SSH Basics

# Install OpenSSH server
sudo apt update
sudo apt install openssh-server -y
 
# Start and enable on boot
sudo systemctl enable --now ssh
 
# Check running status
sudo systemctl status ssh
 
# Check SSH listening port
ss -tlnp | grep ssh

# Ubuntu comes with the client pre-installed. If not:
sudo apt install openssh-client -y

# Generate an Ed25519 key (recommended)
ssh-keygen -t ed25519 -C "your_email@example.com"
 
# Generate an RSA 4096-bit key (better compatibility)
ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
 
# Specify a filename
ssh-keygen -t ed25519 -f ~/.ssh/id_myserver -C "myserver key"

# Method 1: Use ssh-copy-id (recommended)
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@server-ip
 
# Method 2: Manual copy
cat ~/.ssh/id_ed25519.pub | ssh user@server-ip "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"
 
# Method 3: If you already have the public key content, add it directly on the server
echo "ssh-ed25519 AAAA... your_email@example.com" >> ~/.ssh/authorized_keys

# Start the SSH Agent
eval "$(ssh-agent -s)"
 
# Add a key to the Agent
ssh-add ~/.ssh/id_ed25519
 
# List loaded keys
ssh-add -l
 
# Remove all loaded keys
ssh-add -D

# ~/.ssh/config example
Host myserver
    HostName 192.168.1.100
    User ubuntu
    Port 22
    IdentityFile ~/.ssh/id_myserver
 
Host production
    HostName prod.example.com
    User deploy
    Port 2222
    IdentityFile ~/.ssh/id_prod
    ForwardAgent yes
 
Host jump
    HostName jump.example.com
    User admin
 
# Connect to an internal server via a jump host
Host internal
    HostName 10.0.0.50
    User admin
    ProxyJump jump
 
# Default settings for all hosts
Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent yes
    Compression yes

# Connect directly using the alias
ssh myserver
 
# Equivalent to
ssh -i ~/.ssh/id_myserver -p 22 ubuntu@192.168.1.100

# Back up the original configuration
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
 
# Edit the configuration
sudo nano /etc/ssh/sshd_config

# /etc/ssh/sshd_config
 
# Change the default port (reduces scanning risk)
Port 2222
 
# Listen only on a specific address
ListenAddress 0.0.0.0
 
# Disable remote root login
PermitRootLogin no
 
# Disable password authentication (ensure key-based login is configured first)
PasswordAuthentication no
 
# Disable empty passwords
PermitEmptyPasswords no
 
# Enable public key authentication
PubkeyAuthentication yes
 
# Limit maximum authentication attempts
MaxAuthTries 3
 
# Limit maximum concurrent unauthenticated connections
MaxStartups 10:30:60
 
# Set login timeout
LoginGraceTime 30
 
# Disable X11 forwarding (usually not needed on servers)
X11Forwarding no
 
# Disable insecure authentication methods
KbdInteractiveAuthentication no
 
# Display last login information
PrintLastLog yes
 
# Client keepalive detection
ClientAliveInterval 300
ClientAliveCountMax 2
 
# Allow only specific users to log in
AllowUsers ubuntu deploy
 
# Or allow only specific groups
# AllowGroups sshusers

# Check configuration syntax
sudo sshd -t
 
# Reload configuration (does not disconnect existing sessions)
sudo systemctl reload ssh

# Create a custom configuration
sudo tee /etc/ssh/sshd_config.d/hardening.conf << 'EOF'
PermitRootLogin no
PasswordAuthentication no
MaxAuthTries 3
X11Forwarding no
EOF
 
sudo systemctl reload ssh

# Basic connection
ssh user@server-ip
 
# Specify a port
ssh -p 2222 user@server-ip
 
# Specify a key
ssh -i ~/.ssh/id_myserver user@server-ip
 
# Execute a remote command
ssh user@server-ip "df -h && free -h"
 
# Connect in verbose mode (for debugging)
ssh -v user@server-ip
ssh -vvv user@server-ip  # Even more verbose

# SCP: Copy a file to remote
scp localfile.txt user@server-ip:/remote/path/
 
# SCP: Copy a file from remote
scp user@server-ip:/remote/file.txt ./local/
 
# SCP: Copy a directory
scp -r ./local-dir user@server-ip:/remote/path/
 
# SFTP: Interactive file transfer
sftp user@server-ip
 
# rsync: Incremental sync (recommended for large numbers of files)
rsync -avz --progress ./local-dir/ user@server-ip:/remote/dir/

# Local port forwarding: Map a remote service to localhost
# Access local port 8080 to reach remote MySQL
ssh -L 8080:localhost:3306 user@server-ip
 
# Remote port forwarding: Expose a local service to the remote machine
# The remote machine's port 9090 will forward to local port 3000
ssh -R 9090:localhost:3000 user@server-ip
 
# Dynamic port forwarding (SOCKS proxy)
ssh -D 1080 user@server-ip
 
# Run port forwarding in the background
ssh -fNL 8080:localhost:3306 user@server-ip

# Connect to a target server via a jump host
ssh -J jump-user@jump-host target-user@target-host
 
# Multi-hop jumping
ssh -J user1@jump1,user2@jump2 user@target

# Correct permission settings
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_ed25519          # Private key
chmod 644 ~/.ssh/id_ed25519.pub      # Public key
chmod 600 ~/.ssh/authorized_keys     # Authorized keys
chmod 600 ~/.ssh/config              # Client configuration

# Install fail2ban
sudo apt install fail2ban -y
 
# Create local configuration
sudo tee /etc/fail2ban/jail.local << 'EOF'
[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log
maxretry = 3
bantime = 3600
findtime = 600
EOF
 
# Start the service
sudo systemctl enable --now fail2ban
 
# Check ban status
sudo fail2ban-client status sshd
 
# Manually unban an IP
sudo fail2ban-client set sshd unbanip 1.2.3.4

# Allow SSH only from a specific subnet
sudo ufw allow from 192.168.1.0/24 to any port 22
 
# Rate-limit connections (max 6 connections within 30 seconds)
sudo ufw limit ssh

# View SSH service logs
sudo journalctl -u ssh -f
 
# View authentication logs
sudo tail -f /var/log/auth.log
 
# Test connection (verbose mode)
ssh -vvv user@server-ip
 
# Check server configuration syntax
sudo sshd -t
 
# Check the authorized_keys file
cat ~/.ssh/authorized_keys
 
# Check whether SELinux/AppArmor is blocking connections
sudo aa-status

# Check that the client key is correct
ssh-add -l
 
# Check authorized_keys permissions on the server
ls -la ~/.ssh/
ls -la ~/.ssh/authorized_keys
 
# Confirm public key authentication is enabled in sshd_config
grep PubkeyAuthentication /etc/ssh/sshd_config

# Check whether the SSH service is running
sudo systemctl status ssh
 
# Check the port
ss -tlnp | grep ssh
 
# Check the firewall
sudo ufw status

# Remove the old host key
ssh-keygen -R server-ip
 
# Or edit the known_hosts file to delete the corresponding line
nano ~/.ssh/known_hosts