# Cheat Sheet #day37 - SSH

## SSH Cheatsheet

### Basic Usage

* **Connect to a Remote Host**
    
    ```bash
    ssh username@hostname
    ```
    
    Example:
    
    ```bash
    ssh user@example.com
    ```
    
* **Connect with a Specific Port**
    
    ```bash
    ssh -p port_number username@hostname
    ```
    
    Example:
    
    ```bash
    ssh -p 2222 user@example.com
    ```
    
* **Run a Command on a Remote Host**
    
    ```bash
    ssh username@hostname command
    ```
    
    Example:
    
    ```bash
    ssh user@example.com 'ls -l /var/www'
    ```
    

### Key-Based Authentication

* **Generate SSH Key Pair**
    
    ```bash
    ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
    ```
    
    * **Add SSH Key to SSH-Agent**
        
    
    ```bash
    eval "$(ssh-agent -s)"
    ssh-add ~/.ssh/id_rsa
    ```
    
* **Copy Public Key to Remote Host**
    
    ```bash
    ssh-copy-id username@hostname
    ```
    
    Example:
    
    ```bash
    ssh-copy-id user@example.com
    ```
    

### Configuration

* **Edit SSH Config File**
    
    * Path: `~/.ssh/config`
        
    
    ```bash
    nano ~/.ssh/config
    ```
    
    Example Config File:
    
    ```plaintext
    Host example
        HostName example.com
        User user
        Port 22
        IdentityFile ~/.ssh/id_rsa
    ```
    
* **Connect Using Config Alias**
    
    ```bash
    ssh example
    ```
    

### Port Forwarding

* **Local Port Forwarding**
    
    ```bash
    ssh -L local_port:destination_host:destination_port username@hostname
    ```
    
    Example:
    
    ```bash
    ssh -L 8080:localhost:80 user@example.com
    ```
    
* **Remote Port Forwarding**
    
    ```bash
    ssh -R remote_port:destination_host:destination_port username@hostname
    ```
    
    Example:
    
    ```bash
    ssh -R 8080:localhost:80 user@example.com
    ```
    
* **Dynamic Port Forwarding (SOCKS Proxy)**
    
    ```bash
    ssh -D local_port username@hostname
    ```
    
    Example:
    
    ```bash
    ssh -D 1080 user@example.com
    ```
    

### File Transfer

* **SCP (Secure Copy)**
    
    * **Copy Local to Remote**
        
    
    ```bash
    scp local_file username@hostname:/remote/directory
    ```
    
    Example:
    
    ```bash
    scp file.txt user@example.com:/home/user/
    ```
    
    * **Copy Remote to Local**
        
    
    ```bash
    scp username@hostname:/remote/file /local/directory
    ```
    
    Example:
    
    ```bash
    scp user@example.com:/home/user/file.txt /local/directory
    ```
    
* **SFTP (SSH File Transfer Protocol)**
    
    ```bash
    sftp username@hostname
    ```
    
    * **Common SFTP Commands**
        
    
    ```bash
    sftp> get remote_file
    sftp> put local_file
    sftp> ls
    sftp> cd directory
    sftp> pwd
    sftp> exit
    ```
    

### Tunneling and Proxying

* **SSH Tunnel for All Traffic**
    
    ```bash
    ssh -D 8080 -C -q -N username@hostname
    ```
    
    * Use the tunnel as a SOCKS proxy in your browser or application.
        
* **ProxyCommand**
    
    * Use another SSH server as a proxy.
        
    
    ```plaintext
    Host internal
        HostName internal.example.com
        User user
        ProxyCommand ssh -q -W %h:%p gateway.example.com
    ```
    

### Advanced Options

* **Enable X11 Forwarding**
    
    ```bash
    ssh -X username@hostname
    ```
    
* **Enable Compression**
    
    ```bash
    ssh -C username@hostname
    ```
    
* **Increase Verbosity**
    
    ```bash
    ssh -v username@hostname
    ```
    
* **Multiple Levels of Verbosity**
    
    ```bash
    ssh -vvv username@hostname
    ```
    

### SSH Agent Forwarding

* **Enable Agent Forwarding**
    
    ```bash
    ssh -A username@hostname
    ```
    

### Managing SSH Keys

* **List Loaded Keys**
    
    ```bash
    ssh-add -l
    ```
    
* **Remove All Loaded Keys**
    
    ```bash
    ssh-add -D
    ```
    

### SSH Security

* **Disable Root Login**
    
    * Edit `/etc/ssh/sshd_config` on the remote server:
        
    
    ```plaintext
    PermitRootLogin no
    ```
    
* **Change Default SSH Port**
    
    * Edit `/etc/ssh/sshd_config` on the remote server:
        
    
    ```plaintext
    Port 2222
    ```
    
* **Restrict User Logins**
    
    * Edit `/etc/ssh/sshd_config` on the remote server:
        
    
    ```plaintext
    AllowUsers user1 user2
    ```
    

### Additional Tips

* **Use SSH Escape Sequences**
    
    * `~.`: Disconnect
        
    * `~C`: Open command line
        
    * Type escape sequences after pressing Enter.
        
* **SSHFS (SSH Filesystem)**
    
    * Mount a remote filesystem via SSH:
        
    
    ```bash
    sshfs username@hostname:/remote/directory /local/mount/point
    ```
    

This cheatsheet covers essential commands and configurations for using SSH effectively, from basic connections to advanced tunneling and security practices. Adjust commands and paths according to your specific setup and needs.