Secure Shell (SSH) for remote access and secure file transfers.

Basic Connection

Connecting

# Connect with username
ssh user@hostname
# Connect to specific port
ssh user@hostname -p 2222
# Connect with verbose output
ssh -v user@hostname
# Connect with specific key
ssh -i ~/.ssh/id_rsa user@hostname
# Connect and run command
ssh user@hostname 'ls -la'
ssh user@hostname 'uptime && df -h'

Interactive use

# Connect with forwarding agent
ssh -A user@hostname
# Connect with X11 forwarding (GUI apps)
ssh -X user@hostname
ssh -Y user@hostname  # Trusted X11
# Escape sequences (during session)
# ~. - Disconnect
# ~^Z - Suspend SSH
# ~# - List forwarded connections
# ~? - Help

SSH Keys

Generating keys

# Generate RSA key
ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
# Generate ED25519 key (recommended)
ssh-keygen -t ed25519 -C "your_email@example.com"
# Generate with custom filename
ssh-keygen -t ed25519 -f ~/.ssh/id_myserver
# Generate without passphrase (automation)
ssh-keygen -t ed25519 -N "" -f ~/.ssh/id_deploy

Managing keys

# Copy public key to server
ssh-copy-id user@hostname
# Copy specific key
ssh-copy-id -i ~/.ssh/id_rsa.pub user@hostname
# Manual key copy
cat ~/.ssh/id_rsa.pub | ssh user@hostname 'cat >> ~/.ssh/authorized_keys'
# Check key fingerprint
ssh-keygen -lf ~/.ssh/id_rsa.pub
# Change key passphrase
ssh-keygen -p -f ~/.ssh/id_rsa

SSH agent

# Start ssh-agent
eval $(ssh-agent)
# Add key to agent
ssh-add ~/.ssh/id_rsa
# List keys in agent
ssh-add -l
# Remove key from agent
ssh-add -d ~/.ssh/id_rsa
# Remove all keys
ssh-add -D
# Set key lifetime (seconds)
ssh-add -t 3600 ~/.ssh/id_rsa

Port Forwarding & Tunneling

Local port forwarding

# Forward local port to remote
ssh -L 8080:localhost:80 user@hostname
# Access via: http://localhost:8080
# Forward to different remote host
ssh -L 8080:database.example.com:5432 user@jumphost
# Access database through jumphost
# Multiple forwards
ssh -L 8080:localhost:80 -L 3306:localhost:3306 user@hostname

Remote port forwarding

# Forward remote port to local
ssh -R 8080:localhost:3000 user@hostname
# Remote server can access localhost:3000 via its port 8080
# Make service accessible from remote
ssh -R 9000:localhost:9000 user@publicserver

Dynamic port forwarding (SOCKS proxy)

# Create SOCKS proxy
ssh -D 1080 user@hostname
# Use with browser or apps
# Configure SOCKS5 proxy: localhost:1080
# With Firefox
firefox --profile $(mktemp -d) \
  --preferences "network.proxy.type=1;network.proxy.socks=localhost;network.proxy.socks_port=1080"

Background tunnels

# Run in background
ssh -f -N -L 8080:localhost:80 user@hostname
# -f: Background
# -N: No command execution
# -L: Local forward
# Keep alive
ssh -o ServerAliveInterval=60 -L 8080:localhost:80 user@hostname

Configuration

SSH config file (~/.ssh/config)

# Simple host alias
Host myserver
    HostName 192.168.1.100
    User admin
    Port 2222
# With key and options
Host production
    HostName prod.example.com
    User deploy
    IdentityFile ~/.ssh/id_prod
    ForwardAgent yes
    
# Jump host (bastion)
Host internal
    HostName 10.0.0.5
    User admin
    ProxyJump bastion
Host bastion
    HostName bastion.example.com
    User admin
# Wildcard configuration
Host .example.com
    User admin
    ForwardAgent yes
    
# Keep connections alive
Host 
    ServerAliveInterval 60
    ServerAliveCountMax 3

Using config

# Connect using alias
ssh myserver
# Jump through bastion automatically
ssh internal
# Override config options
ssh -o "StrictHostKeyChecking=no" myserver

File Transfers

SCP (Secure Copy)

# Copy file to remote
scp file.txt user@hostname:/path/to/destination/
# Copy file from remote
scp user@hostname:/path/to/file.txt ./local/
# Copy directory recursively
scp -r /local/dir user@hostname:/remote/dir/
# Copy with specific port
scp -P 2222 file.txt user@hostname:/path/
# Copy with compression
scp -C large-file.zip user@hostname:/path/
# Preserve attributes (timestamps, permissions)
scp -p file.txt user@hostname:/path/

SFTP (Secure FTP)

# Connect to SFTP server
sftp user@hostname
# Common SFTP commands:
# pwd          - Remote working directory
# lpwd         - Local working directory
# ls           - List remote files
# lls          - List local files
# cd           - Change remote directory
# lcd          - Change local directory
# get file     - Download file
# put file     - Upload file
# mget .txt   - Download multiple files
# mput .jpg   - Upload multiple files
# mkdir dir    - Create remote directory
# rmdir dir    - Remove remote directory
# rm file      - Delete remote file
# exit/bye     - Quit
# Batch mode
sftp -b commands.txt user@hostname

Rsync over SSH

# Sync directory
rsync -avz /local/dir/ user@hostname:/remote/dir/
# Sync with progress
rsync -avz --progress /local/dir/ user@hostname:/remote/dir/
# Sync with delete (mirror)
rsync -avz --delete /local/dir/ user@hostname:/remote/dir/
# Exclude patterns
rsync -avz --exclude '*.log' --exclude 'node_modules/' \
  /local/dir/ user@hostname:/remote/dir/
# Custom SSH port
rsync -avz -e "ssh -p 2222" /local/dir/ user@hostname:/remote/dir/
# Dry run
rsync -avz --dry-run /local/dir/ user@hostname:/remote/dir/

Security Best Practices

Hardening SSH

# Disable password authentication (edit /etc/ssh/sshd_config)
PasswordAuthentication no
PubkeyAuthentication yes
# Disable root login
PermitRootLogin no
# Change default port
Port 2222
# Use protocol 2 only
Protocol 2
# Limit users
AllowUsers user1 user2
# Restart SSH service
sudo systemctl restart sshd

Connection security

# Check host key
ssh-keygen -F hostname
# Remove old host key
ssh-keygen -R hostname
# Strict host key checking
ssh -o StrictHostKeyChecking=yes user@hostname
# Use specific cipher
ssh -c aes256-ctr user@hostname

Troubleshooting

Debugging

# Verbose output
ssh -v user@hostname
ssh -vv user@hostname  # More verbose
ssh -vvv user@hostname  # Maximum verbosity
# Test connection
ssh -T user@hostname
# Check permissions
ls -la ~/.ssh/
# Should be: 700 for ~/.ssh, 600 for keys, 644 for .pub files

Common issues

# Fix permissions
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
chmod 644 ~/.ssh/authorized_keys
# Clear known_hosts entry
ssh-keygen -R hostname
# Disable host key checking (not recommended)
ssh -o StrictHostKeyChecking=no user@hostname

Advanced Operations

Jump hosts (ProxyJump)

# Connect through bastion
ssh -J bastion.example.com user@internal.local
# Multiple jumps
ssh -J bastion1,bastion2 user@final-destination
# Using config (see Configuration section above)
ssh internal  # Automatically uses ProxyJump

Multiplexing

# Master connection
ssh -M -S ~/.ssh/control-%r@%h:%p user@hostname
# Reuse connection
ssh -S ~/.ssh/control-user@hostname:22 user@hostname
# In config:
# ControlMaster auto
# ControlPath ~/.ssh/control-%r@%h:%p
# ControlPersist 10m

Execute commands

# Single command
ssh user@hostname 'uptime'
# Multiple commands
ssh user@hostname 'cd /var/log && tail -n 20 syslog'
# Pipe commands
cat local-script.sh | ssh user@hostname 'bash -s'
# With sudo
ssh -t user@hostname 'sudo command'

Tips

• Use SSH keys instead of passwords
• Use ~/.ssh/config for frequently accessed hosts
• Enable SSH agent forwarding carefully (security risk)
• Use ProxyJump for accessing internal networks
• Keep SSH client and server updated
• Use fail2ban or similar to prevent brute force
• Monitor /var/log/auth.log for suspicious activity
• Use port knocking or VPN for additional security
• Backup your SSH keys securely
• Use different keys for different purposes

Documentation

Official docs: https://www.openssh.com/manual.html Man pages: man ssh, man ssh_config, man sshd_config