make_tun.sh

#!/bin/bash

# Ensure the script is run with sudo or root privileges
if [ "$EUID" -ne 0 ]; then
  echo "Please run as root or use sudo"
  exit
fi

HOST=$1

# Determine the correct username
if [ -n "$SUDO_USER" ]; then
  USERNAME="$SUDO_USER"
  HOME="/home/$USERNAME"
else
  USERNAME="root"
fi

# Function to check if Host exists in ~/.ssh/config
host_exists() {
  local host=$1
  awk -v host="$host" '
    $1 == "Host" && $2 == host { print "exists"; exit }
  ' ~/.ssh/config
}

# Function to fetch option by Host  ~/.ssh/config
get_ssh_option() {
  local host=$1
  local option=$2
  awk -v host="$host" -v option="$option" '
    $1 == "Host" { in_host_block = ($2 == host); next }
    in_host_block && $1 == option { print $2 }
  ' ~/.ssh/config
}

if [ ! -z "${HOST}" ]; then

  if [ ! -f "$HOME/.ssh/config" ]; then
    echo "$HOME/.ssh/config does not exists"
    exit 1
  fi

  if [[ -z "$(host_exists "$HOST")" ]]; then
    echo "Error: Host '$HOST' not found in ~/.ssh/config"
    exit 1
  fi

  SERVER_IP=$(get_ssh_option "$HOST" "Hostname")
  SERVER_USER=$(get_ssh_option "$HOST" "User")
  SERVER_PORT=$(get_ssh_option "$HOST" "Port")
  SSH_KEY_PATH=$(get_ssh_option "$HOST" "IdentityFile")
  SSH_KEY_PATH="${SSH_KEY_PATH/#\~/$HOME}"  # conver ~ to $HOME
fi

# Prompt for the necessary parameters with default values
if [[ -z "${SERVER_IP}" ]]; then
  read -p "Public server IP (required): " SERVER_IP
  if [ -z "$SERVER_IP" ]; then
    echo "Public server IP is required."
    exit 1
  fi
else
  echo "Server IP: $SERVER_IP" 
fi

if [[ -z "${SERVER_USER}" ]]; then
  read -p "Public server User [user]: " SERVER_USER
  SERVER_USER=${SERVER_USER:-user}
else
  echo "Public server user: $SERVER_USER" 
fi

if [[ -z "${SERVER_PORT}" ]]; then
  read -p "Public server SSH port [22]: " SERVER_PORT
  SERVER_PORT=${SERVER_PORT:-22}
else
  echo "Public server port: $SERVER_PORT" 
fi

if [[ -z "${SSH_KEY_PATH}" ]]; then
  read -p "Public server SSH key path [$HOME/.ssh/id_rsa]: " SSH_KEY_PATH
  SSH_KEY_PATH=${SSH_KEY_PATH:-$HOME/.ssh/id_rsa}
else
  echo "Public server ssh key: $SSH_KEY_PATH" 
fi

if [ ! -f "$SSH_KEY_PATH" ]; then
  echo "SSH key file does not exist at $SSH_KEY_PATH."
  exit 1
fi

# Check if SSH key requires a password
if ssh-keygen -y -f "$SSH_KEY_PATH" >/dev/null 2>&1; then
  echo "SSH key does not require a password."
else
  echo "SSH key requires a password."
  exit 1
fi

# Check if SSH key works and can establish a connection
if ssh -i "$SSH_KEY_PATH" -o StrictHostKeyChecking=accept-new -o ConnectTimeout=5 -p "$SERVER_PORT" "$SERVER_USER@$SERVER_IP" exit; then
  echo "SSH key works and connection can be established."
else
  echo "SSH key does not work or cannot establish a connection."
  exit 1
fi

read -p "Public server forwarding port (required): " REMOTE_PORT
if [ -z "$REMOTE_PORT" ]; then
  echo "Public server forwarding port is required."
  exit 1
fi

read -p "Local receiving port [22]: " LOCAL_PORT
LOCAL_PORT=${LOCAL_PORT:-22}

# Function to check if the service name already exists and prompt for a new one if it does
set_service_name() {
    local service_name
    read -p "Local Service name [backtun-$SERVER_IP.service]: " service_name
    service_name=${service_name:-"backtun-$SERVER_IP.service"}

    if [[ -f "/etc/systemd/system/$service_name" ]]; then
        echo "Service already exists: /etc/systemd/system/$service_name"
        set_service_name # Recursively call the function to prompt for a new name
    else
        LOCAL_SERVICE_NAME="$service_name"
    fi
}
# Initial function call to start the checking process
set_service_name

# Create the systemd service unit file
SERVICE_FILE="/etc/systemd/system/$LOCAL_SERVICE_NAME"

sudo bash -c "cat > $SERVICE_FILE" << EOL
[Unit]
Description=SSH Reverse Tunnel
After=network.target

[Service]
ExecStart=/usr/bin/ssh -i $SSH_KEY_PATH -o StrictHostKeyChecking=accept-new -o ExitOnForwardFailure=yes -o ServerAliveInterval=60 -o ServerAliveCountMax=3 -p $SERVER_PORT -N -R $REMOTE_PORT:localhost:$LOCAL_PORT $SERVER_USER@$SERVER_IP
Restart=always
RestartSec=3
User=$USERNAME

[Install]
WantedBy=multi-user.target
EOL

# Reload systemd to recognize the new service, enable it, and start it
sudo systemctl daemon-reload
sudo systemctl enable $LOCAL_SERVICE_NAME
sudo systemctl start $LOCAL_SERVICE_NAME

echo "SSH reverse tunnel systemd service created and started successfully."
echo "You can control this service with systemctl using name $LOCAL_SERVICE_NAME"