Connect and run command

ssh user@hostname 'ls -la' ssh user@hostname 'uptime && df -h'


### Interactive use
```bash
# 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

Connect and run command

SSH Keys

Generating keys

Managing keys

SSH agent

Port Forwarding & Tunneling

Local port forwarding

Remote port forwarding

Dynamic port forwarding (SOCKS proxy)

Background tunnels

Configuration

SSH config file (~/.ssh/config)

Using config

File Transfers

SCP (Secure Copy)

SFTP (Secure FTP)

Rsync over SSH

Security Best Practices

Hardening SSH

Connection security

Troubleshooting

Debugging

Common issues

Advanced Operations

Jump hosts (ProxyJump)

Multiplexing

Execute commands

Tips

Documentation

SSH config file (`~/.ssh/config`)

ssh-essentials

SSH Essentials

Basic Connection

Connecting