusted hosts | Use ProxyJump instead -- agent forwarding exposes keys to compromised hosts |
| Setting `ControlPath` without `%h`, `%p`, `%r`   | Include all three tokens to ensure unique sockets per connection            |
| Disabling host key checking globally             | Only use `StrictHostKeyChecking no` in trusted, ephemeral environments      |
| Not using `IdentitiesOnly yes`                   | Prevents offering every loaded key to every server                          |

## Security Checklist

- Generate Ed25519 keys with strong passphrases
- Set `PasswordAuthentication no` on servers
- Set `PermitRootLogin prohibit-password` or `no`
- Use `IdentitiesOnly yes` in client config
- Restrict keys with `command=` and `from=` in `authorized_keys`
- Use FIDO2 hardware keys (`ed25519-sk`) for high-security environments
- Install `fail2ban` on servers to block brute-force attempts
- Consider SSH certificate authentication for fleet management

## Delegation

- **Server inventory discovery and connection testing**: Use `Explore` agent
- **Multi-host deployment or bulk file transfers**: Use `Task` agent
- **Network architecture and bastion host planning**: Use `Plan` agent

## References

- [Connections, SSH config, and remote commands](references/connections.md)
- [File transfers with rsync and scp](references/file-transfer.md)
- [Port forwarding, SOCKS proxy, and jump hosts](references/tunneling.md)
- [Key management, FIDO2 keys, agent, and security hardening](references/key-management.md)