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SSH Server Setup

Install and configure OpenSSH servers for StormTunnel connections.

Prerequisites

  • Root or sudo access to the server
  • Network access to the server
  • Firewall permissions for SSH port (default 22)

Operating System Guides

Ubuntu/Debian

# Install OpenSSH server
sudo apt update
sudo apt install openssh-server -y

# Verify installation
ssh -V

# Check service status
sudo systemctl status sshd

Basic configuration (/etc/ssh/sshd_config):

Port 22
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication yes
AllowTcpForwarding yes
PermitOpen any
LogLevel INFO
MaxAuthTries 6
MaxSessions 10
ClientAliveInterval 300
ClientAliveCountMax 2

Apply changes:

sudo sshd -t          # Test configuration
sudo systemctl restart sshd
sudo systemctl enable sshd

CentOS/RHEL

# Install OpenSSH server
sudo yum install openssh-server -y
# Or on RHEL 8/9
sudo dnf install openssh-server -y

# Start and enable
sudo systemctl start sshd
sudo systemctl enable sshd

# Allow through firewall
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload

macOS

macOS includes OpenSSH. Enable via:

System Settings:

  1. Open System Settings → General → Sharing
  2. Toggle Remote Login on

Command line:

sudo systemsetup -setremotelogin on

SSH Key Authentication

Server Setup

# Create SSH user (if needed)
sudo useradd -m -s /bin/bash tunneluser
sudo passwd tunneluser

# Set up authorized_keys
sudo su - tunneluser
mkdir -p ~/.ssh
chmod 700 ~/.ssh
touch ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys
exit

Add Public Keys

From client (easiest):

ssh-copy-id [email protected]

Manual:

Add the public key to /home/tunneluser/.ssh/authorized_keys:

ssh-ed25519 AAAAC3NzaC1lZDI1... user@hostname

Verify permissions:

ls -la /home/tunneluser/.ssh/
# drwx------ .ssh
# -rw------- authorized_keys

Port Forwarding Configuration

Edit /etc/ssh/sshd_config:

# Allow local forwarding only (recommended)
AllowTcpForwarding local

# Or allow all forwarding
AllowTcpForwarding yes

# Prevent remote binding
GatewayPorts no

# Restrict to specific ports (optional)
PermitOpen localhost:5432 localhost:3306
# Or allow all
PermitOpen any

Test port forwarding:

# From client
ssh -L 8080:localhost:80 [email protected]

# In another terminal
curl http://localhost:8080

Security Hardening

Production Configuration

# Authentication - KEY ONLY
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
PermitEmptyPasswords no

# Port forwarding - RESTRICTED
AllowTcpForwarding local
GatewayPorts no
PermitOpen localhost:5432 localhost:3306 localhost:6379

# Logging - VERBOSE
LogLevel VERBOSE
SyslogFacility AUTH

# Limits
MaxAuthTries 3
MaxSessions 5
LoginGraceTime 30
ClientAliveInterval 300
ClientAliveCountMax 2

# Security
X11Forwarding no
AllowAgentForwarding no
PermitTunnel no

# Access control
AllowUsers tunneluser deployuser

Disable Password Authentication

Only after verifying SSH key authentication works!

# Edit config
sudo nano /etc/ssh/sshd_config
# Set: PasswordAuthentication no

# Test and restart
sudo sshd -t
sudo systemctl restart sshd

Warning

Keep a terminal session open until you verify key-based access works!

Logging

Log Locations

OS Log File
Ubuntu/Debian /var/log/auth.log
CentOS/RHEL /var/log/secure
macOS log show --predicate 'process == "sshd"'

View Logs

# Recent SSH activity
sudo tail -50 /var/log/auth.log | grep sshd

# Follow in real-time
sudo tail -f /var/log/auth.log | grep sshd

# Failed attempts
sudo grep "Failed password" /var/log/auth.log

# Successful logins
sudo grep "Accepted publickey" /var/log/auth.log

Troubleshooting

Connection Refused

# Check if SSH is running
sudo systemctl status sshd

# Check if port is listening
sudo ss -tlnp | grep :22

# Check firewall
sudo ufw status            # Ubuntu
sudo firewall-cmd --list-all  # CentOS

Permission Denied (publickey)

# Check authorized_keys permissions
ls -la ~/.ssh/
# Fix if needed
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

# Check ownership
sudo chown -R tunneluser:tunneluser /home/tunneluser/.ssh

# Test with verbose output
ssh -vvv [email protected]

Port Forwarding Not Working

# Check sshd_config
sudo grep -i allowtcpforwarding /etc/ssh/sshd_config
# Should be: AllowTcpForwarding yes or local

# Check if target service is running
sudo ss -tlnp | grep 5432

Configuration Examples

Development Server

Port 22
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication yes
AllowTcpForwarding yes
PermitOpen any
LogLevel INFO
MaxAuthTries 6
MaxSessions 10

Staging Server

Port 22
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
AllowTcpForwarding local
PermitOpen localhost:5432 localhost:3306
LogLevel VERBOSE
MaxAuthTries 3
MaxSessions 5
AllowUsers deployuser tunneluser

Production Server

Port 22
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
AllowTcpForwarding local
GatewayPorts no
PermitOpen localhost:5432 localhost:3306 localhost:6379
X11Forwarding no
AllowAgentForwarding no
PermitTunnel no
LogLevel VERBOSE
MaxAuthTries 3
MaxSessions 3
LoginGraceTime 30
ClientAliveInterval 300
ClientAliveCountMax 2
AllowGroups sshusers
UsePAM yes

Testing Checklist

After configuration:

  • SSH service running (sudo systemctl status sshd)
  • Firewall allows SSH
  • SSH key authentication works
  • Password authentication disabled (if production)
  • Port forwarding works
  • Logs are being written
  • Configuration valid (sudo sshd -t)

Next Steps